Hakan AYKULU20083263Department of Computer Engineering

DEVELOPMENT OF A TEMPERATURE CONTROL SYSTEM

This project is about the development of a

microcontroller based educational

temperature control system

Laboratories are very important part of every engineering course. Students learn the theory in classes and apply their knowledge into practise by using real equipments in laboratory sessions.

For example, electronic engineering students learn the complex theory of transistor amplifiers in the classroom. In order to support theoretical concepts explained in classroom, laboratory sessions must be provided to reinforce this knowledge.

Students retain 25% of what they hear, 45% of what they hear and see, and 70% if they use the “learning-by-doing” method.

INTRODUCTION

The aim of this project was to develop a microcontroller based

temperature control system that can be used for teaching the

principles and the practise of control engineering to

undergraduate students..

An EasyPIC6 type development board is used in the design. This board is developed and manufactured by mikroElektronika. It has many switches, LEDs, jumpers, ICD, and many I/O ports. The output of the temperature sensor is connected to analog input AN0 (RA0) of the microcontroller. The output PORT C of the microcontroller is connected to a D/A connector. The D/A connector drives a high power operational amplifier. The output of the operational amplifier is connected to the fan.

There are many types of control algorithms that can be used in temperature control applications. Some of the commonly used control algorithms are:

•ON/OFF control•Proportional+Integral+Derivative (PID) control•Fuzzy control•Neural network based control

In this project we will be using the ON/OFF based control and the PID based control

Temperature control is very important in process control applications. Accurate control of the temperature is important in many industrial and chemical plants.

Temperature control is professionally performed using a closed loop control system. In such a system a temperature sensor is used to sense the temperature. A digital controller or a computer reads the sensor temperature and provides control actions to obtain the required temperature.

The block diagram of a typical temperature control system is shown below. R is the input, Y is the output.

digitalcontroller Heater

YR error

sensor

D/A Power Amp

The successful implementation of a temperature control system requires a temperature sensor. There are many types of temperature sensors available in the market place. Although there are some digital temperatures available, most sensors give analog outputs.

The outputs of temperature sensors are usually voltages or currents proportional to the temperature being sensed by the sensor. A brief summary of the popular temperature sensing devices is fiven in this section.

A list of the available temperature sensor devices is given below:

•Thermocouples• Resistance Temperature Devices (RTD) Thermistors• Semiconductor temperature sensors

Temperature Sensors

Perhaps the easiest form of control is known as the ON/OFF control, or Bang-bang control. In this type of control the plant output is compared with the desired value. If the plant output is lower than the desired value then the actuator is operated. If on the other hand the plant output is higher than the desired output then the actuator is simply turned OFF.

The ON/OFF type control is used in most simple low-cost temperature control applications. If the plant temperature is lower than the desired temperature then the heater is operated to increase the plant temperature. If on the other hand the plant temperature is higher than the desired temperature then the heater is turned off

Perhaps the most commonly used control algorithm in industry is the PID or the Proportional+Integral+Derivative algorithm or one of its variants, e.g. just proportional (P), or Proportional+Integral (PI).

The PID algorithm gives a smooth and very accurate controlo of the temperature. The theory of the PID algorithm is described below.

The PID algorithm is often referred to as a “three term” controller. İt is currently one of the most frrequently used controller in the process industry. On a PID controller the control variable to the plant is generated from a term proportional to the error, a term which is the integral of the error, and a term which is the derivative of the error.

Proportional: the error is multiplied by a gain Kp. A very high gain may cause instability, and a very low gain may cause the system to drift away. Integral: the integral of the error is taken and multiplied by a gain Ti. The gain can be adjusted to drive the error to zero in the required time. A too high integral gain may cause oscillations and a too low gain may result in a sluggish response. Derivative: The derivative of the error is multiplied by a gain Td. Again, if the derivative gain is too high the system may oscillate and if the gain is too low the response may be sluggish.

Figure shows the block diagram of the classical continuous-time PID controller [8]. Tuning the controller involves adjusting the parameters Kp, Td, and Ti in order to obtain a satisfactory response. The characteristics of PID controlelrs are well known and well established, and most modern controllers are based on some form of PID.     

  Continuous-time system PID controller

dt

tdeTddtte

TiteKtu p

)()(

1)()( (1)

n

kidp kTe

T

T

T

TkTekTeTkTeKkTu

1

)()()(

)()( (7)

The discrete form of the PID is:

Development

Kit

D/A Converter

Heater

Power Amp.

Sensor Fan

 

The algorithm of the ON-OFF Controller is:

BEGINDO FOREVER

Read plant temperature from A/D converter channel 0IF Plant temperature > Set point temperature THEN

Turn ON fanELSE

Turn OFF fanENDIF

ENDDOEND

ON-OFF CONTROL

ON-OFF CONTROL

PID CONTROL

The controller parameters were obtained by forcing the system to oscillate and then the frequency of oscillation and the amplitude of oscillation were recorded and the PID parameters were estimated.

PID Control

The response of the system was as follows:

The developed kit has been successful and the temperature inside the thermal plant has been controlled using both an ON/OFF type control and a PID relay based auto-tuning control.

The results show that with both methods the temperature inside the thermal plant has stabilized at the desired set-point. The PID based design offered a better response as the output was not oscillatory.

The kit developed by the author can be used in teaching the practical aspects of automatic control where students can develop programs on the microcontroller and then control the temperature by implementing various controller algorithms.