Color mark detector with pulsed source and synchronous demodulation
First Claim
1. A mark detector for a mark on a surface having a contrasting background, comprising a framework, said framework having an aperture through one wall thereof, a light source mounted in said framework disposed to direct a light beam through said aperture, a light sensor mounted in said framework disposed to receive light reflected from the surface when the surface is positioned before said aperture, said light source providing a sensor output signal responsive to received light, means for driving said light source at a pulse frequency, thereby producing light source pulses, a gate for sampling said sensor output synchronously with said light source pulses during a gating period and providing a gated output, whereby noise occurring between said light source pulses is barred from said gated output, an integrator coupled to receive said gated output and providing an integral output, a comparator coupled to receive said integral output and said gated output at separate input terminals, whereby a predetermined comparator output state is produced during said gating period when a predetermined difference in levels between said integral and gated outputs occurs, and means for conditioning said comparator output coupled to said predetermined comparator output state and providing a mark output when a surface color passes before said aperture which contrasts with the background.
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Abstract
A detector for marks having a color which contrasts with the mark background employs a solid state light source projecting a light beam having a plurality of wave length components corresponding to a plurality of colored light wave lengths. The light beam may approach the appearance of white light. A pulse modulated detector or sensor is used for receiving light reflected from colored surfaces having contrasting colored marks thereon, wherein high ambient light level rejection is obtained and high signal to noise ratio is provided in a sensor output. The sensor output is gated to eliminate between pulse noise, and the gated output is integrated to obtain a reference signal to which the gated pulse is continuously compared. An increase or decrease in signal is used to provide a light or dark mark indication respectively as a result of the comparison. Feedback from the output suppresses signal instability which may exist at the edges of the detection threshold. Since the comparator reference is self-adjusting to the background condition on the surface carrying the marks, the detection capability is dependent upon detector sweep speed relative to the surface.
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Citations
13 Claims
- 1. A mark detector for a mark on a surface having a contrasting background, comprising a framework, said framework having an aperture through one wall thereof, a light source mounted in said framework disposed to direct a light beam through said aperture, a light sensor mounted in said framework disposed to receive light reflected from the surface when the surface is positioned before said aperture, said light source providing a sensor output signal responsive to received light, means for driving said light source at a pulse frequency, thereby producing light source pulses, a gate for sampling said sensor output synchronously with said light source pulses during a gating period and providing a gated output, whereby noise occurring between said light source pulses is barred from said gated output, an integrator coupled to receive said gated output and providing an integral output, a comparator coupled to receive said integral output and said gated output at separate input terminals, whereby a predetermined comparator output state is produced during said gating period when a predetermined difference in levels between said integral and gated outputs occurs, and means for conditioning said comparator output coupled to said predetermined comparator output state and providing a mark output when a surface color passes before said aperture which contrasts with the background.
- 9. A mark detector for a colored mark on a contrasting colored background, comprising a plurality of semiconductor light emitting devices energized by an electrical signal and providing a substantially unidirectional light beam, each of said plurality of light emitting devices providing light having a different wavelength, whereby any color mark or background reflects some of the light in said light beam, a light sensor mounted to receive light reflected from a surface intercepting said light beam, said light sensor providing a sensor signal related to received light intensity, means for energizing said plurality of semiconductor light emitting devices at a pulse frequency, a charge storing circuit connected to receive said sensor signal and providing a stored signal, said charge storing circuit having a long time constant relative to said pulse frequency period, a comparator coupled to said sensor signal at one input terminal and to said stored signal at a reference input terminal, whereby the reference level for said comparator is automatically adjusted in accordance with the level of said sensor signal, said comparator providing a first comparator output when said sensor signal and stored signal are at a predetermined difference in levels, means coupled to said first comparator output for providing a dark mark output in the absence of said comparator output, an attenuator coupled to said sensor signal and providing an attenuated sensor signal, a second comparator connected to receive said attenuated sensor signal at one input terminal and coupled to said stored signal at a reference input terminal, whereby the reference level for said second comparator is automatically adjusted in accordance with the level of said sensor signal, said second comparator providing a second comparator output when said attenuated sensor signal and stored signal are at a predetermined difference in levels, means coupled to said second comparator output and responsive thereto for providing a light mark output, whereby dark and light marks on the contrasting background on the surface intercepting said light beam each provide a separate output.
Specification