Application Note 3414: Sinusoidal Commutation of Brushless Motors
Contents
1. Figure 18 Commutation by motion controller The advantages of controller commutation are e The motion controller can control phase advance offset and commutation cycle This flexibility allows far greater options for motor and amplifier combinations e All control loop gains are calculated on the controller this allows for single point tuning The disadvantages are e The commutation of two phases requires two analog output signals On many motion controllers this requires a second axis for each brushless amplifier increasing the cost of the controller e A brushless setup routine is required for the controller to determine the phasing Regardless of the method of sinusoidal commutation a brushless servo motor can fulfill environmental and performance specifications that simple brush type servo motors cannot Part Two Commutation Using a Galil Controller Galil controllers produce sinusoidal motor signals by varying the commanded voltage with respect to position The shape of the motor command voltage is a sine wave with a period equal to one brushless cycle A brushless cycle is the number of encoder counts that separate identically charged magnetic pole pairs If it was known that a motor is a four pole pair motor and there are 4000 encoder counts per revolution then there are 1000 encoder counts per magnetic cycle The second phase of the command signal is 19 Galil Motion Control Inc2. M sin 0 120 Kt sin 0 120 M sin 0 240 Kt sin 0 240 14 reorganizing T K M sin 0 sin 0 120 sin 0 120 sin 0 240 sin 0 240 15 From trigonometry 29 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Sin A B sin A cos B cos A sin B 16 Substituting Eq 16 into portions of Eq 15 sin 0 120 sin 0 120 sin 0 cos 120 cos 0 sin 120 sin 0 cos 120 cos 0 sin 120 17 and sin 0 240 sin 0 240 sin 0 cos 240 cos 0 sin 240 sin 0 cos 240 cos 0 sin 240 18 applying F O I L and adding numeric values to Eq 17 and 18 sin 0 120 sin 0 120 25 sin 0 5sin 6 866cos6 bsin 0 866cos0 75cos 0 19 and sin 0 240 sin 0 240 25 sin 0 5sin 0 866cos0 5sin 0 866cos0 75cos 0 20 substituting Eq 19 and 20 back into Eq 15 T K M sin20 25 sin 0 5sin 6 866cos6 5sin 0 866cos0 75cos 0 25 sin 0 5sin 0 866cos0 5sin 0 866cos0 75cos 0 21 canceling terms T Kt M sin 25 sin 0 75cos 0 25 sin 0 75cos 0 22 30 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com And T Kt M 1 5 sin 0 1 5 cos20 23 From Tr
3. E WE MOVE THE WORLD Application Note 3414 Sinusoidal Commutation of Brushless Motors Introduction This application note includes a complete discussion of brushless motors Part One is devoted to an in depth review of both brush and brushless motor theory Part Two relates brushless commutation using a Galil Motion Controller Part Three includes some real world cases of brushless motor examples including tips and tricks to maximize the performance of a brushless application Part One Motor Technology Brush type motor theory Basic principles of physics state that a force F is generated on a current I carrying wire of length L when subject to a magnetic field B results in equation 1 F ILxB 1 When the magnetic field is always perpendicular to the current vector IL equation 1 becomes F l L B 1a Consider Figure 1 As shown a loop of wire with a torque arm R is free to rotate about the z axis Rotational torque is defined as T F R 2 1 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com magnetic field B resultant force F a l Ji wire length L N resultant force F A N y b Y x P Wi sd bus Me desired rotation e K NI current I a asi R D Nh V 1 Figure 1 Current c
4. by hitting phase A with 1 Volt and phase B with 1 2 phase A A negative number causes the axis to stay in the Servo Here SH state 7 Query the current Brushless Degree BD The response 2 88 is within untuned tolerance If this number was greater than expected try issuing a larger voltage on the BZ command f the motor his high friction or heavy inertia 1 Volt may not have been enough to drive the motor to an accurate reading of brushless zero If the system oscillates and times out increase the BZ timeout by adding a n in the second field See the Command Reference for details DMC Terminal i x Communicate with Galil Motion Controller Command Input Gg 1BAZ BI 5 BSZ 2Z 200 Brushless motor setup for the Z axis using the W axis for the Znd DAC Wiring from the ACMD signals to the amp is correct The Brushless Modulus BM is approximately 1001 The Brushless phase offset BB should be set to 60 Input 7 has the correct sensor wired to it Input 6 has the correct sensor wired to it Input 5 has the correct sensor wired to it All wiring is correct and functioning properly BH 1000 BB 60 BZ 1 BD 7 vers 2 68 jn 4 2 Command Repeat Stop Editor crear Send Close Figure 21 Brushless motor setup 25 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Part Three A
5. e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com current flow between phases This keeps the stator field ahead of the rotor The desired lead angle is 90 but because of the limitation of having only three phases the effective lead angle will vary between 60 and 120 Figure 13 shows the rotor at a Hall transition point Up until 0 60 the commutator causes the stator field to be oriented along vector A At 0 gt 60 the commutator advances the field to point B The average of this field switching is therefore maintained at 90 ge Desired rotation S8 Stator field B 7 i N X de Y Ze A A 120 0 B N D va Se Ki e ra 4 P 60 0 v rl N V N Stator field A i Rotor field Figure 13 Stator field orientation vs perceived rotor location Another aspect of the brushless motor technology is that current flows into one coil and out another The direction of the current flow is based on position determined by the Hall effect sensors Al Figure 14 3 Phase brushless motor current schematic 10 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com It becomes apparent that the sum of the current flowing into and out of the system must be zero thus if la is positive either Ig or Ic must be negative Figure 15 details t
6. position in space independent of rotation Due to the action of the motor commutator the armature can be thought of as a wound core with an axis of magnetization fixed in space The axis of magnetization is determined by the rotary position of the brushes For a motor to have equal characteristics for both directions of rotation the axis of magnetization or brush axis 6 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com must be at an angle of 90 with respect to the magnetic field Figure 8 shows the resultant axis of magnetization Currentin Current out 4 up ME Axis of magnetization determined by location of brushes Pe Current into page o Current out of page s Coil shorted Figure 8 Axis of magnetization The major drawback of a brush type motor design is the nature of the design itself The commutation brush as a wear item will eventually need to be replaced As the brushes begin to wear microscopic particles are released invalidating the motor for use in a clean room environment Also due to the switching of the coils some electrical arcing will occur This rules out brush motors for explosive environments Otherwise brush type motors are inexpensive reliable accurate machines that continue to play a role in today s industrial workplace Brushless Motor Fu
7. III MII Z NU Wf l VU ULI LL ULI WAY NN WAY NYY NN N N NN NN N NN Phase B WANN NN N N N Phase C NN NN Resultant Torque SS X VAN N V VALI SII V d W AN NN NN N WWI Af LLL f N N NW YY Wf i YY Mi ANN UU MN 0 60 120 180 210 300 360 Figure 16 Resultant shaft torque with a weak phase C This phenomenon is commonly known as torque ripple and is most noticeable at low speeds 14 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com If a brushless motor relies on the state of Hall effect sensors to commutate the phases the current flowing in a coil can have three discreet states Current can flow into the coil out of the coil or no current can flow The general form of the equation for torque with respect to rotor position within the magnetic field is Ta la Kya sin 0 9 If the position of the rotor is 90 the supplied current is completely converted into shaft torque As the rotor position moves away from 90 the 6 term diminishes the overall available torque As shown earlier the state of the current la will change at 6 60 But at 6 60 the torque is la Kya 0 866 This states that only 87 of the current is utilized The current not converted into torque is wasted as heat Sinusoidal Commutation of a Brushless Motor To fully optimize the conversion of current into sh
8. analog voltage signals per sinusoidal axes These signals labeled MOCMDn are wired into the amplifiers Command Voltage In Encoder feedback Just as with a standard brush type servo the encoder signals are required for the Galil to command motion Encoder feedback is also required for the Galil to properly commutate the sinusoidal brushless motor command signal Hall effect inputs The Hall effect sensors are used in many motors to determine the physical location of the brushless 0 The Hall effects are usually offset at 0 30 or 90 If the offset is not known it can be determined during the brushless setup Hall effect sensors are not required for accurate commutation 22 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Galil Brushless Commands The following is a complete list of the Galil commands relating to the setup and operation of a brushless axis BA Brushless Axis The BA command configures the controller axes for sinusoidal commutation and reconfigures the controller to reflect the actual number of motors that can be controlled Each sinusoidal commutation axis requires 2 motor command signals The second motor command signals will always be associated with the highest axes on the controller For example a 3 axis controller with X and Z configured for sinusoidal commutation will require 5 command outputs 5 axes contro
9. angle 0 the applied torque as measured on the rotor shaft is Tmotor 2 Kt motor Imotor 8 However in a real system the impedances of the coils will most certainly differ albeit slightly The motor constant and applied current can vary by several percent and still be within manufacturer s tolerance As a test consider that every parameter is perfect with the exception of the motor constant on phase C As such at certain points the resultant torque will vary similar to Figure 16 13 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Hall 1 1 0 Hall2 1 0 Hall3 1 o f e 60 120 180 210 300 360 Kt O 7777 777 D lt UNNI WI NY WA Kt ER RA L NS N NN NNNN N NN N NW NY Kt Q BEZZE 120 180 210 e ER C l 95 60 300 360 M P B PL P P P P d Current M M 7777777777 9 60 dao 180 210 300 II Nho 360 UV LL LA EA ih rtr rt_ Current I NN yo NN NN NN NNNNNNN NN N NN N N WWW NN Current lo Applied Torque 77 7 7 VINI M M Mg M B MEAM MM LL Phase A I gl
10. of any safety considerations before attempting this After the axis is enabled the stage should not move The TE Tell Error command should return a reasonable amount of error If the stage jumped check the error If it is greater than 1000 in this case the axis ran away until the Off on Error shut the axis down Check encoder polarity noise or dead shorts Once the motor remains stable the user is ready to perform the standard brushless motor setup as discussed previously 26 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Air bearing Spindles Spindle motors are commonly used at very high velocity 10 000 rpm is not an uncommon requirement Some thought is necessary to ensure appropriate commutation at these speeds As discussed in Section 2 decreasing the TM value can markedly increase sinusoidal resolution Note that all time based commands will have their apparent values modified if the TM is not default To calculate the maximum rpm and counts second apply the following equations Max Rotational Velocity RPM 60 sec 1000 m sec lmagnetic _ cycle 1Re v 1 1000 u sec X RPM 1 min 1 sec of _ desired _ samples 0f _ mag cycles TM u sec lm sec 6 10 X RPM 28 mag _ cycles rev TM of _ desired samples Max Commanded Velocity _ cycle 10 counts t counts ma
11. 329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Brushless Degrees Motor Command magnitude Volts Figure 19b 8 point commutation Often the critical factor in determining the maximum frequency of the sinusoidal output will be determined by the voltage that is supplied to the amplifier The voltage usually defines the maximum rpm of the motor Figures 20a and 20b show a drastic difference between a Galil controller at TM1000 and the amplifier at 24 Volts vs the Galil at TM125 and the amplifier at 48 Volts Time ms Motor Command magnitude Volts Figure 20a TM1000 24 Volt power supply 21 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Time ms Motor Command magnitude Volts Figure 20b TM125 48 Volt power supply The user will want to balance the number of points necessary for smooth motion on the specific system with the command processing time System inertia motor velocity and overall loop gain play an important role in determining the minimum TM value that is required Note that changing the TM value requires recalibration of almost all parameters speed acceleration derivative gain and many others are functions of the TM value For Galil commutation specifications see Appendix B Hardware Setup Motor Command signal As mentioned before the Galil requires two
12. aft torque the amplifier needs to vary the applied current based on a precise measurement of 0 The torque equation for the three phases becomes Ta la Kt sin 0 Ts lB Kt sin 044120 10 Tc Ic Kt sin 0 240 A feedback device such as a quadrature encoder can determine 6 in terms of counts a common resolution is 360 1 revolution 4000 counts The amplifier varies the current in each phase I based on the motor command signal M with respect to 0 la M sin0 ls M sin 04120 11 Ic M sin 0 240 The total perceived shaft torque T is T Ta TB Tc 12 Or T la Kt sin 6 le Kt sin 0 120 lc Kr sin 0 240 13 Substituting Eq 11 into Eq 13 15 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com T M sin 0 K sin 0 M sin 0 120 K sin 0 120 M sin 0 240 K sin 0 240 14 reorganizing T K M sin 0 sin 0 120 sin 0 240 15 Using Trigonometry and canceling terms T Kt M 1 5 sin 0 1 5 cos 23 From Trigonometry sin 0 cos 1 24 Eq 23 becomes T 3 2 Kt M 25 For a full derivation refer to Appendix A Equation 25 states that the torque supplied to the rotor shaft is no longer a function of rotor angle Torque ripple is all but eliminated and the system has linear characteristics quite similar
13. arrying wire exposed to a magnetic field The resulting torque at the axis of rotation is a function of the angle 0 with respect to the magnetic field or T FeRsin 3 Substituting equation 1a into equation 3 T IleLeBeRsin 4 For a given system the terms R B and L are constants In terms of a DC motor these terms can be combined into a common motor constant Kt This results in equation 5 Tzl Ke sing 5 As equation 5 shows the applied torque will decrease as 0 approaches 0 Figure 2 shows the relationship at 0 45 2 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com magnetic field B X magnetic field B resultant force F Figure 2 Coil at 45 At 0 0 the torque will be effectively 0 See Figure 3 MESS N RA NEI ae MAIEN 3 x tic field B NET d rotation O SS AL mpm Figure 3 Coil at 0 If the system has inertia the coil will drive past 0 causing the torque to be supplied in the direction opposite of desired This will cause the system to oscillate around 0 To avoid this situation a process known as commutation has been developed Essentially 3 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com a com
14. axis must be connected to consecutive input lines for example BI 3 indicates that inputs 3 4 and 5 are used for halls sensors The brushless setup command BS can be used to determine the proper wiring of the hall sensors BM Brushless Modulo The BM command defines the length of the magnetic cycle in encoder counts BO Brushless Offset The BO command sets a fixed offset on command signal outputs for sinusoidally commutated motors This may be used to offset any bias in the amplifier or can be used for phase initialization 293 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com BS Brushless Setup The command BS tests the wiring of a sinusoidally commutated brushless motor If Hall sensors are connected this command also tests the wiring of the Hall sensors This function can only be performed with one axis at a time This command returns status information regarding the setup of brushless motors The following information will be returned by the controller 1 Correctwiringof the brushless motor phases 2 An approximate value of the motor s magnetic cycle 3 The value of the BB command If Hall sensors are used 4 The results of the hall sensor wiring test If Hall sensors are used This command will turn the motor off when done and may be given when the motor is off Once the brushless motor is properly setup and the motor confi
15. e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com simply offset by 120 degrees The third phase which is the inverse of the sum of the first two phases is usually determined by the amplifier Before the Galil controller is able to perform this commutation a minimum of three setup commands must be given The first indicates what axis is to be commutated as brushless the second command sets the brushless modulus and the third defines the 0 position From there the Galil controller is able to commutate based on the incoming encoder feedback Brushless phase resolution and frequency If the Galil controller has the servo update time TM set to the default 1000 microseconds the Galil updates the motor command signal every 1 ms The bare minimum number of points necessary to define a sine wave is four This leads to the limitation that with TM1000 the brushless cycle must take at least 4 milliseconds to complete Brushless Degrees Motor Command magnitude Volts Figure 19a 4 point commutation However for smoother commutation the user has the ability to lower the TM value This is not without drawbacks but can quickly lead to a much smoother sine wave at a given motor frequency Figure 19b shows the command voltage based on an update time of 500 microseconds 20 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6
16. ed amplifier may be insufficient for the requested mode of operation Conclusion Galil Motion Controllers provide a robust firmware level brushless servo motor control as an alternative to amplifier based commutation This functionality can be integrated into an innumerable variety of industrial applications If a particular aspect of brushless implementation proves elusive the Applications Department at Galil is more than willing to assist 228 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Appendix A Complete derivation of shaft torque based on rotor position To fully optimize the conversion of current into shaft torque the amplifier needs to vary the applied current based on a precise measurement of 0 The torque equation for the three phases becomes Ta la K sin 0 Ts lp Kt sin 0 120 10 Tc Ic Kt sin 0 240 A feedback device such as a quadrature encoder can determine 6 in terms of counts a common resolution is 360 1 revolution 4000 counts The amplifier varies the current in each phase I based on the motor command signal M with respect to 0 la M sin0 Ip M sin 04120 11 le M sin 0 240 The total perceived shaft torque T is T Ta Te Tc 12 T la Kt sin 0 le Kt sin 04120 Ic Kt sin 8 240 13 Substituting Eq 11 into Eq 13 T M sin 0 Kt sin 0
17. f 90 there will be zero torque Current draw will be normal but all the energy is converted to heat A burned motor is a near certainty 5 If 90 lt 270 the polarity of the current is reversed Positive feedback and wild oscillations will occur Working Eq 26 through T 3 2 K M cos 27 This shows that values over 15 cause a drastic decrease in the available torque To compensate the motor will draw excessive current and could burn the motor At very high values of the motor can actually overdrive the circuitry and enter a mode called positive feedback Such a state can cause catastrophic failure In any given system the perceived lag will increase as the angular velocity increases An intelligent drive system can force into a slightly positive state much as the vacuum advance on an automobile distributor accounts for mistimed spark plug charge due to high RPM As with trapezoidal a sinusoidal commutation amplifier relies on the torque constant Kt being identical in all phases Rarely is this absolutely true to minimize any torque ripple most drives allow a user to vary the amplitude or offset of one or more phases Methods of Sinusoidal Commutation 217 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com There are two common methods of varying the current supplied to the motor windings The h
18. g max velocity 29 sec TM desired _ samples Air bearings with their minimal inherent friction present a somewhat unique dilemma when initializing the axis The BZ command relies on friction to damp out the motion before the command times out If the axis constantly times out increase the timeout value with the lt t argument Refer to the Command Reference for details If the axis still does not settle investigate a velocity based damping algorithm to minimize overshoot during the axis setup Refer to Application Note 3413 for an example of such an operation Low Speed Direct Drives 27 lt Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Direct drive designs can minimize components mechanical play and harmonic resonance However at lower speeds a brushless motor can display torque ripple an effect of imprecise phasing and the quantizing of the phase signal Higher end motors minimize this effect If a design does exhibit a noticeable velocity ripple at slow speeds the operator might consider two things lower the PID settings to 75 of their original values Does this improve or exacerbate the situation A second and more drastic step is to increase the TM value making the servo update time larger Again keep in mind the TM value affects almost all the system parameters If torque ripple is still present the specifi
19. guration has been saved in non volatile memory the BS command does not have to be re issued The configuration is saved by using the burn command BN Note In order to properly conduct the brushless setup the motor must be allowed to move a minimum of one magnetic cycle in both directions BZ Brushless Zero This command drives the motor to zero magnetic phase and then sets the commutation phase to zero This command may be given when the motor is off Essentially the BZ command is a combination of two earlier commands BO and BD If a user issues a BO of 1 unit on the first phase and 1 2 unit on the second phase the motor should move to brushless 0 From there the user can issue a BDO to define this position as zero Creating a custom brushless zero routine may be necessary in low or no friction systems to avoid oscillation A Typical Galil Brushless Axis Setup This section illustrates how an operator would set up a normal brushless motor For this example the motor is a 4 pole three phase rotary motor with Hall effect sensors offset 60 The encoder is 4000 counts per revolution Figure 6 shows a screen shot of a typical brushless axis setup 1 The Brushless Axis is set to Z BAZ 2 The Brushless Input on the Z axis is given as inputs 5 6 and 7 BI5 3 Perform the Brushless Setup and test for polarity Hall effects and offsets Here the motor is energized with a 2 Volt motor command signal for 200 milliseconds in bot
20. h directions BSZ 2 200 The controller returns the statement Wiring from the ACMD signals to the amp is correct If this had stated that the wiring must be reversed the operator would swap the motor command for the first phase with the motor command signal for the second phase The statement The Brushless Modulus BM is approximately 1001 is redundant in this case If the operator had an unknown brushless modulus the controller returns an approximate value The statement The Brushless phase offset BB should be set to 60 states that the Hall effect sensors are off set 60 from brushless zero This information was also previously known The statement Input 7 has the correct sensor wired to it and the two 24 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com following alert the operator to a legal Hall state If the Hall sensors were undetermined the user could rewire based on the suggestion and retry the brushless setup 4 The Brushless Modulus is set to 1000 BM 1000 This explicitly sets the known Brushless Modulus to 1000 counts per brushless cycle 5 The Brushless Offset is set to 60 BB 60 As stated the Hall effect sensors must have an offset assigned Some motors will have offsets of 120 6 Drive the axis to Brushless Zero BZ 1 This statement drives the brushless axis to theoretical brushless 0
21. he relationship between the Hall effect sensors the subsequent switching of current in the coils and the resultant applied torque ze Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Hall 1 Current A Current B Current lc Applied Torque Phase A Phase Phase C Resultant Torque Figure 15 Resultant torque based on position 12 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com As can be seen the digital state of the three Hall effect sensors can determine the rotor position within 60 degrees For example a reading is taken where Hall 1 high 1 Hall 2 low 0 and Hall 3 high 1 This combination 101 places the rotor at 60 0 lt 120 This being true the commutator sets the current flow into phase A positive current and out of phase B negative current The applied torque by each phase is the product of the motor constant Kt and the current I At 60 0 lt 120 Kya positive la positive Ta positive Kye negative Is negative Ts positive 6 Kc indeterminate le zero Ts zero The sum of Ta Ts Tc is shown on the final line For this model assume perfect symmetry in the phases Kt motor Kya Kt Kt c Imotor la lB Ic 7 At any given
22. igonometry sin 0 cos 0 1 24 Eq 23 becomes T 3 2 K M 25 31 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Appendix B Galil Optima Series Controller commutation specifications Number of axes Time step Brushless cycle of points 1 8 TM1000 4 ms 4 7 8 TM625 4 ms 6 7 8 TM500 2 ms 4 5 6 TM500 4 ms 8 5 6 TM375 4 ms 10 3 4 TM375 2 ms 5 3 4 TM250 4 ms 12 1 2 TM250 2 ms 6 1 2 TM125 4 ms 24 Indicates Fast Firmware 32 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com References 1 DC Motors Speed Controls Servo Systems Various authors Electro Craft Corporation Hopkins MN 55343 pp 2 1 2 37 amp 6 1 6 21 2 Command Reference Optima Series Various authors Galil Motion Control Rocklin CA 95765 3 User Manual Optima Series Various authors Galil Motion Control Rocklin CA 95765 pp 2 20 2 22 amp A175 A177 33 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com
23. istorical method has relied on the amplifier to divide the current based on feedback and supply the motor as shown in figure 17 e t10V lp c M otion Commutating Gm dus BA B2 A2 Controller Amplifier M N Figure 17 A Commutating amplifier The benefits of this method of commutation are e Ease of setup simply apply a motor command signal e No initialization needed the amp commutates based on Hall state The disadvantages are e Lack of flexibility the amp and motor are usually paired a brushless motor with a different commutation cycle or Hall angles render the amplifier incompatible e Difficult to tune If there is an offset in one or more of the phases quite often the only method to equalize phases is a potentiometer e The amplifier may contain a velocity loop This effect may interfere with the operation of the controller and must be taken into account during axis tuning The second method involves using the motion controller to commutate the first two phases and allowing the amplifier to determine the value of the third phase Since the sum of the currents at any time is zero the current in the third phase equals the inverse of the sum of the currents la and lb 18 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Motion Amplifier Controller LR i S C 2 p
24. ive current flow and open circuit In this configuration the torque based on rotary position will vary as the current is switched as shown in figure 11 8 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Currenti TorqueT Figure 11 Single phase torque based on rotary position In this model the Torque T is the product of the theoretical motor constant Kt times the supplied current I In a single pole system such as this useable torque is only produced for 1 3 of the rotation To produce useful torque throughout the rotation of the stator additional coils or phases are added to the fixed stator Figure 12 shows a simple three phase brushless motor Hall effect sensor Coil 7 Figure 12 Basic three phase 2 pole brushless motor The key to effective motion is to switch the current in all three phases based on rotor position The use of a physical switch is prohibited because of the timing requirements Instead a digital device known as a Hall effect sensor is used Typically three Hall sensors are placed at 120 apart around the case of the motor Based on the magnetic field produced by the rotor the combination of the three logic signals can determine the location of the rotor to within 60 From this information the commutator switches 9 Galil Motion Control Inc
25. ller where the second outputs for X and Z will be the W and E axes respectively BB Brushless Phase Begins The BB function describes the position offset between the Hall transition point and 6 0 for a sinusoidally commutated motor This command must be saved in non volatile memory to be effective upon reset If no Hall effect sensors are present this command is not necessary BC Brushless Calibration The function BC monitors the status of the Hall sensors of a sinusoidally commutated motor and resets the commutation phase upon detecting the first hall sensor This procedure replaces the estimated commutation phase value with a more precise value determined by the hall sensors If no Hall effect sensors are present this command is not necessary BD Brushless Degrees This command sets the commutation phase of a sinusoidally commutated motor When using Hall effect sensors a more accurate value for this parameter can be set by using the command BC The user can query the current brushless degree value by issuing a BD This command should only be used when the user is creating a specialized phase initialization procedure BI Brushless Inputs The command BI is used to define the inputs that are used when Hall sensors have been wired for sinusoidally commutated motors These inputs can be the general use inputs bits 1 8 the auxiliary encoder inputs bits 81 96 or the extended I O inputs bits 17 80 The Hall sensors of each
26. mall and represent inefficient operation To perform such a function the machine must switch the supply current to these multiple coils based on the rotor angle P gu m aterial eg c eS m aterial V connection to coil Figure 6 A simple commutator 5 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com Figure 6 shows the basic elements of a commutator The machine conductors which constitute the windings on the armature are connected in sequence to the segments of the commutator Figure 7 shows the flow of current through the commutator and armature windings Clockwise rotation Figure 7 Current flow through a motor armature Current flows into the system at brush A and flows out at brush B The small arrows indicate current direction in the individual coil sides If the motor rotation is clockwise it can be seen that 1 7 of a revolution after the instant shown the current in coils 3 3 and 7 7 will have changed direction As the commutator continues to turn the brushes pass over successive segments causing the direction of current flow to change At some points in the armature rotation the brush will be in contact with two segments At this condition the coil connected to these two segments will be shorted through the brush As a result of this switching the current flow in the armature occupies a fixed
27. mutator will reverse the direction of current in the coil providing positive torque at angles larger than 90 Figures 4a and 4b illustrate the principle magnetic field B NN VAN N x q magnetic field B YAAA MW X A S NN VON OK Ae m Se ipee TEL dy SS Ei ZE A RAI SE a L x 7 O AN H desired rotation i ES j resultant force F AIN J DI N magnetic field B No UNS AG SEN a Ae N m agnetic field B q N SKI NN AA x 5 H H RA K 3 N AT Y ci resultant force F Ta gt gt current Y DN CN desired rotation 8 y e H gt d x i SO CN SOY pe fi 2 gt resultant force F d Sg Figure 4b Coil beyond 0 Reversed current Resultant force OK 4 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com To produce any amount of useable rotational torque the system design must rely on multiple current carrying coils of wire to be exposed to the magnetic field B Figure 5 shows the physical design of a rotor insulating material d wire length L current carrying wire radius R Figure 5 Basic armature design In addition to reversing direction of current in the coils the commutator may also shut off the current in the coil when they are at an angle near 90 as the torque produced may be too s
28. ndamentals Many motor types can be considered brushless including stepper and AC induction motors but the term brushless is given to a group of motors that act similarly to DC brush type motors without the limitations of a physical commutator To review a DC brush motor consists of a wound rotor that can turn within the magnetic field as provided by the stator as shown in figure 9 By including the commutator and brushes the reversal of current is made automatically and the rotor continues to turn in the desired direction STE Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com B ar 4 7 DL d se de Commutator Figure 9 A Simple brush type motor To build a brushless motor the current carrying coils must be taken off the rotating mechanism In their place the permanent magnet will be allowed to rotate within the case The current still needs to be switched based on rotary position figure 10 shows a reversing switch is activated by a cam Reversing Switch Figure 10 An inside out DC motor This orientation follows the same basic principle of rotary motors the torque produced by the rotor varies trapezoidally with respect to the angle of the field As the angle 0 increases the torque drops to an unusable level Because of this the reversible switch could have three states positive current flow negat
29. pplication Examples Brushless motor technology can be found in a variety of applications including linear motors high rpm air bearing spindles and low speed direct drives Included here are a few tips and tricks for setting up these systems to run with a Galil Motion Controller Linear Motors Linear brushless motors are essentially a rotary motor that lays flat The stage acts like the stator while the case magnets are laid sequentially along the track Setup and configuration are very similar to rotary motors however linear motors are capable of producing immense amount of torque and due to physical design cannot run freely like a rotary motor can Extreme caution should be taken during initialization Ensure working limit switches if available before applying power to the motor Before any commands are issued the operator must fully understand the functions and consequences of what is typed A common safety routine such as the following might be incorporated into an X axis initialization MO Disables amplifiers OFO Sets offset bias to zero KP Sets proportional gain to 1 KD5 Sets derivative gain to 5 KIO Sets integral gain to 0 TL 5 Sets maximum output voltage to 5 volts 5 of max ER1000 Sets error limit to 1000 ERI Enable Off on Error function At this point apply power to the amplifier The motor should not move MO SHX Enable X axis This command enables the axis Be fully aware
30. to a conventional DC brush motor The calculation of torque based on rotary position assumes an ideal machine The position 0 is read and the calculated current I to be supplied occur at exactly the same instant Because this system is subject to physical reality it is important to understand the effect of an inaccurate position read due to the inherent lag in the system Equation 11 has an additional term defined as the angular offset in the position la M sin 0 lg M sin 0 120 26 le M sin 0 240 can be an undesired offset such as physical motor lag An error in the initial motor setup can introduce into the commutation sequence can also be beneficial an intentional phase advance can allow a motor to turn at marginally higher speeds Some interesting facts about E 1 does not cause torque ripple only torque reduction 16 Galil Motion Control Inc e 3750 Atherton Road e Rocklin CA 95765 USA e 800 377 6329 e Ph 916 626 0101 e Fax 916 626 0102 e www galilmc com 2 If E lt 10 the effect on the torque is minimal This makes the initial phase positioning non critical 3 When E is very large gt 60 Kt is cut in half Cos 60 0 5 For the motor to perform a specified amount of work it will require twice the current Power dissipation will increase 4X This situation will probably burn the motor A thermal sensor is a good safeguard against overheating 4 I
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