Rotary potentiometer with speed reduction gearing

.Iadd.A rotary potentiometer with speed reduction gearing. A multi-tap closed loop distributed impedance is coupled by a mechanical configuration including a shaft to selected taps on an autotransformer. The mechanical coupling configuration permits the output from the wiper on the closed loop distributed impedance to change as a function of shaft position and to jump from one end of the autotransformer to the other while the shaft is continuously rotated in a given direction. .Iaddend.

This invention relates to precision transducers for use in electrical 
apparatus and, more particularly, to such a transducer having novel 
features especially adaptable to miniaturization. 
A novel precision transducer having great utility in many electrical 
applications is disclosed and claimed in my U.S. Pat. 2,843,822 which 
issued July 15, 1958. The apparatus disclosed therein utilizes the 
nutating motion of an eccentric gear for successively connecting the taps 
of a closed impedance loop across various terminals of a potentiometer or 
autotransformer. A wiper is rotated against the impedance loop for the 
purpose of obtaining a voltage output intermediate successive terminals. 
The precision transducer described in the aforementioned patent has great 
resolution, smoothness, and linearity. Due to its inherent capabilities, 
it is of particular significance in applications involving control systems 
for missiles and aircraft. However, the nature of the mechanical movement 
disclosed in the patent is such that the minimum external diameter is 
limited. Reference to the patent, in particular to FIG. 5 thereof, will 
indicate the reason for the limitation. Three radial tongues are located 
on a gear within a gear and extend outwardly to contact the various 
terminals located circumferentially around the periphery of the outer 
annular gear. The diameter of the gears is practically limited by cost 
considerations. The cost of special gear configurations rapidly becomes 
prohibitive as size is decreased. However, the novel nutating gear 
arrangement offers an extremely favorable speed reduction ratio with an 
absolute minimum of parts. The nutating motion of the inner gear, however, 
although particularly well suited to a switching mechanism of the type 
disclosed in the patent, does not produce rotary motion directly about a 
stationary axis. 
It is, therefore, a primary object of the present invention to provide a 
precision electrical transducer capable of a high degree of 
miniaturization. Other objects are to provide such a transducer utilizing 
a nutating gear for speed reduction purposes; to provide such a transducer 
having a cylindrical segmented commutator portion; and to provide such 
apparatus utilizing rotating bridging means contacting the commutator 
segments.

With particular reference to FIG. 3 it will be seen that the apparatus of 
the invention is enclosed in a shell having a base portion 10 and an upper 
portion 12 held together by a threaded portion 14. Shell base 10 encloses 
an autotransformer 16 on a core 18 embedded in a supporting and insulating 
medium 20. 
Upper portion 12 of the shell encloses a stator-rotor combination. The 
stator comprises a hollow cylinder constructed of three solid electrically 
conductive rings 22 alternately stacked with three segmented rings 24. 
Each of segmented rings 24 is constructed of electrically conductive 
segments 26. Tabs 28 are provided for connecting each ring and segment to 
electrical conductors 30. Rings 22 and 24, as well as segments 26, are 
insulated from one another by insulating material 32. A spacer 34 supports 
one end of the stator on a circumferential shoulder 36. Three holes 38 are 
provided in spacer 34 for three conductors connecting rings 22 with 
element 42. The inner surface of spacer 34 is cut in the shape of an 
annular gear 40 as will be more apparent from an examination of FIG. 5. 
Additional stator elements located within shell 12 are a toroidal 
potentiometer coil 42 and a solid collector ring 44. Coil 42 is designed 
as shown in the cross section of FIG. 3. The outer circumference of the 
coil is substantially cylindrical. The inner circumference, however, has a 
convex arched configuration which causes the individual conductors to lie 
closer together to form a more precise wiping surface. A spacer 46 is 
located within shell base 10 and contains holes 48 to allow the passage of 
wires 30 between autotransformer 16 and segments 26. An annular support 
member 50 is supported by spacer 46 and in turn supports bearing 52 and 
the inner end of shaft 54. The outer end of shaft 54 is supported by 
bearing 56 and thrust collar 58 retained in end cap 60 of shell 12 by 
screw plug 62. 
Shaft 54 rotates freely within bearings 52 and 56 and has fixedly secured 
to its surface a circular eccentric 66 and a rotor element 64 which is 
secured by set screw 65 (FIG. 4). Rotor 64 is spool-shaped as shown in 
FIG. 3. The reduced diameter inner portion 68 between flanges 70 will be 
seen to overlap portions of both coil 42 and collector ring 44. A 
spring-like wiper arm 72 (FIG. 4) is offset from its longitudinal center 
line at each end (see (FIG. 3) to contact both coil 42 and collector ring 
44. The wiper arm is retained on rotor element 64 by means of a pin 74 
extending through element 64 and supporting wiper arm 72 as shown in FIG. 
4. To provide proper gripping action it is essential that the center line 
of pin 74 be positioned radially inward from surface 68 of element 64. 
Rotor element 76 (shown partially cut-away in FIG. 3 and in perspective in 
FIG. 7) has an inner bearing surface and rotates freely on the surface of 
shaft 54. Rotor element 76 has three reduced diameter portions separated 
by flanges 78. Each of the reduced diameter portions bridges a solid ring 
22 and a segmented ring 24. On each of the reduced diameter portions is a 
wiper arm 80 having a configuration similar to wiper arm 72 and secured by 
an elongated pin 82 in the manner in which wiper arm 72 is secured to 
element 64. The offset formed at each end of each of wiper arms 80 causes 
the arm to electrically bridge a solid ring 22 and a segmented ring 24. 
Circular eccentric portion 66 of shaft 54 has an external bearing surface. 
Mounted to rotate on this bearing surface is a spur gear 84 which meshes 
with, but has a smaller diameter than, annular gear 40. The relationship 
of gears 40 and 84 will be more clearly seen by reference to FIG. 5. 
Rectangular grooves 86 (FIG. 7) are cut on the inner surface of gear 84. 
Similar grooves 88 are cut on the outer surface of rotor 76. An annular 
coupling ring 90 is located between gear 84 and rotor 76. Ring 90 is 
provided on one side with rectangular shoulders 92 designed to fit within 
and be easily slidable along grooves 86. Shoulders 94 are provided on the 
opposite side of ring 90 but are positioned approximately 90.degree. from 
shoulders 92. Shoulders 94 are similarly designed to fit within and be 
easily slidable along grooves 88. The center opening of ring 90 is made 
large enough to permit the coupling action described below to take place. 
The size of this opening will be seen to be a function of the ratio of the 
diameter of spur gear 84 to the diameter of annular gear 40. 
Input and output terminals 96 are provided on the base of the apparatus as 
shown in FIGS. 1 and 3. 
Enclosed within shell 12 is a resistor 98. Resistor 98 is connected between 
one end tap of transformer 16 and one segment 26 in each of segmented 
rings 24. 
It will be seen that turning shaft 54 will cause direct rotation of rotor 
64 and its wiper arm 72. Since the shaft and the rotor are unitary, the 
wiper will make a complete traversal of potentiometer coil 42 with each 
turn of the shaft. Since arm 72 contacts both coil 42 and collector ring 
44, the collector will be seen to be electrically connected to successive 
points along coil 42. Collector ring 44 is, in turn, connected to one of 
output terminals 96, as will be apparent from FIG. 8. 
Eccentric element 66 is similarly fixed to shaft 54 so that it is rotated 
about the shaft once for each revolution of the shaft. As described above, 
spur gear 84 is rotatable about eccentric 66 and meshes within annular 
gear 40 as shown in FIG. 5. Rotation of shaft 54 causes gear 84 to roll 
about the inner surface of gear 40. Inspection of FIG. 5 will indicate 
that as shaft 54 rotates in one direction, gear 84 will rotate slowly in 
an opposite direction and advance one tooth pitch. Thus a gear ratio of 
12.1 (in the illustrated embodiment) is simply and easily achieved. It 
will also be apparent that the rotation of gear 84 is not concentric about 
the axis of shaft 54. In order to convert this motion to rotation 
concentric about shaft 54, a coupler of the type shown in FIG. 7 is 
employed. Since rotor 76 rotates about shaft 54 as on a bearing, it is 
constrained to rotate about the axis of shaft 54. Element 90 "floats" 
freely within grooves 86 and 88 and changes the speed-reduced, eccentric 
rotation of gear 84 to a concentric rotation at the same speed. 
The electrical operation of this apparatus will be more apparent from FIGS. 
8, 9, and 10. An autotransformer 16 is divided into 32 equal segments by a 
total of 33 taps having conductors numbered consecutively 1'-32' and 36'. 
Each of these conductors is connected to a correspondingly numbered 
commutator segment 1"-32" and 36". In addition, three segments numbered 
33", 34", and 35" are connected by conductors 33', 34' and 35' to resistor 
98. 
Input terminals 96a and 96c are provided one tap removed from each end of 
transformer 16. A center tap 96b is also provided. Output terminal 96d is 
connected to collector ring 44. 
In the illustration of FIG. 8, wiper arm 72 is rotating in a clockwise 
direction about potentiometer 42. Elements 80a, 80b, and 80c are moved 
downwardly (counterclockwise) at a slower rate than wiper 72. At the 
instant illustrated, tap c of potentiometer 42 is connected through slip 
ring 22c, wiper 80c, and setment 2" to transformer tap conductor 2'. Tap a 
is similarly connected to transformer tap conductor 3'. Tap b is in the 
act of switching from transformer tap 1' to 4'. Electrically, therefore, 
wiper 72 is at the midpoint of a potentiometer connected between taps 2' 
and 3' of autotransformer 16. This circuit is illustrated in the enlarged 
schematic of FIG. 9. As wiper 72 continues beyond the midpoint of 
potentiometer c-a, wiper 80b will slide onto segment 4", thus connecting 
tap b to 4' as shown in FIG. 10. The circuit of FIG. 10 will remain until 
wiper 72 passes the midpoint of potentiometer a-b when the transfer of 
wiper 80c from segment 2' to segment 5' will repeat the cycle. 
In order to preserve the slope of output voltage versus shaft rotation at 
the extreme ends of the autotransformer, tap conductors 36' and 32' are 
provided externally of input terminals 96a and 96b. It is also to be noted 
that if there were exactly the same number of segments as transformer 
taps, each of wipers 80a and 80c would have to switch nearly full line 
voltage at some point of its travel. For example, if segment 35" did not 
exist, full line voltage would exist between adjacent segments 32" and 2" 
which could create switching problems. By inserting segments 33", 34", and 
35", not connected to transformer taps, neutral position is provided for 
each wiper element prior to starting a new sequence. 
The mere insertion of segments 33", 34", and 35" is of great help in 
avoiding the problem of switching full line voltage. However, without 
more, segments 33", 34" and 35" are not tied down to a specific ground and 
will tend to "float" without control. In order to prevent this, a resistor 
98 is provided between the end tap of the autotransformer and each of the 
additional segments in order to tie them to a specific reference. In the 
described embosiment, this resistance has a value of 100,000 ohms. 
In the illustrated embodiment, the gear ratio between shaft 54 and rotor 76 
is 12:1. Potentiometer wiper arm 72 makes one complete revolution for each 
switching action accomplished by a wiper 80. It will thus be noted that 
the number of segments in each segmented ring is the same as the gear 
ratio between shaft and rotor. This is also equal to the number of teeth 
on the inner gear 84. Outer gear 40 has one more tooth. 
It will be readily apparent to those skilled in the art that the novel 
precision transducer disclosed herein has many uses in the control of 
electrical quantities. Further, it will be equally apparent that many 
modifications may be made in the apparatus of the invention without 
departing from the spirit and scope thereof. It is to be understood that 
the foregoing is to be construed as descriptive rather than limiting. The 
invention is limited only by the scope of the following claims.