Position control with hydraulics

Hey guys

I'm building a program that has position control with hydraulics. I have three valves: left, right and creep.
I'm using variables: setpoint, actual value, limit low, limit high, deadband, creepband (don't know how else it's written.)
I have two movement speeds: normal and creep.
Setpoint is changed manually by operator.
Machine is cutting wood. Cutting tolerance is bigger than deadband, usually.

In one PLC program i saw, that initially program does not consider deadband. Only after actual position has overshoot setpoint and movement reverses is deadband considered.

Is that correct? I based my program on that. I haven't done any programs for this situation so i lack know-how.

ArviK said:

I'm building a program that has position control with hydraulics. I have three valves: left, right and creep.

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Only one creep/slow speed valve? Does this mean you can only creep in one direction.

Setpoint is changed manually by operator.Machine is cutting wood.

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What kind of machine? Is it in a saw mill? So the operator need to change the set point often?

[quote[
Cutting tolerance is bigger than deadband, usually.
[/quote]
So what are you going to do when the cutting tolerance is smaller than the dead band?

In one PLC program i saw, that initially program does not consider deadband.

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That is OK. At first you must check to see if actuator is in the creep or slow speed band.

Only after actual position has overshoot setpoint and movement reverses is deadband considered.

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That wastes time/production. You should have a high speed and low speed bands. The actuator moves at high speed until the actuator gets within the low speed band. Then only the low speed or creep speed valve is used.

Is that correct? I based my program on that. I haven't done any programs for this situation so i lack know-how.

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If it works well enough it is OK. It would not be acceptable in a modern sawmill.

This is how we did bang-bang setworks systems back in the late 1970s and early 1980s.

Hey Peter

Creep valve is for both directions. Setpoint is changed once, twice per day. Machine is stopped when setpoint is changed. Machine is in saw mill, it saws planks into half. It has more positions that move but they are the same.

Old program where i took the example from is from Siemens S5 system. I changed that to S7. I think that the overshooting thing was built because S5 CPU main cycle was so slow, that machine moved more than deadband lenght in one cycle.

I understand from your post that this is not how things are done today?
I'll build my program without overshooting then. Move in normal speed toward setpoint. In creep position move at creep speed until in setpoint or in really small deadband.

Or are there more cleverer way to do things?

I can't change anything in hydraulics or sensors. Customer would be okay with overshooting. But since i made new program in new CPU, i wanted to make things pretty.

ArviK said:

I understand from your post that this is not how things are done today?

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No, hydraulic motion control is very precise today. Normally servo valves, Temposonic or Balluff rods are used for precise position feed back and a hydraulic motion controller does all the control.

Or are there more cleverer way to do things?

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Yes, but more expensive too. Since the position only moves once or twice a day keep it simple. Normally the saws or logs need to move for each piece and then something much better is required.

I can't change anything in hydraulics or sensors. Customer would be okay with overshooting. But since i made new program in new CPU, i wanted to make things pretty.

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Just make it work.

Veisto is based in Finland. This is a video of a mill they installed in France. They are using our hydraulic motion controllers.
http://deltamotion.com/peter/Videos/VEISTO_DVD.mp4
They tend to sort the logs so they are all the same size then position logs rather than move the saws.