Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-04-26
2003-12-09
Mai, Huy (Department: 2873)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C356S482000
Reexamination Certificate
active
06660997
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to optical encoders. Specifically, the invention relates to a Moiré type optical encoder for use in a control system.
BACKGROUND OF THE INVENTION
Incremental optical encoders are well known devices used to track the relative position and movement of an object along a particular track. Typical optical encoders include a light source that emits a light beam, a modulation means (usually a reticle or grating) for modulating the light beam as the object moves along the track, and a detector assembly for receiving the modulated light beams and converting the optical signal into an electrical signal. Multiple detectors may be used obtain two electrical signals that have a constant phase relationship. Together, the two electrical signals indicate both the change in location and the direction of movement of the object.
A specific type of optical encoder, known as a Moiré type encoder, uses two periodic gratings or reticles to modulate the incoming light signal. A typical Moiré encoder construction is depicted in FIG.
1
. Referring to
FIG. 1
, the light source
21
illuminates the scanning reticle
20
, generating a periodic radiation pattern. Light that permeates the scanning reticle
20
impinges on the object reticle
30
and light sensors
23
and
24
detect light transmitted through the object reticle
30
. As the scanning reticle
20
and the object reticle
30
are translated with respect to one another along the axis indicated by arrows
31
, the intensity pattern (not shown) at the surface of the sensors
23
and
24
varies periodically. This periodic variation of intensity, known as a Moiré pattern, is dependent on the spatial periodicity of the object reticle
30
and the scanning reticle
20
. Conventionally, sensors
23
and
24
are positioned or oriented with respect to the object reticle
30
and to one another, such that the optical signals that they receive have a constant spatial phase difference of ¼ of the Moiré period. Since the optical signals received by the sensors
23
and
24
are phase shifted, the electrical signals (not shown) produced by the two sensors
23
and
24
are also phase-shifted from one another by ¼ period.
The functionality of a Moiré type encoder is shown in FIG.
2
.
FIG. 2-A
shows a magnified view of a portion of an object reticle
30
. The object reticle
30
has a periodic pattern of apertures
32
and opaque portions
33
extending in the y direction. The period (or pitch) of the object reticle
30
is labelled L
y
. For ease of reference, the period L
y
of a reticle
30
is referred to throughout this application as the “pitch”. Any one individual pitch L
y
including both the aperture
32
and the opaque area
33
is referred to in this application as a “cell” of the reticle. The quantity l
y
/L
y
represents the fraction of a cell that is occupied by the aperture and is referred to throughout this application as the “aperture duty cycle”.
As the object reticle
30
and the scanning reticle
20
(see
FIG. 1
) are scanned in the y direction relative to one another, an optical signal is received at each of the two sensors
23
and
24
. The signals A and B of
FIG. 2-B
are idealized representations of the signals produced on sensors
23
and
24
respectively. The plot of signals A and B depicted in
FIG. 2-B
shows the variation of light intensity measured on the sensors
23
and
24
as a function of the relative movement between the object reticle
30
and the scanning reticle
20
in the y direction. It will be appreciated from the plot in
FIG. 2-B
, that signals A and B are phase separated by ¼ period.
Assuming an intensity of I
o
is measured on signal B, the relative position of the object reticle
30
could be y
o
or y
o
′. As a result, typical Moiré encoders measure a second signal A to distinguish between the two possible positions y
o
and y
o
′. For example, if signal B is measured at I
o
and signal A is measured at I
1
, then the system knows that the correct position is yo rather than y
o
′. In most circumstances, a Moiré system can determine the direction of relative motion by measuring either one of signals A or B. For example, if y
o
is the start position, then movement in one direction will cause an increase in the intensity of signal B and movement in the other direction will cause a decrease in the intensity of signal B. Hence, if an increase or decrease in the intensity of signal B is detected, then the direction of motion is known. In some circumstances, however, signal B will be at or near a zero derivative point (i.e. at a maximum or minimum of the signals, such as y
1
, which is a minimum of signal B). In such a situation, both directions of movement will produce similarly increasing intensity profiles for signal B. Signal B is said to be “indeterminate” as to direction; consequently, signal A must be used to determine the direction of motion. With two signals (A and B) differing in phase by a known phase difference, such as ¼ of the Moiré period, at least one signal will always be determinative of the direction of motion.
The principal drawback with incremental encoders, such as the one described above, is that they are only useful for determining relative position and movement. That is, they are only able to determine the position and movement of an object relative to a fixed or predetermined reference position. Often, the reference position used is the start position of the device when the encoder is powered up. Other techniques for obtaining a reference position include using an index signal that alerts the encoder system when the object is at a particular position along its track. This requires that, upon “wake-up”, the encoder searches its track for the index signal, before it is able to locate itself. The dependence of incremental encoders on a reference position is an obvious drawback in some applications, where the start position may not be suitable for a reference, where the provision of an index signal is inconvenient or impossible, or where the time required to locate an index signal is not available.
Some optical position encoders, which do not rely on a reference position are known in the art and are referred to as “absolute position” encoders. A typical implementation for an absolute position encoder is depicted in FIG.
3
. The encoder includes a light source
11
, such as an LED, for emitting light L
a
and a collimating lens
12
to produce collimated light L
b
. A first scale
13
is a specialized grating with a number of grating tracks (t
1
, t
2
, . . . t
n
), each track including apertures
13
A and opaque sections
13
B. For each track (t
1
, t
2
, . . . t
n
), the apertures
13
A and the opaque sections
13
B alternate periodically. However, although the aperture duty cycle is constant for each track (t
1
, t
2
, . . . t
n
), the pitch of each track (t
1
, t
2
, . . . t
n
) is different. A second scale
14
is provided with apertures (
14
A
1
,
14
A
2
. . .
14
A
n
) arranged behind the respective grating tracks (t
1
, t
2
, . . . t
n
). The arrangement of the second scale
14
is such that light transmitted through the apertures
13
A of the first scale
13
is able to pass through the apertures (
14
A
1
,
14
A
2
. . .
14
A
n
). Photodetectors (
15
-
1
,
15
-
2
, . . .
15
-n) are positioned strategically with respect to the apertures (
14
A
1
,
14
A
2
. . .
14
A
n
), so as to convert the light beams passing through the apertures (
14
A
1
,
14
A
2
. . .
14
A
n
) into electrical signals.
Typically, the prior art absolute position encoders use a first scale
13
, which is provided with binary “Gray codes” as shown in
FIG. 4
, wherein grating pitches (P
1
, P
2
, . . . P
n
) between adjacent grating tracks (t
1
, t
2
, . . . t
n
) have a ratio of 1:2. Consequently, the intensities of the light beams (L
e1
, L
e2
, . . . L
en
) received by the respective photodetectors (
15
-
1
,
15
-
2
, . . .
15
-n) change periodically when the first scale
13
moves in a longitudinal dir
Karasyuk Valentin
Laberge Michel
Steiner Thomas W.
Creo SRL
Mai Huy
Oyen Wiggs Green & Mutala
LandOfFree
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