Field strength position sensor with improved bearing tolerance in a reduced space

A position sensor has a shaped dual magnet structure carried upon a pole piece having a generally "c" shaped cross-section. The magnet and pole piece define a generally circular linear field which is concentric about the axis of rotation of the complete rotor. A Hall effect device is inserted into the open portion or gap between the two magnets and is exposed to a well defined field. Through the use of selected geometries and particular magnet materials, a precise, compact and yet tolerant magnetic circuit is produced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains generally to position sensing, and specifically to 
position sensors which are compact, durable and precise for application to 
rugged and demanding environments. 
2. Description of the Related Art 
Electronic devices are advancing technologically at phenomenal rates. The 
cost decreases continually, and is accompanied by almost simultaneous 
increases in capability. These more capable, lower cost devices and 
circuits are applicable to an ever increasing number of requirements. As 
this trend continues, more ways are needed for electronic circuits to 
interface with non-electronic devices and systems. Generally this 
interface is accomplished through a combination of sensors and actuators. 
Position sensing is used to allow an electrical circuit to gain information 
about an event or a continuously varying condition. For example, when a 
sewing machine operator depresses a pedal, a pedal position sensor is used 
to signal a demand for activation of the drive motor. In addition, the 
sensor may be used to establish an amount of demand, or a desired speed at 
which the motor will operate. 
Sensors must endure many millions or even billions of small motions 
referred to as dithers. These dithers are often the result of mechanical 
motion and vibration carried into the sensor. During the life of a sensor 
there may also be a million or more full cycles required. 
There are many applications for sensors, and a wide variety of technologies 
to fill these needs. Each of these technologies offers a unique set of 
advantages and limitations. Of these technologies, magnetic sensing is 
known to have a unique combination of long life components and excellent 
resistance to contaminants. However, in the prior art these devices were 
only applied where little precision was required, such as in proximity 
detection. 
However, magnetic sensors have been limited in application. These 
limitations are generally derived from the need for linearity and precise 
output. In the case of a pedal position sensing application, an operator 
gently depressing the pedal will expect to see a measurable change in 
output of a motor or engine. 
In fact, the first few degrees of rotation may be the most consequential in 
terms of percentage change in motor output. Sensitivity and precision are 
most important close to the zero, or no demand position. Deviations in 
linearity of less than one percent may have very adverse affect on 
performance and even on motor control functions. Sensing a demand for 
motor output when the operator is not depressing the pedal will obviously 
have adverse consequence. Therefore, at the motor zero set point, sensors 
are typically specified for extremely tight and reproducible tolerances 
through all extremes of climate, contamination, and other external 
factors. 
Magnetic circuits offer admirable performance upon exposure to the usual 
contaminants. However, linearity and tight tolerances are another issue. 
Sensors are subjected to forces that change the alignment of the moving 
portion of the sensor with respect to the stationary portion. Somewhere in 
the system is at least one bearing, and this bearing will have a finite 
amount of play, or motion. That play results in false movement between the 
fixed and moving components of the sensor. Unfortunately, magnetic 
circuits of the prior art tend to be very sensitive to the type of 
mechanical motion occurring in a sensor bearing. The problem is heightened 
with poor or worn bearings. 
Typical magnetic circuits use one or a combination of magnets to generate a 
field across an air gap. The magnetic field sensor, be this a Hall effect 
device or a magnetoresistive material or some other magnetic field sensor, 
is then inserted into the gap. The sensor is aligned centrally within the 
cross-section of the gap. 
Magnetic field lines are not constrained anywhere within the gap, but tend 
to be most dense and of consistent strength centrally within the gap. 
Various means may be provided to vary the strength of the field monitored 
by the sensor. 
Regardless of the arrangement and method for changing the field about the 
sensor, the magnetic circuit faces several obstacles which have heretofore 
not been overcome. Movement of the sensor relative to the gap as a result 
of bearing play will lead to a variation in field strength measured by the 
sensor. This effect is particularly pronounced in Hall effect, 
magneto-resistive and other similar sensors, where the sensor is sensitive 
about a single axis and insensitive to perpendicular magnetic fields. 
The familiar bulging of field lines jumping a gap illustrates this, where a 
Hall effect sensor not accurately positioned in the gap will measure the 
vector fraction of the field strength directly parallel to the gap. In the 
center of the gap, this will be equal to the full field strength. The 
vector fraction perpendicular thereto will be ignored by the sensor, even 
though the sum of the vectors is the actual field strength at that point. 
As the sensor is moved from the center of the gap, the field begins to 
diverge, or bulge, resulting in a greater fraction of the field vector 
being perpendicular to the gap. Since this will not be detected by the 
sensor, the sensor will provide a reading of insufficient magnitude. 
In addition to the limitations with regard to position and field strength, 
another set of issues must be addressed. A position sensor must be precise 
in spite of fluctuating temperatures. In order to gain useful output, a 
magnet must initially be completely saturated. Failure to do so will 
result in unpredictable performance. However, operating at complete 
saturation leads to another problem referred to in the trade as 
irreversible loss. Temperature cycling, particularly to elevated 
temperatures, permanently decreases the magnetic output. 
A magnet also undergoes aging processes not unlike those of other 
materials, including oxidation and other forms of corrosion. This is 
commonly referred to as structural loss. Structural and irreversible loss 
must be understood and dealt with in order to provide a reliable device 
with precision output. 
Another significant challenge in the design of magnetic circuits is the 
sensitivity of the circuit to surrounding ferromagnetic objects. For some 
applications a large amount of iron or steel may be placed in very close 
proximity to the sensor. The sensor must not respond to this external 
influence. 
The prior art is illustrated, for example, by Tomczak et al in U.S. Pat. 
No. 4,570,118. Therein, a number of different embodiments are illustrated 
for forming the magnetic circuit of a Hall effect position sensor. The 
Tomczak et al disclosure teaches in one embodiment the use of a sintered 
samarium cobalt magnet material formed into two shaped magnets of opposite 
polarity across an air gap of varying length. 
No discussion is provided by Tomczak et al for how each magnet is 
magnetically coupled to the other, though from the disclosure it appears 
to be through the use of an air gap formed by a plastic molded carrier. 
Furthermore, no discussion is provided as to how this magnetic material is 
shaped and how the irreversible and structural losses will be managed. 
Sintered samarium cobalt is difficult to shape with any degree of 
precision, and the material is typically ground after sintering. The 
grinding process is difficult, expensive and imprecise. 
The device may be designed and ground, for a substantial price, to be 
linear and precise at a given temperature and a given level of magnetic 
saturation, presumably fully saturated. However, such a device would not 
be capable of performing in a linear and precise manner, nor be reliable, 
through the production processes, temperature cycling and vibration 
experienced by sensors. 
Furthermore, devices made with this Tomczak et al design are highly 
susceptible to adjacent ferromagnetic objects. The variation in adjacent 
ferromagnetic material will serve to distort the field and adversely 
affect both linearity and precision. The open magnetic circuit not only 
adversely affects sensitivity to foreign objects, but also sensitivity to 
radiated energies, commonly referred to as Electro-Magnetic Interference 
(EMI or EMC). 
The Tomczak et al embodiments are further very sensitive to bearing play. 
The combination of an open magnetic circuit and radially narrow permanent 
magnet structure provides no tolerance for motion in the bearing system. 
This motion will be translated into a changing magnetic field, since the 
area within the gap in which the field is parallel and of consistent 
magnetic induction is very small. 
Ratajski et al in U.S. Pat. No. 3,112,464 illustrate several embodiments of 
a brushless Hall effect potentiometer. In the first embodiment they 
disclose a shaped, radially magnetized structure which varies an air gap 
between the magnetic structure and a casing, not unlike the last 
embodiment of the Tomczak et al patent mentioned above. However, there is 
no provision for radial or axial motion of the magnet carried upon the 
rotor. Furthermore, the large magnetic structure, like the Tomczak ground 
magnet, is difficult to manufacture and relatively expensive. 
Wu in U.S. Pat. No. 5,159,268 illustrates a shaped magnet structure similar 
to Ratajski et al. The structure illustrated therein suffers from the same 
limitations as the Ratajski et al disclosure. Additionally, the device of 
the Wu disclosure offers no protection from extraneous ferromagnetic 
objects. 
Alfors in U.S. Pat. No. 5,164,668 illustrates a sensor less sensitive to 
radial and axial play. The disclosed device requires a large shaped magnet 
for precision and linearity. The size of the magnet structure places 
additional demand upon the bearing system. No discussion therein addresses 
magnet materials, methods for compensating for irreversible and structural 
losses, or shielding from extraneous ferromagnetic objects. The 
combination of large magnet, enhanced bearing structure, and added 
shielding combine to make a more expensive package. 
Each of the prior art references suffers the disadvantages of high field 
strengths at the zero set point. In the Wu and Alfors patents, a bipolar 
field is utilized. A strong magnetic field is encountered at or near the 
zero point, progressively diminishing in strength to a zero magnetic field 
at or near a mid-point, and then returning to a strong magnetic field. In 
the first embodiments of the Tomczak et al disclosure, a bipolar field is 
also utilized, in that near the midpoint of rotation, the two opposing 
fields are designed to cancel each other. As the sensor is rotated out 
from the midpoint however, the magnetic induction will no longer cancel 
and the sensed field will get progressively stronger. 
In the last embodiment of Tomczak et al a unipolar field is disclosed. 
However, the use of sintered samarium cobalt magnet materials together 
with the necessary grinding operations will force the minimum thickness of 
the magnet to be consequential. 
Attempting to grind too thin a section will lead to large manufacturing 
fall-out due to chipping and breakage. Yet grinding is essential for a 
sintered magnet in order to produce a linear output. In view of the large 
magnetic induction generated by samarium cobalt, the field will be 
substantial even at the thinnest point in the magnet. 
SUMMARY OF THE INVENTION 
The present invention overcomes the aforementioned limitations of the prior 
art and perceived barriers to the use of a precision magnetic position 
sensor through the use of a special geometry magnetic structure. The 
magnet structure includes facing magnets which extend substantially from 
the axis of rotation radially to beyond a pole piece. At the outer 
circumference of the pole piece, the magnet wraps about the edge thereof, 
which tends to linearize the field lines within the region bounded by the 
pole piece and maintain compactness. At a low end of rotation, intended to 
be about a zero set point, an additional means is provided to divert the 
magnetic field lines from linear travel within the gap to perpendicular 
thereto, allowing the measurement of a truly zero field at or near the 
zero set point.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIGS. 1 and 2 a preferred embodiment rotary sensor in accord with the 
present invention is designated generally by the numeral 100. The sensor 
includes a housing 300 and a magnetic structure or assembly 200 of arcuate 
periphery and generally "c"-shaped cross section mounted to the housing. 
Magnet structure 200 includes therein a magnetically permeable pole piece 
210, shaped magnets 212 and 214, an air gap 216 and is supported by a base 
or molded rotor cup 220. 
Pole piece 210 is bonded to magnets 212 and 214 such that the air gap is 
formed between and is bordered by magnets. This use of two magnets 
substantially reduces loss through the air gap which otherwise occurs with 
only a single magnet. The closed magnetic circuit which is completed by 
the pole piece 210 improves performance by being less sensitive to bearing 
play and less sensitive to external ferromagnetic objects. A closed 
magnetic circuit exists, for the purposes of this disclosure, when the 
external flux path of a permanent magnet is confined with high 
permeability material. The pole piece 210, the magnets 212 and 214 and the 
air gap 216 form the closed magnetic circuit. Air is understood to be low 
permeability material. Pole piece 210 further reduces the size of magnets 
212 and 214 required, and may be manufactured from molded or sintered 
metals. More preferably, pole piece 210 is formed from sheet steels such 
as ANSI 430 stainless steel. As shown in FIGS. 1 and 2, the pole piece has 
a generally pie shape configuration in plan view and has a radius from an 
axis 250 to outer edges 261, 263. 
Shaped magnets 212 and 214 are preferably formed by molding magnetic 
materials such as bonded ferrite. Bonded ferrite offers both a substantial 
cost advantage and also a significant advantage over other similar 
magnetic materials in structural loss due to corrosion and other 
environmental degradation. Additionally, bonded ferrite may be produced 
having a very thin, very low field strength region close to the zero set 
point. The advantage of this low field at the zero set point is discussed 
further herein below in reference to FIG. 4. Other magnetic materials may 
be suitable, as will be determined by one skilled in the art. 
Magnets 212 and 214 should extend substantially from the outer edges 261, 
263 of pole piece 210 to a region very close to, or, design allowing, in 
line with the axis of rotation 250. This large extension of magnets 212 
and 214 in the radial direction greatly reduces the effects of radial 
motion of magnetic structure 200. 
Additionally, magnets 212 and 214 are formed with lip structures 474 and 
472 as illustrated best in FIG. 2. These formations extend out beyond and 
partially around pole piece 210. The lips 472 and 474 serve to expand the 
"sweet zone" of operation of Hall effect device 510, by forcing a larger 
area of linear magnetic field lines passing through the air gap and 
coupled between magnets 212 and 214. This larger area of linear field 
lines directly corresponds to greater tolerance for both radial and axial 
play, and yet minimizes the radial extension of the magnets. 
Molded rotor cup 220 includes a surface designed to engage with a shaft 
extending from a device whose position is to be measured. Molded rotor cup 
220 then rotates about an axis identified from end view as 250 in FIG. 1 
and carries therewith the remainder of magnet structure 200. Molded rotor 
cup 220 is retained by housing 300, seal 350, helical spring 360 and cover 
310. 
Cover 310 engages with housing 300 and may, for example, be ultrasonically 
welded in place. Cover 310 is strengthened against warpage and deflection 
through the formation of ribs 312. 
Within the gap formed by magnets 212 and 214 is a hybrid circuit substrate 
500 carrying thereon the Hall effect device 510. Hall effect device 510 
should be positioned somewhere between the outer edges 265, 267 of magnets 
212 and 214, respectively, and the inner ends 511, 513 of the magnets near 
axis 250, but not particularly close to either the edges or the ends, so 
as to avoid the field bulging effect mentioned earlier. 
Hybrid substrate 500 may be attached by heat staking or other similar 
method to the housing 300. Hybrid substrate 500 additionally carries 
thereon electrical circuitry within tray 520. This tray 520 acts as a 
container into which appropriate potting compounds may be placed to 
provide all necessary environmental protection to the associated 
circuitry. Tray 520 should be electrically grounded for protection against 
radiated fields (EMI and EMC). 
Hybrid substrate 500 is electrically interconnected to electrical terminals 
410 through wire bonds 530, though it is well understood that any of a 
large number of electrical interconnection techniques would be suitable. 
Electrical connector terminals 410 emerge from housing 300 at a connector 
body 400, for interconnection to standard mating connectors. 
Magnetic structure 200 rotates about the generally center axis 250 relative 
to housing 300, thereby rotating magnets 212 and 214 together with pole 
piece 210. Hall effect device 510 is stationary relative to the housing 
300. Best illustrated in FIG. 3, magnets 212 and 214 are shaped generally 
helically so as to have a relatively thicker end and a relatively thinner 
end. At the thicker ends 211 and 215, which is at the same angle of 
rotation of magnetic structure 200 for both magnets 212 and 214, there is 
a narrow air gap 217. At the thinner ends 213 and 216, there is a 
correspondingly wide air gap 218. The result is the generation of less 
magnetic induction across gap 218, with more magnetic induction across gap 
217. 
Rotation of pole piece 210 about axis 250 results in changing field 
magnetic induction which is directly measured by Hall effect device 510. 
Proper shaping of the gap will produce a linear output from Hall effect 
device 510. However, such a system will not perform linearly and with 
precision and resistance to bearing play over its life without further 
design considerations. 
In order to stabilize a magnet against irreversible losses, it is necessary 
first to saturate magnets 212 and 214 and then to demagnetize the magnets 
by a small amount. The magnetic structure 200 does not demagnetize evenly 
from magnet ends 211 and 215 to magnet ends 213 and 216, without special 
consideration. Absent the appropriate demagnetization, described in our 
copending application Ser. No. 08/223,474 filed Apr. 5, 1994, now U.S. 
Pat. No. 5,557,493. incorporated herein by reference, the resulting device 
will either lose precision as a result of temperature excursions or will 
lose linearity as a result of stabilizing demagnetization. 
FIG. 4 illustrates the pole piece design 210 having two small extensions or 
dams 482 and 484. These dams serve to attract magnetic flux at the low 
field end of rotation, within gap 218 of FIG. 3, to thereby further reduce 
the strength of the vector portion of the magnetic field that is parallel 
to the axis of rotation 250. This deflection of the magnetic field reduces 
the measured field strength, thereby lowering the output at the low end of 
rotation. The lower the output at this low end, the less the impact of 
non-linearities and variances, such as the effects of temperature and 
irreversible and structural losses. These non-linearities and variances 
are most consequential, as noted hereinabove, at or near the zero set 
point. The use of a zero Gauss field at the zero set point offers much 
advantage in maintaining very tight tolerances by eliminating gain errors 
caused by the magnets, the magnetic sensor 510 and the conditioning 
electronic circuit located on substrate 500. 
The apparatus for measuring angular or rotary position described herein as 
the preferred embodiment is a low cost structure due to the minimal weight 
and reduced demands upon magnetic components. In addition, there are many 
performance advantages not heretofore obtainable, including reduced 
sensitivity to bearing play, resistance to contamination and environment, 
reduced sensitivity to externally located fields, energies and objects, 
durability for motion and dithers, precision, linearity, compactness, 
reduced complexity, and reduced cost. 
While the foregoing details what is felt to be the preferred embodiment of 
the invention, no material limitations to the scope of the claimed 
invention is intended. While a rotary sensor is illustrated for an 
exemplary purposes, one of ordinary skill will readily be able to adapt 
the claimed invention to linear sensors. while a Hall effect device is 
illustrated, magnetoresistive and other magnet sensors will similarly be 
adapted for use herein. Further, features and design alternatives that 
would be obvious to one of ordinary skill in the art are considered to be 
incorporated herein. The scope of the invention is set forth and 
particularly described in the claims hereinbelow.