Registers – Coded record sensors – Particular sensor structure
Patent
1995-11-29
1997-10-07
Hajec, Donald T.
Registers
Coded record sensors
Particular sensor structure
235454, G06K 710
Patent
active
056751362
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to an optoelectronic device for recognizing a mark having a defined contrast pattern wherein the device includes a transmitting element for emitting a beam of light that is passed over the mark and a receiving element for receiving a received beam of light reflected from the mark and producing an analog received signal which exhibits an amplitude modulation dependent on the contrast pattern of the mark. The invention further relates to a method of reducing signal distortion for an optoelectronic device of the above type.
A device of this type is known from EP 0 433 593 A2. The device is embodied as a bar code reader for scanning bar code symbols.
The bar code symbols comprise a succession of light and dark line elements of predetermined width. The bar code symbols are scanned by the device by means of a transmitted light beam, preferably a laser beam. The transmitted light beam has a mean diameter corresponding to its spatial distribution of intensity perpendicular to the direction of propagation. In laser beams, the spatial distribution of intensity ideally corresponds to a Gaussian distribution.
The diameter of the transmitted light beam varies with the distance from the device, depending on the transmitting lens preceding the transmitting element. In the focal plane of the transmitted light beam, the diameter of the transmitted light beam is typically considerably smaller than the width of the line elements. Consequently, the amplitude modulation of the received signal is virtually identical to the widths of the line elements of the bar code symbol, so that the symbol can be reliably recognized by the device.
As the distance between the bar code symbol and the focal plane of the transmitted light beam increases, the diameter of the transmitted light beam rapidly increases. As soon as the diameter of the transmitted light beam is of the same order of magnitude as the widths of the line elements of the bar code symbols, the modulation of the received signal is influenced by the width of the transmitted light beam such that reliable detection of the bar code symbol is impeded or is no longer possible.
As described in EP 0 433 593 A2, as the diameter of the transmitted light beam increases, the high-frequency components of the modulated received signal, which derive from the narrow line elements of the bar code symbol, are increasingly suppressed.
If the amplitudes of the high-frequency components of the received signal are too strongly suppressed in proportion to the amplitudes of the low-frequency components of the received signal, the bar code symbol can no longer be read by the device.
To compensate for this effect, the device of EP 0 433 593 A2 is provided with an analog filter, with which the analog received signal is filtered such that the high-frequency components are amplified to a greater degree than the low-frequency components.
Four frequencies, f.sub.0, f.sub.1, f.sub.2 and f.sub.3, are defined for the purpose of determining the transmission characteristic of the filter.
In each of the regions between two adjacent frequencies, one specific amplification factor is chosen, and the amplification factors become greater as the frequency increases. For a certain diameter of the transmitted light beam, the transmission characteristic of the filter can be permanently set. Alternatively, the transmission function of the filter can be altered by shifting the frequencies f.sub.0, f.sub.1, f.sub.2 and f.sub.3.
This shift is suitably done as a function of the frequency that corresponds to the narrow line elements of the bar code symbol.
One substantial disadvantage of this device resides in the coarse graduation of the frequency spectrum of the received signal. As a result, the influence of the transmitted light beam diameter on the modulation of the received signal can be detected only incompletely. Hence the received signal can only be optimized to a limited extent by means of the analog filter.
Another disadvantage of this device is that the transmission
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Keinath Armin
Worner Jorg
Hajec Donald T.
Le Thien Minh
Leuze electronic GmbH & Co.
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