Absolute encoder avoiding magnetic interference

A correcting signal generator 42 stores the quantitative relation of the magnetic interference between the rotation angle .theta.3 of a resolver 23 and the detected values A3, B3 of a resolver 22, and outputs the correcting values A8, B8 for the magnetic interference according to the rotation angle .theta.3. An adder 41 calculates and outputs the corrected detection values A4, B4 by adding the correcting values A8, B8 to the detected values A3, B3 of the resolver 22 output from a latch circuit 14. A resolver rotation angle calculation circuit 47 calculates and outputs an angle .theta.2 by taking an arc tangent of the quotient A4/B4 of the corrected detection values A4, B4. In the same way, a correcting signal generator 44 calculates and outputs the angle .theta.3.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an absolute encoder for detecting a 
rotation angle of a rotary shaft over a plurality of rotations, more 
particularly to an absolute encoder utilizing reluctance. 
2. Description of the Prior Art 
It is known that an absolute encoder can detect a rotation angle of a 
rotary shaft over a plurality of, for example, several hundred rotations. 
The principle of this detection is to detect the rotation angle of the 
rotary shaft on the basis of the angles detected by the resolvers attached 
to the rotary shaft and the other slowed-down rotary shafts for detecting 
rotation angles during one rotation of the respective rotary shafts. 
In conventional absolute encoders, a plurality of resolvers are used and 
magnetic interference among these resolvers is a problem. That is to say, 
because a magnetic field generated by a detecting resolver reaches 
adjacent resolvers, inductance change depending on the rotation angles of 
other resolvers is added to the inductance change in the detecting 
resolver itself. Consequently, the detected inductance change actually 
corresponds to the resultant inductance change. Therefore, satisfactory 
detecting accuracy can not be obtained. In order to avoid the magnetic 
interference, it is necessary to arrange the respective resolvers apart at 
such a distance that the magnetic interference does not occur which 
results in the problem that the equipment becomes very large. 
SUMMARY OF THE INVENTION 
The present invention solves this problem. An object of the present 
invention is to provide a small size absolute encoder by reducing the 
distance between the respective resolvers through elimination of the 
effect of the inductance change due to other resolvers. 
In order to solve the problem, an absolute encoder according to the present 
invention detects the long-range rotation angles over a plurality of 
rotations of at least one rotary shaft among a plurality of rotary shafts 
on the basis of the angles detected by a plurality of the resolvers which 
are respectively attached to the respective rotary shafts having 
predetermined speed ratios. This encoder comprises a correcting signal 
generator for generating a correcting signal eliminating the signal 
components depending on other resolvers with respect to at least one 
resolver, an adder for obtaining a corrected detection signal by adding 
the correcting signal concerning this resolver to the signal detected by 
this resolver, a resolver rotation angle calculation circuit for 
calculating the rotation angle of this resolver on the basis of the 
corrected detection signal, and a long-range rotation angle calculating 
circuit for calculating the absolute rotation angles covering a plurality 
of rotations of the rotary shafts on the basis of the output from a 
plurality of the resolver rotation angle calculation circuits. 
According to this configuration, it is possible to eliminate the effects of 
the inductance change generated by a resolver other than those to be used 
for detecting the rotation angle of the rotary shaft, to reduce the 
distances among a plurality of the resolvers, and to miniaturize the outer 
shape of the equipment. 
Further, it is also possible to concentrically arrange a plurality of the 
resolvers concerning the calculation of the corrected detection signals. 
With such an arrangement, it is possible to simplify the effects of one 
resolver on other resolvers and thereby simplify the generation of the 
correcting signals. 
Further, it is also possible to generate correcting signals on the basis of 
the signals detected by at least one adjacent resolver. Because only the 
magnetic interference between adjacent resolvers is taken into 
consideration, the generation of the correcting signals can be simplified. 
Further, it is also possible to generate the correcting signals by reading 
out according to the rotation angle correcting signals stored beforehand 
according to the rotation angles of the adjacent resolvers. In this 
method, it is not necessary to calculate the correcting signals each time 
the rotation angles are detected, and circuit load for calculating 
correcting signals can be reduced.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred embodiment of the absolute encoder according to the present 
invention will now be described with reference to the accompanying 
drawings. 
FIG. 1 shows the structure of the absolute encoder using three resolvers 
having different rotational speeds. FIG. 2 shows a cross section of one 
resolver. 
An input shaft 19 of the absolute encoder is mounted in a casing 81 via a 
bearing 90. A non-magnetic substance gear 25 and a rotor 60 are fitted on 
the input shaft 19. A shaft 20 is mounted in casings 82 and 84 via 
bearings 91 and 92. Non-magnetic substance gears 26, 27, 29 are fitted on 
the shaft 20. An axle 80 is supported by the casings 82 and 84. A 
non-magnetic substance gear 28 and a rotor 62 are mounted on the axle 80 
via a bearing 93. A non-magnetic substance gear 30 and a rotor 64 are 
mounted on the axle 80 via a bearing 94. 
A reduction gear mechanism is formed by engaging the gear 25 with the gear 
26, the gear 27 with the gear 28, and the gear 29 with the gear 30. The 
gear 28 is fixed to the rotor 62, and the gear 30 to the rotor 64. 
According to this reduction gear mechanism, the rotor 62 rotates by one 
revolution during 24 revolutions of the input shaft 19, and the rotor 64 
rotates by one revolution during 25 revolutions of the input shaft 19. 
Stators 61, 63, 65 are fixed to the casings 81, 82, 83, 84. The stators 61, 
63, 65 and the rotors 60, 62, 64 are all made of magnetic substance. The 
rotors 60, 62, 64 have cylindrical shapes, and eccentrically rotate about 
their rotational axes. As shown in FIG. 2, each resolver has four tooth 
poles 101, 102, 103, 104 on each stator. Windings 111, 112, 113, 114 are 
wound around these tooth poles. The rotational position of the rotor can 
be detected by sensing the change of inductance of each winding caused by 
the change of the air gap between the rotor and the tooth pole of the 
stator caused by the rotation of the rotor. 
FIG. 3 shows the circuit configuration of the absolute encoder of this 
embodiment. The same elements as those shown in FIGS. 1 and 2 have the 
same numerals as those in FIGS. 1 and 2, and explanation for them are 
omitted here. The resolvers 21, 22, and 23 shown in FIG. 3 correspond to 
the resolvers composed of the pairs of the stator 61 and the rotor 60, the 
stator 63 and the rotor 62, and the stator 65 and the rotor 64 
respectively. 
One ends of the four windings of each winding part are all connected to a 
pulse excitation circuit 52, and the other ends of each of the four 
windings are all connected to each of the four anodes of diode arrays 1, 
2, 3. Each cathode of the diode arrays 1, 2, 3 is connected to each of 
current detecting resistors 5, 6, 7, 8. The pulse excitation circuit 52 
performs the pulse excitation for the winding parts 70, 72, 74 of the 
respective resolvers 21, 22, 23 by pulse excitation signals Pex 1, Pex 2, 
Pex 3. Timing of these pulse excitation signals is controlled by the 
timing signals P1, P2, P3 output from a timing controller 51. Change of 
the inductance of the respective windings of the excited winding parts is 
detected by measuring the current of the windings by means of detecting 
resistors 5.about.8 at a certain time later from the excitation. In 
addition, differential current between two windings are detected by 
differential amplifiers 9 and 10. Signals A and B having the amplitude 
proportional to a sine and a cosine of the rotational angle of the rotor 
are detected. The signals A and B are digitized by A/D converters 11 and 
12 on the basis of a conversion start signal to output from the timing 
controller 51, and converted into a sine wave signal A1 and a cosine wave 
signal B1. 
The sine wave signal A1 and the cosine wave signal B1 are stored into a 
latch circuit 13 by a timing signal t1 synchronized to the excitation 
signal Pex 1, as a detected value of the resolver 21. Similarly, the sine 
wave signal A1 and the cosine wave signal B1 are stored into a latch 
circuit 14 by a timing signal t2 synchronized to the excitation signal Pex 
2, as a detected value of the resolver 22, and the sine wave signal A1 and 
the cosine wave signal B1 are stored into a latch circuit 15 by a timing 
signal t3 synchronized to the excitation signal Pex 3, as a detected value 
of the resolver 23. Thus, the resolver which sent the sine wave signal A1 
and the cosine wave signal B1 can be identified by the timing signal t1, 
t2, t3 output from the timing controller 51. The resolver rotation angle 
calculation circuit 40 calculates and outputs the rotation angle .theta.1 
of the resolver 21 by taking an arc tangent of the quotient A2/B2 of the 
detected values A2 and B2 of the resolver 21 output from the latch circuit 
13. 
In this embodiment, the distance between the rotor 62 and the rotor 64 
shown in FIG. 1 is taken as short as possible. For this reason, the 
resolver 22 composed of the rotor 62, the stator 64, and the winding 72 
interferes magnetically with the resolver 23 composed of the rotor 64, the 
stator 65, and the winding 74. 
The relations between the rotation angle .theta.2 of the detecting resolver 
22 and the detected values A3, and B3 are shown in FIGS. 4 and 5. The 
relations are given by the following Eqs. (1) and (2), 
EQU A3=G.multidot.sin (.theta.2)+K.multidot.sin (.theta.3 ), (1) 
EQU B3=G.multidot.cos (.theta.2)+K.multidot.cos (.theta.3 ), (2) 
where G is a coefficient showing the rate of inductance change, and K is a 
coefficient showing the rate of magnetic interference. A3 and B3 are the 
sum of the signal of the detecting resolver alone and the signal component 
depending on the rotation angle .theta.3 of the adjacent resolver 23. The 
coefficients G and K depend on specifications of the encoder such as the 
arrangement of each resolver, and have values peculiar to each encoder. 
The coefficient K is stored in a ROM 42a in a correcting signal generator 
42. 
Accordingly, the correcting signal generator 42 executes the calculation of 
Eqs. (3) and (4) on the basis of the angle .theta.3 of the resolver 22, 
and outputs the following correcting values A8 and B8 which show the 
amounts of the magnetic interference contained in the detected values A3 
and B3 of the resolver 22, 
EQU A8=-K.multidot.sin (.theta.3), (3) 
EQU B8=-K.multidot.cos (.theta.3). (4) 
An adder 41 calculates and outputs the detected values A4 and B4 corrected 
by adding the correcting values A8 and B8 to the detected values A3 and B3 
of the resolver 22 output from a latch circuit 14. A resolver rotation 
angle calculation circuit 47 calculates and outputs the angle .theta.2 by 
taking an arc tangent of the quotient A4/B4 of the corrected detection 
values A4 and B4. 
In the same way, a correcting signal generator 44 executes the calculation 
of the following Eqs. (5) and (6) on the basis of the angle .theta.2 of 
the resolver 22, and outputs the following correcting values A7 and B7 
which show the amounts of the magnetic interference contained in the 
detected values A5 and B5 of the resolver 23, 
EQU A7=-K.multidot.sin (.theta.2), (5) 
EQU B7=-K.multidot.cos (.theta.2). (6) 
The coefficient K is stored in a ROM 44a in a correcting signal generator 
44. An adder 43 calculates and outputs the detected values A6 and B6 
corrected by adding the correcting values A7 and B7 to the detected values 
A5 and B5 of the resolver 23 output from a latch circuit 15. A resolver 
rotation angle calculation circuit 48 calculates and outputs the angle 
.theta.3 by taking an arc tangent of the quotient A6/B6 of the corrected 
detection values A6 and B6. The angles .theta.1, .theta.2, .theta.3 are 
represented by integers within 0 to 255. An angle .theta.4 is calculated 
by the following Eqs. (7), (8), (9) using a long-range rotation angle 
calculating circuit 50 for the given values of the rotation angles 
.theta.1, .theta.2, .theta.3, 
EQU .theta.2'=.theta.1+256.multidot.{(24.multidot..theta.2-.theta.1+128)/256},( 
7) 
when 
EQU .theta.2'-25.multidot..theta.3.ltoreq.0, .theta.4=.theta.2'+6 
144.multidot.{(.theta.2'-25.multidot..theta.3+128)/256}, (8) 
and when 
EQU .theta.2'-25.multidot..theta.3&lt;0, .theta.4=.theta.2'+6 
144.multidot.{(.theta.2'-25.multidot..theta.3+6 528)/256}.(9) 
Numerical calculation in Eqs. (7), (8), and (9) should be executed with 
integer values. Thus, the rotation angle .theta.4 can be represented by 
integers within 0 to 153599 in the range of up to 600 revolutions of the 
input shaft 19. 
The magnetic interference depends on the relative position between adjacent 
resolvers. Accordingly, it may also be satisfactory to measure the amount 
of the magnetic interference between the angle .theta.3 of the resolver 23 
and the detected values A3, B3 of the resolver 22 in a manufacturing stage 
of this absolute encoder, and store the measured amount into the ROM 42a. 
For the correcting signal generator 44, the situation is the same as that 
in the correcting signal generator 42. Numerical values of a sine wave 
table having an amplitude corresponding to the level of the magnetic 
interference may be used as numerals to be stored into the ROMs 42a and 
44a. In this case, the correcting values A8 and B8 should be output so as 
to be 90.degree. out of phase with each other. 
In the above embodiment, correction between the detected signals of the 
resolvers 22 and 23 arranged in close proximity is conducted. It is also 
possible to take into consideration the interference between other 
resolvers according to desired accuracy.