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Old March 4th, 2019, 12:16 AM   #19
Peter Nachtwey
United States

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Join Date: Apr 2002
Location: Vancouver, WA, US
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Why not use the SSI encoder with the PLC, as well?
This is a very good question. SSI MDT rods are far superior to analog rods. There is no noise but then there is the problem with sample jitter. A motion controller uses and FPGA to ask for the new position every millisecond on the nanosecond. There are no "house keeping" delays or times when interrupts are off. The point is that even if the feedback is SSI it makes little difference if the time between readings is not know. I/we had many arguments about this with Tempsonics in last half the the 1990s about this.

Most people that try to do hydraulic servo control do not know this so they use analog feedback MDT rods.

I'm willing to bet there are people on this forum that would be willing to help you maximize the capabilities of your various attempted PLC solution
I bet there are. I am going to wait for JRW's suggestions when I post more data.

However, there real problem is the control algorithm. The PID function block in in PLCs is not good enough. The hardware can help but it will not get close to a solution.

Note that a 1518 is literally 60x faster (by bit processing speed) than a 1511, so picking the right CPU has a huge effect on what motion is possible.
What it means that a S7-1518 can control more hydraulic cylinders incorrectly.
Th S7-1518 does not have the right algorithms for hydraulic motion control. I can make allowances for the difference in speed. Where I will crucify the competition is when I find things there control algorithm can not do or any flaws in the target generator.


That said, the precanned instructions are mostly puzzle pieces you use to make your own solution. I saw a hydraulics library mentioned on the support site, but it looks like it isn't freely available.
How much do they charge for that? How good is it?

Maybe that's the difference between PLC and motion controller in your definition I'm missing? How much of the application is precanned?
The difference it the attention to detail and control algorithms. I can go into detail but I would lose everybody unless they had a PhD in control theory. This is the honest truth.

There is no way a PLC can compete with a dedicated motion controller. I don't think that is the issue. However, if the application is extremely simple and the tolerance aren't too demanding the a PLC may have a chance but I from our experience the PLC has no chance. For instance the controller for a garbage truck will usually be an embedded micro controller. The control doesn't need to be exact or versatile. I just needs to get the job done.
"Living is easy with eyes closed, misunderstanding all you see...." Strawberry Fields Forever, John Lennon
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