Stepper motor pulse to step/dir

Computers and stuff like that

Stepper motor pulse to step/dir

Postby Jerry » Thu Sep 30, 2010 1:03 am

I have a Haas programable 4th axis that uses a stepper motor to drive the head. The thing is ungodly slow. I would like to replace the stepper with one of the servos I have lying around but I want to be able to still use the original control box.

The control box has the stepper drive built in. Its a Unipolar 6 wire setup. So 4 transistors drive 2 center tapped coils in the motor. The servo drive I have has a couple different input modes. Step/dir, quadrature, and cw/ccw. So I can either take logic input that signal movement with one input for pulse and one for direction, standard quadrature like an encoder, and a mode where one input makes it move a set distance one way and another moves it the other.

And ideas how to convert the 4 phase output of the stepper drive to one of these other methods. I managed to take signals from two of the transistor drives to run the servo more in quadrature but the problem is my resolution is cut in half.
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Re: Stepper motor pulse to step/dir

Postby Doug Coulter » Thu Sep 30, 2010 11:54 am

Jerry, I know some things (a lot) about steppers and their controllers, and somewhat about servos and theirs, but I don't know enough about where you are attempting to glue these two things together to make a system. I presume you have some sort of feedback in the servo system based on a scale of some kind?

Stepper controllers have to control the step rate (and rate of change) to avoid skipping steps in the speed region where the motor is beginning to lose torque, or is trying to accelerate some large mass, else they get lost. So if you want more speed, and are still using a stepper controller to generate the basic signals, you're in trouble unless you can somehow fool it into thinking it can go faster.

Why you would lose half your resolution without having also a corresponding scale error of two I don't know, but you didn't mention. There are various ways to get that back, but if the stepper controller is still in use, it would need to some how be fooled into being willing to "step" faster, which would be an internal software change on the controllers I've designed.

Long story short, I don't have enough into to help, if I can.

Can you not just use the servo stuff "neat" and ditch the whole stepper controller in this case, or is there something missing that the stepper controller provides (like software compatibility)?
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Stepper motor pulse to step/dir

Postby Jerry » Thu Sep 30, 2010 2:43 pm

My servo drives are closed loop drives. An encoder on the brushless motor sends back position info back to the control and everything is self contained. I just supply the drive with 120v and I program the drive to act how I want with a GUI on a PC.

What I want to be able to do is connect the drive up to either my CNC control when I want to use it as a 4th axis or to the Haas controller when I want to use it as a indexer on the surface grinder or something like that. I have a separate stepper drive in my control cabinet to drive the motor itself when it is not connected to the Haas box.

So I dont really care how it acts when it is controlled by the Haas controller, only when I am doing 4th axis stuff on the mill does its slowness and low acceleration become annoying.

To drive the motor in quad mode I need a two square waves 90 degrees out of phase. The haas sends signals to the motor 45 degrees out of phase so I have to pick two lines out of the 4 to drive the pulse input on the servo drive. Since I am leaving out two of the phases I am loosing half the steps to the drive.

Image

Image
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Re: Stepper motor pulse to step/dir

Postby Doug Coulter » Thu Sep 30, 2010 2:59 pm

Riiiiiight -- so they were half-stepping the stepper (which is a lower torque mode so they may have had to go slower yet!). So the question becomes -- why use the Haas at all in that mode, just use whatever normally provides the program -> signal interface when you use that servo elsewhere?

I'm surely missing something here -- are you using actual *power* out of the Hass, or just using as a signal generator?
If you have a schiz on the Hass, there might be a place slightly upstream where you can get the signal you want (if they used a couple flip flops, which is common).
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Stepper motor pulse to step/dir

Postby Jerry » Thu Sep 30, 2010 4:46 pm

The Haas unit can only be programmed from the front panel. You can set it how far to move, how fast, wait for a signal and do this, and so on.. Good for basic indexing but thats about it.

I looked for useful signals elsewhere and couldnt find anything. Somehow I need to detect the rising edges of the pulses and OR them together to create step pulses, and then figure out a way to detect the order of the pulses and switch the direction line high or low depending on the sequence direction.
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Re: Stepper motor pulse to step/dir

Postby Doug Coulter » Thu Sep 30, 2010 5:00 pm

That, I can help you with.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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