fine motion of piston (0.1 mm) 2

[menzulov]

I am a beginner at control system.

Now I design log carriage for vertical saw band. There is 3 independent hydraulic cylinders, that will be travel synchronously. Sometimes the cylinder will be travel for desired value (tolerance around 0.1 mm).
How obtain this properties?

I think, that for this purpores it desire proportioning valve, transducer and control system.

Recommend to me some books for this questions (links). Maybe somebody has done example of this solution.

I am sorry for my bad english.

 

[IceStationZebra]

I don't have any direct experience with type of application, but I can say that if your tolerance is 0.1mm maximum then your measuring system will probably need to be able to measure 0.01-0.02mm. ISZ

 

[PNachtwey]

Now I design log carriage for vertical saw band. There is 3 independent hydraulic cylinders, that will be travel synchronously.

That is odd ( pun ). Usually band saws come in pairs.

Sometimes the cylinder will be travel for desired value (tolerance around 0.1 mm).

That is easy.

How obtain this properties?

I think, that for this purpores it desire proportioning valve, transducer and control system.

Yes, you need a servo valve or a proportional valve with a servo quality spool. Moog, Bosch-Rexroth, or Atos make good valves. The standard way to get feed back in saw mill applications if from a magneto-strictive displacement transducer or MDT for short. These are inserted into the back of the cylinder.

http://www.deltamotion.com/pdf/fluid power basics.pdf

Look at the diagram in the .pdf. You don't need the pressure feedback. Note, valves should be mounted on the top of the cylinder because this minimizes the trapped volume of oil between the valve and the piston.

MDT rods can have one of 4 basic interfaces. Analog, PWM or gated, start stop and SSI. SSI is the most accurate and should be your choice. It is easy to get 5 micron resolution.

The controls are easy. Rockwell has M02AS modules that fit in the Control Logix PLC. Another option is our own controllers which can interface to other brands of PLCs as well as Rockwell's.

http://www.deltamotion.com/products/motion/

Synchronizing axes is easy with now with modern multi axis motion controllers.

The hardest and most important part is choosing the diameter of the cylinder. The diameter, mass and trapped volume of oil determine the natural frequency of the system. The higher the better but like all things in engineering there are price/performance trade offs to be made.

There are system integrators that can install a simple system like this very easily. The integrators can get the right hydraulic designers involved. If you are going to do this on your own I would at least get some training because it will save a lot of time and money in the end.

More reading. I have lots of articles and movies.

http://www.deltamotion.com/pdf/magazine_servohydraulic_sizing_guidelines.pdf

I sell the hydraulic motion controllers. Out of necessity I have had to learn a lot about hydraulics.

Peter Nachtwey
Delta Computer Systems, Inc.

 

[hydromech]

Peter

That was a very comprehensive and helpful response, but I fear that Menzulov may need therapy after he reads it.

It was very good response none the less.

Regards

Adrian

 

[ScottyUK]

Peter,

Are repeaters available for the SSI interface to allow transmission over a distance of (say) 150m?I have looked and come up blank in the past - we used analogue 4-20mA because of distance between the controller and the actuator. It would be useful to know for the future. We would have had to go analogue in the end to interface to the controller (no SSI protocol in the days this was built) but we could have avoided some noise issues by keeping the signal in the digital domain until the signal was in an electrically quiet environment.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[PNachtwey]

Are repeaters available for the SSI interface to allow transmission over a distance of (say) 150m?

Normally one just reduces the clocking frequency. We have the data in the on-line help for our motion controller. It says the clocking frequency should be about 150Hz for 400-600 ft. The documentation also says that this is just an estimate because different cables have different capacitance. I always recommend that you use what the MTS or Balluff documentations says to use. Usually this is a 22 gauge wire.

There are repeaters, we sell them but they do nothing for the capacitance of the wire. One just slows down the clocking frequency as the capacitance goes up. I know we have control gates on locks where the distance was over 1000 ft. There were no problems. One just pays close attention to the wiring.

That was a very comprehensive and helpful response

Actually it just scratched the surface. I stopped where I thought therapy wasn't going to be needed.

BTW, if you guys ever have control issues just post. There is a lot of information on our website. Some of it is marketing but there are application articles.

http://www.deltamotion.com/applications/magazine.php

Something to read if you are stuck in a airport or airplane or want to get to sleep quickly.

If your brain doesn't hurt yet.............
ftp://ftp.deltacompsys.com/public/PDF/Mathcad%20-%20Natural%20Frequency.pdf
The .pdf files are a lot more computationally oriented. There are other files that you may find interesting in the same directory.

Peter Nachtwey

 

[menzulov]

Dear sirs,


thank you for your answers.


Peter, can you tell me in detail, why valves should be mounted on the top of the cylinder? How affect trapped volume of oil on operation of control system?
During treatment, log will become crooked. It causes by internal force. Therefore 2 support point is insufficient. It require support point in centre log lenght.


Adrian, you are right. Therapy will consist of reading books and discussing this questions on forums. I will glad to your assistance.


I am from Russia. There are many books about control system, but all books describe only theory. I have very inportent question: how to pass from theory to practice?
Let us suppose, I was calculate parameters of control system, got hydraulic cylinders, proportioning valves, transducers (e.g.

http://www.givimisure.com/cataloghi/mts_mp200.pdf).

I will need to chose controller, digital readouts, data input device. I will need to program controller. How it makes? Sequence of operations? Are there books for this problem?

 

[PNachtwey]

Peter, can you tell me in detail, why valves should be mounted on the top of the cylinder? How affect trapped volume of oil on operation of control system?

Oil must be compressed to increase the pressure and the resulting force and acceleration. The more oil between the valve and the piston the harder it is to increase or decrease the pressure. Think of the oil like a spring. The longer the spring the more springy it will be and the harder it will be to accurately control a mass on the end of the spring. Look through all the data for natural frequency.

During treatment, log will become crooked. It causes by internal force. Therefore 2 support point is insufficient. It require support point in centre log lenght.

OK, I see. You are holding the log. I was thinking that the axes move the band saws. Normally this is a left and right band saw. Mills that cut bigger logs have two left and two right vertical band saws.

I am from Russia. There are many books about control system, but all books describe only theory. I have very inportent question: how to pass from theory to practice?

With practice! Getting training will be a big help. I can't imagine putting a system together without any help. We I am there is sort of an apprenticeship that engineers go through before being turned loose on their own. I provide training for many of the saw mill OEMs and but am in the Pacific NorthWest in the US. Northwest US and Canada have lots of sawmills and system integrators to keep them going. If you need a system integrator then let me know but you have to be serious about it.

In the US and Canada the standard saw mill PLC is the Rockwell Control Logix 5000. There are two main motion controllers used. The Rockwell M02AS and the Delta Computer Systems, RMC100. Most use the RMC100. There is a new RMC150 but most of the system integrators are just starting to use the newer controller.

These modern controller have auto tuning and will interface directly to Balluff or Temposonic MDT rods. Temposonic MDT rods are standard too with Balluff a distant second. Both the M02AS and the RMCs output +/- 10 volts to drive the valve. The motion commands are relatively simple. A move command consist of a position, velocity, acceleration, deceleration. The motion controller uses this information to generate a motion profile. If you issue the same motion profile to two or more axes at the same time then all of the axes will move together. Each axis will follow its own target but all the targets will be the same. The target position get updated every millisecond or faster. The PID compares the actual position from the MDT feedback with the target position. If the PID and feed forwards are tuned correctly then the actual position will always be very close to the target positions.

After you get the system scaled and the limits set you tune or auto tune the system. Then you try issuing a command with the position velocity and acceleration.

Peter, can you tell me in detail, why valves should be mounted on the top of the cylinder? How affect trapped volume of oil on operation of control system?

Oil must be compressed to increase the pressure and the resulting force and acceleration. The more oil between the valve and the piston the harder it is to increase or decrease the pressure. Think of the oil like a spring. The longer the spring the more springy it will be and the harder it will be to accurately control a mass on the end of the spring. Look through all the data for natural frequency.

During treatment, log will become crooked. It causes by internal force. Therefore 2 support point is insufficient. It require support point in centre log lenght.

OK, I see. You are holding the log. I was thinking that the axes move the band saws. Normally this is a left and right band saw. Mills that cut bigger logs have two left and two right vertical band saws.

I am from Russia. There are many books about control system, but all books describe only theory. I have very inportent question: how to pass from theory to practice?

With practice! Getting training will be a big help. I can't imagine putting a system together without any help. We I am there is sort of an apprenticeship that engineers go through before being turned loose on their own. I provide training for many of the saw mill OEMs and but am in the Pacific NorthWest in the US. Northwest US and Canada have lots of sawmills and system integrators to keep them going. If you need a system integrator then let me know but you have to be serious about it.

In the US and Canada the standard saw mill PLC is the Rockwell Control Logix 5000. There are two main motion controllers used. The Rockwell M02AS and the Delta Computer Systems, RMC100. Most use the RMC100. There is a new RMC150 but most of the system integrators are just starting to use the newer controller.

These modern controller have auto tuning and will interface directly to Balluff or Temposonic MDT rods. Temposonic MDT rods are standard too with Balluff a distant second. Both the M02AS and the RMCs output +/- 10 volts to drive the valve. After you get the system scaled and the limits set you tune or auto tune the system. Then you try issuing a command with the position velocity and acceleration. The following link is the startup guide for the RMC150.

http://www.deltamotion.com/pdf/rmc150startup.pdf

The motion commands are relatively simple. A move command consist of a position, velocity, acceleration, deceleration. The motion controller uses this information to generate a motion profile. If you issue the same motion profile to two or more axes at the same time then all of the axes will move together. Each axis will follow its own target but all the targets will be the same. The target position get updated every millisecond or faster. The PID compares the actual position from the MDT feedback with the target position. If the PID and feed forwards are tuned correctly then the actual position will always be very close to the target positions. There are status bits that can be monitored that tells you when the axis is in position or when an error occurs.