Re: [Emc-users] Should I use Classic Ladder?

[Ray H. <re...@up...>]

Hi Seb

I wrote the little ladder stuff in demo-step as a simple way to
illustrate the flexibility of ladder.  

I agree completely with John that for any kind of serious, servo powered
system, an estop that safely shuts down motion without depending upon
the PC is a good plan.  It also conforms to US/EU directives concerning
machine safety.  

A proper estop system should not depend upon a silicon junction to cause
an estop or prevent an estop from happening.  Most external estop chains
use normally closed elements in a series array.  All of these elements
must say they are ok before the machine is powered up.  

It is my opinion that the PC running CNC software should be involved in
the estop chain.  A charge pump can be a very effective element in the
chain.  A normally open charge pump relay says that the CNC software in
the control computer is able to do it's part of the work.

Ladder has a significant advantage over any single HAL module because it
is designed to sort out logic operations using a very precise pattern,
read all inputs -- apply all logic -- write all outputs.  Many very
complex sets of inputs and outputs can be worked out in very predictable
ways.  It would NOT be appropriate to put the hard limits, the estop
buttons, thermal overloads, and such into the ladder as separate
elements and use ladder to combine them because that places silicon
junctions between an estop button operator and the result that operator
needs.  

There are a class of machines that must be shut down in a specific
order.  Such a machine will use several parallel silicon paths with
voting to produce the needed shutdown. 

That's not to say that a HAL module or a set of HAL modules can't do the
same thing as a ladder or set of PLCs.  You could or you could get
someone else to write a HAL module that takes exactly the set of inputs,
applies exactly the set of parameters, and produces exactly the set of
outputs you desire for your estop chain.  

If you are connecting a set of HAL modules, much like you might write a
set of ladder rungs you have the additional problem of inputs that can
change during the execution of the thread.  The operations in a
collection of HAL modules cascade.  The first may change the signals
presented to the second.  For this reason the user/machine integrator
must be certain that all possible cascaded outcomes are predictable. 

Hope this helps

Rayh




On Tue, 2007-05-29 at 11:04 +0100, John Prentice wrote: 
> Seb
> 
> While you will need to tell EMC and hence your operator that you have had a 
> stop, I hope you are going to have hardwired logic to actually stop the 
> spindle and axes. It is not safe to rely on software for this.
> 
> Best wishes
> 
> John Prentice
> 
> ----- Original Message ----- 
> From: "Seb James" <se...@es...>
> To: "EMC Users List" <emc...@li...>
> Sent: Tuesday, May 29, 2007 10:37 AM
> Subject: [Emc-users] Should I use Classic Ladder?
> 
> 
> > Hello,
> >
> > I'm close to running our Peddinghaus FDB600 CNC drill with EMC2. Digital
> > logic and servo output are all working via a Mesa 5i20 and a custom I/O
> > circuit board. Before I start connecting things up, I want to get the
> > emergency stop logic in place.
> >
> > I plan to use a custom hal component rather than trying to build a
> > classic ladder scheme.... unless anyone here has a good reason for using
> > the classic ladder component? The only possible one I can think of is
> > that you might get more information about what signal has caused your
> > machine to stop.
> >
> > Anything else?
> >
> > regards,
> >
> > Seb James
> >
> > 
> 
> 
> 
> 
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