Acceleration sensors

Position, movements Tilt sensors, acceleration sensors... 

The latest development are micromechanical sensors. Their development began with the large-scale introduction of silicon micromachined pressure sensors to the automotive industry in the nineties, which entailed a massive price reduction. Then acceleration sensors for airbag firing, yaw rate sensors and more were introduced. Many devices are still being discovered. The next step is product evolution, with introduction times between a few years and over a decade, as shown in Tab. 2.2. Once customers in the industry have accepted a product, investment in large-scale production can go ahead. It helps to find more applications for the product The time scale for the product evolution process varies from about five... 

We expect these micromachined sensors to become more and more important in the household industry, in many domestic applications of silicon pressure sensors, acceleration sensors, tilt sensors, infrared detectors and thermopiles, flow meters, as well as gas sensors and liquid constituent sensors. 

Low-Cost Acceleration Sensors in Automatic Washing Machines... 

As a result of sensor development in the automotive industry, low-priced acceleration sensors are now available. Acceleration sensors are fundamentally also suited to observing excursion (Fig. 5.57) of the suds container caused by imbalance. However, static measurement of the weight of the washing, as achieved with a distance sensor, is not possible with acceleration sensors. 

Smart shoes are another new application that will first focus on athletes before the technology is introduced to the public at large. Such smart shoes will include e.g. acceleration sensors for the detection of foot movement and a possible calculation of distances for runners and walkers. 

In irons, acceleration sensors are used that cause the iron to switch off when in an unwanted position or stationary. 

Laine, J.-P., Tapalian, Fl.C., Little, B.E., and Haus, FI.A., 2001, Acceleration sensor based on high-Q optical microsphere resonator and pedestal antiresonant reflecting. Sensors and... 

Samples and Measurements. A scheme for shear measurements is shown in figure 2. Two identical polymer samples of length L, thickness h, and width W are bonded to two rigid metal mounts. The inner mount is attached to an impedance head (a combination force and acceleration sensor), which in turn is attached to a shaker. An accelerometer is attached to the top center of the outer mount. 

Figures 13.7 (a) and (b) show an acceleration sensor also made using an HOD film of 15- im thickness, where the seismic mass was Si 422]. Its resonance frequency was 4.7 kHz, the fracture strength was 4.5 GPa, and the estimated shock tolerance was 7000 G. The decay time of the seismic mass oscillation was within seconds.

Figure 13.7. (a) Acceleration sensor made of HOD film and (b) piezoresistive sensors attached at the suspension points [422]. 

Hardness and anti-abrasiveness AFM probe, MEMS (micro-relays, acceleration sensors, micro-gripper), coating on surgery scalpels, coating on cutting tools, tab tools... 

Very often buoys are used that move with the surface. They have a special elastic mooring, so that the buoy is able to follow the orbital motion of the wave as freely as possible. These measuring buoys contain at least one acceleration sensor for the vertical component. In direction-measuring buoys, the position of the case relative to the horizontal line and to the north has to be determined by additional sensors. The significant wave height and the characteristic periods are then calculated from the variance spectrum of the vertical displacement, and the directional information results from a cross spectral analysis of all components of the buoy movement. A comprehensive description of the measuring instruments and the statistical analysis methods was given by Tucker (1991). 

Although airbags are still the main market for acceleration sensors, other applications require them as well ... 

The total market for acceleration sensors in 2000, according to NEXUS [7], is 467 million. The market will grow to 687 million by 2005. The main drivers for this growth are the increasing rates of equipment with side airbags. Active suspension control, which typically uses 3-5 sensors per system, has the potential to spur further growth if more vehicles are equipped with these systems in the future. 

In practice, when actually manufacturing acceleration sensors by surface micromachining, the overall process flow outline must be filled in with the detailed specific processes - as discussed in the previous sections. This is described below, based on an actual production process used for acceleration sensors [26] and an angular rate sensor made solely by surface micromachining [27, 28]. 

The epi-polysilicon functional layer is patterned and then etched by the trench etch process described in Section 5.3.4.2. The trench etching forms cantilever beams that act as comb fingers in the acceleration sensor, as well as the insulating trench required for electrical separation of the epipolysilicon wiring and bond pad structures (Fig. 5.3.11 e). Straight, unnotched sidewalls are strictly required, especially for all design elements that form springs or capacitive comb structures. 

Surface Micromachined Automotive Sensors 5.3.6.1 Acceleration Sensor... 

Acceleration sensors for automotive applications are manufactured in high volume using the described process flow [26]. The basic working design is illustrated in Figure 5.3.12. 

Fig. 5.3.13 Micromachined acceleration sensor with seismic mass (a), movable comb fingers (b), two fixed-finger outer electrodes (c), two supporting springs (d), and two over-range stops (e)...

M. Offenberg, H. Munzel, D. Schubert, O. Schatz, F. Larmer, E. Muller, B. Maihofer, J. Marek, Acceleration sensor in surface micromachining for airbag application with high signal/noise ratio, Robert Bosch GmbH, SAE 960758, 1996. 

The stress gradient within the thin films. High stress gradients lead to extreme curvatures of the released structures, thereby modifying the functionality of, for example, comb-structure acceleration sensors produced by Bosch or Analog Devices. 

Fig. 5.5.2 Close-up of the proof mass of an acceleration sensor made of a material with a positive and b negative stress gradient...

The suppression of the radial acceleration is realized mechanically by using acceleration sensors that are not sensitive to lateral accelerations, and by keeping an orientation of 90 ° between the drive and detection direction as precisely as possible. 

The sensing element (1) with two acceleration sensors AO, AC2 feeds the Coriolis signals via two capacitance-to-voltage converters (3, 3 ) to a summing node. The different signs double the Coriolis signal and eliminate common mode acceleration signals. 

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