PWM controlled surplus spindle motor

[JFettig]

Hey guys, I have the surplus spindle motor 2.5hp using the 8amp 130v controller found here:
http://www.surpluscenter.com/item.as...tname=electric

It uses 3 hookups to hook up to the pots, I have some hi-rez pics here:
http://jfettig.wc101.com/controll/

I am hoping to use the PWM output in mach2, i have talked to shadow and pminimo(sp?) a very little bit.

Im setting up winxp now and will have mach2 up shortly.

Jon

[shadow]

JFettig described the pot hookup to me. It is a 5K pot and is referenced to 12v and ground. My idea for converting the PWM into a usable signal to replace the pot was to simply use a bipolar transistor, where the PWM on the base would then vary the current passing from the 12V output on the controller to the sense input on the controller. I'm pretty sure I could draw up the circuit but perhaps someone with more experience or a better idea would like to chime in on this, thanks!

[ViperTX]

Well, don't quite know why you want to use the PWM....if the motor is not designed for PWM you will end up with "cogging"....basically the rotor/armature does not have the inertial required to keep turning when the voltage is removed as in a PWM setup. What I would look for is for some type of ladder or step output....I'm not familiar with Mach2...I suspect that Shadow's idea would work if the controller is looking for a current change....I suspect that you really need a variable voltage supply...just kinda depends on what that pot is attached to.....any clues?

[shadow]

The reason for using the PWM out from Mach2 would be to let Mach2 control the speed of the motor. He has a encoder on the spindle to measure RPM, by allowing Mach2 to also set the spindle speed you can have it cut with a precise chipload, and not worry about setting your ipm. I beleive from what I saw of the motor controller board, is that the pot is hooked into the feedback circuit on a linear op amp circuit, andthe pot will give the sense anywhere from 0 to 12V depending on its relative positon of the knob. We want to mimick the potentiometer with the circuit, using the PWM out so that the computer can control it, rather than human control.

Also, we could make our own controller using the PWM, I beleive it is at 20khz which is a very common PWM used in dc motor controllers anyways. We figure it should be fairly simple however to interface the PWM signal into the existing robust linear controller, so we do not need to go to the trouble of making an entirely new controller when a good one is on hand.

[pminmo]

More questions. Does Mach2 have an input for a spindle encoder? Is this to be an open loop setup or closed?

Phil

[JFettig]

yes it has an input, and by closed or open loop, not sure, The encoder I have is from cnc4pc.com and I have a disk ontop of the motor on the extra peice of shaft, Ill be running 1:1 from motor to spindle so itll be good, Basicly it sends pulses from the encoder board to an input on the parallel port, kinda like closed loop encoders on a servo system, only 1 line per rev, I maxed out my spindle at 7500rpm motor at 6k.

I set up mach2 yesterday on my mill and I am extremely pleased, so pleased that I already bought the software

Jon

[ViperTX]

Shadow, I'm quite aware of the interest in using the PWM...but from the software perspective it's just a frequency...it won't drive squat without some type of interface...and the controller is looking for a variable DC voltage to control it's gain....it is most likely some type of VCO.....the ladder logic people in this forum would most likely opt for selecting a set of resistors using a few bits of output to give you a step voltage output (so speeds would be in increments based on the number of resistors selected.) I believe there is the reverse of a VCO....now it you could probably drive off the PWM.....the other option of using things like a Variable DC supply (LM317) still requires you to change resistance. I wonder if you can bypass the OpAmp (if it really is a VCO setup) and feed the PWM via a driver to that other circuitry after the OpAmp. It would be nice to actually see a schematic of that circuitry.....other rambling thoughts changing to a unity gain opamp drive the pwm into an opto and feed the opamp.....rambling on.....

[pminmo]

ViperTX,
Just stating PWM is "just a frequency" is a little misleading. You can vary frequency and not vary duty cycle. PWM really is about varying duty cycle not frequency. There are several ways the pot could be used in the controller, but I suspect it simply is setting a dc voltage level for a comparator such as seen in this schematic:
http://www.solorb.com/elect/solarcirc/pwm1/

How ever the controller Jon is talking about, I believe controls a higher voltage DC motor and uses the 115VAC line switched and rectified to drive the motor. It's more complicated than a simple DC motor.

Jon the real need to know is where the pot fits in. I didn't ask but did the controller come with a manual or schematic?

If it is as I suspect what you will need to do is convert the pwm output to a dc voltage. Do you have a variable power supply?

Phil

[Al_The_Man]

[QUOTE=shadow We want to mimick the potentiometer with the circuit, using the PWM out so that the computer can control it, rather than human control.
..... I beleive it is at 20khz which is a very common PWM used in dc motor controllers anyways. [/QUOTE]
A filtered bridge across the PWM would give a relative dc voltage output, a passive set up would load the PWM signal so it should not be hard to make a simple op-amp precision rectifier out of a couple of op-amps.
Has anyone used these motor amplifiers for universal type motors?
Al

[JFettig]

I do not have a varible power supply,
Nor do I have a scematic.

Ill do some voltage measurements at different spindle speeds.

Jon

[pminmo]

Jon,
When you set a speed, do you find yourself adjusting (tweeking) the speed control while cutting? (i.e. motor slows down under load). The open loop verses closed loop question relates to that. A closed loop system uses feedback to compensate for variances. An open loop system just sets a output and doesn't automatically adjust.

Phil

[JFettig]

It actually speeds up on its own, its kinda slow on doing so but it willl. It does not have a rpm readout or return, just the 2 wires and somehow knows when its really loaded and speeds up to correct it, its actually quite nice if it would go faster, thats what I am hoping to see out of the computer controlled speed controll, have it correct it faster.
when I drill at 20ipm it doesnt even have enough time to correct the speed.

Jon