Sensors Go Systematic （3）

Published:2011/7/28 3:07:00 Author:Amy From:SeekIC

By Helmuth Lemme

Bus connection

In systems with many sensors, it is economical to connect all of them to the computer via a common bus, rather than using individual cabling. This means that each sensor must have a bus coupler, which in many cases can also be housed on the chip. An early example of this type of component was the National Semiconductor LM75, which had a delta/sigma A/D converter and an PC bus interface on the same chip. A large demand for ICs of this type quickly arose, so new types were continuously developed and their range of functions was con­stantly enlarged to meet the wishes of users. They ultimately evolved into complex monitoring components, which (for example) can be used in PCs to monitor not only the tempera­ture but also a whole range of other quantities, such as fan speeds. Figure 2 shows the block diagram of the LM87, which has so many functions that it has become a general-purpose "early warning system’ for all possible types of faults. Here the actual temperature sensor (at the bottom left) is only one of a host of elements.



MEMS

Now that we have learned how to accurately produce mechan­ical structures with micrometre dimensions, it’s possible to integrate sensor elements that operate using moving pans, such as sensors for pressure, force, acceleration, and rota­tional motion. Where we previously encountered metallic or ceramic pans with dimensions in the millimetre or centimetre range, we now find miniature structures made from monocrystalline silicon, which together with their associated electronics form a monolithic block. This combination of micromechanics and microelectronics forms a micro-electro­mechanical system (MEMS).

The first products of this type to be manufactured in large vol­umes were acceleration sensors for triggering air bags in cars. They contain two interleaved comb structures made from sili­con. with one of them being firmly attached and the other being suspended such that it can move. Capacitances in the femtofarad range (1 femtofarad = 10’ ’5 F) are created between these two structures. When the sensor is subjected to a mechanical impact, the movable comb is slightly displaced due to its inertial mass. This causes one capacitance to become larger and (he other one to become smaller, which is sensed using an electronic bridge circuit. Such sensors have proved to be extremely reliable, and more than 100 million pieces have been produced by now. Some of them are even sensitive in two directions, such as the Analog Devices ADXL202 illustrated in Figure 3 (www.analog.com).



Airbags in cars have already saved many lives, but electronic stabilization systems can make an even greater contribution. They sense the rate of rotation of the vehicle about its vertical axis as the primary variable, and if it exceeds the maximum permissible value (as occurs with a skid), they initiate suitable steering and braking manoeuvres. In the event of rotation about the longitudi­nal axis (rollover), the airbags must inflate immediately.

Sensors for use in cars must be produced in large quantities at low costs. The glass-fibre gyroscopes used in avionics and space flight systems would be unaffordable. Here again com­ponents containing movable micromechanical structures have proven to be quite suitable, and they are already being pro­duced by many manufacturers. They are based on the physical phenomenon of (he Coriolis force: if a mass in a rotating sys­tem is displaced linearly in a radial direction, it experiences a deflection in the tangential direction. In the Analog Devices ADXRS150, a flexibly suspended mass is excited into reso­nant vibration by electrostalic forces, with an amplitude of ±3.5 pm and a frequency of approximately 15 kHz. If the chip is subjected to rotation, a force perpendicular to the direction of oscillation is exerted on the vibrating element, and in a manner similar to the previously described acceleration sen­sor, this causes a change in the capacitance measured between two interleaved comb electrodes.

In its practical implementation, the IC uses two sensor cells whose masses oscillate in antiphase. Not only does this neu­tralise the external vibration of the IC, it also makes it less sensitive to disturbance from vehicle vibration. Figure 4 shows a portion of the chip, which measures only 3x3 mm. It is housed in a BGA package with dimensions of 7x7x3 mm.

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