How to implement emergency stop and seq. actions

[angeraer 1]

Dear, I was wondering what's the best method of implementing an emergency stop in a PLC? Add an additional test on each rung that checks the emergency button's state? And second, what is the best way to implement sequential actions that need to be performed by a machine? As I have read in the PLC books it is common to use bit states in each step in the process? Thanks, Andy.

[BobB 58]

There is no best way to do this. The only safe way is to hard wire the E/stop or, better still, implement an IEC type 4 safety relay. Most countries have a Workcover group and they insist on this type of implementation. By the way, Omron have a PLC rack mounted IEC type 4 safety relay for the C200H Alpha and CS1. The PLC cannot control or reset the safety relay but can monitor it. I have used these on the maintenance cranes on the Sydney Harbour Bridge. What do you mean by bit states? Sequential operations can be programmed in many ways, depends on the programmer. For example, I normally use shift registers to step load generators and revesible shift registers, in conjunction with compares of various kinds (ZCP, ZCPL) to operate load control on generators - load goes low - step on more load - load goes high - step off some load. ZCP and ZCPL have a built in differential that is ideal for this purpose.

[PeterRichter 0]

Absolutely - must be hard wired - that goes for just about any code for any application you would encounter. The Estop must be wired in such a way as to remove power ( not just control signal) to any device The program should also be written so that if the E-Stop is released (power back on) that nothing should move until a button or switch has been operated by someone. This info should be taken as a guide only - you should look up the relevant code for the device you are working on. Peter