On Sun, 31 Aug 2008, Jack wrote:
> Date: Sun, 31 Aug 2008 13:07:57 -0500
> From: Jack <[EMAIL PROTECTED]>
> Reply-To: "Enhanced Machine Controller (EMC)"
> <[email protected]>
> To: "'Enhanced Machine Controller (EMC)'" <[email protected]>
> Subject: [Emc-users] Stepper as servo
>
> Yes, this may sound a little 'weird', but has someone tried putting shaft
> encoders on stepper motors, to try to determine if steps are being lost?
>
> If so, I would guess that it would be a SMS (euphemism for a lot of work -
> Small Matter of Software) to generate 'catch-up' steps somewhere in the
> process.
> Either in EMC or in a 'intervening' hardware interface.
>
> Just a wild thought, but I am sure someone else has thought of it before.
>
Step motors with encoder feedback have been used for quite a while (Compumotor
for example), sometimes to simply detect errors and sometimes to create a full
closed loop servo system.
Losing a step is normally a catastrophic event with a step based drive system,
as its most likely to happen at high speeds resulting in a stall, so what you
really want to do is use the encoder to prevent the possibility of lost steps.
You do this the same way AC servos work, by keeping the stator magnetic field
aligned with the step motor rotor (but with a phase lead or lag to generate
torque) This way its not possible to lose steps, though its possible to fall
behind because of lack of torque just like any servo system.
Run this way the step motor is really a 50 pole 2 phase AC servo. The
advantages are the it behaves like a real servo, that is it has ~0 static
power dissipation, cant lose steps etc.
The disadvantage is that its usually not as good as a normal AC servo (except
possible for low speed high torque applications), it requires a high
resolution encoder (1000 lines or more) for best performance and quite fancy
drive circuitry. Our SoftDMC firmware will drive step motors in this mode, so
we have some familiarity with running step motors as servos.
I'm sure EMC could do this as well but it requires:
A high servo thread rate, perhaps 20-50 KHz
The aformentioned fancy drive circuit for the step motor
Software in EMC to generate the 2 phase sine wave drive locked to the rotor
position
Some way to do initial encoder alignment (quite fussy)
Peter Wallace
Mesa Electronics
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