Utilizing Student Computers for Laboratory Data Acquisition in a University-Wide Laptop Environment

Transcription

1 Utilizing Student Computers for Laboratory Data Acquisition in a University-Wide Laptop Environment Lewis G. Frasch, P.E. Lawrence Technological University Session 2559 Abstract Laptop computers are being required of all students starting with the freshmen class of Traditional thermal science labs are being equipped with Analog-to-Digital converters to output the lab's traditional analog signals as a digital signal carried by a RS232 cable. Read-only files with appropriate software can be loaded on a student laptop when they come to lab, and the student's computer becomes the data acquisition system for the lab. Background In the fall of 2000, Lawrence Technological University (LTU) began requiring that all entering freshmen have a laptop computer. In each succeeding year, the next class rank (i.e. sophomores in fall 2001) would be required to have a laptop. The laptops are a standard model leased by the unversity to the students and regularly upgraded. Student laptops that develop problems are fixed by the help desk or exchanged if the problem is too serious to fix quickly. They are preloaded with a standard set of software for each college. For example, all engineering laptops are pre-loaded with a high-end CAD program, programming language, equation solver, and office software. Students who wish to use their own laptop must have one that is compatible with the university's and loaded with the same software. The ability to assume that all students and, therefore, all student lab groups will have a current laptop with predictable software allows two basic lab issues to be addressed. For one, it makes a standard computer available for data acquisition in any experiment. Secondly, if the laptops can be used as stand-alones, it allows LTU to avoid the built-in obsolescence of permanent lab computers. Further, it reinforces the student's understanding of the laptop as a tool. Prototype Context The Thermal Science Lab (TSL) is a senior level lab taken by all ME students. It consists of a series of experiments designed to reinforce the principles learned in thermodynamics, fluid mechanics, and heat transfer in the context of real equipment. Well-designed experiments tend to be used for a very long period of time. In the spring of 2001, a very gifted group of students requested to do a Design Studio redesigning one of the TSL experiments. (Design Studio is a one-semester version of the usual yearlong capstone design course. It is very intense and only allowed by special permission.) Page

2 The experiment assigned to the student group was the Flow Characteristics Lab. In this experiment, students manually took pressure measurements along a four-inch diameter tube and, utilizing Pitot tube and orifice data, predicted K-factors for various entrances and friction factors for the tube. The student project group was tasked with redesigning the experiment for data acquisition using a standard university laptop. The expectation for the new experiment was that a student would bring a laptop to the TSL, be handed a CD, and upon installing the software be able to take and analyze the lab's data. Utilizing analog pressure transducers, an A/D converter, Taltech's WinWedge software, and Visual Basic for Applications (VBA) in Excel, the group succeeded admirably. The system they developed is providing a prototype for data acquisition in the entire lab and, as developments continue, may spread to all of the college. The methodology they developed and the work in progress are the subject of this paper. Current System Figure 1 shows a view of the new experiment. Pressures along the tube for pressure drop information and Pitot traverse pressures are in the 0-10"WC range. Orifice pressure difference is in the 0-25"WC range. The pressure measurements needed were taken with manometers in the old apparatus. Analog instruments now measure pressures. The system is operating in steady state. To get the range of pressures needed differential pressure instruments were used with one side left open to atmosphere, Figure 2. The Auto Tran and the Cole-Palmer pressure instruments used produce a signal 0-5v DC. (A list of the equipment used is included later.) Since the measurements are steady state, a barrel switch controls which instrument is being sent to the A/D converter, Figure 3. Having a student change the barrel switch position when prompted by the VBA program eliminates the need for daisy-chained converters or for resident computer control. Barometric pressure and temperature are input manually by the students when prompted. The analog signals are passed from the barrel switch to the A/D converter. The DGH model 5131 A/D converter chosen has four channels and some programming capability. For this application one channel was used to pass the data from the barrel switch to a RS232 cable. The A/D output crosses the send, receive, and ground pins of a nine pin RS232 cable to com1 on the laptop. The Taltech Corporation produces software called WinWedge. When loaded and operating it places the digital signal from com1 to a cell in an Excel spreadsheet. The call for the data and its manipulation are done by macros programmed in VBA. (WinWedge will work with many software packages, but Excel is a LTU standard and is already both familiar to the students and loaded on their laptops.) The WinWedge software and the Excel VBA program are installed on the student's laptops from a CD. The VBA macros in Excel launch WinWedge. One of the members of the student project team works fulltime doing data acquisition. He wrote an extensive VBA program to process the data. The software operates with "buttons" and is of commercial quality, Figure 4. It produces pressure versus distance plots and actual velocity profiles and power law curves for comparison. It does a linear regression on the fully developed portion of the pressure versus distance curves and predicts entrance K factors and friction factor. Page

3 Certain of the calculations and the power law plots are password protected so the student groups must do these themselves, but the instructor can easily check their results. Comments on the Current System and Lessons Learned The current system has been used for one semester for approximately seven lab groups. Other than some zero drift in some of the pressure sensors no significant mechanical problems were noted. The students developed as part of their work a calibration procedure which will need to be used bi-weekly. The software has functioned well with the exception that some of the original code written for Windows NT stopped working when the campus switched to Windows When results are saved the students tend to simply save the Excel file which the saves all of the VBA program, etc. So, an effort is in progress to limit the information saved. Since the current seniors are not required to have LTU laptops, the instructors are letting the student groups use theirs. Thus, full student hard drives or student computers with viruses have not been an issue. Typically, when students take their data they the results to all the group members as an attachment. So, when student machines are used, as long as one student's machine has hard drive room, etc, there will not be a problem. As a general observation, the student-developed system is excellent. In use, however, it is very complex if any problems occur given that a faculty member that did not write the code must do the troubleshooting. In the future, software will be kept simpler so that anyone familiar with the basic packages can easily troubleshoot it. TalTech provides programs for various software packages in their manual and some sample programs on disk. These form the starting point for the VBA macros for further experiments in the TSL. Obviously, many variations are possible. The most basic would be to use a TalTech macro to get the data to a series of Excel cells and let the students manipulate it from there. It should be noted that the experiments in this lab are either steady state or change relatively slowly so the transient performance of the A/D converters (8 conversions per second) is not an issue. For the methods being used, sampling rate comes from the sample frequency programmed in the VBA macro. Also, the four channels of the converter are adequate for the work done or in progress. If more channels were needed four more converters could be connected, one to each channel of the first. This provides a daisy chain of sixteen possible channels. The DGH manual discusses this procedure. Key Equipment Utilized (Approximate prices are as of spring 2001.) Pressure Transducers: AutoTran Inc. Model 600D-014 (0-10"WC) $100; (800) Cole-Palmer Instrument Company Model (0-25"WC) $247; (847) WinWedge Version 1.2: TAL Technologies, Inc. $180; (800) A/D Converters: DGH Model D5131 for the Flow Characteristics Lab $260; (603) Page

4 Development in Progress At this writing, a second system is being developed. The Transient Cooling experiment in the TSL measures temperature decay and predicts convective heat transfer coefficient based on the lumped capacitance method. This experiment was selected so that a prototype for transient measurements would be developed. The DGH A/D converter model D5321 is being used because it converts the thermocouple signal directly into temperature. Software is being developed utilizing WinWedge and Excel as described earlier. Two thermocouples are being alternately queried, and the transient performance of both is being recorded and plotted. Figure 6 shows the test bed being used with its two thermocouples and A/D Converter. After this project is completed, interested LTU faculty will be invited to a workshop to encourage wider implementation. The author would be happy to supply anyone interested with further information (frasch@ltu.edu) Acknowledgements The author wishes to publicly thank Jennifer Lathi, Edward Reader, Keith Scales, Robert Sonnenberg, and Leann Warner for their phenomenal work on their senior project. Figure 1: Flow Characteristics Experiment Page

5 Figure 2: Pressure Sensors and A/D converter (upper right) Pressure Measurement (analog) Pressure Measurement (analog) Barrel Switch Analog to Digital Converter RS232 Laptop Computer (Excel) (Typical of 22) Figure 3: Block Diagram of Pressure Measurement System Page

6 Figure 4: Typical Flow Characteristics Lab Template Figure 5: Pressure Transducer Wiring Schematic Part One 1 Page

7 Figure 5: Pressure Transducers Wiring Schematic Part Two 1 Figure 6: Transient Cooling Data Acquisition Test Bed References: 1.Thermal Science Lab Fluid Flow Characteristics Final Report; Lahti,Reader,Scales,Sonnenberg,Warner,2001, Final Report for EME4984 Biographical Information: Lewis G. Frasch, P.E. is a graduate of the Ohio State University and Iowa State University. He served as a nuclear submarine officer, a power plant designer, and has been on the faculty at LTU since Page