Whaaaaa???

[keithorr]

Rec'd a new IMS stepper today (3431-6.3) rated at 637 oz.in. to replace an obsolete 3424 rated at 235 oz.in.

So I'm reading the specs and fiddling with the motor turning the shaft (nothing is hooked up). Sometimes the shaft spins with only a slight detent feel, other times the shaft turns kinda stiff.

Then I notice the lead ends are sometimes touching, sometimes not, and if I short the two coil windings (A+ to A-, and B+ to B-) the shaft locks up solid.

I didn't get out pliers or anything to see how much force was needed, just using my fingers. Figure I might screw something up.

Anyways, what is going on inside that makes this happen?

[ESjaavik]

The stepper works as an AC generator when you turn it. If the 2 leads of a winding touches each other the generator is shorted and the force needed to turn it gets much larger. If you short out all windings this way, you should not feel the detent, just a strong resistance to turn. Try to turn it faster and the resistance increases. You don't hurt the motor doing this unless you turn it faster and more forceful than you can do it by hand.

I have understood that one use of surplus steppers is as generators for windmills. It makes sense as it is designed for minimum losses.

[cbcnc]

I forget the term for it but isn't that one of the specs for a dc motor? The ratio of the speed that you turn the shaft to the voltage produced?

Chris

[Greg Fill]

Just for information - because a Stepper motor acts as a generator when rotated it makes a good low cost quadrature encoder for jogging inputs. Take a 200 step motor from a old 5 1/4 inch floppy, one opamp, some diodes and a few resistors and you have a a rotary jogging device. Turning that shaft produces a voltage, the voltage is applified, and the output is quadratured encoded. We drive the stepper motor with a quadrature encoded voltage to make it turn. Just reverse the process, turning the shaft by hand produces a voltage that is quadrature encoded.
Turbocnc has inputs for jogging Channel A and Channel B thats works very nicely with this circuit. To make the jogging circuit work just a little bit better I have tied the quadrature outputs of my stepper motor to a PIC12C508 first. I have coded the PIC so that if you turn the jogging input quickly it outputs a fast jogging pulse stream. Turn the shaft slowly and it outputs a slower stream. This makes fast turns of the shaft mean jog fast and slow moves of the shaft mean jog slow. Your PC's mouse does the same thing when you move it around.

[High Seas]

Greg Fill -
What a cool idea! Now, I'm not much of a "wire-head" , but if you had a hand shetch of a diagram and parts list I'd give that a go! Cool idea.
BTW I seem to be collecting a lot of used steppers form printers etc. What a greta way to use 'em up!
Cheers -- Jim

[ESjaavik]

Greg, that idea also crossed my mind. Did you find out how much electronics is needed to condition the signal for detection by a microcontroller. If not much, then it is a very cheap way of making an encoder. One stepper, preferrably small (free), one PIC/AVR or whatever ($2-3), 2 opamps, 4 diodes and a few resistors? A stepper from a 3,5" floppy is small enough for fitting into a handy enclosure.

I will need such a device, so if you have experience on what goes between the stepper and MCU, please share it with us. I can teach a ATtiny15 to convert to step/dir outputs. It doesn't even need a crystal/ceramic resonator, and can drive an opto input directly.

[Greg Fill]

I have the circuit on my DIY stepper driver board, I'll post the schematic once I do a cut and paste of the appropriate page and figure out how to attach a PDF file in the forum.

The BOM (Bill of materials)

1 - opamp LM358 about the cheapest the DIY can find but any opamp will do
4 - 1N4148 diodes
6 - 3K9 1/4 W resistors
2 - 1M 1/4 W resistors
1 - 10 uf 10 volt electolytic cap

alternatively you can use a LM392 comparator then the BOM would be
1 - LM392
4 - 1N4148 diodes
8 - 3K9 1/4 W resistors
1 - 10 uf 10 volt electolytic cap

The PIC is not required and is only used to condition the quadrature signal to make it into STEP and DIR or do what I did and have an acceleration function like your PC's mouse.

BTW ESjaavik I have completed my chopper controller design and built a few proto PCB's. I drive my 8 amp / phase test stepper motors at 35 volts and 8 microsteps. Max. Pulse rate from TurboCNC is set to 19200 equating to 720 RPM. To get to a higher step rate and RPM I need to use a higher voltage. The FETS I am using are only 50 volt rated so I can't go to a higher voltage until I change them which I probably won't do. 720 RPM equates to 144 ipm and with a planned table length of 24 inches it is fast enough.

[Al_The_Man]

Here is the link for the schematic stepper as a rotary encoder http://members.iinet.au/~richardh/rotenc.htm
Al

[Greg Fill]

Thanks Al, I figured out the circuit myself but I guess there are many ways to do the same thing if you know what you are doing. I was interested in your link to see how they made a jogger but the link seems to be dead. Anyways here is a PDF of my circuit. Pretty straightforward. Any voltage generated by the stepper is clipped by the diodes, amplified and output on Quadrature A and B. The negative input of the opamps are biased midway of the +5 volt power supply. C5 is just a small filter. You can tie the quadrature lines A and B directly to the lines defined by TurboCNC for Jogger channel A and B. I process Quadrature A and B with a PIC12C508 before feeding it to TurboCNC. I use a Unipolar stepper for the input but a 6 wire motor works fine as well. For a unipolar motor the common ties to my J8 pin 5. Winding A goes to pin 1, winding B to pin 2. For a 6 wire bipolar motor tie Winding A+ to 1, A- to 2 and the common of A+ and A- to 5. To help with the jog cogging I also tied my Unipolar motor winding A and D together. This gives a more positive feel when turning the motor.

The part values are quite arbitrary. The 3K9 can easily be anything from 1K to 10K as long as they are the same. The opamp is probably the most widely used general purpose device but any single supply opamp could be used.

Experiment and have fun.

[Al_The_Man]

The guy seems to have abandoned the link but the hard copy I have is just about identical to your pdf.
A typical motor giving 180 pulses per rev. It uses a LM358 for comparator.
Al

[Bent]

Hi
Just lurking by to see what you where talking about. I found this a while ago but you proberly already have figured it out.

http://www.webx.dk/oz2cpu/20m/encoder.htm

Bent

[Greg Fill]

Thanks Bent I did figure out all the same things.
Well it goes to show that nothing is really ever new. Usually someone has already done or designed the same thing. It is a good article and people should read it to get a better understanding what I was trying to say. Lets see how many people actually try the circuit out and improve on it.