Dear users,

I am currently following my internship and my job is to rebuild an airconditioner to cool and heat. I would like to make a system which uses a room temperaturesensor. To use the airconditioner as a heater I used a 4-way valve to reverse the expansion cyclus. The original system has no PLC, so we bought a Siemens CPU 1212C AC/DC/RLY and the SM 1222 RLY module. We have a temperature sensor available and we messured and it gave ohms as unity. My question is: is it possible to connect this sensor directly on analog inputs from the CPU? If not, do I need a converter to make it a voltage or a current?

My second problem is the following: I would like to make a controlsystem to control the room temperature. The software compares the set temperature with the temperature of the room. From there it will heat or cool the room. I found on TIA Portal V13 PID_temp and PID_compact. It looks very complicated and I was wondering if I could use this for my project. If yes, which of those two would be the best solution for this project? Are there any examples available for a situation like this? 

Thank you very much and I appreciate the help very much. It could help my project a lot in progress. Excuse me if my grammar is not perfect. I am trying my best to make it as clear as possible. 

Hello,

you can use for direct connecting a temperature sensor at the S1200 the signal board 1xAI resistor or RTD 1x16 bit -  6ES7231-5PA30-0XB0. At this board you can connect one temperature-sensor e.g. PT100.

For more RTD you can use the AI-card 4 AI, resistor and RTD - 6ES7231-5PD32-0XB0.

Otherwise do you need for "normally" AI a transmitter.

At the programming it is better you use the PID_Temp because ther are more specific functions for temperature controlling and slow reactions at the controlled parameter (like tempoerature).
The question is how exactly do you want control the temperature and how do you control the 4-way-valve (maybe with a analog signal or?).
Please think about a 3step-controller. This could be easier for your application.

I hope this helps.

regards
CaOd

hi KoenTheunissen.

the onboard analogue inputs are only for current or voltage.

For temperature you need a PT100-sensor and the signalboard or signalmodule that CaOd suggested.

SB 1231 RTD
6ES7231-5PA30-0XB0

SM 1231 RTD
6ES7231-5PD30-0XB0
6ES7231-5PF30-0XB0

hope this helps,

regards,

Thank you for your helpfull reply.

If possible I would like the following process: 

1. Temperaturesensor measures the room temperature and the user set the temperature he/she wants to have.

2. The PID compares the room temperature with the set temperature.

3. The temperature will rise or fall. It will start descending or clambing fast and gradually slow down until it comes closer to the setpoint. This regulation will be done by a compressor and the variable-frequency drive powers the compressormotor by increasing of decreasing the compressor power.

4. When the room temperature is equal to the compressormotor speed will decrease of increase in order to maintain the temperature equal. Then compressormotorspeed will increase when the roomtemperature rises or drops again.

Is this explanation clear enough?

The regulation from the 4-way valve will be done by a stepmotor. It only need an angle of 90 degrees forward or backward. It won't be neceserry to make the control very accurate. 

Every semester we see the same problem by students.  Every year we give the same answers. 

Air conditioners have 2 states.  OFF and ON.  You do not need a PID loop to control this.

(Heat Mode) If Temperature is low then turn on heat.  When at temperature is reached, turn off heat.  Add some hysteresis (0.5 degrees) to stop cycling.

Cooling mode is the opposite.

Please don't re-invent the wheel.  The wheel works just fine.