13 Sensor technologies

 

13.9.10 Ultrasonic imaging

The main applications of ultrasound in imaging are found in medical diagnosis and in industrial testing procedures. In both of these applications, a short burst of ultrasonic energy is transmitted from the ultrasonic element into the medium being investigated and the energy that is reflected back into the element is analysed. Ultrasonic elements in the frequency range 1 MHz to 15 MHz are used.

Ultrasound is reflected back at all interfaces between different materials, with the proportion of energy reflected being a function of the materials either side of the interface. The principal components inside a human body are water, fat, muscle and bone, and the interfaces between each of these have different reflectance characteristics. Measurement of the time between energy transmission and receipt of the reflected signal gives the depth of the interface according to equation (13.5). Therefore, in medical diagnosis procedures, the reflected energy appears as a series of peaks, with the magnitude of each peak corresponding to the type of interface that it is reflected from and the time of each peak corresponding to the depth of the interface in the body. Thus, a ‘map’ of fat, muscle and bone in the body is obtained. A fuller account can be found elsewhere (Webster, 1998).

Applications in industrial test procedures usually involve detecting internal flaws within components. Such flaws cause an interface between air and the material that the component is made of. By timing the reflections of ultrasound from the flaw, the depth of each flaw is determined.

 

13.10 Nuclear sensors

Nuclear sensors are uncommon measurement devices, partly because of the strict safety regulations that govern their use, and partly because they are usually expensive. Some very low-level radiation sources are now available that largely overcome the safety problems, but measurements are then prone to contamination by background radiation. The principle of operation of nuclear sensors is very similar to optical sensors in that radiation is transmitted between a source and a detector through some medium in which the magnitude of transmission is attenuated according to the value of the measured variable. Caesium-137 is commonly used as a gamma-ray source and a sodium iodide device is commonly used as a gamma-ray detector. The latter gives a voltage output that is proportional to the radiation incident upon it. One current use of nuclear sensors is in a non-invasive technique for measuring the level of liquid in storage tanks (see Chapter 17). They are also used in mass flow rate measurement (see Chapter 16) and in medical scanning applications (see Webster, 1998).

 

13.11 Microsensors

Microsensors are millimetre-sized two- and three-dimensional micromachined struc[1]tures that have smaller size, improved performance, better reliability and lower production costs than many alternative forms of sensor. Currently, devices to measure temperature, pressure, force, acceleration, humidity, magnetic fields, radiation and chemical parameters are either in production or at advanced stages of research.

Microsensors are usually constructed from a silicon semiconductor material, but are sometimes fabricated from other materials such as metals, plastics, polymers, glasses and ceramics that are deposited on a silicon base. Silicon is an ideal material for sensor construction because of its excellent mechanical properties. Its tensile strength and Young’s modulus is comparable to that of steel, whilst its density is less than that of aluminium. Sensors made from a single crystal of silicon remain elastic almost to the breaking point, and mechanical hysteresis is very small. In addition, silicon has a very low coefficient of thermal expansion and can be exposed to extremes of temperature and most gases, solvents and acids without deterioration.

Microengineering techniques are an essential enabling technology for microsensors, which are designed so that their electromechanical properties change in response to a change in the measured parameter. Many of the techniques used for integrated circuit (IC) manufacture are also used in sensor fabrication, common techniques being crystal growing and polishing, thin film deposition, ion implantation, wet and dry chemical and laser etching, and photolithography. However, apart from standard IC production techniques, some special techniques are also needed in addition to produce the 3D structures that are unique to some types of microsensor. The various manufacturing techniques are used to form sensors directly in silicon crystals and films. Typical structures have forms such as thin diaphragms, cantilever beams and bridges.

Whilst the small size of a microsensor is of particular benefit in many applications, it also leads to some problems that require special attention. For example, microsensors typically have very low capacitance. This makes the output signals very prone to noise contamination. Hence, it is usually necessary to integrate microelectronic circuits that perform signal processing in the device, which therefore becomes a smart microsensor. Another problem is that microsensors generally produce output signals of very low magnitude. This requires the use of special types of analogue-to-digital converter that can cope with such low-amplitude input signals. One suitable technique is sigma–delta conversion. This is based on charge balancing techniques and gives better than 16-bit accuracy in less than 20 ms (Riedijk, 1997). Special designs can reduce conversion time to less than 0.1 ms if necessary.

At present, almost all smart microsensors have an analogue output. However, a resonant-technology pressure-measuring device is now available with a digital output. This consists of a silicon crystal on which two H-shaped resonators are formed, one at the centre and one at the edge. If the pressure to be measured is applied to the crystal, the central resonator is compressed, changing the spring constant of the material and thus reducing its resonant frequency. At the same time, the outer resonator is stretched, increasing its resonant frequency. The resulting frequency difference produces a digital output signal that is proportional to the applied pressure. The device can also give a signal proportional to differential pressure if this is applied between the centre and periphery of the crystal.

Microsensors are used most commonly for measuring pressure, acceleration, force and chemical parameters. They are used in particularly large numbers in the automotive industry, where unit prices can be as low as £5–£10. Microsensors are also widely used in medical applications, particularly for blood pressure measurement, with unit prices down to £10.

Mechanical microsensors transform measured variables such as force, pressure and acceleration into a displacement. The displacement is usually measured by capacitive or piezoresistive techniques, although some devices use other technologies such as resonant frequency variation, resistance change, inductance change, the piezoelectric effect and changes in magnetic or optical coupling. The design of a cantilever silicon microaccelerometer is shown in Figure 13.15. The proof mass within this is about 100 µm across and the typical deflection measured is of the order of 1 micron (10^-3 mm).

An alternative capacitive microaccelerometer provides a calibrated, compensated and amplified output. It has a capacitive silicon microsensor to measure displacement of the proof mass. This is integrated with a signal processing chip and protected by a plastic enclosure. The capacitive element has a 3D structure, which gives a higher measurement sensitivity than surface-machined elements.

Microsensors to measure many other physical variables are either in production or at advanced stages of research. Microsensors measuring magnetic field are based on a number of alternative technologies such as Hall-effect, magnetoresistors, magnetodiodes and magnetotransistors. Radiation microsensors are made from silicon p-n diodes or avalanche photodiodes and can detect radiation over wavelengths from the visible spectrum to infrared. Microsensors in the form of a micro thermistor, a p-n thermodiode

 

 or a thermotransistor are used as digital thermometers. Microsensors have also enabled measurement techniques that were previously laboratory-based ones to be extended into field instruments. Examples are spectroscopic instruments and devices to measure viscosity.