20-07-2012, 01:01 PM
Laboratory system for ultrasonic measurements of distance and speed Description:
ultrasonic measurements.pdf (Size: 54.09 KB / Downloads: 26)
Ultrasonic sensors are used in a large variety of applications such as the measurement of
distance and speed, the detection of objects and object situations, object identification, nondestructive
testing or medical imaging. Owing to their excellent characteristics, which include
high range resolution, high precision and good reproducibility, high mechanical robustness and
long-term stability, a large measuring range that is adjustable with simple means, and a
relatively low price. For communication and object detection under water (sonar), ultrasonic
waves are used almost exclusively due to the large attenuation of electromagnetic waves. Both
the basic metrological principles and signal processing methods are very similar for ultrasonic
and radar measurement technology. Thus, for example, for many applications of noncontacting
distance and speed determination, pulse echo methods are employed. The object
distance to be determined is proportional to the roundtrip time of the emitted, reflected and
received pulse-shaped wave packet; the object speed is the difference between the
transmission and receiving frequencies (Doppler shift). Drawbacks of the simple pulse echo
method (low exploitation of transmission power and thus limitation of range, restriction of
range resolution and measurement precision due to pulse duration) can be eliminated by
applying what is referred to as pulse compression, using transmission pulses with a high timebandwidth
product, e.g. frequency-modulated pulses (termed ”chirps”).
Activities and objectives:
Based on an existing prototype, a modern measuring site will have to be planned and set up
which includes the measuring instruments needed to evaluate and quantify possibilities and
limits of distance and speed measurements by means of ultrasound as well as important
parameters influencing measurement ranges and spatial resolution. This measuring set-up shall
be suitable for use e.g. in studies carried out as part of lab classes. For two modes of operation
(pulse echo and pulse compression methods), echo signals are to be recorded as a function of
the distance, size, number, speed and surface characteristics of different reflectors, employing
several commercial ultrasonic transducers varying with respect to their operating principles,
resonance frequencies and bandwidths, directional characteristics etc. All parameters of the
transmission signal (mean frequency, bandwidth and/or pulse width, pulse shape, amplitude,
repetition frequency) should be adjustable by the operator. In order to achieve almost uniform
sensitivity throughout the coverage area, echo signals must be amplified by a suitable
amplification unit, depending on their roundtrip times and amplitudes. The required signal
processing routines may be implemented either with the help of a microprocessor or a PC;
system control and visualization shall be realized by means of a GUI (graphical user interface)
in Matlab/ LabView.
Desirable knowledge and requirements:
Ultrasonic sensor technology, experience in electronic circuit design and hardware set-up,
programming in LabView or MatLab, basic knowledge of (digital) signal processing.