DRV8234: How accurate is the sensorless position control for brushed DC motors?

Hey Jennifer, 

Jennifer Jordan said:

What is the accuracy and repeatability of position control in DRV8214/DRV8234?

The accuracy will depend on how long the motor is spinning for, as it will miss 1-3 ripples on startup and stop but if properly tuned it won't miss any ripples during steady-state or loaded conditions.  So if you spin it for 100 ripples and it misses 3 then the accuracy is 97%, but if you're spinning for 100,000 ripples then the accuracy is 99.9997%.  

Jennifer Jordan said:

Are there any limitations – low current, stall, etc.?

There is an app note that will be published in the next couple of weeks to address this exact question, I'll link it here once it is published.  Current below approximately 50mA can be hard for the device to detect, and our device struggles to track ripples near stall conditions (95% or more of the stall current - ex. if at stall the motor draws 1A, our device will struggle when the motor is loaded to the point of drawing 950mA).  

Jennifer Jordan said:

I want to vary my supply voltage but maintain constant motor speed, how can I achieve this?  Do you have any motor drivers that help with that?  The motor needs to run at a constant speed even when the VM voltage varies from 14V to 8V.  I would also like it to maintain speed even when a load is applied. 

This would be a perfect application of the Speed Regulation function of this family of motor drivers.  It will use the same ripple counting / speed estimating technology to track the electrical speed of the motor as it spins and will remain steady regardless of voltage or load.  

Best,

Jacob