I have a trouble here and I'm almost sure that what I want to do is impossible.
But to be 100% sure I want to ask all you about your opinions.
I have a DC Brushed motor with permanent magnets.
A very common motor like Fig1.
(Just a pic. from internet, not the real one)
Note: I don't have motor datasheet because I don't know motor model or brand.
But it's like Mabuchi's motors.
I made a H Bridge circuit with PWM to control SPEED and DIRECTION of rotation.
Now I need to implement a torque correction.
When loading the motor, the PWM threshold must increase to keep torque constant.
I did this before using sensors like Hall sensors and optical encoders.
The feedback from Hall/Optic. Encoder tells me when the motor reachs the desired speed meaning that torque is enough for the load applied.
But now I need to do this correction without any sensor attached to the motor.
To do this, first I added a shunt resistor to measure the motor current.
But I need another parameter to determinate the correction behavior.
With the shunt, I can detect if the motor is loaded. The current will rise and based on this I need to increase PWM Threshold in order to increase motor torque, keeping torque constant with load variation. The problem is that when the voltage increases, to correct the torque loss due to the load, the current also increases. And as the current is my feedback about load, that condition cause an infinite grow of voltage because I'm unable to detect when I need to stop increasing motor voltage. Also I can't measure motor speed that is a perfect reference for this correction circuit.
Someone have any idea about how to do this torque control without sensors attached to motor ?
I think that is not possible due the math and physics rules as I described above.
But if some technique to catch motor commutation just by power supply wire is possible then I would be able to measure motor speed solving the problem.
The big question is:
How to measure DC Brushed Permanent Magnet Motor speed by power supply signal ?
Any ideas ?
Thank you all in advance.