Sensor Offers Easy Integration, Long-Range Detection

By using a patented electrostatic transducer rather than a more common piezoelectric, engineers at SensComp (Livonia, MI) have developed an integrated sensor package. The 600 Series Smart Sensor combines the ultrasonic transducer with a circuit board carrying the drive electronics. And, according to Don Widmark, applications engineer, the device's 110-dB output sound pressure level is about three times that of a piezo, and receive sensitivity (-42 dB) is roughly two times better.

The transducer consists of a thin, positively charged gold foil-on-plastic membrane electrostatically driven at 50 kHz to vibrate against a rigid steel, negatively charged back plate. At 1.7 inches in diameter, the sensor is larger than piezo crystal elements, which are on the order of tenths of an inch. Widmark says the larger size permits a narrow-focused beam (15 degrees at -6 dB) that, in combination with the high power output, produces a greater return echo from small, inch-size objects farther away-up to 35 ft. Distance resolution is plus or minus 1%.

But even with a detectible echo, an ultrasonic sensor needs "low ring" characteristics as well, notes Widmark. In other words, after an excite pulse, damping needs to be high enough so that when an echo is returned any vibrations from the sent signal are dissipated in order to sense the return vibrations. Thus the closer a target is, the more critical the need for quick damping. Using a train of pulses to ensure a return, the Smart Sensor can detect an object six inches away thanks to membrane damping. A short single pulse could allow object detection at one inch. With expensive signal processing, piezo sensors could detect objects similarly close, says Widmark.

Surface-mount technology (SMT) allowed for a board size comparable to that of the transducer-resulting in a stacked, integrated device that eliminates wiring from the sensor to the board. Users thus have a reduced footprint sensor that does not need shielded cable to connect a separate board and sensor.

Smart Sensor's internal clock generates pulse trains at 5 Hz or the sensor can be triggered by an external source at frequencies up to 50 Hz. Voltage-regulating circuits allow operation from power sources producing 6 to 30V dc. Applications include level sensing, presence and proximity detection, and robotics. SensComp, Inc.http://rbi.ims.ca/3090-576