measurement: past, present and future: part 2 measurement instrumentation and sensors

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DOI: 10.1177/0020294013485675

2013 46: 115Measurement and ControlBarry E Jones

Measurement: Past, Present and Future: Part 2 Measurement Instrumentation and Sensors

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I. Measurement SystemsTraditional measuring instruments, mechanical and electrical, were manufactured simply with regard to particular requirements and were all treated as separate if not different equipment. This arose because scientists and engineers followed the way their particular branch of science or engineering did things and, if necessary, invented things. The subject of measurement was fragmented towards the specific measurement devices. One just learnt about the measurement in one’s own discipline and for particular quantities. Only people in a given discipline knew how to make measurements in that discipline.

The advent of electronics and its subsequent ubiquitousness, to be followed by microelectronics, microcomputers and software, has changed all this. Today, the study and practice of measurement science and technology has a much more coherent approach, and measurement instrumentation is seen in terms of a common base of measurement theory and a system of mostly common and

interlinked parts. As we shall see later in the article, it is principally at the measuring instrument or measurement system input and output points that both specialist and user knowledge are most helpful.

Because there are so many different types of measurement, measurement quantity ranges and so many different environments in which the measuring equipment must work, the front ends of measurement systems often only apply to relatively small niche applications, and this can affect the sizes of enterprises and markets. How to design to reduce the range of equipment needed to cover a wide range of measurement quantity values and how to use as many common modules of equipment as possible so as to get greater volumes for manufacture of the modules are key considerations for designers of measuring instruments.

Thus, today a measurement practitioner can apply measurement skills to a wide range of applications because much of the basic theory and technology to be used is common to all measurement systems. Of course, specialist knowledge about the quantity to be measured, the technology to be

used at the front end and the desired format of the output of the measuring system is necessary.

In the first article (Part 1 of this themed issue of the journal), fundamental issues applying to all measuring instruments and measurement systems are briefly discussed. There is a well-defined, common and systematic approach to dealing with the different measurement uncertainties and on how to stipulate a measurement result and measuring instrument/system accuracy. The measurement traceability chains are also well defined at any given time.

In Part 2, we will see that there is a great deal of common hardware and software that help in a systematic approach to design, construction, testing and using measurement systems.

II. Functional Measurement InstrumentationIn the year 1977, I set out in a book the coherent and systematic approach to measurement instrumentation. The book was first published in the United Kingdom, then separately in India and also in a German language edition, and


Barry E JonesBrunel University, London, [email protected]

In this article, two general issues are discussed: first, the functional parts of measuring systems and the critical nature of designing primary sensing structures, defining stable functional-part relationships and providing suitable packaging arrangements for the sensory elements, and second, the new sciences and technologies being applied to measure-ment instrumentation. The coherent systematic approach to measurement theory and practice and the multi- disciplinary nature of measurement science and technology are thus illustrated.

485675 MAC46410.1177/0020294013485675Measurement and ControlMeasurement: Past, Present and Future. Part 2 Measurement Instrumentation and Sensors2013

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116 Measurement and Control l May 2013 Vol 46 No 4

remained in print for 20 years. The book outlined the principal functions performed in a measurement system in the generalised manner shown in Figure 1. Then, I highlighted a ‘transducer’ block, which today is called the ‘sensor’ block, while the term ‘transducer’ refers to any transduction process sensor or actuator – the ‘inverse transducer’ in the diagram. The ‘sensor’ transforms non-electrical quantities into electrical quantities, so that the functions that follow can be performed electrically. The measurand is the input that operates directly on the ‘primary sensing element’. This is an element that is likely to determine the system performance. Table 1 lists common primary sensing elements and sensors. In precision dimensional metrology, the primary element is usually either a touch probe or a stylus, but non-contact methods are also used.

The inverse transducer converts an electrical quantity back into the non-electrical quantity for comparison with the input quantity, thus creating a feedback structure. The electrical output of what in 1977 I called a ‘feedback measuring system’ has a direct relationship with the input quantity determined only by the primary sensing input and the inverse transducer. The gain characteristics of the other functional components, in particular, the sensor, do not significantly influence this relationship. Thus, the use of precision actuators can be very useful. This feedback approach has been used for physical quantity monitoring and more recently chemical monitoring.

III. Measurement TechnologiesScience, engineering and technology are all continually on the move, driven variously by basic enquiry, application need and often by serendipity. New measurement equipment often results in new science, while new science and new technologies in turn often result in new measurement systems for a range of applications. Measurement technology has to be engineered on the basis of practical knowledge about what will work

in the particular applications. Thus, measurement practitioners tend to be

technology ‘magpies’ who look across the whole field of technologies looking for

Figure 1. Functional block diagram of a measurement system

Table 1. Measurement quantities and common input devices of measurement systems




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Table 2. Technologies used in measurement systems

• Opticalsensors• Chemicalandgassensors• Ultrasonicandacousticemissionsensing• Sensorsinbiologyandmedicine• Advancesinsensingmaterials• Nanotechnologyforsensorsandactuators• Smartsensorsandinterfaceelectronics• MEMSandsiliconfabricationtechniques• Imaging;Integratedactuators• Thickandthinfilmsensors• Sensormodelling• Sensorpackagingandassemblies

appropriate means to achieve particular measurements.

The past 30 years have provided a multitude of new technologies from which measurement instrumentation can select. Table 2 lists such technologies. Often, it is the clever combination of technologies that provides the best outcomes. Therefore, creativity and lateral thinking, teamwork, measurement experience and practical skills play considerable roles in successful developments of new measurement instrumentation. Tenacity is important because it can take many years to produce successful new measurement instrumentation meeting the required specifications from primary sensing through to indication and recording, and to meet the often challenging environments in which the sensing has to take place.

A. Sensor Housing Materials and Sensor PackagingMost reported sensing methods are solely phenomenological, and this is, of course, why there has been so much sensor research. In the United Kingdom, it is relatively easy to get funding at this level because there are so many phenomena and public funding bodies for the science base that are stuck with the remit for originality and potential patent generation when making these funding decisions about pre-competitive research. Because of these limitations, a high percentage of reported sensor

research in the United Kingdom will never result in commercial gain, even when there are excellent patents and the industry is involved with the research because it is the development and detailed design that must be done by industry to justify the potential private gain.

Besides, it should be realised that in the United Kingdom about 90% of total R&D costs will need to be funded by industry and private investors because the eventual benefits will be largely private. A £400,000 sensors research grant given, for example, to a British public university by the government must be followed by some £2–4 million expenditure by industry and private investors with the understanding that a return on the investment cannot be expected before 5–10 years. Herein lies the difficulty for sensor exploitation in ‘short-term’ Britain and the advantage for ‘long-term’ Germany. This matter is discussed in more detail in the third article (Part 3).

The key to commercialising a new sensor is successful sensor packaging through the application of measurement skills in industry. The packaging of a primary sensing device and the sensor is often difficult because of the harsh working environment for this front end of the measuring system. Material science therefore plays an important role.

Robust sensor housings are normally necessary. For example, Invar – a nickel iron alloy – with very low coefficient of thermal expansion (approximately 1 part per million (ppm)/°C), although expensive, is often used. Inconel – a nickel-chromium alloy – is often used for high-temperature applications because it is oxidation and corrosion resistant. Special stainless steels are also used.

Pressure instruments, pressure transmitters and pressure differential cells are widely used. Measurement of fluid flow in a pipe is often done by measuring pressure drop across an orifice plate, while fluid level in a tank is determined by monitoring pressure at the bottom of the tank. For such a pressure instrument, it is common to use a round diaphragm on which sensors are placed. Under

pressure, the sensors in the central area of the diaphragm are in mechanical tension while those placed around the outer area of the diaphragm are in compression. This positioning of a group of sensors in a Wheatstone bridge circuit arrangement results in greater sensitivity to diaphragm deflection and also allows for rejection of temperature effects on the sensing structure. The sensing diaphragm of a high-performance pressure sensor of relative high cost may well be made of sapphire onto which an oxide layer and then thin-film strain gauges are deposited.

Thick-film technology only requires modest set-up costs and so is suited to budget low-accuracy sensors. For example, thick-film strain gauges are screen printed onto the reverse side of a sensing diaphragm to form a Wheatstone bridge circuit. Insulating and conducting layers are also screen printed onto the diaphragm.

Hermetic sealing of one part of the sensor housing to another part is important to secure against the entry of water vapour and foreign bodies. Various sensor encapsulations are used: silicone or other elastomers and polymers, glass, epoxy or polyimide. The potted structure may be varnished with special coatings.

B. Microelectronics and MicroprocessorsMicroelectronic fabrication at very small scale has resulted in both analogue and digital integrated circuits. Thus, instrumentation amplifiers, analogue-to-digital converters and microprocessors with digital storage can be placed into transducer housings with the sensors. These intelligent or smart sensors can perform many of the functions shown in Figure 2. Outputs can be in the various standardised formats.

C. Microelectromechanical SensorsMicroelectromechanical sensors (MEMS) is a technology of small devices, for example, sensors up to 1 mm in size. Bulk micromachining of silicon using etching processes and anodic bonding of glass and silicon wafers have enabled




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manufacture of high-performance pressure sensors and accelerometers. Surface micromachining has enabled integrated circuits and MEMS to be produced on the same silicon wafer. This has enabled low-cost accelerometers to be manufactured.

This technology requires use of expensive foundry equipment and facilities and therefore is only really appropriate for very high volume manufacture. Designers of measuring instruments and measurement systems usually have to make use of sensory devices sold on the open market, including by competitors, and there may well be no exclusivity. Thus, the potential economic benefits to a company may be reduced, and patent protections may be more difficult.

D. Event Counting, Encoders and Resonant Sensor SystemsIf a measurand can be transduced into a regular series of events, then the measurement process is simply one of counting. An example is counting the number of vortices shed by a bluff body in a flowing fluid stream, as the count rate is proportional to the flow rate. Optical encoders provide a measure in digital format of an absolute position.

There can be an improved signal-to-noise ratio (and measurement sensitivity and precision) and lower power consumption if a measurand is directly converted into a frequency-based electrical signal. A mechanical resonant structure can be used for this purpose. The mass of the structure or tension and compression changes within the structure can be arranged to change with changes in the measurand value. Quartz resonators, metallic and silicon cantilevers and double and triple beam structures have been successfully used. The resonator quality factor needs to be reasonably high, for example, greater than 1000. Very small silicon resonators need to be in vacuum for this to be the case. A capacitance or an inductance sensor can be placed in an electrical resonant circuit to obtain a frequency signal.

E. Fibre Optic SensorsElectromagnetic fields do not affect light waves travelling in a normal glass or polymer optical fibre, and therefore, this is a medium well suited for use in measurement apparatus. Light has energy intensities, wavelengths and wave polarisations, and therefore, these

parameters can be modulated by a sensing structure. Use of wavelength now seems to be the preferred option.

Periodic refractive index grating lines can be inscribed into a short segment of an optical fibre so that it acts as a wavelength-specific reflector; the filter is called a fibre Bragg grating (FBG). If the FBG experiences stress from a measurand, then the wavelength of reflected light changes. If white light is passed through the FBG, then a spectrometer at the receiving end of the optical fibre can be used to observe the missing wavelength of light. There are big advantages in using this approach because a series of FBGs can be sited along a single optical fibre, with each FBG having its own grating characteristic. The measuring system provides a multitude of continuous outputs largely independent of changes and attenuations in light intensity. Thus, a robust point-distributed sensing system independent of electromagnetic interference can be used.

F. Ultrasonic and Acoustic Emission SensingUltrasonic waves generated by sensors travel through solids and fluids and get

InterfaceSignal evaluation

Signal processing

SensorAnalogue

signal conditioning

Digital sensor

correction

Level adjustment

Anti-aliasing

Amplification

A/Dconversion

Linearisation

Auto-calibration

Offset&amplification correction

Driftcompensation

Computation

Trig.Conversion

Digitalfiltering

Auto-correlation

Spectralanalysis (FFT)

Pattern recognition

Eventprediction

Correlation

Sensorfusion

Model-based signal analysis

Figure 2. Functions performed by intelligent measurement instrumentation

FFT: fast Fourier transform. Reproduced with kind permission from AMA Fachverband für Sensorik e.V. (AMA Association for Sensor Technology).2




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reflected at the interfaces between different materials. The waves are attenuated and dispersed as a beam normally spreading outwards in a fairly ‘unfocussed’ manner. This can result in wave reflections from many surfaces so that received return signals can be difficult to interpret. Doppler-shift and transit-time flowmeters use continuous and pulsed ultrasonic waves, respectively, and are non-invasive as they clamp on to the pipes containing the liquid flows.

Acoustic emission (AE) is an event series of short transient elastic waves, which occur when a material undergoes stress and releases energy at cracks within the material or on the material surface. Frequency components of the AE are normally in the range 10 kHz to 1 MHz, and piezoelectric transducers are used to pick up the events from any nearby surface. The shape of the events and the statistics of the events can be used to predict likely failure of a material. AE monitoring is therefore valuable in determining surface wear, such as in bearings, cracks in pressure vessels and pipes, corrosion in concrete and monitoring batched granulation.

G. Signal Recovery and Signal AnalysisThe frequency bandwidth of a measuring system should be that required to reproduce the measurand signal with fidelity, taking into account that bandwidth must be sufficient so as not to create phase distortion. All measurement systems have some bandwidth limitations, particularly at the higher frequency end. The output signal’s signal-to-noise ratio, which determines measurement resolution and contributes significantly to the accuracy of measurement possible, can be improved as bandwidth is reduced to that just required for purposes of fidelity. Reduction of bandwidth reduces the inherent noise level.

Thus, all signal ‘recovery’ or improvement methods are about reducing the noise level. Signal averaging trades time for resolution and is useful for

monitoring transient measurands as in medical applications, while cross-correlation allows natural disturbances to be used such as in flow-rate monitoring.

Analysis of measurement signals takes many forms. Fourier analysis and Fourier synthesis are based on the reality that time waveforms are composed of a series of specific harmonically related frequencies. The Fourier transform converts a time function into a function of angular frequency. Thus, if impulse or step input testing methods are being used for measurement purposes, it is possible to know the frequencies involved. Today, these operations are done digitally.

H. Sensor Fusion and DiagnosisSensor fusion is the process of integration of multiple sensor signals. Weather forecasting requires thousands of measurement inputs (pressure, temperature, wind speed and direction, moisture level and radiation attenuations) taken from atmospheric balloons, sea buoys, land weather stations, aircraft and radar. Video images taken from satellites add information. Algorithms in supercomputers use the data to predict likely weather patterns in the near future.

Diagnostic engineering is concerned with machinery, particularly rotating machines, to monitor condition and detect faults and their causes. Vibration and thermal monitoring as well as oil particle analysis are used. Medical diagnosis is, of course, highly dependent on various measurements taken in medical surgeries and hospitals.

I. Wireless Sensor Networks and Energy HarvestingWireless sensor networks (WSNs) consist of spatially distributed

autonomous sensors, which can cooperatively pass their data through the network to a main location. Figure 3 shows a network ‘node’. The networks can be bi-directional enabling control of the sensors. Star networks and multi-hop wireless mesh networks with routing and flooding are used. Example of the use of such networks include those for environmental, industrial, agricultural and structural monitoring. Energy harvesting is localised to each node. It may take several forms, for example, light and photovoltaics, vibration and electromagnetics or piezoelectrics.

J. Virtual Instrumentation and Sensor ModellingCustomisable software and modular measurement hardware can create user-defined measurement systems called ‘virtual instruments’. Graphical programming languages such as LabVIEW make this a relatively easy process.

Digital representations of physical and functional characteristics of a sensor and housing structure can be implemented by employing building information modelling (BIM) software on a computer. This enables various designs to be tried relatively quickly to obtain an optimised arrangement with performance specifications without having to construct the devices and make real physical measurements.

K. Chemical, Gas and Biological SensorsThe biosensor is an analytical device for detection of an analyte. It combines a biological component, such as tissue, enzymes, antibodies and nucleic acids that will interact with the analyte under study, with a physiochemical detector.

Energy Source

Energy Harvester

Energy Storage

SensorPower

Management

Microcontroller Transceiver

Measurand

Figure 3. Intelligentnodeofawirelesssensorsystem




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The latter transforms the signal resulting from the interaction into another signal easy to measure. The blood glucose biosensor is a good example. Arrays of biosensors provide pattern-responses to give ‘fingerprints’ of a substance.

IV. Trends in Measurement InstrumentationUp until the 1980s, industrial measurement of non-electrical quantities was somewhat restricted and measurement was largely about analogue electronics and bench-top laboratory instruments. Pneumatic instrumentation has remained useful, particularly in combustible environments. During the past 30 years, there has been significant development in the means for measurement of non-electrical quantities by the use of electronics, both for existing applications and for new applications. There has also been change towards measurement systems in-process, in situ, online and for automatic testing. High-specification analogue-to-digital converters have allowed widespread use of digital processing.

Greater use has been made of measurement inspection for product quality control and quality assurance purposes, process plant condition monitoring and now monitoring engineering assets for plant diagnostics and preventative maintenance. Means of measurement by non-contact, non-intrusive and non-invasive methods have been exploited.

Each functional part of the measurement system has advanced through the use of new technologies, although the basic sciences and the basic methodologies of measurement have largely stayed the same. Microelectronics, microcomputers and high-capacity, reliable digital storage have allowed measurement system intelligence and smartness, miniaturisation and low-power operation. Electronic processing units have become localised into sensor packages.

The explosion in sensor technologies has largely resulted from development

and use of physical, chemical and now biological materials. Optics has joined electronics so that the insulating properties of glass and the photon complement the properties of conducting copper and electron. Hand-held instruments, Ethernet and global positioning system (GPS) links and distributed sensing and wireless sensing networks are extending the places from which measurement results can be obtained. Digital data fusion and signal processing, digital data logging and data recording and digital signal diagnostics are extending the ways in which measurement electrical signals are viewed, held and utilised. Printed circuit board (PCB)-embedded ‘instruments’ now allow electronic equipment self-testing.

V. ConclusionThe fundamentals of measurement, as discussed in the Part 1, remain much the same as time progresses. However, during the last generation, there have been significant changes to the technology applied to measurement as discussed in this Part 2 article. There have, of course, been further measuring instrument developments usually of a more specialised kind, for example, those using optical engineering and other radiation-type equipment and those for electrical, electromagnetic and magnetic quantities. Indeed, the total range of measuring instruments and measurement systems is vast. All aspects of physics, chemistry and biology are to be found in measuring instruments of one kind or another.

During the past 30 years, there has been something of a paradigm shift in that monitoring, and measurement has become much more integral to engineering system design, and it seems likely that this trend will continue. Combinations of mechanics, electronics and optics have moved down the dimensional scale from millimetric to micrometric to nanometric.

Basic metrology, instrument calibration and measurement test services have in general steadily

improved to keep pace with the needs of industry and science. The main current and future problem for the United Kingdom though is and will be the lack of people with suitable measurement knowledge and practical skills within the engineering and technology community. The problem is both qualitative and quantitative as will become more evident in Part 3.

AcknowledgementsThe bibliographic sources listed below illustrate well the output of authoritative books on measurement instrumentation and sensors published during the past 30 years or so. The author acknowledges the valuable information, including Tables 1 and 2 from the AMA report.

FundingThis research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

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