Ultrasonics. What is it?
Ultrasonic sensors use the sonar principle in air: they send out an ultrasonic chirp, then convert to the receive mode and wait to detect a return echo from the surface of the target. With the speed of sound in air (or other gas) as a given, the user can calculate distance.
Electrostatic Ultrasonic Sensors
Electrostatic ultrasonic transducers work similarly to capacitive microphones: a thin dielectric film is metal coated on one side only and the moving part of the transducer, which transmits and/or receives the ultrasonic signal. A fixed backplate with concentric grooves, primarily used for beam shaping, activates the film.
Piezoelectric Ultrasonic Sensors
Piezo transducers are constructed using a ceramic piezo element that has been cut and tuned to a specific frequency range. Two electrodes are then bonded to the crystal, and the crystal is then adhered inside a closed or open face housing and sealed from the rear.
The obvious advantage of piezo transducers is the ceramic element can be placed in a variety of housings: aluminum, stainless steel, Teflon, PVC or RTV, to name a few. This allows the design engineer to choose a transducer housing that is compatible with the target being sensed and its associated environment.
The primary disadvantages of piezo transducers are low sensitivity resulting in relatively short range long ring-out after transmit, limiting the short range detection ability, and resonant frequency drift versus temperature. Sensor designers have successfully limited the frequency drift problem by using unique drive and receive circuit designs.
By impulse driving rather than tone driving the transducer with a high voltage, short duration and single cycle, the transducer resonates in a manner analogous to striking a bell with a hammer. The receive circuit is designed with a wider bandpass filter to accept the dynamic range of frequencies that are possible for the transducer over the specified temperature ranges.
The disadvantages of this method are limited drive power and shorter operating range. For applications where the sensor is at a constant temperature (such as indoor applications), tone driving the transducer and using a narrower receive bandpass filter provides better range performance than impulse drive methods.
Long ring-out remains a problem with piezo transducers, and designers usually use two transducers, one to transmit and one to receive if they desire shorter range ability. Low sensitivity also emits the long-range ability of piezo-based transducers, although by modifying the transducer design and using lower-noise/higher-gain receive circuits, many manufacturers have improved the long-range capability of these transducers.