Intelligent Measurement System with Strain Gauge Sensor for Engineering Education DORINA PURCARU*, CORNELIA GORDAN**, ION PURCARU ***, MARIUS NICULESCU* *Faculty of Automation, Computers and Electronics University of Craiova 13 Al I Cuza Street, 0055 Craiova **Faculty of Electrical Engineering and Information Technology University of Oradea 1 Universitatii Street, 41007 Oradea **VIG Impex, Craiova 0 C Brancoveanu Street, 0033 Craiova ROMANIA dpurcaru@electronicsucvro, cgordan@uoradearo, purcaruion@yahoocom http://wwwaceucvro Abstract: - The signal conditioning circuits, data acquisition boards, signal processing methods and sensors studying represent an essential step for engineering education of students in automation, electronics or mechatronics This paper presents an experimental system that consists of strain gauge sensor, signal conditioning module, data acquisition board, TTL/RS-3 converter, power supply and IBM-PC compatible computer This intelligent measurement system ensures the computerized study of the strain gauge sensor using a friendly user interface The studying procedure supposes the assignation of the measurement points, plotting the transfer characteristic and computing some parameters of the force transducer The experimental equipment may be also used as weighing cell for some objects The students apply their knowledge about sensors and transducers, instrumentation amplifiers, microcontrollers and computers for understand the system architecture and the guideline for studying the strain gauge sensor Key-Words: - Education, strain gauge sensor, signal conditioning circuit, microcontroller, computer 1 Introduction The strain gauge is a device for measuring the changes in distances between points in solid bodies that occur when the body is deformed [1,,3] Strain gauges are used either to obtain information from which stresses in bodies can be computed or to act as elements on devices for measuring such quantities as forces, pressure, and acceleration The favorable factors of strain gauges are the small size and very low mass, excellent linearity over wide range of strains, low and predictable thermal effects, high stability with time, relatively low in cost, and resistance change is the circuit output Thermal degradation, relatively low output signals, careful installation procedures and moisture effects are the main limiting factors of strain gauges This paper presents an experimental system for strain gauge sensor studying; it is very useful in engineering education because the students apply the knowledge about sensors and their applications, signal conditioning circuits, signal processing, microcontrollers and computers, for understand how this experimental system works Other experimental system is presented in [4,5] and it enables the rotary incremental encoder study: working modes and methods for resolution improvement, displacement and speed measurement, sense discrimination That system consists in one rotary incremental encoder, an IBM-PC compatible computer, an intelligent interface, a mechanical subsystem and a driving module; it illustrates how the same sensor can measure different physical variables A soft method for resolution improvement and sense discrimination is also presented and verified in [5] The accuracy of rotary incremental encoders is studied in [6] Hardware Architecture and Design Considerations The measurement system presented in this paper consists of strain gauge sensor, signal conditioning ISSN: 1790-5109 75 ISBN: 97-960-474-09-5
module, data acquisition board, TTL/RS-3 converter, power supply and an IBM-PC compatible computer (Fig 1) The main components of this intelligent measurement system are briefly described in this section TTL / RS-3 Converter Data Acquisition Board F Strain- 4 Signal 1V Gauge Conditioning Sensor Module -1V 1V -1V 5V Power Supply 0V/50Hz Fig 1 Hardware architecture of the intelligent measurement system for strain gauge sensor study 1 Strain Gauge Sensor A load cell translates loads or forces into measurable electrical output Strain gauges are placed in the elastic element to sense the strain induced by the load applied on the load cell For strain measurement, a Wheatstone bridge is formed to convert the resistance change to a voltage change Greater sensitivity and resolution are possible when more than one gauge is used A basic full-bridge circuit typically used in strain gauge transducers is illustrated in Fig ; four identical gauges (SG 1, SG, SG 3, and SG 4 ) are connected one to each of the Wheatstone bridge sides The output voltage V IN depends on the resistances of the strain gauges (R SG1, R SG, R SG3, and R SG4 ) and bridge excitation (continuous voltage V S ): 3 3 5V 1V -1V 5V R SG1 R SG4 V = IN VS (1) R SG1 R SG R SG3 R SG4 SG 1 Fig Full-bridge circuit typically used in strain gauge transducers Because the strain-initiated resistance change ( R SGi ) is extremely small for each gauge [3,7], the output voltage of the full-bridge circuit with identical gauges (R SGi =R SG, i=1,,3,4) is given by VS R SG1 R SG R SG3 R SG4 V = IN 4 R SG R SG R SG R SG () The gauge factor (GF) definition [,3] allows us to point out the input-output function of the strain gauge sensor: VS VIN = GF ( ε1 ε ε3 ε 4), (3) 4 where ε i is the strain applied to the gauge SG i If a positive (tensile) strain is applied to gauges 1 and 3, and the same but negative (compressive) strain to gauges and 4, the output voltage will be four times larger: V IN = GF VS ε (4) Bending stress measurement with such 4-gauge system is illustrated in Fig 3; the strain gauges SG 1 and SG 3 are bonded on the top surface, and SG and SG 4 on the bottom surface of the elastic element Fig 3 Bending stress measurement V IN Four resistors (for zero compensation, bridge balancing, output adjusting, and thermal sensitivity reduction, respectively) can be connected in the basic full-bridge circuit from Fig to improve the strain gauge sensor performances [,7] SG 4 SG SG 3 V S ISSN: 1790-5109 76 ISBN: 97-960-474-09-5
Signal Conditioning Module The hardware configuration of the signal conditioning module is illustrated in Fig 4; IA is an instrumentation amplifier (INA114), and NSCA is a noise suppression circuit and amplifier One voltage reference and three regulators prepare the reference voltage for INA114, the excitation of the strain gauge sensor ( V S = 10V ) and the supply voltages of the instrumentation amplifier ( V IA = 9V, V IA = 9V ) INA114 is ideal for a wide range of applications, including bridge amplifier; its gain is set by connecting a single external resistor (R G =100Ω in Fig 4), and the level of the output voltage (V OUT ) is adjusted using the reference voltage V REF, as we can see from the following expression: 50kΩ VOUT = VIN 1 VREF R (5) G internal bus and a memory register (LATCH) is used for A0A7 bits of the multiplexed address/data bus In Fig 5, RTC is the real-time clock, and TXD, RXD are the communication lines between the data acquisition board and TTL/RS-3 converter (Fig1) The hardware resources of this interface enable engendering of some digital or analog commands and also the acquisition of many digital or analog signals for computing some variables or parameters [9] The program modules, at the data acquisition board level, are written in the assembly language and C for 0C55 microcontroller Many possibilities exist for using this flexible and versatile data acquisition board in various industrial applications and laboratory platforms [4,,9] The picture of the intelligent measurement system for strain gauge sensor study is shown in Fig 6 3 Data Acquisition Board The block diagram of the data acquisition board (in detail presented in [,9]) is shown in Fig 5; it is configured around a 16-bit microcontroller from the 0C55 family Philips Semiconductors [10,11,1] The data acquisition board is provided with analog inputs (P50 P57), 16 digital inputs/outputs (P10 P17, P40 P47) and width-modulated digital outputs (PWM0, PWM1) The bi-directional bus driver assures the access to the internal data bus AD0-AD7 The address demultiplexer (DEC) forms selections for the ports that will be connected to the Fig 6 Intelligent measurement system for strain gauge sensor study SIGNAL CONDITIONING MODULE F S Strain Gauge Sensor S- V S V IN R G V IN - V IA IA V IA V OUT 1V NSCA -1V U O Data Acquisition Board V S V REF V IA V IA Voltage Reference & Regulators 1V -1V Fig 4 Signal conditioning module ISSN: 1790-5109 77 ISBN: 97-960-474-09-5
RS-3 RXD/TXD QUARTZ EE K PSEN I C TTL / RS-3 CONVERTER 0C55 RX/TX UART ALE 1 HIGH ADR ADR/DATA WR RD P4 PORT P1 PORT P5 PORT 3 5 P40 P44 LATCH RD WR LOW ADR A0 A7 (ADR) DATA AD0 - AD7 WR RD 16 SRAM 1M CS 7 RTC 4 4 16 P45 ADR A0 A15 SEL EXT I/O PORTS CS 0 CS 7 PSEN DEC 16 16 EPROM/FLASH 3/64K RW RD DRIVER EXT DATA Fig 5 Data Acquisition Board The students can learn to use microcontrollers even if 0C55 is not very fast (because it is older generation) Other data acquisition board may be then easy configured around a new microcontroller, more efficient in applications 3 Procedure for Studying the Strain Gauge Sensor The student must follow three stages for the strain gauge sensor studying: a) Assignation of the measurement points; b) Plotting the transfer characteristic and computing some parameters of the force transducer; c) Application All stages are presented in this section 4) Validate the first measurement point by pressing the Point validation button The gravitation force of the tested object and the output voltage of the signal conditioning module are then displayed for this point, in the fields G[N] and U O [V], respectively 5) Put the first object on the sensor table and introduce its mass (m[g]) 6) Validate the second measurement point using the same Point validation button Number of measurement points Transfer characteristic button Point validation button 31 Assignation of the Measurement Points The dialog window for the assignation of the measurement points is illustrated in Fig 7 This window appears when the student accesses the main program The user must then follow eight steps presented below 1) Specify the number of measurement points (minimum and maximum 10) ) Don t put an object on the sensor table 3) Press the button Start measurements Start measurements button Graphic displaying button Current point Fig 7 Assignation of the measurement points ISSN: 1790-5109 7 ISBN: 97-960-474-09-5
The characteristic values are then displayed for the second point, in the fields G[N] and U O [V] 7) Continue the point assignation until the last measurement point is validated The mass introduced for the current point must be always greater than the mass for the previous point ) Press the Graphic displaying button for advancing to the next stage of the procedure 3 Plotting the Transfer Characteristic and Computing Some Transducer Parameters The dialog window for this stage is that illustrated in Fig In the plane (G,U O ) we can see the measurement points (1,, ) assigned in the previous stage of the procedure In the right side of this dialog window there are two fields ( Processing and Results ) and many buttons for computing some characteristic parameters of the transducer Processing Zero error Linearization Zero error compensation Results Measurement point Computed zero error Fig Measurement points and computed zero error Differential sensitivity Relative sensitivity Experimental measurements Mass of the weighed object Results Computed values of relative sensitivity Linearized transfer characteristic with compensated zero error Fig 9 Linearized transfer characteristic with compensated zero error and computed values of relative sensitivity ISSN: 1790-5109 79 ISBN: 97-960-474-09-5
This second stage supposes five steps 1) Compute and display the zero error by pressing the Zero error button; the computed value can be seen in the field Results (Fig ) ) Linearize the transfer characteristic U O =f(g) using the Linearization button (Fig ) 3) Compensate the zero error by pressing the Zero error compensation button 4) Compute the differential sensitivity using the button with the same name (Fig 9) This parameter is computed for each line segment of the transfer characteristic U O =f(g) The differential sensitivity formula, and the minimum, maximum and ideal values of this parameter are all displayed in the field Results 5) Compute the relative sensitivity by pressing the button with the same name (Fig 9) This parameter is also computed for each line segment of the transfer characteristic U O =f(g) The relative sensitivity formula, the minimum, maximum and ideal values can be all seen in the field Results (Fig 9) 33 Application This experimental equipment becomes an electronic weighing cell when the user presses the button Experimental measurements (Fig 9) There is a small table fixed on the sensor and different objects can be put on this table (Fig 6) The measurement system displays the mass of the weighed object (Fig 9) 4 Conclusion An intelligent measurement system adequate for the strain gauge sensor studying is presented in this paper The main components of this system are the following: sensor, signal conditioning module, data acquisition board, and computer A friendly student interface enables the assignation of the measurement points for plotting the transfer characteristic and computing some parameters of the force transducer The experimental system can become an electronic weighing cell and it displays the mass of the weighed object If the students use this intelligent measurement system and understand how it works, they can apply their previous knowledge about strain gauge sensors, instrumentation amplifiers, signal conditioning circuits, microcontrollers and computers Other data acquisition board may be then easy configured around a new microcontroller, more efficient in applications References : [1] A Helfrick, W Cooper, Modern Electronic Instrumentation and Measurement Techniques, Prentice-Hall International Editions, 1990 [] F Lackner, W Riegler, P H Osanna, M N Durakbasa, High Precision Strain Gauge Based Sensor for Monitoring Suspension Forces at CERN, Measurement Science Review, Volume, Section 3, No, 00, pp 46-49 [3] I Sinclair, Sensors and Transducers (Third Edition), Newness, 001 [4] D Purcaru, E Niculescu, I Purcaru, Experimental Measuring System with Rotary Incremental Encoder, WSEAS Transactions on Advances in Engineering Education, Issue 9, Vol 4, September 007, pp 10-16 [5] DM Purcaru, E Niculescu, I Purcaru, Measuring Method Adequate for Computerized Study of Rotary Incremental Encoders, WSEAS Transactions on Electronics, Issue 6, Vol 3, June 006, pp349-355 [6] P Koči, J Tůma, Incremental Rotary Encoders Accuracy, 7 th International Carpathian Control Conference, Ostrava Beskydy (Czech Republic), 006, pp57-60 [7] H K Tönshoff, I Inasaki (Editors), Sensors Applications Vol1 - Sensors in Manufacturing Wiley-VCH Verlag GmbH, 001 [] I Purcaru, D Purcaru, E Niculescu, C Rusu, M CăpăŃînă, Central Units in Data Acquisition Systems for Engineering Education, Scientific Bulletin of the Politehnica University of Timişoara, Romania, Transactions on Electronics and Communications, Tome 53 (67), 00, pp 37-41 [9] DM Purcaru, E Niculescu, I Purcaru, SD Nedelcut, System Design and Implementation for Some Technological Parameters Measurement, International Review of Electrical Engineering, Vol3, No3, May-June 00, pp 573-579 [10] Philips Semiconductor Data Sheet, Integrated Circuits XA 16-bit Microcontroller Family, 001 [11] V C Petre, The Technician's Point of View in Microcontroller Based Measurements, WSEAS Transactions on Advances in Engineering Education, Issue, Volume 3, 006, pp 779-7 [1] V Vašek, P Dostálek, J Dolinay, D Janáčová, K Kolomazník, Microcontrollers and Modern Control Methods, The nd WSEAS International Conference on COMPUTER ENGINEERING AND APPLICATIONS (CEA'0), 00, Acapulco, Mexico, Proceedings, pp 195-19 ISSN: 1790-5109 0 ISBN: 97-960-474-09-5