Re: [CAD_CAM_EDM_DRO] Estop

wanliker@... wrote:

> Date: Mon, 26 May 2003 13:46:46 -0600
> From: Jim Buslepp <jbuslepp@xxxxxxx
>Subject: Re:disable input and emergency stops
>
>
>My motor drive experience is in the semiconductor process tool industry, so
>the requirements may be different than those in the machine tool industry.
>

Yes, I think that the requirements are quite different.

>- The EPO switch should be used only when there is an immanent threat of
>injury to people or damage to equipment. To allow other uses "dilutes" the
>meaning of the big red mushroom and leads to poor design compromises.
>
>

This may well be against OSHA requirements, in fact. When somebody
crawls inside a
machining cell to clean, inspect or replace tooling, etc. it is probably
required that the
machine be in estop. Most installations want all machines that will be
unattended and
not running to be left in estop. Almost all companies, in my
experience, leave their
CNC controls powered on at all times.

>- The EPO hardware should fail safe. Having a machine down because of a
>fault in the EPO circuit costs money. Having it up with a faulty EPO could
>cost lives. For example, mushroom switches are available with normally open
>and normally closed contacts. With normally open contacts, a broken wire or
>unmated connector can defeat the EPO. This fault would probably be hidden.
>A wire broken during maintenance could disable the EPO for years.
>Therefore, normally closed contacts are preferred.
>
>

Not testing the emergency stop system over a period of years would seem
insane. Most machines
are estopped often in the course of setup changes, shift changes, etc.

>
>- All power supply voltages must be drained to near zero quickly. Bringing
>them down to less than 1V within 1 second seems a reasonable minimum
>standard. Most systems should better that considerably.
>
>With these criteria in mind, I feel that dropping primary power to the
>system is essential. I would not except the controllers as this requires me
>to keep too many components live. Keep in mind that this is not a routine
>event. The EPO should only be activated when there is immanent danger or
>hurting a person or damaging the machine. In this circumstance crashing the
>operating system or having to home the machine is acceptable. If a stop is
>needed for more routine situations it should be implemented separately.
>
>

But, when an operator is INSIDE a 50-ton machine, a software stop is
just NOT good enough!
Read the OSHA reports of all the people crushed in presses, etc. because
somebody else
(or an automation system) started the press with the operator still
inside it.

I really think it should be EASY, and cause no complications, to trip
the E-stop, and that
a complete power down is not required. Separate electrical interlocks
on the control
cabinet can be used for electrical safety. You want the operator to
feel FREE to stop
all possibility of unintended motion, without it causing any difficulty
getting the machine
restarted. If hitting E-stop causes a 2 hour process of recalibrating
the machine, nobody
will ever dare to push the button!

>Asserting disable bits may be a nice extra to stop the motors faster if it
>takes significant time for the power supplies to drop, but this should not
>be part of the primary EPO. If a lot of inertia is involved, motor windings
>can be shorted briefly to act as a brake. This is especially effective with
>stepper motors.
>

Shorting steppers or servo motors can destroy them if excessive currents
flow. A properly
sized braking resistor is a lot better way to go. The short could also
weld or destroy
relay contacts, possibly making the machine FAIL to stop!

> The short should be removed after short time (probably less
>than a second) to allow parts to be moved to free a trapped person. Either
>of these features must be implemented in such a way that they do not
>degrade the reliability or speed of the basic EPO function.
>
>

A properly sized braking resistor will probably cause negligible drag
when manually
backing the machine off of something.

Jon