DRV8889-Q1: Output current is reduced if the torque of motor gets stronger.

Hello Chase,

Thanks for sharing the details. Based on the current waveforms it looks like the supply voltage used in not sufficient to overcome the BEMF at the STEP rate 600 PPS. What microstep setting was used? It is okay to run this way. The behavior looks normal. 

Regards, Murugavel

Dear Murugavel,

Thank you for your support.

My customer uses 1/16 step.

Do you still think that this is normal operation?

or, Do you have the way not to reducing output current even the torque gets stronger?

Best regards,

Chase

Hello Chase,

What was the supply voltage used? Looking at the current waveform I think it can benefit from increasing the supply voltage to get target current in the motor. Thanks.

Regards, Murugavel

Dear Murugavel,

Thank you for your support.

This is the application for automotive, so they use 12.6V car battery. And it is very hard to increase the supply voltage.

Is there any other way not to reducing output current?

Also they have a question about why input current is increasing while supply current is reducing.

Below table is the current measured in power supply. You can see it increasing power supply current while the torque is increasing.

On the other hands, supply current to motor is decreasing while the torque is increasing. How can I explain it?

Is there something happen inside DRV8889?

Best regards,

Chase

Hello Chase,

This is because of the property of the motor itself, L and R values and the BEMF constant. The DRV8889-Q1 will try to regulate the current to the set target value using VREF and TRQ_DAC. However if at a specified STEP rate the difference between VM and BEMF will be such that with the specific motor L and R the current will never rise to the target value and there will be no current regulation. This is typical stepper motor behavior at higher STEP rates. The torque output will be lower at higher step rates because of this behavior. Some stepper vendors provide speed vs torque plots at specific operating voltages. 

"Also they have a question about why input current is increasing while supply current is reducing.". This is a function of power delivered to the motor. For a fixed power in the current regulation zone as the VM decreases the power supply needs to deliver higher current to maintain the power delivered. This is expected electrical behavior.

"Below table is the current measured in power supply. You can see it increasing power supply current while the torque is increasing. On the other hands, supply current to motor is decreasing while the torque is increasing. How can I explain it?". Same explanation as above. The power delivered must be looked at. Higher power delivered = higher torque output of the motor.

"Is there something happen inside DRV8889?". DRV8889-Q1 provides current regulation as long as the difference between VM and BEMF has sufficient headroom to push the desired target current through the coils. If the current does not reach the target current at the given step rate the peak current and the torque would be lower because DRV8889-Q1 would be unable to regulate the current. 

Based on the current waveforms shared in the first post it appears to be reasonable and usable. I do not see anything unusual. 

Regards, Murugavel