Session 2259 LabVIEW Based Instrumentation and Experimental Methods Course Chi-Wook Lee Department of Mechanical Engineering University of the Pacific Stockton, CA 95211 Abstract Instrumentation and Experimental Methods course is required for mechanical engineering program at the University of the Pacific. One of the goals of this course is to incorporate fundamental experimental techniques with modern computer-based data acquisition using National Instruments hardware and software (LabVIEW ). LabVIEW enables data acquisition and control system parameters. With a series of experiments, students learn basic experimental techniques to use sensors for measuring various mechanical system quantities. After the basic experiments, students develop LabVIEW programs working with Signal Conditioning extension for Instrumentation (SCXI) chassis as computer-based data acquisition exercises. The students relate the LabVIEW based data acquisition systems with their other course projects including senior design. Introduction Instrumentation and Experimental Methods is a required junior-level course for mechanical engineering students at the University of the Pacific. This course covers experimental techniques in the measurement of mechanical quantities, statistical analysis, errors in measurements, signal conditioning, and signal processing. The measured mechanical properties through lab exercises include temperature, pressure, strain, and frequency of dynamic systems. Since the outputs of the sensors/transducers used for the lab exercises are voltages, a digital multimeter or an oscilloscope is utilized as a readout device. Then, students convert the basic lab exercises to computer-based data acquisition systems using their own LabVIEW programs to measure and calibrate the sensor/transducer outputs. LabVIEW is short for LABoratory Virtual Instrument Engineering Workbench. LabVIEW programs are called virtual instruments (VIs) and a VI has three main parts: (1) the front panel for the interactive user interface, (2) the block diagram as the VI s source code and actual executable program, and (3) the icon and connector. The icon is a VI s pictorial representation and the connector defines the inputs and outputs of the VI [1]. LabVIEW includes many library functions for data acquisition, signal processing, and statistical analysis. By a gradual exposure to the necessary hardware and software from National Instruments, students learned the fundamental computer-based data acquisition concepts while focusing on experimental techniques. This report describes some of the LabVIEW based lab exercises. Page 5.419.1
Course Content Instrumentation and Experimental Methods is a four-credit course consisting of two eightyminute lectures and a three-hour lab per week. At the beginning of the semester, students learn how to make VIs following the basic structure of a textbook [1]. Students are exposed to the basic programming concept and functions of LabVIEW during this period. After the introduction to LabVIEW programming environment, two students in a group conduct basic lab exercises to measure mechanical quantities. Each lab exercise is followed by the computerbased data acquisition lab exercise to measure and calibrate the same mechanical quantities. Students are required to submit a lab report per group after the completion of each lab exercise. After all general lab exercises, students are required to design and build their own experimental apparatus with the instructor s approval of their project proposals. While students learn to make VIs and execute some lab exercises, the topics including error analysis, fundamental statistics, basic circuit analysis, analogies of dynamic systems, and signal processing/conditioning are discussed in lecture. Temperature Measurement with Thermocouples The lab is designed to enhance knowledge and proper use of thermocouples. The setup for this exercise is shown in Figure 1 and the equipment includes thermocouple wire, voltmeter, ice-bath, beaker, hot plate, glass thermometer, and soldering iron. Figure 1. Thermocouple Experiment Setup Students are asked to do the following steps for this lab exercise: 1. Fabricate a thermocouple with two junctions and to connect the thermocouple wires to a voltmeter. 2. Insert both junctions into an ice bath and record voltages. 3. Remove one junction and place it in a beaker of water. Page 5.419.2
4. Place a glass thermometer in the water near thermocouple. 5. Bring water to a boil and periodically record thermocouple outputs and thermometer temperatures. With the slight modification of the experimental setup shown in Figure 1, LabVIEW can be used to acquire temperature data. In addition to the equipment for the basic thermocouple experiment, Desktop Computer (MS-Windows 95), SCXI-1000 chassis, and SCXI-1321 terminal block are used. Students build their own VIs to take temperature data with computer and display it on the front panel. A sample front panel of the VIs displaying real-time temperature changes is shown in Figure 2. Figure 2. Front Panel of a sample VI For Temperature Display Measurement of Pressure and Calibration Pressure transducers are used widely in industry to directly measure the pressure of gases, liquids, etc. They are also used indirectly to measure airspeed. This laboratory exercise is designed to provide an understanding of the operation and calibration of a transducer designed to transmit pressure data. Apparatus for this lab exercise consists of a five-gallon tank with inlet and outlet valves, a resistance pressure transducer, analog pressure gage, power supply, and digital multimeter. Figure 3 shows the schematic diagram for the experimental setup. The procedure for this lab is: 1. Turn on power supply and set to 5 volts. 2. Connect pressure transducer to the power supply according to the specification sheet. 3. Record output voltage as zero-offset. 4. Fill the tank to a pressure of 25 psi in 2.5 psi intervals closing inlet valve at every interval. Allow time to stabilize. 5. Read and record the output voltage with the pressure reading from analog pressure gage. 6. Release tank pressure from 25 psi to 0 psi in 2.5 psi intervals. Record voltage at each interval. Page 5.419.3
7. Repeat steps 4, 5, and 6 for four times. 8. Turn off equipment and find the calibration factor. Figure 3. Schematics of Apparatus for the Measurement of Pressure and Calibration This basic lab exercise can also be converted to a computer-based data acquisition exercise with PC (MS Windows 95), SCXI-1000, and SCXI-1303 as in Figure 4. Figure 4. Data Acquisition Setup for Pressure Measurement and Calibration While a front panel of VI is to display waveform chart and digital control variables for the interactive user interface, the actual programming is done in a block diagram of VI. One group developed a VI to display the data acquired with alarm systems added for low- and high-pressure warnings. The block diagram of the VI is shown in Figure 5. Page 5.419.4
Figure 5. Block Diagram for Data Acquisition of Pressure with Warning Systems Strain Gage Installation and Measurement Simple resistive strain gages are used for this lab exercise. The resistive strain gage is bonded to an aluminum bar for the strain measurement. The Wheatstone bridge circuit is used for static strain measurement. Ideally, the strain gage is the only resistor in the circuit that varies and then only due to a change in strain on the surface of the beam. In this lab, after students install a strain gage on an aluminum bar, they use P-3500 Strain Indicator from Measurement Group with a quarter bridge setup. For the strain gage installation, students go through degreasing, surface abrading, surface conditioning and neutralizing, bonding, and coating processes [2]. To check installation, Model 1300 Gage Tester from Measurement Group is used. The beam installed with strain gage is placed in flexor as shown in Figure 6. Figure 6. Strain Measurement Setup Page 5.419.5
For strain measurement with LabVIEW, two SCXI modules, the SCXI-1121 and SCXI-1122, are designed for use with strain gages. Students use SCXI-1121 chassis along with a SCXI-1321 terminal block. SCXI-1121 is a four-channel isolation module with transducer excitation. It includes an independent gain amplifier and low-pass noise filter on each channel with complete channel-to-channel electrical isolation. Each channel also includes a completely isolated excitation source and a half-bridge completion circuit. The SCXI-1321 terminal block, used with the SCXI-1121 module, adds manual offset-nulling and programmable calibration for each channel [3]. It is required to modify the hardware setting for SCXI strain gage applications. Figure 7 shows an example of a VI block diagram for strain measurement with National Instruments software and hardware. The strain gage is calibrated with a shunt resistor. Resonance and Vibration Measurement Figure 7. Block Diagram for Strain Measurement VI In distributed mass systems such as beams, knowledge of the distribution of the vibration amplitude becomes necessary before the kinetic energy can be calculated. With Rayleigh Method, it is possible to estimate the fundamental frequency of the beams [4]. In this experiment, students measure natural frequencies of cantilever beams to determine their dependence on the physical properties of the beams. Shake table, piezoelectric accelerometer, vibration meter with integrator, oscilloscope, stroboscope, tachometer, and assorted beams are used for this lab exercise as shown in Figure 8. Page 5.419.6
Figure 8. Apparatus for Vibration Measurement of Cantilever Beams LabVIEW can be used for the measurement of resonant frequencies. With built-in filtering and signal processing library functions, on-line spectrum analysis can be done to determine the natural frequencies. Nyquist sampling theorem and signal conditioning with filters should be emphasized for this type of computer-based data acquisition exercise. This exercise requires careful selection of sampling period. Other Projects Since most of students in this course take the first phase of senior design class, many proposed projects are related to their senior design projects. However, every project in this course is required to have a computer-based data acquisition component. Some of the proposed design projects are listed as: Use accelerometer for suspension design for their mini-baja. Use strain gages to measure transmitted torque for mini-baja. Use strain gage to determine thickness of a floating sphere made of copper. Use thermocouples in series to measure thermal conductivity of an aluminum bar and to display temperature distribution. Use accelerometer to estimate the system parameters of a mass-spring-damper system with a hammer as impulse loading. Use strain gage to measure the internal pressure of soda can. LabVIEW is also used for the Robotics and Systems Analysis and Control classes along with LEGO parts. Tufts University developed LabVIEW based software called Robolab and LEGO Engineer [5]. In Robotics class, students use Robolab software to control LEGO mobile robots. Students in Systems Analysis and Control use LEGO blocks and LEGO Engineer software for the lab exercises. Page 5.419.7
Discussions Students modified all the experimental setups for computer-based data acquisition exercises. The students enjoy the LabVIEW based lab exercises/projects and these projects provide very good learning experiences for the fundamentals of experimental techniques, data acquisition, and instrumentation. The benefits of using LabVIEW and SCXI chassis in Instrumentation and Experimental Methods course can be summarized as: More time for discussion of the fundamental principles related to lab exercises, Easier to understand concepts such as statistics and sampling frequency, Exposure to computer-based data acquisition concept, Enhancement of student s learning experience. Acknowledgements The author would like to thank the University of the Pacific (Long Teaching with Technology Grant) and National Science Foundation (Grant No. 9751111) for funding. Bibliography 1. Wells, L.K., Student Edition User s Guide, Prentice Hall, 1995 2. VISHAY Measurement Group, Student Manual for Strain Gage Technology, Measurement Group, Inc. 1992 3. Application Note 078, Strain Gage Measurement-A Tutorial, National Instruments Corporation, 1998 4. Thompson, W.H., Theory of Vibration: With Applications, Fourth Edition, Prentice Hall, 1993 5. Cyr, M., Miragila, V., Nocera, T., and Rogers, C., A Low-Cost, Innovative Methodology for Teaching Engineering Through Experimentation, p.p. 167 171, Journal of Engineering Education, April, 1997 CHI-WOOK LEE Chi-Wook Lee is Assistant Professor of the Department of Mechanical Engineering at University of the Pacific which he joined in 1998. Prior to 1998, he taught at University of Michigan Flint. He received his B.E. from Hanyang University in 1981, his M.S. from University of Wisconsin-Madison in 1984, and Ph.D. from University of Florida in 1991, all in Mechanical Engineering. His research interests include design of legged robots, dynamic systems, control, and engineering education. Page 5.419.8