Sensorless tach with brushed motors

General topics and discussions about the VESC and its development.
fpga_brad
Posts: 1
Joined: 23 May 2018, 23:22

Sensorless tach with brushed motors

Postby fpga_brad » 23 May 2018, 23:40

I am new to VESC but looking for an ESC that can give me a sensorless Tach output using a brushed motor. It looks like VESC has a sensorless mode that could create a Tach signal, it also looks like it can control brushed motors.

Can it create a Tach signal with brushed motors using back emf techniques?

devin
Posts: 255
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: Sensorless tach with brushed motors

Postby devin » 24 May 2018, 23:21

if you can find a way to observe the back emf voltage then i believe instantaneous back emf voltage * measured kv of the motor should give rpm

rew
Posts: 943
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Sensorless tach with brushed motors

Postby rew » 30 May 2018, 06:45

Let me get this straight.

You have a brushed motor and you want to have SOME feedback on how fast it is going from the motor controller without having to install the hardware that measures this. Right?

With most motors and most dutycycles, the current does not return to zero. So during the PWM OFF period, you can not measure the BEMF voltage as the current through the motor pins the voltage of the lead to the other power supply rails. (If you only need to go one direction you can save a mosfet by using a diode. In practice, there will often be a mosfet there and you'll simply drive the mosfet to reduce losses from 0.6V to near-zero).

But if in advance you have measured the resistance of the motor you could do some simple math.

I = (Vapplied - Vbemf)/ R

so if we know R, Vapplied, and I we can calculate Vbemf = Vapplied - I * R . You could for example write a custom application for the VESC that calculates this 500x per second, and then reports it somehow. If this is accurate enough for your application I cannot determine from here...

P.S. I consider my job "done" when You have the BEMF. That's what you get with a classical tacho: A voltage proportional to the speed of the rotor due to some KV of the tacho generator.


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