Heating and Cooling PID can be implemented using UniStream PLC and UniLogic, this article describes the steps that need to be performed to handle Heating & Cooling PID control.

Temperature PID is suitable for heating or heating-and-cooling applications.

The Temperature PID technology object provides a continuous PID controller with integrated tuning.

Two outputs are required for this purpose, one for heating and the other one for cooling.

Temperature PID can also be used for other PID control tasks like pressure. 

Temperature PID is ascendable and can be used in manual or automatic mode.

The simulation system:

The temperature sensor is a thermocouple type T. 

The heating component is two 24V DC heating elements on a steel plate attached to the heat sink

The cooling element is a 24V DC fan on the heat sink.

Configuration of the PLC:

To implement dual PID, you will need temperature input and UID-0808THS to operate the PWM control of the heating-cooling devices.

The user needs to configure the THS output Block to PWM.

Once you configure the THS, in the I/O there will be new integers for the PWM frequency and Duty cycle.

The frequency is given in milliHz units. 

NOTE: the frequency you need to apply is depended on the physical components.

Auto-tune for dual PID:

You must apply the auto-tune one by one for each PID, and not together.

The auto-tune works by creating three cycles for testing the behavior of the cooling/heating component.

In the process of auto-tune, the system will overshoot and will wait for the system to go back to the set point.

In the process of auto-tuning, the opposed component is active at 20% to create a steady state for the active cooling/heating.

Our auto-tune works when the environment is in a steady state.

For the auto-tune to recognize the ability of the system to active counter heating/cooling and there is a need to apply some counter force.

For example, if you will activate the auto-tune for heating the fan will work constantly at a PWM of 20%.

Only when the auto-tune is complete you can activate the other auto-tune. 

NOTE: when you want to apply the auto-tune for the cooling process you will need to get the temperature above the set point you define, this is for the auto-tune to start properly.

In addition, you need to make sure that the heating element does in fact warm the system when you are in cooling auto-tune, otherwise increase the percentage of the heating PWM until it warms the system. 

Once the two auto-tune processes are complete you can run the two PIDs together. 

The application:

In the attached application, you will need to define the set point. the set point is the same for the cooling and heating.

The user will need to define the process value high and low limits for both of the PIDs.

In addition, you will need to choose which of the PIDs is for heating and which is for cooling.

Run PIDs:

Once you run the PIDs, near the set point the two systems will run at low percentages to maintain the desired temperature.

You can see in the graph the behavior of the PWM.