A force/flash pressure sensor utilizes a stack of annular rings of a magnetoelastic material. One major stack surface is supported at a plurality of points having substantially equal angular displacement therebetween. A member, moving along the axis of the ring stack responsive to an axial force, or pressure, to be measured, presses a different set of points against the opposite stack surface, to distort the ring stack and alter the magnetic flux density saturation magnitude thereof. The plurality of points in abutment with each of the opposite major stack surfaces is advantageously equal; a number of different configurations for applying force (pressure) along the axis of the ring stack are described.

BACKGROUND OF THE INVENTION 
The present invention relates to force/pressure sensors and, more 
particularly, to a novel force/pressure sensor utilizing a stack of 
annular rings of a magnetoelastic material, for sensing the magnitude of a 
force imparted to the sensor substantially along the ring stack axis. 
It is known to measure the magnitude of a force by means of magnetoelastic 
sensing elements. Typically, the magnetoelastic force sensor requires at 
least one tape-wound toroidal ring of a magnetoelastic material, such as 
an amorphous metal alloy; such a sensor operates responsive to distortion 
of the tape-wound ring by a radially-acting force. In addition to the use 
of tape-wound toroidal cores of magnetoelastic material for force sensing, 
other force sensors utilize a flat amorphous metal ribbon which is placed 
under tension, and has the force to be measured applied to a surface of 
the tensioned ribbon. Both approaches are somewhat mechanically complex 
and, therefore, costly and the repeatability of each sensor in a group 
thereof appears to be dependent upon the metallurgical processes utilized 
to originally process the magnetoelastic material. It is therefore highly 
desirable to provide a magnetoelastic force/pressure sensor utilizing a 
relatively simple mechanical configuration, having low cost and relatively 
repeatable sensitivity to displacement, force, or pressure. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the invention, a magnetoelastic force/pressure sensor 
utilizes a stack of annular rings of a magnetoelastic material, such as a 
silicon steel or an amorphous metal alloy. Each of the major surfaces of 
the ring stack is in abutment with a plurality of edges of one of a pair 
of members. Each of the edges in abutment with each major surface is 
advantageously disposed with equal angular orientation with respect to all 
other edges in abutment with the same surface, and with complementary 
orientation to the edges, of the other member, in abutment with the 
opposite ring stack surface. At least one coil means is provided about the 
ring stack for measuring the change in the ring stack magnetic 
characteristics, responsive to the change in magnitude of a force along 
the ring axis and distorting the normally flat ring. 
In presently preferred embodiments, the change in ring stack flux density 
saturation magnitude is measured for a change in axial force (pressure). 
The pair of members are upper and lower members, each including leg 
extensions having edges respectively in abutment with one or the other of 
the stack surfaces. One of the members is maintained substantially 
immovably with respect to the other member, which is responsive to the 
axial force/pressure to be measured. Embodiments utilizing a plurality, 
e.g. 2, 3, 4, . . . , of stack-abutting edges are described, as are 
several different embodiments for forming the measurement coil means about 
the ring stack. Use of the force sensor in a presently preferred 
embodiment of a pressure transducer is also illustrated. 
Accordingly, it is an object of the present invention to provide a novel 
force/pressure sensor utilizing a stack of rings of magnetoelastic 
material and responsive to the magnitude of a force acting along the ring 
stack axis. 
This and other objects of the present invention will become apparent upon 
reading of the detailed description of the present invention, when 
considered in conjunction with the associated drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring initially to FIGS. 1 and 1a, our novel magnetoelastic 
pressure/force sensor 10 utilizes a stack 11 of at least one annular ring 
12 of a magnetoelastic material, such as silicon steel, amorphous metal 
alloys and the like. Preferably, a plurality of thin annular rings, e.g. 
the pair of annular rings 12a and 12b, are aligned in registration one 
atop the other to form stack 11. The rings 12 are supported upon one major 
stack surface 11a at a plurality of points 14 (of which only one point 14a 
is here illustrated). Each point 14 is located at an end of an associated 
leg projection 15 extending substantially perpendicular to the upper (or 
interior) surface 16a, closest to stack 11, of a first member 16. The 
opposite (or exterior) surface 16b of first member 16 is in abutment with 
a fixed, non-movable surface, or member, 18. Each of a plurality of points 
20 is located at an end of an associated leg projection 22 extending 
substantially perpendicular from a lower (or interior) surface 24a, 
closest to stack 11, of a second member 24. Each point 20 abuts upon the 
opposite major surface 11b of the stack 11 of annular rings 12. Each of 
points 20 (e.g. point 20a) is separated from all other points 20 (e.g. 
point 20b) extending from the second member surface 24a; each of points 14 
(e.g. point 14a) is separated from all other points 14 (not shown in FIG. 
1) extending from first member surface 16a. There may be a pair of contact 
points 20 for each contact point 14 (e.g. one of points 20a and 20b on 
either side of a line extending through the leg projection on which point 
14a is formed) or, as will be seen hereinbelow, there may be an equal 
number of points 14 and 20, disposed as discussed with respect to FIGS. 2, 
2a, 2b and 3. 
The force F to be sensed is applied substantially normal to an exterior (or 
upper) surface 24b of second member 24, and is thus applied not only in 
the general direction of extension of leg projections 15 and 22 but 
especially in a direction along the axis 11x of the stack 11 of annular 
rings 12. Coil means 26 is formed about a portion of the annular ring 
stack 11, to provide an electrical sensing means, having a pair of opposed 
ends 26a and 26b, for measurement of the magnetic characteristics of the 
stack 11 of annular rings 12, with respect to the magnitude of applied 
force F. 
In the absence of the application of a force F (applied along stack axis 
llx and against second member surface 24b, when first member surface 16b 
is held substantially immovable), each layer (i.e. each ring 12) of the 
stack 11 is subjected to a low level of mechanical stress and has a 
somewhat "rectangular" magnetic hysteresis loop 30 (FIG. 1a) having a 
first magnetic flux density saturation value .vertline.B.sub.M .vertline., 
as shown at positive and negative saturation limits 30a and 30b. As 
increasing magnitudes of force F are applied to movable member 24, leg 
projections 22 are moved downwardly and the pressure of points 20a and 20b 
(and the counterpressure of point 14a) causes the annular rings of stack 
11 to press against one another and then buckle to assume a somewhat wavy 
shape in a plane orthogonal to axis llx. The distortion of stacked rings 
12 of magnetoelastic material causes the stack 11 to have a different 
hysteresis loop 32, wherein the saturation magnetic flux density magnitude 
.vertline.B.sub.M '.vertline. has positive and negative saturation limits 
32a and 32b which decrease with respect to the saturation magnetic flux 
density magnitude .vertline.B.sub.M .vertline. of the loop with lesser, or 
no, applied force F. Thus, the saturation magnetic flux density magnitude 
decreases, in the direction of arrows C, with application of increasing 
axial force F along stack axis 11x. The decrease in saturation flux 
density magnitude is sensed, by use of sensing coil means 26, and external 
means (not shown) as an effect upon an A.C. waveform signal supplied from 
a source (also not shown) external to the sensor. Sensing may be 
accomplished by use of a single coil 26, utilizing a 
constant-voltage-drive/current-output-monitoring technique, or by the use 
of a pair of coils (as illustrated in FIGS. 2a, 2b and 3) using a voltage 
input/voltage output ratio technique, and may be provided by the 
source/sensing circuitry disclosed and claimed in co-pending U.S. 
application Ser. No. 546,228 filed on even date herewith, assigned to the 
assignee of the present invention and incorporated herein in its entirety 
by reference. 
Thus, electrical signals having a characteristic thereof substantially 
proportional to the force axially applied to at least one magnetoelastic 
annular ring are provided by the force/pressure transducer of the present 
invention. 
While a pair of leg projections 22 and associated points 20 on one 
structured member 24 may be utilized with each leg projection 15 and 
associated point 14 on the opposite structural member 16, we presently 
prefer to use an equal number of distortion-causing respective leg 
projections 15 and 22 and points 14 and 20, respectively, on each of the 
respective fixed and movable members 16 and 24. We have found that three 
leg projections and associated points upon each of the fixed and movable 
members provides a greater magnitude of output signal change, with respect 
to the use of only a pair of leg projections and points from each of fixed 
and movable members 16 and 24. In addition, the use of three extensions 
can provide an inherently symmetrical support system for the annular ring 
stack 11. 
In the embodiment of FIG. 2, the magnetoelastic ring stack 11 comprises a 
plurality of magnetoelastic rings 12, e.g. three rings 12a-12c, aligned in 
registration with each other and with the resulting stack central axis 11x 
arranged vertically. Each ring 12 is formed of a selected magnetoelastic 
material, such as a silicon steel, an amorphous metal alloy and the like; 
each ring has an inner radius r (which may be, by way of illustration 
only, about 1/4" and, more specifically, about 0.225") and has an outer 
radius R (which may be, by way of illustration only, about 7/16" and, more 
specifically, about 0.425"). 
Lower (or fixed) support member 16' comprises a three-legged member having 
one leg projection portion 15a, 15b or 15c , raised from, and in the plane 
of, each of legs 16'a, 16'b or 16'c, respectively, and perpendicular to 
immovable member 18. Each leg projection portion 15a-15c has dimensions 
such that the associated point portion 14a-14c begins at a minimum 
distance r' (less than the inner radius r of the rings 12) from the common 
leg joint 16'x (itse1f substantially aligned with the stack axis 11x) and 
extends to a maximum distance R' (greater than the outer radius R of the 
rings 12), to assure that the lower stack surface 11a bears against all 
point portions 14 even if the stack axis 11x and support member axis 16'x 
are slightly misaligned. The legs 16'a, 16'b and 16'c are provided with 
substantially equal angular disposition, i.e. with an angle of about 
360.degree. /N, where N is the number of legs (e.g. about 120.degree. for 
N=3 legs), between each leg and the remaining two legs of the support. 
Each leg may also include an outer extension portion 36a, 36b or 36c, 
provided at a distance from the support member center 16'x equal to at 
least the exterior radius R of the annular rings 12, to aid in maintaining 
the individual rings 12a-12c in axially-aligned registration, when the 
stack 11 is placed upon the three lower point portions 14a -14c. 
Upper (or movable) support member 24' also comprises a three-legged member 
having one leg projection portion 22a, 22b or 22c, in the plane of each of 
legs 24'a, 24'b or 24'c, respectively; each leg projection portion 22a-22c 
may be of the same height as the remainder of the member leg portion 
24'a-24'c, for a distance R" from the common leg joint 24'x, which will be 
substantially aligned with the stack axis 11x; the distance R" is at least 
as great as the stack ring 12 outer radius R. It should be understood that 
leg portions 16'a-16'c of the lower support member 16' can also be of the 
same height as the leg projection portions 15a 15c thereof, similar to the 
uniform height leg/leg projection portions of upper member 24'. As with 
lower member 16', upper member 24' has the legs thereof provided with 
substantially equal angular disposition, i.e. with an angle of about 
360.degree. /N (e.g. about 120.degree. for three legs), between each leg 
and the remaining two legs of the support. Each leg may also be provided 
with an outer extension portion 38a, 38b or 38c, commencing at a distance, 
from the upper support member center junction 24'x, equal to distance R"; 
the downwardly-directed extension portions 38 serve as a maximum-travel 
stop means, such that application of more than some maximum force F, 
against the "spring" action of the ring stack 11, can only move upper 
member 24' downwardly by that maximum distance necessary to cause the 
lower ends of extensions 38 to abut immovable base member 18 and thus 
prevent additional downward movement responsive to the applied force. 
Sensor 10' is assembled by securing lower member 16' to immovable surface 
18, placing the aligned stack 11 of the at least one annular disk members 
12 upon lower member edges 14, with the stack axis 11x substantially 
aligned with the lower member axis joint 16'x, and then placing upper 
member 24 upon, and substantially in abutment with, the upper stack 
surface 11b. The upper and lower member leg portions 24'a-24'c and 
16'a-16'c are so disposed as to provide substantially equal angular 
displacements between the legs of the opposite members, i.e. upper member 
leg 24'a is placed diametrically opposite to lower member leg 16'b, and 
bisecting the angle between lower member legs 16'a and 16'c, etc. By so 
offsetting each upper member 24' leg (with an offset angle of 360.degree. 
/2N) to bisect the angle formed by the pair of adjacent lower member 16' 
legs, the distortion, responsive to force F, of the stack 11 rings is made 
substantially uniform and somewhat more reproduceable. 
This offsetting arrangement is also shown in FIG. 2a, wherein lower, fixed 
member 16' is attached to a circular immovable support member 18, with the 
lower member edge projections 14a, 14b and 14c positioned to bear against 
the ring surface 11a, at locations referenced by broken lines; the upper 
member 24' (not shown in this Figure) will have its edge portions 22a-22c 
bearing upon upper stacked surface 11b at the solid line positions 22'a, 
22b'and 22c', respectively. 
FIG. 2a illustrates one presently preferred embodiment for forming the 
sensing coil means 26. In applications where coil 26 requires a relatively 
few number of turns, three-quarters of each turn is formed of a 
radially-disposed conductive staple 40a having the opposite ends, e.g. 
interior end 40a and exterior end 40b, thrust into the material of 
immovable support 18. Support 18 is fabricated of an insulative material 
and has a plurality of conductive, i.e. printed circuit, leads 42 
fabricated upon one of the upper surface 18a or the lower surface thereof. 
Leads 42 are so disposed as to connect the interior end 40a of a first 
staple 40 to the outer end 40b of a next adjacent staple 40. In this 
manner, completion of the fourth side of each turn is accomplished. 
Additional conductive leads 43 are a1so fabricated upon member surface 
18a, leading radial1y outwardly from the circle formed by staple exterior 
legs 40b, to interconnect each of a plurality of termination pads 44 to 
the appropriate leg of each staple forming the initial and final turns of 
each of at least one coil. In FIG. 2a, a pair of coils 26-1 and 26-2 are 
formed, respectively between termination pads 44a-44b and 44c-44d. The 
pair of coils may be utilized for a voltage-input/voltage-output ratio 
measurement of magnetic flux density saturation magnitude, or may have a 
jumper wire connected, as between terminals 44a and 44c to provide a 
single coil (wherein the pair of coil terminals 26a and 26b of FIG. 1 are 
formed by terminal pads 44b and 44d respectively) if only a single coil 
for a constant-voltage-drive/current-output-monitoring measurement 
technique of magnetic flux density magnitude is to be utilized. 
Referring now to FIG. 2b, for a sensor having at least one sensing coil, 
with large or small required turns, each coil 26-1', 26-2', etc. may be 
wound upon an insulated bobbin 46, which may itself be formed of a pair of 
complementary semi-cylindrical members 48a and 48b. It will be seen that 
the pair of half-bobbin members 48 are placed about the annular ring stack 
11 and suitably fastened together, and coils 26 then wound within the 
bobbin channel 46a. 
FIG. 2b also illustrates the use of upper and lower members 16" and 24" 
each having only a pair of extension portions 15a" and 15b" or 22a" or 
22b". Thus, the annular ring stack 11 is supported by lower member edges 
14a" or 14b" at a pair points, illustrated by the broken line edge 
indicators 49a and 49b (shown extended to the upper stack surface from the 
actual lines on the lower surface), while the edge portions 24a" and 24b" 
of upper member 24' contact the annular ring stack upper surface 11a at a 
pair of diametrically opposed points, which are disposed along a line 
substantially perpendicular to the line running through lower edge points 
49a and 49b. Thus, it should be understood that each of the opposite major 
surfaces 11a and llb of the annular ring stack 11 have at least a 
plurality of edge portions in abutment therewith, with substantially equal 
annular displacement between the totality of sequential (and alternating) 
edge portions. 
Referring now to FIG. 3, a presently preferred embodiment of a pressure 
sensor 60 is illustrated utilizing a force-sensing magnetoelastic ring 
stack 11. Sensor 60 includes an outer shell top member 62, an outer she1l 
bottom member 64 and a separating annular washer 66. Shell members 62 and 
64 can be fabricated of any material impervious to the "fluid" whose 
pressure is to be measured; many of the common plastics are acceptable. 
Top shell member 62 has a volume 68 formed, as a right circular cylinder, 
into the bottom surface thereof, in communication with a channel 62a. The 
upper portion of top shell member 62 may be extended as a tube wall 62b, 
surrounding channel 62a, for introduction of the fluid, at the pressure to 
be measured, into volume 68. Alternatively, the top shell member upper 
portion 62c may be suitably contoured for attachment of a separate tube 
70, for conveyance of the pressurized fluid to channel 62a and thence to 
volume 68. Advantageously, the volume 68 is cylindrical about sensor 
center-line axis 60a, as are the outer walls 62d of the lower portion of 
top shell member 62, to form an annular wall portion 62e bounding chamber 
68. 
Lower shell member 64 can be formed of the same material as upper shell 
member 62, with a volume 73 formed into the upper surface thereof. 
Advantageously, volume 73 is also in the shape of a right circular 
cylinder, about sensor center line 60a, and has a circular outer periphery 
64a. By utilizing the same diameter for top and bottom shell member outer 
peripheries 62d and 64a, and by making the interior volumes 68 and 73 of 
the same diameter (providing the downwardly-extending annular wall 62e of 
top shell member 62 and the upwardly-extending annular wall 64b of lower 
member 64 with the same thickness), the intermediate washer 66 can have an 
exterior diameter equal to the exterior diameter of both upper and lower 
shell members. Washer 66 includes an aperture 66a located substantially at 
the center of the washer and along the sensor center line 60a. The ends of 
the upper and lower annular wall portions 62e and 64b are sealed to the 
respective upper and lower washer surfaces 66b and 66c, respectively, 
about the entire periphery of the washer and both members. 
Prior to sealing, an O-ring 72, of outer diameter slightly greater than the 
interior diameter of volume 68, is inserted into volume 68 and is then 
overlaid with a slack diaphram member 74, typically of circular shape and 
of diameter at least equal to the inner diameter of wall portion 62e. 
Diaphram member 74 may be formed of any suitable flexible material, such 
as Kapton.RTM. the like. The thickness of O-ring 72 is such that upon 
insertion of 0-ring 72 and diaphram 74 and sealing of washer 66 to the 
ends of upper member annular wall 62e, in pressure-tight manner, along 
the outer periphery of the washer upper surface 66b, the O-ring is 
compressed to form a pressure-tight seal of the periphery of the diaphram 
member 74 to the washer upper surface 66b. 
The fixed member 18, having a circular periphery of diameter less than the 
interior diameter of the lower shell member volume 73, is placed at the 
bottom of volume 73 and first member 16 is positioned thereon. 
Advantageously, lower member 16 will be fastened to member 18, by suitable 
adhesive or the like material, to maintain the center axis of member 16 in 
alignment along the center line 60a of the sensor. The annular 
magnetoelastic ring stack 11 is then positioned upon the appropriate edge 
portions 14 of member 16; the at least one coil means 26, fabricated about 
a portion of the ring stack periphery, is then, if not previously, 
positioned as required to provide clearance for the edges of upper member 
24, to be brought into abutment with the upper surface of stack 11. The 
upper member 24 is then positioned with the appropriate distortion-causing 
edges 22 resting at the appropriate locations upon the top surface of 
stack 11. In the sectional view of FIG. 3, each of upper and lower members 
24 and 16, respectively, are provided with four edge-bearing extensions, 
each disposed at a 90.degree. angle with respect to the adjacent 
edge-bearing extensions of the same member, and each member extension 
disposed at a 45.degree. angle with respect to the adjacent extensions of 
the other member. 
Upper member 24 is provided with an integral extension portion 80, 
extending upwardly along the sensor center line 60a. Extension portion 80 
is of a diameter less than the diameter of the washer aperture 66a, and of 
a length such that extension 80 passes through washer aperture 66a and is 
in abutment with the adjacent surface of diaphram member 74, when the 
upper shell member 62/washer 66 subassembly is placed over the lower 
member subassembly and the washer lower surface 66c periphery is sealed, 
in pressure-tight fashion, to the end of the lower shell member annular 
wall portion 64b. It should be understood that the upper member, washer 
and lower member assembly can be fastened together by other means, such as 
by rolled collar, clamp and the like means. 
It will be seen that an increase in the pressure introduced into the 
chamber formed by upper member volume 68 will exert a force pressing down 
upon that portion of diaphram 74 over washer aperture 66a and cause 
extension 80 to be pushed downwardly. The downward movement of extension 
80 moves first member 24 downwardly, distorting the annular ring stack 11 
and reducing the saturation magnetic flux density magnitude. This change 
in saturation flux density magnitude is sensed through the use of one or 
both coils 26 (which themselves are brought through the bottom shell 
member wall, in pressure-tight fashion, if required, to the coil ends 
26a-26d, as required, available upon the sensor external surface). 
Similarly, a reduction in pressure of the fluid is communicated to the 
chamber formed by volume 68 and causes a relaxation of force upon, or an 
upward motion of, the portion of diaphram means 74 over washer aperture 
66a. The "springiness" of the annular rings of stack 11 is transmitted 
through first member edges 22 to the first member 24, and its extension 
80, causing the first member to move upwardly to keep extension 80 in 
contact with diaphram means 74. Thus, a pressure reduction reduces the 
distortion of the annular rings of stack 11 and results in an increase in 
the absolute magnitude of the saturation flux density of the ring stack, 
which change in saturation flux density is related to the change in 
pressure, and is sensed, again, by use of one or both coil means 26. 
As each of the annular rings 12 in stack 11 is relatively thin, it is 
difficult to ensure that each ring will be substantially flat, such that, 
if the bottom surface 18b of member 18 is in abutment with the bottom 
surface 73a of the chamber formed by volume 73, extension 80 will abut 
against the diaphram means surface in suitable manner. Accordingly, a 
threaded aperture 64c is fabricated through the bottom wall 64d of the 
lower sensor housing member 64, along the sensor center line 60a, to 
receive an adjustment means 85, such as a screw and the like. The bottom 
surface 18b of member 18 may also be provided with a short extension 
formation 87, against which the adjustment means 85 will bear, to allow 
movement of member 18 upwardly and downwardly responsive to rotation of 
screw 85, to adjust the contact force of extension 80 against diaphram 74. 
Equally advantageously, upper member 24 may be provided with formations 
90, about the periphery of the base of extension 80, and of a diameter 
such that formations 90 can be brought to bear against the lower surface 
washer 66c and prevent excessive upward adjustment of first member 24. By 
introducing a reference pressure into the chamber formed by upper volume 
68, adjustment screw 85 can be utilized to compress the rings of stack 11 
by an amount sufficient to provide a calibration reference signal by 
monitoring the output of the appropriate one of coil means 26, with 
formations 90 preventing overadjustment and possible damage of the 
completed sensor during calibration. Formations 90 can be utilized with 
means 85 to remove some of the "springiness" of the disc stack by 
precompression of the stack at initial calibration. The formations can 
also be used to set a minimum sensed pressure, below which pressure 
diaphram 74 will be forced upward to a degree such that extension 80 will 
not be able to follow due to formations 90 coming into abutment with the 
lower washer surface 66. 
While several presently preferred embodiments of our novel magnetoelastic 
pressure/force sensor have been described in detail herein, many 
modifications and variations will now become apparent to those skilled in 
the art. For example, if the spring constant of the rings (in the stack) 
is relatively low, the sensor can be utilized for displacement sensing. 
Other geometric shapes, such as annular triangles, squares, rectangles and 
polygons can be equally as well utilized. Similarly, lower member chamber 
73 can be vented to ambient atmosphere for a gauge (rather than absolute) 
pressure sensor. It is our intent, therefore, to be limited only by the 
scope of the appending claims and not by the specific details and 
instrumentalities presented by of explanation of these preferred 
embodiments herein.