Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2000-12-26
2004-01-27
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S602000
Reexamination Certificate
active
06681635
ABSTRACT:
The invention involves a method to determine a physical quantity by means of acoustic transit time measurement using at least one acoustic transducer as a transmitter and at least one acoustic transducer as a receiver. The invention also includes an apparatus adapted to implement this method.
Such a method and such an apparatus have been described in WO93/0057. The apparatus described there uses acoustic transit time measurement to determine the temperature of a turbulent gas flow through a measuring space. It reduces the effects of noise on the measuring signal by using transmitted signals of a specific, selected frequency and aims to achieve a relatively accurate transit time measurement by means of the corresponding correlation operation. This method, however, is not sufficiently accurate for certain applications.
The present invention attempts, among other things, to overcome this limitation. For this purpose, a method of the type referred to above is characterised by a transit time measurement using an acoustic transmitted signal which is at least partly selected on the basis of the type of signal processing to be used, and at least two points in an acoustic received signal and/or a derived signal, which are also selected at least partly on the basis of the type of signal processing to be used.
Signal processing in order to determine the quantity to be measured involves determining the point in time at which the acoustic signal received shows the expected signal shape. This is achieved by applying a detection method to at least two specific points in the received signal. These points in the signal are representative of the shape of the relevant part of the acoustic received signal.
Since a known transmitted signal is used, and the transfer functions of the transducers and the medium are highly predictable, the detection system can make use of a relatively high level of prior knowledge of the expected received signal. This property, combined with the utilisation in the present invention of at least two points selected at least partly on the basis of the type of signal processing to be used in a received signal which is also selected at least partly on the basis of the type of signal processing to be used, allows considerable improvement of the accuracy of detection and hence of the accuracy of the transit time measurement.
The transmitted signal to be used is preferably selected at least partly on the basis of the geometry, the type of medium and other characteristics and features of the measuring space. In most cases, however, these characteristics solely, or at least largely, determine the frequency range of the transmitted signal, rather than, e.g., the frequency course of the transmitted signal, which means that this frequency course can be set to optimise the results. The accuracy of the transit time measurement can be improved by striving towards a large information content in the transmitted signal, the information content being adapted to the measurement path, the medium and the transducers. As a result of reflections, a known transmitted signal is received by the receiving transducer at the other end as a compound signal, which repeatedly includes the transmitted signal, whether or not distorted.
It should be noted that the transit time of an acoustic signal in a medium over a particular path can only be defined for a monochromatic signal and is then only valid for a homogeneous and isotropic medium in a stationary condition. In a compound signal, each frequency component has its own transit time for a particular path, since the frequency dependence of the velocity of sound results in velocity dispersion. Therefore, using a compound signal necessitates the use of weighted transit times.
In measuring particular physical quantities, such as temperature or distance, weighted transit times can be determined by means of algorithms which use information about the transmitted and detected signals and prior knowledge of other parameters. The functionality of the signal interpretation can be realised by means of algorithms based. on operations such as filtering, correlating, interpolating, curve-fitting, statistical data processing and analysis in a time-frequency domain. Signal interpretation may result in a comparison function which yields, for each point in time, a measure of the similarity between the received signal and the transmitted signal or a signal derived from the latter.
In one preferred embodiment a weighted transit time of an acoustic signal is determined using knowledge consisting of models for the medium and/or the measuring space about the distortion which the signal undergoes on its path through the measuring space. This embodiment preferably uses an aperiodic transmitted signal, of limited duration, in order to reduce the influence of spatial reflections on the signal received. This allows highly accurate weighted transit time measurements for long measurement paths.
The knowledge required for signal generation and interpretation can be divided into initial knowledge, i.e., characteristics and features introduced into the system, and knowledge which changes during the process of measuring. The initial knowledge includes information on system data and system behaviour, parameterised models of the medium and the measuring space, and information on processes taking place in the medium. This initial knowledge is adapted on the basis of the changing process and environmental conditions, including changes in the measuring space, the measurement path and process conditions such as flow.
A further preferred embodiment is characterised by the fact that an acoustic signal, emitted by a transmitter, is acoustically reflected one or more times through the measuring space before being received by the receiver, which may or may not be the same as the transmitter. Thus, the transmitter and the receiver may be one and the same transducer. The transmitted signal is reflected through the measuring space by means of acoustic mirrors before being received by the same transducer, which now functions as a receiver. In addition to the advantage of requiring only one transducer, the use of reflection results in a fairly long measurement path even within a relatively small measuring space. This raises the accuracy of the local acoustic transit time measurement and reduces the influence of external factors such as the environmental temperature on the measuring system and hence on the transit time measurement, provided a suitable construction is used for the measuring system and the housing of the measuring space.
In a further preferred embodiment, the two points selected in a received signal are taken from the slope and the top of the signal, and are used for signal shape detection with the help of specially adapted signal processing based on combined slope and top detection of a received acoustic signal. If, for instance, analogue signal processing uses combined slope and top detection of a selected acoustic signal transmitted by a transmitting transducer, then the use of a transmitted signal selected on the basis of the in this case analogue signal processing technique and the two points in the received signal mentioned above allow detection with a relatively high level of selectivity and prior knowledge of the measuring signal. This results in a simple and extremely sensitive detection technique, unlike conventional signal level detection, in which a received signal, attenuated by damping, is detected at a different geometric part of the signal slope received. A well-known improvement of this latter technique is to repeat the measurement using adjusted amplification levels. The combined slope and top detection system applied in the present invention aims at shape detection based on knowledge of the signal shape, using slope detection as an indication of the expected signal top. The actual shape detection is implemented by determining the position of the signal top corresponding to the detected signal slope. Detection on the basis of the position of the signal top is quicker
Saint-Surin Jacques
Williams Hezron
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