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Browse Prior Art Database

Look-Across Detector System for Height And Jam Sensing

IP.com Disclosure Number: IPCOM000120132D
Original Publication Date: 1991-Mar-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 4 page(s) / 163K

Publishing Venue

IBM

Related People

Arnold, RW: AUTHOR [+2]

Abstract

In a machine which requires the sensing of documents across an open area to control a servo table document height mechanism and to determine whether a jam exists, it is typical to have problems with ambient light flooding the path area. A circuit is presented that combines the following major attributes: (a) rejects very high ratios of ambient-to-signal light, (b) no light path shielding, (c) LED life comparable to machine life, (d) reliability of sensing, (e) detect narrow, fast moving documents, and (f) no adjustments with (g) low initial cost and (h) low field service cost. The photo-transistor and light-emitting diode are referred to as the PTX and the LED; they are typically infrared light-sensitive (no real help with sunlight.)

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This is the abbreviated version, containing approximately 47% of the total text.

Look-Across Detector System for Height And Jam Sensing

      In a machine which requires the sensing of documents
across an open area to control a servo table document height
mechanism and to determine whether a jam exists, it is typical to
have problems with ambient light flooding the path area.  A circuit
is presented that combines the following major attributes:  (a)
rejects very high ratios of ambient-to-signal light, (b) no light
path shielding, (c) LED life comparable to machine life, (d)
reliability of sensing, (e) detect narrow, fast moving documents, and
(f) no adjustments with (g) low initial cost and (h) low field
service cost.  The photo-transistor and light-emitting diode are
referred to as the PTX and the LED; they are typically infrared
light-sensitive (no real help with sunlight.)

      This total circuit provides two identical light path sensing
circuits for stack height control and detection for stack jams.  Fig.
1 shows the block diagram of the common and one of two sensing
circuits.  The second circuit (not shown) consists of a set of blocks
identical to 9 through 28.  The common circuitry consists of voltage
reference VR1 4 and clocking, blocks 5 through 7.  Period 5 and
Strobe 6 form an oscillator at two times the LED pulsing frequency.
The Strobe 6 single-shot, less than one-microsecond duration, tests
the 'light' detect status of one circuit then flips Gate 1 to its
alternate phase.  Gate 1 7 is ON to drive and test circuit 1 and OFF
to drive and test circuit 2.  The document stack 1 may be totally
dark or flooded by sunlight 2 and/or fluorescent light 3.

      The document stack 1 is sensed across the light path 8 by LED 9
and PTX 10.  The high gain, trans-impedance, operational amplifier 12
makes a virtual signal ground, a current node at 11 by keeping its
voltage equal to VR1 4. VR1 4 is about 1.5 volts for the PTX
collector.  The low node impedance as a PTX load increases its
bandwidth so a pulse frequency of three khz can be used.  This allows
the full gain-bandwidth of the PTX to be realized and a simple filter
to reject the harmonics of a fluorescent lighting system.

      All PTX currents are converted to voltage at the output of
amplifier 12; a typical noise filter 13 is across amplifier 12.  Sun
and fluorescent ambient frequencies are less than the cut-off
frequency of filter 14.  Their voltages are passed around a closed
loop to node 11 via filter 14, floating voltage reference FVR2 15,
voltage-to-current amplifier 16 and ambient feedback resistor RFB 17,
typically 50 ohms.  The loop creates a sink for ambient currents that
is 99.9% effective for sunlight, somewhat less for fluorescent light.
Each milliamp of such current raises the output of amplifier 12 by
only 0.05 volt, the voltage needed across RFB 17 to sink that
current. This loop sink current prevents PTX and circuit saturation;
it is limited only by the current capability of the amplifier 16. The
circuit functions...