Capacitive accelerometer

Fig. 5.1.18 Bulk micromachined capacitive accelerometer chip from VTI Hamlin OY showing a massspring system in the center chip and counterelectrodes on both side chips before and after sealing...

D. Lapadatu, Limits in miniaturisation of bulk micromachined capacitive accelerometers, Ph.D. Thesis, K. Univ. Leuven, Belgium, 1996. 

K. Chau, S. Lewis, Y. Zhao, R. Howe, S. Bart, R. Marcheselli, An integrated force-balanced capacitive accelerometer for low-g applications, in Tech. Dig. 8th Ini. Conf. Solid-State Sensor and Actuators (Transducers 95), Stockholm, Sweden, June 1995, 593-596. 

Bryzek J et al 1994 Micromachines on the march IEEE Spectrum 31 20-31 Cole J C and Braun D 1994 Applications for a capacitive accelerometer with digital output Sensor 11 26-34... 

H. Kulah, J. Chae, N. Yazdi, K. Najafi, A multi-step electromechanical/spl Sigma//spl Delta/converter for micro-g capacitive accelerometers, in Sohd-State Circuits Conference, 2003. Digest of Technical Papers. ISSCC. 2003 IEEE International, 2003, pp. 202-488. 

Many of the variations developed to make pressure sensors and accelerometers for a wide variety of appHcations have been reviewed (5). These sensors can be made in very large batches using photoHthographic techniques that keep unit manufacturing costs low and ensure part-to-part uniformity. A pressure differential across these thin diaphragms causes mechanical deformation that can be monitored in several ways piezoresistors implanted on the diaphragm are one way changes in electrical capacitance are another. 

The use of glass is also very common in other sensors than accelerometers using capacitive sensing technology. 

Fig. 5.3.12 Working design of a discrete accelerometer made by silicon surface micromachining the seismic mass forms the movable inner electrode and is suspended on two springs the movable capacitive fingers are interdi-gitated between the outer electrodes 1 and 2, which have fixed (immovable) capacitive fingers...

In addition to the desired dependence on AC, it has a matching-dependent offset and gain that depends on parasitics. Any deviation of the reference capacitor Cref from the nominal value of the sense capacitance Cs appears as offset. Since in many applications AC is much smaller than C0, this offset often exceeds the signal. Offset cancellation should therefore occur early to minimize the dynamic range of the readout electronics. Care should also be taken for the trimming not to introduce a poor temperature coefficient. One solution fabricates the reference with the same process and in close proximity to the sense capacitor. The z axis accelerometer shown in Fig. 6.1.3 [7] utilizes two rnicromachined structures for the sense and reference. The suspension of the reference structure has been made intentionally stiff. 

A monolithic three-axis accelerometer with three independent capacitive readout circuits on a single chip is described elsewhere [7] (Fig. 6.1.14). The circuit is similar to Fig. 6.1.10 and achieves 0.085 aF/v/Hz resolution with 100 fF sense capacitors. The noise is actually dominated by Brownian noise in the sensor itself, as tests in vacuum demonstrate. The actual capacitance resolution is therefore somewhat better than stated. This circuit uses correlated double sampling (CDS) for biasing and to reject flicker noise. 

Most silicon accelerometers are based on a micromachined variable capacitance element (g-cell) that is converted to a voltage using a C-V converter and then amplified, filtered, and buffered to provide an analog output as shown in Fig. 7.1.4. To date, open-loop implementations for capacitive read-out circuits are more widely employed than closed-loop systems, primarily as a result of the stability of such systems [16]. Interface electronics for micromachined sensors depend not only upon the transduction technique (input specification) and the product requirements (output specification) but also on the packaging approach, as parasit-ics are introduced when a multiple-die packaging technique is used. 

Surface micromachining does not usually require two-sided processing of the silicon, rendering it often more CMOS-compatible. A structural material is patterned over the top of a sacrificial material. Subsequently, the sacrificial material is etched, leaving the anchored structural material free to move. Capacitance is used almost exclusively as the transduction technique with surface micromachined devices. An example of a three-layer polysilicon surface micromachined accelerometer is shown in Fig. 7.1.12f. 

For physical sensing applications, micromachined silicon has been used in applications for pressure sensing [22] and accelerometers [23], Thin diaphragms or beams of silicon suspended over the base silicon are formed by chemical etching. Acceleration or applied pressure causes the suspended silicon to move closer to (or farther from) the base causing the capacitance to increase (or decrease). 

Acceleration Accelerometer, gyroscopes Suspended spring, capacitance, resonant piezoresistive effect, diaphragm... 

Rittersma ZM, Splinter A, Bodecker A, Benecke W (2000) A novel surface-micromachined capacitive porous silicon humidity sensor. Sens Actuators B 68 210-217 Sim J-H, Cho C-S, Kim J-S, Lee J-H, Lee J-H (1998) Eight beam piezoresistive accelerometer fabricated by using a selective porous silicon etching method. Sens Actuators A 66 273-278 Steiner P, Lang W (1995) Micromachining applications of porous silicon. Thin Solid Films 255 52-58... 

In the case of actively controlled structural dynamics, sensors may measure a variety of signals, such as accelerations (accelerometers), displacements (Hall sensors, capacitive sensors, laser interferometers, etc.), forces (force transducers), or - typically for adaptronic structures - strain or strain velocities (strain gauges, piezoelectric sensors, etc.). Most of these cases can be represented by the following sensor equation ... 

J. Wu, G.K. Fedder, L.R. Carley, A low-noise low-offset capacitive sensing amphfier for a 50-pg/VHz monolithic CMOS MEMS accelerometer, IEEE Journal of Solid-State Circuits 39 (2004) 722-730. 

An accelerometer is a basic technology that converts mechanical motion into an electrical signal. It is an electromechanical device that measures acceleration force, whether caused by gravity or motion. There are many different types of mechanisms involved in the accelerometers, including piezoelectric, piezoresistive, capacitive. Hall effect, magnetoresistive, and temperature sensors (Table 12.2). Piezoelectric, piezoresistive, and capacitive types are the most common in commercial devices. 

The mass motion has to be measured and recorded by some device. In early accelerometers, a light beam was reflected in a mirror which rotated with the mass motion. Presently, almost all seismic accelerometers use a capacitive transducer, which gives a voltage output proportional to the mass displacement. This type of transducer is very sensitive - it can resolve displacements of the order of pm (10 mm). 

Seismk Accelerometers, Fig. 4 Schematics of variable capacitance displacement transducers, (a) Variable gap. As the central plate (red) moves right, the gap with the left fixed plate (blue) increases (capacitance of left capacitor, yellow, decreases), and the gap with the right plate (blue) decreases (capacitance of the right capacitor, light blue. 

Seismic Acceierometers, Fig. 9 Principal elements of a MEMS (micro-electro-mechanical system) accelerometer with capacitive transducer. The mass is the upper mobile capacitor plate which can rotate around the torsion bars. The displacement, proportional to acceleration, is sensed... 

The two capacitive techniques described above are used variable gap between plates and variable area. The first is quite nonlinear and usually requires electrostatic feedback to linearize the transducer response. Recently, electromagnetic feedback has also been proposed for MEMS accelerometers (Dwyer 2011). On the other hand, the variable area technique is more linear, and some devices using it operate in open-loop mode (e.g., Homeijer et al. 2011). Figure 10 shows a possible arrangement for a MEMS accelerometer. 

Seismic accelerometers sense the ground or stmcture seismic vibrations and, together with a suitable recorder, are called accelerographs. Most modem seismic accelerometers are of force-balance type (EBA), a servo system in which a feedback force is applied to the suspended inertial mass to keep its motion as small as possible. This improves the instmment linearity and dynamic range. Usually the mass motion is measured by a sensitive capacitive transducer. 

Li B, Lu D, Wang W (2001) Micromachined accelerometer with area-changed capacitance. Mechatronics 11 811-819... 

Sensors, Calibration of. Fig. 3 Simplified principle behind the force balanced accelerometer. The displacement transducer normally uses a capacitor C, whose capacitance varies with the displacement of the mass. A current, proportional to the displacement transducer output, will force the mass to remain stationary relative to the frame (Figure from Havskov and Alguacil 2010)... 

In physical sensors, such as pressure sensors and accelerometers, capacitance measurements are a common means of signal transduction. Such methods can also be used with chemical sensors34, however, they have not received extensive attention to date. For sensors applied to the liquid phase, this may be due to the complexities associated with the impedance of the double layer that forms in solution at a charged interface. This tends to make the sensor sensitive to changes in ionic strength, unless the time constant of the selective coating that is relevant to the analytical measurement is considerably different from that of the double layer. Further, many chemical reactions associated with a coating... 

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