Optical encoder incorporating linear light controlling member

Details Optical encoder incorporating linear light controlling member Optical encoder incorporating linear light controlling member

2000-12-21

2003-07-08

Mai, Huy (Department: 2873)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C250S231140, C250S231160

Reexamination Certificate

active

06590201

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to an optical encoder for detecting a position in movement, in rotation and so on.
2. Description of the Related Art
In an inkjet printer for example, printing is performed while moving the printing head. Therefore, it is necessary to accurately detect the position of the printing head in the movement. In such a case as this, use of an optical encoder makes possible to accurately detect the position of the moving printing head.
An example of the optical encoder is disclosed in JP-A-61-292016 (corresponding U.S. Pat. No. 4,691,101). This optical encoder comprises, as shown in
FIG. 24
, an optical unit
103
including a light emitter
101
and a light receiver
102
, and a light controlling member
105
disposed between the light emitter
101
and the light receiver
102
. The optical unit
103
and the light controlling member
105
can move relative to each other longitudinally of the light controlling member
105
. For example, if the optical member
103
is mounted on the printing head of the inkjet printer and the light controlling member
105
is fixed to a case of the inkjet printer, the optical unit
103
moves along the light controlling member
105
when the printing head moves.
The light controlling member
105
is made of a ribbon-like resin film for example, and as shown in
FIG. 25
, formed with a plurality of transparent portions
106
and nontransparent portions
107
alternating with each other. All of the transparent portions
106
have a same length longitudinally of the light controlling member
105
. All of the nontransparent portions
107
have a same length longitudinally of the light controlling member
105
. Further, the length of the transparent portion
106
longitudinally of the light controlling member
105
and the length of the nontransparent portion
107
longitudinally of the light controlling member
105
are equal to each other. In other words, in a pairs of adjacent transparent portion
106
and nontransparent portion
105
, a length L as a sum of the length of the transparent portion
106
longitudinally of the light controlling member
105
and the length of the nontransparent portion
107
longitudinally of the light controlling member
105
is constant in any pair of the transparent portion
106
and the nontransparent portion
107
.
The light receiver
102
includes, as shown in
FIG. 26
, a photodiode group
111
made of four photodiodes
111
a
-
111
d
. These four photodiodes
111
a
-
111
d
are arranged close to each other in the direction of the relative movement between the optical unit
103
and the light controlling member
105
. All of the photodiodes
111
a
-
111
d
have a same length in the direction of the arrangement, and a total of the four lengths is K. In other words, a length of the photodiode group
111
in the direction of the relative movement between the optical unit
103
and the light controlling member
105
is K. In the above arrangement, K and L are exactly equal to each other or generally equal to each other within a manufacturing error. It should be noted here that there is a slight gap between each pair of adjacent photodiodes
111
a
-
111
d
due to technical reasons of manufacture, but these gaps are not illustrated in FIG.
26
.
The photodiodes
111
a
-
111
d
have output terminals connected with input terminals of four adders
113
-
116
as shown in FIG.
27
. Specifically, the input terminals of the adder
113
are connected with the output terminals of the photodiodes
111
a
,
111
b
. The input terminals of the adder
114
are connected with the output terminals of the photodiodes
111
c
,
111
d
. The input terminals of the adder
115
are connected with the output terminals of the photodiodes
111
b
,
111
c
. The input terminals of the adder
116
are connected with the output terminals of the photodiodes
111
a
,
11
d
. The adders
113
-
116
have output terminals connected with input terminals of two comparators
118
,
119
. Specifically, the input terminals of the comparator
118
are connected with the output terminals of the adders
113
,
114
. The input terminals of the comparator
119
are connected with the output terminals of the adders
115
,
116
.
If the light controlling member
105
moves in a direction indicated by Arrow A in
FIG. 25
at a constant speed, or if the optical unit
103
moves in a direction opposite to the direction indicated by Arrow A at a constant speed, the photodiodes
111
a
-
111
d
give output signals as shown in FIG.
28
.
Therefore, outputs from the adders
113
,
114
and the comparator
118
are as shown in FIG.
29
. It should be noted here that the comparator
118
outputs a high-level signal if the output from the adder
113
is greater than the output from the adder
114
.
Further, outputs from the adders
115
,
116
and the comparator
119
are as shown in FIG.
30
. The comparator
119
outputs a high-level signal if the output from the adder
115
is greater than the output from the adder
116
.
As described, in the prior art optical encoder, the photodiodes
111
a
-
111
d
are so manufactured that the photodiode group
111
has the dimension K that is equal to the dimension L as the sum of one transparent portion
106
and one nontransparent portion
107
, thereby obtaining from the comparator
118
and the comparator
119
the output signals having a phase shift of a quarter of the period.
However, according to the prior art optical encoder, in order to make the dimension K and the dimension L as exactly the same as possible, the photodiodes
111
a
-
111
d
must be manufactured at a high accuracy, leading to a problem of increased cost of manufacture. Further, at an occasion when the dimension L is altered for improved detection accuracy, or for manufacture of a plurality of kinds of the product each having a different value in the dimension L, it is necessary to differentiate the size of the photodiode group
111
for each specific value of the dimension L in the manufacture of the optical unit
113
, leading again to the problem of increased cost of manufacture. Further, even if the comparator
118
and the comparator
119
give output signals having the phase shift Of a quarter of the period, advantage of receiving such signals can only be fully enjoyed in a special application. In a general application such as position detection of the printing head in an inkjet printer, the phase shift between the output from the comparator
118
and the output from the comparator
119
may not necessarily be a quarter of the period, but rather it is only necessary that the output from the comparator
118
and the output from the comparator
119
are comparable so as to discern the direction of the relative movement between the optical unit
103
and the light controlling member
105
.
Further, according to the above prior art optical encoder, the photodiodes
111
a
-
111
d
are arranged in a line in the direction of the relative movement between the optical unit
103
and the light controlling member
105
. With this arrangement, if the length L of the pair of transparent portion
106
and nontransparent portion
105
is small, the outputs from the photodiodes
111
a
-
111
d
are small, which leads to deterioration in S/N ratio and a problem to detect accurately.
Specifically, in order to improve detection accuracy of the optical encoder, the length L of the pair of transparent portion
106
and nontransparent portion
107
must be made small, which obviously means the length of the array of the photodiodes
111
a
-
111
d
must be small. However, due to technical reasons in manufacture, there is unavoidably a gap or a region of low sensitivity between each adjacent pair of the photodiodes
111
a
-
111
d
. For this reason, if the length of the array of the photodiodes
111
a
-
111
d
is made small, ratio of the low-sensitivity region to the high-sensitivity region increases, causing a sharp drop in the output from the photodiodes
111
a
-
111
d
. As a result, the S

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