Re: E-Stop switch

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "jeffalanp <xylotex@h...>"
<xylotex@h...> wrote:

> Hi PJ,
> Mach1 is a general purpose control software package that does

not

> really know anything about the hardware that it is driving (very
> loosely coupled system). With such a package, it is best to have

the

> E-STOP switch initiate power-down without relying on software. At

a

> minimum, the E-STOP should cut DC power to the motors(i.e. drives)
> (whether directly through the E-STOP switch, or via the use of a

relay

> (contactor)). Lower DC voltage/amp systems can probably be done
> directly through the E-STOP switch (assuming the switch is rated

for

> the DC current). Larger Voltages/amps may need to have the power
> disconnected by way of a relay(contactor). You want to disconnect

at

> least the DC, because if you just disconnect the AC, your power
> supply may be able to continue to drive the motors after the AC has
> been cut due to charges in caps. Or the E-STOP switch could be used
> as part of a circuit that energizes a relay for normal run mode,

and

> upon entering E-STOP state, the relay circuit is un-energized, thus
> opening up the relay(contactor) path of DC power to the drive. It

is

> also good to let the software know this has happened so it can stop
> issuing pulses. If looser safety requirements are OK, the E-STOP

can

> just signal Mach1, which then would trigger an output that would
> cause the above mentioned relay to open (and cause it to stop

issuing

> pulses, though not a recommended method).
>
> With all of that said, I have read in this news group that E-STOP
> should ALWAYS cut power to the drives(motors) IMMEDIATELY. This is
> not ALWAYS the case. Especially in BIG machines (which aren't

really

> the focus of this group, but the idea should be understood
> regardles.) In a BIG machine with THK type linear ways, and
> ballscrews drives, AND tightly coupled software/hardware you want

to

> delay cutting power to the motors for a short time, yet signal the
> controller that an E-STOP situation has occured. Here's why:
> Imagine the above mentioned BIG machine's table loaded with 500
> pounds (or more) of mass. Add to that the weight of the table
> itself. Now start that table moving at a high rapid rate. Now hit
> the E-STOP switch where you cut motor power immediately and what

you

> get it a free-wheeling heavy mass. It will not stop quickly

because

> of the low friction of the linear way, and little backdrive of the
> screws and motors, and is going to slam right into whatever it is
> that is in the way, be that the milling head, or your hand, etc.
> Instead, in a tightly coupled software/hardware system, the control
> will get the E-STOP signal and immediately place the system into a
> rapid decel/stop. It can stay in this mode for upto half a second

(by

> which time the table has been stopped are greatly slowed down),
> after which point, it will then release power to the drives and let
> the axes 'free-wheel'. Such a system will also have 'fail-

safe/watch-

> dog' hardware built in such that if the software insn't responding
> correctly it will shut down power to the system. In this worst

case

> you would get free-wheeling as mentioned above, but these tightly
> coupled sytems are generally not running under Windows/DOS or PC
> hardware, and are alot more robust than PCs and Windows/DOS so this
> situation isn't generally going to occur. This isn't hypothetical,
> this IS how some BIG machines operate.
>
> Jeff
> http://www.xylotex.com
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "P. J. Hicks"
> <hickspj467@a...> wrote:
> > Hi,
> >
> > When the proper input line (pin) is triggered Mach1 initiates an

E-

> Stop condition.
> >
> > a. Is this (software triggered) the norm? I.e. shouldn't an E-

Stop

> be a physical/mechanical/electrical sort of thing?
> >
> > b. I have read mention of removing all power when E-Stop is
> activated and seen pictures of the Big Red Switch on several
> controller cabinets. Does this mean complete removal of all power

to

> the system both AC primary and DC motor power or one or the other?
> >
> > Progress is slow... and sometimes in the opposite direction!
> > PJH
> >

Seams opinons on this vary somwhat.
I feel that a E-Stop button should not be
used for the normal opperation of a machinetool.
It should be located within easy reach of the Operator
and clearly Marked as such.
When activated it should cut the Power to all devices
attached as close to the Powersource as posible.
Incorporation of a E-Stop in Software is nonsence because
this very software may have caused the reason for the shutoff.
Sence the reason is not known all power should be cut.
On Machines where residual movement after a shutoff may cause
reason for concern a simple bracke clamp should be installed
which clamps onto the axes in case of power shutoff.
Just my 2 cents worth :-)