IlllllIllllllllllllllilllllllllllllllllllllllllIlllllllllllllllllllllllllll
USOO5131154A
United States Patent [191
[11]
[45]
Schierbeek et al.
[54] METHOD AND APPARATUS FOR
[75] Inventors: Kenneth L. Schierbeek, Zeeland;
Kenneth Scho?eld; D811 V- Bui, both
of Holland’ all of Mlch‘
Donne“), Corporation Holland
Mich
5,131,154
Jul. 21, 1992
Attorney. Agent, or Firm-Reising, Ethington, Barnard,
COMPENSATING A MAGNETIC COMPASS
[73] Assignee,
Patent Number:
Date of Patent:
’
Perry & Milton
[57]
ABSTRACT
Method and apparatus are disclosed for compensating a
magnetic compass in a vehicle for magnetic deviation.
With the compass installed, the angle indicated by the
’
'
magnetic compass relative to a direction axis of the
vehicle is measured. A test magnetic ?eld is added at a
[2]] Appl. No: 597,854
known angle relative to the direction axis of the vehicle.
.
The strength of the test magnetic ?eld required to dis
[22] ?led:
oct- 15, ‘990
[51] Int 0 5
[52] Us‘
"""""""""""""""""""
[58] Fi'el'd or:
""""""
"""""" "
001C 17/26
33/363 K
357 ésg 363 R
'33/3’63 K’ 355 R’
-
[56]
’
place the direction indicated by the compass through a
predetermined angle is memorized. Then, the strength
of the resultant magnetic ?eld is calculated using the
measured angle, the predetermined angle and the mem
orized strength ofthe magnetic ?eld. After determining
the strength of the resultant magnetic ?eld with the
References Cited
vehicle oriented in any selected direction, the vehicle is
U s PATENT DOCUMENTS
then reoriented in a direction one hundred eighty de
' '
4,733,179
Hormel ........................... ..
3/1983 Ball?’ e‘ al~ -------- -~
33/356 x
ggh‘grek e‘ 31' "
"""
419371945 7/1990 science??? .
....... .. 33/356
Primary Examiner—l~larry N. Haroian
,4
grees away from the selected direction and the strength
of the resultant magngtic
in that orientation is
determined in the same way. The value of the deviating
magnetic field is calculated as one-half the vector sum
°f the two resultam magnetic ?elds‘
14 Claims, 11 Drawing Sheets
f Nm
E
Q
:
22
18 \ 24
xI
I
26
ELECTRONIC
CKT.
2a
DISPLAY
US. Patent
July 21, 1992
I4
Sheet 1 of 11
5,131,154
f "I"
E
(‘D
I
I8
\
24
x
26
Hag???“
28
DISPLAY
US. Patent
July 21, 1992
Sheet 2 of 11
5,131,154
[52
ELECTRONIC
CIRCL'IT
:J)
DISPLAY
US. Patent
July 21, 1992
Sheet~3 of 11
5,131,154
56
iii
‘in’.
A!» AQ/AOEFJ
Iliad-awn”;
1
1
US. Patent
July 21, 1992
Sheet 4 of 11
5,131,154
I
:PL.---DIRECTION REF AXIS
[
2/4
@206 ORIENTATION AXIS
J72
/ [Fig-I0
IIIHOIOHIIIOIOIIIIII I
cw
,F’g'l3
llj
IIlJIIlI-CCW
||1|11qo|1|1|0|0|1|1|/304 _
cw
ccw
|z
351
357°
3,835,,
_
[Fig-l4
coum
9
62
:1‘
e
12
DEGREES
I20
-
7
H9
265
1/8
2625 - 265%
1/7
3
- 268
559% - 262%
- 271
10
271% - 2714
O0
274
- 277
0/
277
7 2
I/
US. Patent
July 21, 1992
Sheet 5 of 11
5,131,154
m6
vb
muv305Q:
MN
QN
US. Patent
62
July 21, 1992
y
Sheet 7 of 11
61
PA
T
61
62
.X
+X
V
-Y
+X
Fig~15B
+Y
5,131,154
US. Patent
July 21, 1992
Sheet 8 of 11
3%
+Y
3,445
Fig-16
5,131,154
US. Patent
July 21, 1992
Sheet ‘9 of 11
~
START
5,131,154
400
1
IS SWITCH 276
@402
CLOSED '?
1 _
READ
404
HEADING
J
I 408
406
IS HEADING
WITHIN 45° OF
X-AXIS ?
USE X-AXIS
ROUTINE
USING Y-AXIS
COILS FOR TEST
FIELD
412
COILS FOR
N
TEST FIELD
ENERGIZE COIL 32E N414
TO PULL ROTOR CW
1
STORE PHM
cOuNT FOR 1st
@415
TRANSITION AS t1
1
PULL ROTOR
4 MORE
POSITION COUNTS
I
418
N
1
DE~ENERGIZE
COILLSEE
STORE TOTAL
PWM COUNT
AS UI
1
TO 426
N422
424
N
Fig-1 7A
US. Patent
»
July 21, 1992
FROM 424
i
j
Sheet 10 of 11
426
ENERGIZE COIL 32W
To PuLL ROTOR
CC”
5,131,154
I
F1g-17B
CALCULATE x AND Y
COMPONENTS OF
448
D1 A§ D1X,D1Y
I428
sTORE PMM
COUNT FOR 1st
TRANSITION AS t2
8432
/~/
PER Eq. 25. 2a
T
DISPLAY PROMPT M56.
"TURN OAR TO
452
OPPOSITE DIRECTION" /“/
PuLL ROTOR
4 MORE
454
POs. COUNTS
READ
HEAOING
I434
OE-ENEROIZE
COIL saw
‘58
456
IS HEADING
.
‘
1436
usE x AXIS
WITXH_IAL§(I4S5 9 OF
COILS FOR TEsT
STORE TOTAL
PWM COUNT
AS u2
‘?2
442
5
USE Y-AXIS
COILS FOR TEsT
CALCULATE
ANGLES a1 ANO a2
)'
PER Eq, 18 AND 19
46
4
ENEFIGIZE COIL 34N
TO PULL ROTOR CCW
444
T
CALCULATE
_
f
45s
/
ANGLES 61 AND 02
STORE PMM
PER Eq, 20 AND 21
COUNT FOR 1st
TRANs. AS t3
446
CALCULATE
vEcTOR 01
§
PER Eq. 22. 23, AND 24
‘
I468
PULL ROTOR
4 MORE
POSITION COUNTS
l TO 472
US. Patent
July 21, 1992
5,131,154
Sheet 11 of 11
Fig-17C
FROM
468
472
502 1
DE-ENERGIZE
COIL 34N
1
CALCULATE
ANGLES G1 ' AND 62'
PER Eq, 29 AND 30
j
474
504
STORE TOTAL
PWM COUNT
AS U1 '
l
‘
j 475
ENERGIZE COIL 343
TO PULL ROTOR
CALCULATE
VECTOR D2‘
PER Eq, 31. 32. 33
5061 i
CALCULATE x AND Y
COMPONENTS OF
02 As 02x. 02v
78
1 /4
STORE PWM
COUNT FOR 1st
TRANSITION AS t4
)- 492
PULL ROTOR
4 MORE
POS. COUNTS
l f“
PER Eq,
34.35 _
5081
CALCULATE X-AXIS
COMPONENT VX OF
VEHICLE FIELD V
PER Eq. 36
1
CALCULATE Y—AXIS
COMPONENT VY OF
PER Eq. 37
DE-ENERGIZE
COIL 348
l 5496
STORE TOTAL
PWM COUNT
AS U2’
1 I498
CALCULATE
ANGLES a1 ' AND a2‘
S 512
j- 514
OUTPUT VX
TO COIL 32E OR
32W ACCORDING
TO SIGN
j 516
OUTPUT VY
T0 COIL 34N OR ~
348 ACCORDING
TO SIGN
PER Eq. 27 AND 28
\_______
518
END
1
5,131,154
2
the deviating ?eld is stored and subtracted from the
METHOD AND APPARATUS FOR
COMPENSATING A MAGNETIC COMPASS
FIELD OF THE INVENTION
This invention relates to magnetic compasses for
vehicles; more particularly, it relates to a method of
providing deviation compensation for magnetic com
passes.
magnetic ?eld measurements subsequently taken by the
compass in use for direction ?nding to thereby compen- -
sate it for deviation. This method of deviation compen
sation for a flux gate compass is disclosed in the Bauer
et al U.S. Pat. 4,733,179 granted Mar. 22, 1988 and in
the Hormel U.S. Pat. No. 4,720,992 granted Jan. 26,
I988. The one hundred eighty degree method for devia
tion compensation has been known for a long time for
use with ?ux gate compasses for aircraft as shown by
BACKGROUND OF THE INVENTION
A magnetic compass which is installed in a vehicle
must be calibrated in the vehicle to account for the
disturbing effect of the vehicle on the earth's magnetic
?eld. It is well known that automotive land vehicles, as
well as boats and aircraft, produce a magnetic ?eld due
to the presence of ferromagnetic materials, electrical
current carrying wires and the like and this magnetic
?eld interferes with the earth's magnetic ?eld at loca
tions within and adjacent the body of the vehicle. The
magnetic ?eld sensor of a magnetic compass responds
to the ambient or local magnetic ?eld within which it is
immersed for the purpose of direction ?nding with
reference to the earth‘s magnetic ?eld. The magnetic
?eld vector produced by the vehicle, herein referred to
as the deviating magnetic ?eld vector, combines with
the earth‘s magnetic ?eld vector to produce a resultant
magnetic ?eld vector which, without calibration or
compensation, is unsuitable for reliable and accurate
direction ?nding. The resultant magnetic ?eld vector
must be compensated for deviation in order to obtain
accurate direction ?nding by a magnetic compass.
It is known in the prior art to provide deviation com
pensation in a magnetic compass by a pair of compensa
the Baker et al U.S. Pat. No. 3,683,668 granted Aug. 15,
1972.
Heretofore, magnetic rotor compasses, i.e. those hav
ing a magnetic rotor have required orientation at a
known angle relative to magnetic north to obtain devia
tion compensation. It is desirable in vehicle compasses
to provide a magnetic rotor compass with the advan
tages of the one hundred eighty degree method of com
pensation.
A general object of this invention is to provide an
improved method of and apparatus for compensating a
magnetic rotor compass for deviation and to overcome
certain disadvantages in the prior art.
SUMMARY OF THE INVENTION
In accordance with this invention, method and appa
ratus are provided for determining the strength of the
magnetic ?eld in a vehicle which results from the com
bination of a deviating magnetic ?eld originating in the
vehicle and the earth‘s magnetic ?eld. This is accom
plished by measuring the angle between a direction
reference axis of the vehicle and the direction indicated
by the magnetic compass in the vehicle and adding a
test ?eld at a known angle relative to the direction
tion coils which are energized with current to generate 35
a magnetic ?eld which is equal and opposite to the
reference axis of the vehicle. The strength of the test
magnetic ?eld which is required to displace the direc
tion indicated by the compass through a predetermined
this type for an automotive vehicle compass is disclosed
angle is memorized. Then the strength of the resultant
in the Dinsmore U.S. Pat. No. 4,402,142 granted Sep. 6,
magnetic
?eld is calculated using the measured angle,
1983. This type of deviation compensation is also dis 40
the predetermined angle and the memorized strength of
closed in the Schierbeek et al U.S. Pat. No. 4,862,594
the test magnetic ?eld. The resultant magnetic ?eld is
granted Sep. 5, 1989. In this method of deviation com
pensation, the vehicle must be oriented with a heading ' useful for compensating a magnetic compass for the
deviating magnetic ?eld. Deviation compensation of
deviating magnetic ?eld.
aligned with one of the four cardinal directions relative
Further, in accordance with this invention, method
to magnetic north with coil current adjustment being 45
and
apparatus are provided for compensating a mag
made to obtain a compass indication corresponding to
netic compass in a vehicle for magnetic deviation. This
the cardinal point and then the vehicle must be reori
is accomplished by orienting a vehicle in any selected
' ented to a heading corresponding to another cardinal
direction relative to magnetic north and determining
point at right angles to the ?rst and the current in the
corresponding compensation coil is adjusted to obtain 50 the strength of the resultant magnetic ?eld in the vehi
cle using the method described above. Then, the vehicle
compass indication of the second orientation. This pro
is oriented in a direction one hundred eighty degrees
cedure for deviation compensation is usually carried out
from the selected direction and the strength of the resul
by the vehicle driver and even though it may be auto
mated in a computer controlled compass, it results in
tant magnetic ?eld is determined in the same way as
inaccuracy unless the vehicle heading is accurately
aligned relative to magnetic north.
described above. The value of the deviating magnetic
Another method of deviation compensation for mag
?eld is then calculated as one-half of the two resultant
magnetic ?elds.
Further in accordance with this invention, method
and apparatus are provided for compensating a mag
herein as the one hundred eighty degree compensation
method. In this method, the magnetic ?eld which is the 60 netic rotor compass for deviation. The previously
known one hundred eighty degree compensation
resultant of the earth‘s ?eld and the deviating ?eld of
method is utilized in order to avoid the requirement for
the vehicle is measured with the vehicle in a selected
aligning the vehicle in a predetermined direction rela
orientation relative to magnetic north and then the
tive to magnetic north as has been required in magnetic
resultant ?eld is measured with the vehicle in an orien
tation one hundred eighty degrees from the ?rst. Using 65 rotor compasses. According to the invention, the vehi
cle is oriented in an arbitrary direction relative to mag.
the measured values of the magnitude and directions of
netic north and in a second orientation, the opposite
the resultant ?elds, the deviating ?eld is calculated in
direction. In each orientation, a value of the resultant
both magnitude and direction. The calculated value of
netic compasses in vehicles is also known and is referred
5,131,154
3
?eld vector (which is the resultant of earth's ?eld and
the vehicle ?eld) is determined by using a test current in
one of the compensation coils to swing the rotor
through a known angle to obtain suf?cient information
4
14 represents the heading of .the vehicle 10 and the
purpose of the compass 12 is to determine the heading
with reference to the magnetic north direction indicated
by the arrow N,,,. The magnetic heading may be con
verted to the true heading, i.e. with reference to true
to calculate the resultant ?eld vector. The resultant
?eld vectors for the two orientations together with the
memorized values of the currents for swinging the rotor
north, by algebraically adding the value of magnetic
variation for the locality of the vehicle. As used herein,
the term “heading“ means the direction of the longitudi
nal axis of the vehicle relative to the reference direction,
provide sufficient information to calculate the required
compensating currents to compensate the compass for
deviation due to the vehicle ?eld.
Further, in accordance with the invention, a method
such as magnetic north or true north. The term “true
north” as well as the term “geographic north" mean the
is provided for obtaining a high degree of accuracy in
determining the compensating current required even
direction of the pole star polaris. The term “magnetic
north” means the direction of the local magnetic ?eld of
though the compass is provided with an angular mea
surement encoder with relatively low resolution.
Further, in accordance with this invention, the com
pensation method is useful for magnetic rotor com
passes having incremental angular measurement encod
the earth.
The magnetic compass 12 comprises a compass frame
16 with a frame orientation axis 18. A rotor 22 is
deviation according to this invention;
data signal representing the angular displacement of the
mounted on the frame for rotation about a vertical axis.
The compass is installed with its orientation axis 18
ers as well as absolute angular measurement encoders.
aligned with the vehicle direction reference axis 14. The
20 rotor carries a magnet with its polar axis in the horizon
DESCRIPTION OF THE DRAWINGS
tal plane and which seeks to orient its polar axis with the
FIG. 1 is a block diagram showing a magnetic rotor
local magnetic ?eld. The compass is provided with an
compass installed in a vehicle to be compensated for
angular position encoder 24 which provides a direction
FIGS. 2A, 28, 3A, 3B and 4 are vector diagrams 25 rotor from some reference position or direction. The
useful for explaining the principles of this invention;
direction data signal from the encoder is coupled to an
FIG. 5 shows a block diagram of the compass system;
electronic circuit 26 which in turn is coupled to a com
FIGS. 6, 7, 8, 9 and lo show a particular embodiment
pass display 28. The direction data signal from the en
of a vehicle compass for compensation according to this
coder 24 is processed by the electronic circuit 26 to
invention;
'
30
produce a heading signal which represents the heading
FIG. 11 is a diagrammatic representation of the vehi
of the vehicle relative to the local magnetic ?eld. The
cle compass installed in a Vehicle;
heading signal represents the angle between the direc
FIGS. 12A and 128 show a schematic diagram of the
tion of the local magnetic ?eld vector and the direction
electronic circuit of the compass;
reference axis of the vehicle and is suitably displayed on
FIG. 13 is a waveform diagram of the operation of 35 the display 28 in terms of angular degrees.
the compass encoder;
The compass 12 is provided with a pair of compensa
FIG. 14 is a table of values showing operation of the
tion coils 32 and 34 for producing a compensating mag
encoder;
netic ?eld which affects the local magnetic ?eld and
FIGS. 15A, 15B and 16 are vector diagrams for ex
in?uences the orientation of the rotor 22, as will be
planation of the compensation method applied to the
particular compass embodiment; and
40 described hereinafter. The compensation coil 34 has a
FIGS. 17A, 17B and 17C are ?ow charts representing
the program of the microcomputer of the compass.
which is aligned with the orientation axis 18 of the
compass. The coil 32 is disposed with its coil axis in
orthogonal relationship with the Y-axis and de?nes the
coil axis which de?nes a Y-axis of the compass and
BEST MODE FOR CARRYING OUT THE
45 X-axis of the compass.
INVENTION
As illustrated in FIG. 1, the earth's magnetic ?eld is
An illustrative embodiment of the invention will be
represented by a vector E. The vehicle produces a
described with reference to the drawings which depict
deviating magnetic ?eld represented by the vector V.
a magnetic rotor compass which is to be compensated
The deviating ?eld vector has a constant magnitude and
for deviation in accordance with this invention. The 50 direction relative to the direction reference axis 14 and
deviation compensation method of this invention will
may be regarded as a characteristic of the particular
?rst be described with reference to the general case of a
vehicle. On the other hand, for a given locale, the direc
magnetic rotor compass. Following that, the invention
tion of the earth’s magnetic ?eld vector E is constant
will be described with reference to a speci?c embodi
relative to the true north direction. For a given orienta
ment of a magnetic rotor compass. It will be appreci 55 tion of the vehicle relative to the earth‘s magnetic ?eld,
ated as the description proceeds that the invention may
the earth's ?eld E and the deviating ?eld V produce a
be embodied in different forms and may be used in other
resultant ?eld represented by the vector D. It is desired,
applications.
of course, to have the rotor 22 of the compass align
GENERAL DESCRIPTION OF THE DEVIATION
COMPENSATION METHOD
itself with the earth’s magnetic ?eld E and thereby
obtain a heading signal from the compass 12 relative to
magnetic north. However, the presence of a deviating
This invention will now be described with reference
?eld V produces the resultant ?eld of vector D which is
to the general case of deviation compensation for a
displaced from the direction of the earth's ?eld. This
magnetic rotor compass in a vehicle. FIG. 1 illustrates
deviation of the resultant ?eld is variable as a function
in phantom line an automotive vehicle 10 with a mag 65 of the orientation of the vehicle. Accordingly, it is nec
netic compass 12 installed therein. The vehicle 10 has a
essary to remove the in?uence of the deviating mag
direction reference axis 14 which coincides with its fore
netic ?eld V so that the rotor of the compass responds
and aft or longitudinal axis. The direction reference axis
only to the earth's magnetic ?eld E.
5
5,131,154
6
compass reading on the display 28 will be T degrees
which is the angular displacement of the rotor from the
In order to remove the in?uence Of the deviating
magnetic ?eld V, it is necessary to determine the magni
tude and direction of it by measurement and calculation.
V can be derived by what is herein referred to as the
Y-axis of the compass as shown in FIG. 3A. The X-axis
compensation coil 32 is energized with a suf?cient cur
rent to pull the rotor toward the Y-axis through an
one hundred eighty degree compensation method. In
angle a of a predetermined value, say twelve degrees
this method, a vehicle is oriented in any arbitrary direc
tion prior to any compensation and the summation of
the earth's ?eld E and the deviating ?eld V produces a
?rst resultant ?eld D1. Then, the vehicle is turned
through one hundred eighty degrees so that it points in
quired value of current is memorized and is represented
by the vector U in FIG. 3A. The value of the angle G
A useful expression for the value of the deviating ?eld
which results in a displaced ?eld vector D1’. The re
between the resultant ?eld vector D1 and the vector U
can be derived as follows. By inspection of the vector
diagram of FIG. 3A it can be seen that:
the opposite direction thereby producing a resultant
vector D2. The desired expression for the deviating
?eld V in terms of the resultant ?elds D1 and D2 may
be derived as follows.
G=l80'—m
(4)
15
Referring now to FIG. 2A, the direction reference
axis 14 of the vehicle is oriented at an arbitrary angle A
with reference to the earth's magnetic ?eld vector E,
Since angles n and m are complementary, this can be
written as:
i.e. magnetic north. The deviating ?eld produced by the
vehicle represented by vector V is vectorially added to
the vector E to produce the resultant magnetic ?eld
and it can be seen that:
represented by the vector D1. Next. as shown in FIG.
23, it is assumed that the vehicle is turned through one
n = T-a
‘ (6)
‘hundred eighty degrees so that the direction reference
axis 14 (and the deviating ?eld vector V) are reversed in 25 Therefore the angle G can be expressed as:
direction relative to the earth‘s magnetic field vector E.
6:90. + T-e?
(7)
In this orientation, the direction reference axis 14 is at
an angle of one hundred eighty degrees minus A from
Using the law of sines, the value of D1 can be expressed
the earth's magnetic ?eld vector E. In this orientation,
the deviation ?eld vector V is vectorially added to the
as:
earth’s magnetic ?eld vector E to produce the resultant
D1=(U sin G)/sin a
(3)
?eld vector D2. From FIG. 2A, it is apparent that:
The X-axis component of the resultant ?eld vector D1
35 can be written as:
From FIG. 28 it is noted that:
DIX: D1 cos (90°— 7)
D2=E+ V
(9)
(2)
and the Y-axis component can be written as:
It is further noted with reference to equation (2) that the
vector E is negative because the X-Y coordinate system
is rotated one hundred eighty degrees along with the
D1 Y=D1 sin (90' - 7)
vector V relative to that shown in FIG. 2A while the
vector E remains ?xed with reference to the magnetic
north vector Nm. Adding equations (1) and (2) and
dividing both sides by 2 yields:
In a similar manner, the value of the resultant ?eld
vector D2 can be derived, with reference to FIG. 3B,
45
as:
D2=(U' sin G')/sin a’
V=(D1+D2)/2
(10)
(11)
(3)
Equation (3) expresses the value of the deviating ?eld
The X and Y-axis components of the resultant vector
D2 can be written as:
vector V in terms of the resultant ?eld vectors D1 and 50
D2 without need for determining the value of the
earth's ?eld vector E.
As set forth above, the value of the deviating ?eld
and
vector V may be obtained as a function of the resultant
D2Y=D2 sin (90'-7')
(13)
?eld vectors D1 and D2 as shown in equation (3). The 55
values of the vectors D1 and D2 are obtained using the
For the purpose of compensating the compass for
compass 12 after it is installed in the vehicle 10 as illus
trated in FIG. 1. The method of obtaining the values of
deviation, the value of the deviating ?eld vector V is
D1 and D2 will be described with reference to FIGS.
required. Its value, expressed in terms of the resultant
3A and 33. FIG. 3A is a vector diagram showing the 60 ?eld vectors, is known from equation (3). Accordingly,
resultant ?eld vector D1 with the vehicle oriented in
with reference to equation (3) and equations (9) and
any selected direction relative to magnetic north. FIG.
(12), the X-axis component VX of the deviating ?eld
3B is a vector diagram including the resultant ?eld
vector V can be written as:
vector D2 which results from the vehicle being oriented
VX=¢0§ (90'- 7) (D1+D2)/2
(14)
in the opposite direction, as described above with refer 65
ence FIG. 2B. In the ?rst orientation of the vehicle, the
Likewise, the Y-axis component VY of the deviating
compass rotor 22 will be aligned with its polar axis in
?eld vector V can be expressed as follows:
alignment with the resultant ?eld vector D1 and the
7
5,131,154
8
the deviating ?eld vector V and the compensating ?eld
VY=sin (90'- 7) (D1 +D2)/2
(15)
In order to compensate for the effect of the vehicle's
magnetic ?eld vector V, a compensating ?eld vector C,
equal and opposite to vector V, must be generated. This
is done by using the compensation coils 32 and 34
which, as described with reference to FIG. 1, are dis
vector C is zero. Thus, there is no net effect of vectors
V and C on the rotor of the compass. Consequently, the
compass rotor is in?uenced in its orientation only by the
earth's magnetic ?eld vector E and the compass is thus
compensated for deviation.
A particular embodiment of the compass 12 will now
be described.
posed with their coil axes aligned respectively with the
X and Y-axes of the compass. Since the Y-axis is aligned 10 GENERAL DESCRIPTION OF A PARTICULAR
with the longitudinal axis or direction reference axis 14
EMBODIMENT OF THE COMPASS SYSTEM
of the vehicle, and the vehicle heading is measured with
Before describing the compass and the compass elec
reference to magnetic north, the Y-axis is herein termed
tronics
in detail, it will be helpful to consider the man
the north-south axis and the X-axis is herein termed the
ner in which the compass operates to ?nd the heading of
east-west axis. As shown in FIG. 4, the north-south coil
the vehicle.
34 comprises coils MN and 34S and the east-west coil 32
comprises coils 32E and 32W. The required compensat
In general, the compass has a rotor which carries a
to one of the east-west coils which will generate a mag
signal representing the angular displacement from some
magnet which seeks to orient its polar axis with the
ing ?eld vector C is shown in FIG. 4 as being equal and
local magnetic ?eld which, for the present purpose,
opposite to the deviating ?eld vector V. The X-axis and
Y-axis components of the compensating ?eld vector C 20 may be considered to have the same orientation as the
earth's magnetic ?eld and hence has the direction of
are, of course, equal and opposite to the respective
magnetic north. The compass is provided with an angu
components of the deviating ?eld vector V. To obtain
lar position encoder which provides a direction data
this, a compensating ?eld current IVX must be supplied
netic ?eld vector equal to —VX of equation (14). Simi
larly, compensating ?eld current IVY must be supplied
reference position or direction. The compass rotor is
initially oriented in the reference direction and then the
to one of the north-south coils which will produce a
angular displacement of the rotor, either clockwise or
magnetic ?eld equal to -—VY of equation (15). It is
counterclockwise, is measured by the encoder until it
noted that if the value of VX is negative and VY is
settles in alignment with the local magnetic ?eld. The
negative, the vector V lies in the third quadrant and it
net displacement is taken as the heading angle indicated
must be compensated with a compensating ?eld vector
by the compass relative to the reference direction. This
C in the ?rst quadrant, as illustrated in FIG. 4. The
method of ?nding the vehicle heading by measurement
compensation coil 32E is energized to produce an X
of the relative displacement, rather than obtaining a
axis component having a magnitude equal to VX of
measurement in an absolute sense, greatly simpli?es the
equation (14). Similarly, the compensation coil 34N will
‘structure and the electronics of the vehicle compass.
35
be energized to produce a Y-axis component equal to
FIG. 5 shows a compass system with the compass 50
VY of equation (15). The value of energizing current
shown in a bottom plan view partially in section. In
required to produce the compensation ?eld VX is deter
general, the system comprises the compass 50, an elec
mined empirically from the memorized value of current
tronic circuit 52 and a remote compass display 54. The
required to produce the ?eld vector U as discussed with
compass 50, in general, comprises a support body 56
reference to FIG. 3A. The ?eld vector U has a magni
including a cylindrical cup 58 and a base 62. The cylin
tude which is directly proportional to the memorized
drical cup 58 has a longitudinally extending axis 64 and
current value IU and accordingly, the value of current
de?nes a chamber around the axis. A compass magnet
IVX required to produce the X-axis component VX of
68 of cylindrical shape is radially polarized and is
the ?eld vector V can be expressed as:
IVX: VX(lU)/U
45 mounted on a rotor 70 for rotation about the longitudi
(16)
nal axis 64. The compass magnet 68 is freely rotatable
relative to the body 56 and, in the manner of a compass
needle, it aligns itself with the local magnetic ?eld. The
Similarly, the current IVY required to produce the
angular position of the compass magnet is detected by
Y-axis component VY of the ?eld vector V can be
50 an optical position encoder which comprises an encoder
expressed as:
IVY= VYUUV U
(l7)
disk 72 which is mounted for rotation with the compass
magnet 68. The encoder also comprises an optical trans
ducer 74 coacting with the encoder disk 72. In order to
compensate for deviation, a set of compensation coils
For the sake of accuracy, each of the compensating
coils 32E, 32W, MN and 345 should be wound to pro 55 32E, 32W, 34N, 348 are provided. The optical trans
ducer 74 coacting with the encoder disk 72 develops a
duce the same magnetic ?eld strength along the respec
direction data signal which is electrically coupled with
tive axis for the same value of energizing current. Alter
natively, the coils can be calibrated or normalized. In
the electronic circuit 52 which decodes the signals and
order to account for inequality among the compensat
develops a heading signal corresponding to the heading
ing coils, the current requirements are normalized to
of the vehicle. The heading signal is applied by the
produce equal ?eld strengths by using a current cali
electronic circuit 52 to the compass display 54 for visual
brating factor for each of the coils in accordance with
presentation of the vehicle heading. The read-out of the
known techniques.
display 54 may be expressed in degrees or it may be
As discussed above, the compensating current IVX in
expressed in abbreviated notation for the compass
the east compensating coil 32E produces the X-axis 65 points such as N, E, S and W for the cardinal points and
component of the compensating ?eld vector C and the
NE, SE, SW and NW for the intercardinal or secondary
current IVY in coil 34N produces the Y-axis component
compass points. The compass 50 will now be described
of the compensating ?eld vector C. The vector sum of
in greater detail.
9
5,131,154
DETAILED DESCRIPTION OF THE COMPASS
The compass 50 is shown in detail in FIGS. 6. 7, 8 and
9. The compass is adapted for mounting in the orienta
tion shown in FIG. 7, i.e. the axis 64 is aligned in the
vertical direction and the cup 58 is the bottom member
of the compass. The compass 50 comprises, in general,
10
The cover plate 78 is formed with a set of three com
partments 172, 174 and 176 each of which is adapted to
retain an infrared light emitting diode (LED). Each
compartment has a wall con?guration which conforms
to the package con?guration of the LED. Each com
partment is provided with a channel 178 which accom
modates the lens of the LED. The cover plate is formed
a frame including a support body 56 and a cover plate
with an aperture or slot 182 above the compartment 172
78 with the rotor 70 therein. The support body 56 in
cludes the cylindrical cup 58 and the base 62. The cylin~
drical cup de?nes a cavity around the longitudinal axis
64. The rotor 70 carries the compass magnet 68 and is
and in alignment with the slot 122 in compartment 112.
Similarly, the plate is formed With slots 182 which are
located above the compartments 174 and 176, respec
tively, and are aligned with the slots in compartments
114 and 116, respectively. LEDs 192, 194 and 196 are
located in the compartment 112, 114 and 116, respec
tively. As will be described subsequently, the LEDs
192, 194 and 196 and the phototransistors 124, 128 and
134 are electrically connected with the electronic cir
cuit 52.
A pair of alignment pins 136 are formed on the base
rotatably mounted by pivot pins which are supported,
respectively, in bearing elements 84 in the cup 58 in the
cover plate. The optical position encoder comprises the
encoder disk 72 which is integral with the rotor and it
comprises the optical transducer 74. The compensation
coils 32E and 32W are mounted on coil support arm 88
and coils 34N and 345 are mounted on coil support arm
92 on the outside periphery of the cup and are angularly 20 62 and mate with alignment holes 138 on the cover 78
(see FIG. 6). The cover 78 has a set of four pawls 139
which interlock with the base to hold the cover to the
with the compass magnet.
base.
The support body 56, including the base 62 and the
spaced by ninety degrees and are in close proximity
The LED 194 and the phototransistor 128 comprise
cup 58, is a unitary molded body of polymeric material
such as nylon. The base has a semi-cylindrical portion 25 the reference position sensor 106 which produces a
reference signal pulse when the encoder disk 72 is in an
and a flat upper surface on one side of the cup. The base
is formed with a rectangular portion on the other side of
angular position such that the reference aperture 164 is
the cup. This portion is provided with a pair of mount
in alignment with the apertures 122 and 184 of compart
ments 114 and 174. The LED 192 and the phototransis
ing studs 94 which are adapted to mate with mounting
tor 124 comprise a ?rst angular displacement sensor 102
and the LED 196 and phototransistor 134 comprise a
second angular displacement sensor 104.
The compass frame comprising the body 56 and
cover plate 78 has an orientation axis 206 which coin
phototransistors of the optical transducer 74.
35 cides with a line extending through the axis of rotor 70
and the slot 122 of the reference position sensor 106.
The optical transducer 74 comprises a pair of dis
placement sensors 102 and 104 and a reference position
This orientation axis 206 (see arrow FIG. 10) is perpen
dicular to the end of the rectangular portion of support
sensor 106. (See FIG. 9). The transducer includes plural
holes in the electrical connector 96 for support of the
compass. The upper surface of the base, as shown in
FIG. 7, is provided with a cylindrical recess 98 for
accommodating the encoder disk. The rectangular por
tion of the base 62, as shown in FIG. 8, supports the
sets of light emitting diodes (LEDs) and phototransis
tors in an arrangement which will be described subse
quently. For the purpose of retaining three phototran
sistors, the base 62 is formed with three side-by-side
compartments 112, 114 and 116. Each of these compart
body 56.
METHOD OF DETERMINING VEHICLE
HEADING
Before describing the electronic circuit 52 of the
ments has an interior wall con?guration which con
compass system, the method for ?nding the heading of
forms with the package con?guration of a phototransis
45 a vehicle will be described with reference to FIG. 11.
tor; the wall con?guration includes a channel 118
adapted to receive the lens of the phototransistor. As
shown in FIGS. 8 and 9, the compartments 112, 114 and
116 receive phototransistors 124, 128 and 134, respec
tively. The channel of compartment 112 is provided
with an aperture or slot 122 for phototransistor 124.
Similarly, the channel of compartment 114 is provided
with a slot 122 for phototransistor 128 and the channel
of compartment 116 is provided with a slot 132 for
phototransistor 134.
The encoder disk 72 is provided with a single code
track 158 of optical apertures 162 for providing angular
displacement information. The apertures are equally
spaced and, in the illustrative example, there are thirty
apertures in the track. In addition to the angular posi
tion encoder track 158, a reference position aperture
164 is provided at a predetermined angular location
relative to the polar axis of the compass magnet 68; in
FIG. 11 illustrates in phantom line, an automotive vehi
cle 210 having a direction reference axis (coinciding
with its longitudinal axis) indicated by the dashed line
arrow 214. The direction reference axis 214 represents
the heading of the vehicle and it is desired to determine
such heading with reference to the true north direction
indicated by the dashed line arrow 216. The vehicle is
equipped with the magnetic compass 50 which includes
a compass magnet on a rotor so that it seeks to orient the
55 polar axis of the magnet with the direction of the local
magnetic ?eld. The compass 50 has its orientation axis
206 in alignment with the direction reference axis 214 of
the vehicle. It will be assumed for purposes of the dis
cussion of the method of determining vehicle heading,
that the magnetic compass installation in the vehicle is
already provided with deviation compensation, by com
pensation coils, to provide a compensating magnetic
?eld which offsets the in?uence of the magnetic envi
ronment. With deviation compensation, the local mag
65 netic ?eld coincides in direction with magnetic north.
pendicular to the polar axis.
The direction of magnetic north in FIG. 11 is indicated
The cover plate 78 comprises a unitary molded cylin
by the solid line arrow 218. As shown in FIG. 11, the
drical member which is adapted to ?t in a snug relation
polar axis of the rotor 70, as represented by a line be
ship with the cylindrical recess on the support body 56.
the illustrative embodiment it is centered on a line per
11
5,131,154
tween the N-S symbols, is in alignment with the direc
tion of magnetic north arrow 218. Thus, the heading of
the vehicle relative to magnetic north is represented by
the angle Hm. As is well known, the direction of the
local magnetic ?eld of the earth departs from the direc
tion of true north by a variation angle which is different
for different geographic locations. As shown in FIG
URE 11, the variation angle between the directions of
magnetic north and true north is represented by the
12
tion in which the polar axis of the rotor is aligned with
the local magnetic ?eld. This turning motion may in
clude some overshoot of the magnetic north direction
but the rotor will settle into the magnetic north position
after both clockwise and counterclockwise motion. The
turning motion of the rotor from the direction reference
axis position is measured on a continuous basis so that
add the variation angle X.
The magnetic heading H,,, is determined by measur
when it ?nally settles in alignment with the magnetic
north direction, thenet angular displacement, i.e. the
combined clockwise and counterclockwise motion,
represents the heading angle of the vehicle relative to
magnetic north. When the turning motion is counter
clockwise, the measured angular value plus the offset
angle is added to zero degrees (value for magnetic
5 north) tov obtain the magnetic heading I-Im. When the
turning motion is counterclockwise, the measured angu
lar value plus the offset angle is subtracted from three
ing the angular displacement Hm between the direction
hundred sixty degrees (value for magnetic north) to
angle X. Consequently, the heading of the vehicle rela
tive to true north is represented by the angle I-I, between
the direction reference axis 214 and the true north direc
tion which is indicated by the arrow 216. In order to
determine the true heading 11,, it is necessary to ?rst
determine the magnetic heading and then algebraically
reference axis 214 and the polar axis of the rotor 70. For
obtain the magnetic heading H,,,. The offset angle Y is _a
this purpose, a reference line 222 is de?ned by a line 20 negative value when the reference aperture 164 leads
between the axis of rotation of the rotor and the refer
the north seeking pole of the polar axis of the compass
ence position aperture 164 on the rotor. This reference
rotor. The offset angle has a positive value when the
line 222 is offset from the polar axis of the rotor by an
reference aperture lags the north seeking pole.
offset angle Y which is ninety degrees in the example of
This measurement of angular displacement may be
the illustrative embodiment. The offset angle Y may be 25 obtained by various means. The preferred means will be
zero or any other value, depending upon the location of
described in detail subsequently. If the measurement
the reference position sensor 106 relative to the direc
indicates a net clockwise angular displacement, it is
tion reference axis of the vehicle. In the example, the
added to the magnetic north direction of zero degrees.
reference position sensor is on the direction reference
If the measurement indicates a net counterclockwise
axis. The rotor is provided with a relative measurement
rotation, it is subtracted from the north direction of
scale, i.e. a scale which includes a plurality of discrete
three hundred sixty degrees to obtain the magnetic
markers which are angularly displaced a predetermined
heading. After the compass is initialized as described
distance from each other but which does not indicate
above, the vehicle heading is measured continuously
the total angular displacement between any one of the
relative to the reference position. After obtaining the
markers and any given reference line or marker. In the
magnetic heading, the true heading may be obtained by
illustrative embodiment, the relative angular measure
adding algerraically the variation angle X to the mag
ment scale on the rotor comprises the plurality of aper
netic heading Hm. The value of the variation angle X is
tures 162 which were described above.
obtained by orienting the vehicle so that the direction
Using the relative measurement scale of the rotor 30
reference axis is aligned with the true north direction
and the reference position sensor 164, the magnetic 40 and reading the measurement of the magnetic heading.
heading Hm is determined as follows. The rotor is
THE ELECTRONIC CIRCUIT
turned between the angular position in which the refer
ence line 222 is in alignment with the direction refer
The electronic circuit for the compass system will be
ence axis 214 and the angular position in which the
described with reference to FIGS. 12A and 12B. The
polar axis of the rotor is aligned with magnetic north 45 electronic circuit 52 comprises a signal processing cir
and the turning motion between these angular positions
cuit 232 shown in FIG. 12A and a compass sensor cir
is measured to obtain the magnetic heading.
cuit 234 shown in FIG. 12B. The sensor circuit 234 is
Preferably, the method is carried out by allowing the
located in the compass housing and is electrically con
rotor 70 to assume an arbitrary angular position relative
nected, through a connector having complementary
to the direction reference axis. Typically, this angular 50 parts 233 and 235, with the signal processing circuit 232
position would be that in which the reference axis is
which is suitably disposed with a display at a remote
aligned with magnetic north; however, the method
location in the vehicle. The signal processing circuit 232
allows for the possibility that the rotor may be in any
includes a microcomputer 236. In addition, it comprises,
angular position. Then, the rotor is turned from such
in general, a power supply circuit 238, a reset circuit 242
arbitrary angular position relative to the direction refer
for the microcomputer, and an electrically erasable
ence axis through a suf?cient angle for the reference
programmable read-only memory (EEPROM) 244 for
aperture 164 to reach the reference position sensor 106
storing data to be utilized in the signal processing. The
which, in the example, is aligned with the direction
vehicle heading display 54 is controlled by the mi
reference axis 214. This turning motion may be in either
crocomputer and suitably takes the form of a liquid
direction, clockwise or counterclockwise. (When the
crystal multi-segment alphanumeric character display.
reference line 222 reaches or passes through alignment
The heading direction data signals are supplied from the
with the direction reference axis 214, the compass is said
sensor circuit 234 to the microcomputer 236. The mi
to be "initialized" since that alignment serves as the
crocomputer includes a read-only memory (ROM) 248
reference position for starting measurement of displace
for storing software including the operating program
ment relative thereto.) Then, the rotor is allowed to 65 for the compass system. The microcomputer also in
turn, under the influence of the local magnetic ?eld,
cludes a random access memory (RAM) 252 for use in
between the position in which the reference line is in
performing calculations, for ?ags and registers and
alignment with the direction reference axis to the posi
other functions required for signal processing. The mi
13
5,131,154
crocomputer is operative, under program control, to
process the direction heading data signals from the
sensor circuit 234 and to control the energization of the
display 54 to indicate in abbreviated symbols, the head
14
ing of the vehicle.
The microcomputer 236, in the illustrative embodi
R41 and R40 of the microcomputer. A manually actu
ated switch 276 is serially connected between the power
supply output terminal and the input pin R43 of the .
microcomputer to initiate the execution of deviation
compensation as will be described.
The coil circuit 264 comprises a set of four coils 34N,
ment, is a Hitachi DP-42 microcomputer available from
34S, 32E and 32W which were described above as the
Hitachi Corporation of Japan. The microcomputer re
ceives a regulated supply voltage on pin VCC from the
output terminal of the power supply circuit 238. This
power supply circuit includes an integrated circuit volt
compensating coils. (As shown in FIGS. 5 and 6, the
age regulator and has an input terminal connected to
with the 13+ or battery supply voltage of the vehicle,
east-west compensating coils 32E and 32W are shown
as being mounted on the same support arm on one side
of the rotor and the north-south compensating coils 34N
and 345 are similarly mounted.) The coils 34N and 345
have one terminal connected through a resistor 278 to
suitably through the ignition switch 254 of the vehicle.
the output of the power supply 238. The other terminal
The common terminal of the power supply 238 is con 15 of coil 34N is connected to ground through the collec
nected directly to chassis ground.
tor and emitter of the switching transistor 282. The base
The reset circuit 242 is connected between the reset
of the transistor 282 is connected through a resistor to
pin of the microcomputer and the pin VCC of the mi
pin D7 of the microcomputer 236. The other terminal of
coil 345 is connected to ground through the collector
momentary logic high signal to the reset pin each time 20 and emitter of a switching transistor 284 which has its
the microcomputer is powered-up to reset the mi
base connected through a resistor to pin D6. In a similar
crocomputer circuits.
manner, coils 32E and 32W have one terminal con
The EEPROM 244 is utilized for storage of certain
nected to the output of power supply 238 through a
data for use by the microcomputer. In particular, the
resistor 285. The other terminals of the coils are con
value of the variation compensation is stored in the 25 nected to ground, respectively, through switching tran
EEPROM. The voltage supply terminal VCC of the
sistors 286 and 288 which have their bases connected
EEPROM is connected directly with the output termi
through respective resistors with pins D5 and D4. A
nal of the power supply and the ground terminal GRD
protective diode is connected in parallel with each of
is connected directly with the ground terminal of the
the coils. This circuit permits selective energization of
voltage regulator. The data pins D0 and D1 of the BE 30 each of the coils individually under control of the mi
PROM are connected directly with the correspond
crocomputer 252 to provide a predetermined compen
ingly designated pins of the microcomputer. The clock
sating current for deviation compensation. A variable
pin CLK is connected directly with the pin D2 of the
compensating current is provided for each coil by pulse
microcomputer and the pin CS of the EEPROM is
width modulated (PWM) DC pulses controlled by the
crocomputer. The reset circuit is operative to apply a
connected directly with the pin D3 of the microcom 35 microcomputer 236 at the respective output pins D4,
puter.
D5, D6 and D7. The pulse width is variable to provide
The display 246 is a liquid crystal display of the multi
a variable duty cycle ranging from a minimum duty
segment type for alphanumeric characters. Supply volt
cycle, the duty cycle being referred to herein as a PWM
age for the display 246 is provided by the power supply
count to a maximum duty cycle or PWM count. Addi
238. The six signal input terminals of the display 54 are
tionally, the four coils may be energized sequentially to
connected respectively with output pins R01 through
provide for swinging of the compass rotor to initialize
R3 and R10 through R13 of the microcomputer 314.
the compass as described as above.
A clock circuit 256 is connected between the clock '
pins 05C] and OSCZ of the microcomputer 236. Other
pin connections for the microcomputer will be de 45
scribed in conjunction with the compass sensor circuit
212 which will be described presently.
The compass sensor circuit 234, as mentioned above,
OPERATION OF THE OPTICAL ENCODER"
The operation of the optical encoder will now be
described with reference to FIGS. 10, 13 and 14. For
purposes of explanation, it will be assumed that the
encoder disk 72 is in an angular position such that the
reference line 222 thereof is in alignment with the direc
tion reference axis 214. The reference line 222 is merely
is located in the housing of the compass 50. In general,
it comprises an optical transducer circuit 262 (of optical
transducer 74 of FIGS. 5 and 6), and a coil circuit 264
which is used for both deviation compensation and
rotor swinging.
it is de?ned by the location of the reference aperture
164 relative to the axis of rotation of the disk. With the
The optical transducer circuit 262 comprises the pair
encoder disk 72 in this position, the reference position
a direction pointer, not a physical or structural line, and
of displacement sensors 102 and 104 and the reference 55 sensor 106 produces a reference pulse which signi?es
position sensor 106. The displacement sensor 102 com
that the encoder disk is in its “home” or reference posi
prises the LED 192 and the phototransistor 124 which
tion. With the encoder disk in the reference position, the
are optically coupled together as previously described.
apertures 162 thereof are positioned relative to the dis
The displacement sensor 104 comprises the LED 196
placement sensors 102 and 104 as indicated in FIG. 10.
and the photo transducer 134 which are optically cou
The code track 158 comprises a cyclically repeating
pled with each other. Similarly, the reference position
sensor 106 comprises LED 194 and phototransistor 128
which are optically coupled, as previously described.
pattern around the circumference of the disk 72 of alter
nate apertures 162 and opaque sectors of equal angular
width. Thus, an aperture 162 and an adjoining opaque
The LEDs are energized continuously by the regulated
sector may be considered to be one cycle in the pattern.
voltage from power supply 238 through a resistor 274. 65 The sensors 102 and 104 have an optical beam width
The phototransistors have their collectors connected
which is effectively one-half the angular width of each
with the output of the power supply 238 and their emit
of the apertures 162 and hence, one-half the width of the
ters connected respectively with the input pins R42,
opaque sector between adjacent apertures. With this
15
5,131,154
relationship, the sensors 102 and 104 are spaced from
each other an angular distance corresponding to an
integral number of cycles plus one-fourth cycle. This
spacing of the sensors results in the generation of a pulse
train by sensor 102 which is phase displaced from the
pulse train generated by the sensor 104. More particu
larly, the pulse train from sensor 102 will lag behind the
pulse train from sensor 104 when the encoder disk 72 is
rotated in the clockwise direction and it will lead the
pulse train from sensor 104 when the disk is rotated in
the counterclockwise direction. The phase difference is
one quarter cycle. This phase displacement is indicative
of the direction of rotation of the encoder disk.
FIG. 13 is a graphical representation of the pulse
signals generated by the sensors 106, 102 and 104. In this
graphical representation, the abscissa axis represents
angular displacement of the encoder disk 72 and the
ordinate axis represents relative signal amplitude from
the respective sensors. For explanatory purposes the
pulses of the pulse trains, as shown in FIG. 13, are ideal
ized as rectangular pulses. The pulse trains 302 and 304
are each alternately high for one-half cycle and low for
one-half cycle. The logical pulses are each one quarter
cycle wide. The reference pulse 306 is at a logical low
16
noted that this would be the magnetic heading indicated
by the compass if the encoder disk remained in the
reference position. This value of magnetic heading is
obtained, as described above, by subtracting the offset
angle Y of ninety degrees from the measured angular
value of three hundred sixty degrees (corresponding to
a count of one hundred twenty). As indicated in FIG.
14, the logic signal will remain at l—l over an angular
range of three degrees spanning from two hundred
sixty-eight and one-half degrees to two hundred seven
ty-one and one-half degrees. If the encoder disk is ro
tated clockwise past two hundred seventy-one and one
half degrees, the logic signal will change to l—O and
remain at that value until it reaches an angular displace
ment of two hundred seventy-four and one-half de
grees. When the displacement angle reaches that value,
the logic signal will change to 0—0 and remain at that
value until angular displacement is two hundred seven
ty-seven and one-half degrees. At that value the signal
will change to 0—-1 and remains at that value until the
angular displacement is two hundred eighty and one
half degrees. Then, the sequence of logic signal values is
repeated. For counterclockwise motion, the sequence
of values of the two bit logic signals is l—l, 0—l, 0—0
value or “0“ when the encoder disk is in the reference
and l—O as shown in the table of FIG. 14 for corre
position shown. The pulse train 302 produced by the
sponding angular increments of displacement. This se
quence of logic signal values is repeated for continued
rotation. With the encoder disk 34 in any angular posi
displacement sensor 102 is at a logical high or “1" and
the pulse train produced by the sensor 104 is also at a
logical high or "I" in the reference position of the en
coder disk 72. When the encoder disk is rotated from
the reference position in a clockwise direction, the dis
tion, its direction of rotation from that position can be
placement sensor 102 will produce logic signals, as
represented by the pulse train 302, in the sequence of
determined by comparing the two bit logic signal at its
current value with the previous logic signal value. For
example, with the encoder disk in the reference posi
tion, the logic signal is l—l. If the next logic signal is
l-l-O-O as indicated by that notation on FIG. 13. When
l—O the rotation is clockwise and if it is 0—1 the rota
it is rotated in the counterclockwise direction, the logic 35 tion is counterclockwise. In the illustrative embodi
signal will have the sequence of l-OaO-l. At the same
time, the sensor 104 will produce a logic signal having
ment, there are thirty apertures 162 in the code track.
This, as can be seen from the table of FIG. 14, provides
the sequence of l-0~0-l for clockwise rotation and the
sequence of l-l-O-O for the counterclockwise direction.
quently, the number of discrete angular positions which
for an angular resolution of three degrees. Conse
The displacement angle from zero degrees (or three
hundred sixty degrees) is indicated on the lowermost
abscissa axis of FIG. 13. The logic signals in the pulse
the reference position may be designated by a number
trains 302 and 304 which are read simultaneously by the
ranging from zero to one hundred nineteen with a dif
can be determined for the encoder disk 34 is one hun
dred twenty. Therefore, the angular displacement from
sensors 102 and 104 are combined as a pair, as shown in
ferent number for each three degree increment. The
the table of FIG. 14, for each angular increment of 45 reference position is designated either as a count of zero
displacement.
or the equivalent count of one hundred twenty. The
FIG. 14 shows the two bit logic signals produced by
angular position measured from the reference position
displacement sensors 102 and 104 as a function of angu—
may be designated as the cumulative count of the
lar position or displacement. For purposes of explana
changes in the two bit logic signal values, taking into
tion, it will be assumed that the encoder disk 72 has been
swung by the coil circuit 264 to the reference position,
as shown in FIG. 10. In this position, the logic signal is
l—l. When the coil circuit 264 is no longer energized,
the encoder disk will be free to rotate under the in?u
ence of the local magnetic ?eld so that the north seeking
account the direction of rotation as being either clock
wise or counterclockwise. FIG. 14 shows in the clock
wise displacement count column the cumulative count
shows the cumulative count for each position for coun
pole thereof will become aligned with the magnetic
terclockwise rotation. Therefore by starting the count
north direction. With reference to FIG. 14, it is noted
that the displacement count with the encoder disk in the
at zero or one hundred twenty and incrementing it by
for each angular position for clockwise direction and
the counterclockwise displacement count column
one for each change of logic signal value in the clock
reference position is taken as a value of one hundred
wise direction and decrementing it by one for each
twenty or a value of zero depending upon whether the 60 change in the counterclockwise direction, the net or
rotation away from the reference position is in the
clockwise or counterclockwise direction. It is further
noted that with the encoder disk in the reference posi
tion, the angular position in degrees, as indicated in
FIG. 14, is in the range of two hundred sixty-eight and 65
one-half degrees to two hundred seventy-one and one
half degrees. This corresponds nominally with the di
rection of two hundred seventy degrees, it being further
cumulative count will represent the unique angular
displacement or position of the encoder disk 72 relative
to the reference position.
DEVIATION COMPENSATION OF THE
PARTICULAR EMBODIMENT
In the particular embodiment of the compass system
described above, the optical encoder for measuring