I am attempting sensor-less speed control of a brushed DC motor using PWM and measuring the BEMF (motor-induced voltage proportional to speed). The motor is powered through a H-bridge IC (TI DRV8801).
I've noticed the BEMF is only observable in
fast decay mode, where the motor is basically reversed until the current is zero and then the switches turn off.
This leaves the motor connected to power rails through the free-wheeling diodes. Plus the resistor-divider. Plus EMI filter caps on the motor.
So the circuit might look like this:
I assume the oscillation shown in the following image is a result of the residual energy of the motor inductance and the connected capacitance (filter, wire)?
- yellow: motor -
- cyan: motor +
- blue: center point of voltage divider: 10 kΩ to GND and each motor contact
- pink: current (large offset with DRV8801; might also be due to bad layout)
The long settling time limits the PWM duty-cycle and the frequency. Is there a way to shorten it? Maybe with a load resistor?