>
> Stuart:
>
> Regarding scales + encoders. I think I'm one of the people who says you
> could use scales if the PID loop got its velocity info from the encoders
> and its position info from the scales. There might still be
> complications, its one of those things that just needs to be tried. I'm
> willing to write the modified PID loop that would be needed.
>
> Of course, you'd have to be able to read both scales and encoders at the
> same time - that means twice as many encoder reading channels. If you
> have the hardware to do that, its not a big deal, but if if means buying
> another card it gets less attractive.
>
The extra encoder feedback is already in the machine. It will be
hooked up next week. I am hoping to get the G&L running next week. I
thought I would have it moving yesterday. Not quite yet but very
close.
> Regarding using scales for screw comp - again you need to be able to
> read both at the same time. EMC can't change the comp data on the fly,
> and it would be a very serious rewrite to change that. Right now the
> comp points are read from the file in ascending order, and EMC computes
> the slope between successive points at startup to simplify interpolating
> between the points. Changing a point on the fly would also change the
> pre-computed slopes on both sides of the point. If you wanted to change
> the location of a point, not just the amount of correction at that
> point, it gets even worse.
>
This is and is not the answer I wanted. Of course, if using the
scale in the PID loop works then the idea of lead screw compensation
is moot.
I had no thought of changing the compensation point location on
the fly just the compensation amount.
The thermal growth of ball screws is nothing short of amazing.
They just don't seem to quit growing as the day progresses. Scales can
remove the ball screw thermal error from the machine positioning.
Thermal growth of the machine and scales is still a problem but it is
a very much easier and predictable measure.
For most machining projects the thermal compensation is not much
of an issue. On a part with +/-.010 inch tolerance the compensation
may may not even be necessary, depending on the size of the machine
and the part. But, the machine (G&L) I am working on is supposed to
have jig borer accuracy. It is large (X 100 inches, Y 100 inches, W 47
inches, and Z (quill) 36 inches) and will do large parts and tools
(steel, graphite and aluminum). Thermal considerations are a must.
> However, it _would_ be rather straightforward to gather the comp data
> automatically. A HAL sum block can be used to subract encoder position
> from scale position to get the screw error, and the sampler component
> can capture that error, as well as the nominal position, to a file at
> preset intervals. So you could just start the sampler while the machine
> is at one end of travel, and do a slow G1 move to the other end, while
> sampler records the data. I'd go slow to minimize the chance of error
> due to time-skew between readings - maybe 5-10 minutes to go from one
> end to the other, capturing a reading every second or so.
>
For all but the very most insane accuracies needed the time skew
would be a minimal error issue. .500 inch increments is adequate for
almost all machines. In all honesty, if the machine needs increments
less than 1 inch then the machine has mechanical problems. When the
compensation amount is interpolated between the points, the accuracy
is very good. If a machine has error problems necessitating step
compensation between 1/2 inch increments it has problems the lead
screw comp cannot fix.
Even with the magic of EMC2 it is still not possible to make a
silk purse out of a sow's ear.
> Regards,
>
> John Kasunich
>
I offer the use of the G&L as a test bed for your software.
thanks
Stuart
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