Re: Re: Closed loop steppers

Ian Wright wrote:

> From: "Ian Wright" <Ian@...>
>
> Thanks for the explanation Ray. I still feel a bit uneasy about the idea of
> using a 'free running' motor as a servo really is to get ultimate accuracy.
> While I can see that missing steps on a stepper system could be easily
> overcome by using a suitably sized motor, I don't quite understand how a
> servo system would avoid stalling if it hit a tough bit as it was ramping
> down to a stop - or do the servo boards all use some kind of pulsed power
> feed to maintain maximum torque at such slow speeds

Yes, modern servo amps use PWM (Pulse Width Mdulation), and increase
pulse width when needed to increase torque. I have what would be
considered very undersized motors, servo amps and leadscrews. I have
knocked my Bridgeport head out of tram and sent half of a 3/8" end mill
flying across the room several times, while moving around at 45 IPM
during setup. When a clamping bolt or something hits the stationary tool,
the servo amps usually don't even trip off on overcurrent, and the first
indication of trouble is the loud ping when the tool hits the wall!

Now, nobody expects the machine to hold accuracy during such an event.
But, the important thing is that NO STEPS are missed, because the encoder
and its counters aren't overloaded during such an event. With stepper
motors, the power device (stepper) and the position accuracy determining
device are the same thing - not so in a stepper system. Now, none of this
makes any difference if your whole machine is knocked out of alignment,
but if the crash was not that severe, you just go on without worry that
you've lost your position reference.

> - if they do then surely
> they are going to jerk as much as a stepper in half or microstepping mode?

Well, yes, there is a jerk during a crash, but only then. I will tell you that
a servo doesn't jerk under normal conditions. I so often have to look at the
scrren to see if the numbers are changing, to know whether the program is
running or not! The motion is so smooth at low speeds you can literally
have your hand on the table and not be sure it is moving.

In my own EMC / servo setup, EMC is recomputing velocity 1000 times
a second, but the servo amp is actually providing higher bandwidth than
you would get from EMC, which the Nyquist theorem would limit to
less than 500 Hz. First, EMC assures smooth motion through acceleration
and deceleration ramps, and the servo amp smooths out between the
servo updates. This is, of course, a full, analog servo system, not the
servo lite approach, where a CNC control designed for steppers is
used with a servo drive.

Jon