Cannot obtain output from my controller

[maxidivine]

I am using a Eurotherm 3504 controller to controll a valve based on input pressure.

See details:

PV – Pressure transmitter input (0-5 bar), 4-20mA (i/p to controller)

Module 1 – Remote setpoint (0-5 bar), 4-20mA (i/p to controller)

Module 2 – Analogue output of valve actuators position to control room (0-100%), 4-20mA (o/p from controller)

Module 3- Analogue output to valve actuators (0-100%), 4-20mA (o/p from controller).

---Proportional action with gain of 40%/bar

(I interprete this to mean ...a 1 bar change in pressure results in 40% valve movement when proportional acts on its own. This gives a gain of 2 and PB of 50% I believe).

---Integral action time – 300 secs

---No derivate

Problem: I do not get any response in the output. Modules 2&3 are both reading their lowest vales – 4mA as output, which corresponds to 0% open in the actuators.

I am using a set point of 2.5 bar (midway of the 0-5bar pressure transmitter scale). When I vary the i/p current I am using to simulate the PV, (4-20mA), I do not get get any output in the valves.

Have I got my interpretation of the PB wrong?

I wish I could post screen captures here.

[Dennis_Knutson]

I don't know what those screen captures are supposed to represent. What software are you using? Anything related to NI?

[maxidivine]

Sorry about that.

I am using a real controller (Eurotherm 3504). The plan is to test the controller using simulated 4-20mA pressure input and see the resulting output.

Later on, I will use labview to simulate different pressure inputs to feed into the controller. So my question is for the first part, i.e feeding the controller with simulated 4-20mA. So, you can say that my question is a simple control loop question.

I have used 50% as gain and I get no output. I thought that the higher the PB, the slower will be the response. I have tried reducing the PB to 2.5% and I still do not get any output response.

The screen shots represent the configuration for each contributors to the control loop, e.e PV, input and outputs.

Thanks

[Jun']

Hi Maxidivine,

Wouldn't it be wiser and more efficient to contact Eurotherm for a question that does not imply any NI software or NI hardware?

Thanks,

M-Junior

[maxidivine]

Hello,

Thanks for your suggestion. I have contacted them before posting here.

Basically, it is more of a control issue than a 'Eurotherm' issue.

I have been able to get the valves to work now, thanks to the explanation below:

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Proportional Band: The amount of change in the controlled variable, expressed in engineering units, required to move the actuator from one end of its stroke to the other.

%PB: Percent proportional band is the proportional band expressed as a percentage of the sensor span.

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I set the PB as 2.5% instead of 2.5 engineering units. I have corrected this and it works fine now.

Kind regards.