Re: Geckodrive

Well, then I guess I'm not seeing what I'm seeing on my lab bench.:-)

1) Parametric resonance is caused by a system (motor + drive) phase
lag of 180 degrees. It is corrected by adding a derivative component
(phase lead) to the system.

We do this in our drives using analog techniques, the G100 uses
digital PID feedback. Either way the motor will not resonate having
+80 degrees phase margin.

2) There is nothing "chaotic" even about a stalling motor. It simply
looses phase-lock with the rotating excitation field. The servo
summing node develops an increasing lag angle (where it is minus
where it should be). The controller decelerates the rotating field
until the motor "catches-up" and resynchronizes.

The trick is to keep this from happening in the first place. A step
motor's torque is a sine function of the lead/lag angle; constrain
this angle to +/- 1.8 degrees by how you control the rotating field
(accelerating/decelerating it in a PID loop) and the motor cannot
stall.

3) A step motor when operated PID closed-loop becomes an AC
servomotor just as a DC servomotor becomes an ordinary DC motor when
operated without a PID servo loop.

Mariss


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Jack Hudler" <yahoo@...>
wrote:

>
> Unless I'm missing something in your post (you said "step motor").
>
> A stepper motor by its very nature is stallable via load, inertia,

and

> resonance. Unless you're suppressing mid range instability caused

by parametric

> oscillations then you're always in danger of stalling regardless of

loading.

> These suppression or damping features are usually found on high end
> micro-stepper drivers.
> You can fix this some what by having a stepper with extra torque

than needed but

> that's a wasteful and expensive route and all you've really done is

changed the

> rotor inertia.
>
> Once a stepper stalls it enters a very chaotic state and at this

point the

> pulses from the incremental encoder are all invalid. You always

have to re-index

> after a stall to remove the chaotic error. Still the end effect is;

you're part

> is screwed.
>
> At any rate an incremental encoder is not going to give you advance

warning of a

> stall in order to avoid it. You might try putting a tachometer on

it and

> programming a DSP to sense the instability but that's not a cheap

route either.

>
> This technique might be useful on a servo motor using current and

tach/encoder

> feedback. This is also something you find on some high end servo

drivers.

>