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

Jeff Barlow wrote:

>
> 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 didn't state that clearly enough. Writing things from the outside in,
rather than the inside out : The position error is used to modify the
velocity command sent to the servo amp. The difference between
actual velocity read by the tach and commanded velocity is used to
chenge the torque command. This needs to be integrated (another name
for negative AC feedback) due to the mass of the drive system.
So, although this (integrated) error actually DOES work out to
be (roughly) proportional to the position error, due to the source
that it comes from, it is not truly position error. The reason is if
the machine hits a bump, the velocity servo detects it and compensates
before the CNC control even gets to see the change. That's because
the velocity loop is inside the position loop, the bandwidths involved,
and the (doubly) discrete nature of position reports from the encoder.

Jon