Process Control

Process Control 1.1 THE PURPOSE, CONTROLLING THE REFRIGERATION CYCLE

Figure 12: Control structure for refrigeration cycle

The purpose of this experiment is to control the level and the temperature of the evaporator in the refrigeration cycle, see figure 12. The expansion valve is used to control the level in the evaporator, see the blue control loop. LT is the level transmitter, while LC is the level controller. The setpoint of the level is given to the level controller. The temperature in the evaporator is controlled by the speed of the compressor, see the red control loop. TT is the temperature transmitter and the TC is the temperature controller. The setpoint is given to the temperature controller.

1.2 STARTING UP To be able to run the experiment, the equipment have to be turned on, this is done by activating the three buttons “Power”, “Compressor” and “Pump” in the control panel. The process is running when all three buttons have got green colour, you may have to push the buttons several times. This is due to some security mechanisms in the physical process.

1.3 SETUP Set the level controller to automatic and the temperature controller to manual.

Set the cooling water valve to 75% opening and the speed of the compressor to 50%, this is done in the Manual Control Panel, push the update button “Push Button to Update Process Parameters” afterwards, this is done to update the values on the server. Set the PID controller parameters and setpoints in the Automatic Control Panel as follows: Level: P = 10, I = 200, D = 0 and setpoint = 50 Temperature: Setpoint = -2 The process should then run until the level in the evaporator is approximately at the setpoint. This can be seen from the control panel or in Process Explorer, the historical database tool.

1.4 CONTROLLER TUNING – LEVEL To tune the level controller we want to apply the Ziegler-Nichols method. Setup for Ziegler-Nichols tuning method: Bring up Process Explorer, click on APIS and Ctrl_L. Here you select LT2_filter and TC03_intset; these parameters give you historical data over the level in the evaporator and the setpoint for the level. To get updated values in the historical database plot, you need to manually push the “Pan to Now” button. Go to the Automatic Control Panel and set the P-parameter to 20 and the I-parameter to 0 for the level control, and then push the button “Push Button to Update Process Parameters”. You are now going to increase the P-value for the level control until you get standing oscillations for the level in the evaporator. (Remember to push the submit button after updating the P-values!) You have to look at the LT2_filter value in Process Explorer to know if you have standing oscillations or not. When you have standing oscillations you measure the period for the oscillations, this period is used to decide the controller parameters, see figure 13. When you have the gain, the P value, and the period of the standing oscillations, these can be used to decide the controller parameters for level control, see table 1. Use table 1 to calculate a PI controller for the level in the evaporator.

More information about Ziegler-Nichols parameter tuning can be found at the following site: http://www.remotelab.ntnu.no/refrig/pdf/controller_tuning.pdf. Update the level controller with the new controller parameters. Run the process for a while to see whether or not the new control parameters give acceptable performance. If not, the Ziegler-Nichols tuning should be run again with a new gain, i.e. a new value for P.

Figure 13: Process Explorer with standing oscillations

1.5 CONTROLLER TUNING – TEMPERATURE To tune the temperature controller we will use a step response experiment. First of all the system should get time to settle, which means that the level controller should get time to stabilize the level around the setpoint and the temperature should be fairly stable. Apply a step in the speed of the compressor, for example form 50 to 60, and plot the speed of the compressor (TC02_pdr) and the temperature in the evaporator (TE1) in Process Explorer. A first order model with time delay is going to be established, look at the course material, controller_tuning.pdf. Based on this model the PI control parameters can be found from the table information given below and in the course material.

Figure 14: Estimating model parameters from the process reaction curve.

The response in Figure 14 is the result of a step of size B at time 0 in the manipulated variable, the model parameters are found as follows: 1. Locate the inflection point, i.e., the point where the curve stops curving upwards and starts to curve downwards. 2. Draw a straight line through the inflection point, with the same gradient as the gradient of the reaction curve at that point. 3. The point where this line crosses the initial value of the output (in Figure14 this is assumed to be zero) gives the apparent time delay ?. 4. The straight line reaches the steady state value of the output at time T + ?. 5. The gain K is given by A/B.

1.6 TEST OF CONTROLLER TUNING At last the new controller parameters are going to be tested. Set the new controller parameters for both temperature and level in the Automatic Control Panel and push the submit button. Set both the controllers in automatic control, green colour. Run the process to see that the controllers are able to stabilize the process around the selected setpoints. (Level: 50 and Temperature: -2) After the process has stabilized around the setpoints, apply a step in the setpoint to the level, for example a setpoint equal 55, and push the submit button. Look at the measurements in Process Explorer and see how well the controllers handle this step. Afterwards, try with a step in the setpoint for the temperature, for example -4.