Re: Controls theory

Mike Romine wrote:

Jon Elson wrote:

> > Well, I beg to differ. A machine tool is NOT a steady-state system, as
> > the one you describe above. It also has friction (which changes sign
> > when velocity does) which is somewhat protortional to velocity plus
> > the static friction. Also, there are cutting forces, which vary all over
> > the place.
>
> True, but it is steady state from a controls perspective anytime the
> one of the set points is constant, be it position, velocity or
> acceleration.

No, that's not true! There are external events (most specifically, the
entry and exit of cutting teeth into the work) that are ocurring, as well as
various other non-linearities of less magnitude, such as variation in
friction at different machine locations. But the big one is the cutting
forces. So, you can never be really sure anything is truly steady state.

> > All of this is why old errors are not relevant to the current
> > state of the system. You have to find a reasonable time window to
> > look back through, and keep a rolling sum of those errors - unless
> > you want digital filtering, in which older errors become less significant.
>
> Sounds like a good idea to me. The beauty of doing the control loop in
> software (as opposed to op-amps) is that you can do all sorts of cool
> tricks that non-linearize the control system like the one you mention
> above.

But, the biggest point I want to make is that old errors sampled when
the system is moving left are a DETRIMENT when the system is
moving right. They actually make the error LARGER! (In the
particular PID implemented in the EMC program.)

Jon