Step motor with load feedback - free running like a brushless motor - somewhat

Christof Eb.

Step Motor with Load Feedback

Intention
The idea of this project is to have a method to use a step motor like a brushless motor. It will run with high speed at low torque and will automaticly run with low speed and high torque. – And loose no steps.

Feedback wellcome! I am now thinking of a method to reduce the power consumption at low speeds. Use a step motor for robots?
Have fun,
Christof

Lawson

FYI, a six wire stepper can be run off of a sensorless brushless DC motor controller if the end of coil A is shorted to the center tap of coil B. The free ends of coil B and end of coil A are then hooked to the three motor wires of the sensorless BLDC controller. In effect the 2-phase stepper has now been rewired as a 3-phase star winding. (with a "wild leg") I suspect that a variation on the sensing circuits used in sensorless brushless controllers would work with a stepper motor too.

Marty

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Lunch cures all problems! have you had lunch?

Graham Stabler

Brushless sensorless controllers will not work well at very low speeds.

The idea has also been put forward by Mariss of Geckodrive on their Yahoo group, the "unstallable stepper drive". Have a search and you should find info.

Of course it just means everything slows down until the motor has enough torque. Personally I like to use big enough motors for the job but I can see the attraction especially for none CNC applications like robotics as you mentioned.

Graham

Christof Eb.

Thanks for your input, Marty, I had not been aware of this possibility.

Thank you too, Graham, for your comment.
Yes, that is a wise thing to do, install motors, that are just big enough. - I have moved in this direction too, after first lowest cost trials. Of course everything is steeply growing more expensive: Motors, drivers, voltage supply. At my lathe, I think I have motors which are big enough. - At least when it is cutting metal. When it is positioning in rapid movement, is there any "fast enough"? When I work with wood, there could be more speed too, but I think, you will see changing speeds on the surface, what you might not be happy about. (I am using an old pc with a bought program "pcnc" for it.)

As far as I have read, Mariss wants to use encoders. The method, that I have posted, does need only a resistor that you have anyway to control the current, and an opamp and some propeller pins together with some software, not even assembler. Perhaps I will use it for a single axis positioning system. I have got a mill, where I would like to have a controlled vertikal axis to bring the head into position for changing tools and shorter and longer tools.

Best regards, Christof

Graham Stabler

All you can do with this is ensure that you don't loose steps but you shouldn't do that anyway with the correct sized motors and appropriate speeds and feeds. If you are using a lathe and the spindle is open loop then this would lead to a poor finish anyway.

As far as your rapids go all this will do it reduce the speed until the motor can manage it, that can also be achieved by setting up the rapid speed/accell carefully. The only way to go faster is a different motor and generally higher voltages, or servos!

The other thing is that this needs to be integrated in to your motion controller somehow, that could be the propeller as well but if you want a smooth transition from CAD through CAM to machining that might be a lot of work.

Graham

Chris Savage

This thread is off topic in the Propeller Forum and is being moved to the Sandbox Forum.

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Chris Savage
Parallax Engineering