Temperature Control System (Oven)

Objective:

To study the performance of various types of controllers used to control the temperature of an oven.

Apparatus Required:

Graph sheet

System Description:

Temperature control is one of the most common industrial control systems that are in operation. This equipment is designed to expose the students to the intricacies of such a system in the friendly environment of a laboratory, free from disturbances and uncertainties of plant prevalent in an actual process. The ‘Plant’ to be controlled is a specially designed oven having a short heating and cooling time. The temperature time data may be obtained manually, thus avoiding expensive equipment like an X-Y recorder or a pen recorder. A solid state temperature sensor converts the absolute temperature information to a proportional electrical signal. The reference and actual temperatures are indicated in degree Celsius on a switch selectable digital display.

The controller unit compares the reference and the measured signals to generate the error. Controller options available to the user consists of ON-OFF or relay with two hysteresis settings and combination of proportional, derivative and integral blocks having independent coefficient settings. A block diagram of the complete system is shown below:

Block Diagram

The Plant (Oven):

Plant to be controlled is an electric oven, the temperature of which must adjust itself in accordance with the reference or command. This is a thermal system which basically involves transfer of heat from one section to another. In the present case we are interested in the transfer of heat from the heater coil to the oven and the leakage of heat from the oven to the atmosphere.

Controllers:

Basic control actions commonly used in temperature control systems are

• ON-OFF or relay
• Proportional
• Proportional and integral
• Proportional-Integral-Derivative

All the four controllers have been used here. The parameter constants are listed below:

Proportional Gain (K_P) = 0 to 0.1V/­⁰C

Integral Gain (K_I) = 0 to 0.024/sec

Derivative Gain (K_D) = 0 to 23.5 sec

Sensor:

The oven temperature can be sensed by a variety of transducers like thermistor, thermocouple, RTD and semiconductor sensors. In the present set up, the maximum oven temperature is around 90⁰C which is well within the operating range of a semiconductor temperature sensor like AD590. Further these sensors are linear and have a good sensitivity, viz. 1µA/⁰K. Associated electronic circuits convert this output to 10mV/⁰C which may be easily measured by a DVM. The time constant of the sensor has however been neglected in the analysis since it is insignificant compared with the oven time constant.

Experimental work:

A variety of experiments may be conducted with the help of this unit. The principal advantage of the unit is that all power sources and metering are built in and one needs only a Watch to be able to note down the temperature readings at precise time constants. After each run the oven has to be cooled to nearly the room temperature, which may take about 15-20 minutes.

1. Identification of oven parameters:

 Plant identification is the first step before an attempt can be made to control it. In the present case, the oven equations are obtained experimentally from its step response as outlined below:

In the open loop testing, the oven is driven through the P-amplifier set to its maximum gain of 10. The input to this amplifier is adjusted through reference potentiometer (the one next to switch 2). This input can be seen on digital display, so that when you set 5⁰C, the input to proportional amplifier is 50mV (@ 10mV/⁰C) and its output (which acts as input to driver/actuator) is 0.5V.

Procedure:

• Keep switch S₁ to ‘WAIT’, S_2­ to ‘SET’ and open ‘FEEDBACK’ terminals.
• Connect P output to the actuator input and switch ON the unit.
• Set P potentiometer to 1. Adjust reference potentiometer to read 5.0 on the DPM. This provides an input of 0.5V to the driver.
• Put switch S₂ to the ‘MEASURE’ position and note down the room temperature.
• Put S₁ to the ‘RUN’ position and note temperature readings every 15 sec., till the temperature becomes almost constant.
• Plot temperature-time curve on a graph paper. Referring to the fig shown below, calculate T₁ and T₂ and hence write the transfer function of the oven as:

\[G(s) = \frac{{K{e^{ - S{T_2}}}}}{{(1 + S{T_1})}}\]

Open Loop Response

2. ON-OFF Controller:

• Switch S₁ to ‘WAIT’ position and allow the oven to cool to room temperature. Short ‘FEEDBACK’ terminals.
• Keep switch S₂ to the ‘SET’ position and adjust reference potentiometer to the desired output temperature, say 55.0⁰C, by seeing on DPM.
• Connect R output to the actuator/driver input. Outputs of P, I and D must be disconnected from the actuator input. Select ‘HI’ or ‘LO’ value of hysteresis.
• Switch S₂ to ‘MEASURE’ and S­₁ to ‘RUN’ position. Read and record oven temperature every 15-30 sec., for about 20 minutes.
• Plot a graph between temperature and time and observe the oscillations in the steady state. Note down the magnitude of oscillations, rise time, percent overshoot and steady state error.

3. Proportional Controller:

Ziegler and Nichols suggest the value of K_P for P controller as

K_P= (1/K) X (T­₁/T₂)

K = Oven Temperature at steady / Input voltage (V).

% of P controller value = K_P/0.1 X 100%

• Starting with a cool oven, switch S₁ to ‘WAIT’ position and connect P output to the actuator input. Keep R, D and I outputs disconnected. Short ‘FEEDBACK’ terminals.
• Set P potentiometer to the above calculated value.
• Select and set the desired temperature to say 55.0⁰C.
• Switch S₁ to ‘RUN’ position and record temperature readings as before.
• Plot the temperature-time data on a linear graph paper and observe the rise time, steady state error and percent overshoot.

4. Proportional-Integral Controller:

Ziegler and Nichols suggest the value of K_P and K_I for P-I controller as

K_P = (0.9/K)X (T­₁/T₂), K_I =1/T₁=1/3.3T₂.

Keep T₂ constant and T₁=3.3T₂

% of I controller value = K_I/0.0244 X 100%.

• Starting with a cool oven, switch S₁ to ‘WAIT’ position and connect P and I output to the actuator input. Keep R and D outputs disconnected. Short ‘FEEDBACK’ terminals.
• Set P and I potentiometers to the above calculated values.
• Select and set the desired temperature to say 55.0⁰C.
• Switch S₁ to ‘RUN’ position and record temperature readings as before.
• Plot the temperature-time data on a linear graph paper and observe the rise time, steady state error and percent overshoot.

5. Proportional-Integral-Derivative Controller: 

Ziegler and Nichols suggest the value of K_P, K_D and K_I for P-I controller as

K_P = (1.2/K)X (T­₁/T₂), K_I =1/T₁=1/2T₂, K_D = 0.5T₂.

Keep T₂ constant and T₁=2.0T₂

% of D controller value = K_D/23.5 X 100%.

• Starting with a cool oven, switch S₁ to ‘WAIT’ position and connect P, D and I output to the actuator input. Keep R outputs disconnected. Short ‘FEEDBACK’ terminals.
• Set P, D and I potentiometers to the above calculated values.
• Select and set the desired temperature to say 55.0⁰C.
• Switch S₁ to ‘RUN’ position and record temperature readings as before.
• Plot the temperature-time data on a linear graph paper and observe the rise time, steady state error and percent overshoot.
• Compare the results of various controller options.