Using 4 to 20ma output transducers w/h PLC

[Katy's Super tech]

I am new to programming and not very knowlageable about PLC capabilitys. What I am wondering is can one program set points to turn on and off outputs with a plc or would a separate controler be needed that the PLC would tranfer the information from the sensor to and then outputs from the second source controler routed back to the plc. I would like to utalize a single transducer,pressure,to turn on outputs which will load and unload one or more compressors basicly replacing multiple pressure switches with a plc connected to a single transducer any help in this matter would be greatly appriciated. I am trying to build a sequencer which would tranfer lead compressor at a preset timed interval and dependant on the pressure bring online lag or secondary compressor if the current output of the transducer drops to a certain leval as well as of course as loading and unloading the lead compressor with a higher output. I hope this makes sence. Please Email reply to [email protected] thanks

 

[leitmotif]

I am new to programming and not very knowlageable about PLC capabilitys. What I am wondering is can one program set points to turn on and off outputs with a plc or would a separate controler be needed that the PLC would tranfer the information from the sensor to and then outputs from the second source controler routed back to the plc.

I would like to utalize a single transducer,pressure,to turn on outputs which will load and unload one or more compressors basicly replacing multiple pressure switches with a plc connected to a single transducer any help in this matter would be greatly appriciated.
REPLY using a PLC to "sense" the pressure via the transducer (4 - 20 mA output) to control the unloading valve should be fairly easy. You will need a PLC that has one or more inputs that are analog (4 to 20 mA 1 to 5 V etc etc)
AND you need to ensure the analog on PLC matches that of tranducer (each must be 4 to 20 mA etc etc)

I am trying to build a sequencer which would tranfer lead compressor at a preset timed interval and dependant on the pressure bring online lag or secondary compressor if the current output of the transducer drops to a certain leval as well as of course as loading and unloading the lead compressor with a higher output. I hope this makes sence. Please Email reply to [email protected] thanks

YOu are talking two things
1. Switching units to equalize time.
Fairly easy depending on your criteria. COuld do this with timer functions in PLC compare time run of 1 vs 2 then start the lesser of the two.
Could do this based on pressure and time interval

Bringing on a second unit to keep pressure up
can be done with the same transducer and input. All you do is set the set point of the output at a value less than taht of the unloader.

We would need to know what PLC you have in mind.

Dan Bentler

 

[Tim Ledwith]

Katy,

I have done something similar on some Hydraulic Pumps using a single 4-20mA pressure transducer. The PLC was an A-B PLC5. Let me know if this is any good to you, and I will send you a copy of the program.

Regards, Tim.

 

[Lancer]

Katy's

Hi hows things?, the answer to all your questions is yes, PLC are capable of receiving analog input from various Field devices, these can be ranged from 0-20ma 4-20ma(zero offset) 0-10v etc.
Basically the PLC will transfer the ma signal into an integer value or word address within its addressable memory. This then allows the PLC to preform maths functions, in your case depending upon the PLC you would probably compare (CMP) the analogue pressure input with a static value or memory address and then... oops, I don't want to spoil all your fun by tell you how to do all of it. This site has some invaluable tutorials relating to learning how to operate PLC's check this link for starters http://www.plcs.net/chapters/math22.htm

Lance

 

[Stephen Luft]

With regards to your questions placed on PLCs.net, what you are looking to accomplish can be done with a PLC. It will require the programming of some internal timers and set points within the PLC to compare the pressure transducer readings.

How many outputs will you require?

Have you chosen a PLC yet?

We do have compressor customers that we supply controllers that do a lead lag pump. There are several ways to accomplish this.

Let me know if we can be of assistance to you.

Hope this helps with your requirements.

Please note that I am employeed by Entertron - an embedded PLC manufacturer.

God Bless,

 

[TConnolly]

Dont program a single pressure for your setpoint. Program an on pressure and an off pressure, or a pressure + a hysterisis band. If you progam only a single pressure setpoint, say 100 psi, and you are fluctuating between 99.9 and 100.1 psi you will short cycle the compressor off and on. This is very hard on the compressor. It would be better to program a bit to turn on at 100 psi and shut off at 105 psi.