MECH 322 Instrumentation Feedback Temperature Control Performed: 04/20/06 Pablo Araya : I believe I performed 100% of this lab Participation (__/50 points)

ABSTRACT The goal of this work is to construct a feedback system to control the temperature of water in a beaker, and evaluate its performance. A heater connected to a solid state relay and a thermocouple are placed in a beaker of water. A LabVIEW program is constructed to monitor the water temperature and close/open the relay if the temperature is below/above a user-specified set point value. Three different temperature set points are examined. The rate at which the heater/natural convection system increases the steady state temperature is greater than the rate it decreases it. The amplitude of the steady state temperature increased as the set point temperature increased, but its oscillatory period decreased.

Fig. 1 Type-J Thermocouple Temperature versus Output Voltage A third order polynomial appears to accurately represent the data. The relation between signal conditioner output and the thermocouple voltage is V TC = (V OUT / V/mV) mV These relations are used to interpret the voltage measured by the data acquisition system to determine the thermocouple temperature.

Figure 2 VI Front Panel

Figure 3 VI Block Diagram

Figure 4 Measured Temperature versus Time for Three Set Points The steady state error amplitude increases and period decreases as the set point temperature increases. The temperature rise rate when the set-point temperature is increased is greater than the settling rate when the set-point is decreased.

Table 1 Time to Reach Set Point For each set-point change, this table gives data for the temperature change  T = T SP -T I, the time to reach the set point P 1 = t R –t C,and the average rate of temperature change dT/dt =  T/ P 1. In steps 1 and 2 the 200 W immersion heater is used to increase the temperature. In step 3, natural convection to the surroundings is used to lower the set-point temperature. The rate of temperature decrease due to convection is slower than the rate of increase from the heater

Figure 5 Temperature vs Time for T sp =45 o C The unsteady waveform, period and amplitude are somewhat regular. The temperature spends more time above the set point than below it. The maximum error is E MAX = 0.7°C The oscillatory amplitude and period range from T PP = °C and P 2 = s, respectively.

Figure 6 Temperature vs. Time for T sp =85 o C The unsteady waveform is somewhat irregular, but its period and amplitude are roughly constant The temperature spends about the same amount of time below and above the set point. The maximum error is E MAX = -1.3°C The oscillatory amplitude and period range from T PP = °C and P 2 = s.

Figure 7 Temperature vs. Time for T sp =70 o C The unsteady waveform, period and amplitude are not regular The temperature spends more time above the set point than below it. The maximum error is E MAX = 1.4°C The steady state oscillatory amplitude and period vary between T PP = °C and P 2 = s, respectively.

Table 2 Steady State Behaviors after the Set Point is Reached The rate at which the heater increases the temperature is greater than the rate natural convection decreases it. The steady state behavior is regular at the first two set points and irregular at the final set point. The steady state peak-to-peak temperature variations increase with set-point temperature, but the oscillatory period decreases. The convection heat transfer from the beaker to the surroundings is larger at the hotter set points.