DRV10975EVM: non-linearity in motor frequency

Stefan Lintner

Part Number: DRV10975EVM

Hi Team,

a customer observed a non linear behavior when controlling the speed via I2C on the DRV10975EVM.

i.e. 100 or 200 or 250 let to the same motor frequency. (at no load) i.e 252Hz and with 300 it change to 273Hz.
Customer used the frequency from the GUI and checked his correctness with a scope and a current probe on one phase.

What could be the issue here?
How should the speedcmd register be interpreted?

BR,
Stefan

over 4 years ago

0 Isaac Lara over 4 years ago

TI__Mastermind 23075 points

Hello Stefan,

Thank you for posting to the MD forum. The non-linearity issue is actually a known problem that stems from the GUI and do not recommend customers to read the motor speed in RPM from the GUI. Instead we recommend reading the motor electrical period and calculate the RPM that way.

I will link these two threads that contain more info on how to overcome this issue:

https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/933806/drv10975-speed-vs-rpm

https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/944353/drv10975evm-drv10975-speed-vs-rpm/3495421?tisearch=e2e-sitesearch&keymatch=drv10975%20I2C%20SPEED%20NOT%20CHANGING#3495421

I hope this helps!

Best,

Isaac

0 Stefan Lintner over 4 years ago in reply to Isaac Lara

TI__Expert 5235 points

Hi Isaac,

customer did no rely on the GUI reading but also measured with a scope and a current probe on one phase.
He still observed this behavior.

I also already discussed the two threads above with him.

Can you think of any other reason for this behavior?

BR,
Stefan

0 Stefan Lintner over 4 years ago in reply to Stefan Lintner

TI__Expert 5235 points

Update: When the motor has a load attached, the speed register matches the measurements. Where is the mismatch with no load coming from though?

0 Isaac Lara over 4 years ago in reply to Stefan Lintner

TI__Mastermind 23075 points

Hello Stefan,

Thanks for the update. This is interesting since this is something we would suspect to occur with a load attached and not what the customer is seeing on their end. Do you think the customer could provide the following:

EEPROM Register Settings

SpeedCmd and spdCmdBuffer values for both situations where the motor has a load attached and with no load. If they could do a sweep from 0 to 512 on the GUI that would be very helpful, they dont not need to get all the values but just enough to get a better understanding of how the speed is behaving through the range. If they could include their bench measured value as well, this should help us understand what is going on a bit more.

Best,

Isaac

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DRV10975EVM: non-linearity in motor frequency