Hello everyone,
I have program which controls the moving of an hydraulic driven conveyor in increments that are setup
by operator interface (panelview 550 k5a5) in tenths of an inch. At present this system works very well
(most of the time) but like all of you I desire more from my products.
Here's a brief example of how it's measured out: 1st. a numeric entry is made. there can be up to 24
different positions entered for any new part chosen. 2nd. a math conversion is made via logic and converted
into counts that are loaded into the high speed counter.Each time this machine reset for a new part to be made
it moves to a set point, stops and does a function and moves there after in a sequenced pattern preset via
interface.3rd. At the end of each new part made logic stops the part and positions the piece that is placed.
Just so you know I'm measuring in an step method. (1.5-1.5-1.5------6.0-1.0----3.0-1.0------6.0-1.5-1.5).
The issue I'm experiencing is sometimes the conveyor seems to move beyond the preset points and the
results are the new part has moved beyond it's desired position. NOT GOOD!
Now the question. What logic could I use to minus the overflow of stopped point from the next set point so
the conveyor will respond accurately?Also, What would be better the step method or conventional method of
measuring?

Thank you all for any response to these questions.
Jeff Purnell

"How far, how fast ?"

I'm guessing it's a hydraulic 2-position valve and a hydraulic motor driving a chain or belt.

Does this over-shoot happen all the time ? Only on some parts ? Does it happen more when the hydraulic fluid is hotter ? How much overshoot are we talking about in inches ? How many counts does that equal ?

Conveyor is moving in only one direction @ 52 fpm. Everything is a new component.
Yes, it is a Parker valve all ports closed center position . chain.
Yes, on all parts.
The temperature is maintained at 120 degrees during operation.
Anywhere from dead on up to 1/2" overshot. This would equal 11 pulses. (22 pules per inch)
Using a BEI Encoder 550 ppr.

First let me make sure I understand, you are using an a plain jane on/off type valve? Not a proportional or servo? Correct?

If that's so then it's going to be tough to do this consistently. I'm not saying it can't be done but you may save time and money looking at your hardware. Now I guess it could do some calculations to figure out how accurate your system could be but I'm more of a show and tell type of person. Figure out the minimum the output must be on for the part to move at all consistently. Measure this. This is your absolute maximum accuracy. If it is already out of range then your not going to be able to do anything with it. If it is you could look for the overshoot and incorporate this bumping method to get back. This is as crude as you can get, but if it is all you have to work with then it might limp you along.

Other options could be...

Servo but it sounds like that is out of your budget

Proportional valve but personally I've had a lot of trouble out of these in harsh environments

Slow/Fast valve. This might be an option for you. Pretty much what you do is parallel two valves and put flow controls in the "Slow" valve. Use the fast valve until you get close then switch to the slow valve for positioning. Definitely a roll your own approach but...

BTW - What type of accuracy are you looking for. Looks like your system is good for about an 1/8"

Some questions about your problem.
1. What is your scan time (Min, Max, Average)
2. Is the overtravel always at a specific step or set of steps, or related to any parallel operation.
3. Are you using the built in counter or a high speed counter module on non local I/O.

Answer to your question is: (x100us) Max.= 36
Min.= 10
Avg.= 30 x100us
No, the overtravel is throughout the coarse of the run.
Yes,I'm using the built in HSC.
Just so you know this machine is making Wooden Pallets and was controlled by a proximity switch sensing a
cam on a timing chain to achieve the stop positions. Also this machine used all ice cube relays and 1sec. timers. So in trying to upgrade to PLC control I wrote this program based on using existing hardware with the
addition of a pilot operated check sandwhiched under the dirictional control valve. The conveyor is already
two speed controlled going high speed between products then at slower rate during assembly.This system
produces around 2000 units in a 8 hour shift so, around 20 units are effected by this overtravel issue it's
enough that I would like to address it somehow.

Thanks
Jeff Purnell

This the logic from one station which produces the bottom half of a pallet.[attachmentid=4508]


My logic here is simple, HSL a interger in a counted step, once a number is loaded pulses are counted up to the preset number, then stops,
does a function and HSL a new number and so on until the max length is reach.Then reset to zero via prox.But watching the HSC acc it will always overshot the peset by around 10-15 pulses.I was thinking I could minus the overflow from the next number in line to try and keep
accuracy.I've tryed a few math conversions but just can't get past a good way of doing it yet.Any ideas?

Thanks