Pneumatic to Electronic Control

The law sez that if you change from pneumantic to electronic in a control circuit, you have to rip out tubing and install wires. ;^)

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Honestly, your question is too broad and too vague. No one here is going to write you a thesis on laws and theories. The best I can do is tell you that properly configured, 3 to 15 psi in gets 4 to 20 mA out, in most cases. Foxboro, Rosemount, Endress + Hauser and hundreds of others can sell you the magic box to do the deed.

And if you want to send that 4-20 mA signal without having to run wire, you can do it remotely using radio frequency (RF). www.remotecontroltech.com/Product/AnalogWireless.aspx

1 Fully understand what the pneumatic circuit is doing.

2. Fully understand what needs to be done (this may not be the same thing, as compromises are often made when designing things for cost or safety or laziness).

3. Implement what needs to be done as an electrical system.

Sorry to be vague, but it's an open question. In general, use something flexible like a PLC, connectall the inputs/data you can, then connect all the outputs/control devices, then write a structured control program. It's usually cheaper to do it this way than start from a minimum system and have to add bits. Jeff

Dear Sir,

If the circuit is simple, only a few switches and I/O which does not justify a PLC, then simply use micro-switches and relays.

To my thinking, you are using pneumatic logic controls such as AND and OR gates, Flip-Flops and so on. These are the past generation system which is compelled by the safety restrictions of the environment. The machine which these pneumatic logic control sensors inputs and outputs may be in explosive environment, which is classified as Explosive environment, Class 1 Div. 1 or 2. So, if you want to convert to electrical system, you must provide the barriers for the electrical signals in order to comply to the safety requirement of the hazardous environment.

Are we talking in the same wavelength?

Tijit

From 3-15psi to 4-20mA, use a P/I converter.

From on/off pneumantic to on/off electric use a pressure switch.

Pneumatic controllers can be replaced with electronic controllers, though any potentially explosive atmosphere hazardous area implications need to be complied with under BS EN 60079. If the final control element is pneumatic then an I/P converter will be needed also.

There are such things as pneumatic to electrical transducers as well as electrical to pneumatic transducers. You can find them on Globalspec.com.

FESTO is one of the largest companies in the field of pneumatics. You may want to read about their pneumatic controls and electronics on www.motioncontrol.com . When you're ready to ask a specific question(s) of them you should visit www.festo-usa.com . Good luck!

Do you mean "electronic" or "digital control". Electronic controls are almost as dead as pneumatic.

For going from pneumatic to electric, start with the basic formula P=F*A. Now think of the different ways you can detect the force, such as strain where you can use a wheatstone bridge, Piezo Resisitive Technologies (PRT), Anderson loop, or capacitive sensing where force deflects a diaphragm. You can also detect displacement from force, such as the movement of a plunger through a coil.

These are technologies that exist today with their advantages and disadvantages, but they all were derived by the formulas that apply to pneumatics. Then determine the types of electronic circuits that are capable of detecting the variables within the formulas.

Sometimes they may not be obvious. I worked with an inventor of a device that would electronically measure viscosity for a customer that builds diesel engines. If you looked at the formulas that define viscosity, it appeared that its mainly a mechanical function through friction. Well he used a PRT design, where the oscillation of a crystal was affected by the changes in viscosity. The customer dropped the effort due to the cost to manufacture, but later the inventor sold the idea to Mercedes, who had Lucas Engineering take over the project. The idea worked very well and if they were able to reduce the cost of growing the crystals and some of the processing issues it might be in vehicles today.

Iam unable to understand, What exactly your doubt is. In the vast field of process automation, pneumatic signals are used to operate pneumatically operated mechanical objects & to create motion controls (CNC). Pneumatic signals are controlled by electrical signals generated by electronic modules.

Anyway with a small example, I will explain things to keep in mind while designing a process control automation.

- First have clear views in, what is the process to get controlled.

- Then List out following:

1.Process parameter sensors, 2.Final control objects, 3.Input signals, 4.Interface module, 5.Out put signals to activate Final control objects, 6.Feed back signals . 7.Type of process controller.

For Example, Assume thet you want to control temperature of a liquid filled in a jacketed tank. Inlet & outlet of tank jacket is connected to steam pipeline from boiler. Here steam is the indirect media used to heat the liquid in tank. So to control the temperature of liquid you must tune the flow of steam w.r.t time & temperature setpoint.

Now from above process, summerize the following:

1.Process parameter sensors: RTD or Thermocouple.

2.Final control objects : Pneumatically controlled valve which operates with 3-15 psi pressure varied via electrical signal of 4-20 mA.

3.Input signals : 4-20 mA (i.e., o/p of signal from sensor via signal convertor)

4.Interface module : Signal convertor with RTD or Thermocouple I/P & 4-20 mA O/P.

5.Out put signals to activate Final control objects : 4-20 mA (i.e., O/P signal from signal convertor)

6.Feed back signals : 4-20 mA (O/P signal from Control valve)

7.Type of process controller : PID controller or PLC as per your wish

Now do the installations, design a closed loop interconnecting above all above, set the temperature set value & after observations tune the PID settings & stabilise the system.

Regards,

Vinod.P, India.

Encl: Diagram of closed loop attached below: