PID control

Hi.
I have an application where i want to control the temperature in a room with a resistive load, Controlling the load with a PLC (PID closed loop control).

Ok so im going to use a powercontroller. The power controller takes 4-20mA and controls the load 0-100%. To be able to control the temerature due to a temperature reference, then Im going to use a closed loop PID control approach.

So I have a PLC, a temperature probe (PT100), and a powercontroller.

So I have some basic control question, starting with this:

1.
The setpoint to PID is in degree Celsius. Lett say the setpoint is set to 30 degree Celsisus, then if actual value (temperature probe) is 30 degree celsius as well, then there is no error and the output of the PID is what? 0, 30?

2.
How can I make the output of the PID into 4-20mA signal? So if im in steady state (no error, then i like to output 4mA to the power controller.


Thanks!
Best Regards
Hans

1. More then likely your PID output wouldn't be 0%. It would stay at some level to compensate against heat loses in the room. Normally we refer to PID output in terms of %. So 0% output corresponds to 4 mA and 100%-> 20mA.
2.To make your PID loop output 4-20 mA you have to scale it. Depends on PLC you are using and analog output card, it may be different scale factors.
To get specifics you need to provide your PLC and analog card info.

Thanks Val, I had to reed up some old school lectures. I get it now and it makes sense

hapetter said:

Hi.
So I have some basic control question, starting with this:

1.
The setpoint to PID is in degree Celsius. Lett say the setpoint is set to 30 degree Celsisus, then if actual value (temperature probe) is 30 degree celsius as well, then there is no error and the output of the PID is what? 0, 30?

Click to expand...

If the output were zero then the temperature would drop to ambient temperature. The heater raises the temperature some amount above ambient. Say the heater can raise the temperature 20 degrees with 100% power. The heater has a gain of 0.2 degree/%. Now if ambient is 20 degrees and you want to heat to 30 degrees the heater will require 50% power. When there is no error the proportional term will be 0% but the integrator term will wind up to 50% as the heater heats the room. In your case 50% will be 12ma.

I bet where you are ambient is much less that 20 degrees.

hapetter said:

Hi.
I have an application where i want to control the temperature in a room with a resistive load, Controlling the load with a PLC (PID closed loop control).

Ok so im going to use a powercontroller. The power controller takes 4-20mA and controls the load 0-100%. To be able to control the temerature due to a temperature reference, then Im going to use a closed loop PID control approach.

So I have a PLC, a temperature probe (PT100), and a powercontroller.

So I have some basic control question, starting with this:

1.
The setpoint to PID is in degree Celsius. Lett say the setpoint is set to 30 degree Celsisus, then if actual value (temperature probe) is 30 degree celsius as well, then there is no error and the output of the PID is what? 0, 30?

2.
How can I make the output of the PID into 4-20mA signal? So if im in steady state (no error, then i like to output 4mA to the power controller.


Thanks!
Best Regards
Hans

Click to expand...

If this is a room that's occupied by people, and has heaters but no air handler (say, baseboard heaters around the edges of the room) you're going to have a hard time controlling the temperature with a PID loop. Just write a few rungs of logic that turn the heaters on when temperature drops a degree or two below setpoint, and turns them off when temperature equals setpoint. Then, put your temperature probe someplace where opening and closing doors or windows is not going to drive the system nuts.

Since you already have the probe and 4-20mA and all, you could be more sophisticated by modulating the heater output based on deviation from setpoint. But, if this is space heating in an occupied space, I doubt even that much is necessary.

If you have a very small test cell or something, and you're trying to maintain temperature to within 1/10 degree, then set up the PID. But if you have a large space to heat, it's at best overkill and at worst you won't be able to tune it very well.