How to drive a DC brushed servo with PWM?

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19 Mar 2017 07:02 #89878 by chrisalbertson
I've acquired a few brushed DC servo motors. These are basic, they take DC in two brushes and have a quadrature encoder. I want to use these with LinuxCNC. The PC can read the encoders either directly or via a Mesa FPGA card. The PC will also generate PWM and direction signals to control the motors. Again either directly or via a Mesa card.

So what else is needed? I figure a DC power supply that can supply about 50 volts and then "something" that lets the PWM control the power supply's 50V output. It seems that is would be just a basic dual H-bridge.

I could build a simple h-bridge but I bet there are better commercial ones available

I'm asking for suggestions. What make and model parts should I be looking at for driving mid to small size DC brushed servo motors using the PWM/Direction signals from either a parallel printer port of Mesa FPGA? I'm OK with buying used on eBay or new parts. Cost does matter as this is for hobby/education
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19 Mar 2017 12:58 #89880 by tommylight
There are PWM drives on ebay for 23 euro a piece but it has 2 full bridges on it so it drives 2 motors for that price.
That is for education only as they have no current control, no enable pin so they will start spinning in one direction as soon as you power them.
AMC has plenty, GeckoDrive has DC servo drives with step and direction, leadshine has plenty etc, this is the 80 to 150 dolar range.
What do you intend to do with them?
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21 Mar 2017 12:27 #89996 by andypugh
When you say "small" how small are you talking? What sort of voltage and current?
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21 Mar 2017 12:51 #90000 by tommylight
Do these count as smal? They are 56W 24V servos with 200 CPR encoders on them.

I also do not know what type of motors or size the OP has.
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21 Mar 2017 13:44 #90004 by Muzzer
Another option from CNCDrive.com Here . 160V / 35A and around the £100 mark. Is that beefy enough? I have one of these but haven't been able to try it yet.

Although you could take the encoder signal and output a digital PWM signal using a Mesa card, you would need to take care of all the hardware and protection as well as set up and optimise a positioner / PID controller. On the other hand, a servo drive takes care of all of that. I guess it would depend if you want to play at developing a servo drive or implementing a machine.
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