Best way to measure motor load?

I have a motor that runs at a fixed speed, controlled by a pwm and I would like to measure the load. My first thought was to use a resistor in series and measure the voltage drop, but I'm worried that the pwm and the inductance of the motor would require a rather complex filter.

What are your thoughts?

A moving average value of the motor current should be reasonable to use in measuring the power input to the motor.

Hall effect current sensors are available that introduce negligible voltage drop.

You may want to give this a read: What’s the relationship between current and dc motor output torque? Depending on your specific application for a commercial application where you may wish to actually measure load torque you can consider a rotary torque sensor.

Ron

@jremington Thank you for your answer. The load is changing very slowly so your suggestion would be easy to implement. A Hall effect current sensor would do the trick.
@Ron_Blain Thanks for the link. Very interesting.

klurige:
I have a motor that runs at a fixed speed, controlled by a pwm and I would like to measure the load. My first thought was to use a resistor in series and measure the voltage drop, but I'm worried that the pwm and the inductance of the motor would require a rather complex filter.

What are your thoughts?

Its the opposite, the inductance means the current is smoothed, so the voltage across the resistor
will be approximately steady, assuming a high enough PWM frequency.

A problem is if you are using an H-bridge, the sense resistor cannot be at ground potential for both
movement directions.

Its wise to low-pass filter the thing though, as the motor winding inductance may not be large enough.
A simple RC filter is all you need.