Need help reading 4-20ma pressure transducer

[rsdoran]

Y'all correct me if I am wrong

Since you are using an ML1500 you will need to obtain an analog input card, possibly 1769-IF4 and an analog output card, possibly 1769-OF2.

In RSL500 you will need to go into the project tree under Controller to I/O Configuration and install the cards, the slot you place them in will determine their addresses, the manual(s) should provide those details.

Take a look at the SCP instruction, it is possible to make the input transducer to directly work the vent valve, the precise "how" depends. From what you have mentioned so far I doubt PID would be necessary, as mentioned not sure about the analog being necessary.

You may get a lot of help on the project but you have to provide details before anyone can provide anything substantial.

 

[TConnolly]

I'm with everyone else, use a standard air pressure regulator. The compressibility of air makes it very difficult to control and a mechanical regulator is going to give you very good control for a lot less money and hassle, despite the fact that you are interlocking it with other things.

However, that said, the set up of the PID loop is a trivial thing. Whats going to be much more difficult is tuning it to achieve the performance you desire.

I have attached a sample program. You can see how simple it is to set up. The analog input and outputs are configured to scale the raw input and output for PID (0-16383). Enter the range of your pressure transducer in SMIN and SMAX, thus a 0-10 bar transducer is entered as 0 to 145 to get PSI, and the setpoint, 80PSI is entered in .SPS. If you want to adjust any of these values programatcially then use a MOV instruction (or Math instructions if any value is computed) to move the value into the appropriate PD word.

Thats the easy part. Then it needs to be tuned and you probably cannot do that in advance of connecting the PLC and hardware. Unless you are very good at PID tuning you may find that you will spend several days getting the loop dialed in by trial and error, and that is expensive.

Good luck. You're gonna need it.

 

[Stationmaster]

There is more to this operation that's why I'm using a plc but the only part that I'm having trouble is controlling the pressure at +-10%. I need to use a proportional valve to let air in and vent out gradually, other wise I will have a sudden increase or decrease in pressure and I'll be out of tolerance which will show on the chart recorder.

In an earlier post you said: "there is no requirements as to how long it would take to fill the vessel". Smaller lines will cause slower filling.

Right now we are regulating air pressure manually through air regulators and an operator constantly monitors the long process.

I agree with your assessment that A PLC would help automate the process, but largely with the SEQUENCING, ie: opening and closing charge or discharge lines, monitoring safety interlocks, monitoring/regulating temperature, allowing the operator to set a "time", etc.

This operation involves blowers, safety interlocks, cooling, vacuum, air pressure, heating and everything is recorded through a chart recorder. By using a plc would automate the whole procedure.

My interpretation is that some process is being performed inside the tank, that is: an operator puts something in the tank, closes it up, the process is performed, the pressure is vented off, the tank is opened, and the part is removed.

I still don't see the need for analog control of pressure, unless you have a strict RATE at which you need to pressurize or depressurize. Then you may consider a walking setpoint arrangement (whole NEW can of worms).

Stationmaster

 

[tiopaeng]

We get a job once in a while that requires different pressure settings and we don't want the operators to be adjusting the air regulator because at one time we had to scrap a $15,000.00 part. We just want the operators to enter the parameters per job and push the start button.

Originally I was just going to use solenoid valves to open, close and vent but I'm afraid that might have an effect with the pressure jumping up and down on the chart recorder.

Thanks for explaining the PID Loop and for the sample file this will really help me.

My thanks to everbody,
TioPaeng

 

[TConnolly]

what safeguards keep the operator from entering the wrong number? I see more potential for error that way than with a regulator. You could create a set of password protected recipes, but then there is still the chance the operator picks the wrong recipe. These kinds of safeguards are ultimately not an automation issue but a mangement and procedure issue. Do you have a set up check procedure in place as a part of your quality plan? If not then you should, given the price of the parts - and replace the quality director while you are at it. Shame on him/her. Seriously, if you scrap that much because of a silly set up error then it is not the operator's fault, it's your lazy butt quality director's fault.

You could still use a regulator but have the pressure transducer check to see that the pressure is set correctly and trigger an alarm to alert the operator to double check it. But then, someone still has to enter a number.