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LE 11 - Controlling temperature with a PID controller

 WARNING

Make sure the whole control station is not energized before touching or starting your exercise!

This experiment involves measuring temperature and controlling it using a controller. For this purpose, a tank filled with water is to be used along with a thermocouple probe. The controller manual must be read first to program it. Take a look at the instruction manual and check for the right path in the menu.

Objective

Temperature measurement with a thermocouple and its control by means of a controller while exploring the configuration menu of the controller.

Material

  • Temperature controller
  • Thermocouple probe
  • Tank with water
  • Heating element (electric resistance)
  • Multimeter
  • Test station
Fig. 01 Tank to be filled with water showing ports for the heating element and the temperature well


What to do

For this activity, you will need to use the same ladder diagram used to turn on the PID controller to measure the temperature with a thermocouple. Remember that your installation should have the following features,
  • The PID controller should turn on with the green button. Also, the green pilot light should turn off, and the red pilot light should turn on as this button is pushed.
  • The PID controller should turn off with the red button. Also, the green pilot light should turn on, and the red pilot light should turn off when this button is pressed.
  • There should be an emergency stop button so that when it is activated, a yellow pilot light turns on, indicating an alarm.
  • There should be an alarm, such as a yellow pilot light or buzzer, to indicate that the heating element is on.

Read the PID controller electrical diagram to learn in which terminals connect the thermocouple sensor. Recall that in the test stations there are four PID controller models: ESM-9930, XMT-A808, TAL-B4RJ4C and REX-C900FJ02. You may download the instruction manuals in the following links:

Fig. 02 These are the models of PID controllers available

Each PID controller has different features, so not all have the same functions and capabilities. Confirm the model in the physical component and check it in the instruction manual.

Another component should also be considered to enable the heating element via the PID controller. This new component is the solid-state relay.
Fig. 03 Sample image of a solid-state relay. You should check the real features of the one installed in the test station


For reference on installing the temperature control system, see the schematics in Fig. 04.

Fig. 04 Schematics of wiring for a PID controller aiming to control the temperature via a heating element. Notice that in this case, the SSR needs to be powered by a low DC voltage.


How to do it

Step 1. Identify terminals for thermocouple

Look carefully at the PID controller your station has. Then take a look at the electric diagram. The electric diagram can be found either in the instruction manual or printed on the controller case.

Be aware of not wiring the voltage supply to these terminals or you could blow up the controller.

Step 2. Identify the terminals for the SSR

Look for the process output connections for the solid-state relay. Recall that this device is to be powered with a low DC voltage. You can also look at the small red led in the SSR to verify when it is on or not.

You should also identify the DC and AC terminals (the latter for the heating element).

Step 3. Turn-on the PID controller

You are advised to check as follows. Connect an AC bulb in place of the heating element and verify that it stays on (it should never turn off). If it works, proceed to change the bulb by the heating element.

This is easily done by just pushing the start button. You will need to give the controller a few seconds to fully initialize.

Step 4. Set-up the corresponding thermocouple and units

The PID controller does not know the type of thermocouple you are connecting. Therefore, this must be set up before any measurement.

For this step, you will need to read the corresponding instruction manual and follow the steps to navigate through the controller menu. Below, Figs. 4-5 give this help for one of the controllers.

Fig. 05 Menu for thermocouple set-up for the EMKO-9930 controller. Follow the coloured lines.


Fig. 06 Options for setting-up different types of thermocouples for the EMKO-9930 controller. Notice that for each thermocouple two options are available. The first option gives temperature readings without decimals while the second option provides one digit after the point for more precision.


Step 5. Program the PID controller for a heating process

You will need to follow a similar series of steps to program the PID controller. Here are some hints that can help:

  • The PID controller may work either to heat or to cool. You need to choose the corresponding option in the menu
  • Some PID controller models have: on/off and PID control modes. You should choose the correct option
  • Most modern PID controllers can automatically estimate their control constants, but these can also be introduced by the user. Be aware of the selected option
  • You will need to set the goal temperature (set point)

Step 6. The control process is coming to life

Changes in the system are not instantaneous and may take some time (several minutes).

Be aware that the temperature changes and is kept at its set point. Avoid perturbing the system so the controller can do its job.

Carefully review the other instruction manuals to find the corresponding path to navigate the menu and set-up the controller properly.

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