U.S. patent application number 10/848525 was filed with the patent office on 2005-02-10 for load measurement device.
This patent application is currently assigned to BBNT SOLUTIONS LLC. Invention is credited to Madden, Richard, McCarthy, Daniel Joseph, Thomas, Gary L..
Application Number | 20050028606 10/848525 |
Document ID | / |
Family ID | 25287651 |
Filed Date | 2005-02-10 |
United States Patent
Application |
20050028606 |
Kind Code |
A1 |
Madden, Richard ; et
al. |
February 10, 2005 |
Load measurement device
Abstract
A variable reluctance load cell for measuring the static or
slowly fluctuating load, or tension, on devices is contained in a
support tube. A sensor in the tube utilizes opposing C and I shaped
magnetic cores attached to opposing ends of the support tube. A
magnetic circuit is formed having an inductance defined by the size
of the gap between the magnetic cores with the reluctance dominated
by the gap. The sensor inductance is coupled with a fixed,
predetermined capacitance in a resonant LC circuit, and the
resonant frequency is a function of the gap. The sensor is in a
cavity within the tube, and the cavity is sealed in a manner that
prevents water or other damaging agents from entering the sensor.
In this manner, mounting the sensor and tube to a static device and
measuring the AC voltage at the sensor, the amount of load, or
stress can be determined.
Inventors: |
Madden, Richard; (Groton,
MA) ; McCarthy, Daniel Joseph; (Cambridge, MA)
; Thomas, Gary L.; (Hingham, MA) |
Correspondence
Address: |
ROPES & GRAY LLP
ONE INTERNATIONAL PLACE
BOSTON
MA
02110-2624
US
|
Assignee: |
BBNT SOLUTIONS LLC
Cambridge
MA
|
Family ID: |
25287651 |
Appl. No.: |
10/848525 |
Filed: |
May 17, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10848525 |
May 17, 2004 |
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10138272 |
May 3, 2002 |
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6752029 |
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10138272 |
May 3, 2002 |
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09842564 |
Apr 26, 2001 |
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6422089 |
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Current U.S.
Class: |
73/779 |
Current CPC
Class: |
G01G 3/12 20130101; G01L
5/101 20130101; G01L 5/102 20130101 |
Class at
Publication: |
073/779 |
International
Class: |
G01L 001/00 |
Claims
What is claimed is:
1. A load cell comprising: a link, said link constructed of a first
material, said link having a link longitudinal axis; a tube having
a tube-longitudinal axis and attached to a surface on the link such
that said link longitudinal axis and said tube longitudinal axis
are approximately parallel; first and second members, said first
and second members extending transversely across an interior
portion of the tube at different locations and forming a sensor
cavity within the tube; a first magnetically permeable core
attached to said first member and extending transversely to said
tube longitudinal axis and providing a pair of first spaced pole
ends, each first pole end having a first spaced face; a second
magnetically permeable core attached to said second member and
extending transversely to said tube-longitudinal axis and providing
a pair of second spaced pole ends, each second pole end having a
second spaced face, each said second spaced face opposing its
corresponding each said first spaced face and separated by a
magnetic gap width; a first excitation coil wound around one of the
first pole ends and a second excitation coil wound around the other
first pole end using conductive wire, one end of the wire from each
of the first and second excitation coils being conductively
connected and the other end forming an excitation coil pair; said
excitation coil pair connected to a cable extending through the
tube, connectable to a supply voltage and predetermined capacitance
in a resonant inductance-capacitance circuit, providing connection
to an inductance which is variable as a function of the magnetic
gap width and also as a function of relative displacement of said
first and second members resulting from strain on the link along
the link longitudinal axis.
2. The load cell of claim 1, further comprising an electrical
connector mounted on the tube, an inside portion of the connector
facing the sensor cavity, said excitation coil pair of wires
electrically connected to the inside portion of the electrical
connector.
3. The load cell of claim 1, wherein said first spaced faces are
substantially perpendicular to the tube-longitudinal axis.
4. The load cell of claim 1, wherein said second spaced faces are
substantially perpendicular to the tube-longitudinal axis.
5. The load cell of claim 1, wherein the voltage source is an
external voltage source.
6. The load cell of claim 1, wherein the first magnetically
permeable core is generally C-shaped.
7. The load cell of claim 1, wherein the first magnetically
permeable core is generally I-shaped.
8. The load cell of claim 1, wherein the external voltage is
AC.
9. The load cell of claim 1, wherein the electrical connector is an
underwater mateable connector.
10. The load cell of claim 1, wherein the excitation coil pair of
wires are between 24 and 0 gauge.
11. The load cell of claim 1, further comprising an encapsulation
material that encapsulates the first and second excitation
coils.
12. The load cell of claim 10, wherein the encapsulation material
is polyurethane.
13. The load cell of claim 1, further wherein the first material is
steel.
14. The load cell of claim 1, wherein the sensor cavity is sealed
to prevent water or damaging agents from entering the cavity and
damaging any of the first magnetically permeable core, the second
magnetically permeable core, the first excitation coil, the second
excitation coil, the conductive wire, the excitation coil pair, and
the electrical connector.
15. The load cell of claim 1, wherein the sensor cavity is filled
with a second material that protects at least one of the first
magnetically permeable core, the second magnetically permeable
core, the first excitation coil, the second excitation coil, the
conductive wire, the excitation coil pair, and the electrical
connector from environmental agents.
16. The load cell of claim 15, wherein the environmental agents are
one of fresh water, seawater, air, pollutants contained in air,
pollutants contained in fresh water, pollutants contained in
seawater and marine growth contained in sea water.
17. The load cell of claim 15, wherein the second material is a low
durometer elastomeric potting material.
18. The load cell of claim 17, wherein the low durometer
elastomeric potting material is one of a solithane and an RTV.
19. The load cell of claim 15, wherein the second material is an
oil.
20. The load cell of claim 19, wherein the oil is a silicon
oil.
21. A load measurement device comprising: a link, said link
constructed of a first material having a link longitudinal axis; a
hollow structure fixedly attached to said link along the link
longitudinal axis, said hollow structure constructed of a second
material having substantially repeatable elastic properties, said
structure having a longitudinal axis; first and second members,
said first and second members extending transversely across an
interior portion of the hollow structure at different locations and
forming a cavity within the structure; a first magnetically
permeable core attached to said first member and extending
transversely to said longitudinal axis and providing a pair of
first spaced pole ends, each first pole end having a first spaced
face, said first spaced faces substantially perpendicular to the
longitudinal axis; a second magnetically permeable core attached to
said second member and extending transversely to said longitudinal
axis and providing a pair of second spaced pole ends, each second
spaced pole end having a second spaced face, each of said second
spaced faces substantially perpendicular to the longitudinal axis
and opposing corresponding first spaced faces and separated by a
magnetic gap width; a first excitation coil wound around one of the
first spaced pole ends and a second excitation coil wound around
the other first spaced pole end using conductive wire, one end of
the wire from each of the first and second excitation coils forming
an excitation coil pair; and an electrical connector mounted on the
hollow structure, an inside portion of the connector facing the
cavity within the hollow structure, said excitation coil pair
electrically connected to the inside portion of the electrical
connector, an outer portion of the connector connectable to an
external voltage source and predetermined capacitance in a resonant
inductance-capacitance circuit, providing connection to an
inductance which is variable as a function of the gap width and
also as a function of relative displacement of said first and
second members resulting from strain on the cylinder along the
longitudinal axis.
22. The load measurement device of claim 21, wherein the first
material and the second material have similar coefficients of
thermal expansion.
23. The load measurement device of claim 21, wherein the first
material and the second material are the same material.
24. The load measurement device of claim 21, wherein the first
material and the second material are steel.
25. A load cell for measuring the load on a link constructed of a
first material and having a link longitudinal axis, said load cell
comprising: a tube having a tube-longitudinal axis and attached to
a surface on said link such that said link longitudinal axis and
said tube longitudinal axis are approximately parallel; first and
second members, said first and second members extending
transversely across an interior portion of said tube at different
locations and forming a sensor cavity within said tube; a first
magnetically permeable core attached to said first member and
extending transversely to said tube longitudinal axis and providing
a pair of first spaced pole ends, each first pole end having a
first spaced face; a second magnetically permeable core attached to
said second member and extending transversely to said
tube-longitudinal axis and providing a pair of second spaced pole
ends, each second pole end having a second spaced face, each said
second spaced face opposing its corresponding each said first
spaced face and separated by a magnetic gap width; a first
excitation coil wound around one of said pair of first pole ends
and a second excitation coil wound around the other of said pair of
first pole ends using conductive wire, one end of said wire said
first excitation coil and said second excitation coil being
conductively connected and the other end forming an excitation coil
pair; said excitation coil pair connected to a cable extending
through said tube, connectable to a supply voltage and
predetermined capacitance in a resonant inductance-capacitance
circuit, providing connection to an inductance which is variable as
a function of said magnetic gap width and also as a function of
relative displacement of said first and second members resulting
from strain on said link along said link longitudinal axis.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/138,272, filed on May 30, 2002, which is a
continuation of U.S. patent application Ser. No. 09/842,564, filed
on Apr. 26, 2001, the specifications of which are incorporated by
reference herein in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to load cells for measuring
static and slowly fluctuating load. More particularly, the present
invention relates to a variable reluctance load cell for measuring
the load, or tension, on static devices in an environmentally
hostile environment (e.g., determining mooring line tensions of
offshore oil platforms).
[0004] 2. Description of the Related Art
[0005] Offshore deepwater platforms utilize various mooring systems
for stationkeeping. A number of offshore platforms e. g. spars,
deep draft caissons, semisubmersibles and floating production,
storage and offloading vessels (FPSOs) are equipped with a means to
jack the mooring chain and maintain tension on the line, reducing
the amount of slack. Passively moored platforms or vessels that are
not required to move may not be equipped with a permanently mounted
tensioning system. As the mooring lines experience fatigue and
stretch, the platform can twist, leading to increased friction
between the links, and accelerating fatigue and failure. The amount
of tension on the mooring line determines the amount of slack, and
consequently the amount of relative movement of the platform or
vessel.
[0006] Fairleads are used to attach the mooring chain to the deck
of the platform or vessel. In one configuration, a chain stopper
latches the chain outboard of the fairlead and allows the stopper
to rotate freely about two perpendicular axis. All motion change
between the mooring line and the vessel occurs on proper bearing
surfaces, and not between the fairlead and the chain.
[0007] Tension in the vertical chain leg between the fairlead and
the deck level stopper, combined with the rotation of the fairlead
caused by yawing of the platform or vessel, promote undesirable
wear in the chain links.
[0008] Similarly, suspension bridges rely on large cables to
maintain support for the bridge span. The amount of tension on the
suspension cables is indicative of the stress placed on the cables,
and the amount and rate of cable wear and or fatigue.
[0009] Various devices are available to measure the amount of
tension, or applied tensile force, placed on fasteners and securing
lines, including strain gauge bridges, differential transformers,
capacitance sensors and variable reluctance load cells.
[0010] Mooring line tensions have been measured with instrumented
chain links that employ strain gauged shear pins, strain gauges and
strain gauge load cells. Generally, these devices have not been
reliable for long term applications in hostile environments, for
example, marine and aerospace environments. Strain gauges require
adhesive attachment to the surface being measured. In
non-controllable environments, strain gauges are subject to drift
caused, for example, by adhesive breakdown, requiring
recalibration. In environmentally hostile environments, the
frequency of recalibration, repair or replacements becomes
expensive, and may even be dangerous to perform.
[0011] Variable reluctance load cells use a variable reluctance
transducer to measure force. For example, a core and winding can be
used to sense changes in proximity to a cantilevered spring.
Changes in inductance, caused by changes in the gap between the
core and the spring, are reflected in the frequency of an
oscillator circuit. In a previous load cell sensor utilizing a
variable reluctance transducer, the load cell sensor responded
primarily to forces along a pre-selected axis, while being
relatively insensitive to both forces along axis transverse to the
pre-selected axis, and to bending moments. Accordingly, the load
cell sensors were mounted in the middle of the structure under
observation to compensate for bending forces. Additionally, the
load cell sensor was internal to a load-measuring unit, and
contained all the components. Intrusion of contaminants into the
sensor region could lead to premature aging of the components,
including corrosion, making the readings unreliable.
[0012] Accordingly, there is a need for a load measurement device
that is less prone to the various effects of exposure to hostile
environments and can take into account effects of bending.
SUMMARY OF THE INVENTION
[0013] This invention provides a variable reluctance sensor for
measuring the load, or tension, on static devices in an
environmentally hostile environment.
[0014] A sensor in accordance with the invention uses opposing
magnetic cores contained in a support tube. Each of the magnetic
cores is attached to opposing ends of the support tube. Thus, as
the support tube expands along the tube axis, the ends of the
support tube, which are perpendicular to the tube axis, separate. A
magnetic circuit is formed having an inductance defined by the size
of the gap between the magnetic cores. Accordingly, when the
magnetic cores attached to the tube ends separate, the size of the
gap between the magnetic cores is increased. Thus, when the
inductance is altered, the amount of expansion that has occurred
can be determined. Knowing the elastic characteristics of the
support tube material, the amount of force applied to the support
tube can be calculated. Similarly, contraction of the support tube
results in a change in inductance that is indicative of the amount
of stress reduction. Alternatively the support tube can have very
little stiffness relative to the structure that it is mounted on so
that no load passes through the support tube and it merely
displaces the same amount as the structure displaces in the region
between the attachment points. The combination is tested under
known loads to provide the calibration.
[0015] Preferably, one of the magnetic cores is generally C-shaped,
and attached to an end plate by way of a bracket. The end plate may
be one of the tube ends, or another plate that is in turn attached
to the support tube. The C-shape is preferred for one of the
magnetic cores so that the windings can be placed at the ends of
the C-shaped gaps. The other magnetic core is preferably I-shaped,
and is attached to a second end plate by way of a second bracket.
The second end plate, like the first end plate, may be the other
tube end, or another plate that is in turn attached to the support
tube. Thus, a cavity within the support tube containing the sensor
is formed. Preferably, the cavity containing the sensor is sealed
in a manner to prevent water or other damaging agents from entering
the cavity and damaging the sensor or its wiring. The cavity can
also be filled with a low durometer elastomeric potting material,
silicon oil, or any other suitable material for protection of the
components from environmental agents such as water. The choice of
the elastomeric potting material can be selected according to the
anticipated environmental exposure of the sensor. For example, a
low out-gassing material may be appropriate if the sensor is used
at high altitude or space while a low compression material may be
better if the sensor is used below sea level, such as underwater or
underground.
[0016] An excitation coil is wound around the poles on one of the
magnetic cores, and provides electrical connection for an
inductance whose value is variable as a function of the widths of
the gaps, and also the axial distortion of the support tube. In the
preferred embodiment, there are two excitation coils, each
surrounding a separate end of the C-shaped core. This arrangement
minimizes non-linearity of response due to fringing effects. The
wires from the two coils are twisted and attached to cabling that
connects them to external circuitry. Thus, when excited by an
external AC voltage, the C-core, the I-core and the gap between the
C and I cores form an element of a magnetic circuit. The reluctance
of this element is dominated by the gap because the C and I cores
are fabricated from high permeability magnetic materials having
very little reluctance. The sensor inductance is coupled with a
fixed, predetermined capacitance in a resonant
inductance-capacitance (LC) circuit. The resonant frequency of the
LC circuit is a function of the gap between the C-shaped and
I-shaped cores. Accordingly, changes in the gap dimension results
in a change in oscillation frequency. Since the only active
component in the sensor is the number of excitation coils, the
sensor is immune to drift.
[0017] To measure the load on a static device, for example, a chain
that moors a marine platform, the support tube is fixedly attached
to the surface of a sensor link, and the sensor link placed as a
link in the chain. The support tube can be attached to the surface
of the sensor link using bolts, by welding, or any other suitable
attaching means. In order for the sensor to measure the load on the
sensor link, it is preferred that the sensor tube material and the
sensor link material are compatible, more preferably the same
material or material having the same or similar coefficient of
thermal expansion. In the preferred embodiment, the support tube
and the sensor link are made of steel. When used in marine
applications, it is preferred that a protective coating is applied
to the support tube and the sensor link.
[0018] These and other features and advantages of this invention
are described in or are apparent from the following detailed
description of the preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The preferred embodiments of the present invention will be
described in detail, with reference to the following figures,
wherein:
[0020] FIG. 1 is a top view of a variable reluctance sensor
assembly having a variable reluctance sensor in a support tube
attached to a sensor link;
[0021] FIG. 2 is a close-up top-view of the variable reluctance
sensor;
[0022] FIG. 3 is a side view of the variable reluctance sensor
assembly; and
[0023] FIG. 4 is close-up side view of the variable reluctance
sensor.
[0024] Throughout the drawing figures, like reference numerals will
be understood to refer to like parts and components.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] FIG. 1 is a top view of a variable reluctance sensor
assembly 10 having a variable reluctance sensor 12 in a support
tube 14 attached to a sensor link 16. FIG. 2 is a close-up top-view
of the variable reluctance sensor 12. FIG. 3 is a side view of the
variable reluctance sensor assembly 10, and FIG. 4 is a close-up
side view of the variable reluctance sensor 12.
[0026] The sensor 12 in accordance with the present invention uses
an opposing C-shaped magnetically permeable core 18 and an I-shaped
magnetically permeable core 20 contained in the support tube 14.
This forms a magnetic circuit having an inductance defined by the
size of the gaps 22, 24 between the magnetic cores 18,20.
[0027] The C-shaped core 18 is mounted on a first mounting plate 26
by a first bracket 28. Likewise, the I-shaped core is mounted on a
second mounting plate 30 by a second bracket 32. The first mounting
plate 26 is preferably positioned transverse to the longitudinal
axis of the support tube 14. Similarly, the second mounting plate
30 is also preferably positioned transverse to the longitudinal
axis of the support tube 14. As the support tube 14 expands along
its longitudinal axis, the first and second end plates 26, 30
separate. Since the C-shaped core 18 and the I-shaped core 20 are
attached to their respective mounting plates 26, 30, the size of
the gap 22, 24 will be lengthened. This alters the inductance
between the C-shaped core 18 and the I-shaped core 20, and is
indicative of the amount of expansion that has occurred, and the
force needed to cause the expansion. Likewise, as the support tube
14 contracts along its longitudinal axis, the first and second end
plates 26, 30 move towards each other, resulting in the shortening
of the gap 22, 24. This alters the inductance between the C-shaped
core 18 and the I-shaped core 20, and is indicative of the
reduction in the force applied to the sensor link 16.
[0028] Mounted around each end of the C-shaped core 18 is a first
excitation coil 34 and a second excitation coil 36. The two
excitation coils 34, 36 are connected in series at coil wires 38,
and subsequently attached to cabling 40. The excitation coils 34,
36 provide electrical connection for an inductance whose value is
variable as a function of the widths of the gaps 22, 24, and also
the axial distortion of the support tube 14. This arrangement
minimizes non-linearity of response due to fringing effects. The
coils 34, 36 are preferably encapsulated in a non-conductive
material, for example, polyurethane.
[0029] When excited by an external AC voltage, the C-core 18, the
I-core 20 and the gaps 22, 24 between the C and I cores 18, 20 form
an element of a magnetic circuit. The reluctance of this element is
dominated by the gaps 22, 24 because the C and I cores are
fabricated from high permeability magnetic materials having very
little reluctance. The sensor inductance is coupled with a fixed,
predetermined capacitance in a resonant inductance-capacitance (LC)
circuit, not shown. Many different LC circuits are known in the
art, and the actual layout can vary. Accordingly, LC circuit design
need not be discussed further. The resonant frequency of the LC
circuit is a function of the gap between the C-shaped and I-shaped
cores. Accordingly, changes in the gap dimension results in a
change in oscillation frequency. Since the only active component in
the sensor is the number of excitation coils, the sensor is immune
to drift.
[0030] Cabling 40 can be any electrically conductive wires, and is
preferably low capacitance twisted pair whose dielectric constant
varies little with temperature. Cabling 40 connects the sensor 12
to a connector 42. The connector 42 can be any suitable electrical
connector. In this invention, when used near or under water,
connector 42 is preferably an underwater mateable connector. The
connector 42 allows the cabling 40 to be connected to external
electrical circuitry that supplies electricity as well as
connecting the sensor to a resonant LC circuit, not shown.
[0031] In the preferred embodiment, mounting plates 26, 30, in
conjunction with the support tube 14, forms a sensor cavity 44. The
sensor 12 is enclosed in a protective cavity that can be sealed, to
provide additional protection against water, marine growth or other
agents from damaging the sensor 12 and its cabling 40. In the
preferred embodiment, the sensor cavity 44 is filled with a low
durometer elastomeric potting material that encapsulates the sensor
12 and its wiring. The potting material can be any of a number of
known potting materials, for example, solithanes and room
temperature vulcanizing silicones, commonly referred to as RTV.
Alternatively, the sensor cavity 44 can be filled with an oil,
preferably a silicon oil. Additionally, the external surface 46 of
the sensor cavity 44 can be covered with an elastomeric boot and
shroud, not shown, forming a rugged sheath around the sensor cavity
44 to protect the sensor 12 and its components from damage caused
by dropped or sharp objects impacting the support tube 14 or the,
sensor cavity 44.
[0032] The support tube 14 contains the sensor 12, and is used to
mount the sensor 12 to the sensor link 16. Having a support tube 14
longer than the sensor 14 increases the gauge length of the sensor
assembly 10, providing additional displacement and increasing the
accuracy of the device. The connector 42 is mounted on the support
tube 14, and allows for connection of the sensor 12 to the external
LC circuit.
[0033] The support tube 14 is fixedly attached to the surface 48 of
a sensor link 16. The support tube 14 can be attached to the
surface 48 of the sensor link 16 using bolts, by welding, or any
other suitable attaching means. In order for the sensor 12 to
accurately measure the load on the sensor link 16, it is preferred
that the support tube material and the sensor link material are
compatible. More preferably the support tube material and the
sensor link material should be the same material or material having
the same or similar coefficient of thermal expansion. In the
preferred embodiment, the support tube 14 and the sensor link 16
are made of steel. When used in marine applications, it is
preferred that a protective coating is applied to the support tube
and the sensor link.
[0034] The primary purpose of the support tube 14 is to increase
the gauge length of the assembly and thereby provide additional
displacement to increase the accuracy of the device.
[0035] When assembled, the variable reluctance sensor assembly 10
can measure steady state and slowly fluctuating loads in the range
from DC to 10 Hz. This device is capable of measuring displacements
to as low as 0.0001 inches. Multiplying the displacement by the
stiffness of the link yields the load measurement capability. The
amount of measured displacement is dependent upon the area of the
poles and the opposing area of the I-core, and can range from
10.sup.-4 and larger, including inches and feet.
[0036] The device is particularly appropriate for measuring loads
on deployed structures that require long term monitoring and do not
provide access to the sensor for maintenance or replacement.
Applications include building monitoring, bridge monitoring, tower
monitoring, marine moorings monitoring, and weighing hoppers of a
vehicle. Marine moorings monitoring includes large marine platforms
such as spars, or a buoy that floats vertically in the water and
moored to the seabed by anchors, as well as floating production,
storage and offloading vessels and semi-submersibles and
submersibles.
[0037] Another embodiment is to mount at least 3 sensors away from
the axis in a single housing or in multiple housings and to use the
displacements measured by these sensors to define a plane whose
equation specifies both the displacement along the longitudinal
axis and the rotation thus separately defining the axial tension or
compression and the bending load.
[0038] While the invention has been specifically described in
connection with certain specific embodiments thereof, it is to be
understood that this is by way of illustration and not of
limitation, and the scope of the appended claims should be
construed as broadly as the prior art will permit.
* * * * *