Using PLC to control hydraulic movement

I am new and know very little about PLCs. I have a sawmill and the head moves vertically. I want to be able to move it in 1" or so movements at a time. Say I am sawing a 12" in log I want to push a button and have the saw head move 1" and stop. Now I have to do it manually with a ruler scale. Takes time. I have seen this done using a rotary encoder and a plc with an electric hydraulic valve. The encoder reads the movement and shuts off the valve. I would need to be able to program different thickness's and store the program. Not sure what kind of encoder and PLC I would need for this. Doesn't have to be deadly accurate since we are dealing with wood. I am just not good at electronic things. Mechanical devices are my thing. But I am willing to learn. Thank you for your time.
Jim

If this is for a home/hobby use you could use a stepper motor with an Arduino, as seen in this video: https://www.youtube.com/watch?v=JzR6FV-sdH4

You could program one push button to raise the blade 1" and another push button to lower the blade 1".

dragonx556 said:

If this is for a home/hobby use you could use a stepper motor with an Arduino, as seen in this video: https://www.youtube.com/watch?v=JzR6FV-sdH4

You could program one push button to raise the blade 1" and another push button to lower the blade 1".

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The head weighs about 500 lbs. This is for sawing logs. I want to stay with the hydraulic since it is in place,

Piston or screw

How is the head moved? A piston and slide then you would need a linier encoder. If screw drive then a rotary is good.

Automation direct has a PLC called DL05 that will take a encoder input without a high speed card. You may be able to program it with the free software. They also make a C more screen with free software. Tec support is good and the have free videos on how to. Maybe $500 in hardware total.

If this is the way you go, get the hardware and play with it. Then try to make a program and post it here someone will help if you make the effort to try.
Good luck

I work in the wood industry and we have a lot of hydraulics doing exactly what you need.

Usually to do this you would use a hydraulic servo valve and a transducer measuring the position of the cylinder. Then a controller would shift the valve to maintain the target set point. New all this stuff would costs you in the neighborhood of 5 to 10k.

There are lots of options to do this cheaper depending on what you want to accomplish.
Is there already a cylinder and valve present to raise and lower the saw head, but you have to actuate them manually? Let me know what you have and I can make some suggestions as to how to do this.

Here is a Parker solution:

http://www.parker.com/literature/Hydraulics Group US/Intellinder_CAT_HY18-0035.pdf

https://www.youtube.com/watch?v=QkEvE7SqsMI - another

Whatever solution you use be it encoder, linear transducer you should install a stop valve in the hydraulic line to keep the thing from moving when you are sawing.
If it does move while sawing it will usually wreck a fair amount of stuff.

In the sawmill I worked in the Chip N Saw used allen bradley I/O working with Temposonic transducers:

http://www.mtssensors.com/

A lot depends on how long you're willing to wait to complete a positioning move. If you can use a flow control valve to slow the flow of hydraulic fluid you may be able to get the accuracy you require by simply monitoring the position and turning off the solenoid controlling movement when you get close to the target position. You may have to tweak the flow control valve in the course of a day's work to compensate for changes in the temperature of the hydraulic fluid or buildup of sawdust or other debris in the mechanism.
To get a faster response you could use "coarse" and "fine" valves, each with different flow control settings. Start motion with the coarse valve open and switch to the fine valve when you get close to the target position. Then shut off the fine valve as described above.
For faster response and improved repeatability, use a servo valve. That allows you to change the flow rate while the axis is moving. As you approach the target position reduce the flow rate so that you arrive at zero flow when you arrive at the target position. A PLC can recalculate the difference between the actual position and the target position every few milliseconds. A motion controller can recalculate more frequently as well as being able to use a more sophisticated control calculation.

sorry for the delay. computer problems. I know what kind of hardware it takes and how to mount it. They use a rotary encoder,electric hydraulic valve and a PLC. I just don't know what encoder works with what PLC and how to program them. Willing to learn I have just never done it. They are already an option on many mills but it is propriety info. It should be fairly simple with off the shelf components. Once the encoder sends the signal to the plc it shuts off the valve therefore stopping the movement of the head.

Automation Direct DL05 with a high speed counter card and one of their encoders. Programming software is $395.
Or you could go with a Productivity 2000 PLC from Automation Direct. PLC costs more, and is a lot better, but software is free.

How do you plan on converting the linear motion of the saw head to rotary motion for the encoder?

How much total stroke?

I second the MTS Temposonics, they are tough. I would feed the transducer output to an analog input on the PLC. Overall stroke would determine the resolution in this case.

Steve Bailey said:

A lot depends on how long you're willing to wait to complete a positioning move. If you can use a flow control valve to slow the flow of hydraulic fluid you may be able to get the accuracy you require by simply monitoring the position and turning off the solenoid controlling movement when you get close to the target position. You may have to tweak the flow control valve in the course of a day's work to compensate for changes in the temperature of the hydraulic fluid or buildup of sawdust or other debris in the mechanism.
To get a faster response you could use "coarse" and "fine" valves, each with different flow control settings. Start motion with the coarse valve open and switch to the fine valve when you get close to the target position. Then shut off the fine valve as described above.

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This is how we did it in the late 70s and early 80s.
In this case I would recommend my competitor ( sometimes they buy our controller when forced to )
http://www.pawtaw.com/
They can make a complete control system for you. Chances are they already have something put together to do exactly the OP wants.

A good positioning system will cut the wood with less waste and faster. If the wood is cut too thick the planer mill will plane off the excess wood. If it is too thin then the wood can't be sold for the desired dimension.

People under estimate the difficulty in controlling a hydraulic cylinder well. Often they just get it to work somewhat and put up with the short comings.

For faster response and improved repeatability, use a servo valve. That allows you to change the flow rate while the axis is moving. As you approach the target position reduce the flow rate so that you arrive at zero flow when you arrive at the target position. A PLC can recalculate the difference between the actual position and the target position every few milliseconds. A motion controller can recalculate more frequently as well as being able to use a more sophisticated control calculation.

Click to expand...

Steve, do you know that a hydraulic cylinder is limited in how it can respond based on the natural frequency and damping factor? If a PI or PID is optimally tuned the best the one can do is reduce the error as a function of exp(-damping_factor*natural_frequency*time/2). There is NOTHING one can do to improve on that using normal control methods. That means if the hydraulics and or mechanical design results in low numbers in the natural frequency or damping factor the PLC programmer is simply screwed. This is why i have little respect for most hydraulic and mechanical designers. There are algorithms that allow the control person to exceed the limitations of normal PI and PID control.

Steve, do you know that a hydraulic cylinder is limited in how it can respond based on the natural frequency and damping factor? If a PI or PID is optimally tuned the best the one can do is reduce the error as a function of exp(-damping_factor*natural_frequency*time/2). There is NOTHING one can do to improve on that using normal control methods. That means if the hydraulics and or mechanical design results in low numbers in the natural frequency or damping factor the PLC programmer is simply screwed.

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No, I didn't know that. But then, that's your life's work, not mine.
I'm glad you've joined the discussion on this thread. Now the OP can get an idea of what can be done with a top-of-the-line control system and make an informed decision about how sophisticated a control system he wants to implement.

KB1GNI said:

How do you plan on converting the linear motion of the saw head to rotary motion for the encoder?

How much total stroke?

I second the MTS Temposonics, they are tough. I would feed the transducer output to an analog input on the PLC. Overall stroke would determine the resolution in this case.

Click to expand...

There is a roller chain attached to the frame and it goes around a sprocket attached to the encoder. When the head moves up and down the chain is stationary but the encoder moves with the head thereby turning as the head moves. Stroke is 24".

Steve Bailey said:

I'm glad you've joined the discussion on this thread. Now the OP can get an idea of what can be done with a top-of-the-line control system and make an informed decision about how sophisticated a control system he wants to implement.

Click to expand...

The OP doesn't need a top of the line control system but he does need one that is good enough. I could do the control in the PLC but I have a lot of experience at this and that makes a HUGE difference. The application isn't that difficult. The problem is that the gains are different going up and going down. The PLC has no target generator.
I just got back from China. Over there they use simple PID control and limit the output to get the speed limit. This way they don't need a target generator. Feed forwards aren't commonly understood or used.

An RMC75S-QA1 would probably be overkill, but I could get it running in just a few minutes.
http://deltamotion.com/products/motion/rmc70/
The PawTawJohn control relies on using a MDT so that is out.

There is a sawmill trade show in Portland this week.