A serpentine robot designed for efficient
Contents
1. 25 4 1 Segment Processor Board C U ns apne f si Sir ii ey ee Figure 21 Segment processor PCB Each segment of the robot contains a circuit board responsible for monitoring sensors driving servos and performing computations related to that segment Our robot has seven segments and as many processor boards Each board contains a Microchip PIC18F4685 RISC processor 17 operating at 5 MIPS The device contains a hardware Controller Area Network CAN transceiver that manages all network traffic It also has internal FLASH memory giving us the ability to reprogram the board in circuit after firmware modifications Currently the main task of the processor is to calculate a two axis PID control algorithm responsible for driving the two motors located in the segment To complete the servo motor loop the PCB also includes two motor drive ICs and two quaderature encoder counter ICs to manage motor position The board accepts four analog inputs two of which are dedicated to motor current measurement The remaining two channels can be used for other analog or digital sensor inputs Figure 22 is a block diagram of the hardware contained on the processor board 26 Microcontroller analog inputs motor current Figure 22 PCB block diagram 4 2 Drive Motor Processor Board Figure 23 Drive motor PCB In addition to the robot s segment processor boards there is one drive motor processor loca2. In this configuration the output voltage is linearly proportional to change in resistance through strain of the specimen and its sensitivity is twice that of the linearized quarter bridge Half Bridge Strain Gauge RI R2 R R3 R AR R4 R AR _ R AR OR RIOR FAR di 2R 2R LZR 10 Finally in the full bridge configuration all four resistors are replaced with strain gauges Equation 11 shows that this configuration provides an output that is linear with strain and it is four times as sensitive as the half bridge Full Bridge Strain Gauge RI R3 R AR R2 R4 R AR R AR OR R ER E 2Ry 2Ro LR 11 4 5 2 Motor Current In addition to being able to measure the external forces exerted on the snake it 1s important to know the internal forces acting on the robot s segments Specifically we would like to know what types of torques are being applied to the segments by the servo motors We measure these torques indirectly by taking advantage of the fact that the torque applied by a motor is directly proportional to the current drawn by the motor In this section we will discuss two simple methods of current measurement that were considered for use in the robot 41 Vsense LOAD AD CONVERTER Figure 33 Typical Current Sense Amp Application 14 The most common method of measuring small currents is by placing a small resistance in the current path and measuring the voltage drop ac
3. Tension Force Force Figure 29 Quarter Bridge Configuration An alternative wiring method is the half bridge configuration 18 figure 30 This device uses two strain gauges mounted such that when one is in tension the other is in compression The result is that the resistance of one element increases due to applied strain while the resistance of the other element decreases Figure 30 shows the physical mounting of the strain gauges and a wiring diagram for the half bridged The half bridge configuration has several major advantages over the quarter bridge 37 configuration 18 The first is that the output voltage swing using this method is approximately twice that of the quarter bridge Additionally the output signal is now a linear function of strain The final benefit of this configuration is that it has built in temperature compensation If both sensors are in the same environment as is the case here then ambient temperature changes will effect both sensors equally Since the resistance of the two sensors scale equally with temperature change the resistance ratio for a given strain remains constant The obvious drawback of this method is the need to mount two gauges in locations that experience equal and opposite strain In some cases this may not be possible but the design of the foot bracket makes this type of mounting simple 1 Strain Gauge in Compression 1 Strain Gauge in Tension Force Force Figure 30 Half B
4. This robot uses the industry standard Controller Area Network CAN 6 to move data between processors CAN was originally developed for use in automotive applications and accordingly has been designed for noise immunity and fault tolerance The physical bus is a differential pair which inherently provides immunity to electrical noise because any noise induced in one wire will also be induced in the other and this common mode signal is easily removed by the differential receiver amplifier The interface between the bus and the microcontroller is accomplished with a CAN bus transceiver This chip serves several important functions It electrically isolates the bus from the processor boards to prevent bus noise and electrical faults 28 from effecting the processor board s operation Additionally the transceiver has the ability to automatically detect faults and disconnect the offending processor board from the bus so that it cannot render the entire bus inoperative CAN is a peer to peer protocol meaning there is no bus master Any node can initiate a data transfer with any other node This is an important feature because certain distributed control topologies require the ability of segments to communicate directly with each other Master slave protocols like I2C 21 require that all communications are initiated by a pre designated master device and thus direct communication between two slaves is not possible One slave would h
5. back into service An added benefit of this type of modular design is that robot is essentially composed of many copies of the same mechanism simplifying the development process To see how these issues have been addressed we first take a system level look at the robot s mechanical structures and their interactions Subsequent sections provide a detailed look at individual component operation and design Motor Figure 1 Computer Rendering of Robotic Snake Figure 1 shows a rendering of the robotic snake The robot contains fourteen ribs and is approximately 30 long At the bottom of each rib is a pair of feet that carry the robot forward Forward motion is powered by a drive motor located at the tail of the snake Processor Segment Motor Segment One Segment Articulating Motor Figure 2 Zoomed in look at three adjacent ribs Feet Figure 2 shows a zoomed in view of three adjacent ribs From this picture it is clear that the robot is composed of two types of ribs The first contains two servo motors used for articulation while the second contains a circuit board and batteries These are named motor ribs and processor ribs respectively Starting from the head of the snake ribs alternate between motor and processor type There are a total of seven motor ribs and seven processor ribs The design is such that referring to figure 2 the motors located on a particular motor segment are driven by the circuitry lo
6. command is used to turn these LEDs off identifier 0010s 1 aaaaa data bytes 0 data N A notes With s 0 the Green LED is turned off With s 1 the Red LED is turned off _UPDATE Set new integral gain type general call description changes integrall gain of motor s identifier 0100s 1 aaaaa data bytes 4 data 32 bit float IEEE 754 format notes At power up this value is retrieved from non volatile memory The contents of this memory can be modified with the STORE operation STORE Store current parameters ot ROM type general call description copy the P I and D constants for motors 0 and 1 from RAM to ROM identifier 0111s 1 aaaaa data bytes 2 data byte 0 0x55 byte 1 OxAA notes At power up the P I and D gain coefficients are loaded from ROM to the motor control routine This command provides a means of modifying these values so they are restored on power up 62 LED_ON Turns LED on type addressed description Each drive board contains two uncommitted LED that can be used for diagnostics This command is used to turn these LEDs on identifier 0001s 1 aaaaa data bytes 0 data N A notes With s 0 the Green LED is turned on With s 1 the Red LED is turned on P_UPDATE Set new proportional gain type general call description changes proportional gain of motor s identifier 0011s 1 aaaaa data bytes 4 data 32 bit float IEEE 754 format notes At power up
7. interest in such robots is in having them conduct tasks that require complicated locomotion and cognition The biological creatures after which the human made robots are designed manifest a remarkable degree of efficiency and agility when compared to what we have been able to mimic so far in human made designs For example the small cross section and low center of gravity of most biological snakes coupled with their large repertoire of possible motion sequences make their bodies very efficient when navigating confined spaces and rough terrains To date no artificial snake has been able to come close to duplicating these navigational characteristics In this study we concentrate on a set of motions observed in medium size 1 4m biological snakes There are currently several robot designs that attempt to reproduce the movements of such snakes Almost all of these designs require the robot to articulate segments of its body in a repetitive sequence to achieve locomotion and some even attach passive wheels to the snake s body in order to facilitate movement As a result of these design decisions the artificial snakes are generally slow and most especially those with wheels are not well suited for travel over rough terrain We offer an alternative design that propels the snake using many small feet attached to disk like body units ribs Due to the superior flexibility that this design provides the resulting robot which we have built a
8. must be equal 14 Figure 9 Constant Velocity Joint The kinematic analysis is repeated using the modified cams Again there are seven cams each with a bending angle of 25 degrees Applying a constant angular velocity at the first u joint we measure the angular velocity of each downstream joint The results of this test are shown in figure 10 where angular velocity is plotted verses time Note that now the non ideal behavior is confined to even cams while the odd cams move with constant angular velocity The velocity ripple that remains in the odd numbered cams cannot be removed but at the joint angles produced by the snake it will be confined to about 10 drive motor velocity Angular Velocity vs Time CV joints Q a o je e o D a o Odd Cams a Even Cams Velocity deg sec e a 8 o o o a o Time s Figure 10 Constant Velocity Joint Analysis 13 3 4 Foot Mechanism As the drive motor rotates it imparts motion to the snake s universal joints section 4 3 As the u joint rotates so to do the integral cams see figure 5 which couple drive motor power to the foot mechanism The foot mechanism is a group of mechanical linkages which convert rotational motion from the cam into an orbital walking motion in the snake s feet Figure 11 shows a series of snap shots of the foot mechanism over one full rotation of the cam Viewing the figure left to right to
9. software reset Same effect as issuing a herdware reset Node does nothing and waits for START command identifier 00010 1 00000 data bytes 2 data byte 0 0x55 byte 1 OxAA notes In RESET mode the node s CAN REPORT_FREQ address may be changed Once START command is issued address change is locked out Set new data report frequency type general call description Changes the frequency at which the node broadcases its current motor position and motor current draw values identifier 00100 1 00000 data bytes 1 data byte 0 report frequency in hertz notes The report frequency is sent as an unsigned integer Data reporting can be disable by setting this value to 0 CAN_ADD Change node s CAN address type general call description Modifies the CAN address of a node identifier 00110 1 00000 data bytes 3 data byte 0 0x55 byte 1 OxAA byte 2 address 1 to 31 notes This command can only be issued immdiatly after node reset Once the START command is issued this feature is locked out The address must be in the range of 1 to 31 SETPOINT Set new motor command position type addressed description Motor drivers are disabled but all control acquisition and communication processes remain active identifier 00001 1 aaaaa data bytes 0 data N A notes LED_OFF Turns LED off type addressed description Each drive board contains two uncommitted LED that can be used for diagnostics This
10. the objective of these tests is to demonstrate basic capabilities of the robot we will present performance measures where appropriate e g time to complete maneuver maximum current draw In each test a predefined trajectory in space was defined and a MATLAB routine was used to calculate the articulations required for the snake to adhere to and crawl along the curve In this sense the behavior of the robot is open loop It assumes perfect knowledge of the terrain over which is it crawling and that there is no slip as it 52 moves over the surface In future experimentation sensor input will be used to allow the robot to cope with uncertainties in its surroundings 6 3 1 Motion in the Plane In this test the robot follows a predefined planar path The S shaped path shown in figure 39 has a bending radius of approximately eight inches Figure 39 Trajectory followed in planar movement test Figure 40 Robot during planar movement test 53 During this maneuver the robot crawled a distance of sixty 60 inches in approximately forty 40 seconds The average current draw was 1 2 amps 6 3 2 Cantilevering Gap Crossing In this maneuver the robot crawls across a seven 7 inch gap figure 40 This behavior is important because it arises frequently in situations where the robot attempts to traverse uneven terrains e g crawling over large rocks While this test shows that this robot can successfully cross gaps of app
11. with respect to each other while the latter can expand and contract like an accordion He further outlines several locomotion algorithms for robots of these types but does not address their construction and testing We observe that robots of this family are expected to be slow and require extensive operator input In addition the precise interaction between segments which is required for efficient locomotion can be difficult to achieve Downing 1 goes into great detail discussing the possible construction of inextensible snake like robots but does not settle on a particular design Our approach developed through the study of these papers as well as several prototypes that were built earlier in Philadelphia area laboratories proposes an alternative design for robotic snakes This design does not fall into any of the previous classes Our robot propels itself using many small feet with locomotion similar to that of a millipede Additionally the power to drive all of the snake s feet is derived from a single motor located in the tail of the robot Moving the snake forward is a simple matter of controlling this drive motor Comparing this approach to locomotion with that used in most other robotic snake designs we see that most other robots of this class accomplish forward motion through a complex sequence of body movements called a gait While this articulation is effective in moving the robot forward even over rough terrains the forw
12. www sharpsma com Appendix A Mechanical Components Figure 42 Motor Frame 5 o A A Figure 43 Foot Assembly 57 Figure 45 Motor Mount Figure 46 Pinion Figure 47 Cam U Joint 58 59 Le A a Figure 48 Lower Rocker Figure 49 Upper Rocker Figure 50 Transfer Arm Assembly 60 Coupling Figure 51 Spool Figure 52 mer Cable Retai Figure 53 Appendix B Instruction Set STOP Disable Motors type general call description Motor drivers are disabled but all control acquisition and communication processes remain active identifier 00001 1 00000 data bytes 0 data N A notes START Start motors type general call description Enables motor drivers The first time this command is issued after reset it also causes entry into normal operation mode identifier 00011 1 00000 data bytes 2 data byte 0 0x55 byte 1 OxAA notes This command causes the command motor position to equal the current motor position preventing the robot from jumping to a prior setpoint upon motor re enable PID_FREQ Set PID update frequency type general call description Changes PID loop execution frequency identifier 00101 1 00000 data bytes 1 data byte 0 update frequency in hertz notes The report frequency is sent as an unsigned integer 61 RESET Place node into reset type general call description Executes a
13. 0406 Carl Stahl Sava Industrial Inc Riverdale NJ www savacablr com Dual Axis 1 7g Accelerometer with SPI Interface Analog Devices Inc 2005 www analog com Low Cost 300 Ysec yaw Rate Sensor with SPI Interface Analog Devices Inc 2005 www analog com 1 and 2 axis Magnetic Sensors HMC1001 1002 HNC 1021 1022 Honeywell Sensor Products 2000 www ssec honeywell com magnetic Single Dual Quad High Side Current Sense Amplifiers with Internal Gain Maxim Integrated Products Sunnyvale CA 2001 www maxim ic com 15 16 17 18 19 20 21 22 56 Fully Integrated Hall Effect Based Linear Current Sensor with Voltage Isolation and a Low Resistance Current Conductor Allegro Microsystems Inc www allegromicro com Dual Full Bridge PWM Motor Driver 3966 Allegro Microsystems Inc www allegromicro com 28 40 Pin High Performance Enhanced Flash Microcontrollers w CAN Module Microchip Technologies Inc 2004 www microchip com Strain Gauge Measurement A Tutorial Application Note 078 National Instruments December 1995 www ni com Coreless DC Motors Data Sheet Series 2232SR Faulhaber Group Sch naich Germany www faulhber com LPC2109 2119 2129 Product Data Sheet NXP Semiconductor 2007 www nxp com UM10204 I C bus specification and user manual NXP Semiconductor 2007 www nxp com GP2D120 General Purpose Distance Measuring Sensor Sharp Microelectronics 2000
14. A Serpentine Robot Designed for Efficient Rectilinear Motion A Thesis Submitted to the Faculty of Drexel University by Richard Anthony Primerano in partial fulfillment of the requirements for the degree of Master of Science September 2008 Copyright 2008 Richard A Primerano All Rights Reserved ill Table of Contents E A Ate ous nats Ak ds Aa aca alien asians havin his v List OU ACTON GANS A vii Eist Of CLOT YING ieren aE o E vii ADMI A a es Vili HOC A A 1 A a A dit es ais 2 2 Mechanical LOSS WOT RE RE nec Me Nine 5 2 1 OA 5 2 2 Frame nn a TEREE EAE Ne NA EN nn unir 8 2 2 1 Design Options Future Considerations oooooccnnocononccooncconcconnnonn nono nocanacnnos 9 23 Cam Tommie yee NE o e en 10 2 3 1 Kinematics of the U Joint and CV Joint 11 2 4 Foot Mechas a 15 24 1 Design Options Future Considerations oooooconocccconoconnnonnnconncconocannconnnos 16 25 a Re ann 16 2 5 1 Design Options Future Considerations 22 O e A i E A E waive aol rt deal 23 3 1 Segment Processor Board 5 sy disses Re id A 25 312 Drive Motor Processor Boards dt usant 26 3 2 1 Design Options Future Considerations ooooccnoccnoncnononconncconoconn nono ncnnncnns 26 3 3 COMMUN Cation BUS pic 27 3 4 Control Topology esei Sn rt enue ees dl Soe QE nn 29 SAM Instruction SERRE sc ences ost ee alone era acemases 30 3 5 A chest task cutee E dct ats Re nf nt dm ne ate yank AY 32 A TO aay a et
15. Figure 26 shows a segment experiencing a force on each of its feet This force causes a small deflection in the foot bracket which is sensed by the attached strain gauge Note that the left and right feet have their own strain gauges and can measure force independently Foot Bracket Force Figure 26 Force Measurement and Foot Bracket Location 34 Figure 27 shows a foil strain gauge 18 similar to the one used on this robot Strain gauges of this type are made by embedding a foil measurement element in a polyimide substrate The device is approximately 50um thick and is bonded directly to the specimen whose strain is to be measured As the specimen experiences strain so too does the foil element This strain subsequently causes a change in the element s resistance which is converted to a voltage and amplified by a conditioning circuit As was mentioned earlier we measure strain on the foot bracket to determine applied force To determine the optimal location of the strain gauge we performed a finite element analysis FEA on the bracket This test shows graphically where the largest strains are experienced Naturally to get the best sensitivity from the device we place the gauge at the location that experiences the largest strain under applied load Figure 28 shows the results of a FEA performed in ProEngineer In simulation force was applied at the bottom of the bracket while the top of the bracket was held fixed Under applied l
16. actile sensors the snake continually monitors the force it exerts on the ground If the snake senses an uneven force distribution over its segments it will alert the operator 48 Figure 37 shows an example of stability monitoring When the snake is lying firmly on a surface each of its feet exerts an equal force on that surface panel 1 As the robot begins to cantilever for example over the edge of a table fewer feet are supporting it and the force distribution is no longer uniform The second panel shows this case As a result the forces exerted by the feet closest to the edge are largest If the snake proceeds much further it will fall over the edge of the table panel 3 By monitoring the forces exerted on its feet the snake can prevent itself from falling of the table 5 1 2 Ground Hugging Figure 38 Ground Hugging As the snake moves forward as instructed by the operator it should naturally follow the contour of the ground beneath it This process much like stability monitoring requires reading the force sensors and driving servo motors so that the robot maintains uniform contact with the ground Figure 38 shows a case when all of the snake s segments are touching the ground As the snake crawls forward it monitors these sensors to ensure its segments maintain uniform contact Data reported by these sensors can also be used to form a map of the underlying ground As the snake bend 49 to maintain contact with th
17. al has much better impact resistance than acrylic allowing us to use thinner cross sections for the rib construction Not only will the ribs be stronger but they will considerably lighter The newly designed rib is approximately 35 lighter than the original rib This change alone will translate into a 20 reduction in weight of the overall robot The table below compares acrylic with injection molded ABS For our purposes the most important of these values are the impact test results For example the unnotched Izod impact test shows that ABS can absorb at least four times more impact energy than acrylic before failure Crack propagation in acrylic tends to be very rapid Small cracks that may form during the life of the robot can propagate quickly and lead to failure of the part This behavior is associated with brittle failure On the other hand ABS exhibits ductile failure which means that cracks and defects to not spread quickly but rather the part bends before if ultimately fails These classifications are 10 reflected in the notched Izod impact test Here the impact strength of the material is reduced substantially due to the addition of a notch ABS is available in many grades with varying mechanical properties For the second version of this robot we will choose as ABS that exhibits good impact strength and elasticity 2 Property Acrylic L ABS Density 0 0415 0 043 Ib in 0 0368 0 0437 Ib in Elongation
18. al or all simultaneously Of the current commercially available low level serial protocols CAN seems the most appropriate for this application The second generation of this device will also use this standard 4 4 Control Topology CAN bus Centralized high level control Torque sensors Distributed low level control Figure 24 Control Network Topology Figure 24 shows the control hierarchy used by the robotic snake The robot s processor boards are networked to each other and to a personal computer PC In the current configuration the PC issues commands to the robot and reads sensor data from the robot Gait generation sensor processing and other intelligent tasks are executed centrally by the PC The robot s local processors are responsible for executing commands issued by the PC but are not given the ability to process or react to sensor data This division of labor with the PC handling high level tasks and the snake s processors handling low level tasks is inefficient in the sense that the 30 snake s local processing power is not being fully utilized Despite its inefficiency however this division has several benefits especially in the early stages of control algorithm development For example the robotic snake consists of fifteen 15 actuators and approximately thirty 30 sensors For this system it is considerably simpler to implement a centralized control strategy than a dist
19. al pair of gears and a spool As the motor rotates the spool winds a control cable which ultimately pulls on neighboring segments providing articulation of the robot The control cable is 0 030 diameter nylon coated pre stretched aircraft cable This cable has a breaking strength of 80 lbs and since it is pre stretched its elongation under load will be negligible Figure 14 shows the complete servo gear train Figure 14 Servo Gear Train For control purposes it is necessary to relate encode counts to control cable take up i e the length of cable would around the spool First we will develop the input output relationship between the encoder and spool E counts 43 men ns _ 63 508 counts 63 508 cpr 2 revolution revolution 13 revolutions revolution From equation 2 every rotation of the output shaft spool requires 63 508 encoder counts We are now interested in relating control cable take up in inches to spool rotation In other works we will find how many inches of cable are required to wrap 19 around the spool one time The radius of the take up spool is 0 063 This will not be used in the calculation of the spool s effective circumference however Instead we will take into account the thickness of the control cable which wraps around the spool The effective radius of the spool will be a pitch circle that lies on the centerline of the cable as it wraps around the spool Figure 15 we see tha
20. ally made for larger current spans Using such a device in our design would require an additional amplifier before A D conversion and the signal chain would suffer from low SNR Using a device such as the MAX4376 on the other hand allows us to sense current and amplify the signal in one process Additionally resistive current sensors provide excellent noise performance For these reasons we have decided to use resistive current sensing We had originally intended to implement high side current sensing but because of the construction of the motor driver IC we were forced to use the low side sensing technique The disadvantage of this method added impedance and noise in the ground path will not be an issue for us because the motor return is not shared by any of the sensitive circuitry 43 Pn3 Fins IP P Voltage Regulator P P Pin 1 Fin 2 Figure 34 Block diagram of Allegro Hall Effect current sensor 15 4 5 3 Heading Orientation An important feature of many autonomous robots is the ability to accurately measure its heading and orientation This information is necessary for both navigation and control The robotic snake accomplishes heading orientation sensing with an inertial measurement unit IMU mounted in its tail An optional second IMU can be placed in the head of the snake This device consists of a solid state six degree of freedom 6DOF sensor with 3 axis magnetometer The 6DOF sensor consists of six components th
21. ard motion of these types of robots is generally very slow The main objective of our design was to construct a robot that could efficiently perform straight line motion with minimal power consumption and computation Since this robot will actually walk rather than dragging itself we expect it to be capable of navigating rough terrains more efficiently than existing prototypes 3 Mechanical Design 3 1 Overview Before development of this device began several main design objectives were established These included 1 ability to perform efficient rectilinear motion 2 small cross section and 3 ability to easily lengthen or shorten the robot The first of these needs arose from the observation that all current robot designs that are capable of operating in rough terrains perform forward motion through gaits that tend to be very slow measured in inches per minute We sought to develop a device that could perform in these same environments but move at higher forward speeds measured in inches per second The second objective arose from the desire to develop a robot that could access confined spaces in applications such as pipe inspection and exploration The final objective is desirable because a robot that can easily be lengthened to suit a particular application will be more versatile and adaptable Additionally a design of this type suggests that if a segment were to malfunction or break it could be removed and the robot be placed
22. arts and performing interference analysis on the finished assembly to ensure that parts would not interfere with each other under normal operation The snake has been designed such that its length can be increased or reduced to fit the particular application Each segment is identical to all others It carries its own batteries and processor 3 2 Frame forks dE rib Figure 4 Snake Processor Rib The frame of the snake is constructed from 0 231 thick acrylic ribs as illustrated in figure 4 Four forks two on each side have been solvent welded to the rib The nominal pitch between the ribs is 2 With fourteen ribs in the prototype the robot is approximately thirty inches long Acrylic was chosen because it is economical and easy to shape by machine 3 2 1 Design Options Future Considerations Acrylic was ideal for building a proof of concept device because of it good machinability characteristics and because the ease with which composite parts figure 4 can be assembled The four forks shown to the rib in figure 4 are attached with a solvent based acrylic adhesive that melts the joining surfaces and forms a bond that is as strong as the base material The ability to create such strong bonds was another factor in choosing this material for the prototype robot The second version of this robot still in the deign stage will have ribs constructed of injection molded Acrylonitrile butadiene styrene ABS plastic This materi
23. ave to send data to the master and the master would have to relay that data to the second slave Obviously this arrangement is inefficient in a network that requires direct communication between any two nodes such as ours CAN has a unique method of specifying which node is the recipient of an information packet In many low level protocols each node would have a unique address If node A wants to communicate with node B A would transmit a packet onto the bus that contained node B s address in the packet header All nodes on the bus would inspect the packet If the packet destination address corresponded to that node s address the node would store and process the packet if not the packet would be ignored This concept is taken a step further in CAN Rather than have a unique address CAN nodes have multiple acceptance filters Each transmitted packet has an identifier in its header that all nodes on the bus inspect If this identifier matches one of the values stored in that node s acceptance filter register the packet is received and processed CAN allows different identifiers to have different priority levels both in the receiving 29 node s buffer and during bus arbitration This addressing scheme allows for great flexibility in defining the upper layers of the communication protocol that the snake uses More urgent messages can preempt less urgent ones and we now have the ability to send messages to one node sever
24. bit stuffing rules of the CAN network see 6 for details The final subfield is the five bit address subfield which identifies the node to which the instruction is being issued In our system which consists of seven segment processors and one drive motor processor 32 snake processor boards are assigned addresses in ascending order with address 1 given to the processor at the head of the snake and address 8 given to the drive motor processor at the snake s tail Address 0 is reserved for general call instructions When an instruction is issued to this address it is acted on by all nodes in the network Referring to figure 25 the listed instructions are divided into two groups Instructions 1 through 6 are general call instructions while 7 through 13 are addressed instructions Due to the encoding scheme we have chosen the system can support up to thirty two 32 general call instructions and thirty two 32 addressed instructions The network supports up to thirty one 31 nodes A detailed description of each instruction is given in Appendix B 4 5 Sensors A robot of this complexity requires a variety of sensors to monitor stability motor torque battery life obstacles and various environmental variables The ability to measure these quantities allows the robot to sense its environment and adjust its operation accordingly One of the main design goals of this project is to make a robot that is not only flex
25. botic system is summarized in the table below Byte Byte Byte 1 Byte high 00001100000 o AN 2 Reset place nodes into reset 0001010000 2 oaa oss C5 San Jenable motors ooott100000 2 __ AA os A Report feq set new data report frequency 00100100000 1 __ i freq ri LS PD freg set PID update frequency 0101100000 fie ri 6 CAN add enter CAN addres set routine 0110100000 4 address ma 055 priority Ps ted ON fumiEDson Os 0 o ted OFF ram LED om 0010staaaaa 0 set new Integral gain for motor 0100sTaaaaa O10Islaaaaa low store current parameters 10 ROM o1100taaaaa 2 PO AA xs Figure 25 Robotic Snake Instruction Set Data Frame Format frame Every instruction issued on the CAN bus is packaged into a data frame with each frame containing three fields as shown above The first is the identifier field the second is the number of data bytes in the frame and the third is the data payload Note that some instructions require no data payload identifier The identifier field is the portion of the data packet that describes the instruction and recipient of that instruction We have divided this eleven 11 bit field into three subfields as shown above The five bit opcode subfield is the binary encoding of the instruction being issued This subfield is followed by a delimiter field containing a binary 1 This is added so that the data frame conforms to the
26. break 1 30 2 110 Elongation Yield 4 5 1 7 6 Izod Impact Notched 0 225 0 375 ft lb in 7 12 ft lb in Izod Impact Unnotched 5 06 ft lb in 20 ft lb in to NB Charpy Impact Unnotched 9 04 28 6 ft Ib in 23 8 114 ft Ib in Material properties of Acrylic and ABS plastics 3 3 Cam U Joint Device Figure 5 Cam U Joint Device A 0 500 hole is located in the center of each rib see figure 4 and two flanged ball bearings are pressed into it These bearings support the cam universal joint device shown in figure 3 This component has two functions the device 1 transmits power from the drive motor located in the rear of the snake to all upstream segments while 11 allowing flexure between segments and 2 converts the drive motor s rotational motion into an orbital motion in the snake s feet by way of a set of cams When designing this device the non ideal behavior of the u joint must be taken into account Universal joints have the undesirable property of being non constant velocity This means that for a constant input shaft velocity and non zero joint angle the output shaft velocity varies approximately sinusoidally with time This sinusoidal speed variation worsens as joint angle increases If left uncorrected the velocity ripple would quickly multiply as rotary motion traveled down the fourteen u joints found in the snake The result is that feet at the rear of the snake near the motor would o
27. cam was plotted over time The results plotted in figure 8 demonstrate the severity of velocity distortion seen in downstream cams As expected cam one rotates at the same speed as the servo motor while each successive cam rotates with a more distorted motion than the 13 previous one Cam seven s velocity varies by 80 45 of the drive motor velocity each cycle If this behavior were left uncorrected the snake would not operate properly Angular Velocity vs Time Cam 1 Cam 2 Cam 3 o Cam 4 Cam 5 Cam 6 Cam 7 Velocity deg sec 0 0 0 2 0 4 0 6 0 8 1 0 1 2 1 4 1 6 1 8 2 0 Time s Figure 8 U Joint analysis results In the arrangement described above the non constant velocity behavior at each universal joint in the series is the accumulation of the non CV behavior of all universal joints that precede it With a minor revision to the design however this non ideal behavior can be made to cancel between adjacent universal joints In an arrangement known as a constant velocity joint two universal joints are placed back to back see figure 9 In this configuration the input and output shafts rotate at the same velocity while the linkage between the two shafts exhibits a non constant velocity behavior In order for the non ideal behavior of the two u joints to exactly cancel each other the bending angle of each joint in figure 9
28. cated on the processor board immediately behind it For this reason addition and removal of ribs must be done as a pair The motor processor rib pair is referred to as a segment Thus far we have introduced two types of motors found on this robot The first is the drive motor figure 1 located at the rear of the snake This motor transmits power to all upstream ribs to power a series of feet on the bottom of the snake Figure 3 shows the components responsible for this and how they are positioned on the robot s rib The basic function of this mechanism is to take rotary motion applied to the cam and convert it to orbital motion in the feet The detailed operation of this device is discussed in section 4 3 U Joint Cam Assembly and section 4 4 Foot Mechanism Cam Foot Figure 3 Components on a typical rib The second type of motor is the articulation motor figure 2 Each motor segment contains two such motors These are responsible for bending articulating the segment on which they are located These two motors provide two degrees of motion per segment The prototype snake fourteen articulation motors Their operation is detailed in section 4 5 Servo Mechanism The cross section of this robot is 3 5 The necessity to integrate a large number of moving parts in such a small area was aided by computer design and modeling ProEngineer was used exclusively for developing solid models of each of these parts assembling these p
29. detection on all sides of the robot The sensor gives an analog output signal that can be digitized directly with one of several available analog to digital converter ADC channels on each of the snake s segment processor boards Many infrared IR proximity sensors provide a binary output signifying only if an object is present not its distance Since this robot will function as a path planning and object avoidance platform it is often necessary that we not only detect the presence of obstacles but how far they are from the robot 45 The ability to feel the ground and detect objects gives this robot enough information to make rudimentary control decisions For example if the operator instructs the robot to move forward the robot will automatically adjust its joint positions so that it hugs the ground as it moves forward In addition if the robot sees an obstacle in its path it can automatically lift its head in an attempt to climb over the object While these basic behaviors free the operator from many laborious control tasks the robot still relies heavily on the human operator to provide basic instructions e g move forward and intervene when the robot gets stuck This requires that the operator be able to see clearly what lies in the robots path In future versions of the robot we intend to include a miniature CMOS video camera in the head of the snake to relay video to the operator Figure 36 shows the placement of IR sens
30. e ground over which it crawls its shape reflects the contour of the underlying ground 5 1 3 Motor Torque Limit Limiting the amount of torque produced by the motors is done primarily to protect the motors and other mechanical components from damage This is a straight forward task of monitoring the motor current sensors and taking action when the current draw exceeds some set threshold This process can be augmented by understanding the dynamics of the robot For example if the snake cantilevers over the edge of a table figure 37 large torques are required by the motors located near the table s edge Knowledge of the robot s dynamic model can predict the amount of torque required thereby setting a maximum cantilever distance determined by maximum motor current 5 2 Foreground Processes 5 2 1 Kinematic Equation Calculations Several mathematical models that describe the physical behavior of this robot have been developed The kinematic model relates joint angles to segment positions For example if given the angles of all segments in the robot the kinematic model can calculate the position in Cartesian space of any segment The inverse kinematic model can take a desired segment position and determine the required joint angles to place the segment correctly The inverse kinematic equations are necessary in performing basic locomotion and curve following tasks In a typical scenario the operator would specify a curve in space along wh
31. e sai acs E ee a S 33 39 2 Motor C rrent alos os cca dat 40 3 5 3 He di g Orientation series tn ste nue 43 3 54 Object Detection Sensors scan di tienne 44 3 5 5 Special Purpose Sensors scare sde a drid 45 3 6 Power and Communities 46 4 SOM ca 46 4 1 Back erounid Processes ass 47 4 1 1 AB MIN e a uals aut ba enr 47 AA OMT AAS Ne ve Siesta oe eda 48 AAS Motor Torque TT bal 49 4 2 Foreground PROCESS id 49 4 2 1 Kinematic Equation Calculations oooonnccinncinococonoconononnnconncconocnnncnnnno 49 5 Specifications and Performance dd 50 5 1 Mechanical Specifications cid n annee 50 5 2 Electrical Specifications A a 51 5 3 O O Pa els 51 5 3 1 Motion in ela lea 52 5 3 2 Cantilevering Gap COS Mi ida 53 6 Conclusion 12 A A ARE LE A 54 Appendix A Mechanical Components Appendix B Instruction Set iv Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 List of Figures Computer Rendering of Robotic Snake 6 Zoomed in look at three adjacent ribs o ooonocccnoccnicccinccconoconcnonn conan ono coco nocnno nono 6 Components on a typical dt es ds 7 Snake Processor Rib A A A A ras 8 Came Jorn DeViCe is As 10 Illustrated U Joint and Assembled Snake U Joint oooooooccinocinococoncconccinnconnccnns 11 LIGNE TSP A A 12 U Jointanalysis SR da 13 Constant Velocity JO
32. ed through the Controller Area Network CAN bus A PC is also connected to the network and can be used to display sensor data and issue commands Due to the network topology employed here 24 figure 17 we can implement the robot control strategy in a centralized manner in a distributed manner or using a hybrid technique This will be discussed in detail in Section 5 4 CAN bus Centralized high level control Torque sensors Torque sensors sensors sensors Distributed low level control Figure 20 High level electrical system view The robot can be equipped with many navigational sensors including heading torque and proximity sensors Auxiliary sensors include temperature and humidity pressure and vibration In all the robot can support approximately thirty 30 sensor inputs With this number of sensors and the amount of data they produce it becomes advantageous to process and act on the data locally rather than send it all to a central processor for analysis For example tactile sensors in the feet are continually sampled to monitor the robot s stability If processors detect that the robot is not on a level footing they can drive the servo motors in the proper direction so that the robot can achieve more uniform contact with the ground Having much of the low level control done locally means that not all of the sensor data needs to be relayed to the operator only that which is relevant
33. ept of Stability Monitoring cooonocnnccnoccnocconcnoncnononnnconacononononnnonncnnn ns 47 Figure 3 Ground Hugging oes i cnn auc SS 48 Figure 39 Trajectory followed in planar movement test 32 Figure 40 Robot during planar movement test ooooccnocnnccnoccnocconcnoncnononnnonncnononononnnonncnnnons 32 Figure 41 Robot crossing a seven 7 inch Dn ia 53 PIES 42 Motor Frame a YA anni 57 Fig re 43 Foot Asma tas 57 Figure 44 Guide Block Assembly ac 57 Figure 45 Motor Mount A a a ais 58 Figure 46 Figure 47 Figure 48 Figure 49 Figure 50 Figure 51 Figure 52 Figure 53 vi PINON O he Rt ee de E tea ccateaseahsaine 58 Cam WV ONM e Ad Dn mine ne se 58 Lower ROCKED a A tance A a A AA A 59 Upper ROCKET i n Rs a ica 59 Transfer Arm O E E aaa E E 59 COUP IS A an 60 SOOO Rs spall Pans ee ne ne an geet 60 vil List of Acronyms ABS acrylonitrile butadiene styrene ADC analog to digital converter CAN controller area network CMRR common mode rejection ratio DOF degrees of freedom IR infrared MEMS microelectromechanical systems MIPS million instructions per second PID proportional integral derivative PLL phase locked loop PWM pulse width modulation viii Abstract A Serpentine Robot Designed for Efficient Rectilinear Motion Richard Anthony Primerano Moshe Kam and William Regli Robots that mimic the natural motions of animals have long been of interest in science and engineering The primary engineering
34. foot Moreover we needed a device that would allow two feet to move 180 degrees out of phase from each other so that the rib was always in contact with the ground The details in figure 11 shows that the final solution to this problem is relatively complicated The mechanism being designed for the next version of this robot will provide the same resulting foot motion except that the foot mechanism has been simplified We have reduced the part count from sixteen to eleven parts 3 5 Servo Mechanism While the drive motor and foot mechanism along with integrated universal joint are responsible for forward locomotion the robot still requires additional mechanical components to allow for turning cantilevering and other functions we associate with biological snakes 17 Figure 12 DC brush motor To accomplish these functions the prototype contains fourteen DC servomotors which allow the snake to articulate its segments with respect to each other These motors from Faulhaber 19 integrate a DC motor four stage stainless steel planetary gearbox and 400 count per revolution quadrature encoder This assembly along with a Proportional Integral Derivative PID control algorithm provides a complete closed loop position controller Figure 13 shows a block diagram of the complete servomechanism electrical mechanical Figure 13 Servomechanism block diagram 18 The entire servo assembly consists of the servo motor an addition
35. g Radius Notes 1 While the diameter of each segment s rib figure 4 is 3 5 inches the addition of the foot increases its effective diameter to 3 7 inches 51 2 This maximum forward speed has been imposed during experimentation to limit wear on mechanical components The absolute maximum forward speed is determined by choice of drive motor 3 The cantilevering operation is depicted in figure 37 Cantilever distance is defined as the maximum overhang distance for which articulating motors are powerful enough to support the snake The actual distance that the snake can successfully cantilever is also dependant on its length In the absence of motor torque limitations the snake can cantilever at maximum half of its body length 6 2 Electrical Specifications Parameter i Units Notes Supply Voltage Standby Current motors off Running Current Processor Speed Communication bandwidth Notes 1 In standby mode all electrical systems are powered with the exception of motor drivers 2 Running current is dominated by motor current draw Certain maneuvers require greater current demands than others In experimentation this number is the largest observed current draw 6 3 Test Results One of the main goals of this project was to design a robot capable of accessing confined spaces and operating on rough terrains Several tests were conducted to assess the robot s capabilities in such scenarios While
36. greater pulling force we had to select a pulley with a significantly smaller radius The second issue that is that the cables sustain abrasion during operation The control cables pass through Teflon guide tubes that route the cables past obstructions on the segment After prolonged operation the nylon coating on the cable and the Teflon guide tube will become worn and require replacement Regularly replacing all cabling and guide tubes would be very labor intensive 23 Although the control cable method is acceptable for a proof of concept model it is clearly not adequate for a second generation prototype Several solutions to this problem have been considered and one has been chosen as the method to be used in the second version of the robot Each set of control cables will be replaced with a single push pull rod driven by a lead screw As the lead screw rotates the attached nut moves up or down retracting or extending the attached control rod This action causes articulation in the segment The alternative design is depicted in figure 19 Not only does this method eliminate the shortcomings of the control cable method but it also makes assembly of the robot much simpler Figure 19 Push pull rod design 4 Electrical Design The top level view of the robot s electrical system is shown in figure 20 The robot consists of eight processor boards seven segment processors and one drive motor processor Each of these processors is connect
37. ible and maneuverable but also semiautonomous Complex tasks such as maintaining a stable footing when moving over uneven terrains or ensuring that the torque exerted by a motor is within safe operating limits should happen automatically This allows the operator to concentrate on coarse grain control of the robot such as telling it to move forward look up etc It is the robot s job to ensure that while moving forward it maintains contact with the ground or that in looking up it does not move its center of gravity to a location that would cause the robot to fall over This section outlines the sensors found on this robot 33 4 5 1 Tactile Several of the semiautonomous behaviors that the snake will perform require that it be able to accurately measure the amount of force it exerts on a surface The ground hugging behavior forces the snake to adjust its body to maintain even force distribution across all of its feet The stability monitoring behavior requires that the snake detect situations where several of its segments begin to lift off of the ground To accomplish tactile sensing the foot brackets located on the bottom of each segment have been constructed to behave as miniature electronic scales Force applied to the foot causes a deflection in the bracket This deflection is measured by a foil strain gauge bonded to the bracket The amplified signal from the strain gauge represents the force exerted on the ground by that foot
38. ich the snake is to crawl This curve would be fed into a fitting algorithm that uses the kinematic equations to determine 50 how each of the snake s joints must be bent in order to fit to the curve As the snake crawls forward the fitting algorithms continually re fits the snake with a slightly advanced starting point At this point tactile sensor data can be used in order to fine tune the snake s joint angles so that uniform contact is maintained with the ground Simultaneously the curve that was originally fed into the fitting algorithm is also fine tuned to reflect the contour of the ground over which the snake crawls Currently the processes of curve generation and snake fitting are executed on the host PC and joint angle commands are issued to the snake incrementally as it crawls forward We are currently working to divide the algorithm in a way that makes it suitable to execution in the distributed processing architecture of the robotic snake 6 Specifications and Performance The robot was designed so that its length can be adjusted to fit a particular mission The mechanical and electrical specifications given below assume a robot seven 7 segments long 6 1 Mechanical Specifications Parameter Length i seven segments and drive motor Diameter Mass single segment Mass robot including drive motor Forward Speed Cantilever Distance i based on motor strength Bendin
39. lementation of distributed control algorithms A second version of the device is also being designed This version will feature wireless operation both power and communications overall reduction of weight and reduced mechanism complexity 55 List of References 1 2 3 4 5 7 6 8 9 10 11 12 13 14 K Dowling Limbless locomotion Learning to crawl with a snake robot doctoral dissertation Carnegie Mellon University Pittsburgh PA 1997 I Erkmen Snake Robots to the Rescue JEEE Robotics and Automation Magazine September 2002 pp 17 25 G S Chirikjian The Kinematics of Hyper Redundant Robot Locomotion IEEE Transactions on Robotics and Automation December 1995 pp 781 793 C W Eno Jormungand an Autonomous Robotic Snake technical report Machine Intelligence Laboratory University of Florida 1998 H Choset W Henning A Follow the Leader Approach to Serpentine Robotics ASCE Journal of Aerospace Engineering 1999 TR1000 916 50 MHz Hybrid Transceiver Datasheet RF Monolithics Dallas TX 1999 www rfm com CAN Specification Version 2 0 Robert Bosch GmbH Postfach Germany September 1991 F Hartwich A Bassemir The Configuration of the CAN Bit Timing 6th International CAN Conference 2000 B Miiller et al Fault Tolerant TTCAN Networks 81h International CAN Conference 2002 Cable Design Guide DG
40. n lessons learned through the construction of this first prototype 2 Background Most biological snakes employ one of four major types of locomotion each of which is outlined below 1 Lateral Undulation The snake s body forms a series of S shaped curves The back portion of each of these curves pushes against the ground propelling the snake forward Concertina the snake expands and contracts sections of its body alternately while planting others on the ground firmly This motion is similar to that of an inchworm Sidewinding the snake contacts the ground at only two points while moving its body in a sinusoidal motion The result is that the snake moves sideways rather than forward Rectilinear the snake propels itself in a straight line by moving scales on its stomach in a wave like motion Rectilinear motion allows snakes to access very confined spaces It would appear that if a robotic snake could undergo rectilinear motion it would be very maneuverable in constricted areas Currently robotic snakes are available which can undergo rectilinear motion by articulating each of their segments in a repeated sequence A 1995 paper by Chirikjian 3 outlines various methods for accomplishing this task The approach is to create rectilinear locomotion through body movements alone Chirikjian classifies snake like robots as either inextensible or extensible The former are capable of only bending their segments
41. nd tested can actually walk over obstacles and therefore will be much more maneuverable than existing prototypes 1 Introduction Snake like robots 1 are believed to offer several advantages over conventional wheeled or legged robots For example robotic snakes have a low center of gravity which makes them very stable when moving on inclines In addition if a snake like robot were to fall over it may recover by articulating its body in the proper way Unlike their walking or wheeled counterparts snake like robots spread their weight out over a large area thus exerting less force per unit area over the surface on which they are moving This characteristic means that robots of this class are better suited for moving over soil or sand compared to wheeled and legged robots that are more likely to get stuck in such environments This study details the design and construction of a robotic snake prototype that addresses many of the shortcomings found in previous robots of its type The key element of this design is the robot s ability to execute rectilinear motion straight line by simply controlling a single drive motor as opposed to articulating its body in complex motions gaits as is typical of many other serpentine robots During the design process many options had to be considered and throughout this document these alternatives will be presented and addressed Plans for a second version of this robot will also be presented based o
42. nt A la dts EN ia 14 Constant Velocity Joint Analysis 14 Foot Mechanism Detail EII ANA 15 DC prush Motoi nissan tee 17 Servomechanism block dara iii 17 SONO Gear A at tii eect ea et am greet aries ut 18 Spool VINE Sra aces Since tesa a A O oe voles se etiueks 19 Control cable attachment points on a typical segment 20 Control Cable Ri ds 21 Figure 18 Geometric relationship between cable length and joint angle 21 Fig te O 23 Figure 20 High level electrical system View hist dades 24 Figure 21 S ament processor PCB eras 25 Figure 22 PCB DIO ck diag rain pasos gas a e 26 Figure 23 Drive motor PC Be haces ease cn dettes 26 Figure 24 Control Network Topology inca diia 29 Figure 25 Robotic Snake Instruction Set 31 Figure 26 Force Measurement and Foot Bracket Location 33 Figure 27 Strain Gauge Image and Schematic 18 cc ccccccscessccstsesseessencsssessevessenenss 35 Figure 28 Finite Element Analysis of Foot Bracket 6 Ib applied 35 Figure 29 Quarter Bridge Configuration cal da 36 Figure 30 Half Bridge CONO o re 37 Figure 31 Full Bridge Contar OO a Ne 38 Fig re 32 Wheatstone Bridge ni oniga iio 39 Figure 33 Typical Current Sense Amp Application 14 41 Figure 34 Block diagram of Allegro Hall Effect current sensor 151 43 Fioure 35 Sharp GP2D120 Sensor lid dns 44 Figure 36 IR Sensor and Camera Field of View 45 Figure 37 The Conc
43. oad the software calculates the amount of strain experienced at numerous points on bracket and the results are shown as a superimposed color gradient The colors at the top of the color chart on the right hand side of figure 28 represent regions of highest strain Not surprisingly we see the greatest strain at the location where the vertical and horizontal sections of the bracket meet This is the location where the strain gauge is bonded PAL Figure 27 Strain Gauge Image and Schematic 18 188e 05 856e 05 237e 06 917e 96 97 06 276e 06 956e 06 635 06 315e 06 1 1 O 7 6 De 3 es ea Le Figure 28 Finite Element Analysis of Foot Bracket 6 lb applied 36 Resistive strain gauges can be wired in one of three ways The simplest method is to apply a single strain gauge to the specimen and have that element act as one arm in a Wheatstone bridge as shown in figure 29 This is known as the quarter bridge configuration 18 The advantage of this method is that it only requires a single sensing element which reduces device cost and complexity The quarter bridge sensor has several drawbacks including non linear output and temperature sensitivity These effects can be corrected in hardware or software In practice the nonlinearity of the signal is very small and can often be ignored If accurate measurements are required temperature compensation is generally required Ly Ro R 1 Strain Gauge R3 in
44. of the robot At this stage of development however we provide power and communication via a tether This was decided so that issues of communication bandwidth and battery runtime would not interfere with control algorithm development As the design matures we will transition to an un tethered robot 5 Software The discussion thus far has centered on the physical design of the robot This section discusses the software components being developed to control the robot and manage data flow between the robot and the user The processes that reside on the robot s processors are divided into two categories background processes and foreground process Background processes are those that provide low level control of the robot 47 by monitoring tactile and torque sensors while making incremental adjustments to individual servo motors These processes include stability monitoring ground hugging and torque monitoring Foreground processes are those that provide more advanced control mechanisms to allow the robot to avoid obstacles and negotiate rough terrain Software development is still in its early stages and this section introduces the main applications that are being develpoed 5 1 Background Processes 5 1 1 Stability Monitoring U UUUUUUUUNUUN PERT TTT Oho et tt ebay ee r feasted Figure 37 The Concept of Stability Monitoring The first background process we look at is stability monitoring Through the use of its t
45. ors and camera along the robot s length Four forward looking IR sensors and one forward looking camera are the robot s primary means of object detection Eight additional side looking IR sensors are also included in the design IR Sensors Video Camera hi Field of View Field of View ul ide A A o BARBAAARARARADA no Object Detection Sensor Placement Top View Object Detection Sensor Placement Side View Figure 36 IR Sensor and Camera Field of View 4 5 5 Special Purpose Sensors The sensors discussed in previous sections were necessary for the basic operation of the robot Depending on the applications the snake will be used in it may be desirable 46 to include other special function sensors intended mainly to provide the operator with environmental information following is a partial list of sensors that could reasonably be incorporated into the snake per specific application In addition to this list any sensor with a small form factor less than approximately one inch cube and a compatable output can be interfaced to the robot e Relative Humidity Temperature e Gas CO CO Methane etc e Flame and Smoke 4 6 Power and Communications During the robot s design provisions were made to allow the device to operate un tethered The robot can carry batteries that give it a run time of approximately 1 5 hours In addition a wireless communication module can be added to the CAN bus to allow radio control
46. p to bottom observe that as the cam rotates clockwise the left foot begins in the downward position frame 1 and sweeps an orbital path until it returns to its original position Following the motion of the right foot from frame to frame its motion is 180 out pf phase with respect to the motion of the left foot When one foot is touching the ground the other is elevated Additionally the robot designed such that the feet on neighboring ribs operate with 180 phase difference In other words when the left foot of one rib is touching the ground the right foot of its neighbor is touching Figure 11 Foot Mechanism Detail 16 3 4 1 Design Options Future Considerations Before settling on this mechanism to impart forward locomotion several alternatives were considered The first and most obvious choice was to use wheels or treads to propel the snake forward however wheels and treads were abandoned because they generally do not work well on rough surfaces Another option was to use feet that were rigidly attached to the ribs and move the snake forward by articulating its body in specific sequences gaits This option was discounted because robots of this type move slowly with forward speed measured in inches per minute After the decision was made to give the snake feet several methods were considered to power them The main challenge was in converting rotary motion from the drive motor to the orbital motion observed in the
47. perate smoothly while those at the front of the snake would move with very abrupt motions This condition would render the snake inoperable Fortunately there is a simple solution to this problem know as a constant velocity joint CV Joint 3 3 1 Kinematics of the U Joint and CV Joint Figure 6 Illustrated U Joint and Assembled Snake U Joint Figure 4 shows a conventional universal joint compared with one of the u joints found on our snake The latter is formed when two of the snake s cams are joined at their ends The input output relationship for a u joint is defined as 12 1 tan dg 1 cos 0 1 p tan Note that when 6 0 the equation reduces to 2 1 Physically this means that when there is no bend in the joint the input and output shafts move with same velocity As the joint angle varies however the output shaft undergoes periodic velocity fluctuations even when input velocity is constant Computer simulation shows the degree of speed variation that can be found in the segments of the robotic snake for even moderate joint angles The graphic shown in figure 7 was used as the input to ProEngineer s motion analysis software This figure represents a snake seven segments long with each segment bending at an angle of 25 degrees Figure 7 U Joint Test Setup For our analysis a constant angular velocity 180 rad sec was applied to the first cam in the assembly and the angular velocity of each downstream
48. ree microelectromechanical system MEMS accelerometers measuring linear acceleration in the x y and z directions and three MEMS rate gyroscopes measuring angular acceleration about the x y and z axes This sensor measures acceleration in all six degrees of freedom Finally the three axis magnetometer has three mutually orthogonal magnetic field sensors that together resolve the direction of magnetic north When the robot is relatively stationary the 44 accelerometers can be used to resolve the direction of gravity With these two directions determined the robot can completely determine its heading and orientation while stationary When the robot is moving heading data is derived primarily from the rate gyroscopes Signal processing techniques are used to determine how best to combine input from all nine sensors to accurately determine the robot s heading 4 5 4 Object Detection Sensors As the robot navigates through its environment it is necessary that it be able to detect obstacles in its path The sensors that have been discussed so far are primarily used for autonomous control of the robot We will now introduce several proximity sensors that are useful in detecting obstacles in the robot s path Figure 35 Sharp GP2D120 sensor 22 Figure 35 shows the Sharp GP2D120 IR proximity sensor 22 This device has a 1 5 12 sensing range Several of these devices will be placed along the snake s length providing object
49. ributed one In a centralized control scheme a single control algorithm processes all sensor data and generates all actuator signals Numerous methods from classical control theory e g lead lag compensation PID control and modern control e g pole placement observer based controllers can be implemented in this framework Distributed control schemes however are often difficult to implement partly because coupling of the system s dynamics necessitates communication between distributed control components As a first step we have employed a hybrid control approach where the snake s distributed microprocessors execute a PID motor control algorithm while the PC issues various commands to the snake and processes sensor data from the snake 4 4 1 Instruction Set The control hierarchy currently implemented on the robotic snake divides high level and low level control tasks between the PC and segment processors respectively The PC performs path planning and inverse kinematics computations and issues joint angle setpoint commands to the robot s distributed processors These processors in turn take those commands and perform closed loop control of the robot s motors In this sense there is a master slave relationship between the high level controller the PC and the low level controller the robot s processors with the PC issuing 31 instructions and the snake responding to them The instruction set currently supported by our ro
50. ridge Configuration The final configuration that can be used for the strain sensor is the full bridge This device uses four active elements and is wired as shown in figure 31 The benefits of this method are similar to those of the half bridge By using four strain gauges we can obtain an output signal that is four times the amplitude of the linearized quarter bridge sensor 38 2 Strain Gauges in Compression 2 Strain Gauges in Tension Force Force Figure 31 Full Bridge Configuration In this design we decided to use a half bridge configuration because it provides a good balance between sensitivity and complexity While the quarter bridge configuration is the simplest alternative the output voltage from the device is not linear with applied strain so additional compensation and calibration would need to be performed We also ruled out the use of the full bridge transducer because our space constraints will not allow us to fit four strain gauges in the area of limited area of the foot bracket On the other hand the two element design provides a linear output and can easily fit on the foot bracket It has twice the sensitivity of the quarter bridge design but only half the sensitivity of the full bridge design We conclude our discussion of strain gauge configurations by deriving the equations that relate change in sensor resistance due to applied strain to output voltage methods The equations are derived based on the Whea
51. ross that element Several manufacturers offer current sense amplifiers made specifically for this Figure 33 shows the MAX4376 current sense amplifier 14 in a typical application This device is made to operate as a high side current sense amplifier This means that the sense resistor is placed on the positive side of the load An alternative method is to use a low side configuration where the sense resistor is placed on the low potential side of the load The former method has the advantage that it does not introduce impedance into the ground path a necessity for noise minimization 15 The disadvantage of this configuration is that one must use an amplifier with high common mode rejection ratio CMRR The latter method has the advantage that it can be constructed of inexpensive amplifiers but because of the location of the sense resistor the ground path resistance is increased This can potentially cause excessive noise in the circuit 42 There is another simple method of current sensing that circumvents the disadvantages of resistive current sensing This method uses a magnetic field sensor often a Hall Effect sensor convert magnetic field that results from the flow of current in a conductor into an electrical signal Figure 34 shows a block diagram of the Allegro ACS704 Hall Effect current sensor 15 The currents measured in our application are relatively small on the order of tens of milliamps Hall Effect current sensors are gener
52. roximately seven inches laboratory tests have shown that articulating motor strength is sufficient to cantilever approximately eight 8 inches of the robot s length If the robot is not sufficiently long its center of mass will prevent it from crossing gaps of this length In these cases the robot will tip over before the actuators saturate This illustrates a situation where the operator may want to adjust the robot s length to suite a particular mission As the robot crosses a gap its current draw steadily increases due to the increased torque required by the articulating motors to keep the cantilevered section of the snake extended horizontally In experimentation the robot required a maximum torque of three 3 amps during this maneuver Figure 41 Robot crossing a seven 7 inch gap 54 7 Conclusion This paper presents the design analysis and construction of a snake like robot During the design stage simulation tools were used to verify proper operation of the mechanism and after construction several experiments were conducted to verify performance A robot of this type is well suited to exploration of confined spaces and rough terrains This type of robot also serves as a useful research platform for testing object avoidance and path planning algorithms distributed control algorithms and sensor fusion algorithms Future work with this platform will include the addition of tactile and object detection sensors and the imp
53. t the effective radius is 0 093 With this dimension we can calculate the spool take up per revolution equation 3 and the overall relationship between motor position and cable length equation 4 Pitch Circle Figure 15 Spool Illustration 584i T 27x0 093 0 5841 284 in_ 3 revolution 4 0 584 in 109 000 revolution E counts a 109 000 count in gt Al From equation 4 C is the number of encoder counts from home position and Al is the resulting change in control cable length The choice of these definitions will become clear in the following section 20 Control cable attachment points X Y Figure 16 Control cable attachment points on a typical segment Now we seek to develop an input output function relating motor position to the joint angle of the segment being driven by the motor We will first relate control cable length to joint angle The control cables that articulate the snake s segments are oriented at forty five degrees 45 from the horizontal and vertical axes Referring to figure 16 the cables attached at points X and X operate as a pair When the X axis servo motor is activated it retracts the X cable and extends the X cable or vice versa This causes the segment to bend in the X axis Figure 17 shows a side view of how these cables are rigged The Y axis servo operates in exactly the same manner 21 Figure 17 Control Cable Rigging Under applied motor torq
54. ted in the tail of the snake The main task of this processor is to control the operation of the drive motor In many ways this PCB is a reduced function version of the segment processor The code that runs on this processor is very similar to the code residing on the segment processors with the major difference being that the drive motor board controls one motor while the segment board controls two 4 2 1 Design Options Future Considerations 21 The microcontroller chosen for this design was chosen for its simplicity rather than processing power This device is fast enough to provide the snake with basic functions but does not have the resources to solve the complex kinematic and dynamic equations needed it our future investigation Due to these limitations the next generation of the robotic snake will contain a more powerful set of processors The microcontroller currently being considered for use in the next generation robot is the LPC2119 from Philips 20 This is a 60MHz ARM based microcontroller with built in dual CAN transceivers pulse width modulation PWM outputs and several counter timer channels The 32 bit core will enable us to perform complex calculations quickly and the onboard peripherals allow us to reduce the circuit board s part count This microcontroller also has extensive power management features including clock rate reduction power down modes and the ability to disable unused peripherals 4 3 Communication Bus
55. this value is retrieved from non volatile memory The contents of this memory can be modified with the STORE operation D_UPDATE Set new derivative gain type general call description changes derivativel gain of motor s identifier 0101s 1 aaaaa data bytes 4 data 32 bit float IEEE 754 format notes At power up this value is retrieved from non volatile memory The contents of this memory can be modified with the STORE operation
56. tstone bridge in figure 32 Equation 8 gives the input output relationship of the bridge with four arbitrary resistor values 39 R4 R3 Figure 32 Wheatstone Bridge Wheatstone Bridge p ay R4 __ RI _ R4 RI R2 RI R3 R4 8 R3 RA RI R2 R1 R2XR3 R4 In a strain gauge application one or more of the resistors R1 R4 are replaced with resistive strain gauges resulting in the four possible configurations shown in figures 29 31 In the quarter bridge configuration resistor R4 is replaced with a strain gauge having nominal resistance Ro Under applied strain the resistance of the device varies by AR Resistor R3 is set to the strain gauge s nominal resistance Ro while R1 and R2 are set to any equal value R Equation 9 shows the input output relationship for the quarter bridge Note that the relationship is non linear but for small resistance variations can be approximated by a linear equation Quarter Bridge Strain Gauge RI R2 R R3 R R4 R AR 9 _ Ry AR GR R CR AR _ 1 AR _ AR ae 2R 2R AR 22R AR 4R In the half bridge configuration resistors R3 and R4 are both replaced by strain gauges Note that when the specimen is strained one gauge experiences tension its resistance increases while the other experiences compression its resistance decreases The resistors R1 and R2 are again set to an equal value Equation 10 40 shows the input output relationship for the half bridge
57. ue t the length of the control cables vary such that the following relationships are held L P Al L P Al gt L L 2P 5 where P is the pitch between two ribs 2 in our design and A is the variation in cable length caused by the applied motor torque The problem has been redrawn in a simplified form in figure 18 Figure 18 Geometric relationship between cable length and joint angle 22 From this it is simple to relate cable length to joint angle The result is given as 6 2sin L 2V2 6 for L 2 0 76 79 0 0 76 7 6 2sin L 2V2 6 76 7 6 Combining 4 5 and 6 we obtain the overall input output function relating motor position in encoder counts to joint angle es dain EE 242 6 os 109 000 2 2 7 sin 2 Le 2 rer 7 3 5 1 Design Options Future Considerations While the mechanism outlined on the previous section is effective it has several drawbacks The first issue is that control cable lifetime is greatly affected by bending The 1 16 bending radius that this cables experiences in wrapping around the spool will have a significant effect on both the cable s load carrying ability and the cable s life Control cable manufactures recommend that pulley diameters have a radius in the range of sixteen times the cable diameter for adequate operation lifetime 10 For a 1 32 cable the spool radius would ideally be larger than half an inch Due to size restrictions and the need for
Download Pdf Manuals
Related Search