Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2001-02-15
2001-11-20
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S001820, C073S591000, C073S618000, C073S648000
Reexamination Certificate
active
06318180
ABSTRACT:
The present invention relates to the determination of a frequency, particularly the resonant frequency of a resonator.
To determine the frequency at which a resonator resonates one has to drive the resonator over a range of frequencies with a loudspeaker for example whilst detecting the amplitude of the signal in the resonator with, for example, a microphone for the frequency at which the resonator is currently being driven. The loudspeaker is scanned through the possible range of frequencies at which resonance may occur. To determine the resonant frequency accurately the loudspeaker must be scanned through the possible range of frequencies at which resonance may occur in small steps. For example, to achieve an accuracy of +/−0.5 Hz, the loudspeaker must scan through the range of frequencies in steps of 1 Hz. However, if the range of frequencies that must be scanned is large this will take a long time which may be inconvenient.
According to a first aspect of the present invention a method of determining a frequency at which a resonator resonates comprises:
driving an acoustic transmitter for applying an acoustic signal to the interior of a resonator by scanning through a first range of frequencies in substantially equidistant steps of a first size;
detecting a signal from an acoustic receiver arranged to detect the amplitude of an acoustic signal within the resonator produced by driving the acoustic transmitter over the first range of frequencies;
determining a frequency at which a maximum occurs in the detected signal for the first range of frequencies;
driving the acoustic transmitter by scanning through a second range of frequencies containing the determined frequency from the first range of frequencies, the second range being smaller than the first range, and scanning through the second range in substantially equidistant steps of a second size which is smaller than the steps of the first size;
detecting a signal from the acoustic receiver produced by driving the acoustic transmitter over the second range of frequencies; and
determining a frequency at which a maximum occurs in the detected signal for the second range of frequencies.
According to a further aspect of the present invention an apparatus for determining a frequency at which a resonator resonates comprises:
means for driving an acoustic transmitter arranged to apply an acoustic signal to the interior of a resonator by scanning through a first range of frequencies in substantially equidistant steps of a first size;
means for detecting a signal from an acoustic receiver arranged to detect the amplitude of an acoustic signal within the resonator produced by driving the acoustic transmitter over the first range of frequencies;
means for determining a frequency at which a maximum occurs in the detected signal;
means for driving the acoustic transmitter by scanning through a second range of frequencies containing the determined frequency, the second range being smaller than the first range, and scanning through the second range of frequencies in substantially equidistant steps of a second size which is smaller than the first size;
means for detecting a signal from the acoustic receiver produced by driving the acoustic transmitter over the second range of frequencies and
means for determining a frequency at which a maximum occurs in the detected signal for the second range of frequencies.
By driving the acoustic transmitter over a first relatively broad range of frequencies with a relatively large frequency step size a coarse value for the resonant peak is quickly obtained for the relatively broad range of frequencies scanned. Having obtained a coarse value for the resonant peak the acoustic transmitter is driven over a second narrower frequency range containing the coarse value for the resonant peak detected earlier to determine the resonant frequency more precisely. The resonant frequency may thus be detected quickly and precisely.
However, a problem with this is that the task of determining the frequency at which the acoustic transmitter is driven at the time that the resonant peak is detected is complicated by the fact that the hardware takes a finite time before a change in the frequency driving the acoustic transmitter results in a change in the detected acoustic receiver amplitude. This results in an error in the detected resultant frequency.
This is solved in a still further aspect of the present invention by scanning each range of frequencies in a first direction and determining a first frequency at which a maximum occurs and then scanning in the opposite direction and determining a second frequency at which a maximum occurs and determining the average of the first and second frequencies at which maxima occurred.
A final value for the frequency at which the maximum occurs in the detected signal is preferably obtained by summing a predetermined number of samples at each frequency at which an acoustic transmitter is driven over a further scan. By summing a predetermined number of samples the effects of random errors such as noise are reduced to produce a more dependable result. As summing a number of samples at each frequency is slower than previous scans, the range of frequencies scanned in the further scan is preferably smaller than that of previous scans to reduce the time taken to perform the scan. The summing scan is preferably the last scan preformed after the resonant frequency has already been substantially identified.
The actual frequency which produces resonance when driving an acoustic transducer is preferably measured by counting the number of its cycles in a predetermined period or by measuring the time taken to produce a predetermined number of cycles.
REFERENCES:
patent: 4233843 (1980-11-01), Thompson et al.
patent: 5074419 (1991-12-01), Stearns
patent: 5210718 (1993-05-01), Bjelland et al.
patent: 5211054 (1993-05-01), Muramatsu et al.
patent: 5251482 (1993-10-01), Bates et al.
patent: 5406503 (1995-04-01), Williams, Jr. et al.
patent: 5528924 (1996-06-01), Wajid et al.
Byrne David
Humphrey Francis Alan
Price Barry Leonard
Holt William H.
Lattice Intellectual Property Ltd.
Saint-Surin Jacques M.
Williams Hezron
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