Optics: measuring and testing – Velocity or velocity/height measuring – With light detector
Patent
1992-11-06
1994-08-30
Buczinski, Stephen C.
Optics: measuring and testing
Velocity or velocity/height measuring
With light detector
356 5, 324 7648, G01C 308, G01P 336, G01R 1900
Patent
active
053432851
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an electrical circuit for measuring the frequency of laser Doppler signals, having at least one counter for counting the signal pulses and having a microprocessor for evaluating the counter reading produced within a measuring interval.
Especially when measuring the speed of a moved surface by means of laser beams which are guided onto the moved surface and the reflected dispersed light thereof is measured and evaluated to determine the speed information, there are usually produced laser Doppler signals having an undesirable amplitude, phase and frequency modulation which is based on interference in the measuring volume and caused by changes in the moved surface. When determining the frequency of laser Doppler signals which contain speed information, this modulation has a very disadvantageous effect. This is because for frequency determination the number of signal pulses which occur within a predetermined measuring interval is counted for example by means of a standard frequency counter, in particular in rigidly predetermined asynchronous intervals. However, as a result of modulation, no countable pulses occur during certain periods of time, so that the determined frequency is defective, i.e. is below the actually existing frequency. This is because the counter counts the number of pulses within the predetermined time interval, and the frequency is calculated from the number of counted pulses by taking the average. If there arises within the predetermined time interval a relatively large gap within which no countable pulse occurs, then the established average value becomes inaccurate.
In practice, the remedy is to use as small as possible a division of the measuring intervals, so that intervals in which no pulses are counted are discarded and are not taken into account in the frequency calculation. However, even in the case of dividing a measuring interval into as many small intervals as possible the measuring method is ultimately the same, the only difference being that as a result of the higher quantization ultimately a higher degree of accuracy is achieved, but without basically removing the error. This is because to do this would necessitate a division of the measuring intervals of such smallness that much more calculating capacity is involved in the evaluation of the measuring intervals than exists today.
If a signal were derived which provides information on whether pulses are present, for example derived from the amplitude of the laser Doppler signal, then information can be lost, since the measurement of an entire interval is discarded although the additional signal has given the message "no pulses" only for the smallest part.
Instead of frequency measurement, there is also possible period length measurement from the result of which the frequency can be calculated with the aid of the equation f=1/T. Here, only a single valid oscillation is necessary to determine the frequency. This is also done very fast. However, this method is ultimately inaccurate, since the period length is subject, as a result of the phase and frequency modulation mentioned at the outset, to greater statistical variation. Moreover, this method requires high measuring expenditure and above all very high-performance hardware.
It is therefore the object to provide an electrical circuit by means of which the accuracy of the frequency measurement from laser Doppler signals can be very substantially increased without a disproportionately high level of expenditure being necessary for this.
According to the invention, the object is achieved in that for controlling the pulse count a blocking element, such as an And-gate, which is controlled by an additional signal, for example by an automatic band pass, is connected in series with the laser signal pulse counter, in that there is arranged a time pulse counter which is connected for example to a quartz pulse generator and determines the measuring interval by means of a predetermined number of pulses, and in that the laser signal pulse counter and the time pulse co
REFERENCES:
patent: 3689921 (1972-09-01), Berry
patent: 4297030 (1981-10-01), Chaborski
patent: 4715706 (1987-12-01), Wang
patent: 4974960 (1990-12-01), Dopheide et al.
patent: 5198750 (1993-03-01), Proklin
Futterer Martin
Gondrum Uwe
Grebe Volker
Milbradt Frank
Buczinski Stephen C.
Mesacon Gesellschaft fur Mebtechnik mbH
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