[ringram2077]

Hi
I have the bare bones drives with no current limiting. I can either use resistors to limit motor current or add a chopper circuit to the 12 volt supply. I was thinking about using a DC motor speed control like those used in RC electric planes. They are inexpensive and come in a variety of current capacities. Simply feed the correct pulse length to the speed control to set the current. Any reason why this would not work ? I have 5.9 amp bipolar steppers with a 2 volt rating.

Thanks

[kiwichris]

Urrrr,

I suppose this would work at low speeds, but you'd not be looking for model aircraft controllers, as they are not reversable, you'd have to use a couple of car controllers which have two full H bridges in them.

At low speeds it would probably work, and with the right control electronics you could even get a reasonable microstep driver going.

But. (there always is a but..)

Most R/C controllers use a mirco that has a fairly low-resolution PWM output, so your current control is going to be fairly coarse, and the update time is only 50-70hz, so your maximum step rate would only be 70 steps per second, and even if you were full stepping that would equate to only 21rpm, which would make using most machines painfully slow.

You'd also probably find that RC Car controllers come with breaks and other features that would reduce the performance of the stepper by clamping the current in the winding every time you changed direction (every step) which might cause some interesting effects..

Also, the car controllers are generally designed for 6-10 cell packs, so only realistically 15V max, which is not generally considered high enough for good performance on steppers. (R/C aircraft controllers come in much higher voltage ratings, but are not reversable.)

Interesting idea, and the specs on RC controllers are certainly attractive, but I think youd be pushing the preverbial up hill to get any real use out of a setup like that, and the cost would be prohibitive by the time you got two car controllers plus control electronics per motor...

Cheers, Me.

[ringram2077]

Just to clarify my question, I want to add a chopper to my bare bones drives to allow for current limiting. I thought I could PWM the supply to the drives and accomplish current limiting. I am new to this so I may be headed down a dead end street.

Thanks

[kiwichris]

OK, that sorta makes more sense....

You're still going to have problems with the responce time of the R/C interface..

The update time for plane speed controls is controlled by the PWM servo interface the use, it has an update frequency of 50-70hz, which is no where near fast enough for a chopper design..

For a chopper to be effective in a stepper design you need to turn the chopper hard on, and then when the current in the stepper gets to your set point, turn off the chopper.. This period might be as short as a few microseconds if the motor is at rest. The R/C controller is going to have a best case response of one period of its control signal, which would be 14 to 20 ms. At best you'll get a seriously hot motor, at worst you'll over-current the driver bridges and let the magic smoke out of the driver or powersupply..

What type of controller do you have now? It may be relatively trivial to build a chopper for it, depending on the design, and depending on how brave you are with a soldering iron.

[ringram2077]

I am really good with the soldering iron. I have a driver made by kitsrus.com . It is kit 158 and is really just a collection of mosfets that does the stepper thing. I would appreciate any idea you might have as to a solution for a chopper. I will be out of town for a few days ( party in Key West, FL) so I will check back on the return.

Thanks

[NC Cams]

There were a number of NE544 based electronic speed controls (ESC) at one time or another. Delta and Tekin were two that I used regularly at the time. Novak made some using a Misubishi 51660 IC but the NE544 based ones were a bit easier to use/program/adapt to current limiting.

You could add a current sensing shunt to the power switching side of the FWD direction and feed that signal to one of the pulse stretching pins voltage reference pins. When the current reached a presettable limit, the op amp would automatically fold back the duty cycle to limit the current to whatever you programmed it for.

If you can still locate some of the iC's, you could still make an ESC that would run at whatever duty cycle (1% to 100%) you wanted depending on what the minimum pulse width you wanted/needed and current limit as well.

If you want to get more sophisticated, you could feed the drive signal of the 544 to an oscillator constructed of a simple op amp and uprate the switching frequency of the ESC from 50 hz or so (what they worked with via analog signals of the day) to 3600hz current switching frequency without too much difficulty.

To drive the 544 based ESC, a 556 based dual oscillator circuit was all that was needed - noe phase created the timer pulse and the other created the reference pulse that you'd stretch or shorten to drive the ESC from 0 to 100% duty cycle (single speed) or +100% to 0% to -100% speed (bidirection drive with H bridge)