Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-04-07
2002-05-28
Kim, Robert H. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S231160, C250S231180
Reexamination Certificate
active
06396052
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ink jet printers, and, more particularly, to a paper position sensing system for an ink jet printer.
2. Description of the Related Art
With printers which use a columnar array of print elements or nozzles, a typical mode of operation requires that the column of nozzles be swept horizontally across the paper while the nozzles are selectively fired according to points in a bitmap which represent printed pixels. At the end of such a swath, the paper is indexed vertically by the height of the printhead and the printhead is again swiped across the paper. With this process, there are inherent print defects introduced by such things as paper feed inaccuracies and nozzle-to-nozzle variations in drop size or placement.
Studies of inkjet printer print quality indicate that the paper positioning system must be able to control the location of the paper within 4 micrometers to eliminate paper positioning artifacts in single pass printing. A control system capable of this resolution requires a position sensor with resolution in the sub-micrometer range. Conventional digital encoders, the usual sensor for this type of control system, are not capable of achieving resolution this fine.
An optical encoder
10
(
FIG. 1
) includes a light source
22
, a code wheel
24
, a light detector
26
, an optical mask
28
and a rotating shaft
30
. Mask
28
discerns the spatial location of the shadows produced by code wheel
24
. Another method of discernment is to carefully control the size and location of light detectors
26
relative to the lines and windows of code wheel
24
.
Code wheel
24
is mostly translucent with a series of opaque radial lines
32
near the periphery. Code wheel
24
is attached to rotating shaft
30
. The stationary mask
28
has a matching set of opaque lines
34
. Light from source
22
passes through the translucent portions of code wheel
24
and then through the translucent portions of mask
28
, eventually falling upon detector
26
. The amount of light falling upon detector
26
depends upon the alignment of the translucent portions of code wheel
24
to the translucent portions of mask
28
. When the translucent portions of code wheel
24
align with opaque portions
34
of mask
28
, light is blocked from detector
26
. When the translucent portions of code wheel
24
align with the translucent portions of mask
28
, light passes through to detector
26
. The amount of light falling on detector
26
is a direct indication of the location of code wheel
24
relative to mask
28
. The output of light detector
26
is a periodic function of the rotational angle of code wheel
24
and might look like the waveform shown in FIG.
2
. It is to be understood that the term “rotational angle” may also be referred to herein as “angular displacement”, “rotational position”, or “angular position”.
Position changes can be coarsely tracked by counting the number of cycles traversed. Finer resolution can be obtained by observing the level of the output of detector
26
within each cycle. If the output of detector
26
were an ideal triangle wave, the entire fine resolution portion of the position measurement could be accomplished with one encoder channel. With current technology, this is not a realistic expectation. There is also some ambiguity with just one channel since two different positions within each cycle produce the same output magnitude. This ambiguity is overcome by a two channel encoder
36
(
FIG. 3
) including a second mask
29
and a second light detector
40
which are aligned with respect to the first mask
28
and light detector
26
so as to produce a signal that is 90 electrical degrees out of phase with the first signal shown in FIG.
4
. The phase relationship of these two signals also helps determine direction of motion. Optionally, a second light source
38
in alignment with the second mask
29
and second light detector
40
may be included.
A two channel encoder is useful in tracking relative position changes of a rotating shaft. In the case where the absolute position of the shaft is required, an additional reference mark is needed. This is commonly accomplished by adding a third channel called the index channel with an associated light source
43
and index light detector
42
(FIG.
5
). A single mark
44
detectable by optical sensor
42
is added to code wheel
24
. A pulse occurs on the index channel once per revolution of code wheel
24
as index mark
44
passes sensor
42
thus indicating the absolute position of shaft
30
.
Historically, most optical encoders have provided digital outputs. This is accomplished by “squaring up” the light detector outputs as shown in FIG.
6
. Not “squaring up” the sensor signals, but instead processing them while they are still in their analog form can produce higher resolution.
In summary, given an optical encoder with two quadrature analog outputs and an index signal, absolute position is determined in the following manner. First, index mark
44
is found. The encoder is advanced until a pulse is seen on the index channel. Upon seeing this pulse, a count is started to keep track of the number of cycles of either channel A or channel B that have been traversed. In between discrete cycle counts, fine position resolution is achieved by examining the strength of the signals on both channels A and B.
What is needed in the art is a method and apparatus for converting the outputs from an analog position encoder into high precision, digital position data.
SUMMARY OF THE INVENTION
The present invention provides a high precision analog encoder system which uses the same basic optical sensor employed by other digital encoders but achieves hundreds of times the resolution.
The invention comprises, in one form thereof, a method of determining a feed position of a print medium in an imaging apparatus. A feed roll with a central feed shaft carries the print medium such that a rotational position of the feed shaft has a predetermined relationship with the feed position of the print medium. An encoder device, connected to the feed shaft, produces a first periodic signal and a second periodic signal approximately 90 degrees out of phase with the first. Each periodic signal is a function of the rotational position of the feed shaft. Each period of each signal corresponds to a predetermined rotational distance of the feed shaft. A modified first signal is created. The slope of the modified first signal has the same sign at each rotational position of the feed shaft. The magnitude of the slope of the modified first signal is equal to the magnitude of the slope of the unmodified first signal at each rotational position of the feed shaft. A modified second signal is created. The slope of the modified second signal has the same sign at each rotational position of the feed shaft. The magnitude of the slope of the modified second signal is equal to the magnitude of the slope of the unmodified second signal at each rotational position of the feed shaft. The modified first signal is added to the modified second signal to thereby create a summation signal. The summation signal has a plurality of local minimum values, a plurality of local maximum values, and a plurality of substantially linear segments. Each linear segment interconnects a corresponding local minimum value with an adjacent local maximum value. A periodic modified summation signal is created by adding a corresponding constant value to each linear segment to thereby create a plurality of shifted linear segments such that each shifted linear segment extends between a substantially equal minimum value and a substantially equal maximum value. Each shifted linear segment represents one cycle of the periodic modified summation signal. A number of completed cycles of the modified summation signal are counted. A value of the modified summation signal at a selected point in time is determined. The feed position of the print medium is calculated based upon the number of cycles counted and the determ
Barry Raymond Jay
Dutton Todd Alan
Rice Steven Andrew
Kim Robert H.
Lexmark International Inc.
Taylor & Aust P.C.
Thomas Courtney
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