Posted 15 December 2004 - 06:46 AM
I'm new here. I need to learn how to set up shift registers. They are used on machines that are multistation to identify a station that contains a defective product. The defective product is identified then ejected from the machine down the line at the eject station. A rather poor explanation I admit.
Can someone offer some pointers.
Posted 15 December 2004 - 11:00 PM
Today I spoke with the Allen-Bradley PLC support people, a co-worker, and I looked over the links supplied to this post. They all recomended BSL or
bit shift left and tried to explane to me how it works. I still don't get it.
Allen-Bradley's example was a bottle filling production line. I guess bit shift left is what I need to use.
I do not understand the logic in this. A defect is detected then ejected from the machine several machine cycles later. How does the machine know a certain position has a defective part in it for that many cycles? I just do not understand how this flows
Posted 16 December 2004 - 05:46 AM
My explanation may be off a bit, but I'll give it a go.
Imagine you a multi step process for filling bottles, a value of 1 indicates a good bottle.
A bottle is detected at step 1, a value of 1 is written to a tracking register at bit 0.
Then the machine indexes to step 2, we then use BSL to move the 1 from bit 0 to bit 1. At step 2, the value of bit 1 is evaluated, since it is one, the bottle is processed and then inspected, it passes so a value of 1 remains (good bottle). Index machine, and use BSL to move the 1 from bit 1 to bit 2, the value of bit 1 is evaluated, since it is one, the bottle is processed, it then fails inspection and a value of 0 is written to bit 2. Index machine, and use BSL to move the 0 from bit 2 to bit 3, the value of bit 3 is evaluated and since it is a 0, no action taken (bad bottle).
When the bottle gets to the final stage, since it has a value of 0 (bad bottle) it is rejected.
While we were following this one bottle, others were entering the same machine and going through the same steps. So when we shifted the 1 from bit 0 to bit 1, another bottle's value was written to bit 0, so that each time a bottle shifts stations, it's status bit follows along. Using one register you can have upto 16 different stations. If you need more than that it gets a little complicated.
Hope this helps, like I said, I'm a little rusty with the bit shift instruction and your actual PLC code will vary, you will have to work out when to trigger the BSL and how to handle start up and shut down. What to do if you have power failure in the middle of a step etc...
Edited by Ken Moore, 16 December 2004 - 05:49 AM.
Posted 16 December 2004 - 06:37 AM
Thanks, you made yourself very clear. Let me look at this. I'll be back latter.
Thanks to you I'm closer
Posted 16 December 2004 - 08:16 PM
I think I got it. BSL (bit shift left) is a little like a counter.
It is quite amazing to see it work. Three lines of programming was enough for a demo. One input makes it shift to the next bit. A second input makes the bit true or false, an important point.
In short here is how it works. BSL begins at a check station (sensor). A reject station is 10 machine cycles away. The first bit or machine cycle (b3:0/0) is true. The next cycle b3:0/0 jumps to b3:0/1 (remains true) continues cycling to the reject station. By this time the original b3:0/0 (true) is now b3:0/10 (true) and the reject station is tripped. Now if the part just behind b3:0/9 is false it remains false when it jumps to b3:0/10 remains false, so the reject station is not tripped.
A demonstration is worth a thousand words.
Edited by jglass, 19 December 2004 - 07:07 AM.
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