Re: Enocders and stepper feedback method?

Dave,

I think I know of a simpler (maybe?) method to make a stepper system
closed loop without encoders. This is the way I was told Emco-Maier
did it on their stepper production machines.

The lead screws have a plastic disk with single marker slot which is
sensed every full revolution of the screw. The control keeps a
running total of all the steps commanded, both forward (plus) and
reverse (minus) between senses of marker. When it senses the screw's
marker it compares the total commanded steps to the number required
for one screw revolution. If they don't agree, the control errors
out with an "out of position" message.

An interesting aspect of this system is it won't detect a problem
until the screw's marker is sensed. For instance, if the machine
is "crashed" (and steps are lost) the event may not be detected until
the tool moves away from the part a distance equal to one revolution
of the lead screw. I've seen this happen many times. The delayed
error notification isn't a problem.

This seems to me to be a fairly simple system and is quite reliable
and accurate on the Emco lathes since these machines will
hold "tenths" all day long turning steel.

The same sensed marker is used to reference the machine. The machine
is manually jogged near some reference arrows on the slide, the REF
key is pressed and the machine moves slowly towards the reference
(the marker) position. It seems to slightly oscillate back and forth
over the marker to establish position. It's able to establish
reference position to within the machine's resolution of .0001".

Doug

PS, I don't understand your method well enough to comment on it.

--- In CAD_CAM_EDM_DRO@y..., daque@s... wrote:
> Anyone ever consider something like this?
>
> 1)Have an 8 bit counter that counts both the encoder outputs and
the
> requested steps.
>
> 2) start the count at 128
>
> 3)If the ratio of encoder(x4) to stepper pulses is for example
> (2.5pulses/step) and if cw rotation is considered positive add 5 to
> the count for each cw stepper pulse requested, subtract 2 for each
cw
> encoder count received.
>
> 4) If the count gets to 130, ( >1/2 step ) step the motor 1 step
CW.
> the same for ccw, if < 126 step the motor ccw.
>
> No need for a 32 bit counter and you still could have up to 24
steps
> missed and it still catch up. It would seem lots of ratios of
encoder
> to stepper could be used by just changing the values for both count
> increments and the up and lower step points.
>
> I am trying to figure out how to do that with a PIC chip. The
problem
> I see is the random timing of the pulses would likely end up
loosing
> a step now and again. I will likely need a pic and another chip.
>
> I just wanted to bounce this off the list, to see what you guys
thing
> of the general approach. Sound like a reasonable thing to try?
>
> Dave WC0H