DC Motor Torque Related Question

Thread Starter


Joined Feb 16, 2010

Attached is my idea for monitoring Torque.

The Idea

Firstly, I wish to monitor torque as a sudden force is applied to the bottom of the arm.

At present the red off center mass will want to pull the arm in the anti-clockwise position. I want to program a micro-controller (PID controller or Fuzzy Logic) to maintain constant current so that the motor is held against the switch in the clockwise direction.

Therefore there will be a current maintained in the motor. When a force is applied as shown, I want to program the micro-controller to increase the current to match the force being exerted onto the arm so that the switch is in constant contact.

NOTE: the force is an impact and sudden therefore the micro will need to react fast enough to the impact.

Ideally, I want to know if my train of thought is correct in saying that once the impact is over, if I was monitoring the current, I could also monitor the torque applied to the arm by using the motor constant etc?

Any help would be great :)



Joined Apr 20, 2004
If that is a basic switch, it will exert its own force on the experiment. Substitute a couple of photointerrupters and use a flag on the lower arm as an indication of the zero position. At zero, neither photointerrupter will be obscured. The one that gets blocked will indicate the direction of displacement of the arm. Of course, that can also let you dispense with the mass.

The magnitude of the impact and the definition of "sudden" may have an effect on the experiment.


Joined Jan 11, 2009
You seem to be confusing the force during the impact and the force of the motor with constant current. The constant current time does extend into the time of impact as well. In one case the angular velocity is constant 0 and in one it is not. During the time of accelleration the force is mostly used to change the momentum in the system, when the velocity is 0 you are measuring the force of the motor alone.

One good way to do the whole thing is connect the motor to a flywheel of known moment of inertia. Apply a constant current ( or constant voltage ) and measure the the voltage ( or current ). Finally measure the angular velocity over time. Change in the angular velocity is proportional to torque. You thus can get the torque for any current or voltage at any angular velocity ( including 0 if you spin the motor in one direction and then reverse the polarity )

As you appartus stands I do not see how you can get much useful data from it. But I would be interested in know how are you going to apply the force, I do no see any apparent way.