Methods for Measuring Resistance - Report Example

Design ReportDesign Report Template Measurement of Unknown Load Using a Strain Gauge Instrumentation and Measurement ENM2104 Engineering Design Process Confidentiality Notice Access to this document and referenced documents is provided to the recipient under the following conditions: 1) The contents are to be used solely for the purposes of the ENM2104 Unit at The School of Engineering, Edith Cowan University 2) The document will not be made accessible to any external party other than (if necessary) lecturers currently engaged by ECU under a contract which addresses confidentiality 3) Any requirement to vary these conditions is to be referred to the ENM2104 Unit Coordinator © Edith Cowan University 2014 Except as provided by the Copyright Act 1968, no part of this document may be reproduced, stored in a retrieval system or transmitted in any form or by any means without the prior written permission of the Edith Cowan University. Enquiries should be directed to the ENM2104 Unit Coordinator. Executive Summary What is known to many people when it comes to the measurement of resistance is the use of voltmeter and ammeter. That method is very direct and therefore easy to undertake. The biggest problem however, with this kind of measurements is the fact that they are prone to the internal resistances of these meters. In an ideal situation, it is expected that the internal resistance of a voltmeter is infinity whereas that of the ammeter to be zero. However, this is not usually the case; hence there are always instrumental errors that emerge as a result. This experiment therefore was meant to measure the resistance with a lot of accuracy using the Wheatstone bridge. Background The project (experiment) was prompted by the inaccuracies surrounding the measurements done by use of an ammeter and a voltmeter directly to find the resistance. Moreover, there is also need to measure the weights of objects using electrical means prompted by the strain in the strain gauge. Due to these considerations, there was a need to carry out such an experiment. Scope This experiment is aimed at ensuring that precise and accurate values of resistance (other than those obtained by measuring directly using an ammeter and a voltmeter) are obtained by varying the loading of the strain gauge. Additionally, the experiment had an objective of using that variation in loading to find the value of the unknown load through a Wheatstone bridge. TABLE OF CONTENTS 1.Introduction 5 2.Implementation 5 3.Sensor Selection 8 4.Measurement System Implementation 9 5.Conclusion 10 6.Reference 10 1. Introduction When a force or weight is applied, some strain is produced as a result. That strain produced is literally sensed by a strain gauge using the electrical resistance variation. Strictly speaking, the strain gauge is extremely ideal in measuring such parameters as pressure, displacement or even weight. It can be inferred that strain does exist virtually in all matters almost at all times arising from the external weights or its own weight. It is in this regard that this experiment sought to use the strain in the strain gauge to measure the mass of an unknown load. 1.1. Description of the Project The apparatus used were a strain gauge, 4- digit DVM, a Wheatstone bridge, three 120Ω resistors, power supply and weights of 1kg, 2kg, 3kg, 5kg and 6kg. Basically, the experiment involved connecting the strain gauge with no load and then recording that value in the table. Through Wheatstone bridge, the resistance of the strain gauge using the loads 1kg, 3kg and 6kg was determined and then recorded in the table 1. This experiment was ideally meant to find the effect of loading on the resistance of strain gauge by use of Wheatstone bridge, to find the effect of loading on strain gauge and find the voltage difference using bridge circuit and finally to calculate the value of the unknown load using the results found. 2. Implementation 2.1. Abstracting the Problem What is known to many people when it comes to the measurement of resistance is the use of voltmeter and ammeter. That method is very direct and therefore easy to undertake. The biggest problem however, with this kind of measurements is the fact that they are prone to the internal resistances of these meters. In an ideal situation, it is expected that the internal resistance of a voltmeter is infinity whereas that of the ammeter to be zero. However, this is not usually the case; hence there are always instrumental errors that emerge as a result. This experiment therefore was meant to measure the resistance with a lot of accuracy using the Wheatstone bridge. 2.2. Division of Tasks The tasks under this experiment were divided into three main categories i.e. the use of a Wheatstone bridge, use of a bridge circuit and the determination of the unknown load using the resultant graphs. First, the strain gauge was connected with the Wheatstone bridge and then the resistance of the strain gauge found with no load. The results were then recorded in table 1. Using the loads 1kg, 2kg, 3kg, 5kg and 6kg, the resistance of the strain gauge were found using the Wheatstone bridge. The graph of Resistance against Load was then plotted and analysed accordingly. Load (Kg) 0 1 2 3 5 6 Unknown Resistance (Ω) Table 1 Secondly, the strain gauge was connected with the bridge circuit as shown in figure 1:Where RB is the strain gauge. Figure 1 The voltage difference across nodes 2 and 4 were determined using the DVM (in between) with no load and then the value tabulated. The same was done with loads 1kg, 3kg and 6kg and the values tabulated in table 2. A graph was eventually plotted using the measurands. Loads (Kg) 0 1 3 6 Unknown Voltage Difference (mV) Table 2 Finally, the two graphs plotted using results in Table 1 and Table 2 were used to find the value of the unknown load as follows: Unknown Load A = Kg (Read from the graph) Unknown Load B = Kg (Read from the graph) 2.3. Problems Faced During the entire experiment process the two major problems that were experienced were the frequent power outages due to the interruptions from the nearby substation which was undergoing some minor repairs and maintenance and the second problem was the lack of better equipment (especially the Wheatstone bridge was obsolete). 3. Sensor Selection The sensor used in this experiment was the strain gauge. Strain gauge is well known for its function as a sensing element for a variety of categories of sensors e.g. pressure and temperature sensors. Most strain gauges in the market are foil types which are available in a variety of shapes and sizes to suit certain applications. Their mode of operation is founded on the principle that when the foil is exposed to some stress, then its resistance somewhat changes. In this case therefore, the strain gauge used was selected based on the consideration of the test specimen. 3.1. Identification of the Sensor Type The sensor (the strain gauge, in this case), had a number of characteristics that were looked at when it was being identified. The four major characteristics that were considered while identifying the sensor type included but were not limited to the format of the attachment of the strain gauge, the backing material of the strain gauge, the configuration of the gauge and finally the sensitivity of that strain gauge. In this case, the strain gauge used was electrical in nature. This implies that the change in gauge resistance and the change in length are important aspects. Their relationship produces the ‘gauge factor’ given by: This gauge factor can be said to be a measure of the amount of resistance for a given change in length; hence it is an index of the strain sensitivity of the gauge. Since the higher the gauge factor, the more sensitive the gauge and the better the electrical output, the strain gauge used was of higher gain factor. 3.2. Performance Test of the Sensor As may have been mentioned in the sections above, the strain gauge basically works on the principle that when a piece of wire is stretched, its resistance automatically changes. Inside a strain gauge, there is a thin wire that is wound systematically. If a load is applied to the material on which the wire is wound, then the wire stretches hence increasing its length while reducing its cross-sectional area. This would mean an increase in its resistance since: 4. Measurement System Implementation A strain gauge bridge circuit shows the strain which is measured by the degree of imbalance and uses a precision voltmeter at the core of the bridge to establish an accurate value of that imbalance. For this experiment, the measurement system was implemented by recording the values that were obtained using the Wheatstone bridge and the bridge circuit in the tables 1 and 2. This was followed by an analysis of the graphs that were plotted using those results. From then, the values of the unknown loads were determined; out of which a conclusion was made. 4.1. Hardware Configuration Borrowing from previous discussion, it was seen that the experiment utilized six key apparatus in its quest to achieve its primary objectives. The physical apparatus used were the strain gauge, 4- digit DVM, Wheatstone bridge, three resistors of 120Ω each, power supply and three different weights of 1kg, 3kg and 6kg. The said hardware were configured as shown in the figure 2 below: Where RB is the strain gauge Figure 2: Hardware Configuration 4.2. Software Configuration DAQ software is commonly used software in the field of engineering. For this experiment, this software was necessary in order to help acquire data from the strain gauge sensor all under computer control and to eventually bring the data together for storage as well as manipulation if need be. The DAQ software used, as is expected, returned values of voltage which would eventually be converted into the values of resistance that the experiment was interested in. 5. Conclusion In conclusion therefore, it can be said that though this method is a more precise one for measuring resistance, in the process, it is possible that a lot of discrepancies may occur. This is due to the fact that generally, compared to the direct measurement of the resistance using a voltmeter and an ammeter, this method uses a number of processes which are naturally prone to human errors. That said, it still remains one of the most precise methods for measuring resistance ever known. 6. Reference Kirkup, J. (1994). Experimental Methods: An Introduction to the Analysis and Presentation of Data. Brisbane: John Wiley & Sons. Read More