Self-calibrating system for detecting media movement by using capacitors as sensors

A sheet movement detection system for apparatus having a sheet feed path includes a first capacitor sensor including a first pair of parallel plate members located in opposing relation on opposite sides of the feed path plane at an upstream position and second capacitor sensor including a second pair of parallel plate members, which are correspondingly sized and spaced to said first plate members, in opposing relation on opposite sides of the feed path plane at a downstream position. The sensors are coupled in a detection circuit which applies potentials across the pairs of plate members so that output signals of the first and second sensing means are subtracted from one another. A position circuit detects the output of the detection circuit during movement of a sheet lead edge through the second sensor and provides a sheet lead-edge position signals indicating the position or velocity of the sheet lead edge.

FIELD OF THE INVENTION 
The present invention relates to systems for detecting the movement 
(position and/or velocity) of media along a use path and, more 
particularly, to a system employing capacitive sensors to provide signals 
that are accurately indicative of the movement of media lead edges vis a 
vis a feed path. 
BACKGROUND OF the INVENTION 
A wide variety of detection systems have been employed to sense conditions 
of sheet feed along a use path, e.g. the feed path of original or copy 
sheets in copier apparatus, the feed path of documents to be scanned by 
input scanner devices or the feed path of print sheets within printing 
machines. Such systems have been used to detect (i) sheet position error 
(e.g. longitudinal, lateral or skew error), (ii) double sheet feed and 
(iii) improper sheet type (e.g. transparency versus opaque sheet). 
U.S. Pat. Nos. 3,230,519; 3,591,922; 3,646,372 and 4,258,326 disclose the 
use of capacitive sensing devices to detect the presence or absence of 
dielectric objects, a variation in thickness because of a web splice and 
the existence of a double sheet feed. Those systems are not used for 
accurate movement detection. 
In many printing, copying and scanning applications, it is important that 
movement of fed sheets be very accurate. For example, in situations where 
image portions of different colors are to be placed on print or copy 
sheets during successive passes, accurate sheet movement positioning is 
necessary to assure proper register of the different color image portions. 
When data is to be printed in proper register on a preprinted form or when 
personalized printing is to be added to advertising literature, accurate 
movement of the blank form or advertising signature sheet is important. In 
systems where input scanners merge data from separate input documents, 
accurate relative location information can require accurate movement of 
the input and output sheets. 
Commonly assigned U.S. Pat. No. 5,035,415, of July 30, 1991, entitled 
"System for Detecting the Accurate Positioning of Sheets Along a Feed 
Path", filed in the names of Lee, Kriegel and Stephany describes a system 
employing a pair (s) of capacitive sensors disposed in a bridge circuit in 
a manner which allows precise position detection of a fed sheet vis a vis 
the feed path. 
SUMMARY OF THE INVENTION 
One important purpose of the present invention is to provide improved sheet 
detection systems, which employ capacitive sensors such as described in 
the above cited application, for detecting movement of a lead sheet edge 
along the sheet feed path. The system of the present invention is 
advantageous from the viewpoint of compactness. Moreover, it affords sheet 
by sheet detection signal calibration so as to avoid errors incident to 
dielectric variations between individual sheets. A further advantage of 
the present invention is its versatility in operating with different sheet 
lengths without locational adjustment of sensors. 
In one aspect the present invention constitutes an improved movement 
detection system for apparatus having a sheet feed path along which sheets 
are moved to a use location(s). The system comprises first capacitor 
sensing means including a first pair of parallel plate members located in 
opposing relation on opposite sides of the feed path plane at an upstream 
position; and second capacitor sensing means including a second pair of 
parallel plate members, which are correspondingly sized and spaced to said 
first plate members, in opposing relation on opposite sides of the feed 
path plane at a downstream position such that a fed sheet can sequentially 
pass between the first and second pairs of plate members. The sensing 
means are coupled in detection circuit means for applying potentials 
accross the pairs of plate members in a circuit configuration such that 
output signals of the first and second sensing means are substracted from 
one another. Position circuit means detects the output of the detection 
circuit means during movement of a sheet lead edge through the second 
capacitor sensing means and provides a sheet lead-edge position signals 
indicating the position of the sheet lead edge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 illustrate one embodiment of the present invention 
constructed to operate with a sheet feed device 10, which can be a 
subsystem of a copier, printer, scanner or similar apparatus. The sheet 
feed device 10 comprises a pair of rollers 11, 12 by which vacuum belts 
13, 14 are driven to transport a sheet S along a feed path P, from right 
to left as viewed in FIGS. 1 and 2. For example, roller 12 can be driven 
by a drive control system 15, such as a stepper motor and associated 
machine control circuitry 100. 
The position detection system of the FIGS. 1 and 2 embodiment comprises 
first and second sensing means 21, 22; and as more clearly seen in FIG. 4, 
each of these sensing means comprises a pair of equally sized capacitor 
plates spaced in parallel, opposing relation on opposite sides of the feed 
path &. In FIG. 4 plates comprising respective sensing means are denoted 
with sub-characters "a" and "b", e.g. plates 21a and 21b comprise sensing 
means 21. In the preferred construction for the sensing means shown in 
FIG. 4, the plates are formed of phosphor-bronze alloy, have major 
dimensions of about 1".times.7", and are spaced about 0.80 inches. 
Desirably the plates 21a and 22a are photofabricated on common dielectric 
substrates 23, and plates 21b and 22b are fabricated in the same way on 
dielectric substrate 24. To eliminate field fringing the sensing means 
preferably have a photofabricated boundary electrode portions 25 
surrounding each sensing plate portion and portions 25 are coupled to a 
source of AC potential that is synchronized in phase angle and magnitude 
to the potential applied to portions 21a, 21b, 22a, 22b. As shown, the 
portions 25 are not connected to the sensing plate portions. Also grounded 
backing electrode surface coatings 26 can be provided to effect noise 
immunity to and from the sensing plates and effect enhanced field contact. 
In general, the capacitive sensing means should be sized and spaced so 
that, when a potential is applied across the spaced plates, the 
capacitance of the unit varies in direct proportion to the amount of sheet 
material therebetween. Specifically, the capacitance of each sensor means 
will increase as sheet material moves increasingly into the gap between 
the plates. 
One preferred mode for the first and second sensing means 21, 22 to 
function can be understood by reference to FIG. 3, which shows schematic 
details of a preferred detection circuit 30. The detection circuit 30 
comprises a bridge circuit 40, a differential amplifier circuit 50 and, 
preferably, a rectifier 60 and filter circuit 70. As shown in FIG. 3, the 
circuit 41 comprises a potential source 48 and is constructed to apply 
potential to sensors 21, 22 in a capacitive bridge, which also 
incorporates capacitor 41 and balance adjust capacitor 42 as shown. The 
output lines 44, 45 from the bridge circuit are applied to the inputs of 
differential amplifier 50. 
The bridge circuit configuration allows the outputs of sensors 21, 22 to be 
substracted from one another by differential amplifier 50, whose output is 
amplified, rectified by circuit 60 and filtered by circuit 70 to produce a 
detection voltage signal Vo. In the FIGS. 1 and 2 embodiment, the signal 
Vo goes from zero to a positive maximum voltage level Vmax as the sheet 
lead edge progresses into the first sensing means, completely covering the 
first sensing means area, and then back to zero then as the lead end of 
the sheet progresses to gradually cover the second sensing means. 
Referring now to FIGS. 1 and 5, one preferred mode of operation in accord 
with the present invention can be described. As illustrated the output of 
detection circuit 30 is coupled to microcomputer machine control 100, 
which comprises microprocessor 101 with related timing control and 
interrupt interface sections 102, 103 and cooperative read only memory 
(ROM) 104 and read/write memory (RAM) 105. An output interface section 106 
of machine control 100 is coupled to drive control 15, as well as other 
control units (denoted, e.g. "Y") of the overall machine in which the FIG. 
1 transport system is utilized. The ROM 104 contains programs whereby the 
microcomputers is adapted to start-up and perform the various operations 
of the overall mchine, such as operations of the FIG. 1 subsystem, in 
coordination with other machine functions as described hereinafter. 
Thus, during each sheet feed as directed by machine control 100 in 
accordance with a program of ROM 104, another stored program in that ROM 
directs that output from detection circuit 30 be sampled at known 
intervals and stored in RAM 105. As the lead portion of a fed sheet moves 
progressively from right to left through the first sensing means 21, the 
signal Vo will increase progressively to a value Vmax (see FIG. 6), when 
the sheet lead end completely covers the area between the plates of first 
sensing means 21. The signals stored during sheet movement to cover the 
area between the plates of the first sensing means can be utilized in 
several modes, according to the present invention. 
In a first mode, the signals are processed by microcomputer 101, in 
accordance with a program in ROM 104, to determine the value Vmax and 
subsequent signals are divided by the value Vmax to calibrate the signals 
sampled during movement of the lead edge of the sheet through the second 
sensing means. More specifically, in this mode the ROM 104 is programmed 
with nominal Vo values indicative of a known sheet positions during 
movement through the second sensor means. These values can be obtained by 
sampling the values Vo at different accurately measured (e.g. with a 
micrometer) positions of a test sheet lead edge (through the first sensing 
means) and dividing those values by the Vmax signal corresponding to the 
test sheet. During actual sheet feed operations, different sheets will 
yield different position voltage values due to differences in their 
dielectric characteristics (caused e.g. by different compositions, 
different "weights" or different moisture content). This first mode of the 
present invention dynamically calibrates for each different sheet by 
dividing the values Vo during movement through the second sensing means 
and comparing the dynamic quotient signal values to the stored nominal 
quotient signals. In th is mode, the machine control can detect accurately 
the position of the sheet lead edge at each stage of movement through the 
second sensing means. Moreover, by sampling at a plurality of 
predetermined intervals the velocity of the lead edge can be determined 
and drive control 15 adjusted accordingly to introduce the lead sheet edge 
at a predetermined position at a desired time, e.g. in register with an 
image transfer drum of a copier apparatus. 
In another mode of operation, the sensors and detection circuit can be 
adjusted so that the value of the signal Vo .div. Vmax during movement of 
the lead edge through the second sensor means is precisely inversely 
proportional to the distance which the lead edge has progressed. For 
example, when the lead edge is one-fourth into the second sensor means 
gap, the signal Vo is 0.75 Vmax and when three-fourths into the gap, Vo is 
0.25 Vmax, etc. In this second mode, the value Vmax is stored for each 
sheet during movement through the first sensor, and microprocessor 101 
computes calibrated percentage Vmax values for that particular sheet, e.g. 
3/4 Vmax, 1/2 Vmax, 1/4 Vmax, for comparison to signals sampled during 
movement of the sheet lead edge through the second sensor means. 
In another mode of operation the microprocessor can simply sample the 
signal Vo through the condition Vmax, and then periodically until the 
signal Vo returns to 0 volts. At the instant of return to the Vo =0 value, 
the drive control 15 can be stopped to leave the lead sheet edge precisely 
at the end of sensing means 22, or another operation can be intiated, e.g. 
the commencement of a printing or scan operation vis a vis the sheet. 
In still another mode of operation the sensors means 21, 22 can be spaced 
further apart in the path length direction than the sheet length. In this 
mode the signal from the detection circuit 30 is sampled and stored as it 
increases from zero to Vmax. After the sheet trail end passes from between 
the first sensing means, the sheet lead end will move between the second 
sensing means and the signal, gradually increasing from zero to Vmax, is 
compared to the stored signal in analogous modes as described above. 
The invention has been described in detail with particular reference to 
certain preferred embodiments thereof, but it will be understood that 
variations and modifications can be effected within the spirit and scope 
of the invention.