Re: [CAD_CAM_EDM_DRO] CNC hydraulic test

Eric Keller wrote:

> > What you need is to set up most of a velocity servo amp, except for the
> power
> > driver. This means, the CNC control sends a velocity command to the servo
> > amp. The amp measures velocity with a tach or derives it from the
> encoder,
> > and produces a velocity error. The velocity error goes to a small power
> amp
> > which drives the proportional servo valve. generally, an AC signal is
> added
> > to the servo valve drive, called dither, to keep the valve from
> 'sticking',
> > which is really more deflection from the hydraulic pressure in the valve.
>
> I think dither may be marketing hype from MTS. In my simple experiments
> with
> dither level, I have found no advantages to dithering at all. However, this
> may
> be a result of the fact that I don't feel like adding the dithering code in
> to my real
> time control program -- I'd rather use the time doing better control.

Allen-Bradley also used dither, although it was generated by analog means
in the DAC / servovalve driver board and added to the DAC value.

>
> As a controls weenie aside, the reason that hydraulics have an interesting
> instability
> is that they have at lease one zero in the right hand side of the s-plane.
> Thus at high
> gain, you are guaranteed to be unstable. It also means that it is not a
> minimum phase
> system so all the stuff you learned in undergraduate controls is off buy a
> negative sign.
> The cylinders are pure integrators, which means you are marginally stable
> open loop,
> but in practice, they will not stay still open loop. Finally, there is a
> pole at the "nominal
> fill frequency" ( for lack of a better term). A digital lead-lag control at
> a fairly high
> sample frequency does a great job of controlling, given that you have a good
> feedback
> sensor.
>
> Hydraulic systems are non-linear, but not in a real interesting way.
>
> The following comments by Jon are a matter of personal taste. I
> will not describe to you the experimental method by which I determined
> that DTE25 hydraulic oil looks emerald green as it is flying thru the air.
> I would say that a good digital feedback system gets rid of the jiggles.

Well, my little experience with hydraulic controls is mostly based on 25-year
old technology, where it was considered real impressive to have a 100 Hz
servo update cycle. By moving the CNC control up an order of magnitude in
update frequency, these things become much more easily managed.

Jon