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Getting Started with an InstaSpin Kit for Low Speed High Torque Application

Aaron Yurkewich
Prodigy 120 points
Other Parts Discussed in Thread: DRV8302, DRV8301, DRV8301-Q1

Hello E2E commuinty,

I developed a sensorless BLDC motor controller board to control the XK4082 1500KV 6cell Brushless Inrunner motor from Hobbyking using back EMF sensing, but was not able to reliably control the motor at <50RPM due to very small back EMF signals (<100mV). Our application requires a robust low speed (<20RPM), high torque (70A) sensorless BLDC controller as the motor is used in lower limb rehabilitation. 

My questions are:

What is the difference between the DRV8302 Kit and DRV8301 Kit and which would you recommend?

Is there a kit for the DRV8301-Q1? Will it be more robust as it is automotive qualified?

Are encoders required? I did not think they would be as these boards are for sensorless control, but when I called the TI representative he was very adamant that encoders are needed for the DRV83xx kits to work for any motor.

Are there inputs on the kit boards for encoders?

Can encoders be used with the open source INSTASPIN-MOTION code to improve the low speed velocity control issues caused by guessing velocity based on time between commutations?

What are the breaking (stopping) capabilities with these kit boards (i.e. 1000rpm to 0rpm in 10 miliseconds)?

What are the speed reversal capabilities with these kit boards (i.e. 1000rpm to -1000rpm in 10 miliseconds)?

Thank you in advance for your help,

Aaron



  • 0
    TI__Mastermind 21320 points

    Hi Aaron,

    The kits are the same hardware wise except for the DRV83xx chip. The chips have a few differences, the main one being that the DRV8301 has a SPI interface and the DRV8302 does not. You can check the respective datasheets for more info.

    There is no kit for the DRV8301-Q1 at the moment.

    Encoders are not required. I do believe the board has inputs for hall sensors/encoders though. The kits do run just fine with sensorless algorithms

    I am unsure here. I will ask our InstaSPIN team to look at this question.

    Breaking and reversal capabailities will depend on many things. It is impossible to provide one number. The main issue will be surges to the power supply during these periods pushing up the voltage level.

  • 0 in reply to Nicholas Oborny
    TI__Guru** 119560 points

    Aaron,

    "Are encoders required? I did not think they would be as these boards are for sensorless control, but when I called the TI representative he was very adamant that encoders are needed for the DRV83xx kits to work for any motor."

    Encoders are not required for torque or velocity+torque applications.  An Encoder would be required if you needed position control wrapped around the speed+torque loop.

    "Are there inputs on the kit boards for encoders?"

    Yes, there is a header for 5V supply encoder.

    "Can encoders be used with the open source INSTASPIN-MOTION code to improve the low speed velocity control issues caused by guessing velocity based on time between commutations?"

    Yes, for continuous very low speed control the sensorless observers do not work. There are other "sensorless" techniques that may work (high frequency injection) or you can use a mechanical sensor like an encoder.  InstaSPIN-MOTION includes a proj_lab12 in www.ti.com/tool/motorware that does exactly this.  You will see some additional -MOTION capability for sensored feedback in a release scheduled for late November.

    "What are the breaking (stopping) capabilities with these kit boards (i.e. 1000rpm to 0rpm in 10 miliseconds)?What are the speed reversal capabilities with these kit boards (i.e. 1000rpm to -1000rpm in 10 miliseconds)?"

    The rate at which you can accel/decel will first be determined by the motor itself.  Magnitude of the short circuit current  (Flux in V/Hz dividied by 2pi divided by stator inductance) gives you a good idea about it's capabilities.

    You will also want very tight current control.  If using InstaSPIN-FOC or InstaSPIN-MOTION you will want to take the current control Kp values (which are divided by 4 to soften them for initial evaluation) and multiply them by 4 to stiffen them too your limit.  If using a speed controller you also have to test it's limitations that it isn't overly aggressive at any point and drives into oscillation. Using InstaSPIN-MOTION makes this much simpler.

    But beyond that  YES, you need to take either some HW or SW action to handle what the motor does to the bus in a major deceleration event.  Certainly if at high speed you just brake the inverter (turn on all HS or LS switches) it is possible to cause an over voltage event.  A common way to deal with this is to add a braking resistor to the hardware.  This is likely what you want to do for a fast deceleration.  You can also potentially add your own software logic to dissipate some of the voltage into the Id current to limit the amount that gets put onto the bus.