Ok, i'm new enough so apologies if this is so basic but please do respond if you know! I must set up an electro-pneumatic postioning system (done) and control it remotely using Siemens S7 and SCADA (WinCC). My main question is how do you actually change the pneumatic signals to a "controlled" signal on the PLC. Sorry if it sounds confusing... it's because I am confused.

 

This sounds like and I-P application, where I-P is I (Current) to P (Pressure). There are several devices on the market to do this conversion. For instance Fischer makes an I-P device. Basically, the I-P device recieves a 4-20ma signal and positions a regulator device to a calibrated pressure based on the current input. For instance, if the desired output pressure was the range 3-15psi, when the current signal was at 4ma the pressure output would be 3 psi where when the current input is at 20ma the pressure output would be 15 psi. Between 4 and 20ma the pressure output would be proportional to the current input signal. I-P regulators are commonly found in the ranges of 3-15psi, 6-30psi but other ranges are available, and most devices can be calibrated to other ranges, within limits.

On the other end of the spectrum. Other devices, known as transmitters, have a pressure input and convert the pressure to a proportional current output.

Hope this helps. If a better explanatin is needed or this is not what you were asking, contact me at [email protected] if you think I could help you.

David Farris
[email protected]

 

send the pneumatic signals via a pressure switch if digital - or an p/i converter if analogue and then connect these signals into the plc.

 

I know nothing about Siemens PLCs but if you have a pneumatic signal and you need to get it into an electronic type control you will need a P/I. A pressure to current converter the opposite of the I/P.

 

Dear Daryl,

I cant seem to understand what you mean by "how do you actually change the pneumatic signals to a "controlled" signal on the PLC".

Interfaces between pneumatic and electrical systems are done with devices like "i/p (current to pressure) converters, pneumatic pistons or solenoid valves" for analog and digital outputs respectively. Devices like
"valve positioners (linear feedback), pressure transmitters and pressure switches" are used respectively for analog and digital inputs respectively. It really depends on which physical parameter you're trying to
measure/monitor and consequently control.

Hope this helps.

Tomy Zacharia

 

Not sure what your pneumatic signal is. Three to fifteen PSI is an industry standard. This signal is converted to an analog 4-20 mA signal with a transmitter called a P to I or P/I (pressure to current)transducer. Other signal pressure spans and electrical signal spans are available. The net should show you many brands of these devices.

 

PID algorithm in S7 software

PID ---> Analog Output [ 4-20 ma dc , 0-10vdc ]

4- 20 madc -----> Pressure signal.

This PID algorithm would change the output[control] current output/pneumatic signal/position.

Few years back in marketing the reply given to
this question " Does microprocessor based control
give pneumatic output , YES provided you couple
with I/P converter.

 

thanks for posting...

The pneumatic signal is just a constant 2Bar (25psi) but as i said this is constant. Using various diiferant types of sensors the system (positioning system) is controlled and is fully automated on site. Problem is how to control it remotely, do i have to solder on connections to a RTU (PLC) and use an external modem to transmit a phone signal or can i use a simple 2-core cable (i.e E34972)?

 

But the signal is a digital signal. A P/I converter can't really be used, can it...

 

OK i think we need to clarify a couple of points to help with the design here.

when you say remotely are you talking a few meters or a few miles. ie another location on site or another site all together.

It is unclear wether you need to control the positioning system with an analogue or digital type signal. What are you trying to tell the positioning system to do here. Simply move from one extreme to the other (open/close?) or move to a desired position over within a set range (positional or modulating type control)

I gather that an electro pnuematic positioner is already used on the site and this is receiving a signal from some electrical device and converting it to a pneumatic signal for the valve or whatever is positioned. Is this correct.

Is the PLC already used in this system or are you looking to add one to acheive your goal?

What is your area of experience (this is not meant to be derogatory, only so that we can answer your question in terms you will understand)