PLC SEQUENCERS

[Pivotman]

I am familur with Ladder logic but do not have much experience with sequencers. Does any body know of a good book that explains how they work? Or sources for any simular info.

 

[johnnyplang]

Are you talking A/B? If so, sequencers are just file to word function blocks. Its gets the same result as using an indirect address in a MOV block. We use them all the time for our servo programs which consists of 40 steps. Each step consists of a position, speed, dwell time and some output functions. When the servo reaches the programmed we drive the sequencers to output the next step's data (position, speed, etc..). We didn't have to use a sequencer but I guess you can see what is going on a little easier since the sequencer has more information displayed. Some people use them to control the machine outputs (I think sort of like the way an old music box works) but I would never do that.

 

[FrancisL]

AB Sequencers are wonderful in my opinion. You can use them to create state matrices for example, where the possible states of a module (machine, equipment module, units etc) are the rows of the matrix and each column provies the setting for the outputs to the devices in the module.
EG
State Valve1 Valve2
Idle Closed Closed
Fill Open Closed
Empty Closed Open

Then you just have to set the row number to set all the devices.

You can do similar things with inputs, for example to get a boolean that tells you whether all the devices are in the right state.
This way you can create sequences where the steps and transitions can be changed just by changing the data rather than the code.

And because they are very fast and memory efficient the resuting program is small and fast.

You can do similar things with other PLC's by the way, but maybe not so easily.

 

[Electromechanical30]

Sequencers are easy to write up on AB systems, but a bit cumbersome using Siemens.

 

[OldPfart]

I think this may go back a bit further than the replkies to date. What is the brand and model of your sequencer? I have not ued one of these for 15 years.