How to measure current on DC servo motor.

Need to measure current on DC servo motor. What is the best way to measure the current on the motor as it moves to position? Using PXI-7344 controller and MID-7654 drive unit.

This is not an easy task. Here are some ideas:
Direct current measurement:
Add shunt resistors into the motor cabling and measure the voltage over the resistors. As the 7654 doesn't provide DC current but PWM current (32 kHz) you will have to acquire the data pretty fast (at least with 200 kHz). This could be done with a DAQ-board like the PCI-6220. Additionally you will have to do some math to calculate the RMS value of the current.
Control voltage measurement:
You could measure the output voltage of the 7344 as it is proportional to the duty cycle of the PWM current signal of the 7654. You will also need some additional measurement hardware but you wouldn't have to use shunt resistors and RMS calculations. The major disadvantage of this option is cabling as there is a single cable connection between the controller and the drive and you would have to use e. g. two external connector blocks and an additional cable in order to be able to connect your measurement hardware to the voltage output of the 7344.
Torque measurement:
Torque is also proportional to motor current so you could add a torque sensor to your motor and measure the output signal with e. g. a PCI-6220. Depending on your hardware setup this might be the best option.
I hope that helps,
Jochen Klier
National Instruments Germany

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How to measure current on DC servo motor.

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