Capacitive ultrasonic transducers and imaging systems: Dissertation

The principles of ultrasonic ranging system are introduced first. The problems of the basic range measurement system are discussed with methods to improve the range precision. In addition the use of the beam pattern of a single transducer is studied. The ultrasonic imaging systems in mobile robot consist usually of multiple ultrasonic transducers around the vehicle or arrays of ultrasonic transducers. In a basic ultrasonic imaging system consisting of a group of transducers the map of the environment around the robot is built up from distances along the acoustic axis of each transducer. Because the transducers point in different directions, it is possible to fire some transducers simultaneously which increases the imaging rate. The most advanced imaging system is an ultrasonic beamformer that yields the image from the whole beam area at a single pulse making these systems very attractive in mobile robots. However, there are at least two drawbacks which restrict the widespread use of ultrasonic arrays operating in air. First, the beamforming algorithm needs high processing power and second, the distance between array elements needs to be less than about half of the wavelength making the construction of such systems difficult. In this thesis the principles of a transducer array Lepakko are presented. This ultrasonic system is based on a non-linear amplitude detection method and because the phase is not used as in a conventional array, the element interval can be large compared with the wavelength. However, the operation of the beamforming and echo detection algorithms in real time involves many calculations requiring the use of signal processor circuits. Because the beamformer uses the amplitude of echoes, wide bandwidth, wide beam elements are needed to get distance and direction at high precision across the whole beam. The specifications of the transducers gave an impetus to study transducer elements needed in the array Lepakko, and this work resulted later a model for the capacitive ultrasonic transducers. In this thesis a new equivalent circuit for capacitive ultrasonic transducers is introduced. The properties of capacitive ultrasonic transducers were studied in extensive measurement series. The resonant frequency, the bandwidth and sensitivity of ultrasonic transducers were measured and the results were published in many articles. This experimental research combined with theory of capacitive acoustic transducers gave a simple equivalent model for capacitive ultrasonic transducers. The model was used to improve the performance of the transducer array Lepakko and in a simulator that includes ultrasonic transducers, acoustic path and electronic circuits.