What hardware do I need to control a DC motor?

[lopezj50]

I would like to control the speed of a DC motor using pulse width modulation.  It's not the LabVIEW programming that I'm worried about, it's getting the correct hardware to interface with the motor.  What hardware would I need to control the speed of a DC motor?  I'm guessing something with some sort of analog output and then goes to a power drive or amplifier?  A power supply as well (unless the motor connects directly to the power drive where it can get voltage?) ?  Thanks.

[Jochen]

The easiest way to control a standard DC motor is using a motion control board like the PCI-7342 that runs the control algorithm onboard. As this board outpus a +/-10 V control signal a power drive (e. g. MID-7602) is needed to generate the PWM current signal for the motor. If the MID-7602 doesn't meet your needs the PCI-73xx boards can also interface to most 3rd party drives through a UMI.

There are even more options, e. g. building your own motion controller with real-time hardware or FPGA based devices but for standard motion control tasks this is typically not needed.

Please let me know if you need more specific information. In this case please provide some insight into your application.

Best regards,

Jochen Klier
National Instruments Germany

[lopezj50]

Jochen,

Thanks, I was exploring those devices last night.  Would it also be reasonable to generate a pulse train and send it through the digital outputs of a USB-6210 (since I already have that device)?  From there I could send those pulses through a drive or amplifier circuit, then send that to a mechanical relay which could give me the current I need to supply the motor?  I don't see why this would not work but please let me know.  And thanks for the suggestions.

Joe

[Jochen]

This approach won't work for position and velocity control as it doesn't provide closed loop control. A DC motor acts very differently from a stepper motor. You can control a stepper motor by sending step and direction pulses to a drive and the motor will move the number of steps you command it to do. With a DC motor you need a feedback device like a quadrature encoder to be able to control it's velocity and position.

Now you could think of using the USB-6210 to read the position feedback with your PC and run the control loop (e. g. PID) in software but as Windows isn't a real-time OS and the USB-6210 couldn't be used with a real-time OS at all you won't get a stable system behavior with this approach.

Depending on your application you might also need some security features like limit switches and following error monitoring which is also hard to accomplish with a USB DAQ device.

Jochen

[lopezj50]

Jochen,

Ok, so I guess I will have to go with the controller.  That way I can have more control over speed and close the loop.  Does the motion controller provide the quadrature encoder and such devices to keep track of position and velocity?  The reason that I'm using a DC motor is because we're using a worm gear reducer for a heavy load lifting application.  It is hard to find a cheap stepper for high torque applications.  I had anticipated spending a lot of money for computer hardware but not for mechanical parts such as a motor and gear reducer.  Now, if I get the motion controller and power drive, will I need anything else?  Do the leads from the motor go straight into the power drive?  Can I switch polarity easily so that the motor can go forward and reverse?  Thanks.

Joe

[Jochen]

Joe,

• The motion controller provides inputs for quadrature encoders. The quadrature encoder is typically mounted on the shaft of the motor.
• You shouldn't need more components than a controller, a drive and the motor. A UMI may be needed to connect the controller to a 3rd party drive.
• The motor leads connect directly to the drive
• With a controller and a drive the motor can be commanded to move in both directions.

I hope that helps,

Jochen

Message Edited by Jochen on 10-19-2006 03:05 PM

[lopezj50]

Excellent, you're information was most helpful.  If I have anymore questions then I'll just post again.  Thanks a million.

Joe

[Brian_Beal]

You've kinda got 2 threads going on this same subject. 

How tightly do you need to control the speed?  Do you care about position, other than at the end of travel limits?

[lopezj50]

I need to control speed relatively tightly.  I will have to adjust speed of the motor based on the rate at which a temperature is changing.  I don't really need to know position other than the travel limits, but an encoder would be a nice added feature for this project.  I will be using a 1/2HP DC motor, 90 V, and full load amperage is 5.  The gear reducer has a 30:1 gear ratio and maximum torque is 442 in/lbs.   This should be sufficient since we will not be lifting anymore than 200 lbs.  I think I'll need to get a third party power drive to get the amperage for the motor.  Any suggestions?  Thanks, and sorry about the double thread.

[BME_genuis]

I have a very similar motor system setup to what you have.