Session T2E COMPUTERIZED LABORATORY PRACTICE FOR FUTURE SCIENCE AND TECHNOLOGY TEACHERS Slavko Kocijancic1 Abstract The attitude of pupils at secondary school level towards science and technology is significantly influenced by their teachers. Since studying engineering and science has become less popular in recent years it is important to make science and technology lessons more attractive to an extended range of pupils. The project described in this paper is concerned with the modernization of school laboratory apparatus and methods, with emphasis on the use of computers for data acquisition, data analysis and control. An important element in the practice of computerized laboratory work in schools is to adequately train future science and technology teachers as well as teachers already working in schools. Related courses are based on projectoriented work integrating science, computing, robotics, mathematics, etc. Examples of such projects are presented. Index Terms Science teaching, technology teaching, computer based laboratory, project work.
INTRODUCTION It is a worldwide phenomenon that the study of engineering and science has become less popular in the recent years. Secondary school teachers could have a significant role in changing this tendency by improving the attitude of pupils towards science and technology. Laboratory exercises with some hi-tech elements appear to motivate a majority of pupils. For this reason and others, computers as a part of classroom technology are regularly used for data acquisition (DAQ) [1]. Software is used to support DAQ to display data numerically and graphically and for data analysis [2-4]. In some cases, it is convenient to control the experimental conditions by driving motors, heaters, generating electrical signals or to build models of computer controlled devices [5]. For the effective use of computer supported laboratory work in school from a pedagogical and technical viewpoint, it is essential to adequately train future teachers of science and technology, as well as to permanently prepare courses for in-service training of present teachers. The Department of Physics and Technology at the Faculty of Education in Ljubljana (Slovenia) has been coordinating the development of computer based instrumentation, software and courseware for the last ten years. A multifunction computer interface, named CMC-S2, with analogue and digital input/output (I/O) functions was designed and manufactured in Slovenia. The interface is designed as a compromise between its cost, its usage in 1
measurement applications (eight channel analog input with relatively high sampling rate of 200 k/second per channel) as well as its facility to support various control possibilities through a 16-bit digital output and a two-channel analog output. The CMC-S2 interface comes complete with various sensors, amplifiers, rectifiers, transducers, models of robots and apparatus for some specific experiments. In parallel to the hardware and instrumentation, software was developed continuously to support practical work in school laboratories. The software supports real time data acquisition, can represent the data numerically and graphically. It includes some extensive numerical tools for analysis of the sampled data such as fast Fourier transform, smoothing, derivatiation in time and curve fitting. Sampled data can be exported in text form compatible with software packages for data analysis. Standard spreadsheet software packages are usually used for this purpose because of their easy availability and widespread applicability. Traditional laboratory manuals have been replaced by html based manuals integrating Java applets, video clips and hyperlinks to the data acquisition software [6]. Related courses for future science and technology teachers are based on project oriented work integrating electronics, computing, robotics, science, mathematics, etc. Examples of such students' projects include gas laws, wave phenomena of sound and ultrasound, properties of optical sensors, principles of infrared remote control, computer based control of light intensity, bicycle generator as an example of a synchronous machine, ultrasonic distance measurement via printer port, voltage current curves, transient phenomena in RLC circuits, straight line and rotational dynamics, thermal conduction and light absorption spectra. More detailed information can be found on our website [7].
EXAMPLE PROJECTS The trainee teachers are initially introduced to computerized laboratory work with an overview of basic data acquisition principles. Through practical work with the computer interface CMC-S2, students become aware of the fundamental characteristics important for the application of digital and analog I/O. Digital output is used for on/off control of various actuators (DC and stepper motors, heaters, bulbs, etc.) involving transistor circuits and relays. The functionality is compared with the use of analog output where actuators are controlled via power amplifiers. Digital
Slavko Kocijancic, University of Ljubljana, Faculty of Education, Dept of Physics and Technology, SI -1000 Ljubljana, Slovenia,
[email protected]
0-7803-6424-4/00/$10.00 © 2000 IEEE October 18 - 21, 2000 Kansas City, MO 30 th ASEE/IEEE Frontiers in Education Conference T2E-13
Session T2E inputs are connected to mechanical switches as well as to more sophisticated digital on/off sensors (light gates, incremental shaft encoders). The rest of the time is dedicated to the use of the analog input where various analog sensors (temperature, force, position, pH, etc) are utilized. After the introduction, the laboratory work is focused on particular projects. Some of the projects are common to all students; others depend on individual trainee teachers' subjects (e.g. physics, technology). In what follows, examples of such projects are outlined. Characteristics of Optical Electronic Components The project is presented as a study of light sources and light detectors (photo sensors). In an initial discussion students tell what they already know about optical components - what they are called, what are their characteristics and how they can be used in practical applications. Applications where a light beam emitted by an LED is directed onto a photo sensor are more closely discussed. Reference is made to the use of a photo gate as a stop clock and the general principles of optical communications. Students are asked to design their own experiments. Some of the suggested topics include plotting V-I curves of LEDs, V-I curves of a light dependent resistor (LDR) at controlled illumination. The last objective in the project is to construct a circuit, which can transform a voltage signal at the input of the circuit to light intensity, which can then be converted back to a voltage. It can be concluded from the previous examples that the intensity of the emitted light is proportional to current through the LED. A V to I converter seems to be a relevant choice of circuit to convert an input voltage to light intensity. For detecting light intensity two possibilities come into are usually considered either (a) converting resistance or (b) converting conductance to a proportional voltage. At this point the photoelectric effect is briefly explained as a transfer of energy from incident light on a substance to free electrons within the substance. Such clarification is necessary for a decision to convert conductivity to voltage. The complete circuit with VFG as the input voltage derived from a function generator and VADC2 as the output voltage is shown in Figure 1.
FIGURE 1.
The ideal relationship for analog optical transmission of voltage would be linear conversion of voltage into light intensity and linear conversion of light intensity to conductance. Students are asked to derive a mathematical relationship between VFG and VADC2 . Finally the proposed relationship is checked experimentally by connecting triangular wave as the input voltage VFG . The voltage VFG and the output voltage VADC2 are connected, respectively, to analog input channels of the computer interface. Results are compared for all basic photo sensors, namely LDR, transistor and photodiode. The combination of an LED with photodiode (Figure 2) provides a relationship which is closer to linear compared to that of an LED combined with an LDR. Students use a standard spreadsheet application to numerically analyze the relationships.
FIGURE 2. CONDUCTANCE OF PHOTO DIODE DEPENDING ON CURRENT THROUGHLED
An interesting phenomenon can be observed at this point. One can couple two LEDs, one as a light source and one as a photo sensor (reverse biased). Students study various combinations of pairs of LEDs emitting different colors (infrared, red, orange, green and blue). For example, using a green LED as the source and a red LED as sensor works, whereas opposite combination does not. Comparing turn-on voltages with energy of photons, the appearance of photoelectrons in semiconductors is briefly explained.
FIGURE 3. DYNAMIC CHARACTERISTI CS OF COUPLED LED AND LDR AT 70 H Z
COUPLING AN LED WITH DIFFERENT PHOTO SENSORS
0-7803-6424-4/00/$10.00 © 2000 IEEE October 18 - 21, 2000 Kansas City, MO 30 th ASEE/IEEE Frontiers in Education Conference T2E-14
Session T2E The circuit in Figure 1 is intentionally investigated at low frequency and students are encouraged to study its dynamics. With the LDR, the signal at the output is clearly deformed at 70 Hz (Figure 3), while the photodiode continues to behave normally even at 7 kHz (Figure 4).
Figure 1 is checked by replacing the triangular waveform input with square waves. Students are asked to compare the time response of the circuits using an LDR and using a photodiode as the photo sensor (Figure 5). Ultrasonic Wave Phenomena Two ultrasonic sources are placed at a fixed, but variable, distance apart on a horizontal plane, directed upwards and connected to 40 kHz voltage supply to provide a source of parallel (coherent) ultrasonic waves. A stick, which can be rotated at one edge around an axis on the plane (the angle of rotation is measured by potentiometer - see Figure 6) has a receiver mounted at the other edge connected to an amplifier with AC to DC conversion. Rotating the receiver above the sources, a typical interference pattern can be obtained (see Figure 7). One source can be removed and replaced with the vertical plate to show interference of an ultrasonic source with its reflected waves.
FIGURE 4. DYNAMIC CHARACTERISTI CS OF LED AND PHOTODIODE AT 7 K HZ.
FIGURE 6. APPARATUS FOR ULTRASO NIC WAVE PHENOMENA .
FIGURE 5. SQUARE WAVES TRANSMITTED THROUGH COUPLED LED WITH LDR (TOP) AND LED WITH PHOTODIODE ( BOTTOM). NOTE THE DIFFERENT TI ME SCALES.
Because of the considerable importance of digital optical communications systems, the dynamics of circuit in
FIGURE 7. I NTERFERENCE PATTERN OF ULTRASONIC WAVES EMITTED BY TWO COHERENT SOURCES.
0-7803-6424-4/00/$10.00 © 2000 IEEE October 18 - 21, 2000 Kansas City, MO 30 th ASEE/IEEE Frontiers in Education Conference T2E-15
Session T2E Students are required to determine the wavelength of the ultrasound and its speed in air from data obtained by varying the distance between sound sources.
By analyzing the data using spreadsheet software and applying curve fitting methods, students are required to determine constants k and a in the following equation:
Gas Laws
pV k = a ,
A pressure sensor with a range from 0.05 bar to 2.00 bar is used to measure pressure in a plastic glass cylinder with movable piston (see Figure 8). The cylinder has a built-in temperature sensor and electric heater. A potentiometer connected to the piston is used to determine the volume of gas confined within the cylinder. Pressure, temperature and volume are simultaneously displayed. Relations between p, V and T can be investigated and a p-V curve plotted. The change is almost isothermal if the piston is moved slowly and almost adiabatic for rapid changes of the piston position. Starting with an adiabatic expansion, waiting for the gas to absorb heat at constant volume and then performing an isothermal compression, the cyclic change can be shown (see Figure 9).
where p is gas pressure and V is its volume. Direction Sensitive Photogates The operational use of a photogate is as follows. A light beam from a source is directed onto a photogate. The logic level at the output of the electric circuit is changed when the light beam is interrupted. Photogates of that type are available from most of suppliers of educational equipment. Fixing plates with alternate transparent and non-transparent areas (see Figure 10) to an object in motion enable its velocity and/or acceleration to be determined and curves of x(t), v(t) and/or a(t) can be plotted on the computer screen. A consequence of having only one photo sensor is that it is not possible to determine the direction of motion of the moving body. By using only a single photogate it is not possible to distinguish a change in direction during the motion, as happens, for example after an upward throw or during oscillatory motion. The use of one photogate alone, therefore, cannot help to overcome the very frequent misconception that there is always a positive acceleration at increasing speed and a negative acceleration at decreasing speed.
FIGURE 10. DETERMINATION OF VELOCITY , ACCELERATION AND PLOTTING X( T) CURVES
FIGURE 8. APPARATUS FOR GAS LAWS EXPERIMENT.
Thus detection of the direction of motion requires two sensors, for example optical incremental shaft encoders. As shown in Figure 11, two sensors connected to two digital inputs of a printer port (LPT) can be used in this way.
FIGURE 11. T HE PRINCIPLE OF A D IRECTION SENSITIVE P HOTOGATE
FIGURE 9. P -V CYCLIC CHANGE IN A G AS.
The photogate circuit can be constructed from commonly available low-cost components. Light emitting diodes with narrow light beam and high illuminating power (3cd) are adequate for use as light sources. Using LED's in
0-7803-6424-4/00/$10.00 © 2000 IEEE October 18 - 21, 2000 Kansas City, MO 30 th ASEE/IEEE Frontiers in Education Conference T2E-16
Session T2E the visible spectrum (red) is helpful from a didactic point of view. The characteristics of the photo sensors used need to have suitable response time. A phototransistor or photodiode can be combined with a voltage comparator or SchmittTrigger to obtain a digital signal at the output of the photogate. Even simpler is the use of a logic gate detector IC with a built in photodiode, amplifier and Schmitt-Trigger such as the SFH 5440 [8]. An electrical schematic and instructions on how to interface the light gate via the LPT can be found on the website from which software supporting such light gate can also be downloaded [7]. Results obtained for uniformly accelerated motion using photogates detecting direction are shown in Figure 12. FIGURE 13. T IME DEPENDENCE OF POSITION, VELOCITY AND ACCELERATION OF A PENDULUM ( LEFT) AND THE CORRESPONDING VELOCITY VS. POSITION CURVE ( RIGHT).
CONCLUSIONS
FIGURE 12. VELOCITY AND ACCELERA TION IN UPHILL AND D OWNHILL MOTION
Ultrasonic Distance Sensor An ultrasonic distance sensor connected to a computer is a convenient way to measure the position of a body in motion. A change of logical level from 0 to 1 at the digital input of the sensor triggers the emission of a 40 kHz ultrasound pulse. The echo of the ultrasound is detected by the sensor and the logical level at the sensor's digital output is changed form 0 to 1. The time between the trigging of the pulse and the detection of its echo is used to determine the distance to the moving body. A commercial motion sensor produced by the Vernier company [9] can be interfaced to a computer via the printer port or via the CMC-S2 interface. Sp ecial software supporting the ultrasonic distance sensor was designed. Besides sampling of position of a body in motion, the software determines its velocity and acceleration curves, presents curves of one variable against the other and fits selected portions of the data to linear or quadratic functions of position in time. Technical information on how to connect the sensor to the printer port can be found on the website from where the software can be downloaded [7]. Typical curves obtained for the oscillatory motion of a ball attached to a string are shown in Figure 13.
After about ten years of introducing computerized laboratory techniques to trainee teachers of physics and technology, the results are encouraging. The first observable outcome was that instructors in the physics laboratories, although not particularly enthusiastic about computerized laboratories, modified some experiments based on traditional instrumentation by applying the computerized data acquisition system. We have analyzed these modifications and have come to some conclusions. In most cases the instructors utilized the computer based oscilloscope for slow processes where relatively expensive storage oscilloscopes were necessary before (gas laws, ultrasound wave phenomena, variation of temperature at various experiments, etc). Traditional stop clocks controlled by sensors were also replaced by computer based stop clocks. Some computer based sensors were introduced to physics laboratories where they had not applied before (sonic motion sensor, electronic gas pressure sensor, acceleration sensor, etc). The second group of traditional experiments which were modified by introducing computers in science laboratories were ones where numerical analysis was invoked for large numbers of samples, such as FFT in sound experiments and differentiation and curve fitting in various kinematic experiments. Since sampled data can be saved in text data files, students are encouraged to engage in further analysis of the data using computer software packages. Students are observed to develop more initiative during computerized laboratory work compared to traditional laboratories. Significant proportions of lesson times are saved by computerized data acquisition and display when compared with the time needed for traditional manual data recording and graph drawing. Immediate feedback is available through 'real-time' presentation and analysis of the data. Students who used computers focused more on general trends in the data rather then giving a significance to individual measurements.
0-7803-6424-4/00/$10.00 © 2000 IEEE October 18 - 21, 2000 Kansas City, MO 30 th ASEE/IEEE Frontiers in Education Conference T2E-17
Session T2E Since project aims are emphasized, students are motivated to apply their knowledge of electronics, computing, physics, and mathematics and to get new information that is required by particular project from the literature and the www. Feedback from students after their practical computerized laboratory course was particularly positive. To find out how frequently on-line experiments are utilized during physics lessons in secondary education in Slovenia, a survey was carried out among approximately 200 first-year students at the Faculty of Education and the Faculty of Mathematics and Physics (both University of Ljubljana, Slovenia). Half of those sampled were future teachers at primary school level and the other half were either students of physics or future physics teachers at secondary level. Over 61% of students sampled remembered on-line experiments from upper-secondary school and about 15% of students gave positive answer for lower-secondary schools. Unfortunately, in such cases, the computer seems to be mostly handled by a teacher and infrequently by pupils (3%). This is in contrast with other results of the survey, which showed that pupils were more appreciative of handson laboratory work than of demonstration experiments. The reason that pupils cannot work with the computer during science laboratory classes is that, in most schools, there is only one computer in the science laboratory. Laboratories equipped with several computers are mostly confined for use in computer studies and, as such, are not equipped with data acquisition systems. The main aim of our efforts to develop some low-cost devices connected to the printer port (photogates, ultrasonic distance sensor), therefore, is to enable hands-on experiences for pupils in such laboratories.
[6]
Kocijancic S., "Integrating Computer Based Science Lab and Multimedia", Proceedings of the Seminar Experiments and Measurements in Engineering Physics Education, Technical University Brno, October 1998 pp. 20-24,
[7]
http://www.pef.uni-lj.si/slavkok/cbe/
[8]
http://www.infineon.com/products/opto/3703.htm
[9]
http://www.vernier.com
ACKNOWLEDGMENT I gratefully acknowledge the support by Ministry of Education and Sport and National Education Institute, Republic of Slovenia. Further progress has been enabled by incorporating the computerized laboratory to the Ro − Computer Literacy Program. The general aim of Ro is to promote all aspects of the application of information technology in education.
REFERENCES [1]
Preston D.W. and Good R.H., "Computers in the General Physics Laboratory", Am . J. Phys. 64 (1996), pp.766-772
[2]
Newton L., "Graph talk: Some Observations and Reflections on Students' Data-logging", Sch. Sci. Rev. 79 (1997), pp. 49-54
[3]
Rogers L., "New Data-logging Tools - New Investigations", Sch. Sci. Rev. 79 (1997), pp. 61-68
[4]
Barton R., "How do Computers Affect Graphical Interpretation", Sch. Sci. Rew. 79 (1997), pp. 55-60
[5]
Martin F.G., "A Toolkit for Learning: Technology of the MIT LEGO Robot Design Competition", Proceedings of the Workshop on Mechatronics Education, Stanford Univ ersity, July 21-22, 1994
0-7803-6424-4/00/$10.00 © 2000 IEEE October 18 - 21, 2000 Kansas City, MO 30 th ASEE/IEEE Frontiers in Education Conference T2E-18
