Angle transducer unit of the multiturn measuring type for absolute measurement of the angular position of a rotatable shaft. A first transducer (10) of the absolute angular measurement type with a drive shaft (3) is non-rotatably mounted on the rotatable shaft. A second angle transducer (11) of the absolute angular measurement type has a driven shaft (4). There are gear wheels (1,2) mounted on the drive and driven shafts, these gear wheels being mutually in mesh to form a gear for up or down gearing of the rotational rate of the drive shaft. The two angle transducers, e.g. resolvers, are intended for being conventionally connected to an apparatus for measuring the differential angular position of the shafts. Novel to the angle transducer unit is that the gear wheels include an internally toothed gear wheel (2) and an externally toothed gear wheel (1). The latter rolls on the internally toothed gear wheel (2) and has a number of teeth which is only some few teeth, preferably only one tooth less than the number of teeth of the internally toothed gear wheel, and is mounted on one of the shafts 3 and 4. The internally toothed gear wheel (2) is mounted on the other shaft. Cylindrical housing parts are disposed for mounting the shafts mutually eccentric.

The present invention relates to angle transducers of the multiturn type, 
where the absolute angular position of a shaft which can rotate is 
measured, i.e. the total number of revolutions plus an angle less than 
360.degree. is measured from a fixed initial, reference position of the 
shaft. 
For reasons of safety, angle transducers of the kind mentioned above are 
coming into greater and greater use with industrial robots. The reasons 
for this is that should a current failure occur it is essential to keep 
track of the absolute angular positions of the respective rotating shafts 
in the robot, because when power returns the robot must continue its 
motion from where it left off when current failed. In other words, a 
current failure must not mean that the respective angle transducers record 
zero values, since this would mean that when current returns the 
respective shafts in it would start from their respective zero angular 
positions, which could give the robot a completely incorrect motion and 
could mean danger to life for persons in the vicinity of the robot. Such 
undesired zero setting of angular position occurs if the respective angle 
transducer does not measure the shaft angle absolutely but only relative 
to the angle at which the shaft happens to stop. 
Angle transducers of the kind mentioned are known, and usually comprise two 
transducers, e.g. resolvers or electro-optical transducers arranged side 
by side, and mutually connected via a spur gear. One of the transducers is 
connected to a rotating shaft of the robot. The gear comprises two gear 
wheels, one of which has 64 teeth, for example, and the other 65 teeth. 
The differential rotation of the resolvers enables absolute measurement of 
the total angle passed through by the rotating shaft. Any angular position 
of the two resolvers corresponds to an unambiguously determined total 
turning angle of the rotating shaft. The differential turning angle is 
determined by conventional electrical circuits, which may be of the 
phase-sensitive type or the amplitude sensitive type. 
The angle transducer unit described in the last paragraph has plurality of 
disadvantages. The transducers in it are situated side by side and are 
mutually connected to the gear, resulting in that the cylindrical housing 
into which the whole transducer unit is usually built has a large 
diameter. In certain applications, the space taken up by the transducer 
housing on the machine detail where the rotating shaft is situated is 
disadvantageously large. The gear, which solely comprises two externally 
toothed spur gears in mesh, is not free from backlash. To prevent this 
backlash, one of the gear wheels is provided with a springbiased gear 
wheel which urges the gear wheels in the gear into mesh with each other 
during all rotational phases of the shaft. The teeth of the gear are 
subjected to large loads due to the large moment of inertia of the driven 
gear wheel and the high momentary accelerations occuring for rapid 
positional changes, or when the rotating shaft changes its direction of 
rotation. Such large accelerations cause wear on the teeth after a while. 
Only one tooth on one gear wheel is simultaneously in engagement with a 
tooth on the other gear wheel of the gear, resulting in that only these 
two teeth take up the acceleration stresses. This causes wear on the teeth 
of the gear. The meshing factor of a conventional spur gear is normally 
about 1.6-2. Both gear wheels in a conventional spur gear rotate at high 
absolute speed and the mutual, relative tooth speed is very high. The high 
relative tooth speed causes wear on the teeth. All this reduces the life 
of the known angle transducer. Due to the high tooth speeds and thereby 
the high centrifugal force, the gear lubricant will be urged away from the 
teeth. To obtain good, durable lubrication it is necessary for the gear 
wheels to be disposed in an oil bath. 
The present invention has the object of providing an angle transducer of 
the kind mentioned in the introduction, which, while avoiding the above 
mentioned disadvantages has long life, and for this purpose has a gear 
with small moment of inertia, low relative tooth speed and a high meshing 
factor as well as the least possible backlash without using spring biased 
bear wheels. 
The characterizing features of the invention are disclosed in the 
accompanying claims. 
From a study of the claims it will be understood that since the externally 
toothed gear wheel only has few teeth, preferably only one tooth, less 
than the internally toothed gear wheel there is obtained: 
a) in the case where the internally toothed gear wheel is mounted on the 
shaft of the driving transducer and the externally toothed gear wheel on 
the shaft of the driven transducer that when the driving shaft has rotated 
one revolution, the driven shaft has rotated one revolution plus the angle 
taken up by the difference in the number of teeth, a small upward gearing 
thus being obtained, 
b) in the case where the externally toothed gear wheel is mounted on the 
driving shaft and the internally toothed gear wheel on the driven shaft, 
that when the rotating shaft has rotated one revolution the driven shaft 
has rotated one revolution minus the angle occupied by the difference in 
the number of teeth. 
If the driving rotational rate is high, both gear wheels do indeed rotate 
at high rotational rates, but the relative tooth speed of the gear wheels 
is very low, more specifically the rotational rate of the driving shaft 
divided by the number of teeth on the gear wheel mounted on the driving 
shaft. Due to this low relative tooth spaced, the wear on the teeth is 
reduced considerably, compared with the gear conventionally used. 
As the externally toothed gear rolls against the internally toothed gear, 
there are a plurality of teeth in mesh simultaneously, i.e. the gear has a 
greater meshing factor, which is about 8. In the inventive gear a large 
number of teeth will thus take up the acceleration stresses in the meshing 
teeth. By distributing the load caused by the acceleration over so many 
teeth, the life of the teeth, and thereby that of the gear is increased. 
By turning the houses relative each other, the backlash can be brought to a 
least possible value, which is advantageous from the manufacturing aspect, 
since the teeth do not need to be mashined with particularly tight 
tolerances. The situation is actually the reverse, i.e. rather large 
tolerances can be permitted and these can be compensated by turning the 
houses relative each other so that the externally toothed gear wheel comes 
into a desired degree of engagement with the internally toothed gear 
wheel. There is thus avoided the use of separate, spring biased gear 
wheels. 
Due to the end-on-end mounting of the angle transducers, the amount of 
space taken up by the transducer units on the machine detail where the 
output shaft is situated is reduced. In other words, the inventive 
apparatus requires extension in length rather that in width.

It will be seen from FIG. 1 that the apparatus in accordance with the 
present invention includes an externally toothed gear wheel 1, an 
internally toothed gear wheel 2, a drive shaft 3, a driven shaft 4, a 
first housing 5, a second housing 6 and two cover plates 7 and 8. 
The externally toothed gear wheel 1 has 63 teeth and the internally toothed 
gear wheel 2 has 64 teeth 9 against which the externally toothed gear 
wheel 1 is to mesh. 
The gear wheels are fabricated in the manner described in my Swedish patent 
application 87 00291-1. The units 1-6 form a gear of a nature such that 
when the gear drive shaft 3 has rotated 64 revolutions its driven shaft 4 
has rotated 63 revolutions. The drive shaft 3 of the gear is a part of an 
angle transducer 10 of conventional kind, e.g. a resolver with a 
cylindrical housing. The shaft 3 is preferably straight through and the 
part of its projecting out to the left from the housing in FIG. 1 is 
intended to be non-rotatably connected to the unillustrated rotating shaft 
which is to have its absolute angular position determined. In the same 
way, the driven shaft 4 of the gear is a shaft in an angle transducer 11, 
e.g. a resolver. This transducer 11 also has a cylindrical housing and is 
intended for fitting into the other housing 6. 
Booth housings 5 and 6 are in the form of cylindrical sleeves. In the first 
cylindrical housing 5 there is a cylindrical shoulder 12, defining the 
depth to which the first transducer 10 can be thrust into the first 
housing 5. In the second housing 6, there is correspondingly a cylindrical 
shoulder 13 defining the depth to which the cylindrical transducer 11 can 
be thrust. Both transducers are fixed non-rotatably in the respective 
housing 5, 6 e.g. by glue or cement. FIG. 2 illustrates the step in 
assembly where the transducer 10 is fitted into the housing 5, the cover 
plate 7 placed over the drive shaft of the gear, subsequent to which the 
externally toothed gear wheel is non-rotatably attached, e.g. by glue, on 
the shaft 3. FIG. 3 illustrates how the transducer 11 is fitted into the 
other housing 6, after which the cover plate 10 is placed over the driven 
shaft 4 of the gear and finally the internally toothed gear wheel 2 is 
fixed, e.g. by glue, on the shaft 4. The cover plates 7 and 8 serve as 
barriers to the penetration of glue along the respective shafts 3, 4. The 
gear is lubricated. 
The shafts 3 and 4 are parallel with their centre lines 15a and 15b at 
mutual spacing e, see FIG. 5. This spacing e enables the externally 
toothed gear wheel 1 to roll round the internally toothed gear wheel 2, 
and can thus be regarded as the measure of an eccentric mounting of the 
driven shaft 3, where e is the difference between the pitch radii of the 
gear wheels. As will be seen from FIG. 2, there is a cylindrical bore 14a 
in the housing 5. As will be seen from FIG. 5, this bore 14a is eccentric 
relative the longitudinal symmetry axis 15a of the housing. The 
eccentricity is denoted by f. As will be seen form FIG. 3, there is a 
cylindrically machined surface 14b on one end portion of the housing 6. 
This surface 14b is similarly arranged eccentrically relative the 
longitudinal symmetry axis 15b of this housing 6, as will be seen from 
FIG. 5. The eccentricity is the vectorial sum (magnitude and direction) of 
e and f relative the longitudinal symmetry axis 15b. 
The surface 14b is intended to be a light drive fit in the bore 14a, as 
will be seen from FIG. 4, which illustrates the two transducer parts 10 
(FIG. 2 and 11) in FIG. 3 assembled together such that the gear wheels are 
in mesh and a closed space 16 has been formed, which is axially defined 
partly by the cover plate 7 and partly by the gear wheel 2. The cover 
plate 7 thus prevents departure of the lubricant from the space 16 and 
thus has the double function of preventing penetration of glue and 
departure of lubricant from the space 16 as a result of the action of 
centrifugal force. In turn, this considerably extends the time intervals 
at which the transducer must be lubricated. The cover plate 7 should have 
a radial extension such that it covers the end wall 17 of the internally 
toothed gear wheel 2. Due to the low relative rotational rate of the two 
gear wheels 1 and 2, the cover plate 7 is only subjected to minimum wear. 
By turning the housing 5 and 6 relative to each other, the gear wheels are 
displaced in mutual relationship, due to the eccentrically arranged fit 
14a, 14b, and in this way the backlash between the two gear wheels is 
reduced to a minimum. After adjusting this relative angular relationship, 
the housings 5, 6 are fixed to each other, e.g. with the aid of glue. 
The embodiment of the invention described hereinbefore can be modified in 
many ways and varied within the scope of the inventive concept. Instead of 
the eccentric fit 14a, 14b, which has, inter alia, the function of 
allowing radial displacement of the gear wheels 1 and 2 relative each 
other, a linear displacement of the gear wheels can be carried out with 
the aid of other means. 
Glueing, or cementing has been proposed above as means for fixing the gear 
wheels to the shafts, for fixing the transducers in their respective 
housings and for fixing the housings relative each other, but it will be 
understood that other means, e.g. set screws, can be used. The number of 
teeth has been stated to be 63 and 64 respectively, but it will be 
understood that other values can be used, as well as having a difference 
in the respective number of teeth other than 1, e.g. 2, 3 or more. 
The housing parts 5, 6 have been shown above as separate units, but it will 
be understood that a housing 5' can be integral with the housing of the 
transducer 10', and a housing 6' can be integral with the housing of the 
transducer 11' as depicted in FIG. 6. 
In addition, several apparatuses in accordance with the invention can be 
connected in series for increasing the measuring range. For example, as 
depicted in FIG. 7, an extra angle transducer 11" can be provided. Angle 
transducer 11" includes a gear 2", similar to gear 2, affixed to a driven 
shaft 4" similar to driven shaft 4. In addition, angle transducer 11 
includes a gear 1", similar to gear 1, affixed to a drive shaft 3" similar 
to drive shaft 3. The additional angle transducer 11" is coupled to angle 
transducer 11 through an eccentric means of the type already described 
herein including sleeves 5", 6" which are similar to sleeves 5, 6.