Sequencer Design

Hello,

What is the most common way to implement a sequencer of say solenoid valve zones, christmas lights, whatever?

I have been playing around with an old Omron Micro 1 and a MicroSmart PLC, and an old version of Windldr. So far I have experimented with cascaded timers, a timebase and counters (with compare if equal etc), and shift registers; loading a "1" and rotating it round and round.

How would the ability to skip a zone, jog through the zones manually, or change timings for some/all zones influence your choice?
What about the difference between zones that are on for seconds/minutes each versus zones that are on for several hours each? Obviously mixtures of the above complicate matters.

An online search revealed that Automation Direct has PLC's that have built in drum sequencers, but I want to know how to do it from scratch.

If anyone is familiar with a 4017 chip, that's one of the things I'm trying to duplicate. In a Digital Electronics class I took years ago, we used the timebase and frequency divider approach....

Thank You,
Steve

What is the most common way to implement a sequencer of say solenoid valve zones, christmas lights, whatever?

Click to expand...

For Christmas lights, there are several commerical controllers available, designed with electronic chips programmed for the purpose (Light-O-Rama comes to mind).
http://www1.lightorama.com/

For industrial PLC programs, most PLCs have sequencer instructions, drum controller instructions, FIFO and LIFO stacks, as well as bit-shift registers. I see that WindLDR version 5.1 has only bit-shift instructions.

How would the ability to skip a zone, jog through the zones manually, or change timings for some/all zones influence your choice?

Click to expand...

For what I know about PLC sequencer instructions, skipping a step can usually be done by creating a sub-routine that advances the sequencer pointer by 2 steps, skipping one step. The time for each zone can be the same, or different.

Having a differernt time for each zone or step slightly complicates the program but not as much as you think. In fact, all zones can be timed with the same timer, but using comparison instructions in parallel. Each comparison instruction (for a different time period) causes the sequencer to advance one step.

If you use one timer and comparison instructions in parallel, each comparison instruction can use variable On and Off times that are set manually or by another program routine.

Here is an example (using the LogixPro simualater of Allen Bradley RSLogix software) of flashing lights controlled by a sequencer, with variable time steps. Rungs 0, 1, and 2 are for the sequencer, rung 3 sets the timer values one time at PLC startup, and rung 4 sets the sequencer pattern one time at startup.