DC Motor with Shaft Encoder Learning Objectives Components
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1. void init void s32 positionO s32 speedO Begin Code void main void s32 startpos initQ initialize everything vt100ClearScreen clear the terminal screen print a little intro message so we know things are working rprintf r nWelcome to the Wheel Encoder LAB r n while 1 startpos positionO timerPause 500 speedO positionO startpos 2 rprintfProgStrM Encoder0 Position print encoder0 position use base 10 10 chars signed pad with spaces rprintfNum 10 10 TRUE positionO rprintfProgStrM Speed gan Jos State University Department of Mechanical and Aerospace Engineering rev 2 1 060CT2007 Encoder Laboratory Encode 6 rprintfNum 10 10 TRUE speed0 vtl00SetCursorPos 5 1 end main void init void initialize our libraries uartInit initialize the UART serial port uartSetBaudRate 9600 set the baud rate of the UART timerInit initialize the timer system rprintfInit uartSendByte initialize rprintf system vtl00InitQ initialize vt100 library setup external interrupt cbi ENCO_A_PORT ENCO_A_PIN encoder phaseins are inputs with pullups off cbi ENCO_B_PORT ENCO_B_PIN EICRA BV ISC00 IBV ISC01 rising edge triggered EIMSK BV INTO fintO enabled sei end initQ SIGNAL SIG_INT2. metal tab and by its leads The MOSFET you will be using in this lab is the IRL520 It is specifically designed to be fully turned on by logic level circuits 5 V which makes it ideal for controlling medium power devices such as dc motors using a microcontroller Its package style is an industry standard TO 220 This package is somewhat awkward to use in a solderless breadboard because its leads are so large To avoid damaging the solderless breadboard insert the IRL520 so that its metal tab is to one of the 5 hole rows but with each lead in a separate row To do this you will have to bend the leads slightly see Figure 1 The 1 MQ resistor is used to make sure that charge can bleed out of the gate to ground to turn off the MOSFET in the event that the microcontroller pin it connects to inadvertently changes from being an output with logic high asserted to an input high impedance 12 V 1N4148 Motor IRL520 G S D Figure 1 DC Motor Driver Circuit The circuit uses an IRL520 power MOSFET MOSFETs are very static sensitive so handle them by the metal tab Why is the diode needed across the motor leads Note use of the 1N4148 diode is a concession to the limitation of the solderless breadboard to accommodate large wire diameter It would be more appropriate to use a standard rectifier diode with larger forward current capacity such as a 1N4003 however the lead diameter for the 1N4003 is too large for the solderless breadbo
3. to keep track of the encoder pulses from the motor An ISR is a special kind of function often used in applications with microcontrollers which is executed when a specific kind of event occurs Such events might be a rising edge low to high transition a falling edge high to low transition on a specific pin an overflow of a counter etc When one of these events occurs the regular program operation is interrupted and the program jumps to the ISR code to handle the situation triggered by the event After the ISR instructions are completed the program returns to what it was previously doing Interrupts are powerful tools for embedded systems programming They allow the microcontroller to perform other tasks such as send or receive data across the serial port without having to be tied up in waiting for an input state to change ISRs must be kept short and care must be taken in their use so that timing and reliability are not compromised For more information on interrupts see http www nongnu org avr libc user manual group__avr__interrupts html In this lab we will have the encoder Phase A trigger an external interrupt while Phase B is routed to another input pin The ISR will check the state of the Phase B input pin to determine the direction of the encoder and then increment or decrement the encoder s position variable With the encoder position known we can count the number of encoder ticks within a given amount of time Th
4. ERRUPTO if ENCO_B_PORTIN amp BV ENCO_B_PIN position0 else positionO Reconnect the red mini hook of the FG to the gate of the IRL520 Explain how this program works Compare the speed you measure with the scope with the value your program outputs How well do these values agree 18 Suppose the encoder were mounted to the shaft of a motor or the wheel of a vehicle If the diameter of the wheel is 3 inches write a program that will indicate the rotational speed of the wheel and tell how far the vehicle has traveled How accurately can you calculate the speed and distance Quantify your answer Run your program with the setup you just used In order to accomplish this you may want to use floating point math Yes the Atmega can handle this However to print a floating point number using rprintf you may need to change a small line of code in the rprintfconf h header file This file should be located in the AVRLIB directory on your hard drive Uncomment the line that defines RPRINTF_FLOAT This will allow the rprintfQ function to print floating point values See the rprintf h file and the rprintf example program included with AVRLIB for help on printing various variable types using rprintf gan Jos State University Department of Mechanical and Aerospace Engineering rev 2 1 060CT2007
5. Encoder Laboratory Encode 1 DC Motor with Shaft Encoder Learning Objectives By the end of this laboratory experiment the experimenter should be able to e Explain how an encoder operates and how it can be use determine rotational speed and angle of a motor shaft e Use the IRL520 power MOSFET to control power devices e Explain the concept of Pulse Width Modulation to control the speed of a DC motor e Interface the Atmel Atmega 128 microcontroller to an encoder Components Qty Item 1 Atmel Atmega 128 microcontroller STK500 and STK501 interface boards and serial port cable 1M Q resistors Solderless breadboard 1N4148 diode IRL520 power MOSFET DC motor with encoder p ee pb ph Introduction In this lab you will investigate how rotational speed and rotational angle can be determined using a rotary encoder A rotary encoder consists of a disk with alternating opaque and clear radial regions In operation a light source is positioned on one side of the disk and a photosensitive device such as a phototransistor is positioned on the other side of the disk As the disk rotates the passage of the opaque and clear regions of the encoder disk alternately block and allow light to impinge on the receiver which produces corresponding voltage pulses The rotational speed of the encoder disk can be determined by counting pulses during a known time period The angle of rotation corresponds directly to the number of pulses since the n
6. ard which is suited for 22 ga solid core wire Leads with significantly larger diameters when forced into the breadboard holes will permanently bend the internal contacts Subsequent use of the breadboard hole with 22 ga wire can then lead to unreliable contacts A work around to this problem is to solder 22 ga wire to the oversize lead The current draw of the motors used in this lab is low enough that the 1N4148 will marginally suffice See Figure 2 for more information on rectifier diodes gan Jos State University Department of Mechanical and Aerospace Engineering rev 2 1 060CT2007 Encoder Laboratory Encode 3 Rectifier diode I_ oe o Figure 2 Rectifier Diode Representations The band on the diagram to the left corresponds to the vertical line on the schematic symbol for the diode on the right and can be used to determine the polarity of the diode A diode acts like a check valve for current flow The diode is said to be forward biased when its anode voltage is higher than its cathode voltage Which lead is the anode and which is the cathode The check valve opens up when the forward bias voltage is approximately 0 6 V 1 V 7 With the FG set up from Step 5 clip the red mini hook lead from the function generator cable to the gate of the IRL520 and the black mini hook lead to the common ground Vary the duty cycle of the signal from the FG and observe what happens to the motor Explain why the mot
7. eering rev 2 1 060CT2007 Encoder Laboratory Encode 4 10 11 12 Now connect the scope to the Ch A and Ground pins of the encoder Power the STK500 Re connect the red mini hook of the FG to the gate of the IRL520 and run the motor as in Step 7 Observe the output of channel A on the scope Determine the speed of the motor at 5 different duty cycles that span the full range that the FG can output Plot motor speed vs duty cycle in your report and discuss your results Atmega 128 Measurement of Encoder Pulses 13 14 15 16 Disconnect the red mini hook of the FG and turn off the 12 V power supply Make sure that the Atmega 128 controller is OFF Connect channel A of the encoder to pin PDO of the Atmega 128 and channel B of the encoder to pin PB4 channel B can really go to any other unused input pin however you need to keep track of which pin and make necessary changes in your code At this time connect the serial cable to the COM port on your computer and to the CTRL RS232 COM port connector on the STK500 After DOUBLE CHECKING all of the connections to the Atmega 128 turn on the 12V power supply to power up the device Check to make sure that the power LED on the STK500 is on it should turn red then yellow then flash green and finally stay green If the light does not come on see the TA for help Do not proceed if the board does not power up We are going to use an interrupt service routine ISR
8. is can be translated into the motor s speed in encoder ticks per second Finally we will display the values for encoder position and speed by sending them to the serial port You can use a terminal program such as HyperTerminal to view the output from the serial port If you use HyperTerminal configure the COM port to 9600 8 N 1 N and the ASCII settings should not append line feeds to incoming line ends gan Jos State University Department of Mechanical and Aerospace Engineering rev 2 1 060CT2007 Encoder Laboratory window Tachometer Program Put your name here Put the date here Include Files Encode 5 17 Now enter and run the following program Then open a COM control or HyperTerminal include lt avr io h gt include I O definitions port names pin names etc include lt avr interrupt h gt include interrupt support include global h include our global settings include uart h include uart function library include rprintf h include printf function library include timer h include timer function library timing PWM etc include vt100 h include VT100 terminal support define ENCO_A_PORT PORTD define ENCO_A_PORTIN PIND define ENCO_A_PIN PDO define ENCO_A_DDR DDRD define ENCO_B_PORT PORTB define ENCO_B_PORTIN PINB define ENCO_B_PIN PB4 define ENCO_B_DDR DDRB Function Declarations
9. or speed varies with the duty cycle Using an Encoder with the Atmel Atmega 128 External Interrupt 8 Unhook the red lead of the FG from the gate of the IRL520 9 Using the information provided in Figure 3 below connect 5V and Ground from the STKS500 interface board to the corresponding pins of the motor s encoder Hint You may want to transfer the pins from the encoder to your breadboard by using jumper wires then you can easily connect the pins on the Atmega128 to the desired pins of the encoder via the breadboard This will power the encoder s optointerrupter and provide 0 5V signals on channels A and B as the encoder wheel turns Since this is a quadrature encoder the signals from channels A and B are 90 degrees out of phase Explain how the two signals from channel A and channel B can be used to determine the direction that the motor spins jpe L max 595 655 p 2 020 307 he i WOLEX CENTER CRIMP TOGE TERMINAL HOUSING 2695 a SERIES Witt ACCEPT an MOLEX MATIN 3H TERMINALS 2759 al E HEWLETT PACKARD HEDS 9100 HEDS 9140 OPTICAL ENCODER Figure 3 DC Motor with Encoder The view on the right depicts the motor encoder from behind i e the motor shaft is pointing into the paper and from this orientation Pin 1 of the encoder is the furthest one to the right Use the table to make the proper connections San Jos State University Department of Mechanical and Aerospace Engin
10. umber of pulses per revolution is constant We will also look at controlling the speed of a DC motor using the concept of Pulse Width Modulation PWM Procedure Function Generator Output to Control Duty Cycle 1 Set up the function generator FG to output a 1 kHz square wave remember to set the output termination to HIGH Z Look at the signal on the scope 2 Set the amplitude to 8 V p p Offset the waveform by 4 V so that you have a 0 to 8 V square wave 4 Select the Duty function on the FG by pressing the Shift key then the Offset key gan Jos State University Department of Mechanical and Aerospace Engineering rev 2 1 060CT2007 Encoder Laboratory Encode 2 5 Rotate the knob and see what happens to the output waveform when you vary the duty cycle What are the limits you can set the duty cycle to What does duty cycle mean Describe in your own words DC Motor Speed Contol Using Duty Cycle 6 Build the circuit shown in Figure 1 Don t forget the diode Important note MOSFET s are very sensitive to static electricity Make sure that you are not carrying static charge before you handle these devices It is best to work on a properly grounded anti static surface with an anti static bracelet on your wrist If these precautions are not available then discharge yourself by touching a grounded metal surface such as the frame of the bench before you handle a MOSFET Always handle a MOSFET by its large
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