DRV8837: How do we measure rotor speed if we don't use magnetic encoder

Hi Miles,

There are multiple ways to implement sensorless position monitoring. One of the most popular is the current ripple counting method. Below are some resources on how to implement ripple counting for position control and stall detection. There are some drawbacks to this method. Since the current ripples are directly related to the motor BEMF, during initial start-up or motor slow down, the ripples will be unstable and can lead to accuracy problems over time. Your firmware needs to take this into account and constantly calibrate the system. So this solution is firmware intensive.

• https://e2e.ti.com/blogs_/b/behind_the_wheel/posts/the-seat-remembers-brushed-dc-motor-ripple-counting-drives-innovation-in-full-featured-memory-seats
• https://www.ti.com/lit/ug/tidud30a/tidud30a.pdf?ts=1634095117188&ref_url=https%253A%252F%252Fwww.ti.com%252Fsitesearch%252Fdocs%252Funiversalsearch.tsp%253FlangPref%253Den-US%2526searchTerm%253Dripple%2Bcounting%2526nr%253D4725

Miles,

One thing I forgot to mention. This method may not work well with the DRV8837 since it is an integrated H-bridge driver. This solution is best for gate drivers like the DRV8706 with inline current sensing (inline = bidirection current sensing).

You can still use the current ripple counting method with the DRV8837. Below is how you can add a sense resistor to generate a small voltage drop on the source of the low side FETs. This small voltage can be amplified using a current sense amplifier like the INA181 (in bidirectional current sensing topology) and passed through the circuit described in the reference design (linked in previous reply).

Let me know if you have any questions and I'll be happy to help.

Hi Miles,

i suggest reading this app note(https://www.ti.com/lit/an/slva321a/slva321a.pdf?ts=1634097909811&ref_url=https%253A%252F%252Fwww.ti.com%252Fsitesearch%252Fdocs%252Funiversalsearch.tsp%253FlangPref%253Den-US%2526searchTerm%253DSlva321%2526nr%253D25). In short summary, either decay mode can be used but it depends on your application needs. In certain cases, fast decay is the best option ( specially if your application requires fast decay of the current). But It comes at the cost of higher ripple (which may be a good thing if you are trying to implement current ripple counting). Brake is the opposite. The current decays slower but will have lower current ripple. 

I hope this answers your question.