Stepping Sequencer backwards

[R_C]

Using a Sequencer like the example from this link http://forums.mrplc.com/index.php?showtopic=6455

It is working great for controlling a Pick and Place unit, very simple discrete I/O. When an input is made go to the next step.

My question is, I have a customer who would like Step Forward and Step Backward buttons from my HMI. I would like to know how you guys handle stepping a sequencer backwards and do you leave grippers open on backwards steps?

 

[brstilson]

I know this can be done, but I haven't done it before. I think you may be able to specify the step number to go to in the sequencer, perhaps by changing the address that contains the current sequence step number?

 

[Lancie1]

I would like to know how you guys handle stepping a sequencer backwards and do you leave grippers open on backwards steps?

The formal Sequencer instructions are not really designed to step backwards. To make a SQO go backwards takes a lot of fiddling and finagling. If you need to Step both directions, it would probably be better in the long run just to create your own sequencer by using a memory word as a Step Indicator. For example, N7:0 = Step Number. If Step Number = 10, the do all the rungs for Step 10. If N7:0 = 9, then go back to Step 9 and do it again. If you do go back, you have to have checks to see that you do not allow something dangerous or upsetting to happen. For example, if a gripper is holding something, and you go back, you might not want the gripper to drop the object. Each Step then has to be set up to allow for the backward possibility.

...I have a customer who would like Step Forward and Step Backward buttons from my HMI.

When customers say things like this, sometimes they don't really mean what it sounds like. For example, Step 9 may consist of a robot taking a part from Step 8, doing some operation on the part, then passing it on to Step 10. What would constitute a backward step from 9 to 8? Would you have the robot pick the same part from Step 10, "undo" whatever it previously done to the part, then pass it back to Step 8? Or what?

 

[TConnolly]

Instead of using an SQO instruction use a counter (or an integer word). Use an up counter to sequence forward, a down counter to the same counter address to sequence backwards. Right after the counter use your own masked move with the counter .ACC as an indexed address to move the matching word from your source file to your destination.

 

[KB1GNI]

We use counters as sequencers extensively in the automated equipment here in our plant. We have an application where we jump back to a previous step at times but not step backwards sequentially.

Some jobs require for example steps 4,5,and 6 to be performed X number of times. On the first pass through, when step 6 is completed a counter is examined. If the counter is not satisfied, a value of 4 is written to the sequencer's accumulator and it jumps back. Once the counter is satisfied, the sequence is allowed to go past step 6.

I wish more programers used sequencers for machines or processes that have sequential operations. It makes it SO much easier to troubleshoot.

If it makes any difference, I'm talking about Automation Direct PLCs and their predecessors. (GE, TI, Siemens in the case of the 305s)

 

[R_C]

The formal Sequencer instructions are not really designed to step backwards. To make a SQO go backwards takes a lot of fiddling and finagling. If you need to Step both directions, it would probably be better in the long run just to create your own sequencer by using a memory word as a Step Indicator. For example, N7:0 = Step Number. If Step Number = 10, the do all the rungs for Step 10. If N7:0 = 9, then go back to Step 9 and do it again. If you do go back, you have to have checks to see that you do not allow something dangerous or upsetting to happen. For example, if a gripper is holding something, and you go back, you might not want the gripper to drop the object. Each Step then has to be set up to allow for the backward possibility.

Yeah, I started the programming for stepping the sequencer backwards and it got ugly real fast. I have reverted back to moving a value into an integer at each step since each step will see a set of inputs made before the next step.

When customers say things like this, sometimes they don't really mean what it sounds like. For example, Step 9 may consist of a robot taking a part from Step 8, doing some operation on the part, then passing it on to Step 10. What would constitute a backward step from 9 to 8? Would you have the robot pick the same part from Step 10, "undo" whatever it previously done to the part, then pass it back to Step 8? Or what?

I wish that were true. These guys really want Step forward and Step backward functionality. Stepping backwards can really be a head scratcher.

 

[jstolaruk]

I wish more programers used sequencers for machines or processes that have sequential operations. It makes it SO much easier to troubleshoot.

In the auto industry, sequencers are strictly forbidden in the GM, Ford, Chrysler controls specifications because of the opposite - a b$tch for electricians to troubleshoot.

 

[KB1GNI]

^ That's bizarre. I guess their electricians can't troubleshoot using a laptop, then. Probably a union thing.

Our oldest moldng presses (long since gone to Mexico) had mechanical stepper switches, where a solenoid would lift a weight and thus index what looked like a ear of corn with individual "kernels" that could be removed or inserted to change the program. Each row of kernels actuated a set of electrical contacts.

I think we built them in the '60s... they are probably still chugging away.