New at PLC please help

Hello
This is the first time i am even doing PLC programming.
I am basically trying to ocntrol a proportional valve whihc takes in a continuous input of 4-20 mAmp. Thus at 4 mAmp it is completely closed and at 20mAmp it is fully open.
For the input i have a temperature sensor, downstream of the valve where the stream is mixing with a hot water stream. now i need the valve to close or open to change th emixing temperature according to a curve i have.
Well i just need to know how to begin. i have an idea however i do not know how to get the results in a aprticular amount of time since the curve is strictly time dependant.
Any help would be highly appreciated.
Thank you

You don't need to do this with a PLC, you can use a stand alone instrument with built in PID manufactured specifically for this function.

Hello
My boss wants to buy a PLC to do this. I cant figure out how to program a curve into a PLC.
ALso jsut as an option, where would i get a quote on such a PID controller?

I hate to say this but you've stepped into some advanced programming concepts for a beginner trying to program a PLC while the concept of what you are trying to do sounds simple the reality is not for the faint of heart. Essentially your going to have to program an analog input for the temperature senser. This is a value that your going to need to scale the range (for this we will need the make and model of the PLC, type of temp senser). then your going to need an analog output module to control the pneumatic valve.(any info you can provide on that would be helpful. This is where it gets worse, the other step is your going to need to set up a PID control to control the time based rate of the pneumatic valve. I would do a search on threads by Ron Beauford he's posted a well written explanation of PID (its in several sections). The amount of innformation were going to need for this is going to be difficult to obtain ( rate of loss of temperature, open or closed loop system, etc there is more but there are better experts here on PID than myself

edit using a PID controller as suggested by the other would be far easier to setup

http://www.trerice.com/pdfs/Regulator/TR890_Series_206-207.pdf

http://www.trerice.com/pdfs/Regulator/910T_3Way_Valve_Stainless_Steel_238.pdf

http://www.trerice.com/pdfs/Regulator/IP_Transducer_TA901_216.pdf


I work in a poultry processing plant, we use this for our scalders and high pressure pumps. The life of the valve is about 5 years, we have never had one of the controlers fail. The thermocouple needs to be at least 10 feet down stream if mounted in the pipe. The scalders are held at 131.5 degrees with 140 birds a minute going through them, with no problem on accuracy.

Lots of options and probably cheaper and easier than using a PLC.

You would only really need a PLC if you wanted to do other functions as well as the temperature control.

Then you would need to additionally buy the programming software, learn to program and commission the software yourself.
All PID controllers have the software built in and most have self learning functions (autotune).

Some examples

National Instruments

Siemens

TDS

Newport

Red Lion


and there will be a whole lot more..

In case you boss absolutely wants a PLC here is one article more to follow
http://www.plctalk.net/qanda/attachment.php?attachmentid=6103

and a pictorial description with examples of calculations

here

plus another controller I used to use a lot when I wa sin the food business

Eurotherm

Ok this does not sound too good.

Well the things is im going to be using the PLC to control a VFD for an inverter motor as well.
There will also be a differential pressue monitor and flow rate monitor.
A PLC is preferred since it can be put to use in other projects as well. We're a small R&D and product validation center here, so that is important.
Is this PID loop programmable within the PLC.
Were most probably going to be using a EZPLC touch PLC board from EZAutomation. model number is EZPLC--A-64E. I dont even know if the PLC is capable of allt his. i really am a begginer and i have to learn fast. I'll probably need to read the article 10 more times to make head and tail out of it.

one article from Ron

http://www.plctalk.net/qanda/showthread.php?t=11969

he has two other section one was what is "I" in pid
the other was I beleive what is "P" still searching for the other two

Well have a copy of the files however to large to post here so will have to search Ron's threads PS I misspelled his name its Ron beaufort

Well i have some idea about PID control. Im a co-op student and did control theory last semster but then they dont really go into too much detial. we made a matlab program to control an azimuth radar system and minimize the transient response. Even then, my knowledge is not very indepth

I just talked to my boss. Just to add to the complexity and the burden on my life , there might be thre temperature sensors, one in the cold pipe, one in the hot pipe and one in the mixed pipe to create a time dependant temperature profile. then the output needs to be on two valves which are complimentary to each other so that the downstream flow rate is constant.
thus he wants to input all three temp inputs so as to be more accurate.
I am however strongy going to suggest use of a PID controller to do this.

Hello Wirepuller
THe specs for the controller says that input is multi (switchable). Does this mean it can take in multiple inputs at the same time?

Hi Morded
Well i can calculate the rate of heat loss. Estimating that wont be too much of aproblem. the link for the PLC Manufacturer is as follows:
http://www.ezautomation.net/index.php

the temp sensor is by omega
TH-44004-1/4NPT-80
http://www.omega.com/toc_asp/frameset.html?book=Temperature&file=TH-44000-NPT

i can attach the valve information link
http://www.ascovalve.com/common/pdffiles/product/8290_v7393r3.pdf
http://www.ascovalve.com/common/pdffiles/product/8290r1.pdf

Never used an EZPLC, its certainly cheap and cheerful, which does not make it bad, maybe value for money.

Other more expensive PLC's would offer PID instructions or blocks.

Of course a lot more expensive, but here you would have to write your own PID control from scratch which isn't a simple undertaking, as I cannot see a PID instruction in their documentation.

It may still be cost effective and easier to get a PID controller as well as the EZPLC. Use the PLC to send setpoints and control the other things.