MEMS Mechanical Sensors - Stephen Beeby Part 9 pot

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Jones and Dr. Tinghu Yan 7.1 Introduction In a highly mechanized world, force and torque are among the most important of all measured quantities [1–4]. They play a significant role in products from weighing machines and load cells used in industrial and retailing applications, to automotive and aerospace engines, screw caps on medicine bottles, and nut and bolt fasteners. Forces and torques can range from greater than 10 kN to less than 1 µN, and from 50 kNm to below 1 Nm, respectively. Measurement accuracy levels required can vary widely from, say, 5% to better than 0.01% of full scale ranges, depending on the application. Hysteresis and nonlinear effects in the mechanical structures of measuring devices need to be small, and measurement resolutions need to be high. Measurement devices need to be robust to withstand changing environmental influences such as temperature, vibration, and humidity, and they must also provide reliable measurement over long periods of time. Mechanical interfacing of the devices can be difficult and can influence final measurement. The forces and torques may change rapidly, and so the devices must have adequate frequency and transient responses. There are several methods to measure forces and torques. Often, the force to be measured is converted into a change in length of a spring element. The change in dimensions is subsequently measured by a sensor, for example, a piezoresistive, a capacitive or a resonant sensor. It is not so surprising, therefore, that most force and torque measurement devices utilize the long and well-established resistance strain gauge technology. Unfortunately, the metallic resistance strain gauge is relatively insensitive such that in use it is normal to obtain only several millivolts of analog voltage before amplifi - cation, and the gauges must not be significantly overstrained. The rangeability and overloading capabilities are seriously restricted. Also, the gauges consume relatively high electrical power (e.g., 250 mW). In general, measurement instrumentation now needs smaller sensing devices of lower power consumption and with greater rangeability and overload capabilities. Greater compatibility with digital microelectronics is highly desirable. Noncontact and wireless operation is sometimes needed, and in some cases batteryless devices are desirable. Production of measurement devices using metallic resistance strain 153 gauges can be relatively labor intensive and skilled, and may require relatively ineffi - cient calibration procedures. In recent years some instrument manufacturers of force and torque measure - ment devices have moved away from using resistance strain gauges. Already, one leading manufacturer of weighing machines for retail and industrial applications now uses metallic and quartz resonant tuning fork technologies, and smaller compa - nies have established niche markets using surface acoustic wave (SAW) technology, optical technology, and magnetoelastic technology. Further commercial developments are taking place to enhance device manufac - turability and improve device sensitivity and robustness in operation. Measurement on stiffer structures at much lower strain levels is now possible. The worldwide sen - sor research base is very active in exploring MEMS for sensing force and torque, and the rest of this chapter will review the current situation and future prospects. 7.2 Silicon-Based Devices Strain gauges based on semiconductor materials such as silicon have been used for a long time, and although they are rather more expensive and more difficult to apply to a surface than metal strain gauges, their big advantage is a very high gauge factor of about ±130, allowing measurement of small strain (e.g., 0.01 microstrain). It should be noted that the same factor for metal strain gauges is about 2. In semicon- ductor gauges most of the resistance change comes from the piezoresistance effect [5]. This gauge is rather nonlinear at comparatively high strain levels—that is, the gauge factor varies with strain. For example, if the gauge factor is 130 at 0.2% of strain, then it is about 112 at 0.4% of strain, which is the elastic limit of the gauge. Also, the gauge factor varies significantly with temperature about –0.15%/°C, which is more than 10 times worse than the metal gauges. This temperature sensitiv- ity can be substantially reduced by using two gauges, each consisting of two pieces of semiconductor material having almost equal but opposite sign gauge factors. The two gauges are mounted with their axes at right angles on the member to be strained by a force and the four resistances are connected in the bridge as shown in Figure 7.1 [6], all these resistances have very similar temperature coefficients of resistance. The bridge output is proportional to strain, but little unbalance occurs due to 154 Force and Torque Sensors V o Output R 2 R 4 R 1 R 3 V s R+ 1 R 4 − R+ 3 R 2 − Figure 7.1 Temperature-compensated semiconductor strain gauges (the plus and minus signs indicate positive and negative gauge factors). (From: [6]. © 1977 B. E. Jones, Inc. Reprinted with permission.) temperature change. Other gauge arrangements are also used. Semiconductor strip strain gauges can be very small, ranging in length from 0.7 to 7 mm, and having width typically a tenth of the element length; thus, they are useful in the measure - ment of highly localized strains. In a diffused semiconductor strain gauge (Figure 7.2), an n-Si base has a p-Si diffused layer, and this layer works as a stress-sensitive conductor when its resis - tance is measured between leads attached to deposited metallizations. A cantilever with four C-shaped diffused gauges is stretched and compressed at its upper and lower surfaces, respectively, when the cantilever undergoes bending deformation under force F. All the gauges are identical since they are made on the same die and during the same technological cycle. MEMS technology makes use of silicon as a mechanical structural material because of its excellent mechanical properties and the relative ease of fabricating in high volumes small mechanical devices by the process of micromachining [7, 8]. Silicon is an excellent piezoresistive material, with good mechanical properties. Amorphous silicon can be deposited directly on a mechanical part, for example, glass or plastics. The basic structure of such a sensor is shown in Figure 7.3 [9]. A thin amorphous silicon layer (n-, p-, or micro-compensated) acts as the sensitive area, with size 300 × 300 µm, and four metallic contacts. Two of these contacts are used to apply a fixed current to the sensing element, while the other two, orthogonal to the previous ones, provide as output a voltage proportional to the mechani- cal stress. When a mechanical stress is applied, an anisotropic modification of resistivity occurs. 7.2 Silicon-Based Devices 155 F 1 2 3 4 5 Figure 7.2 Cantilever integrated strain gauge element. F = force, 1 = cantilever, 2–5 = C-shaped strain gauge. a-Si Flexible support V out V in I in Figure 7.3 Structure of the sensing element. (From: [9]. © 2003 IEEE. Reprinted with permission.) A silicon piezoresistive force sensor has been used in a tonometric transducer [10]. A plunger is positioned with silicon gel-like glue to press onto the force sensor. The other end of the plunger has a disposable protecting latex cap to touch the eye - ball cornea. The simultaneous use of silicon bulk-machined components and miniaturized high precision mechanical structures in a hybrid configuration can solve industrial measurement problems elegantly. As one example, a micro-torque sensor based on differential force for use in the watch industry has been developed [11] with a resolu - tion better than 0.5 µNm over the range –200 to 200 µNm; it has a volume 3×3×1 cm. The torque sensor is schematically represented in Figure 7.4. It consists of two piezoresistive force sensors. A 100-µm-thick spring blade made of copper beryllium and mounted perpendicular to the torque axis converts the torque to a force acting on the two force sensors. The force sensors are micromachined silicon cantilevers. A perpendicular bar mounted on the torque axis acts on the spring blade by way of two adjustable screws. The spring blade acts through two points on the two cantile - ver force sensors. A torque applied on the axis will increase the pressure on one force sensor and decrease the pressure on the other. Load cells are force sensors that are used in weighing equipment [3]. In most conventional load cells the spring element is made from steel or aluminum, and metal resistance strain gauges are used as the sensor elements. Silicon does not suffer from hysteresis and creep, and therefore, a load cell made from silicon might be a good alternative to traditional load cells made from steel. Bending beam structures may be used for loads up to 150 kg, but for high loads, certainly above 1,000 kg, a load cell has to be based on the compression of silicon as shown in Figure 7.5 [7]. This sensor consists of two bonded silicon wafers. The edge of the sensor chip is compressed under the load, and the amount of compression can be measured by measuring the change in capacitance between two capacitor plates located in the center. An improved design to apply the load homogeneously will be discussed in Section 7.5. Another design of silicon load cell for loads up to 1,000 kg has been reported [12]. Besides large forces/torques, very small quantities can be sensed; a micro-torque sensor based on differential force measurement was reported more than 10 years ago [11]. 156 Force and Torque Sensors Adjust screws bar Torque axis Spring blade Force sensor chip Figure 7.4 Schematic representation of the micro-torque sensor. (From: [11]. © 2003 IEEE. Reprinted with permission.) 7.3 Resonant and SAW Devices Sensors utilizing a frequency shift as an output are highly attractive. They can be extremely sensitive and possess a wide dynamic range. The nature of the output signal makes these devices easy to integrate into digital systems and provides a rea - sonable immunity to noise. For these reasons, metallic and quartz tuning fork reso- nators have been successfully applied in industry [13–17], and sensors using bulk silicon technologies have also been demonstrated [18–21]. Recently, metallic digital strain gauges have been developed [22]. The metallic triple-beam resonator with thick-film piezoelectric elements to drive and detect vibrations is shown in Figure 7.6. The resonator substrate was fabricated by a double-sided photochemical-etching technique, and the thick-film piezoelectric ele- ments were deposited by a standard screen-printing process. The resonator, 15.5 mm long and 7 mm wide, has a favored mode at 6.2 kHz and a Q-factor of 3,100, and load sensitivity about 13 Hz/N. Other means of resonator drive and detection are possible, for example, the use of an optical fiber to reflect light from a beam edge, and an electromagnetic drive [23]. A surface-micromachined force sensor using tuning forks as resonant transduc - ers has been successfully demonstrated [24]. Figure 7.7 shows the basic design of a micromachined DETF. One end of the structure is anchored to the substrate and the other is left free for the application of an axial force. The dimensional design of the DETF determines the desired operating frequency and sensitivity [25]. In the center of each of the lines is an electrostatic transducer, such as a comb or parallel plate drive. When this tuning fork is used as an oscillator (lateral balanced mode), the 7.3 Resonant and SAW Devices 157 15.5mm15.5 mm Figure 7.6 Photograph of metallic resonator. (From: [22]. © 2003 IEE. Reprinted with permission.) cover plate (stainless steel) cover plate (stainless steel) bottom plate (stainless steel) bottom plate (stainless steel) silicon silicon silicon silicon capacitor plates (NOT LOADED) (LOADED) Figure 7.5 Principle of a load cell based on compression of silicon. (From: [7]. © 2001 Springer-Verlag Berlin Heidelberg. Reprinted with permission.) resulting frequency is a function of the applied force. The change in this frequency is the output of the device. The force sensor constructed used two tuning forks in a dif- ferential or push-pull structure, such that the output of the device was a shift in the frequency difference between them. This arrangement cancelled out temperature effects and allowed the force being measured to be amplified by mechanical leverage to the connection point of the two forks. In vacuum with closed loop feedback the fork frequencies were each close to 228 kHz and sensor sensitivity was about 4,300 Hz/µN. A fully integrated silicon force sensor for static load measurement under high temperature has been demonstrated [26]. In this case load coupling, the excitation and detection of the vibration of the microresonator were integrated in one and the same single crystal silicon package. The complete single crystal design together with a single-mode optical fiber on-chip detection method should allow measurement to high temperatures well over 100°C. A perforated mass was suspended on two beams of 25-µm thickness and 0.5-mm length (Figure 7.8). Tests in a vacuum showed the 158 Force and Torque Sensors Anchor Comb drive actuators i sense V drive F tf Figure 7.7 A basic tuning fork design using surface micromachining technology. (From: [24]. © 1995 ASME. Reprinted with permission.) Electrodes Electrodes Resonator Frame Figure 7.8 Resonant structure: perforated mass suspended on two beams. (From: [26]. © 2000 SPIE. Reprinted with permission.) resonant structure vibrated with an amplitude of 100 nm in resonance at about 104 kHz with a Q of 30,000. Load sensitivity was about 4,000 Hz/N. Relatively small SAW resonators can be used for noncontact torque measure - ment [27–35]. The sensitivity of SAW devices to strain is sufficient to perform meas - urements on a shaft that has not been weakened. Usually two SAW devices are used in one sensor, as shown in Figure 7.9 [30], and differential measurement of either phase delay or resonant frequency is performed in order to achieve temperature compensation and eliminate sensitivity to shaft bending. Both types of SAW sensors rely on the fact that the torque M applied to the shaft creates two principal components of strain,s xx = −s yy = s. As a result, one of the SAW devices is under ten - sion and the other one is under compression, causing the opposite change of phase delay or resonant frequency in the devices. The resonators have the same or better performance for the same size of substrate and are less demanding in terms of the receiver bandwidth and sensitivity. Resonator Q factors are about 10,000. The torque sensor interrogation system can employ continuous frequency tracking of reflected frequencies from the two SAW resonators, having slightly different fre - quencies, for example, 200 and 201 MHz. For torque of ±10 Nm, and using ST-X quartz SAW resonators, device sensitivity to torque at room temperature has been measured as 4.65 kHz/Nm. This torque sensitivity has a temperature coefficient of 0.2%/°C. Therefore the sensor needs to measure both torque and temperature to allow for the temperature compensation of the measured results. SAW devices can break if the strain in the substrate is more than approximately 1,500 microstrain. If the sensor has to withstand a 30-fold overload, then the nominal strain can be equal to 50 microstrain. As a consequence, interrogation error gives torque measurement error of about 1%. 7.4 Optical Devices Measurement of torque has always been an important challenge for numerous industries like aerospace and automotive. In particular there is increasing interest in electric power-assisted steering (EPAS) systems among vehicle manufacturers and component suppliers [36–39]. One of the key components of an EPAS system is a torque sensor with a basic specification as follows: torque measuring range of around ±10 Nm, an overload torque capability (nonmeasuring) of about ±110 Nm, and maximum rotational speed of around 90 rev/min. The sensor must meet the appropriate environmental and electromagnetic compatibility (EMC) 7.4 Optical Devices 159 M x y s xx s yy ( a ) M x y s xx s yy To RF couplers (b) Figure 7.9 Torque sensing element based on (a) SAW reflective delay lines and (b) SAW resonators. (From: [30]. © 2003 IEEE. Reprinted with permission.) [...]... Publishing, 199 9 [16] Hauptmann, P., “Resonant Sensors and Applications,” Sensors and Actuators, Vol A26, No 1–3, 199 1, pp 371–377 [17] Bailleu, A., and W Thelen, “Doppelplattenresonanzsensor zur Kraftmessung,” Conf SENSOR 97 , Nuremberg, Germany, May 13–15, 199 7, Vol A5.1, pp 7–12 [18] Greenwood, J C., “Silicon in Mechanical Sensors, ” J Phys E: Sci Instrum., Vol 21, 198 8, pp 1114–1128 [ 19] Greenwood,... Instrumentation, Measurement, and Feedback, New York: McGraw-Hill, 197 7 Elwenspoek, M., and R Wiegerink, Mechanical Microsensors, Berlin, Germany: SpringerVerlag, 2001, pp 97 –106 7 .9 Future Devices 1 69 [8] Sharpe, W N., Mechanical Properties of MEMS Materials,” in The MEMS Handbook, M Gad-el-Hak, (ed.), Boca Raton, FL: CRC Press, 2002, pp 3.6–3.16 [9] Biagiotti, L., et al., “A New Stress Sensor for Force/Torque... 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