Lab 1: The ON/OFF Controller ENGG*3410 Instructor: Dr. Vale Lab Performed By: Vithursan Thangarasa and Graham Thoms Date: Monday, January 25, 2016
1
Introduction
The purpose of this lab is to investigate an ON/OFF controller, with different setpoints, hysteresis values, and disturbances that simulated the control of the temperature of a water tank. This ON/OFF controller uses a closed-loop feedback system, and is widely used such as in home heating systems where a heater is turned off above a certain temperature threshold and on when below a temperature threshold. In order to simulate the water tank a thermistor was used and a ADC was interfaced with labview to read and set the heater output (in volts).
2 2.1
Analysis Processed Data
The graphs are presented in Appendix A, B, and C. Appendix A contains the graphs of the different experiments conducted for a set point value of 4.00. Appendix B contains the graphs of the different experiments conducted for a set point value of 6.00. Appendix C contains the graphs of the different experiments conducted for a setpoint of 8.00.
2.2
Effect of Disturbance
The effect of turning the disturbance ON/OFF can be observed by analyzing the amplitude of the temperature sensor output and the phases of the heater output/temperature sensor output graphs. It can be seen that when the disturbance is ON, the amplitude of the temperature sensor increased. In addition, a phase change occurs for the heater output and temperature sensor output graphs. A phase change (inverted) occurs because after the effectors transmit a corrective response, the disturbance follows. So, due to this sequence of events, the amplitude is always higher than what it should be. As a result, the actuator is in an OFF state for a longer period of time. The amplitude of the output is still higher than the expected output, even though the actuator is in an OFF state for a longer period of time. This is primarily because the disturbance generates temperature changes by adding heat to the system that was not there initially. 2.3
Effect of Set Point
The effect of changing the setpoint values was not significant based on the heater output and temperature sensor output graphs. It can be seen from the graphs that the set point value is the temperature the system needed to maintain. Therefore, by increasing or decreasing the set point value, one can increase or decrease the temperature that the system was to maintain. Consequently, the system exhibited the same behaviour and response to hysteresis and disturbances at each of the different set points (4.00, 6.00, and 8.00). The systems tested in this lab was a fairly ideal system, resulting in similar system behaviour at each of the set points. However, in a real world application where the controller needs to
maintain the temperature of a water tank, differences in set points would be more visible due to the nature of heating or cooling water. 2.4
Effect of Hysteresis
The hysteresis produced by the potentiometer represented the approximation of the limit cycle of the heater output. Essentially this represents the heat capacity of the plant. The frequency of the heater output cycle was directly proportional to the set hysteresis. Thus, decreasing the hysteresis decreased the period and amplitude of the heater output and increasing the hysteresis increased the period and amplitude of the heater output. This effect can be seen in all the graphs as expected, since low hysteresis switches the heater on/off more often but needs to change the controlled variable less while with high hysteresis the heater switches on/off less often however it has to change the heater output a lot more (higher amplitude). 2.5
Pros and Cons of Synthesis
The effect of having a very low hysteresis would mean that the heater is switched on/off more often and is not the best approach for controlling a plant. This can be seen as inefficient since a lot more power is being used by the heater to set the plant temperature compared to a slower on/off heater (higher hysteresis). Furthermore the a higher frequency of the heater output would mean more wear and premature failure to the device, as the rapid switching can negatively affect the components of the system, adding more cost in the long run.
3 Conclusion In the final analysis, the ON/OFF controller lab showed how changing the disturbance, set point value, and hysteresis value can affect the behaviour of the system. When the disturbance was introduced, the ON time of the heater decreased because the source was heated more quickly. Whereas, the OFF time increased because the source slowly cooled down. Also, it was easily observed that increasing the set point value, increases the temperature the system needed to maintain, and decreasing the value, decreased the temperature. Moreover, the system behaved proportionally to change in values of the hysteresis. The period and amplitude of the heater output decreased when decreasing the hysteresis value, and increased when increasing the hysteresis value. Finally, it was determined that a lower hysteresis value would result in a higher frequency of the heater output, thus more wear and premature failure of the system’s components.
Appendix A
Figure 1 Heater Output (V) vs. Time (s) graph for Set Point = 4.00, Hysteresis = Minimum, and Disturbance = OFF.
Figure 2 Temperature Sensor Output (V) vs. Time (s) graph for Set Point = 4.00, Hysteresis = Minimum, and Disturbance = OFF.
Figure 3 Heater Output (V) vs. Time (s) graph for Set Point = 4.00, Hysteresis = Maximum, and Disturbance = OFF.
Figure 4 Temperature Sensor Output (V) vs. Time (s) graph for Set Point = 4.00, Hysteresis = Maximum, and Disturbance = OFF.
Figure 5 Heater Output (V) vs. Time (s) graph for Set Point = 4.00, Hysteresis = 5.00, and Disturbance = OFF.
Figure 6 Temperature Sensor Output (V) vs. Time (s) graph for Set Point = 4.00, Hysteresis = 5.00, and Disturbance = OFF.
Figure 7 Heater Output (V) vs. Time (s) graph for Set Point = 4.00, Hysteresis = 5.00, and Disturbance = ON.
Figure 8 Temperature Sensor Output (V) vs. Time (s) graph for Set Point = 4.00, Hysteresis = 5.00, and Disturbance = ON.
Appendix B
Figure 1 Heater Output (V) vs. Time (s) graph for Set Point = 6.00, Hysteresis = Minimum, and Disturbance = OFF.
Figure 2 Temperature Sensor Output (V) vs. Time (s) graph for Set Point = 6.00, Hysteresis = Minimum, and Disturbance = OFF.
Figure 3 Heater Output (V) vs. Time (s) graph for Set Point = 6.00, Hysteresis = Maximum, and Disturbance = OFF.
Figure 4 Temperature Sensor Output (V) vs. Time (s) graph for Set Point = 6.00, Hysteresis = Maximum, and Disturbance = OFF.
Figure 5 Heater Output (V) vs. Time (s) graph for Set Point = 6.00, Hysteresis = 5.00, and Disturbance = OFF.
Figure 6 Temperature Sensor Output (V) vs. Time (s) graph for Set Point = 6.00, Hysteresis = 5.00, and Disturbance = OFF.
Figure 7 Heater Output (V) vs. Time (s) graph for Set Point = 6.00, Hysteresis = 5.00, and Disturbance = ON.
Figure 8 Temperature Sensor Output (V) vs. Time (s) graph for Set Point = 6.00, Hysteresis = 5.00, and Disturbance = ON.
Appendix C
Figure 1 Heater Output (V) vs. Time (s) graph for Set Point = 8.00, Hysteresis = Minimum, and Disturbance = OFF.
Figure 2 Temperature Sensor Output (V) vs. Time (s) graph for Set Point = 8.00, Hysteresis = Minimum, and Disturbance = OFF.
Figure 3 Heater Output (V) vs. Time (s) graph for Set Point = 8.00, Hysteresis = Maximum, and Disturbance = OFF.
Figure 4 Temperature Sensor Output (V) vs. Time (s) graph for Set Point = 8.00, Hysteresis = Maximum, and Disturbance = OFF.
Figure 5 Heater Output (V) vs. Time (s) graph for Set Point = 8.00, Hysteresis = 5.00, and Disturbance = OFF.
Figure 6 Temperature Sensor Output (V) vs. Time (s) graph for Set Point = 8.00, Hysteresis = 5.00, and Disturbance = OFF.
Figure 7 Heater Output (V) vs. Time (s) graph for Set Point = 8.00, Hysteresis = 5.00, and Disturbance = ON.
Figure 8 Temperature Sensor Output (V) vs. Time (s) graph for Set Point = 8.00, Hysteresis = 5.00, and Disturbance = ON.
