Re: [CAD_CAM_EDM_DRO] CNC hydraulic test

> 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.

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.

> The problem with hydraulics is they are extremely noisy, waste lots of

power

> with variable displacement pumps or waste 100% of power all the time with
> constant displacement pumps and pressure regulators, require vastly more
> maintenance than servo motors, occasionally have spectacular failures with
> flexible hoses blowing off under 1500 PSI, and tend to have 'jiggles' due

to

> little fluctuations in the valves. I have worked with some MTS material
> testing machinery, and the noise is absolutely unbelievable. Of course,

they

> had a concrete block room about 15' cube, with a 25 Hp const. displacement
> pump right next to the machine. That kind of setup would get an OSHA
> inspector on your back real quick, today!

> Jon
>
>
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