Hi, I am pretty new to the world of control engineering.

Anyway I am using siemens PLC 315-2DP to control heating ovens via PID. There are four banks of heaters in the ovens. The ovens are baking steel rods.

Basically the ovens are supplied with 3 different temperature setpoints for one baking session. The temperatures vary between 100 degress celcius and 500 degrees celcius depending on the baking pattern.

What is the best control for this situation. Currently they are using ON OFF control.

I would like to use PI control -
I think this should be the best form?

IS this plant an integrator - someone told me that if one applies a constant input to a plant and the output increases then the plant is an integrator. This makes sense, but then would i need to have PI control or can I just use P control

 

PID control is generally preferable to a simple ON-OFF control, but the choice is driven by the type of actuator you have. If you only have an ON-OFF heater, then it is possible to use a PID algorithm to generate an ON-OFF cycle time but this is a little more complex.

For example. if the PID output is 100% then the heater is always on, if the output is 0% the heater is allways off, if the output is 50% the heater is on for 30 seconds and off for 30 seconds.

PI control is usually done by just turning off the D in a PID controller. This is because it is hard to tune Deriviative unless you use software or are comfortable with some math.

No, your plant is not an integrator. However, temperature controls are typically very slow to respond, and hence look like an integrating process if you only watch them for a few minutes. In some situations you can get perfectly good PID control tuning values if you treat the loop as an integrator. - This usually means using large gains (P) and long reset times (I). Of course, doing some bump tests and running the results through some PID tuning software will give you better results

Rob

www[.]lymac.co.nz

 

PI control will work well, you will need Integral action for sure. If you just use P control your oven will never reach the correct temperature. The I term will slowly change the output to compensate for any error between the actual temperature and setpoint.
Some would advocate using D (derivative also known as Rate) but I don't think in your case it will be required.

Most electric heaters are controlled with a time duration scheme meaning that the Output of your PI controller controls the percentage of time the elements are energized.

In your PLC has one timer (Cycle Time) with a fixed time (e.g.20 sec) and another (On time) with the setpoint derived from the controller output/5 to give you a variable 0 - 20. When the controller output is at 10% the elements will be on for 2 seconds off for 18 then on again. At 50% they will be on for 10, off for 10 and so on. You can make the cycle time longer or shorter but I find 20 seconds is good for most applications.

If you have solid state relays they are normally zero crossover switching type.

Hope this helps
Roy