Wheel speed sensor

A wheel speed sensor including an annular rotor fixed to a wheel for rotation therewith and having circumferentially distributed surface interruptions. A sensing device mounted near the rotor is fixed against rotation and includes a sensing portion opposable across an air gap to the surface interruptions of the rotor to produce a signal representing rotation of the surface interruptions past the sensing portion. The rotor and sensing portion are of complementary generally V-shaped cross section and are separated by a V-shaped air gap such that the output signal is substantially insensitive to relative motion between the rotor and sensing device in an axial plane of the rotor.

FIELD OF THE INVENTION 
This invention relates to an improved wheel speed sensor for sensing the 
angular velocity of a rotating body, such as a vehicle wheel. 
BACKGROUND OF THE INVENTION 
While the present invention is applicable to a variety of rotation sensing 
uses and environments, one specific application of the present invention 
is as a wheel speed sensor system in automotive and truck vehicles. It 
provides an output signal suitable for driving an antilock system on such 
a vehicle. 
Examples of prior wheel speed sensing systems may be seen in U.S. Pat. Nos. 
3,854,556, 3,938,112, 3,961,215 and 4,029,180, assigned to the Assignee of 
the present invention. Such systems typically mount a rotor on the wheel 
of which the speed is to be sensed, and a speed sensing device is fixed 
with respect to the axle housing. The sensing device is typically of 
electromagnetic type, whereas the rotor is of ferromagnetic material and 
is provided with surface interruptions such as apertures, teeth or 
ripples. 
The aforementioned systems have provided the rotor and sensing device with 
interacting radial portions axially opposed across an air gap. Variable 
reluctance-type sensing is achieved by providing a magnetic flux source, 
in the form of a magnet, in the sensing device to establish a flux path 
through pole pieces extending from the magnet to the radial surface of the 
sensing device, across the air gap, and into the slotted radial portion of 
the rotor. Rotation of the rotor moves the slots past the pole pieces to 
periodically change the magnetic flux in the path, which change is sensed 
by a coil wound around the magnet to provide a corresponding electrical 
output signal of frequency related to the rotation rate of the rotor. 
Since such devices are dependent solely on flux change for generation of 
an output signal, it is important that the flux change resulting from 
rotation of the rotor be significantly larger than any flux change 
resulting from radial or axial movement of the rotor relative to the 
sensing device, since these latter flux changes cause signal aberrations 
normally characterized as "noise". Thus, to obtain an accurate indication 
of the angular velocity of the rotating body, it is necessary that the 
sensing system have a high signal-to-noise ratio. This is particularly 
important when the wheel speed sensor provides the wheel speed signal for 
a vehicle antilock system, so that a skid condition can be detected and 
corrected in a proper manner. 
Various of the aforementioned patents propose structures for obtaining an 
improved signal-to-noise ratio. A difficulty in such prior structures in 
presented by the requirement that the air gap be minimized in order to 
maximize the electrical output signal from the sensing device. However, as 
the air gap is reduced in the interest of greater system sensitivity, the 
more noticeable, in comparison, become even relatively moderate axial 
and/or radial movement of the rotor as it rotates relative to the sensing 
device, as due to cocking of the rotor with respect to its rotational 
axis, or flexing thereof during operation. 
Accordingly, the objects and purposes of this invention include provision 
of: 
(1) A rotation sensing system particularly for sensing the speed of a 
vehicle wheel, and intended to provide an increased signal-to-noise ratio. 
(2) A system, as aforesaid, providing a rotor and opposed sensing device 
configured to reduce sensitivity to radial and axial motion of the rotor 
relative to the sensing device. 
(3) A system, as aforesaid, in which the interacting faces of the rotor and 
sensing device, opposed across the air gap, are relatively large for 
permitting a relatively large air gap thickness without reducing system 
sensitivity, and for providing a more favorable ratio as between air gap 
thickness and expected radial and/or axial vibration or runout of the 
rotor. 
(4) A system, as aforesaid, wherein the effect of rotor runout toward and 
away from the sensing device is substantially reduced by a tapered 
interfitting configuration of the rotor and sensing device, and wherein 
relative axial displacement of said rotor and sensing device results, in 
proportion of such taper, in a much smaller change in the thickness of the 
air gap. 
(5) A system, as aforesaid, in which a two-legged air gap as provided 
between the tapered sensing device and rotor, cancels the effect of rotor 
runout in a direction transverse to such air gap, due to a reduction in 
the thickness of one air gap leg which is compensated for by a 
corresponding increase in the thickness of the other air gap leg, whereby 
the effective output signal is not disturbed. 
(6) A system, as aforesaid, in which a rotor of relatively simple 
configuration is readily formable in relatively thin sheet metal while 
obtaining sufficient rigidity and resistence to deflection during 
rotation, by reason of its cross sectional configuration. 
(7) A system, as aforesaid, which permits an improved signal-to-noise ratio 
despite use of only a single coil and substantially a single flux path. 
Other objects and purposes of this invention will be apparent to persons 
acquainted with apparatus of this general type upon reading the following 
specification and inspecting the accompanying drawings. 
SUMMARY OF THE INVENTION 
The objects and purposes of the invention are met by providing a wheel 
speed sensor generating signals to indicate rotation of a body. The sensor 
includes an annular rotor fixed to the body for rotation therewith and 
having circumferentially distributed surface interruptions. A sensing 
device mounted near the rotor is fixed against rotation with it and 
includes a sensing portion opposable across an air gap to the surface 
interruptions of the rotor to produce a signal representing rotation of 
the surface interruptions past the sensing portion. The rotor and sensing 
portion are of complimentary generally V-shaped cross section as seen in 
an axial plane (i.e. a plane containing the axis) of the rotor and are 
separated by an air gap of correspondingly V-shaped cross section such 
that the output signal is substantially insensitive to relative motion 
between the rotor and sensing device in the axial plane.

DETAILED DESCRIPTION 
FIG. 1, by way of example, illustrates an embodiment of the invention 
installed on a vehicle wheel-axle assembly 10 to provide a wheel speed 
signal. The assembly 10 comprises a nonrotatable axle 11 on which a wheel 
unit 12 is rotatably supported by bearings 14. Conventional brake 
mechanism 16 is, as schematically indicated in FIG. 1, suitably mounted on 
a brake mounting flange 18 fixed to axle 11 and is actuable to brake 
rotation of wheel unit 12 in a conventional manner. The wheel unit 12 
conventionally incorporates a brake drum or disk, as well as suitable 
mounting means for wheels. 
The rotation indicating system embodying the invention, generally indicated 
at 20, comprises a rotor 22 coaxially mounted for rotation with the wheel 
unit 12 and a sensing device 24 fixed with respect to the axle 11, hereby 
secured to the flange 18, in sensing opposition to the rotor 22. As wheel 
unit 12 rotates, rotor 22 rotates relative to sensing device 24 producing 
a flux change which creates an induced voltage in the sensing device 24 as 
hereafter discussed. As seen in FIGS. 1 and 2, rotor 22 and the opposed 
portion of sensing device 24 are complimentary in cross section, one being 
convexly profiled and the other concavely profiled, the rotor and sensing 
device being interfitted so as to be spaced by a double or V-shaped air 
gap, i.e. an air gap with two legs extending transverse to each other. 
Turning now more specifically to the structure of the rotor 22, same is 
formed as a ring of material capable of continuing a magnetic flux path, 
i.e. a ferromagnetic material, conveniently sheet steel. As seen in radial 
cross section (FIG. 2), the rotor 22 comprises spaced annular walls 31 and 
32 which converge leftwardly away from the sensing device 24 to form a 
tapered recess 33 opening concavely toward the sensing device 24. The 
walls 31 and 32, here the radially outer and inner walls respectively, are 
preferably substantially straight in cross section and are connected at 
their adjacent leftward ends by a rounded bight portion 35. 
The rotor walls 31 and 32 are provided with evenly circumferentially 
distributed surface interruptions 38 which extend substantially the full 
width of each wall to maximize the flux changing area of the rotor and 
hence the output signal of the sensing device 24. While the surface 
interruptions 38 may take other forms, e.g. teeth, ripples, etc., such 
interruptions are shown as slots which extend through the thickness of 
walls 31 and 32 and extend widthwise of such walls. To permit a 
conveniently high frequency and a desirably uniform substantially 
sinusoidal wave form for the output signal of the sensing device 24, the 
slots 38 are preferably of elongate rectangular cross section wherein 
their dimension axially of the rotor substantially exceeds their dimension 
circumferentially of the rotor. The slots 38 are preferably evenly spaced 
apart by webs 39 of the same circumferential width as the slots. The slots 
38 terminate short of the free edges of the walls 31 and 32, thereby 
leaving circumferentially continuously annular rims or edge portions 41. 
The rotor 22 may be formed from a blank of thin sheet steel. Due to its 
ring-shape and substantially V-shape cross section, the rotor will be 
relatively rigid and free from any tendency to deflect during rotation. 
The rotor 22, as seen in FIG. 1, is fixed coaxially to the wheel unit 12. 
In the example shown, the rightward face of wheel unit 12 is coaxially 
grooved at 47 to receive the bight portion 35 of the rotor 22 which is 
secured thereto, as by welding. 
In the embodiment shown, the V-shaped recess 33 defined between rotor walls 
31 and 32, as seen in cross section, is bisected by a line parallel to the 
axis of axle 11. 
Turning now to sensing device 24 (FIGS. 2 and 3), same includes a sensing 
portion 51 having a cross sectional shape which compliments that of the 
rotor 22 and is spaced from the rotor walls 31 and 32 by legs 53 and 54 of 
an air gap 56. The sensing portion 51 is formed as a tapered protrusion 58 
which extends into the recess 33 generally in axial alignment therewith, 
with its radially outer and inner surfaces 61 and 62 received between and 
spaced from the opposed inner surfaces of the rotor walls 31 and 32 by the 
substantially uniform air gap legs 53 and 54, respectively. The leftward 
end 65 of protrusion 58 is normally spaced from the opposed surface of the 
bight portion 35 of the rotor by a distance exceeding the average 
thickness of the air gap legs 53 and 54, and preferably conforms generally 
in shape to the bight portion 35. 
The sensing device 24 comprises a magnet 68 (FIGS. 2 and 3), conveniently a 
permanent magnet of generally cylindrical form centered on and displaced 
rightwardly from the protrusion 58. Ferromagnetic pole pieces 71 and 72 
extend from the ends (north and south poles) of the magnet 68 in 
side-by-side relation leftwardly toward and into the rotor recess 33. The 
leftward ends of pole pieces 71 and 72 form the protrusion 58 
aforementioned. 
A sensing coil 74 is preferably wound around the magnet 68 between pole 
pieces 71 and 72 for sensing changes in the magnetic flux path afforded by 
the magnet, pole pieces and rotor, due to rotation of the rotor. As shown 
in FIG. 3, the coil 74 is connected electrically at 75 and 76 to a 
suitable load or driven device 77 to apply an electrial signal thereto 
varying in frequency in accord with the rotative speed of the rotor. The 
load 77 may for example be the control circuitry of a vehicle antiskid 
system of known type. 
The sensing device 24 further includes a nonmagnetic, electrically 
insulative housing or encapsulation 81 which serves to interlock and 
protect the components 71, 72, 74 and 68 therein. The sensing device 24 
may be mounted upon the brake mounting flange 18 and, for sake of example, 
such is accomplished providing housing 81 with integral side flanges 83 
having elongated mounting holes 86 to permit movement of the sensing 
device toward and away from the rotor. A bracket 87 (FIG. 1) secured by 
screws 88 to the brake mounting flange 18 may be used to support the 
sensing device 24, with further screws 91 extending through holes 86 for 
securing the sensing device 24 in desired axial position with respect to 
the rotor 22. 
The housing 81, which may be of conventional synthetic resin material, 
everywhere surrounds elements 68, 71, 72 and 74, preferably even including 
a protective skin of such material over the surfaces 61 and 62 of pole 
pieces 71 and 72. On the other hand, the surfaces 61 and 62 of pole pieces 
71 and 72 (i.e. the protrusion 58) may be left uncovered as in FIG. 2. As 
seen in FIG. 2, housing 81 extends rightwardly from protrusion 58 
initially along the slope or taper line of their surfaces 61 and 62 and 
then diverges, as seen at 92 and 93, to form shoulders which upon 
rightward deflection of the rotor would contact the rotor edge portions 41 
and 42, respectively, prior to bottoming of the protrusion 58 within the 
rotor recess 33. The housing 81 thus assists in preserving the pole pieces 
and rotor from self-destructive contact. It will be noted that the 
cross-sectional thickness of the pole pieces is a maximum at the rightward 
part (FIG. 2) of protrusion 58, with the cross-sectional thickness of the 
pole pieces 71 and 72 being reduced, as shown at fillets 101, where the 
material of the housing begins to enclose the top and bottom walls of the 
pole pieces. 
As seen in FIGS. 3 and 4, the protruding leftward ends of pole pieces 71 
and 72 correspond in width and circumferential spacing to the width and 
circumferential spacing of the rotor slots 38. Whereas it is contemplated 
that each pole piece 71 and 72 may terminate in a single protruding part 
58', for greater apparatus sensitivity, it is preferred that the pole 
pieces 71 and 72 be bifurcated (FIG. 3) so as to each provide a pair of 
protruding parts 58' and 58". These parts 58' and 58" are separated by a 
spacing corresponding to the circumferential width of web 39, and adjacent 
protruding parts 58" and 58' of adjacent pole pieces 71 and 72 are 
separated from each other by a spacing corresponding to a multiple of the 
circumferential spacing of adjacent web 39. 
OPERATION 
In operation, the rotor 22 rotates with the wheel unit 12 and thereby moves 
its alternating slots 38 and webs 39 circumferentially past the protruding 
parts 58', 58" of the pole pieces 71 and 72 of sensing device 24. With 
magnet 68 oriented as shown in FIG. 3, the path of magnetic flux therefrom 
can be traced along the broken line F in FIGS. 2-4. Thus, the magnetic 
flux F is shown to proceed from the north pole N of magnet 68 into pole 
piece 72 (FIGS. 3 and 4), thereafter splitting to enter parts 58' and 58" 
of pole piece 72. The flux passes from both the radially outer and inner 
surfaces 61 and 62 throughout the exposed, tapering length W thereof, and 
across the air gap legs 53 and 54 and thus into the rotor webs 39. The 
flux is transferred along the webs 39 to the circumferentially continuous 
bight and rim portions 35 and 41, so as to pass circumferentially along 
the rotor to the webs 39 nearest the protruding parts 58' and 58" of the 
remaining pole piece 71. In a manner oppositely ordered, but otherwise 
similar to the above-described flux transfer, the flux is transferred from 
the rotor to the pole piece 71 and thence to the south pole S of magnet 68 
to complete the flux loop. The magnitude of flux transferred is maximized 
with the rotor webs 39 directly opposed to the protruding parts 58', 58", 
and is minimized when the rotor slots 38 are directly opposed to such 
parts 58', 58". These conditions alternate during rotor rotation, inducing 
a corresponding alternating signal at the output 75, 76 of coil 74, having 
a frequency corresponding to the rotational speed of the rotor 22. 
The sensing device 24 is insensitive to radial motion of the rotor, as due 
to vibration or eccentricity thereof. This is because the sensing 
protrusion 58 faces radially outer and inner parts of the rotor, rather 
than across a single gap. Thus, should the rotor shift downward to widen 
the air gap leg 54, it simultaneously and compensatingly narrows the outer 
gap leg 53. Accordingly, the reduction in flux travel across the widened 
gap 54 is compensated for by the increased magnetic flux travel across the 
narrowed gap 53. Accordingly, a double or two-legeed gap cancels the 
effect of radial motion of the rotor 22 with respect to the sensing device 
24, insofar as the output signal of the sensing device is concerned. 
Moreover, the sensing device 24 is also relatively insensitive to axial 
motion of the rotor. More particularly, the elongate (here 1 to 4) taper 
means that for a given amount of axial shifting of the rotor with respect 
to the sensing device, the average thickness of the air gap leg 53 and 54 
will change by only a fraction of that amount (here one quarter of that 
amount). Further, the slotted width of rotor 22 (extending generally 
axially of the rotor) exceed the width W of the exposed or active portion 
of the protruding parts 58', 58" such that the full width W of the parts 
58', 58" will still oppose the slotted portion of the rotor despite 
substantial rightward or leftward motion of the rotor from its position 
shown in FIG. 2. 
As compared to prior devices in which the rotor and sensing device have 
flat radial faces opposed axially across a gap, this inventive device can 
be provided with greater axial clearance between the sensing device and 
rotor to substantially reduce the danger of collision therebetween and 
consequent damage thereto. Further, in the inventive devices, the 
substantial area of the surfaces of the rotor 22 and sensing protrusion 58 
permits the average working thickness T of the air gap legs 53 and 54 to 
be relatively large, without loss in sensitivity as compared to 
conventional radially faced rotors and sensing devices of similar radial 
cross sectional area. 
It is also contemplated that the free end 65 of the sensing projection may 
be leftwardly extended, to a smaller radius than the rightward face of 
rotor bight 35 to provide blocking engagement between the free end 65 and 
the interior surface of bight 35 while the average thickness of the air 
gaps 53 and 54 remains greater than zero. 
Although in the above-discussed FIGS. 1-4 embodiment, the rotor 22 is 
arranged to axially face the sensing device 24, other orientations are 
contemplated. For example, FIG. 5 shows a rotor 22A with its walls 31A and 
32A diverging radially, rather than axially, and radially opposed by 
sensing device 24A. Thus, sensing device 24A is insensitive to axial rotor 
motion, since widening of one air gap leg 53A or 54A is compensated by 
narrowing of the other. Sensing device 24A is also substantially 
insensitive to radial motion of rotor 22A, in view of its elongate taper 
wherein a given change in radial spacing of rotor and sensing device 
results in a substantially smaller change in air gap width. 
Although the normally preferred embodiment of the invention here shown 
includes a concave rotor and convex sensing device, also contemplated, 
within the broader aspects of the invention, is the reverse configuration 
(convex rotor, concave sensing device). 
Although a particular preferred embodiment of the invention has been 
disclosed in detail for illustrative purposes, it will be recognized that 
variations or modifications of the disclosed apparatus, including the 
rearrangement of parts, lie within the scope of the present invention.