STEP 7 and Encoder

We have a project where the "begin-product" as well as the "end-product" is on the conveyor. There are robots to detect the "begin-product" and after the robotcyclus, the robot lays the "end-product" again on the conveyor.
Now to detect where the products are on the conveyor I used a DB with the value of distance of each product. I use pulses of 0.01s: every time if there is a pulse, i decrease the value in the DB.
Now I want to use an encoder to have more pulses, so I can work more accurate.
I was told that I have to use the FM350-2 counter module. I don't know this card, but why should I use it, and why can't I just put the encoder on an input of my PLC ?
If I have so much pulses, and every pulse the plc should decrease the values of my DB's: what about the cycle-time?

Lot's of questions, I hope it's a litlle bit understandable.

Thanx,

Greet

Well, you problem deserves a much more thorough explanation, but I'll at least get you started....

1. The first problem I see is the way you are doing it already. You say that the existing pulses are 10ms? If so, I assume 10ms on, 10ms off. Are you sure that you are not missing pulses already? 10ms is very short, and when you add in the latency of the input and the scan time, I can see how pulses could be missed. But if the program is small, and you aren't using much IO, then I guess it would work. You might want to check this though.

2. You wouldn't be able to connect the encoder to the inputs (unless it's a very low resolution encoder) because of what I just described: Because of the scan times, you would miss a bunch of pulses. The FM350-2 will count these pulses asynchonously to the scan, so you could connect a very high resolution encoder to it.

3. Keep in mind that the FM350-2 can be used a couple of different ways. You can simply connect the encoder and then read the accumulated value on each scan. If you do it this way, you still won't have more accuracy than what your scan time allows. A better way is to parameterize the card to use the interrupt IO to trigger external events separate from your PLC.

There is a lot more to say on this subject, but that should get you thinking.

If you're wanting accuracy at 10 ms intervals, you certainly need an FM of some sort. If it's as simple as you imply then an FM350 or FM352 should do the trick, but as S7Guy says there's more to this than meets the eye.

If things turn out to be more complicated, then you may need to look at the FM352-5, which is a full blown autonomous parallel processor, with a 1 micro-second cycle time. If you're on your own, the learning curve is like Mount Everest (or is Mont Blanc steeper, although not so high), but in this case you would have the good fortune to take advantage of my painful experiences, which you can follow here .

While the learning curve is steep, once you get into it, this is one really powerful beast!

Thanx. Indeed lots of thinking to do.

But still I want to try to explain myself a little better.

There is a conveyor (speed=8 m/min). And the thing that I want (must) to do is to know exact where my pieces are, and what sort of piece it is (beginproduct or endproduct). At first I did it with a pulse of 0.01s, but that means a precision of 15mm and that is not correct enough. Now I want to work with a higher precision, thus this means more pulses...

... more pulses means a higher resolution of Encoder...
but you have to check whether your system supports (currently) and/or in future purchase of FM.

Another thing, my personal experience though, sometimes there have been problems that had been solved by a mechanical alteration rather than a PLC/electronic.

Regards,