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

Jeff Barlow wrote:

> It seems obvious to me that independently measuring position, velocity,
> acceleration, and time, is, at least theoretically, redundant. Since we
> are using digital electronics we "know what time it is" for free. So,
> then, it would seem that all we really need is one of: a tach, a
> position encoder, or an accelerometer. (No, I'm not seriously suggesting
> we stick an accelerometer on the table).
>
> The setup that Jon and others are using clearly works, but, to me at
> least, it seems redundant and confusing. If I've got this right, there
> are two servo loops controlling each motor. One consists of the amp,
> motor, and tach. The other is EMC, amp, motor, and position encoder.
> This "extra" complexity makes it harder to think about, and I wonder if
> it doesn't make the system harder to tune.

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.

> At very low motor speeds I suspect that neither a tach nor an encoder
> give us much useful information. My guess is that cutting nice smooth
> interpolated circles depends more on getting the servo loop damping
> tuned just right than anything else.

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