Re: [CAD_CAM_EDM_DRO] Re: Machine security and safety

ccs@... wrote:

> > ballendo wrote:
> > >
> > > Chris,
> > >
> > > Good points.
> > >
> > > I'm hoping you're in MDI when you're doing this.
> > > Do you limit rapids, and turn the F/R over-ride down before you get
> > > close?
> > >
> > > This brings up an issue to consider for control writers: Just what
> > > can be done to increase operator safety in this type of "human
> > > intervention needed" situation? Thoughts, anyone?
> > >
> > > (and before someone says "use a probe routine"; besides that!)
> >
> > How about a foot operated power switch?
>
> I'm afraid none of these things are really any use if an encoder falls
> off a motor - the broken servo loop is going to cause the motor to
> take off at full speed regardless of any limits normally imposed by
> the software, and on any machine with hefty drives for fast rapids the
> user would probably not be able to react in time. And there won't
> even be a 'following error' to shut it down unless it was supposed to
> be moving when this happened.

There are ways of handling this. My machine will fault if it goes open
loop. If the servo gain is high enough, then the normal operation of
the machine never saturates the servo amps. When it goes open
loop, the servo amps DO saturate, the current drawn from the power
supply is very large, and the amp trips on overcurrent. I know some
commercial machines can be quite dangerous when the run away -
I've heard stories of vises, and even entire machine tables thrown
through walls!

Allen-Bradley had a scheme in the 7320 controller that compared
the tach signal with a velocity signal derived from the encoder. If
the tach was 10% greater than the encoder-derived value, the control
would cause an E-stop. With old, light-bulb encoders, this was
necessary.

Jon