ARTICLE
Year : 2010 | Volume
: 56 | Issue : 4 | Page : 189--192
Design of Intel 8751 Microcontroller-based System for Monitoring and Control of a Thermal Process
Francis Enejo Idachaba
Department of Electrical and Information Engineering, College of Science and Technology, Covenant University Ota, Ogun State, Nigeria
Correspondence Address:
Francis Enejo Idachaba
Department of Electrical and Information Engineering, College of Science and Technology, Covenant University Ota, Ogun State
Nigeria
Abstract
An Intel 8751 microcontroller-based system was developed to monitor and control the temperature of an oven. The IN4148 signal diode was used as the temperature sensor and the on-off control algorithm was utilized with the system switching off the heating process whenever it attains the preset value. The system provides a digital readout of the system temperature and a status/blinker indicator showing whether heating is in progress or terminated. The control programs were loaded onto the memory of the microcontroller and the enable the selection of the different temperature values. The system improves accuracy by eliminating human participation and saves operator time. The system is easy to operate, maintain, and upgrade.
How to cite this article:
Idachaba FE. Design of Intel 8751 Microcontroller-based System for Monitoring and Control of a Thermal Process.IETE J Res 2010;56:189-192
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How to cite this URL:
Idachaba FE. Design of Intel 8751 Microcontroller-based System for Monitoring and Control of a Thermal Process. IETE J Res [serial online] 2010 [cited 2013 Jun 20 ];56:189-192
Available from: http://www.jr.ietejournals.org/text.asp?2010/56/4/189/70624 |
Full Text
1. Introduction
Industrial processes are pivotal in the advancement of technology. These processes over time have been refined from manually controlled systems to automated computer controlled systems. This has led to more efficient manufacturing processes with very high precision levels [1] .
Physical processes require measurement of the variables involved in the processes and the control of these processes based on the measured variables. Examples of these variables include temperature, flow rate, pressure, etc.
A combination of measurement and control can only be achieved when a form of intelligence is introduced. Intelligent instrumentation can thus be defined as the use of a measurement system to evaluate a physical variable employing usually a microprocessor/microcontroller-based system to perform the information processing [1],[2] . The result of the information processing can then be used to control the physical process based on the stored program in the system memory.
1.1 System Description
The block diagram of the temperature monitoring and control system is shown in [Figure 1]. The system hardware consists of a signal diode (1N414 8), a differential amplifier, a flash comparator analog to digital converter, the Intel 8751 microcontroller, seven segment display interfaced to the microcontroller to enable the display of the set (desired) temperature (ST), and process temperature (PT), which is from 25°C to 99°C. Other circuits are introduced for the blinker and heater system control. The keypad serves as the input device for the ST.{Figure 1}
1.2 Software System
The software governing the operation of the system was developed in assembly language and then complied before being loaded onto the onboard erasable programmable read only memory (EPROM). The algorithm utilized is listed below.
Initialize portsSTSample PTCompare ST and PTIf ST > PT then continue heatingIf ST = PT turn off the heater
The flow chart for the system is shown in [Figure 2].{Figure 2}
1.3 The Transducer Circuit
The general purpose silicon diode IN41 48 was used as the sensor as shown in [Figure 3].{Figure 3}
The 1N4148 signal diode generates a forward voltage of approximately 600 mV when a fixed current of 1v mA is passed through it [3] .
The Zener diode is used to maintain a fixed voltage across the diode such that any variation in the current flowing through the diode will be due to the external fluctuation in temperature, which is converted to voltage by the diode's −2 mV/°C temperature coefficient. This negative coefficient leads to the generation of a voltage with negative slope. This voltage is amplified and inverted by the succeeding stages of operational amplifiers connected to the sensor circuit.
1.4 Microcontroller and ADC
The Intel 8751 has four 8-bit ports, which can be used for data transfer to and from the microcontroller [4] . The system is designed for a temperature range of 25°C-99°C. Within this range, the temperature is sampled at 5°C interval giving a total of 16 discrete readings. The parallel comparator ADC is used for conversion of the sensor output from the analog format to a digital format [5] . Sixteen operational amplifiers in four LM 324 chips are used to implement the parallel comparator ADC [6],[7]. With the microcontroller being an 8-bit chip, the temperature ranges were divided into 8-bit ranges with the lower 8 bit (25°C-60°C being labeled as low temperature and 65°C-99°C being labeled as high temperature). The outputs of the different op-amps are fed to port 1 of the microcontroller using the Tristate 74LS245 octal bus transceivers. The use of the transceivers is to enable the selection of the required range of temperatures to be transmitted to the microcontroller since the microcontroller ports can access only 8 bits at a time. The selection of the desired range access to be read is determined by the activation of the 74LS245 bus transceiver, and this is achieved by the use of the 74LS138 1-8 demultiplexer.
The control program was stored in the onboard EPROM. Port 1 of the microcontroller with the aid of latches (74LS245) serves as the input port for low and high temperatures, and as an output port for driving the seven-segment display. This eliminates the need for a display driver for the seven-segment display.
The port 3 pins addressing capabilities are enhanced by the use of the 74LS138, which increases the addressable pins of the microcontroller. The other pins in port 3 are assigned the up, down, and enter push buttons and the relay. Port 0 and 2 are used for accessing external memory and these pins are unconnected since no external memory is used. The pins 29 and 30 are also left unconnected due to the nonutilization of external memory. The circuit is shown in [Figure 4].{Figure 4}
1.5 Control Electronics for Heater and Blinker Circuits
The control electronics is responsible for the actual ON-OFF switching of the heater, the status indicator and the blinker circuits. The circuit is given in [Figure 5].{Figure 5}
1.6 Switching/Indicator Algorithm for the Heater/Blinker Controller Circuit
At power on, Ra and Rb are de-energized.Rc is energized through the normally closed contact of Ra.With Rc energized, the status Light Emitting Diode (LED) stays on.With ST entered, Ra is energized.Rb is also energized through the normally open contact of Ra; this switches ON the heater.Rc is de-energized as it is connected to closed contact of Ra, which is now open.The status Led is connected to the astable multivibrator, which flashes at 0.73 Hz, indicating that heating is in progress.When ST equals PT, the Ra is de-energized by the microcontroller via P3.3. and this de-energizes Rb switching off the heater.The Rc is energized and status LED stays on, indicating standby mode.
2. Results and Discussion
During the development stages, the various units of the hardware were evaluated individually and as an integrated unit. The software was also tested using the A51 macroassemblers. All the errors found were corrected and the system was tested using several water heating cycles and a mercury thermometer for temperature measurement. The system was calibrated and the results are shown in [Table 1].{Table 1}
[Table 2] shows the heater and status LED test results. These results show that when the heater is on, it stays on until the ST is attained and after that, the heating process stops.{Table 2}
3. Conclusion
The results in [Table 1] and the output after calibration show a change from the diode's negative temperature coefficient to a positive temperature coefficient, which eases system calibration; the device control mechanism shows a switching of the heater in line with the control algorithm. This switches the heater off automatically when the ST is attained. The digital readout also provides a means of observing the temperature of the process as it rises up to the ST when the heater is switched off.
References
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| 6 | J Graeme. Application of Operational Amplifiers Burr-Brown Research Corporation. New York: McGraw Hill; 1973. |
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