Mechanical measurements and Measurement systems


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This presentation gives the information about mechanical measurements and measurement systems of the subject: Mechanical measurement and Metrology (10ME32/42) of VTU Syllabus covering unit-5.

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Mechanical measurements and Measurement systems

  1. 1. UNIT-5: Measurements and measurement systems: • Definition, significance of measurement, generalized measurement system. • Definitions and concept of accuracy, precision, calibration, threshold, sensitivity, hysterisis, repeatability, linearity, loading effect, system response-times delay. • Errors in measurement, classification of errors. • Transducers, transfer efficiency, primary and secondary transducers, electrical, mechanical, electronic transducers, advantages of each type transducers. Hareesha N Gowda, DSCE, Blore-78 2
  2. 2. MEASUREMENT SYSTEM: • Measurement means determination of anything that exists in some amount. • If those things that exist are related to mechanical engineering, then the determination of such amounts are referred to as mechanical measurements. • An engineer is not only interested in the measurement of physical variables but also concerned with their control. • These two functions are closely related because one must be able to measure a variable such as temperature, or flow in order to control it. • The accuracy of control is essentially dependent on the accuracy of measurement. Hence, a good knowledge of measurement techniques is necessary for the design of control systems. Definition of Measurement : • Measurement is defined as the process or the act of obtaining a quantitative comparison between a predefined standard and an unknown magnitude. Hareesha N Gowda, DSCE, Blore-78 3
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  4. 4. SIGNIFICANCE OF MEASUREMENT SYSTEM • Measurement provides the fundamental basis for research and development. Development is the final stage of the design procedure involving the measurement of various quantities pertaining to operation and performance of the device being developed. • Measurement is also a fundamental element of any control process, which requires the measured discrepancy between the actual and the desired performances. • Many operations require measurement for proper performance. For example : In modern central power stations, temperatures, pressures, vibrational amplitudes etc., are monitored by measurement to ensure proper performance. • Measurement is also a bias of commerce, because the cost of the products are established on the basis of amounts of materials, power, expenditure of time and labour, and other constraints. Hareesha N Gowda, DSCE, Blore-78 5
  5. 5. THE GENERALIZED MEASURING SYSTEM • Most measuring systems fall within the framework of a general arrangement consisting of three phases or stages: • Stage 1. A detection-transduction, or sensor-transducer, stage • Stage 2. An intermediate stage, which we shall call the signal- conditioning stage • Stage 3. A terminating, or readout-recording, stage • Each stage consists of a distinct component or group of components that performs required and definite steps in the measurement. These are called basic elements: their scope is determined by their function rather than by their construction. Figure 1.2 outline the significance of each of these stages. Hareesha N Gowda, DSCE, Blore-78 6
  6. 6. First, or Sensor-Transducer, Stage • The primary function of the first stage is to detect or to sense the measurand. • At the same time, ideally, this stage should be insensitive to every other possible input. • For instance, if it is a pressure pickup, it should be insensitive to say, acceleration; • if it is a strain gage, it should be insensitive to temperature; • if a linear accelerometer, it should be insensitive to angular acceleration; and so on. • Unfortunately, it is rare indeed to find a detecting device that is completely selective. • Unwanted sensitivity is a measuring error, called noise. Hareesha N Gowda, DSCE, Blore-78 7
  7. 7. Second, or Signal-Conditioning, Stage • The purpose of the second stage of the general system is to modify the transduced information so that it is acceptable to the third, or terminating stage. • In addition, it may perform one or more basic operations, such as selective filtering to remove noise, integration, differentiation, as may be required. • Probably the most common function of the second stage is to increase either amplitude or power of the signal, or both, to the level required to drive the final terminating device. Hareesha N Gowda, DSCE, Blore-78 8
  8. 8. Third, or Readout-Recording, Stage • The third stage provides the information sought in a form comprehensible to one of the human senses or to a controller. • If the output is intended for immediate human recognition, it is, with rare exception, presented in one of the following forms: – As a relative displacement, such as movement of an indicating hand or displacement of oscilloscope trace – In digital form, as presented by a counter such as an automobile odometer, or by a liquid crystal display (LCD) or light-emitting diode (LED) display as on a digital voltmeter Hareesha N Gowda, DSCE, Blore-78 9
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  10. 10. • To illustrate a very simple measuring system, let us consider the familiar tire gage used for checking automobile tire pressure. Such a device is shown in Fig. (a). • It consists of a cylinder and piston, a spring resisting the piston movement, and a stem with scale divisions. • As the air pressure bears against the piston, the resulting force compresses the spring until the spring and air forces balance. • The calibrated stem, which remains in place after the spring returns the piston, indicates the applied pressure. • The piston-cylinder combination constitutes a force-summing apparatus, sensing and transducing pressure to force. • As a secondary transducer, the spring converts the force to a displacement. Finally, the transduced input is transferred without signal conditioning to the scale and index for readout. Hareesha N Gowda, DSCE, Blore-78 11
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  22. 22. Transducers • Transfer Efficiency: • The first contact that a measuring system has with the measurand is through the input sample accepted by the detecting element of the first stage . This act is usually accompanied by the immediate transduction of the input into an analogous form. • The medium handled is information. The detector senses the information input. Iin, and then transduces or converts it to a more convenient form. Iout,. The relationship may be expressed as Hareesha N Gowda, DSCE, Blore-78 23
  23. 23. THE PRIMARY AND THE SECONDARY TRANSDUCER • Consider the Bourdon lube pressure gage as shown in Fig. • The primary detector-transducer element consists of a circular tube of elliptical cross section. • When pressure is introduced the elliptical cross section tends to become circular section. This causes the free end A to move outward and the resulting motion is transmitted by the link to sector gear and in-turn to pinion causing the pointer to move over the scale. • In this example Bourdon tube serves as the primary detector- transducer, changing pressure to linear displacement. • The link, sector gear and pinion acts as secondary transducer and as an amplifier to give magnified output. Hareesha N Gowda, DSCE, Blore-78 24
  24. 24. Passive Transducer • A component whose output energy is supplied entirely or almost entirely by its input signal is called a passive transducer. • The output and input signals may involve energy of the same form (say both mechanical) or there may be energy conversion from one form to another (say mechanical to electrical). • In other words, a passive transducer requires an auxiliary source of energy. Example : Bonded wire strain gage. Active Transducer • An active transducer has an auxiliary source of power which supplies a major part of the output power while the input signal supplies only an insignificant portion. • In other words, active transducers are self powered. • Further, in these transducers, there may or may not be a conversion of energy from one form to another. • Example : electronic amplifiers, piezoelectric transducer. Active transducers are those which require electric current (a source of energy) for working, while passive transducers are those which does not need an external source. Passive transducers directly produce electric signals without an external energy source. Hareesha N Gowda, DSCE, Blore-78 25
  25. 25. Classification of first stage devices Hareesha N Gowda, DSCE, Blore-78 26
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