Note: Descriptions are shown in the official language in which they were submitted.
CA 02338637 2001-O1-24
WO 00/06973 PCT/AU99/00590
1
ANGLE ENCODER
Field of invention
This invention relates to angular position sensors, particularly the sensing
of the absolute
rotational angle of a rotatable body without requiring counting from a
reference mark.
Background
Conventionally, angular position sensors have been used for sensing the
rotational angle
of a rotatable body. These conventionally consists of a detector unit and a
graduated
scale of material with contrasting bars formed of alternatively transparent
and opaque or
reflecting bars, the displacement of which is detected by the detector unit,
comprising
photoemitting, photodetecting and optical means.
The scale is illuminated by the photoemitting means, being a source of electro-
magnetic
radiation (EMR), typically UV, visible or IR light, that generates patterns on
one or more
arrays of photodetectors sensitive to the EMR. Such arrays include CCD
devices, VLSI
vision chips, one and two dimensional photodetector arrays and lateral effect
photadiodes (commonly referred to as PSD's or position sensitive devices). The
output
of the one or more arrays is processed to produce a measure of the angular
position of
the rotatable body. The scale can be arranged axially or radially about the
axis of
rotation of the body, and is of such a nature that allows a continuous output
of the arrays
regardless of the angular position of the body, as the limited array
dimensions may not
allow the complete circumference or radial face to be viewed by the arrays at
any instant
in time.
Such sensors commonly provide a signal based on the incremental angular
position of
the scale, and absolute angular position is determined by counting from a
known
reference position. The accuracy of incremental sensors is often substantially
improved
by the use of well known techniques such as quadrature interpolation. Such
quadrature
methods require a non-varying bar angular spacing.
CA 02338637 2001-O1-24
WO 00/06973 PCT/AU99/00590
2
Alternatively, the sensor may provide a signal based on absolute position by
the use of
bar codes applied to the scale. These bar codes generally do not have constant
angular
bar spacing as each set of bar codes are unique for each angular position to
be sensed,
and such absolute position sensors generally do not provide the position
measurement
accuracy provided by incremental sensors as they cannot use quadrature
interpolation
techniques.
However, if an absolute position sensor is required having high accuracy, two
scales are
required. The first measures coarse absolute position by interrogation of a
bar code, and
the second provides a fine relative position by quadrature interpolation of a
regular bar
pattern.
The prior art which provides a high accuracy absolute position measurement and
which
is most closely related to that of the present invention is described in US
Patent
5,235,181 (Durana et. al.) This describes a sensor composed of 2 scales, a
pseudo-
random bar code scale for coarse absolute position and a regularly spaced
scale for fine
position.
The position sensor described in US Patent 5,235,181 has several inherent
drawbacks.
The use of two scales necessitates the use of multiple photodetector arrays
which is of
increased cost compared to a single array. Also, the scales and the arrays
need to be
positioned relative to each other very accurately which also increases cost
and limits the
maximum accuracy of the sensor. In addition, inevitable changes in mechanical
deflection and assembly clearances in service cause uncertainty of the
relative position
of the two arrays which further limits the maximum accuracy possible.
The essence of the present invention resides in the provision of both coarse
resolution
absolute position detection and fine resolution incremental position sensing
with a single
scale which provides all the necessary information. Preferably this is
achieved by the
use of a bar code which has a constant bar pitch and a varying bar width or,
alternatively,
special forms of bar codes with varying bar pitch. Thus, the bar code provides
the binary
information necessary for absolute position sensing and also provides a
regular bar
CA 02338637 2001-O1-24
WO 00/06973 PCT/AU99/00590
3
pattern enabling fine resolution interpolation of position. In addition, the
sensor
preferably relies on reflective principles, where the photoemitting means and
photodetecting means are located on the same side of the rotating body, and
the scale
comprises regions of high and low reflectivity.
There are several advantages of such a design compared to that described in US
Patent
5,235,181. Firstly, as only a single scale is used only one photodetector
array is
necessary, reducing cost. Secondly, as both scales are combined, inaccuracy
due to
relative scale misalignment is eliminated providing better measurement
accuracy.
Thirdly, the combination of the two scales makes the sensor less sensitive to
mechanical
distortion, tolerances ar bearing clearances, as the variation in the relative
position of two
scales and arrays described in the prior art is eliminated. Fourthly, the use
of a reflective
scale allows simpler and more compact construction as it allows the
photoemitting and
photodetecting means to be packaged in the same assembly with further savings
in
space and cost in the sensor. Fifthly, another advantage with the use of
reflective scales
compared to transmissive scales is that the EMR is reflected from the surface
of the
scale and is not affected by edge scattering as is the case with apertures, or
other
problems from internal reflection, diffraction or degradation over time as is
the case with
transparent materials where the EMR has to travel through the medium in the
transmissive regions. Such effects would otherwise limit the maximum
resolution of the
sensor.Finaily, the combined scale is less complex than two separate scales
hence can
be produced quicker and at lower cost, especially if applied by direct writing
techniques
such as laser marking.
Summary of Invention
The present invention consists in an angular position sensor comprising at
least one
body at least partially surrounded by a housing, the body rotatable about an
axis of
rotation fixed with respect to the housing, the body having a grating element
attached
thereto or integral therewith, the grating element comprising a surface of
revolution about
the axis of rotation, the surface comprising a pseudo-random distribution of
regions of
high and low EMR reflectivity arranged in the form of an endless succession of
individual
CA 02338637 2001-O1-24
WO 00/06973 PCT/AU99/00590
4
binary bar codes, the sensor also comprising at least one EMR source and at
least one
array of EMR sensitive detectors, the source irradiating the surface and the
array
receiving incident EMR reflected from the surface, the source and the array
fixed with
respect to the housing, a pattern thereby produced by incident EMR on the
array
resulting from the alternating regions of low and high reflectivity on the
surface of the
grating element, the pattern on the array processed by a processor to derive
the absolute
angular position of the regions with respect to the housing, and hence provide
a measure
of the absolute angular position of the rotatable body with respect to the
housing.
In one embodiment the at least one body comprises two rotatable bodies each of
which
has a respective grating element, the two bodies connected by a member of
predetermined torsional stiffness, and at the at least one array of EMR
sensitive
detectors receiving the incident EMR reflected from the surfaces of the
grating elements,
the pattern or patterns processed to derive the absolute angular position of
the regions
on the surfaces of the grating elements with respect to the housing, and the
difference
between the angular positions further processed to derive the relative angular
displacement of the grating elements, and hence provide a measure of the
torque
transmitted by the member. The at least one array of EMR sensitive detectors
may be
two arrays of EMR sensitive detectors, each of which is associated with a
respective
grating element. The at least one EMR source may be two EMR sources, each of
which
is associated with a respective grating element.
It is preferred that the surface of revolution may at least be partially
cylindrical or partially
conical.
It is preferred that either the regions of high EMR reflectivity or the
regions of low EMR
reflectivity comprise bars having a constant centreline pitch and varying
thickness. It is
preferred that the varying thickness bars comprise bars of at least two
discrete
thicknesses. It is preferred that the bars have only two thicknesses ie. wide
bars and
narrow bars.
Alternatively, it is preferred that either the regions of high EMR
reflectivity or the regions
CA 02338637 2001-O1-24
WO 00/06973 PCT/AU99/00590
of low EMR reflectivity comprise bars with varying centreline pitch and the
pitches are an
integer multiple of a fundamental pitch. It is preferred that the bars of
varying centreline
pitch are of constant thickness. Alternatively the bars may be of varying
thickness. It is
preferred that the varying thickness bars comprise bars of at least two
discrete
thicknesses. It is preferred that the bars have only two thicknesses ie. wide
bars and
narrow bars.
In some embodiments the surface of revolution has a plurality of castellations
protruding
radially therefrom. It is preferred that the regions of high reflectivity
correspond to areas
of maximum protrusion of the castellations, and the regions of low
reflectivity are
angularly aligned with the areas of lesser protrusion between the
castellations.
It is further preferred that the areas of maximum protrusion are smoothly
machined,
moulded or sintered, or surface treated with paint or material deposition to
impart high
reflectivity, and the discontinuous gap areas or areas of lesser protrusion
are machined,
moulded or sintered, or surface treated with paint or material deposition to
impart low
reflectivity.
It is preferred that the regions of high reflectivity are metallised, shiny or
light coloured,
and the regions of low reflectivity are substantially transparent, matt,
roughened or dark
coloured, thus forming a reflective scale.
The at least one array of EMR detectors is positioned radially inside or
outside of the
surface.
It is preferred that the at least one array of EMR detectors comprises a one
dimensional
or a two dimensional array, a CCD, a VLSI vision chip or a lateral effect
photodiode.
It is preferred that the pattern is also processed by the processor to derive
the angular
velocity of the rotatable body with respect to the housing.
CA 02338637 2001-O1-24
WO 00/06973 PCT/AU99/00590
Brief Description of the Drawings
6
The present invention will now be described by way of example with reference
to the
accompanying drawings, in which:
Fig. 1 a is a diagrammatic sectional view of an angular position sensor
according
to a first embodiment of the present invention showing the rotatable body
consisting of
regions of high and low reflectivity provided by radially protruding
casteilations, and a
radially disposed photodetector array,
Fig. 1 b is a larger scale view of a portion of the grating element shown in
Fig. 1 a,
Fig. 2a is a diagrammatic sectional view of an angular position sensor similar
to
that shown in Fig. 1 a employing axially protruding castellations and an
axially disposed
photodetector array,
Fig. 2b is a larger scale view of a portion of the grating element shown in
Fig. 2a,
Fig. 3a is a diagrammatic sectional view of an angular position sensor
according
to a second embodiment of the present invention showing a rotatable body
consisting of
a cylindrical scale surface with regions of high and tow reflectivity and a
radially disposed
photodetector array,
Fig. 3b is a larger scale view of a portion of the grating element shown in
Fig. 3a,
Fig. 4a is a diagrammatic sectional view of an angular position sensor similar
to
that shown in Fig. 3a employing a disc shaped scale surface with an axially
disposed
photodetector array,
Fig. 4b is a larger scale view of a portion of the grating element shown in
Fig. 4a,
Fig. 5 is a diagram illustrating the pattern incident on the photodetector
array and
a technique employed providing both coarse resolution absolute angle
measurement and
CA 02338637 2001-O1-24
WO 00/06973 PCT/AU99/00590
7
fine resolution interpolated incremental measurement, and
Fig. 6 is a diagrammatic sectional view of an angular position sensor
according to
a third embodiment of the present invention where the sensor comprises two
rotatable
bodies connected by a torsional member, and the sensor providing for
measurement of
the torque transmitted by the torsional member.
Mode of Carrying Out Invention
Figs. 1 a & 1 b show an angular position sensor according to a first
embodiment of the
present invention. Rotatable body 1 comprises grating element 2 with a
discontinuous
outer cylindrical surface l4~composed of alternating regions of high and low
EMR
reflectivity, arranged in the form of a succession of individual binary bar
codes. Grating
element 2 comprises radially protruding castellations 3 interposed between
radially
extending cavities 4. The regions of high reflectivity on cylindrical surface
14 correspond
to areas of maximum radius 12 of castellations 3 with respect to axis of
rotation 8 of
rotatable body 1, and may be smoothly machined, moulded or sintered, or
surface
treated with paint or material deposition to impart the required high
reflectivity. The
regions of low reflectivity on cylindrical surface 14 correspond to
discontinuous gap areas
13, and are substantially non-reflective due to the presence of cavities 4,
comprising
areas of minimum radius 15 which are disposed at lesser radius than
aforementioned
areas 12, and are ideally machined, moulded or sintered, or surface treated
with paint or
material deposition to impart low reflectivity. Rotatable body 1 is enclosed
in housing 5
and supported in bearings 6 and 7, and is able to rotate about-axis of
rotation 8. EMR
source 10 and EMR sensitive photodetector array 9 are fixed in housing 5 and
arranged
such that EMR source 10 illuminates discontinuous surface 14, which reflects
EMR to
the substantially radially disposed array 9. Thus a pattern is produced on
array 9, which
is processed by processor 11 to provide a measure of the absolute angular
position of
rotatable body 1 with respect to housing 5. It should be noted that the words
"reflection",
"reflected" and "reflectivity" in this specification are relate to specular
and/or diffused
reflection.
Figs. 2a & 2b show an alternative angular position sensor according to the
first
CA 02338637 2001-O1-24
WO 00/06973 PCT/AU99/00590
8
embodiment of the present invention. Rotatable body 1 comprises grating
element 2 with
a discontinuous radially oriented flat disc surface 14 composed of alternating
regions of
high and low EMR reflection, arranged in the form of a succession of
individual binary
bar codes. Grating element 2 comprises axially protruding castellations 3
interposed
between axially extending cavities 4. The regions of high reflectivity
correspond to areas
of maximum axial protrusion 12 of castellations 3 with respect to axis of
rotation 8 of
rotatable body 1, and may be smoothly machined, moulded or sintered, or
surface
treated with paint or material deposition to impart the required high
reflectivity. The
regions of low reflectivity correspond to discontinuous gap areas 13, and are
substantially non-reflective due to the presence of cavities 4. Rotatable body
1 is
enclosed in housing 5 and supported in bearings 6 and 7, and is able to rotate
about axis '
of rotation 8. EMR source 10 and EMR sensitive photodetector array 9 are fixed
in
housing 5 and arranged such that EMR source 10 illuminates discontinuous
surface 14,
which re-radiates EMR to the substantially axially disposed array 9. Thus a
pattern is
produced on array 9, which is processed by processor 11 to provide a measure
of the
absolute angular position of rotatable body 1 with respect to housing 5.
Figs. 3a & 3b show an angular position sensor according to a second embodiment
of the
present invention. Grating element 2 of rotatable body 1 comprises a
continuous
cylindrical surface in the form of graduated scale 20 composed of alternating
regions of
high and low EMR reflectivity, arranged in the form of a succession of
individual binary
bar codes. A metallised coating, or other shiny or light coloured material or
surface
treatment, provides substantially axially aligned regions of high reflectivity
21. A
substantially transparent, roughened or dark coloured material or surface
treatment
provides the interspaced regions of low reflectivity 22. Rotatable body 1 is
enclosed in
housing 5 and supported in bearings 6 and 7, and is able to rotate about axis
of rotation
8. EMR source 10 and EMR sensitive photodetector array 9 are fixed in housing
5 and
arranged such that EMR source 10 illuminates the regions of high and low
reflectivity 21
and 22 which re-radiates EMR to the substantially radially disposed array 9.
Thus a
pattern is produced on array 9, which is processed by processor 11 to provide
a measure
of the absolute angular position of rotatable body 1 with respect to housing
5.
CA 02338637 2001-O1-24
WO 00/06973 PCT/AU99/00590
9
Figs. 4a & 4b show an alternative angular position sensor according to a
second
embodiment of the present invention. Grating element 2 of rotatable body 1
comprises a
continuous radially oriented fiat disc surface in the form of graduated scale
20 composed
of alternating regions of high and low EMR reflectivity, arranged in the form
of a
succession of individual binary bar codes. A metallised coating, or other
shiny or light
coloured material or surface treatment, provides substantially radially
aligned regions of
high reflectivity 21. A substantially transparent, roughened or dark coloured
material or
surface treatment provides the interspaced regions of low reflectivity 22.
Rotatable body
1 is enclosed in housing 5 and supported in bearings 6 and 7, and is able to
rotate about
axis of rotation 8. EMR source 10 and EMR sensitive photodetector array 9 are
fixed in
housing 5 and arranged such that EMR source 10 illuminates the regions of high
and low
reflectivity 21 and 22 which re-radiates EMR to the substantially axially
disposed array 9.
Thus a pattern is produced on array 9, which is processed by processor 11 to
provide a
measure of the absolute angular position of rotatable body 1 with respect to
housing 5.
In the case of both first or second embodiments, it will be appreciated that
processor 11
can readily be programmed or hardwired to calculate the rate of change of
absolute
angular position of rotatable body 1 as a function of time, and therefore also
provide a
measure of absolute angular velocity of rotatable body 1 with respect to
housing 5.
Fig. 5 shows an example of a pattern produced by incident EMR on array 9
according to
the first or second embodiment of the present invention (also according to a
third
embodiment described below). The individual bits 30a-a represent dark areas of
the
pattern on array 9 caused by reduced levels of reflection from the regions of
low
reflectivity 13 (first embodiment) or 22 (second embodiment). Array 9 is a one-
dimensional "linear" array, for example a Texas Instruments TSL1410 Black &
White
Linear Array chip with 128 pixels and an active window length of approximately
8 mm.
This array is adapted to provide both an absolute angular position measurement
and a
fine resolution incremental angular position measurement. The absolute angular
position
measurement is performed by the reading of at least one complete word formed
by a
predetermined number of successive bits, in this case word 31 comprising five
bits, so as
to permit the identification of the word of the pseudo-random sequence
representing the
CA 02338637 2001-O1-24
WO 00/06973 PCT/AU99/00590
absolute angular position measurement. The disposition and use of such pseudo-
random sequences are generally well known in the art of image analysis, and
are
described in US Patent 5,576,535 in reference to the measuring absolute linear
displacement. Another example of one combination of such sequences is
described as
an Ouroborean ring in "Game, Set and Math" by Ian Stewart, Penguin Books,
1989.
The disposition of the regions of high and low EMR reflectivity employed in
this
embodiment of the present invention differs, however, since the pattern
produced on
array 9 comprises a sequence of bits of a constant centreline pitch "a" (ie.
the spacing
distance between the centreline of adjacent bars) with varying width "p" and
"qu. Fig. 5
shows five bit word 31, with binary number "1 " represented by bits 30a and
30d having
width "p" and binary number "0" represented by bits 30b, 30c and 30e having
width "q".
The complete word 31 is thus 10010 (ie. 18 in base 10), which is processed by
processor
11 to provide a unique absolute angular position of rotatable body 1.
Importantly, the
disposition of regions of high and low EMR reflectivity, which results in a
pattern on array
9 with constant pitch, allows the same pattern, and hence array, to be used
for the
measurement of fine resolution incremental angular position. One such
interpolation
technique is also shown in Fig. 5. The EMR intensity pattern on array 9 is
denoted by
P(x) where x is the horizontal scale representing angular displacement and P
is a
function of x.
If the EMR intensity pattern is sinusoidal, then:
P(x) = sin[2n(x-d)/a]
Where a = pitch of the pattern, and
d = displacement of the pattern
The pattern P(x) is sampled by the individual pixels of array 9. Let P; denote
the i-th
sample. Thus the "pattern vector" of n samples can be denoted as P = [P1, P2,
P3,...Pn].
Two weighting functions are now defined, being the sine and cosine weighting
vectors:
CA 02338637 2001-O1-24
WO 00/06973 PCT/AU99/00590
K~; = sin(2ni/a) for i = 1....n
11
K2; = cos(2ni/a) for i = 1....n
Hence phase angle a is given by:
a=arctan [(EP;Ki;) / (EP;K2;)] for i = 1....n
The resulting phase angle a is a measure of the incremental displacement of
the pattern
relative to the sine and cosine weighting vectors and provides a fine
resolution angular
position measurement that is, on a statistical basis, many times finer than
the width of
one bit of the pattern. The coarse resolution absolute angular position
measurement and
fine resolution incremental angular position measurement is combined to
provide an
absolute angular position detector with fine resolution requiring only one
detector array
and with low susceptibility to mechanical deflection and misalignment.
The use of other styles of bar codes with constant pitch can be similarly
processed
according to this "convolution algorithm", for example where the binary bit
information is
coded as a difference in length of the bar rather than width. Also, the binary
bit
information can be encoded as a difference in the level of attenuation of the
re-irradiated
EMR such as by the use of a greyscale code. Moreover, although this embodiment
demonstrates the convolution algorithm based on a bar code with constant bar
pitch and
variable bar width, it should be appreciated that the algorithm will also
function equally
successfully for a variable bar pitch situation, providing that that the bar
pitching selected
is an integer multiple of a "fundamental pitch". For example, referring to the
terminology
used in Fig. 5, the centreline pitching separating bits 30a-a may be arranged
as
respectively "a", "3a", "2a", and "a" (with a fundamental pitch of "a") rather
than the
constant pitch of "a" as shown in Fig. 5. Indeed any integer multiple of "a"
may be used
for the centreline pitch between successive bits. In the situation where such
a varying
pitch format of bar code is selected, the bar code encryption can be achieved
via the
varying pitch spacing rather than via bar width (as shown by the bit pattern
in Fig. 5),
thus it is feasible in this situation to use a constant bar width and still
achieve satisfactory
bar code encryption for coarse resolution absolute angular position
measurement.
CA 02338637 2001-O1-24
WO 00/06973 PCT/AU99/00590
12
It should also be noted that the succession of bar codes could have reverse
reflectivity
compared to the embodiments described, that is high reflectivity regions
imposed over a
low reflectivity background.
Also in the present specification "high reflectivity" and "law reflectivity"
is broadly defined
in reference to the particular EMR source selected. For example, if a red
light EMR
source was used, the regions of high and low reflectivity of the surfaces of
the reflective
gratings may consist of regions which are painted (or otherwise coloured by
some
means) with a red and blue surface coating respectively.
Fig. 6 shows an angular position sensor according to a third embodiment of the
present
invention. The angular position sensor comprises two rotatable bodies 1 a & 1
b which are
connected by torsion bar 23 of predetermined torsional stiffness. Grating
elements 2a &
2b are respectively attached to or integral with rotatable bodies 1 a & 1 b
and arrays 9a &
9b respectively receive incident EMR re-radiated from surfaces 20a & 20b. In
certain
other embodiments (not shown) arrays 9a & 9b may be combined as a single
array. This
single array will therefore necessarily be a 2D array, and will receive EMR
reflected from
both surfaces 20a & 20b. Similarly, in certain other embodiments (not shown),
EMR
sources 10a & 10b may be combined as a single EMR source.
Surfaces 20a & 20b are shown as similar to surface 20 in Figs. 3a & 3b, that
is these
surfaces are cylindrical and each comprise a graduated scale composed of
alternating
regions of high and low EMR reflectivity, and arranged in the form of an
endless
succession of individual binary bar codes. It will be recognised that other
types of
"surfaces of revolution" could alternatively be employed in place of these
continuous
cylindrical surfaces 20a & 20b, for example continuous flat disk surfaces
(similar to
surface 20 in Figs. 4a & 4b), discontinuous cylindrical surfaces (similar to
surface 14 in
Figs. 1 a & 1 b), or discontinuous flat disk surfaces (similar to surface 14
in Figs. 2a & 2b).
A "surface of revolution" of a body in this specification is defined as a
surface which is
equally disposed about the axis of rotation about which the body rotates.
CA 02338637 2001-O1-24
WO 00/06973 PCT/AU99/00590
13
The patterns on arrays 9a & 9b, or the pattern on the earlier mentioned single
array (not
shown), are processed in processor 11 to derive the absolute angular position
of the
regions of high and low reflectivity (or transmissibility in other
embodiments) on surfaces
20a & 20b of each grating element 2a & 2b respectively with respect to housing
5. The
difference between these absolute angular positions is further processed by
processor
11 to derive the relative angular displacement of grating elements 2a & 2b,
and hence
provide a measure of the torque transmitted by torsion bar 23.
Thus this third embodiment of the angular position sensor not only provides a
measure of
the absolute angular position of each of the two rotatable bodies 1 a & 1 b
(and potentially
their angular velocity as described earlier) with respect to housing 5, but
also provides a
measure of the torque applied between rotatable bodies 1 a & 1 b (which is
reacted by
torsion bar 23).
It will be appreciated by those skilled in the art that numerous variations
and
modifications may be made to the invention without departing from the spirit
and scope
of the invention.
