Re: [CAD_CAM_EDM_DRO]CNC Pulse jitter?

Rich,

Pulse jitter has three effects. Badly spaced pulses places an
acelleration load on the motor. The motor must speed up or slow down
to match the pulse spacing. This robs the load of the torque that is
now spent in acelleration.

Second, poorly spaced pulses pump the motor into mid-band resonance
due to repetative acel/decel cycles.

Third, is acceleration ramp "stair-casing". Not strictly a pulse
jitter problem, it is still a problem with marginal systems. Stair-
casing refers to the discreet jumps in step pulse rates while ramping
the speed. All digital systems have this effect, just that poor ones
have large changes in speed per ramp step.

Jitter can be expressed as a percentage of pulse period or in units
of time. Put your scope on the step output and command a constant
speed. Jitter will show up as a pulse period to period time
variation. Very slowly sweep the speed. Jitter will be more
pronounced at some speeds and be absent at others. Any period to
period variation of more than 1-2% (of full step) can be trouble.

Mariss

--- In CAD_CAM_EDM_DRO@y..., RichD <cmsteam@s...> wrote:
> I have seen talk of computer pulse jitter causing axis drive
problems
> on stepper driven systems.
> This may be a difficult question but, how much is bad enough to be
> on the threshold of being a problem?
> Is there a rule of thumb or easily perceptible symptom to look for?
> What is the std method/spec to measure jitter?
>
> I'm trying to troubleshoot a mill and apparently I have a bad case
of
> resonance but, in looking at the incoming pulse train on the scope
> (Tek 2236) really makes me wonder of other bugs as well, not helping
> the situation.
> RichD