Extruder Controller as X-Y-Z Axis Controller

Good Morning!

I'm wondering if anyone has tried to use an extruder controller to drive one or more of the X-Y-Z motion axis with a DC servo motor and a quadrature encoder for position feedback. I would think it should be possible, one would really only be concerned with how much current the DC motor draws.

What do you guys think?

P.S. when I talk about DC Servo Motors I do not mean hobby style servos. I'm really referring to a Brushed DC Motor.

Edited 1 time(s). Last edit at 06/23/2009 09:06AM by labmat.

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Matthew LaBerge
[reprap.iceboxrobotics.com]

I don't know if anyone in this forum is doing it, but in the industry, it's quite common. DC motors can rev a lot higher and require less attention to detail.

As far as the encoder side goes, there's a really simple implementation where you take one of the encoder lines and wire it to an edge sensitive configured interrupt pin, the other wires to a standard input. Every time you get an interrupt, you count up if the standard input is say.. high, and down if it's ..low.

The time overhead in software is pretty insignificant.

The difficulty is fairly obvious... with the motors potentially moving very fast, you are going to need to consider active braking and acceleration, deceleration and synchronisation of the axis a lot more carefully.

However, to get high performance form stepper motors, many of the same considerations apply, only from a slightly different perspective.

If you want to try this, and are prepared to writ some software to make it happen, then you will need to have separate software processes handling each axis, building mass profiles for each axis, and synchronising high speed axis with the low, so that for example when you are drawing a circle, the sides, top and bottom will have one motor running very slowly or not at all, and the other at maximum speed. Or... in the case of a square for instance, one motor has to stop completely before the other starts. a 45 degree angle would be equal motor speeds, but what if one motor is loaded higher than the other ? Your software will have to constantly track the target position for each axis and know how to fix discrepancies of actual versus required positions.

labmat Wrote:
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> Good Morning!
>
> I'm wondering if anyone has tried to use an
> extruder controller to drive one or more of the
> X-Y-Z motion axis with a DC servo motor and a
> quadrature encoder for position feedback. I would
> think it should be possible, one would really only
> be concerned with how much current the DC motor
> draws.
>
I spent over a year using solarbotics gearmotors to control x, y and z axes in Tommelise 1.0. It sort of worked. Unfortunately, there was so much play and inertia in the gearboxes that it was impossible to get resolution sufficient for fine detail work. I finally dropped the gearmotors in favour of linear stepper motors for Tommelise 2.0. I've never considered going back to that.

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

Grael,

Thanks for the information, I am going to have to do some further investigation into what it will take to make this happen. I currently have zero experience with programming as I am a mechanical designer and haven't really had a need to learn any kind of programming.

Stage accuracy is not a problem for me as the X-Y stage I have is from a piece of high performance semiconductor equipment. The stage currently uses servo motors coupled with precision ground leadscrews to drive the stage. I posted information on my setup here previously [forums.reprap.org] any guidance I might be able to get is much appreciated.

The first revision of my reprap will use Gecko drives to convert the step and directions commands to servo control. These were recommended to me by two separate people one of them on this forum.

I would however like to use the servos and encoders with a custom designed controller to gain closed loop position feedback.

Servos are a good choice, especially those with quadrature encoders, just wire an encoder pin to the brake of the servo controller, then all ou have to do is send out streams of pulses to the motor and to the brake to get an instant stepper motor of sorts.

Biggest problem though is torque, servo motors have massive differences in torque, and its something to look at.
Acceleration and deceleration may be a problem also, that you'll have to experiment with.