Measuring the varying Resistance of a DC motor

It isn't motor resistance that changes. It's back emf. As a motor is loaded the back emf decreases, drawing more current.

Search the web for 'current shunt' -- that should help. Basically you want to measure a small voltage across a very small resistance with a big current flowing through it. You keep the resistance small so your measurement doesn't disturb the system, which means you need a good low-offset amplifier in the DAQ or out.

Perhaps even searching for 'current measurement' would be helpful.

Motor resistance does change, as the coils heat up.

Back-emf doesn't depend on load, but on motor speed and - to a lessor extent - motor temperature. The magnetic field generated by a permanent magnet decreases as the magnet gets hotter, and with it the back-emf per unti motor speed.

thanks. a current shunt seems to be exactly what i'm looking for.

Which comes first, load or speed? Put on a load and the motor slows etc, etc. I would think the motor resistance change is negligible. Stall a motor and it probably burns due to lack of back emf.

Back emf is created by the 'moving motor winding cutting the magnetic field generted by the fixed magnet/excitation coils. Speed thus coomes first.

If you've got a servo-controlled motor that maintains the same speed, you see the same back emf independent of the torque being generated.

Small - +0.3% per degree Celecius.

No, it burns out due to escessive curren through the coil, which intelligent driver design can prevent.