Study in Dynamics and Control of Temperature in a Mixing Tank

Transcription

1 Study in Dynamics and Control of Temperature in a Mixing Tank Particular safety issues: (1) Read the procedure below and then come back and re-read this paragraph. Always make sure that there is water running through the heater before you activate the heating element. Otherwise residual water in the heating element will be heated to steam and the hoses and tubing carrying the hot water will burst. Also, make sure that the water is turned on at the source, that the outlet tube is secured in a sink and that the water lines are not kinked. (2) Before touch anything each team member identify the real components in the experimental setup with their corresponding icons (symbols) in Figure 1. (3) Ensure that the water streams are flowing before you touch anything else. Control system components: The schematic of a control system is shown in Figure 1. Cold water from the RO unit (Reverse Osmosis) unit is used. It passes through a filter to remove any solid particles and then is split into two streams. One goes through a 1200 electric heater which heats it up to about 110 F. The second goes to a control valve. The two streams are mixed together in a mixing tank. Flow and temperature of the hot and cold water streams and the tank temperature are monitored. The tank temperature is sensed using a Type K thermocouple and fed to the PID controller. The controller output is sent to the control valve. PID Tank Temp TT Hot water line V-3 V-1 V-2 E-2 To Drain E-1 Figure 1- A control scheme for simple system For this experiment station the heat is supplied by a 1200 watt heater and a 30 psig supply water pressure is used. Air supply pressure to the system should be regulated at 20 Chemical and Biomedical Engineering Department USF page 1 of 5

2 psig. It is used to convert the signal from the valve (a current signal, 4-20mA) to a pneumatic signal (3-15psig). It will be necessary to hook up the sensors to the Vernier Lab Pro so the flows and temperatures can be logged. The suggested operating conditions for this experiment are: Cold water valve, V-3 should be 2.5 turns open Hot water valve, V-2 should be open about ½ turn A 1200 watt heater is used Before touch anything identify the real components in the experimental setup with their corresponding icons (symbols) in Figure 1. Initiating the sensors: With the flows off, zero the potential probes. Connect the three temperature probes to the Lab Pro device and change the units to degrees F for convenience (the temperature readings on the controller will be in degrees F). Open valves V-3 and V-2 as indicated above. Start LoggerPro and start monitoring the temperatures and flow rates. Activate the heater. Making a Step Change (FOPDT Identification) It is best to make a sizable step change of -30%CO for a measurable change in bath temperature to result. The negative step change is made because the controller decreases faster than it increases set points in manual mode. The controller must be set to manual mode. The number displayed on the controller is then percentage of controller output, or percentage of 4-20mA. To get the controller on manual mode, navigate through the menu as follows: Cnfg Out1 Self Enbl Exit the menu and the controller will be in manual mode. Hold down the up arrow until the controller is around 65% (this takes a few minutes, poor controller design) then let the system reach a steady state temperature. While collecting temperature and flow data, drop the CO to around 35% by holding the down arrow. A step change of -30% will take 11 seconds to achieve, but it is assumed to be instantaneous since the system time constant is large in comparison. Once the system has reached a new steady state, cease data collection, save your data, and transfer it to another computer. Leave the water running but deactivate the heater. On the other computer, import your data into EXCEL. Find the process gain K, the process time constant and the dead time θ. Then, use these parameters to estimate K C and I for both the Ziegler-Nichols and Smith-Corripio methods. Note that we are going to run the PID controller as a PI controller. So D will be set to zero. Once these parameters are computed, return to the experimental apparatus. After ensuring the water streams are flowing, activate the heater. Set up a LoggerPro run for 3600 seconds, 1 sample per second. Start the run and let the system approach steady state. In the meantime, follow the instructions below for preparing the controller. PID Controller Software Setup For changing tuning parameters in the PID controller, the computer connected to the controller should be used. The software to change parameters in the controller is Newport iseries Configuration 1.8 Basic, and it should be installed on the computer Chemical and Biomedical Engineering Department USF page 2 of 5

3 connected to the controller. The controller is attached to a computer via RJ-45 ethernet cable and the computer has been previously set up for communication with the controller. If this is not the case, see the experiment resources listed for instructions on the necessary connection settings. Start the Newport iseries and MICROCONFIG program from the Newport group in the Windows Start menu. The first step is to make sure the correct controller is selected. Follow the process outline in the following figures. (a) Click the Hardware button in the upper-right hand corner of the main window (picture of a wrench). Make sure the settings match the settings in figure 2. Figure 2 Hardware Settings (b) Make sure the settings on Comm tab are set as in figure 3. Figure 3 Communication Settings (c) Go to the Input tab and make sure Thermocouple Type K is selected. Chemical and Biomedical Engineering Department USF page 3 of 5

4 (d) Go to the Out1 tab and notice the PB, Reset, and Rate settings. These are the PID controller tuning parameters for proportional band (PB = 100/K c ), integral time (τ I ), and derivative time (τ D ), respectively. Set the parameters to the calculated controller tuning values. See figure 4. Figure 4 Output 1 Settings (e) After the system has stabilized, make a step change in the set point of 2 degrees and observe the response. Click the Set points tab to define the controller set point. Make both set points equal to the desired controller set point. See figure 5. Figure 5 Setting the Controller Setpoint (f) Send the new settings to the controller by clicking the Write button on the right hand side of the main window. Click the Network button to send. The new set point will show on the controller s display by SV. Chemical and Biomedical Engineering Department USF page 4 of 5

5 Figure 6 Sending the Changes to the Controller. (g) Test the performance of the controller by making a set point change of 2 o F and observing the response. Plot both CV (controlled variable) and CO (controller output, ie flow). (h) Repeat with second tuning rule. Before you leave the lab, make a sketch of the Process Instrumentation Diagram, identifying all sensors, valves and other devices used in the process. Data Analysis 1. From the data log, identify the step response and estimate the parameters of the FOPDT model. Note that due to the large amount of fluctuations in temperature, you will need to draw smooth curves before estimating the parameters. What is the estimated error in the model parameters? 2. Make a comparison of the model prediction with the observed data by plotting them side by side. 3. Determine the tuning constants using both tuning rules given above. What are the estimated uncertainties in the tuning constants? 4. Graph the response of the controller for a step change in set point for both tuning methods. 5. Draw the Process Instrumentation Diagram using Visio. Chemical and Biomedical Engineering Department USF page 5 of 5