Sunday, April 30, 2006


More work with the z-axis

Simon spent a bunch of time with me this afternoon getting his java stepper exerciser code working on Godzilla. It's a lot nicer than the old exerciser, loads more torque from the stepper and adjustable, too.

As mentioned in the documentation the slider for stepper speed is VERY nonlinear. From 0-240 you go up to about 1 rps. For every step above about 250 though, the stepper tries to get a LOT faster. It's kind of sad because the speeds we'd like to use run from 250-255, which means we don't have a lot of resolution. :-(

For what it's worth, when I was working on routers there was a torque chart for the motors which you had to accelerate and decelerate through to avoid losing steps.
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I expect that the control routines on the PIC16F628 cpus will have to take that sort of thing into account if we stay with steppers. Right now, we haven't had enough experience with how steppers interact with the other parts of the extruder positioning system to know just what we ought to be worrying about in that regard.

For my own part I'd rather use a DC gear motor with a shaft encoder rather than a stepper. You get a lot more torque for the energy you invest with that approach. For now, though, we're staying with steppers. :-)
For the moment we can always alter the polymer deposition speed to compensate for any lack of motor speed control.

Vik :v)
How is the stepper speed controlled? Is it a counter with a step on overflow?
Hmmm... I'm thinking of a feedback loop with some sort of electronic tachometer to boost the resolution in your target speed range.
Just what you need, more wiring and comms routines. Maybe you could incorporate the tach and feedback circuit transparently in the driver circuit.
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