Heater Control

Hi Everyone,

I need some ideas to program a room air heater controller. I have s7-1200 and will be using temperature sensor with 0-10 volts analog output. Output will be ssr to control power to the heater.
Not familiar with which instruction will be more suitable for this kind of application. I only need a simple but working program. I am reading the PID stuff and looks too complicated and wondering there might be a simplier way to achieve the same result.
Thanks in advance.

I suggest a simple deadband control like a home thermostat. Heater on at a low temperature setting, heater off when temperature rises to a slightly higher temperature. Use a time delay on each to avoid excessive cycles on an off from short duration temperature fluctuations.

franchois13 said:

...wondering there might be a simplier way to achieve the same result.

Click to expand...

If we were talking Allen-Bradley, the simpler way for the same result would be to directly implement the PID calculation directly in logic instead of using the PID instruction. There are examples of how to do this in other PLCS.net threads, and it would probably not be necessary to use the "D" part.

If you are willing to give up performance, in terms of how tightly the controlled temperature holds to target, you can go with a more simple approach such as on/off control with a deadband around the target.

However, since you already have an analog output, and presumably the SSR takes 0 - 10 VDC as proportional to power output, you are well set up for PI control. One added complexity with PI control is the need to determine effective tuning gains, and a poorly tuned loop could do worse than on/off with deadband.

Thank you guys for the inputs. I probably going to go for dead band control given the required control is not that tight.
I would also want to give a go for the PI control. I have not done any loop tuning so this will give me a bit of challenge.
Do you Guys have any suggested readings for this? could not seem to find basic information.

If you are using a Solid State Relay for control you need to think about your system design. A SSR is an on/off device and probably won't accept a 0-10 VDC analog signal. Maybe something like this: https://www.omega.com/pptst/PCM4.html

S7-1200 has 0-10V AI but no AO so OP is on the right track. On-off control with hysteresis (deadband) is the simplest way to do this.

Search for relay pid tuning method, which is an ideal way of finding out the process model for an on off heater.

Once you have your process model, you should be able to select some good tuning constants, depending on the lag/deadtime of your system. The lag of your system can also be used to determine an on off rate, typically around 1 minute. Output of your pid loop will control the duty cycle of this on off period.

I hope this limited information helps you make a better system than the on off deadband controller.

I like the idea of having a pid involve on my controller, though temperature accuracy is not that critical. I have tried the simple on off but temperature still overshoots a few degrees after turning off.
Can I use the pid compact instruction?
By the way the SSR I am using is only for turning heater on and off connected to digital output.