Control of Force for a test facility

Hello everyone, I have a question concerning the control of a force in a test facility. First I will explain what exactly is the task.
There is a pneumatic cylinder that is controlled via a proportional valve. This pneumatic cylinder is supposed to press a pedal with a certain force (up to a maximum force of 500N), then holding the pedal for 2 seconds in that position and then releasing the pedal again (cylinder moves back in its original position). This is repeated exactly every 10 seconds. The force exerted on the pedal can be measured with a force sensor. As far as I know the set up has a proportional valve (to set the right pressure that actuates the cylinder) and a switching valve to switch the pressure from the proportional valve to the pneumatic cylinder and to release the cylinder again.
I don?t really know what the best way is to control this thing. I was thinking about a PID controller, but maybe there is something simpler or something better. Also I don?t exactly know how to manage the switching. Should the switching valve be inside the control cycle? Or should I control a constant force and measure only at those times when the force is applied? The switching could be done independently of that. But I really don?t know what is the best way. I would appreciate any hints you have for me. Sincerely, Walter
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Walter Mller wrote:

What do you mean by "proportional valve"? Do you mean a pressure regulator, or do you just mean that there's a valve from some high-pressure supply who's opening you control? It sounds like a pressure regulator from your description of the behavior.
If the system has good repeatability from one pedal press to the next, and it never varies, then you don't need closed-loop control at all -- you just need to find the right regulator setting, then spend your effort sequencing the switching.
If the system has good repeatability from one pedal press to the next but it slowly varies over time then you may be able to get by with measuring the pressure of each pedal press and using the information collected to update the strength of the next pedal press. This will end up being a sampled time system, with one control system iteration each pedal press. This would probably be implemented by something that you could describe as a "PID controller", although just using something called a "PID controller" wouldn't necessarily lead to success.
If the system acts differently each and every time the pedal is pressed then you'll need some sort of closed-loop control that acts _while_ the pedal is being pressed. This will be difficult to pull off, because you're going to want something that acts fairly quickly, you probably don't want any overshoot, and you probably _do_ want high precision. You could probably make this work, but I suspect that the best pneumatic architecture* would be a truly proportional valve, possibly from a regulated supply, with a bit of bleed available during the down stroke to allow the gas pressure to bleed off after an overshoot. This would all have to be closed by a controller containing a PID algorithm that's had lots of work done on it.
* But I'm not an experienced pneumaticist, as someone will no doubt point out.
--

Tim Wescott
Wescott Design Services
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Tim Wescott schrieb:

Hi Tim, thanks for your detailed answer. It will take me some time to figure everything out that you just wrote. By proportional valve I mean a valve that can set the pressure depending on the supply voltage (which can be varied). So it is some sort of pressure regulator. I think the second case that you mentioned is true: The repeatability should be quite good from one press to the next, but since the setup us run for a period of a couple of days or even weeks, the force could slowly vary over time. That?s why I think a closed loop control would be the best thing... I?ll come back to what you wrote later. Greeting, Walter
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Walter Mller wrote:

The force won't vary if the cylinder doesn't seize from lack of lubrication and the pressure is held constant. Depending on the upstream pressure, a one- or two-stage regulator might be needed to meet the pressure tolerance specification. http://www.fisherregulators.com /
Jerry
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Tim Wescott wrote:
...

I've done several projects with pneumatic actuators, and I can tell you that they're very difficult to control. Air is springy, so close positioning is out. (Using position feedback even with very little stick-slip friction in the cylinder almost guarantees overshoot. In a few cases, I changes over to hydrolics,in most of them using most of the original pneumatic components. In another case, I replaced the pneumatics with a linear actuator consisting of a hollow-shaft stepper and a leadscrew. (I recently mailed the unused spare to a student.)
Walter might well have success with regulated pressure to produce the needed force, and a needle valve (or other flow restricter) so that the cylinder moves slowly enough to avoid overpressure from inertia if the miving parts. Then a simple 4-way shuttle valve is all the control he will need.
Jerry
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Walter Mller wrote:

You can control position or force but not both at the same time. You can limit force and position at the same time.
If you get to 500N and try to stay at that postion, what do you do if the force changes due to leaks or the pedal compressiing or moving. Do you maintain position or maintain force? You need to define what your needs are a little better. It is possible to go to a position or a force, what ever is reached first.
The pedal better be able to provide 500 N of opposing force. If you are controlling only force. If you are force/pressure limiting the actuator will apply upto 500N of force until it reaches the position set point. Position/Force limiting is what I think you want to do.

This is repeated exactly

A switching valve is not necessary if you are using a proportional valve because it can let the air in or out. One proportional valve can control position or force or limit both at the same time. You want a valve with a zero over lapped spool.

http://www.deltamotion.com/products/motion/rmc70 / would do nicely. It has an ethernet interface so you can upload the recorded data. Our product has be used in many test systems.

Don't use the 'switching' valve.

Peter Nachtwey
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Peter Nachtwey schrieb:

That?s right. Actually I only want to control the force.

Yes. The scenario is as follows: the cylinder presses the pedal until the pedal provides 500N of opposing force. The position where this force is reached is not important. Although for security reasons there should be a position limit, but I think this is already provided by the pneumatic cylinder.

I didn?t quite understand what you were trying to say. I just thought that?s the way tasks like this are usually done: one proportional valve to control the pressure and one switching valve to let the air pass to the cylinder and another one to release the cylinder again. What exactly means "a zero over lapped spool"? Thanks in advance, Walter
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Walter Mller wrote:
...

A proportional valve allows flow to both ends of the cylinder, depending on the position of the spool. To avoid a dead zone, some spools allow pressure to both ends simultaneously; the openings to the ports overlap. A non-overlapped spool is the equivalent of a break-before-make double pole switch, the cylinder being immobilized when the spool is at center stroke because all ports are closed. The spool valve functions like the slide valve in a steam engine. (As a matter of fact, I built a spool valve rather than the traditional slide valve into a 1/4 Hp steam engine I built because the balanced design eliminates friction due to pressure, and because the circumference of the spool can easily exceed the widest conceivable slide valve.)
Jerry
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