DRV8434AEVM: Technical questions

Hi Roy

Thanks for posting in this forum. 

Roy Chou1 said:

1. How can we measure the back-emf when motor was operating?

It is not possible to measure BEMF explicitly because the driver outputs will be active while the stepper is running. With certain steppers, step mode setting and step rates observing xOUTx voltage may show superimposed BEMF sine wave. Generally speaking it is not possible to measure the BEMF explicitly. For stall detection we use indirect approach with smart tune ripple control STRC decay mode variable TOFF. The variations in TOFF is affected by BEMF. See this application note, https://www.ti.com/lit/an/slvaei3/slvaei3.pdf.

Roy Chou1 said:

2. We were using DRV8434AEVM to drive customer's motor. Customer's motor was moving up and down as picture showing. Why we met stall issue when motor moved up, but no stall issue when motor moved down. (We set the same VREF)

It is possible the motor be over driven or stall threshold not setup correctly.

Roy Chou1 said:

3. When we used DRV8434AEVM, we produced the stall issue manually. But why start steps motor was still spin when stall occurred. It looks like that we didn't have stall protection, is it correct?

The DRV8434A device provides stall detection and reporting only. The MCU controlling the DRV8434A should make a decision based on this reporting of stall. See Table 7-7. Fault Condition Summary in the current datasheet page-27.

Roy Chou1 said:

4. What does VREF voltage mean? Does it mean the current? May you help check if below equation is ok or not? The stepper motor spec shows the coil resistor is 8R and we use 12V. 

12/8 = 1.5A(motor current). Because we used two phase and so one phase current should be 0.75A. Follow the datasheet, IFS = VREF/1.32. VREF = 0.99V. But motor is hot when we set the value.

Yes, VREF is used for setting IFS (full-scale) current with TRQ_DAC = 100% setting. I have used similar stepper for headlight leveling applications commonly known as PL35L stepper motors. These are rated between 0.4 to 0.5 A IFS. The stepper drivers such as the DRV8889-Q1 or the DRV8434A are current mode drivers VM voltage does not affect the motor current as it will be regulated by the stepper driver. Usually the stepper manufacturer provides the IFS specification. It is not calculated like you described. Overdriving will heat the motor and reduce the motor life significantly. I'd recommend 400 mA for this motor. You can see the same type of motor in the stall detection application note I mentioned in #1.

Roy Chou1 said:

5. How to set Torque? I am not sure the relation between N.m and voltage. Can we set it by using the motor spec "PULL-OUT TORQUE=180gf-cm and PULL-IN TORQUE=150gf-cm"?

Stepper motor output torque is a function of IFS setting. Motor vendor current specification will be for rated torque output. TRQ_DAC is used for reducing the current if needed. For example in hold mode to reduce the IFS such that the motor heat can be reduced if in hold torque mode for long period of time. Thank you.

Regards, Murugavel 

Hi Murugavel,

Thank you for your comments. Regarding to no. 5,

1. I saw that DRV8434-Q1 doesn't have TRQ_DAC pin, but the other motor driver having like DRV8889-Q1.

2. Can we input PWM pulse way into VREF to reduce the motor driver temp?

Regards,

Roy

Hi Roy,

Roy Chou1 said:

1. I saw that DRV8434-Q1 doesn't have TRQ_DAC pin, but the other motor driver having like DRV8889-Q1.

This is correct. The DRV8889-Q1 has a SPI register for TRQ_DAC.

Roy Chou1 said:

2. Can we input PWM pulse way into VREF to reduce the motor driver temp?

VREF input must be DC reference voltage. So a PWM based DAC, reference https://www.ti.com/lit/an/spraa88a/spraa88a.pdf?ts=1733238201002&ref_url=https%253A%252F%252Fwww.google.com%252F with an RC filter at the output can be used to vary the VREF voltage. So, yes, VREF can be set to a lower value during hold mode to reduce motor driver temp. Another easy approach, if you need only two levels of voltages, would be to add an additional resistor to GND from the VREF potential divider to reduce the VREF - see below. Thank you.

Regards, Murugavel