Position Control of a Pneumatic Actuator

[jmc241]

I am looking for a method to accurately control the position of a pneumatic actuator. The selected approach must be integrated in a closed loop control system (using actuator position as the the feedback signal).

So far I have found three possible methods:

#1) A commercial proportional servo-pneumatic system (Enfield, Textron, etc.) with the electronics integrated with the valve body. (This tight integration of the electronic and mechanical parts is undesirable for my application.)

#2) A configuration that uses a 3/2 solenoid valve connected to each side of the actuator. Pulse width
modulation (PWM) is used to adjust the duty cycle of each solenoid valve and raise/lower the pressure in each cavity. This approach looks more promising than #1 for our application

#3) A configuration that uses a 5/3 double solenoid valve. The literature seems to indicate that these valves are specifically made for actuator position control. However, I can't find information about the control methodology for these valves. I understand the basic function of the valve, but not how it would be integrated into some higher level control logic.

My questions are:
a) What's the best way to integrate #3 into a closed-loop control system?

b) What are the pros & cons of different options for the center position of a 5/3 valve (open center, closed center, pressurized center)?

c) What are the advantages/disadvantages of Options #2 (two 3/2 valves) and #3 (a 5/3 valve)?

d)Are there any other options available for my application? My main constraints are:
* 24 VDC power
* 100-150 psig supply gas (helium)
* Control electronics must be separated from the
mechanical portion of the valve.
* Integrate into a closed-loop control system

Thanks for your help.

JMC241

 

[tsenthil]

You can use a positioner/Position transmitter combination to control the position of Pneumatic Actuator. Normally Positioner like Siemens PS2 or Fisher DVC2000 or Metso ND9000 are available with inbuilt position transmitter which will give you 4-20 mA output related to your Actuator position.

 

[israelkk]

Is this for aerospace/aircraft use? I assume it is otherwise why use Helium?

If it is for aerospace use I fail to see why use 100-150psi for aerospace use where weight and size are greatly matter. The main advantage of pneumatic actuator for aerospace/aircraft use is the ability to concentrate high load actuators in a small space and light weight i.e. high pressure and small actuators.

The drawback is that industrial valves will not fit and custom designed valves are needed. But if weight and space are important then custom designed valve will be much smaller and faster than industrial valve. Not to mention that they will fit for the harsh environment of aerospace where industrial valve will not.

 

[Compositepro]

I've used Moore pneumatic controls with good results. I don't if it would be suitable for helium though.

http://download.a-tcontrols.com/pdf/IOM/2005.PDF

 

[swyw]

It seems to me that you are very worry about electronic stuff. Perhaps there is some potential danger of explosion. If this is the case a pneumatic-pneumatic positioner may be your best choice. There is some commerically available spool valve that may be used for feedback control.

My employer sell these kind of devices. Unfortunately this forum do not allow any commerical activities. All I can tell you is I work for a German industrial automation device manufacturer and some of our competitors in Germany also make similar products.

All the best.

 

[bheart790]

I think a electrical-pneumatic positioner will be fit to you, The positioner can accept 4-20mA input signal,and feedback 4-20mA position signal, it is easy to design a close loop systerm,and the solenoid will be not necessary.

 

[gerhardl]

All suggestions above seems sensible, but depending of what you actually are trying to do. Size? One off or series, or mass product? Accuracy required? Speed of adjustment important? Loss of air/helium significant? Lifetime/number of cycles requirement?

On your original question: a 3/2- way valve is cheaper, more redily comercially available in a wide range of sizes and qualities, and ready-made solutions does exist.

A 5/3 more limited in varities and sizes, but exists from several suppliers, generally the better the quality the higher the price.

If 5/3 I would recommend mid position all ports closed to stabilize and lock the air on the one side of the actuator while increasing pressure on the other, then locking this side nand let out on the other a bit and so on.

A 3/2 valve will have two positions: either letting air in or out of the given cavity connected to the solenoid valve.

To lock the cavity you will have to use two 3/2 way valves on one side. One for inlet/outlet, the other (or in stead a 2/2) for locking or opening the outlet of the first valve.

The commercial solutions incorporate a rather delicate balancing logic, I presume, and it would best to buy this, as a program, if possible.

I would suggest direct acting, not pilot operated solenoid valves, to minimize operational loss of helium, with good solenoid valve throughlet/capacity to have sufficient adjustment speed and capacity, and if necessary additional throttle valves on connection lines (where allowed seen from solenoid operational limitis).

Are you sure a pure electrical solution is not available on the market, covering your needs?

 

[ScottyUK]

Have you looked at, for example, the Fisher Fieldvue range of electro-pneumatic positioners for the process industries? They're primarily used for valves but could reasonably be fitted to many linear or rotary actuator designs.

Quantify the following if possible:

How much separation do you require between your actuator and the controller?
What positional accuracy are you looking for?
Are you looking for a full closed loop controller, or to integrate this in to an existing one?
How big (or small) do you need this device to be?


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