Re: [CAD_CAM_EDM_DRO] CNC hydraulic test

> ----------
> >From: "Sven Peter, TAD S.A." <peteryco@...>
> >To: "CAD_CAM_EDM_DRO@egroups.com" <CAD_CAM_EDM_DRO@yahoogroups.com>,
> "cnc-de@yahoogroups.com" <cnc-de@yahoogroups.com>
> >Subject: [CAD_CAM_EDM_DRO] CNC hydraulic test
> >Date: Mon, Jun 18, 2001, 10:58 PM
> >
>
> > Hello Friends,
> > For years I dreamed of converting the hydraulic control of
> > my copy lathe into CNC.
> > Right now I have 2 electronically driven proportional valves
> > sitting on my table together with the driver boards.
> > The client permits me to make a test for my project before I shall
> > install them in his machine.
> > The input is -10V...0V...+10VDC but I have no computer with interface
> > cards.
> > Has somebody an idea how I could make me a simple test set-up to find
> > out
> > if these proportional valves would do an acceptable job in an CNC
> > application?
> >
> > In THOSE days when I studied it was said it would go OK to make a CNC
> > machine with hydraulic pistons. Now everybody goes with ball screws and
> > server or stepper motors. Might be I am stubborn but at least I want
> > want to have made
> > a test before I go to ball screws.

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.

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