Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-07-14
2002-06-11
Allen, Stephone B (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S23700G, C356S395000, C356S398000
Reexamination Certificate
active
06403950
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for generating a carrier frequency-modulated signal for the evaluation of n>2 photoelectric measurement signals which are generated by imaging a structured surface onto a spatial frequency filter and are phase-shifted with respect to one another.
Methods in which measurement signals are generated by imaging a structured surface onto a spatial frequency filter are known. Optical lens systems are usually used for the imaging. The structure of the surface may be regular or else stochastic. The spatial frequency filter comprises a regular grating structure in the form of an amplitude grating or a phase grating. The light fluxes passing through the spatial frequency filter are usually imaged onto a photoelectric receiver by a field lens. However, it is also known to design the structure elements of the spatial frequency filter as photoelectric receiver areas, so that measurement is effected directly in the plane of the spatial frequency filter.
In the event of a movement of the structured surface relative to the structure of the spatial frequency filter, alternating signal amplitudes are produced at the photoelectric receivers. The movement may take place parallel to the plane of the spatial frequency filter. The signal change at the photoelectric receiver is then proportional to the distance of movement or speed of movement of the imaged surface. Systems having a regular surface structure which operate according to this system are known as incremental transmitters for linear or angular movements. Systems having a stochastic surface structure are also known as correlation optical distance and speed measurement systems.
However, the movement may also take place perpendicularly to the plane of the spatial frequency filter, in which the case the signal change depends on the quality of the imaging on the spatial frequency filter. In the event of sharp imaging, the spatial frequency of the imaged surface correlates optimally with the spatial frequency of the filter.
One problem in optically modulated measurement methods generally consists in high continuous light components in relation to the modulated light component. It is known, for the purpose of suppressing continuous light, to generate two signals which are phase-shifted by 180° with respect to one another, and to superpose said signals on one another, so that the signal components corresponding to the continuous light cancel one another out.
In the case of an amplitude grating as spatial frequency filter, it is possible to provide different filter areas in which the grating structures are offset in phase by 180° with respect to one another. In the case of phase gratings, it is possible to choose e.g. prism-type grating structures in which the light fluxes defracted at the two prism flanks are geometrically separated and phase-shifted by 180° with respect to one another. In the case of photosensitive spatial frequency filter structures, alternately successive receiver strips can be connected together and the signals can be electronically phase-shifted by 180° with respect to one another.
A further problem in such measurement systems is that of identifying the direction for the movement of the structured surface relative to the structures of the spatial frequency filter. For this purpose, it is known e.g. to generate two measurement signals which are phase-shifted by 90° with respect to one another and define a rotating electric field.
In the case of amplitude gratings as spatial frequency filters, it is again possible to provide, to that end, different filter areas in which the grating structures are offset in phase by 90°. In the case of phase gratings, it is possible to provide e.g. prism-type grating structures having more than two flanks inclined differently with respect to one another. The different light fluxes are assigned respectively separate photoelectric receivers. The light fluxes are modulated when the structured surface moves perpendicularly to the structure of the spatial frequency filter. The direction of movement can be derived from the comparison of the modulation phases.
In the case of a chessboard-like or pyramidal structure of the spatial frequency filter, modulated light fluxes can be generated for two coordinate directions of the movement and be measured by suitably arranged photoelectric receivers.
The light fluxes imaged onto said photoelectric receivers represent an image of the pupil of the system which images the structured surface. It is known to assign e.g. two separate receivers to each pupil image, with the result that said receivers each receive light fluxes which have passed through different pupil areas. By suitably evaluating the spatial frequency-filtered signals assigned to the different pupil areas, it is possible to determine the position of the focal plane of the image of the surface relative to the plane of the spatial frequency filter. The distance between the surface and the imaging system can then also be derived from this.
All of the systems mentioned previously have the disadvantage that the signal modulation becomes worse and worse in the event of slow movements and a measurement signal can no longer be obtained at a standstill. In order to solve this problem, it is known to move the spatial frequency filter at a known speed in one direction or periodically in oscillations, and hence to modulate the actual measurement signal. This method, which is known as carrier frequency modulation, allows e.g. signal evaluation by phase-sensitive rectification with the carrier frequency as reference.
This method is particularly advantageous in the evaluation of the signals for the setting of the focusing of the imaged surface onto the spatial frequency filter. The carrier frequency-modulated measurement signals of the photoelectric receivers assigned to the different pupil areas have the same phase relative to the carrier frequency only in the event of sharp imaging, with the result that either the correspondence of the carrier frequency-modulated measurement signals or the correspondence of the phases gives a very sensitive setting criterion.
In the case of a spatial frequency filter arranged on a circular disk, the movement may be effected by rotation. Owing to the grating constant, which becomes coarser in the radially outward direction, however, different carrier frequency modulations are produced in the measurement field and make the signal evaluation more difficult. In general, a linear movement in one direction can be realized only over a very limited length. If the spatial frequency filter is arranged on the curved surface of a rotating drum, the spatial frequency filter is curved, with the result that sharp imaging is not possible over the entire filter surface.
Therefore, an oscillatory movement is preferred in practice. For this purpose, the spatial frequency filter may, for example, be suspended from piezoelectric bending devices or be moved back and forth by a suitable linear drive (DE 23 30 940 C2). The disadvantage of this arrangement resides in the signal evaluation, since, depending on the position of the structures on the imaged surface relative to the displacement of oscillation of the spatial frequency filter given an amplitude of oscillation of ± half a grating constant, the carrier frequency modulation is effected both with the fundamental and with the first harmonic of the carrier frequency. Therefore, the phase-sensitive rectification must be carried out with regard to the carrier frequency and twice the frequency with respect thereto (DE 27 36 583 C2). A further difficulty arises in measurement arrangements which measure simultaneously in two coordinate directions. In this case, the oscillatory movement has to be performed at 45° with respect to the coordinate axis in order to obtain the same carrier frequency modulation for the two measurement directions.
A summarizing account can be found in the paper “New developments in optical grating technology for machine vision and industrial sensors”, by R.
Schaefer Roland
Schneider Eckart
Stamer Jan
Allen Stephone B
Corrsys-Datron
Foley & Lardner
LandOfFree
Method of producing a carrier frequency modulated signal does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of producing a carrier frequency modulated signal, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of producing a carrier frequency modulated signal will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2939971