Optics: measuring and testing – By light interference – Having light beams of different frequencies
Patent
1999-12-27
2000-11-14
Font, Frank G.
Optics: measuring and testing
By light interference
Having light beams of different frequencies
356450, 356349, G01B 902
Patent
active
06147763&
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a signal processing circuit for reference and measured signals occurring in a heterodyne interferometer according to the definition of the species of the independent claim. Heterodyne interferometry is used, for example, for contactless distance, angle, or velocity measurements. An optical measuring beam is directed onto grid structures or mirrors placed on the measured object. The surface of the measured object may occasionally even serve as a diffraction grid or a reflector.
BACKGROUND INFORMATION
European Patent Application No. 0 461 119 (corresponding to International Patent Publication No. WO 09/10195) describes a heterodyne interferometer in which a laser diode is used as the radiation source. The injection current of the laser diode is periodically modulated using a saw tooth or triangular modulation signal, for example. Injection current modulation results in periodic changes in the frequency of the optical beam produced by the laser diode, which is split by a beam splitter into two beam paths. The optical measured route is in the first beam path, and an optical monostable element is provided in the other beam path. Due to the periodic modulation, a frequency difference, corresponding to the heterodyne frequency, always appears between the two beam paths. The two partial beams are brought to a beam receiver for interference after traveling through the measured route. The measured signal generated at the beam receiver has a sinusoidal signal shape. The frequency of the measured signal is equal to the heterodyne frequency. The information about distance, angle or velocity is contained in the phase angle of the measured signal with respect to a reference signal. The reference signal can also be made available optically in an optical reference route. In this case, the two partial beams are brought to an additional beam receiver for interference after traveling through the optical reference route. The optical reference route may be omitted if the reference signal is derived electrically from the modulation signal of the laser diode.
European Patent Application No. 0 729 583 describes a device for measuring the phase difference between two electric signals occurring in a heterodyne interferometer. Each predefined edge of a first signal and each predefined edge of a second signal trigger counting sequences that allow the determination of the phase difference from multiples of 360.degree. and high-resolution measurement of the angle from 0.degree. to 360.degree.. No instructions are given regarding the pre-processing of the signals sent to the known device.
The object of the present invention is to provide a circuit for pre-processing signals occurring in a heterodyne interferometer.
SUMMARY OF THE INVENTION
The circuit according to the present invention has the advantage that a periodically occurring sign change between a reference and a measured signal does not affect the determination of the phase difference. Such a periodically occurring sign change arises due to the modulation of the radiation source with the predefined modulation signal. A phase jump occurs if the sign of the slope of the frequency variation changes.
The circuit according to the present invention assures that the reference signal and the measured signal, between which the phase difference to be measured occurs, have the same heterodyne frequency and that the heterodyne frequency is defined as an integer multiple of the modulation frequency of the radiation source. This can be achieved by suitably dimensioning an optical monostable element or by appropriately defining the amplitude of the modulation signal.
A gate signal, whose period corresponds to the modulation period, is derived from the modulation signal. Using the gate signal, the reference signal is filtered by multiplication so that those signal components of the reference signal having the same phase sign are forwarded. The measured signal is also filtered by multiplication using the gate signal so that only those
REFERENCES:
patent: 4594000 (1986-06-01), Falk
patent: 5293215 (1994-03-01), Pfendler et al.
patent: 5402230 (1995-03-01), Tian et al.
Font Frank G.
Natividad Philip
Robert & Bosch GmbH
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