Tachogenerator having a magnetoresistance stator coil

A tachogenerator has a magnetoresistance film coil formed on a stator surface along a circular path concentric with a rotor axis. The coil has a spatially periodic structure, with multiple spaced-apart lateral components series connected with longitudinal components. The stator surface is opposite to a rotor surface having multiple equiangularly spaced magnetic pole faces with alternately opposite polarities. Current passed through the coil develops voltage variations. Undesirable small voltage variations resulting from manufacturing tolerances are averaged to zero at the coil output terminals.

BACKGROUND OF THE INVENTION 
The present invention relates to a tachogenerator. 
Tachogenerators currently in use include a type of device having a single 
magnetoresistance element mounted in proximity to rotor pole faces. While 
this type of tachogenerator is advantageous for detecting extremely low 
angular velocities due to the speed-independent nature of the 
magnetoresistance element, there are undesirable voltage fluctuations 
resulting from mechanical tolerances. This significantly degrades the 
quality of products and prevents the tachogenerator from meeting 
requirements imposed in high-precision speed-control applications. 
SUMMARY OF THE INVENTION 
Therefore an object of the invention is to provide a precision 
tachogenerator. 
This object is obtained by forming a magnetoresistance film on a surface of 
the stator along a path extending concentrically about the rotor axis over 
an arc of at least 180 degrees. The film has a spatially periodic 
structure having multiple angularly spaced-apart, lateral components 
series connected with intermediate longitudinal components between output 
terminals. The lateral components of the magnetoresistance coil average 
out voltage variations resulting from manufacturing tolerances, so that 
errors are cancelled at the terminals of the coil. 
According to one feature of the invention, the tachogenerator of the 
invention has a flat pancake configuration. This is achieved by forming 
the coil on a flat surface of the stator in a periodic structure having 
multiple radial components and forming the pole faces on a flat surface in 
opposition to the surface of the stator. 
According to a further feature of the invention, undesirable voltage 
variations which occur as a result of electromotive forces at one-half the 
frequency of resistance changes are eliminated by arranging the lateral 
components of the coil so that pairs of adjacent components are spaced 
apart an even integral multiple of the angular pole face spacing.

DETAILED DESCRIPTION 
A tachogenerator 10 represented in FIG. 1 is attached to an electric motor 
20. 
Tachogenerator 10 comprises a rotor coil and a stator 12. Rotor 11 is 
mounted on motor shaft 13. Rotor 11 is shaped as a disc having plural 
equally divided radial segments 14 which are alternately oppositely 
magnetized in the direction of the axis of shaft 13 to present pole faces 
14a and 14b in a common flat plane 11a which is parallel to the surface of 
stator 12. Magnetic boundaries between adjacent pole faces thus extend 
radially and are angularly spaced apart at equal intervals. 
Stator 12 comprises an annular insulative support 4 formed of epoxy resin 
or Bakelite mounted on motor 20. Shaft 3 extends through hole 21 and is 
secured to rotor 11 of motor 20 so that ple face plane 11a is axially 
spaced in opposition to the surface 16a of support 16. 
On the surface of support 16 is a ferromagnetic strip 17 film of 
magnetoresistance material such as permalloy, nickel-cobalt alloy or the 
like. Such ferromagnetic material is deposited on support 16 as a 
single-turn flat coil having a thickness of from 500 Angstrom units to 1 
micrometer using electroless plating, sputtering, or vacuum evaporation 
technique. The magnetoresistance material is deposited along a path 
concentric with the axis of rotation of rotor 11 in a spatially periodic 
structure having multiple lateral components 17a which extend radially and 
equiangularly and are spaced apart and connected in series with outer and 
inner intermediate longitudinal, or arcuate components 17b, 17c. Radial 
components 17a are spaced apart at equal angular intervals at which the 
pole faces are spaced apart so that each radial component coincides 
axially with each of the magnetic boundaries between the pole faces. As 
illustrated in FIG. 3, current is supplied from the positive terminal of a 
DC voltage source 18 through a load impedance 19 to coil 17 and thence to 
the negative terminal of the voltage source to detect voltage variations. 
Changes in resistance occur in radial components 17a in proportion to the 
magnetic field strength, which changes combine to produce a total 
resistance change which generally takes the shape of a substantially 
constant-amplitude sinusoid varying at a rate proportional to the rotation 
speed of rotor 11. Voltages are also induced by an electromotive force 
(EMF) in radial components 17a in proportion to the rate of change in 
magnetic field strength, which voltages combine to produce a sinusoidal 
voltage having an amplitude and frequency that varies in proportion to the 
rotor rotation speed. The frequency of the EMF is one-half the frequency 
of the resistance variations. These voltage variations are summed and 
appear across terminals 22 and 23. 
Because the voltage variations are summed, the tachogenerator of the 
invention averages out small undesired variations which might occur as a 
result of manufacturing tolerances. Because of the multiple lateral 
components 17a corresponding with multiple pole faces 14a, 14b, coil 17 
extends over an arc of at least 180 degrees. In a practical embodiment, 
the number of pole faces range from 60 to 1000 depending on the 
tachogenerator diameter and the degree of precision required. 
When the rotor speed is relatively low, the induced EMF components have a 
relatively small magnitude, whereby the magnitude of voltage variations 
attributed to the resistance changes is much greater than the EMF voltage 
components. Since the resistance is predominant at low speeds, the 
tachogenerator of the invention provides constant-amplitude voltage 
variations at twice the frequency of the EMF. This is particularly 
advantageous to high precision angular velocity or angular position 
sensors for low speed applications. At high speeds, on the other hand, the 
induced EMF components, which are proportional to rotor speed, become 
dominant and increase in magnitude. High amplitude voltage variations are 
thus available. Although the frequency is one-half the frequency of the 
resistance change, the high speed operation may compensate for the 
frequency reduction. 
The embodiment of FIG. 1 is advantageous for forming the coil 17 in an 
efficient manner and provides a flat, pancake-like construction. 
However, it is also possible to arrange the pole faces and lateral coil 
components on opposed cylindrical surfaces, as illustrated in FIGS. 4 and 
5. A tachogenerator 30 of the cylindrical arrangement comprises a rotor 31 
and a stator 32. Rotor 31 is a disc having an axial dimension sufficient 
to induce magnetic resistance changes in the stator coil. Rotor disc has 
plural equally angularly spaced-apart radially magnetized segments 33 
which are alternately oppositely poled as seen in FIG. 5, so that pole 
faces 33a and 33b are presented on the cylindrical surface of the rotor 
disc. Rotor 31 is mounted on shaft 34 of the motor 20. 
Stator 32 is formed of an insulative cylindrical support 35 having a center 
hole 36 through which the rotor shaft 34 extends into the motor, so that 
the cylindrically arranged pole faces of rotor 31 are uniformly spaced in 
opposition to the inner cylindrical surface of stator support 35 on which 
is formed a magnetoresistance coil 37. As in the previous embodiment, coil 
37 is a spatially periodic structure having multiple equiangularly 
spaced-apart lateral components 37a which extend parallel with the axis of 
the rotation of rotor 31 and are connected in series with upper and lower 
intermediate longitudinal components 37b and 37c, the axial components 37a 
being provided in equal number to the pole faces 33. The coil 37 is 
deposited in the manner described in the previous embodiment. A current 
supply and voltage sensing circuit of FIG. 3 is connected to the coil 37 
as in FIG. 3. 
In cases where it is desired to detect the direction of rotation of motor 
20, the dual coil structure shown in FIG. 6 is used instead of the single 
coil structure of FIG. 3. The dual coil structure comprises an outer coil 
40 and an inner coil 50 having radial components 50a equal in number to 
the radial components 40a of outer coil 40. Radial components 50a are 
angularly displaced by 90-degree electrical angle, or 1/4 the angular 
spacing between adjacent pole faces, with respect to radial components 
40a. Inner coil 50 is coupled to voltage source 18a through load 
resistance 19a and outer coil 40 is coupled to voltage source 18b through 
load resistance 19b. A speed detector 41 may either be coupled to the 
outputs of the inner or outer coil. The direction of rotation of motor 20 
is detected by coupling a phase detector 42 to the outputs of inner and 
outer coils. 
In a modification of the present invention, the lateral components of the 
coil structure are arranged in pairs and the lateral components in each 
pair are angularly spaced apart from each other by an even integral 
multiple of the angular spacing of the pole faces to cause EMF-induced 
currents to develop in opposite directions in the lateral components of 
each pair. 
FIG. 7 is a top view of coil 60 in such a modification. Coil 60 has 
multiple radially extending lateral components 60a which are formed into 
pairs. The lateral components of each pair are angularly spaced from each 
other by twice the angular spacing of pole faces 14a, 14b and the lateral 
components of adjacent pairs are angularly spaced from each other by twice 
the angular spacing of the pole faces. A positional relationship between 
pole faces 14a, 14b and coil 60 is shown in developed form in FIG. 8. The 
EMF voltages induced in radial components 60a of each pair have the same 
polarities as indicated by arrows 62 and hence cancel each other out. The 
output signal available from terminals 61 contains only the voltage 
variations resulting from resistance changes. Because the induced EMF 
components are cancelled, the output voltage is a purely sinusoidal 
waveform having a constant amplitude independent of the rotor speed. 
FIG. 9 is an illustration of an alternative form of the embodiment of FIG. 
7. Coil 70 comprises multiple pairs of radially extending lateral 
components 70a, the lateral components of each pair being angularly spaced 
from each other by twice the angular spacing of pole faces 14a, 14b; the 
lateral components of adjacent pairs are spaced from each other by the 
angular spacing of the pole faces. The EMF voltages developed in the 
lateral components 70a of each pair have like polarities, but are opposite 
to the polarities of the voltage developed in adjacent pairs as indicated 
by arrows 72 and 73 in FIG. 10. Thus, voltage cancellation occurs in each 
pair.