DRV10987: DR10987 EVM close-loop operation not possible

Hello Louis47356,

Current limit during start up usually results from 3 sources: Tcontrol, incorrectly programmed Phase Resistance, and incorrectly programmed Kt.

Looking at your motor parameters, the phase resistance seems to be incorrectly programmed. Phase to Phase resistance is 1.3 ohms, and phase resistance will be half that (R_ph_ph / 2 = R_ph_ct) to equal 0.65 ohms.

For Kt, we highly recommend finding that number by taking a scope probe between two of the phases, spin the motor, and measure Voltage (peak to peak) and frequency of the BEMF. Then, Kt will come from dividing the peak to peak voltage by to 2 and dividing by frequency:

Kt = (V_pk_pk/2)/(f_BEMF)

For Tcontrol, it seems you've already followed that in the tuning guide but just incase you haven't, go to the Control Advance Time Fine Tune section and follow the instructions there. From prior experience, 360us seems rather long for motor of this inductance. 

In addition, this motor may have too low of an LR time constant to spin for our devices. The limit is shown in the Recommended Application Range table in the DRV10987 data which I've copied below. L/R = (76u)/(1.3) = 58.4us

Best,

-Cole

Hey Alois,

I'll split this up into 2 problems:

L/R Constant and Tuning:

You can try to double the PWM frequency of our driver to 50kHz to see if it helps at all. Or you can disable current limit and let the device's overcurrent protection (OCP) (about 4Apk) protect the motor, Speed Abnormal Lock1 or OCP will probably trigger next if there is a problem with spinning the device like this. I will say, low inductance motors are known to cause problems with this family of devices which should be addressed in the next generation of devices.

Allow me to get in contact with someone else who might be able to comment on a different device family if your problems continue.

GUI Problem:

Unfortunately, we have seen the "commas only issue" before but have not been able to replicate in the office at all. Can you go to the "region and language" settings on your computer, switch it to "English: United States", restart the program, and see if you are able to enter decimals?

Otherwise, we'll have to use the scripting tool in the GUI and type in the values in Hex. To do this, make a go to the Script menu at the top of the GUI, launch window, and in the Run menu of the scripting window, run module (.py file). To record and write a script, go to Apendix A.3.2.2 in the DRV10987 EVM User's Guide. You'll have to manually configure the registers like shown below:

This is a script that changes tcontrol to 120µs and reads back the value in GUI. It seems you already know how to shift the values so the same would apply to this register.

GUI.write_register("DRV10987","CONFIG2",0x2C)
GUI.read_register("DRV10987","CONFIG2")

Best,

-Cole

Hey Alois,

This is good news. I agree that it sounds like some tuning is needed.

At this point, I highly encourage you to make sure you have found the most optimized Tcontrol or lead time setting for your motor. Though I've mentioned this before, it is very critical for the motor to have this optimized at high RPMs. This tuning is found in the DRV10987 tuning guide, as mentioned before.

If current limit Lock0 is somtimes being hit, then I encourage you to look at the closed loop acceleration setting on the basic settings tab in the closed loop settings section. The lower it is, the less likely current will surge and hit the current limit.

You can also change the Control Coefficient Settings and ClkCycleAdjust settings. These settings are highly situational and may or may not improve performance. If you find that changing the setting does not help, be sure to change it back.

Finally, observe the "Motor Velocity Constant" (or Kt) measured by the device on the Display tab in the Display section. If you find the numbers are widly different at your high speed, you may want to change your programmed Kt value.

Hope this helps,

-Cole