Re: [CAD_CAM_EDM_DRO] Servo systems (was: re:cleaning, lovejoy, black box tach....)

Jon,

Yeah, I sort of knew bandwidth was part of the problem. I'm more used to
stuff with bandwidth requirements of not much over DC. I hope it's
obvious that I'm not suggesting we try to run a high speed servo loop
through a PC, though.

It's just this sort of functional partitioning of the system that I'm
trying to get a handle on. The conventional wisdom is that you do the
slow stuff in software and the fast stuff in hardware. A corollary is
that slow is cheap, fast is expensive.

The current (torque) servo loop is a given, I was ignoring it. The need
for position control is also given, though it need not be closed loop
(i.e. steppers). The need for a separate velocity loop is what I'm
having trouble with.

When EMC is run in it's stepper flavor it outputs position commands
(step, dir.). Mariss uses this to drive servos, also. When EMC talks to
servo-to-go cards it outputs velocity commands and expects position
feedback. Hummm.

When you say: "The current command comes from the velocity error
(integrated by an op-amp with a feedback capacitor)", I think "yes, but,
that's really position error then, isn't it?". This whole velocity deal
starts to seem kind of artificial to me. Am I missing something here?

I'm still trying to get this clear in my head.

Jeff

On Wed, 29 Nov 2000 18:13:45 -0600, Jon wrote:

>
>OK, the trick here is that one CPU CNC controls (like EMC does it,
>no DSPs in the servo card, etc.) have somewhat limited bandwidth.
>To get more bandwidth in the velocity loop, we can use an op amp.
>They sure are cheaper than a DSP processor and a fast, high resolution
>ADC!
>
>The traditional servo and EMC has 3 servo loops. The inner one is a
>current servo. The current sense circuitry is in the servo amp.
>The current command (which closely approximates the torque
>command) comes from the velocity error (integrated by an op-amp
>with a feedback capacitor). Velocity comes from a DC tach, and
>this loop can have greater bandwidth than the CNC positioning
>loop. (Maybe it HAS to have greater bandwidth?) The velocity
>command comes from the CNC control, which uses only the encoder
>signals, but computes a lower-bandwidth velocity value internally
>from delta-position. The CNC control uses essentially only position
>error to generate a velocity command, but adds in additional
>terms to compensate for desired velocity, rate of change of
>position error, etc.
>
>
>My ordinary DC tachs give useful information down below .01 IPM,
>at which point stick slip friction overcomes the gain of the servo loop.
>The tach is actually still working down there.
>
>Jon
>