Ultrasonic Range Finders

Wednesday, November 15th, 2017 - Piezoelectric, Transducer/Sensor

Ultrasonic Range Finders

Ultrasonic imaging has come to the fore in recent years as an important alternative to X-rays in the field of medicine (Karrer, 1983; Waaft and Gramiak, 1976). In spite of this success, however, the ultrasonic technique has yet to provide any new solutions to the problem of range finding in the field of robotics. This is thought to be due mainly to the large mismatch between the impedance of the ceramic transducers and that of the air medium, as well as the wide emission angle of most ultrasonic transducers.

An interesting research program is, however, being carried out at the University of Canterbury (NZ) by Prof. L. Kay and his team on the development of an ultrasonic sensor suitable for robotic applications.

The present prototype is only a single point rangefinder but the small size of the transducers indicate the possible expansion to a limited 2-D linear array.

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Figure 1. Exploded view of an ultrasonic transducer (courtesty of Polaroid)

Because of their wide angle of operation, single point ultrasonic rangefinders have, in fact, useful applications in obstacle detection in such fields as mobile robotics (Seals, 1984). The best known single point distance sensor currently on the market is the Polaroid ultrasonic sensor developed primarily for photographic camera applications. Since the Polaroid sensor is, in principle, very similar to others in this field, it will be used as the mean of explanation. The basic structure of the front end ultrasonic transducer is shown in Figure 1.

Ultrasonic Range Finders Principle Of Operation

The principle of operation of an ultrasonic sensor is to measure the time delay t between the transmitted and reflected sound pulses which, assuming a constant velocity v for sound propagation, is related to the obstacle distance d by the simple formula d = vt.

Ultrasonic sensor functional block diagram

Figure 2. Ultrasonic sensor functional block diagram (courtesy of Polaroid)

Figure 2 illustrates the main steps of the ultrasonic distance measuring method. A sound pulse is produced by the ultrasonic transducer (pulse length approximately 1 ms and frequency spectrum from 50 kHz to, typically, 60 kHz) which, after a time delay proportional to the object distance from the sensor, also receives the reflected sound pulse or ‘echo’. The hardwired local intelligence then processes these two signals (emitted and reflected pulses) and calculates the obstacle distance from the sensor.

Since the received ‘echo’ could have been produced by any suitable within the incident sound pulse ‘cone’, whose output beam pattern is shown in Figure 3, a scan of the required area needs to be carried out in order to obtain a range image if required, as in the case of obstacle recognition. This can be achieved either electronically, by using an array of ultrasonic transducers, or mechanically, by moving the single transducer in a suitable pattern. It is interesting to note that, since the transmitting cone changes with frequency, a small degree of electronic ‘scan’ is achieved by incorporating a frequency sweep within the emitted sound pulse; a typical frequency range would be 50-100 kHz.

Typical ultrasonic transducer output

Figure 3. Typical ultrasonic transducer output (@ 50 kHz) (courtesy of Polaroid)

Another useful outcome of the frequency sweeping (also achieved by using multiple discrete frequency values, as in the case of the aforementioned Polaroid distance sensor) is that there is less chance of one object being particularly transparent (or absorbent) to the transmitted frequency thereby┬ácausing faulty operation, since this object would not be ‘detected’ by the ultrasonic sensor.

I hope this information about “Ultrasonic Range Finders” is easy to be understood.