Motion Control and Motor Drives

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BLDC Motor Speed from Controller

In the tester I am building, I am having troubles acquiring a stable motor speed.  I need a stable speed so that I can control the motor speed to 1100 +/- 50 RPM.


We have a compressor connected to a Dart 700BDC controller, and a cFP-CTR-502 counter input connected to one of the hall sensor inputs on the controller.  The counter is set up just to continually counting the number of pulses (counter overflow is handled within the test application).  The test application gets the counter value and timestamp from the Fieldpoint VI's approximately every 500 milliseconds. It compares these two items to the data from the previous iteration to generate an RPM.  I've attached a VI that has the routine that I am currently using.


When I run the motor, I have RPM values that fluctuates beyond my 50 RPM window.  Do I need to tweak my routines to display a more stable reading?  Or is a different solution required?

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Hi OlderJohn,


The first you should try is lowering your sample time and perhaps even your proportional gain. Are you familiar with PID control. I recommend reading a great article on it here

Joe Daily
National Instruments
Applications Engineer

may the G be with you ....
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