Design of LVDT Based Digital Weighing System

Transcription

1 International Journal of Electronics and Computer Science Engineering 2100 Available Online at ISSN Pratiksha Sarma 1, P. K. Bordoloi 2 1,2 Department of Applied Electronics and Instrumentation Engineering Girijananda Chowdhury Institute of Management and Technology, Guwahati pratiksha.sarma@yahoo.com Abstract- Linear variable differential transformer (LVDT) has evolved into a highly accurate and reliable form of displacement transducer and has found widespread application in industry for the measurement of displacement, force or pressure. This paper discusses the design consideration of an LVDT based digital weighing system capable of weighing up to 1000 grams (1 kilogram). Atmel 89S52 microcontroller is used to acquire, process and display the weight into 16x2 LCD. The system has been designed and calibrated using standard weights and is found to be simple and of low cost. Keywords Weight measurement, Linear Variable Differential Transformer (LVDT), microcontroller, LCD. I. INTRODUCTION A system can be defined as a way of performing task according to some predefined plan, program and a set of rules. With the advancement of technology, the processes or systems are becoming more and more complex. Due to this increase in complexity, for efficient analysis of process, emphasis is given more in design considerations of the system so as to have an effective and user friendly system. Considering the new trends in Instrumentation, effort has been made to design and fabricate a displacement based weight measuring system using LVDT. A linear variable differential transformer (LVDT) is a type of electro-mechanical transducer capable of measuring linear displacement with a high degree of accuracy. Such a system can be used as a force measuring transducer and can be employed to measure spring deformation in weighing system. It offers many advantages over potentiometric linear transducers such as frictionless measurement, long mechanical life, excellent resolution and good repeatability [2,4,5]. LVDT is used as a secondary transducer in various measurement systems. In the present setup, spring is used as a primary transducer to convert the measurand into a displacement. The LVDT is then used to measure that displacement and it gives a corresponding out-of-balance voltage. The present system has been designed with a digital display in a 16x2 LCD panel using microcontroller AT89S52. The system design of our scheme is divided into two parts: Hardware Implementation Software Implementation A. HARDWARE IMPLEMENTATION II. DESIGN IMPLEMENTATION Our hardware design specification mainly requires: a) Construction of the LVDT b) Design of the signal processing circuit c) Construction of a programmable hardware circuit for digital display. Construction and working of the LVDT: The physical construction of a typical LVDT consists of a movable core of magnetic material and three coils comprising the static transformer [2,7]. Here, we have constructed the LVDT in a plastic spool where three windings (one primary and two secondaris) are made according to requirement. The windings are made by using enamel Cu-wire of 31 SWG (Standard Wire Gauge), each winding consisting of 2000

2 2101 turns. The primary of the LVDT is excited from a 50 Hz, 230 V AC supply through a , 500 ma step down transformer. Two terminals of the secondary windings are connected in series opposition. The output is rectified by a full wave rectifier and the DC voltage output is measured between the other two terminals (Fig 1). Figure 1. Design of the LVDT section Signal Conditioning Circuit: The signal conditioning circuit (fig 2) consists of an Op-Amp OP07. The Op-Amp OP07 works as a voltage level shifter and amplifier circuit [1, 5]. A reference voltage of about 1.6V is given to the inverting terminal of the op-amp through a voltage divider circuit. The circuit is so designed that it compares the reference input with the rectified output of the LVDT and accordingly produces an output ranging from 0 to 5V, suitable for the ADC input.. Figure 2. Signal Conditioning Circuit Construction of programmable hardware circuit: The digital circuitry (Fig. 3) mainly includes the interfacing of the ADC 0804 and LCD with the microcontroller AT89S52 [3,6]. A zener diode is used across the input of the ADC to protect it from over voltage.

3 IJECSE, Volume 1,Number 4 Pratiksha Sarma and P. K. Bordoloi 2102 Figure 3. Digital Circuitry for processing and display III. BLOCK DIAGRAM AND OPERATION The block diagram of the system is shown in Fig 4. The output voltage from the secondary terminals of the LVDT is rectified by a bridge rectifier (4 diodes of 1N4007) and the rectified voltage is filtered by a shunt capacitor of 220µF. Also a resistance of 1K is connected across the shunt capacitor. The rectified output from the bridge is given to the signal conditioning circuit. The output of the signal conditioning circuit is then applied to the input pin (pin no 6) of ADC0804. As input to the ADC cannot exceed 5.25 V, so a zener diode of 5.1V has been used across the input for over voltage protection. The analog input is converted to a digital 8-bit data. Therefore, for an analog input of 0 to 5V, the 8-bit ADC produces an output from 0 FFH or (decimal). This digital data from the ADC is fed to port1 of the microcontroller AT89S52. The microcontroller processes the hex value from port 1 and converts it into a suitable decimal value from 0 to 1000 g in 256 steps. The microcontroller then displays this output on the 16x2 LCD panel (JHD162A).

4 2103 B. SOFTWARE IMPLEMENTATION Figure 4. Block Diagram of the system The software design includes developing algorithm for the system, allocating memory blocks as per functionality, writing separate routines for different interfacing devices and testing them on the designed hardware. Interfacing the microcontroller with ADC, LCD, memory etc. has been carried out using software modules. All the control programs are written in C language. The code for the project is developed in Keil C51 IDE. START INITIALIZE LCD AND ADC PINS SEND THE CONVERSION TO THE MICROCONTROLLER BY WRITING 0 BIT START THE INFINITE LOOP IN MICROCONTROLLER READ THE DIGITAL DATA FROM ADC PROCESS THE INPUT DIGITAL DATA IN THE MICROCONTROLLER FOR DISPLAY DISPLAY TO THE LCD END Figure 5. Flow Chart

5 IJECSE, Volume 1,Number 4 Pratiksha Sarma and P. K. Bordoloi 2104 Figure 6. Photograph of the circuit Figure 7. Photograph of the LCD displaying weight in grams III. EXPERIMENTAL OBSERVATION After the operation of the circuit, it is calibrated for experimental observation. Calibration is done by using standard weights. The system has been calibrated for 0 to 1000g with five standard weights of 200g each and observation is done for 5 sets of readings within the range g with an interval of 200g. After taking these readings, the calibration curve has been plotted between standard weights and LCD reading, displaying weight in grams (fig 8). From the figure, the curve is found to be almost linear.

6 2105 Figure 8. Experimental Graph IV. RESULTS AND DISCUSSIONS Range: The system is designed for the range of 0 to 1000 grams. But the range can be increased by choosing spring with different spring constant. The range of the system depends on the spring deformation. Resolution: The resolution of the ADC used is 19.5 mv when V ref is 5V as it is an 8 bit ADC. So with a change of 19.5mV in the input of the ADC, the digital output of the system changes by 3.92 g. Therefore the resolution of the system is 3.92g. Accuracy: The output of the designed system is found almost linearly proportional to the input displacement. There are a number of factors affecting the accuracy of the system such as unstabilized input AC voltage, the spring may be non-linear, presence of noise in the circuit components. Though accuracy of the system is somewhat reduced but according to its design it gives quite satisfactory results. Also by using 12-bit or 16-bit ADC, the resolution and accuracy of the system can be increased to a great extent. V. CONCLUSION With the present design of the hardware and software, the weighing system is found to be working satisfactorily with accuracy of ±4 gms. The system is compact, have fast response and system linearity, however, is maintained within maximum spring deformation (or LVDT core movement) of 10mm. VI. FUTURE IMPROVEMENT AND SCOPE The present system with minor modifications and firmware incorporation, can be used successfully for laboratory and industrial use. More elaborate systems using multiple springs or compound cantilevers can be built that may have impact on temperature and hysteresis effect as well as linearity. There is scope for design of the mechanical structure of the LVDT to reduce the effects of side loading, vibrations and damping. Acknowledgement: Author is thankful to N. Manoranjan Singh, Research Scholar and DST Inspire Fellow, Department of Instrumentation and USIC, Gauhati University for his valuable advice, help and encouragement during the period of the work.

7 IJECSE, Volume 1,Number 4 Pratiksha Sarma and P. K. Bordoloi 2106 V. REFERENCE [1] Coughlin Robert F. & Driscoll Frederick F., Operational Amplifiers and Linear Integrated Circuits, 6 th Edition, Prentice-Hall of India Private Limited, pp [2] Sawhney A.K., A Cource In Electrical & Electronics Measurement And Instrumentation, Dhanpat Rai & Sons Publications [3] Mazidi, Mazidi and McKinlay, The 8051 Microcontroller And Embedded System Using Assembly and C, 2nd Edition, pp [4] Signal Conditioning an LVDT, Application Report SPRA946 - August 2003, Texas Instrument [5] N. Mathivanan, PC-Based Instrumentation Concepts and Practice 1 st Edition, PHI Learning Private Limited, pp 12-28, [6] [7]