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L6470: Switching between two speeds using Run command causes current to slowly drop

Question asked by cocks.jim on Sep 1, 2015
Latest reply on Nov 18, 2015 by Enrico Poli
Hello,
I'm using an L6470 over SPI to change between two speeds every few seconds (say 1000 steps/s <-> 1100 steps/s). To achieve this I am issuing the Run command alternating between my two set points.

I have noticed that every time I swap speeds the current reduces very slightly. After my application has been running for an hour the current has reduced to such a point where there is no longer enough torque to turn the motor. Setting the motor to 0 steps/s (again using the Run command) and back causes the current to return to normal. If I leave the motor to run at a constant speed, there is no change in current over the time period. Stopping the motor between each speed change is not ideal.

To debug this problem I have tried to remove all status polling and that did not help. I also ensured that there is no erroneous SPI commands being sent using a logic analyser. I have also ruled thermal problems by using freeze spray on the L6470 chip. And I on't think it is a power supply problem. The rate of decay of the current through the motor is proportional to the rate I send through the run command.

Any ideas what could cause this behavior?

Thanks

Edit:
I was able to reproduce this behavior using dSPIN and EVAL6470H-DISC. I have attached the script and register configuration.

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