Created Date: November 13,2010

Created by: milldrone

Hello all,

I am working on a "setworks " for a sawmill application.

Ok.

Does this seem like an application for a PID loop? Would that help at all?

Yes, but the question might be: Will it help enough ?

Question: you stated:

The problem that i have is that the hydraulic cylinder is overshooting the stop position by a considerable amount. My initial program would reverse the cylinder to try to get it back, but it would then overshoot in the reverse direction. This cause the cylinder to jump back and forth continuosly. I 've tried to program an earlier stop point, and can get the cylinder to stop repeatably in the right position, unfortunately, i have to stop the cylinder 1/2 inch early. That works fine for larger moves, but for smaller moves, it does not work.

Based on your above description, this sounds like you could achieve acceptable performance from adjusting the stop point according to the move distance.

Classically, positioning is done with proportional valves, like Bernie suggested. And some sort of PID control.

You may have to invest in proportional hydraulic valves.

And not done with hydraulic needle flow controls. Also if a higher level of precision or speed is needed, a separate motion controller CPU may be required.

Created Date: November 13,2010

Created by: billf_Clark_Pulley

Using a prop valve with accumulator and hard tubing the connections to the valve and cylinder you will correct this issue. Using a Delta Motion Controller would be your best bet. I use them all the time to control multiple axis on hydraulics and I can control the tolorence with 0.002 ". I'm currently working on 4 machines which all move hydraulic cylinders two of the them are to position only and one is position to pressure control.

Created Date: November 13,2010

Created by: KPrice

amotomura, it all depends on the accuracy and repeatability you are looking for. I agree with all the posts that you will Not get +/- .002 " unless you go with the proportional/servo valves/Delta controller, etc.

However, we have done setworks projects - where +/- .030 " was acceptable. Use 2-speed setup. When close to target, go into slow. Use a temperature/pressure compensated flow control valve for slow. Use Stage programming and Immediate instruction. And you must compensate for "coast " of the hydraulics. And if you are also doing "stacks ", always set from same direction.

With this "on/off " control you can get better than .030 ", but there is a trade-off between accuracy and speed and cost. If you need the higher accuracy and speed, then you'll need the higher cost systems.

Created Date: November 15,2010

Created by: milldrone

this was a completely manual saw. It used push buttons with some relays to fire the valves.

I think the .030 " tolerance is closer to our budget!! I will look into the flow control valve to get the slow speed.

This helps define your project a little better, both precision wise and budget wise.

milldrone, my next try with the machine will be using different stop positions depending on the distance needed. I will also give the PID loop a try.

One question first. You mentioned flow controls earlier, and I'm presuming this is a "woodmiser " or "woodmiser clone horizontal mill ". Do you know if your flow controls are "meter in " or "meter out "? The reason I ask is because "meter in " when lowering an "overhauling load " can cause cavitation in the cylinder, and you will not get repeatable results.

My advice is to try the "short stopping point when there is a long move " first. This will only be programming and wont cost you any hardware dollars. Start by measuring how progressively longer moves increases you error. Use this error factor to adjust your "stopping early target "

If you do not get "good enough " results try Kprice's method with two valves. I'm going to make a presumption that you have a constant volume pump and not a pressure compensated pump and you are using a DO5 or DO3 directional valve. This would mean that you would have tandem center spool hydraulic valves. If this is the case. A valve similar to this http://www.grainger.com/Grainger/HALDEX-BARNES-Electric-Lowering-Valve-5PG83 would allow full flow around the flow control valve. Then you can adjust the flow control to a creep (presuming the flow control only affects the head lowering speed). If you are lucky enough to have a pressure compensated pump, then a second directional control valve could give you three speeds. AKA 1 valve slow, second valve medium, both valves full speed.

Created Date: November 15,2010

Created by: KPrice

amotomura, Stage programming are very powerful programming instructions that can be used with your DL06. (You will love it.) It allows you to turn on or turn off various sections (Stages) of ladder logic. One of many reasons it can be helpful to your project is that you can have only a small amount of ladder logic rungs "running " and thereby reduce your scan time. In your first post you asked "any way to improve reaction times on the PLC side? ".

Some people use the word "stacks " to refer to an absolute position to move the log (or saw) - so that they can cut a given number of boards a certain size - and wind-up with a cant the correct size.

By saying "stacks ", I was thinking of any reason to move to an absolute position. Another reason to move to an absolute position: prior to rotating the log, push a button to "save " the absolute position, so when you return to that side (or opposite side), you can push a button to return to that position.

By moving to an absolute position from the same direction, you improve accuracy.

Created Date: November 15,2010

Created by: doug6949

A variable displacement pressure compensated pump is one solution. Unfortunately, they are very expensive and still require multiple valves.

Another possible solution is to use a two stage pump. The first stage is typically three to ten times the displacement of the second stage. Both stages are used for high speed. Shifting to low speed simply involves bypassing the first stage.

Two stage pumps tend to be expensive but are readily available as new surplus. Surplus Center of Nebraska is one common source.

Doug

Created Date: November 15,2010

Created by: billf_Clark_Pulley

Yes, Delta units are a little more costly. An easy and cheap fix for this would be to use a directional valve with flow controls on the A & B port. Then shift the valve for High and Low speeds. The high speed valve would have no flow control and the slow speed would. It should get you within the tolerance you want. As for Prop valves they are not cheap, I like the ATOS brand, but you will drop around $1,800 per valve if you need one with a 20 GPM flow rating.

Created Date: November 15,2010

Created by: Bob S BN

I just have to throw this out there...

I don't have any idea what this cylinder is doing in application, or how much force it must exert, but a couple of you do.

But, because it has a hydraulic system, are we stuck in a paradigm that it HAS to be hydraulic system?

I replaced a hydraulic system that moved a platen on a drill press with a servo roller screw actuator. The one I used was from Exlar, has the motor built "around " the screw assembly, and wasn't any bigger around than the 4 " bore cylinder, and only a couple inches longer, and can push almost 4,000 pounds. This method was cheaper than our other choice of upgrading the hydraulics with prop valves and better pump. Plus, we got a whole pump and tank assembly off the floor and no more hydraulic leaks!

There are other companies making roller screw and ball screw actuators that may be cheaper if you don't have much for size limitations.

Just something else to think about, and my $.02,