Re: [CAD_CAM_EDM_DRO] EMC Following Error

Jon,

> David Micklethwaite wrote:
>
> > I am trying to get EMC working with a small mill. I have Gecko G320

drivers

> > and am using steppermod. I can jog the axes back and forwards by the

amounts

> > I expect and in the right directions. However, I occasionally get

following

> > errors and do not understand why.
>
> Steppermod does not simulate a servo system, as freqmod does, so it can't

get the

> following error from anything having to do with the acceleration, PID

gains, etc.

> But, it DOES have a limit of how fast it can put out pulses. Your input

scale is very

> high, 32000 steps/inch. For 1.2 IPS (Inch/Second) velocity, that requires

38,400

> steps/second to be issued from the computer.

I knew it was high - it is the result of using what was available (2000 cpr
encoders, 8tpi leadscrews and 2:1 ratio because I was concerned about the
motors ability to move the table). I am making new pulleys to get rid of the
2:1 ratio. If the motors can move the table I will halve the steps/inch and
increase the speed.

> You don't report what your PERIOD
> parameter is set to, but it needs to be REALLY high.

I thought I had read somewhere that the period parameter was not needed by
steppermod and hould be commented out. I have done that. Is it required?

> I think it needs an absolute
> minimum of 38.4K x 2 = 76800 per second, or 0.00001303 seconds/interrupt.
> But, this will make the jump between the next lower velocity and top

velocity a

> 2:1 jump (from 19200 steps/sec to 38400) which will drive steppers nuts.
> Maybe the G320 will not have a problem with this, but it might still cause
> problems in getting a steady speed, as the steps will be jumping around in
> timing. Anyway, that period comes out to 13 microseconds, meaning the CPU
> gets an interrupt for the stepper task every 13 uS. I think you'll need

an 800+

> MHz Pentium III, minimum, to run at this rate and still keep the rest of

EMC and

> Linux functioning.

Hmmm. Don't have an 800+ MHz P3. This is a 700 MHz Celeron. I have noticed
that only EMC runs, screen updates and mouse tracking are very slow. I guess
I need to reduce that 32000 steps/inch. I reduced the velocity to 0.5 ips
and also dropped the acceleration to 1.0 ips**2. This appears to have got
rid of the following errors and the table moves around at reasonable cutting
speeds (rapids are slow).

> If by following errors, you mean that the Gecko drives trip out, then your

acceleration

> parameter is WAY too high. 20 Inch/sec/sec is More appropriate for a

high-end

> pen plotter than a machine tool!

No, the Geckos don't trip out. EMC goes into e-stop and pops a window saying
following error. This puzzled me because it has no feedback. I didn't think
that it would stop because it couldn't handle the step rate. I guess I
expected it would reach some speed (less that I asked for) and run at that.

Regards,

Dave.

David Micklethwaite