Displacement sensor

A sensor (1) for sensing the displacement of a body (12) comprises a deformable strip (2) enclosed in a chamber 3. The strip is constrained under longitudinal and transverse constraint such that end portions (9, 10) of the strip are parallel to a direction of movement of the body, the strip being locally deformed in the form of an S-bend. A magnet operable between the strip and the body varies the location of the S-bend along the length of the strip according to the position of the body. Optical or electronic sensors are provided for indicating the sensed position. The sensor may be used for sensing liquid levels or displacement of moving bodies such as valve elements particularly in hazardous environments including aircraft.

BACKGROUND OF THE INVENTION 
This invention relates to a sensor for sensing the displacement of a body 
and in particular to a sensor which is suitable for use in hostile 
environments where direct contact with the body is undesirable. 
It is known for displacement to be sensed using a follower device such as 
disclosed in U.S. Pat. No. 3,065,635 in which a magnetic follower is 
isolated from the body but moves in unison with the body by virtue of 
magnetic coupling. The displacement of the follower device is then taken 
as an indication of the body's displacement. 
Such sensors have particular application to measurement of liquid levels 
where the body constitutes a float. 
It is also known from U.S. Pat. No. 4,637,254 to provide a position sensor 
in which a deformable strip adopts an S-bend configuration at a localized 
position along its length and the position at which the S-bend is formed 
is determined by the position of a body whose displacement is to be 
sensed. The position at which the S-bend is formed is then detected as an 
indication of body position. A disadvantage of this system is that the 
body is biased into contact with the strip in order to deform the strip 
and a flexible seal arrangement is required to protect the sensor from the 
environment in which the body is movable. 
SUMMARY OF THE INVENTION 
According to the present invention there is disclosed a sensor for sensing 
the displacement of a body comprising a deformable strip, a rigid 
enclosure defining a chamber within which the strip is received and 
isolating the strip from contact with the body, constraining means 
operable to maintain the strip under longitudinal and transverse 
constraint such that first and second end portions of the strip extend 
substantially parallel to a longitudinal direction in which the body is 
movable relative to the sensor and locally deforming the strip in a 
transverse direction orthogonal to the first and second portions such that 
a deformed portion in the form of an S-bend integrally and unitarily 
connects the first and second end portions, and first magnetic means 
operable between the strip and the body to apply a magnetic force 
non-uniformly to the strip such that the location at which the strip is 
deformed to constitute the deformed portion is longitudinally variable in 
registration with a reference portion of the body whereby the longitudinal 
displacement of the deformed portion is representative of the displacement 
of the body. 
An advantage of such an arrangement is that the rigid enclosure avoids the 
need for any flexible seal arrangement and no contact is made between the 
body and the strip thereby reducing frictional drag effects on body 
movement. 
Preferably the strip is comprised of ferromagnetic material and the body 
comprises a magnetic element arranged in closer proximity to the first end 
portion than to the second end portion thereby constituting the first 
magnetic means. 
Conveniently the length of the length of the chamber is less than the 
undeformed length of the strip whereby end walls of the enclosure 
constitute longitudinally constraining elements of the constraining means. 
Preferably the enclosure comprises longitudinally extending first and 
second walls contacted by the respective first and second end portions of 
the strip and wherein the first magnetic means is operable to bias the 
first end portion into contact with the first wall. 
Preferably the sensor further comprises biasing means operable to bias the 
second end portion into co-planar contact with the second wall. 
The biasing means may comprise a second magnetic means operable to apply 
substantially uniformly to the strip a magnetic force in opposition to and 
of smaller magnitude than the magnetic force applied by the first magnetic 
means. 
Alternatively the biasing means may comprise electrostatic means operable 
to apply an electrostatic force in opposition to and of lesser magnitude 
than the magnetic force applied by the first magnetic means. 
Alternatively the biasing means may comprise a clamp operable on the second 
linear portion to apply a transverse deformation such that the second 
linear portion exhibits an arcuate profile when viewed in longitudinal 
projection. 
Conveniently the magnetic element extends longitudinally in a direction 
parallel to the direction of motion and wherein the magnetic element is 
transversely magnetized along its length. 
Advantageously the enclosure comprises a window through which the strip is 
visible. 
The chamber may be filled with a liquid. Where an opaque liquid is used in 
combination with a transparent window against which the second end portion 
of the strip is biased, the strip will be visible to the extent that it is 
in contact with the transparent wall and thereby provides a visible 
indicator of the position of the deformed portion. A viscous liquid within 
the chamber may also be utilized to provide damping. 
The sensor may further comprise sensing means operable to sense the 
position of the deformed portion and to produce an electrical signal 
representative of the longitudinal displacement of the deformed portion. 
Preferably the sensing means comprises at least one electrode associated 
with the enclosure and wherein the sensing means is operable to produce an 
electronic signal responsive to the capacitance between the electrode and 
the strip. 
Alternatively the sensing means may comprise two or more inductive elements 
associated with the enclosure and wherein the sensing means is operable to 
produce an electronic signal responsive to inductive coupling between the 
elements. 
Alternatively the sensing means may comprise a transmitter of 
electromagnetic radiation operable to transmit longitudinally through the 
chamber, a detector operable to detect reflective radiation, ranging means 
operable to generate an output signal representative of the range at which 
the radiation is reflected and reflecting means associated with the 
deformed portion whereby the output signal is representative of the 
displacement of the deformed portion. 
In such an arrangement the constraining means may conveniently be arranged 
to induce an S-bend at the deformed portion in which the strip is deformed 
through substantially 90.degree. relative to its longitudinal extent. 
The sensing means may alternatively comprise a longitudinally extending 
fluorescent light guide, the sensor further comprising a light source 
operable to illuminate the deformed portion and an optical detector 
operable to produce a signal representative of the longitudinal location 
of the light guide at which the fluorescence is detected. 
In such an arrangement the constraining means is preferably operable to 
deform the strip such that the strip is deformed through substantially 
45.degree. at the S-bend whereby a longitudinally directed light beam is 
transversely deflected through substantially 90.degree. at the deformed 
portion. 
The sensing means may alternatively comprise a scale extending 
longitudinally of the enclosure and optical reading means operable to read 
the portion of scale in registration with the deformed portion. 
In such an arrangement the scale may comprise markings applied to the strip 
and wherein the deformed portion comprises an S-bend in which the strip is 
bent through substantially 90.degree. relative to its longitudinal extent. 
The scale may alternatively comprise markings applied to a wall of the 
enclosure and wherein the deformed portion comprises an S-bend in which 
the strip is bent through substantially 45.degree. relative to its 
longitudinal extent. 
In such an arrangement the optical detector comprises one or more lenses or 
mirrors mounted on a sledge coupled to the strip so as to be localized to 
and movable with the deformed portion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In describing the following embodiments, the same reference numerals will 
be used where appropriate for corresponding elements of different 
embodiments. 
In FIG. 1 a sensor 1 comprises a deformable strip 2 of resilient 
ferromagnetic sheet material which extends longitudinally within a chamber 
3 defined by an enclosure 4. The enclosure 4 is generally rectangular 
having opposite and parallel first and second walls 5 and 6 respectively 
extending horizontally with the second wall uppermost and side walls (not 
shown) which are removed in the drawing for clarity. The chamber is 
bounded by end walls 7 and 8 against which the ends of the strip 2 abut, 
the end walls 7 and 8 being longitudinally spaced apart by a distance 
which is less than that required to allow the strip to extend linearly to 
its full extent so that, in order to accommodate the strip within the 
chamber 3, the strip must be deformed. 
The shape adopted by the strip 2 is such that a first portion 9 extends 
linearly in contact with the first wall 5 and extends into abutment with 
end wall 8. A second portion 10 extends linearly in contact with the 
second wall 6 so as to extend parallel to and spaced above the first 
portion 9 and extends longitudinally into abutment with end wall 7. A 
deformed portion 11 connects the first and second portions 9 and 10, the 
first, second and deformed portions thereby being formed integrally and 
unitarily and merging smoothly from one to the other. 
The enclosure 4 provides longitudinal constraint upon the strip 2 by virtue 
of the abutting end faces 7 and 8 which restrict the longitudinal extent 
of the strip and furthermore the enclosure provides transverse constraint 
in a direction orthogonal to the walls 5 and 6 and to the plane of the 
strip by virtue of the first and second walls 5 and 6 being spaced apart 
by a fixed distance determined by the end walls 7 and 8. The position at 
which the strip 2 is deformed to constitute the localized deformed portion 
11 is not uniquely determined by this constraint and by applying an 
external force to the strip it is possible to shift the location of the 
deformed portion longitudinally in a continuously variable manner between 
the end walls 7 and 8. In referring to the first, second and deformed 
portions 9, 10 and 11 therefore it is to be understood that these 
references apply to the instantaneous shape of the strip 2 since for 
example an element of the strip at its mid-point could at various times 
lie within the first portion, the second portion or the deformed portion 
depending on the location of the S-bend induced by constraining the strip. 
A magnetic element 12 constituting part of a body whose position is to be 
sensed has a major surface 13 exending parallel to and in proximity with 
the first wall 5 of the enclosure 4. The magnetic element 12 is 
longitudinally displaceable by movement of the body in a direction 
parallel to the longitudinal extent of the enclosure 4 and the strip 2 and 
the enclosure is positioned such that the available range of movement of a 
leading edge 14 of the element 12 lies between limits which are between 
the end walls 6 and 7 and which are inwardly spaced from the end walls by 
at least the longitudinal extent of the deformed portion. 
The magnetic element 12 is permanently magnetized in a transverse direction 
parallel to the end walls 6 and 7 such that the strip 2 is magnetically 
attracted towards the element by a magnetic force which in the orientation 
of FIG. 1 is directed vertically downwards. 
Consequently the first portion 9 of the strip 2 is biased by the magnetic 
force in a downward direction and rests in linear contact with the first 
wall 5. 
A second magnetic element 15 is located externally of the enclosure 4 
adjacent the second wall 6 and is permanently magnetized in the same sense 
as the first magnetic element 12 but to a lesser strength. Consequently a 
weaker attractive magnetic force acts vertically upwardly on the strip so 
that the second portion 10 of the strip is biased vertically upwards and 
rests in linear contact with the second wall 6. 
The deformed portion 11 is constrained to extend between the first and 
second portions 9 and 10 at a location which is longitudinally localized 
in registration with the leading edge 14. 
Longitudinal displacement of the element 12 to the left or right as viewed 
in FIG. 1 will be accompanied by movement of the strip such that the 
deformed portion 11 remains in registration with the leading edge 14. The 
longitudinal displacement of the deformed portion 11 is therefore 
representative of the longitudinal displacement of the body associated 
with the magnetic element 12. The position of a body may thereby be sensed 
using the arrangement of FIG. 1 by attaching such a magnetic element 12 to 
the body in proximity with the sensor 1. 
The position of the deformed portion 11 may be visually observed by 
providing the enclosure 4 with a suitable window which preferably provides 
viewing through the second wall 6. The chamber 3 may if required be filled 
with an opaque liquid such that a visual demarcation is viewable in the 
window between that portion of the window contacted by the strip 2 and 
that portion which is separated from the strip and therefore contacted by 
opaque liquid. Various arrangements of illuminating such an enclosure and 
providing measurement scales can also be incorporated into such a sensor. 
The second magnetic element 15 may alternatively be replaced by an 
electrode to which an electrostatic charge is applied by a suitable 
circuit in order to provide electrostatic attraction between the strip 2 
and the electrode in a direction which will bias the strip into contact 
with the second wall 6. 
The magnetic element 12 in a further alternative arrangement may comprise a 
magnetized portion located at the leading edge 14 of sufficient strength 
to obviate the need for the element to be magnetized along its entire 
length. 
An alternative embodiment is shown in FIG. 2 in which an alternative sensor 
20 has a strip 2 which is resiliently deformed so as to remain biased into 
contact with the second wall 6. An end portion 21 of the second portion 10 
of the strip is held in a clamp 22 which grips the end portion such that 
it is deformed into adopting an arcuate profile as viewed in longitudinal 
projection. The strip 2 will then tend to remain biased against the second 
wall 6 in opposition to the downward magnetic force applied by the 
magnetic element 12. The second wall 6 is formed of transparent material 
constituting a window through which the strip 2 can be observed and the 
chamber 3 is filled with opaque liquid. 
The position of the deformed portion 11 may be sensed by other means 
instead of or in addition to visual means. The first and second walls 5 
and 6 may for example comprise insulated conducting plates. By connecting 
the plates and the strip to a suitable circuit the capacitances C.sub.1 
and C.sub.2 between the first plate and the strip and between the second 
plate and the strip respectively can be sensed and compared. An output 
representative of C.sub.1 /(C.sub.1 +C.sub.2) will for example provide a 
signal representative of the fractional distance from one end at which the 
deformed portion is located. 
FIG. 3 illustrates a further alternative means of sensing the position of 
the deformed portion 11. Conductive tracks 25 and 26 are arranged to 
extend in coaxial loops over the first and second walls 5 and 6, the 
tracks being arranged such that magnetic flux generated by current through 
one track will be coupled to the second track. By connecting the tracks to 
a suitable circuit and energizing one track with an alternating current 
the inductive coupling between the tracks can be sensed and will be 
influenced by the proximity of the electromagnetic strip (not shown in 
FIG. 3 for clarity). 
The strip 2 may itself be marked with a scale which can be read by a 
detector arranged to read only that portion of the scale corresponding to 
the deformed portion 11. An optical arrangement using light or non-visible 
radiation can be arranged such that an illuminating beam is passed 
longitudinally through the chamber 3 so as to be incident upon the 
deformed portion 11 as illustrated schematically in FIG. 6 where a light 
beam 60 is shown being transmitted and received from a transducer 61. In 
the arrangement of FIG. 6 the strip is constrained such that its S-bend 
configuration deforms the strip to a maximum deflection of 90.degree. 
relative to the longitudinal extent. There is therefore a central portion 
of the deformed portion 11 which is substantially at 90.degree. to the 
light beam 60 thereby enabling light to be returned to the transducer. The 
scale illuminated at the deformed portion 11 may be read by a suitable 
optical arrangement. The effectiveness of such reading techniques will be 
enhanced by increasing the size of that reflective part of the strip 2 
which is maintained at substantially 90.degree. to the longitudinal extent 
of the strip and further improved by providing a positive powered lens in 
proximity with the scale to be read. In such an arrangement it is 
therefore advantageous to utilize a movable lens element 30 as illustrated 
in FIG. 4 which is mounted on a carrier 31 arranged to move the lens 
element in unison with the deformed portion 11. 
The carrier 31 is mounted for sliding movement longitudinally of the 
chamber and includes first and second guide rollers 32 and 33 which engage 
and guide the strip 2 in a manner which constrains the deformed portion 11 
to extend linearly between the guide rollers in a direction orthogonal to 
the walls 5 and 6 and hence orthogonally to the beam 60. 
The guide rollers 32 and 33 are arranged to contact upper and lower 
surfaces 34 and 35 respectively of the strip and are spaced apart in a 
direction orthogonal to the plane of the strip so as to constrain the 
strip into its deformed state. The carrier 31 is formed of a low friction 
material which in the preferred embodiment is PTFE so that sliding contact 
with the walls of the enclosure 4 incurs minimal resistive drag. 
Scale markings 36 applied to the strip 2 provide a binary code which for 
each illuminated portion of the scale markings uniquely defines the 
position of the illuminated portion with respect to the longitudinal 
extent of the scale. In the preferred embodiment the scale is represented 
by a pseudo random binary sequence of black and white bars. Alternative 
codes such as Gray code may be utilized. 
Such a scale may alternatively be applied to the second wall 6 and may be 
read by illuminating a portion of the scale using a beam which is 
deflected from the deformed portion through 90.degree.. Such an 
arrangement requires the deformed portion to extend at substantially 
45.degree. to the longitudinal extent of the enclosure. This can be 
accommodated by suitable spacing of the end walls 7 and 8. The area 
available for reflection may be enhanced by using a carrier 31 in which 
the guide rollers 32 and 33 are offset in longitudinal position so as to 
dispose a substantially planar surface of the deformed portion 11 
extending at 45.degree. relative to the longitudinal extent. 
Such deflection through 90.degree. may alternatively be utilized in an 
arrangement shown in FIG. 7 in which a fluorescent light guide 70 is 
placed along the second wall 6 and is illuminated by means of a 
longitudinally directed beam 60 which is reflected through 90.degree. so 
as to be incident upon the light guide in registration with the deformed 
portion 11. The location along the light guide 70 at which fluorescence 
occurs is thereby representative of the position of the deformed portion 
11 and may be visually or otherwise sensed to provide an indication of the 
displacement of the body. 
A further alternative method of sensing the displacement of the deformed 
portion utilizes an arrangement of the type shown in FIG. 6 in which a 
light beam 60 transmitted so as to be reflected from the deformed portion 
is detected by a transducer 61 of an optical ranging device utilized to 
sense the range of the deformed portion from the transducer. A signal 
representative of the range is therefore representative of the position of 
the deformed portion 11 and hence is representative of the displacement of 
the body. 
Sensors of the type disclosed above have numerous applications such as the 
sensing of liquid level as illustrated schematically in FIG. 5. A float 40 
is arranged to be vertically movable on the liquid surface and in 
proximity with the enclosure 4. The strip 2 received within the enclosure 
has a deformed portion 11 which moves in registration with a magnetic 
element 12 forming part of the float. 
The position of the deformed portion 11 may be sensed by any of the 
techniques referred to above to provide a liquid level measurement. 
The use of an enclosure which seals the chamber 3 containing the strip 2 
from external environments renders the sensor particularly useful for 
applications in aircraft for measurement of liquid levels and displacement 
of other moving bodies. Valve position elements may also be remotely 
sensed using such sensors. 
The chamber 3 may be filled with a suitable damping fluid to remove 
vibration effects should this be necessary in a particular environment. 
The magnetic means associated with the body may alternatively comprise an 
electromagnet. 
Alternatively the strip itself may be permanently magnetized and the 
magnetic means associated with the body may be a ferromagnetic element 
which is acted upon by the strip. 
In a further alternative, the strip may be placed in the field of a fixed 
permanent magnet, a soft iron element being associated with the body so as 
to distort the field in a manner which is depended on displacement.