Proportional Valve Positioning problem

InControl · Mar 28, 2008

Gentlemen,

I am a systems integrator with 25 years experience in PLC programming. I have recently been asked to do a saw setworks application. No Big Deal (Normally) I have done alot of this in the past using a QS or QB and Temposonics for feedback.

The problem is that the customer wanted to get his electronis from a particular vendor. This vendor sell Idec PLCs and Turck Positioning Sensors. So what i decided to do is use a Turck Sensor similar to a Temposonic but 4-20mA out.

The hydraullics guy specified a Vickers KBSDG4V type proportional valve. Last week I went to the site and tried to control the cylinder. We hooked everthing up and the cylinder extended all the way out. The valve is controlled with +/-10VDC so I gave my analog output card on the Idec PLC a positive 1 volt..no response then I gave it a negative 1 volt it slowly retracted all the way so i gave it + 1 volt it extended all the way. Come to find out, at 0 volts the valve ports are not equal pressure so it extends. I tried to control the valve with a PID but it is constantly searching for setpoint so it overshoots just slightly and then corrects and overshoots the other direction and continually hunts for SP. I spoke to the Hydraulics people and they said that I really need a model KBFDG4V valve with built in deadband. The problem is that the lead time on the valve is 6 weeks minimum and the customer is not willing to wait that long. I am thinking now about posibly trying to determine exactly at what voltage the cylinder is nulled to stay still and simply using logic to tell it to ramp voltage down as it approaches setpoint and then at setpoint move the null value out. Any help or suggestions would be appreciated.

Chuck Woodbury · Mar 28, 2008

Check out the RMC's at Delta Motion. They provided me one overnight at one sawmill I worked for and walked me through setup and training via web.
http://www.deltamotion.com/

TConnolly · Mar 28, 2008

InControl said:
Come to find out, at 0 volts the valve ports are not equal pressure so it extends.

I looked up that valve to see what kind of a spool it has. It is more accurate to say that the A and B ports are at equal pressures when command is 0. The cylinder extends because the piston end area is greater than the rod end area, resulting in a net force that extends the piston. The distinction between that and your original statement is everything when understanding what is really happening.

I am thinking now about posibly trying to determine exactly at what voltage the cylinder is nulled to stay still and simply using logic to tell it to ramp voltage down as it approaches setpoint and then at setpoint move the null value out. Any help or suggestions would be appreciated.

This is exactly why you need to understand what is really happening with the valve spool and the cylinder - that spool has no position that does not deliver some pressure to A or B.

I tried to control the valve with a PID but it is constantly searching for setpoint so it overshoots just slightly and then corrects and overshoots the other direction and continually hunts for SP. I spoke to the Hydraulics people and they said that I really need a model KBFDG4V valve with built in deadband. The problem is that the lead time on the valve is 6 weeks minimum and the customer is not willing to wait that long.

The PLC probably is not fast enough to do what you are trying to do. You need a motion controller. With any decent motion controller the valve you have should do just fine - in fact, it will give superior control to the valve with an overlapped spool. The controller linked by Chuck is an excellent choice.

InControl · Mar 28, 2008

Thanks

Chuck thanks for the fast response, I checked out the delat site earlier today and was looking at the RMC70 the problem is that it dosen't seem to be compatible with the Idec PLC.

A little background.. I have written some VB code to run in the background in Excel the office at the mill enters the orders in excel then hit a button which transfers the orders and saw sets to the saw via an OPC Server KEPServerEX for Idec. There is also a Redlion G3 touchscreen involved. So I have written all that code already for the transfer of information from the PC in the office to the Idec and the Redlion and I don't know for sure if I can send info to the Delta RMC70, but from what I read it is not able to communicate with the Idec or the Redlion.

CroCop · Mar 28, 2008

InControl said:
Chuck thanks for the fast response, I checked out the delat site earlier today and was looking at the RMC70 the problem is that it dosen't seem to be compatible with the Idec PLC.

A little background.. I have written some VB code to run in the background in Excel the office at the mill enters the orders in excel then hit a button which transfers the orders and saw sets to the saw via an OPC Server KEPServerEX for Idec. There is also a Redlion G3 touchscreen involved. So I have written all that code already for the transfer of information from the PC in the office to the Idec and the Redlion and I don't know for sure if I can send info to the Delta RMC70, but from what I read it is not able to communicate with the Idec or the Redlion.

The DeltaRMC70 can outright replace the Idec as long as you IO requirements aren't extremely high, & doesn't Redlion support Modbus TCP/IP? If so, bop, bing, badda boom.

The RMC70 also comes with some great VB example code. Very easy to communicate with via VB.

InControl · Mar 28, 2008

Alaric said:
the A and B ports are at equal pressures when command is 0. The cylinder extends because the piston end area is greater than the rod end area, resulting in a net force that extends the piston.

Thanks Alaric, you obviously have a better grasp on hydraulics than I do. So given what you said the null value needed to drive the cylinder back to a "non-moving" position will change through the stroke depending on the volume change / stroke position.

I realize that PLC PID is not the way to control for positioning a cylinder. But with your knowledge of hydraulics would you say that they are correct in saying that a valve with deadband is what I need. If deadband means that at 0 volts the cylinder stays put then speed is not an issue. They also talked about adding a pilot operated check valve. I am intrested in your responses..

Thank you very much.

InControl · Mar 28, 2008

Alaric,
I am sorry I didn't read all the way down on your message about this valve being better that the overlapped one.

Can you give me an idea as to how much te RMC70 cost. As for the I/O, I am using quite a bit on th Idec So I will have to keep the PLC and use the motion control also it appears.

I just need to communicate set positions to the RMC.

Thanks again

Peter Nachtwey · Mar 28, 2008

The motion controller will not fix poor hydraulics!

The problem is that the customer wanted to get his electronis from a particular vendor. This vendor sell Idec PLCs and Turck Positioning Sensors. So what i decided to do is use a Turck Sensor similar to a Temposonic but 4-20mA out.
[/quote[
This is very non-standard for saw mill applications. So what was wrong with the QS and Temposonic? QB are very old and the QS is getting there. However, it is a tried and true method of doing hydraulic servo control.

4-20ma out is not a good idea. First there the analog noise and drift issue. More importantly the motion controller never knows when the data was valid. Modern hydraulic motion controller, even the QS, have a FPGA that ensures the data is collected at regular intervals. It is important to not only know the position but the difference in time between samples so velocities can be calculated.

Alaric gave a good explanation as to why the piston extends with 0 volts. You can tweak the null. I have customer that tweak the valve null so the cylinder will retract with 0 volts. This way the cylinders retract to a safe position when power goes off, in theory. In reality the null is never perfect and drifts so the motion controller must always be adjusting.

I tried to control the valve with a PID but it is constantly searching for setpoint so it overshoots just slightly and then corrects and overshoots the other direction and continually hunts for SP.

Click to expand...

That this is a tuning issue and a performance issue. It will be hard for a PLC to match a QS or RMC70.

I spoke to the Hydraulics people and they said that I really need a model KBFDG4V valve with built in deadband.

Click to expand...

The hydraulic people don't know what they are talking about. If you think you have control problems now just wait until the valve with dead band replaces the good valve. KEEP THE VALVE YOU HAVE!!!

The problem is that the lead time on the valve is 6 weeks minimum and the customer is not willing to wait that long. I am thinking now about posibly trying to determine exactly at what voltage the cylinder is nulled to stay still and simply using logic to tell it to ramp voltage down as it approaches setpoint and then at setpoint move the null value out. Any help or suggestions would be appreciated.

Click to expand...

Use a RMC70. I don't know what you mean by being incompatible. The RMC70 communicates using Modbus and the Idec should be able to speak Modbus over a serial line. Better yet would be to use Modbus/TCP over Ethernet but I didn't see where Idec supports that. Unless you have a lot of I/O then who needs the Idec anyway? The RMC70 can communicate directly to the G3 and to the PC.

A little background.. I have written some VB code to run in the background in Excel the office at the mill enters the orders in excel then hit a button which transfers the orders and saw sets to the saw via an OPC Server KEPServerEX for Idec. There is also a Redlion G3 touchscreen involved. So I have written all that code already for the transfer of information from the PC in the office to the Idec and the Redlion and I don't know for sure if I can send info to the Delta RMC70, but from what I read it is not able to communicate with the Idec or the Redlion

Click to expand...

WHAT????!!! We certainly do communicate extremely well with Red Lion HMIs. We have example programs already done for Red Lion G3s. If the Idec can some how manage to speak Modbus then it can talk to the RMC75. The are on our website in the download area. We like Red Lion's use the Redlion's at trade shows. We also have a ActiveX control that allows Excel, VB or .net programs to communicate with the RMC over Ethernet or a RS232 or RS485 serial line.

In short, the RMC70 plays well with others and keep the valve you have.

Peter Nachtwey
President
Delta Computer Systems, Inc.

InControl · Mar 28, 2008

Peter,

I certainly didn't mean to **** you off. I just found out about your product today and when I read the specs I didn't see Redlion listed.
I just went to the download area and found the example code for the Redlion...I appologize.

I also contacted the local distributor and got a quote for a RMC75E with 2 axis control and analog input and 8 Discrete I/O. Now I have to tell the Mill owner that his Brother-in-law's components are not going to work.

Thank you for the input... It looks like the Delta Motion Controller is the way to go.

CharlesM · Mar 28, 2008

I certainly didn't mean to **** you off. I just found out about your product today and when I read the specs I didn't see Redlion listed.
I just went to the download area and found the example code for the Redlion...I appologize.

I also contacted the local distributor and got a quote for a RMC75E with 2 axis control and analog input and 8 Discrete I/O. Now I have to tell the Mill owner that his Brother-in-law's components are not going to work.

FYI, I am in Bentonville and I have a RMC75E & a Redlion HMi on my bench now. I used the example project to get the Delta and the HMi talking ethernet without any problem. Not only was it no problem it was very very easy. I have been using the RMC controllers for a few years now but I am just getting started with the Red Lion. One thing I have seen but have not tried is to pass I/O through the Redlion. They show this on their web site but I have not tried it. It may be fast enough to pass data thru the Red Lion from the PLC to the RMC.

I get my RMC's from Industrial Electrical Supply however I do know of some hyd distributors that are selling them as well.

You may have already downloaded RMC Tools programming software. If not you should. When you do you can connect to a motion controller online. I have done this with my computer only but I bet you could connect with the Red Lion as well.

Delta also has a forum you may want to check out.

http://forum.deltamotion.com/index.php

Hope this helps.

TConnolly · Mar 28, 2008

InControl, with all due respect, I don't see where Peter said anything that looks like he was ticked off.

InControl said:
If deadband means that at 0 volts the cylinder stays put then speed is not an issue.

There is no guarantee that it will. If there is a net positive force in any direction then the piston is going to move. On any piston there are three forces at work (assuming there is no non-parallel load to the piston axis). These forces are the piston area * pressure, rod end area * pressure, and load forces. (Rod end area is piston area - rod cross section) For the piston to remain stationary (piston area * pressure) - ((rod end area * pressure) + load foces) must equal zero. Change any one, be it through a drip, leakage past the valve spool, or a shift in the load force, or some other factor, and that piston is going to move.

They also talked about adding a pilot operated check valve. I am intrested in your responses..

P.O. checks can help hold a piston in place on a bang/bang valve - but I think you're asking for unnecessary difficulties. The RMC-70 is the way to go.

Peter Nachtwey · Mar 28, 2008

InControl said:
Peter,

I certainly didn't mean to **** you off.

I wasn't but I did want to make it clear that we work well with others. Most PLCs can communicate using Modbus or DF1 and we support both.

I also contacted the local distributor and got a quote for a RMC75E with 2 axis control and analog input and 8 Discrete I/O. Now I have to tell the Mill owner that his Brother-in-law's components are not going to work.

You really shouldn't use analog MDTs. Use Start/Stop, PWM ( gated ) or SSI MDT rods. If prefer SSI MDT rods. SSI MDT rods may cost a little more but there are three BIG advantages to using SSI MDT rods.
1. Higher resolution. This helps with the velocity regulation and the ability to increase the derivative gains which adds stability.
2. The motion controller initiates the interrogation so the control knows when the position data is valid. This is necessary to calculate accurate speeds.
3. Easier replacement. Most of my saw mill customers use 5 micron SSI rods. If a MDT rod must be replaced then it is simply replaced and they are all calibrated to return 1 count per 5 microns. Analog rods will not be calibrated the same so every time a rod is replaced the calibration tool will need to be used. This requires physically and accurately measure the position of the actuator at 2 locations. This is a pain in the rear. It is bad enough to get out wires and dial indicators once during initial start up but to do it every time a rod needs to be replaced is unsatisfactory.

Note, if you do decide use SSI MDT rods the simple thing to do is to get Temposonic rods. If you decide to get Balluff SSI rods then you must make sure they are SYNCHRONOUS SSI rods. This is important. Getting back positions at 5 micron resolution makes no difference if the motion controller doesn't know when the position is valid. Synchronous SSI rods latch or estimate the position synchronously to our clock signal.

kamenges · Mar 28, 2008

Originally posted by Peter Nachtwey:

If prefer SSI MDT rods.

Why do you prefer SSI over Start/Stop? I would think that Start/Stop would provide you with the most direct and immediate measurement.

Keith

CharlesM · Mar 28, 2008

Why do you prefer SSI over Start/Stop?

My guess is higher resolution and faster.

kamenges · Mar 28, 2008

But what can be faster than initiating your own interogation pulse and waiting for the reflection? The resolution limit would be on the controller side with the clock frequency used to measure the pulse return time. Unless the sensor adds significant delays to the interface signals.

Keith

Proportional Valve Positioning problem

InControl

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CroCop

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Peter Nachtwey

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