axis clamps/brakes was Re: Measuring backlash more accurately?

Jon,

I understand that is the case in your "real" servos (or any "real"
servo system). What I was wondering is if it's still true for a "step-
servo" system?

Thank you in advance,

Ballendo

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> ballendo wrote:
>
> >Juha,
> >
> >My reply on this was thinking steppers, even though you mentioned
> >servos... I believe the rutex drives close the loop within the
drive,
> >like geckos? In this case, I think you can treat the system like
> >steppers...
> >
> >Jon, Won't the "violent jump" be limited to the "hunting"
error/size
> >in these type servo drives? I would think that clamping an axis
> >driven by these would be nearly the same as clamping a stepper
axis?
> >
> >
> It depends on the amount of backlash between the motor and encoder
as to
> how many
> encoder counts can build up with the axis clamped. If the encoder
is an
> integral
> part of the motor, it may be no problem. If the encoder is on the
> leadscrew, and
> the motor drives it via a belt reduction, it could cause all sorts
of
> problems, such
> as overheated drives and motors. I know that in my system, which
uses
> the above
> mechanical setup, that if the motors are locked, the servo amps
will
> build up to
> high current in about one second and then cause an overcurrent
trip.
> This is because
> the open-loop gain is very nearly infinite, and it requires the
closing
> of the loop
> through the mechanical coupling to keep the system stable.
>
> I also know from texts that any discuntinuity or abrupt non-
linearity in
> the transfer
> function of any component in the system can lead to instability. I
> actually ran
> across this in a precision rotor speed control in my photoplotter.
I
> didn't have
> a negative power supply for the servo amp, so it could accelerate
the
> motor but
> not brake it. (The motor's back-EMF was less than 3 V, and
Darlington
> transistors
> were used, so that back-EMF was not enough to sink current to
ground.)
> It caused bursts of instability that causes the whole system to go
haywire.
> When I changed the connection of the - terminal from ground to -5 V
for
> the servo amp, the system became VERY stable, because the
discontinuous
> transfer function was fixed.
>
> Jon