DC motor as a servo motor

[kenny]

I am in the process of converting my manual bridgeport knee mill to CNC. I have an older Anilam 1400M controller with X,Y and Z axis servo driver cards Which put out 24 volts DC. Would brushed DC motors work instead of servos If I use some type of encoder? I was thinking about using digital readout scales for linear encoders.

[ViperTX]

Yes, as long as they are reversible....which they should be....search the net someone converted some brushed DC motors to use as a servo motor.

The only problem that is visible is the use "digital readout scales"....are these actual digital output and not quadature signals? Also, if you have any amount of backlash between your DC motor and the movement of the table....you'll most likely be complaining about the DC motor "hunting and pecking" for position.

[kenny]

I have read that article, it is informative. do you suggest I use those shaft mounted enconers rather than the DRO scales?

[ViperTX]

kenny....if you have the DRO scales...I'd try them and you can give us a first hand experience....come to think of it...I believe Bubba did something similar...mounted the encoders on the lead screw.

[Al_The_Man]

Non-servo type DC motors usually have less poles than a servo motor, therefore cogging is greater at low speeds, with the cost of servo on ebay it may not be worth it unless you can inspect the motors to see the construction, the amount of commutator bars will indicate the quality for servo use. Also they should be permanent magnet field.
Also non-servo motors do not usually have an extension shaft at the rear for encoder etc.
Al.

[wilfredtak]

mmm, i still got a 0.75Kw inverter along with a 3 phase motor.
with connections for RUN/STOP/BRAKE/EXT-POTM.
Maybe there's a possibility to connect a controller of some kind and use that as a sort of stepper/servo?

[ESjaavik]

And there is one more "feature" of a DC servo motor which very few other motors have:
It's torque/current line is linear, and it is so almost from zero speed. OK Al, so cogging is also non-linearity, but a normal DC motor is not linear even on a larger scale.

One that could be used is some old computer tape reel motors with 4 brushes. I've seen someone say they have successfully tuned them after putting on an encoder. This sounds reasonable, as they are really servo motors. Just that the feedback device (when in the tape station) was the tape going down a column (square tube) with air sucked through it, and the tape tension measured by how far down it went. This was then fed back to the motor current controller to keep the constant tension.