Sensor scene bustles in Beantown

Sensors Expo, held here in September, showed developments still advancing in sensor technologies-those companies supplying and supporting pressure, contact, and inertial sensors had many new products on display. Here are just some of the noteworthy developments on the show floor:

Motorola (Phoenix, AZ) was highlighting its newly released low-g (1.5-8.0g) MEMS (microelectromechanical systems)-based inertial sensors. What impressed me was the small 16-pin SOIC package size (6.0 x 9.9 mm), which neatly takes up minimal board space for embedded applications. Rod Borras, manager of the company's Semiconductor Products Sector, said that the design engineers came up with a device able to sense accelerations in three orthogonal directions using the change in capacitance between surfaced-machined flat silicon plates in the sensor-all in one flat package rather than multiple, orthogonal ones. Starting with a single silicon plate, this is divided in two, with the interface between the halves a series of interdigitated "fingers." Any acceleration parallel to the plane (x, y axes) changes the capacitance between the fingers laterally or longitudinally. Parallel to this plate is a second plate. Any acceleration at right angles to the plates (z axis) produces a capacitance change between the two. And, because the plates are parallel, the result is a device enclosed in a single flat package on a pc board.

Uses for the accelerometers include vibration measurement, seismic detection, bearing wear monitoring, inclinometers, and security. The sensors could also find use in automotive applications to detect rollovers, sense braking deceleration, and control active suspensions. Healthcare and fitness equipment could benefit from applications in physical therapy and rehabilitation equipment sensing force and in use as pedometers to measure distance.

Allegro MicroSystems' (Worcester, MA) new, open loop Hall effect current sensor, is designed to free engineers from having to develop their own magnetic circuits and comes in a small package size easy to fit on a pc board. The ACS750ECA-100 is a fully integrated plus or minus 100A sensor with a 13 kHz bandwidth and low thermal drift (see figure). HITEC Corp. showed its nifty MMT^TM Bolt Insert Strain Gage Assembly-an internal strain gauge assembly for insertion into existing bolts for monitoring tensile loads.

Sensors are sometimes only as good as their supporting electronics. Toward that end, Texas Instruments (Tucson, AZ) announced a new 1.8V op amp in its Burr-Brown product line. Frank Haupt, strategic development engineer in Advanced Analog Products, told me the big feature was rail-to-rail input without crossover between input stages to reduce distortion and improve common mode rejection ratio (CMRR) to 100 dB, typical. In more general engineering terms, this means that only one input stage is needed in the amplifier used to drive the sensors. Thus the usual crossover distortion in a signal when switching between the two input stages usually found in an amplifier is eliminated-a simple but elegant design change. The 1.8V amp's low voltage uses include devices powered by two battery cells.