Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
1999-04-12
2001-06-26
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
Reexamination Certificate
active
06250159
ABSTRACT:
The invention relates to a method and a measuring device for characterising objects by means of ultrasound according to the precharacterising portions of the independent claims. In particular the thickness of walls, of layers on substrates and in composite layers can be measured, as can heights of steps on surfaces, as well as surface profiles.
When an ultrasonic wave hits a boundary surface of two media of different acoustic impedance, then in general it is partly reflected. This phenomenon is inter alia exploited for thickness gauging. An ultrasonic impulse is admitted to the object to be measured, for example a layer carried by a substrate. Then the ultrasonic impulses (echo impulses) which have been reflected at the front and rear of the object are detected as a measuring signal. From the delay time difference &Dgr;t between the two echo impulses, the thickness d of the object can be calculated by suitable calibration, e.g. if the acoustic velocity c is known:
d=c&Dgr;t/2.
Conversely, if the object thickness d is known and the delay time difference &Dgr;t has been measured, the acoustic velocity c can be calculated.
Such an ultrasound measuring signal can be taken as convolution of a measuring-object-dependent reflection sequence with a system reply depending on the measuring system. Accordingly the detected measuring signals are not ideal, sharp peaks (delta functions) but prolonged high-frequency impulses, due to the limited bandwidth of the test system. Inter alia, this results in the following difficulties for the thickness gauging described above. Where the requirements for accuracy are high, the delay time difference &Dgr;t must partly be determined with an accuracy below the duration of an ultrasonic impulse, for example by means of a curve adaptation function or by means of a characteristic position (threshold value, zero passage, slope) of the ultrasound impulses. In the case of small thicknesses, i.e. in the case of small delay-time differences, the two echoes overlap, resulting in difficulties in their resolution. The relative terms “thin”, “small” thickness of the layer to be measured or “high” accuracy which are used here, are always to be understood in relation to the high-frequency impulse duration (carrier frequency or centre frequency) applicable depending on the materials and methods. These difficulties manifest themselves increasingly in technologically interesting thickness-gauging problems, due to the ever thinner layers applied and/or due to acoustic delay specific to the material and the medium of propagation, which acoustic delay increases at higher frequencies.
For example, printed patent specification DE 44 14 030 C1 describes a method for determining the thickness of a layer in the case of overlapping echo impulses. In this, the build-up time of the detected enveloping curve is used as a measure for the layer thickness. To this effect, the measuring device must first be calibrated by means of measurements on layers of known thickness by storing a correlation between build-up time and layer thickness. The necessity of such demanding calibration presents a first disadvantage of this method. A further disadvantage arises from the fact that a layer can only be measured from the direction of the substrate side which in most cases is not accessible. This places extreme limits on the application range of this method. In addition, the method only functions for strongly overlapping echo impulses of amplitudes similar in size, which presents a further serious limitation.
Published application DE 44 34 688 A1 discloses an ultrasound thickness-gauging device with digital data acquisition and an evaluation unit which subjects the echo impulses to an evolution analysis. Evolution takes place by means of a division in the frequency domain (Wiener filtering). Fast Fourier transformation (FFT) or inverse FFT is used for the transition from the time domain to the frequency domain and back. Evolutions in the frequency domain suffer from the disadvantage of being very sensitive to unknown noise present in the spectrum of the measuring signal.
Both the above-mentioned documents describe ultrasound thickness-gauging devices which operate only in contact with the object to be measured or by means of a special coupling medium, but not in a non-contacting way. However, non-contacting thickness-gauging would be desirable for various applications, for example to avoid leaving any traces on the objects to be measured; to avoid damaging the objects to be measured; or to avoid their contamination by a coupling medium. Gauging the thickness of a freshly applied layer of powder, lacquer or paint, prior to baking, drying or hardening provides one example where only a non-contacting method of measurement can be considered.
It is the object of the present invention to describe a method for characterising objects to be measured by means of ultrasound, in particular for thickness-gauging, measuring the acoustic velocity, or measuring a surface profile. The method is to be suitable for non-contacting or contacting measurements, according to choice. It is to be applicable to objects with thicknesses or steps below, equal to, or above the sound-wave length used. For example a homogenous object, a self-supporting layer system or one or several layers on a substrate may be an object to be measured. In the latter case, the measuring method should be able to be applied from the direction of the side with the layer or the substrate, according to choice. Furthermore, it is the object of the present invention to provide a measuring device for implementing the method.
This object is met by the measuring method according to the invention and by the measuring device according to the invention, as defined in the independent claims.
With the measuring method according to the invention, at least one ultrasonic wave is transmitted by a measuring device; echo waves reflected by the boundary surfaces of the object to be measured are detected as a measuring signal by the measuring device. The measuring signal is digitalised and subjected to an evolution analysis in the time domain. The evolution algorithm used is based on the assumption that the quantity of “physically sensible” reflection sequences can be limited by using realistic a-priori value ranges orientated towards the test problem. The measuring signal is taken to be a convolution of a reflection sequence depending on the object to be measured, with a system reply depending on the measuring system. This approach in the time domain leads to a tree-based maximum-likelihood sequence estimation of exponentially increasing complexity. However, in the case of real interesting definitions of the problem, this complexity can be reduced by incorporating existing a-priori values orientated towards the test problem, and by respective data concerning the reflection sequence.
The measuring method according to the invention uses a new evolution algorithm in the time domain which is particularly computation efficient by orthogonalisation and by an implemented search strategy corresponding to the existing a-priori value ranges. In the evolution algorithm according to the invention, a given measuring signal is approximated by weighted additive overlapping of temporally shifted base impulses (wavelets). The shape of the base impulse can be derived from suitable reference signals (calibration, gauging) and is taken as being known. In this, dispersion and frequency-dependent acoustic decay can be considered by a shape of the base impulses changeable with the delay time. Weighting and temporal shift of the base impulses is for example optimally determined in the least-square-sense, i.e. by minimising the error square sum (sum of the squares of the differences between measuring signal and evolved signal). However, other optimisation algorithms may also be used for this purpose. In the case of white measuring noise, this represents the solution with the largest probability; the so-called maximum likelihood solution.
When characterising another object to be m
Kalin August
Kreier Peter
Larkin Daniel S.
Miller Rose M.
Oppedahl & Larson LLP
Ramseier Hans-Ulrich
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