chapter2

chapter2 - CHAPTER 2 STATIC CALIBRATION A measurement...

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CHAPTER 2 STATIC CALIBRATION A measurement system is made up of many components. At the input is the quantity that you wish to measure, let's denote this by I for input. I may be changing with time. Examples of the input could be temperature, acceleration, flow rate, particle size or displacement. The measurement system output will usually be a voltage. Let's denote this by O for output. In Figure 1 is shown a block diagram of components that may be part of a measurement system. In some systems not all components will be present. The three building blocks are: (1) The Transducer This will produce some electrical quantity such as voltage, current, charge, resistance, inductance, or capacitance that is related to the input, I. Examples include an RTD which converts temperature to resistance, a piezoelectric force transducer that changes force to charge, a microphone that converts pressure fluctuations to capacitance fluctuations, a thermocouple which changes temperature differences into voltage differences, a strain gage which converts strain into a change in resistance. (2) The Signal Conditioner This stage can often be split into further sub components. One component may convert resistance to a voltage change as in a bridge circuit (see Chapter 9). Another component may be a demodulation circuit as in an LVDT (see Appendix of Beam Lab). Amplifiers are often part of the signal-conditioning block, and are used to raise the signal level above the noise level. Filters are also commonly used in signal conditioning to remove noise. This stage conditions the signal so that it is a voltage within the input range of the data acquisition and display device. This input range is part of the acquisition device specifications and will be provided by the manufacturer. (3) The Data Acquisition and Display Device This could be an analog to digital converter attached to a computer on which the data can be graphed and analyzed. This could be a digital multimeter (DMM), which will display RMS voltage in a digital display, or it could be an ammeter, or an oscilloscope. These components may be connected by leads or, when measurements are being made at a remote site, there may be transmitters or receivers between these components, or between their subcomponents.
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2-2 Input, I Output, O Figure 1: Components of a measurement system. The Calibration Procedure and Some Definitions It is important that we determine what a suitable operating range is for the transducer. We are ultimately concerned with frequency response characteristics and the static calibration characteristics. The frequency response characteristics will be discussed in the System Identification Chapter. In this chapter we will focus on the relationship between the output (O) and the input (I) as the level of the input is slowly increased or decreased. Usually known constant values of input are put into the measurement system, the system is allowed to come to steady state, and the level of the constant output is recorded. A plot of measured output level versus known input level is known as a calibration curve. An example of such a curve is shown
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This note was uploaded on 12/26/2011 for the course ME 365 taught by Professor Merkle during the Fall '07 term at Purdue.

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chapter2 - CHAPTER 2 STATIC CALIBRATION A measurement...

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