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" <[EMAIL PROTECTED]> > To: "EMC Users List" <emc-users@lists.sourceforge.net> > 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 > > > > > > > > > ------------------------------------------------------------------------- > This SF.net email is sponsored by DB2 Express > Download DB2 Express C - the FREE version of DB2 express and take > control of your XML. No limits. Just data. Click to get it now. > http://sourceforge.net/powerbar/db2/ > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
