U.S. patent application number 09/863204 was filed with the patent office on 2003-01-23 for torsional sensing load cell with overload protection.
Invention is credited to Bruns, Robert W..
Application Number | 20030015041 09/863204 |
Document ID | / |
Family ID | 25340537 |
Filed Date | 2003-01-23 |
United States Patent
Application |
20030015041 |
Kind Code |
A1 |
Bruns, Robert W. |
January 23, 2003 |
TORSIONAL SENSING LOAD CELL WITH OVERLOAD PROTECTION
Abstract
A torsional sensing load cell, suitable for mounting at support
locations of an automotive seat in order to determine weight and
sitting position of an occupant of a motor vehicle. The load cell
has the shape of a tuning fork, with one arm fixed to a foot
attached to a chassis and a second parallel arm, not contacting the
first arm, arranged to support a quadrant of a seat by means of a
flange on the side of the second arm, causing torsion in the arm. A
stop pin arrangement is provided in the load cell to prevent
overloading the cell in a high force situation such as a
collision.
Inventors: |
Bruns, Robert W.;
(Carmichael, CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER
EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Family ID: |
25340537 |
Appl. No.: |
09/863204 |
Filed: |
May 22, 2001 |
Current U.S.
Class: |
73/849 |
Current CPC
Class: |
B60R 21/01516 20141001;
G01L 1/2206 20130101; B60R 21/0152 20141001; G01G 19/4142 20130101;
G01G 3/10 20130101; G01L 1/048 20130101 |
Class at
Publication: |
73/849 |
International
Class: |
G01N 003/20 |
Claims
What is claimed is:
1. A load cell for use as a force indicator comprising: a first
arm; a second arm in parallel relation to and proximate said first
arm; a joining section integrally connected to an end of said first
arm and an end of said second arm, so that there is torsion in said
joining section when torsion is transmitted between said first and
second arms; a force transducer connected to produce an electrical
output signal representing torsion between said first and second
arms; and an electrical circuit connected to receive said
electrical output signal, said first arm having a stop pin affixed
thereto at an end distal to said joining section, said stop pin
disposed in intersecting relation to said second arm, wherein
deflection of said first arm relative to said second arm is limited
by said stop pin.
2. The apparatus of claim 1 wherein said force transducer comprises
a pair of spaced apart strain gauges, one strain gauge associated
with each of said first and second arms.
3. The apparatus of claim 1 wherein said first and second arms and
said joining section are arranged in a shape of a tuning fork.
4. The apparatus of claim 2 wherein said joining section has an
axis of symmetry and said strain gauges are symmetrically located
relative to the center line.
5. The apparatus of claim 1 wherein said first arm is connected to
a post anchored to a rigid platform.
6. The apparatus of claim 5 wherein said rigid platform is part of
a vehicle.
7. The apparatus of claim 6 wherein said vehicle is a motor
vehicle, said post is a fixed foot rising above said platform.
8. The apparatus of claim 1 wherein said first and second arms are
integrated into a supporting rail.
9. The apparatus of claim 1 wherein said first and second arms each
has a linear geometry.
10. A load cell comprising: a generally U-shaped member having a
main body portion and first and second extending portions extending
away from said main body portion, said second extending portion in
parallel relation to and proximate said first extending portion; a
first transducer coupled along said first extending portion and
proximate said main body portion; and a second transducer coupled
along said second extending portion and proximate said main body
portion, said first extending portion having a lateral member
distal said main body portion, said second extending portion having
an opening into which said lateral member is disposed, wherein
deflection of said first extending portion relative to said second
extending portion is limited by said lateral member contacting a
side of said opening.
11. The load cell of claim 10 wherein the first and second
extending portions are substantially coplanar.
12. The load cell of claim 10 wherein the main body is subject to
torsion when opposing forces are applied to the first extending
portion and to the second extending portion.
13. A load cell comprising: a first member; a second member; a
third member integrally coupling together said first and second
member in a shape of tuning fork so as to produce torsional stress
in said first, second, and third members in response to a first
force on said first member and a second force on said second
member; a first force transducer connected to said third member
proximate said first member to detect torsional stress in said
first member; and a second force transducer connected to said third
member proximate said second member to detect torsional stress in
said second member, said first member having a stop pin fixedly
attached thereto at an end distal said third member, said second
member having an opening therethrough and disposed at an end distal
said third member, said stop pin disposed within said opening.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to load cells, and in particular to
load cells for use in sensing weight and position of a seated
occupant in a motor vehicle for deployment of safety devices, such
as air bags.
[0002] Since the development of the air bag and its inclusion in
automobiles a problem has existed with the relative deployment
force used for various individuals. Air bags have been a
requirement on new vehicles since 1992. Air bags are made to arrest
the forward momentum of the driver or passenger in the event of a
collision. If one designs a universal air bag for all passengers,
then it must have sufficient force to stop the largest of the
expected passengers. Smaller passengers have less momentum, and so
do not require the same momentum change as the larger passenger. In
addition, smaller passengers are shorter, and sit closer to the
dashboard, and therefore experience more of the bag's explosive
force than a average adult male, sitting further back. As a result
of the current air bag deployment force, there have been a number
of injuries and fatalities associated with air bag deployment. As
of mid 1998, 105 deaths have been attributed to the deployment of
air bags with a small adult or a young child when no air bag
deployment would not have resulted in any injury to the
occupant.
[0003] This situation has caused NHTSA, the National Highway
Traffic Safety Administration, a branch of the U.S. Department of
Transportation, to propose rules which will change the criteria for
air bag activation, as well as the deployment force, in order to
protect such small occupants. In addition to these requirements,
the NHTSA has also identified "out of position" occupants as a
source of concern. Thus a system must be able to modulate or reduce
the air bag deployment force if the occupant is in a position so as
to be injured by the air bag, even if that occupant is a full size
adult.
[0004] There are several methods which can sense the presence and
weight of an occupant. In U.S. Pat. No. 5,573,269, Gentry et al.
teach an apparatus which uses weight measurements, using a sensor,
in an automobile seat as an input to a controller which operates
air bags. This sensor, described in U.S. Pat. No. 5,494,311, is a
thin structure that resides in the bottom seat cushion. As is
recognized by Gentry, much of the occupant's weight is also
directed into the seat back, thereby bypassing the weight sensing
pad and traveling directly through the seat structure to the
chassis of the vehicle. An incline sensor, which measures the tilt
of the back of the seat is also provided to compensate for this
effect.
[0005] There are two problems with this system. First it assumes
that the weight can be determined only by the pressure on the seat
cushion bottom and by the angle of the seat. That is not always the
case. Consider an occupant who puts horizontal pressure on the
floorboard in front of the seat. This increases the force on the
back with a resulting decrease on the bottom cushion. At some point
this pressure can be great enough that nearly all of the occupants
weight is on the back cushion. This problem is also present in U.S.
Pat. No. 5,474,327. In this device a set of pressure sensitive pads
is placed beneath the surface of the seat cushion. While this
device is adequate for the detection of a child seat, it does not
give adequate information for small adults and out of position
occupants.
[0006] Blackburn et al. teaches in U.S. Pat. No. 5,494,311 a system
where pads are placed in both the lower and rear seat cushion. This
gives a better weight measurement under all conditions, the obvious
downside is the cost.
[0007] One of the problems of prior systems is that they cannot
read negative weight, i.e. when forces are present that would cause
the force on the seat support to go negative. This can occur when
the occupant places force, via his feet, on the front of the
passenger compartment.
[0008] Yet another difficulty is that since the pressure is sensed
on the seat, the seat belt tension adds to the reading. A 40 pound
car seat could then, with sufficient tension on the seat belt, put
200 pounds of force on the seat surface, causing a false
reading.
[0009] An object of the invention is to devise an apparatus for
accurately sensing weight of an occupant in an automotive seat for
deployment of restraint devices.
[0010] Another object of the invention is to determine where a
passenger is seated in an automotive seat.
SUMMARY OF THE INVENTION
[0011] The above object has been achieved with a torsional sensing
load cell having the shape of a tuning fork with two arms. The load
cell is configured to handle overload by the use of a deflection
stop pin. For example, in an automotive application, one arm of the
cell supports part of the load of a car seat and the other arm is
fixed to a foot attached to the automotive chassis. Torsion exists
in the load cell as the load arm deflects relative to the fixed
arm. A pair of strain gauges measure the torsion in the load cell
and produce an electrical signal which is reported to a circuit
which converts the electrical signal to a weight measurement. By
placing a load cell at each of four corners where car seat support
feet are located, the entire load in a car seat can be measured and
the position of a seated person can be determined by observing
weight distribution among the four corners of the seat. Since the
support feet are insensitive to the manner in which loads are
generated, the load cells sense true load, even where unexpected
loads are created, for example by a car passenger pushing against a
dashboard by means of his feet.
[0012] An automotive car seat is usually moveable by means of an
electric motor and is not directly mounted to the automobile
chassis. Rather, the car seat is mounted on two parallel moveable
glide rails which are movably supported on rollers by two parallel
fixed guide rails. The guide rails are fixed in place by rigidly
connecting each guide rail between two support feet, one at the
front of a seat and one at the back. The moveable glide rails
transmit force to the fixed guide rails. Since the load cells of
the present invention link the fixed guide rails to the fixed feet,
torsion is allowed to develop between a guide rail and a fixed
foot. Torsion then exists in the bridge section of the load cell,
between the two arms of the load cell. Here is where strain gauges
are mounted for torsion measurement. Electrical signals generated
by the strain gauges ar sent to a circuit which produces a force
signal. Signals from four load cells associated with an automotive
seat are directly proportional to the weight of an occupant in the
seat. The fractional distribution of weight between forward load
cells associated with the front of the seat and rearward load cells
associated with the rear of the seat indicate where an occupant is
seated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The teachings of the present invention can be readily
understood by considering the following detailed description in
conjunction with the accompanying drawings:
[0014] FIG. 1 is a perspective view of an automobile seat employing
the apparatus of the present invention using a vertical rail
support;
[0015] FIG. 2 is a top view of the apparatus of the present
invention;
[0016] FIG. 3A is a top view of the apparatus of FIG. 2 attached to
fixed automotive structures;
[0017] FIG. 3B is a perspective view of the apparatus of FIG.
2;
[0018] FIGS. 4A-4B and 5A-5C are schematic diagrams of the bridge
circuitry coupling the transducers shown in FIG. 3 to output
lines;
[0019] FIG. 6 is a front view of an automobile seat employing an
alternate embodiment of the apparatus of the present invention
using a horizontal rail support;
[0020] FIG. 7 is a side view of the apparatus of FIG. 4;
[0021] FIGS. 8A-8D are top diagrammatic views of a process for
making the present invention;
[0022] FIG. 9 is an exploded perspective view of an alternate
embodiment of the apparatus of FIG. 2;
[0023] FIGS. 10A-10C are front, side, and bottom views of the load
cell of FIG. 9, mounted to an upright support for an automotive
seat within the support structure;
[0024] FIG. 11 is a detail of a load cell mounting taken along
lines 11-11 in FIG. 10A;
[0025] FIG. 12 is a side view of the load cell of FIG. 9 mounted
within an automotive seat above an automotive seat support
structure;
[0026] FIG. 13 is a front, partial cataway, detail view of the load
cell mounting shown in FIG. 12;
[0027] FIG. 14 is a perspective view of another alternate
embodiment of the load cell of FIG. 2;
[0028] FIG. 15 shows a basic stress-strain diagram of a
material;
[0029] FIG. 16 illustrates the typical forces that arise in a
rear-end collision event;
[0030] FIG. 17 illustrates another embodiment of a load cell in
accordance with the present invention; and
[0031] FIG. 18 shows a load versus strain curve typifying the
strain characteristic of the load cell of FIG. 17.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0032] With reference to FIG. 1, an automotive car seat 11 is seen
connected by flanges 13 to moveable glide rails 15. The flanges are
connected to sides of the glide rail so that weight from a quadrant
or portion of the seat causes a slight amount of twisting of the
rail, proportional to weight on the seat. The glide rails move in a
telescopic relation relative to fixed guide rails 17, with bearings
or rollers transferring the load from the glide rails to the fixed
guide rails. The fixed guide rails are supported by feet 19 which
are fixed to an automotive chassis or similar structure. Each of
the feet 19 is an L-shaped bracket with a base welded or bolted to
the chassis and an upwardly extending portion which is welded or
bolted to the guide rail.
[0033] Each guide rail 17 is C-shaped and is housed partially
within a larger glide rail 15, so that the glide rail slides over
the guide rail in the embodiment of FIG. 1. Other geometries are
possible as will be seen below. A motor (not shown), carried by car
seat 11, moves the glide rail relative to the guide rail, using a
gear which engages a gear rail 55 that is fixed and parallel to the
guide rail.
[0034] Weight or downward force on seat 11, carried by the rail
system is transferred to the feet by means of the torsional sensing
load cell 21 of the present invention. The torsional sensing load
cell is the only connection between the rail support system for the
automotive car seat 11 and the feet 19. The torsional sensing load
cell has transducers which measure a torsional force and produce an
electrical signal carried by cable 23. The purpose of the load
cells is to measure the torsional force on seat 11 applied by a
seated person in order to apply the appropriate amount of gas
pressure to an air bag A or similar safety device. If the seated
person is of very low weight, it is assumed that the person is a
child and the air bag A is not deployed. Air bag deployment based
upon seat weight is known, as previously described, but by
different mechanisms.
[0035] With reference to FIG. 2, torsional sensing load cell 21 is
a metallic bar, preferably steel, but also aluminum or titanium,
with a generally rectangular cross section, seen to have the shape
of a small tuning fork with a first arm 25, having a pair of
mounting holes 27 and 29 therein, spaced apart from a second arm
31, also having a pair of mounting holes 33 and 35. Although the
holes in the two arms are aligned for manufacturing ease, bolts
passing through one arm do not contact the other arm. Rather, each
arm is independent of the other so that torsion can develop through
the load cell and across the bridge section connecting the two
arms. The actual size of the torsional sensing load cell is
slightly smaller than tuning forks commonly used in piano tuning,
approximately one inch in overall width by 0.625 inches in height
by almost 3 inches in length. The cell may be stamped, cast or
forged. The arms 25 and 31 are joined by a bridge section 37. The
transition zones 39 and 41, indicated by dashed lines, where the
bridge section 37 joins the arms 25 and 31 serve as places for
surface mounting of strain transducers 43 and 45. The bridge
section may have cut-outs for mass relief. The cut-outs may define
I-beam shapes so that mass relief does not affect structural
strength. Each strain transducer is able to sense torsion of the
underlying member and produce an electrical signal representative
of the strain. The electrical signal is carried out by cable 23 for
both strain transducers.
[0036] In FIGS. 3A and 3B the second arm 31 is seen to be fixed to
foot 19 by means of bolts 47 which are secured to an upwardly
extending portion of foot 19, namely riser 49. It is important to
note that second arm 31 is fixed relative to an automotive chassis
to which foot 19 is connected. On the other hand, first arm 25 may
be considered to receive cantilever support from the first arm and
is connected to guide rail 17 by means of bolts 53. Weight on the
seat is transferred to the guide rail 17, as explained above, which
in turn transfers the weight to the riser 49 with some twisting or
torsion of the torsional sensing load cell. Note that there is a
slight amount of clearance, roughly one millimeter, between the
guide rail 17 and riser 49. The clearance is necessary to allow for
independent movement of first arm 25 relative to second arm 31 as
the first arm provides cantilever support to guide rail 17. It is
this independent movement which creates a torsion through the load
cell. Gear rail 55 may be seen to be connected to second arm 31.
The torsion in the bridge section 37 is reported by the two strain
transducers 43 and 45 which are connected in a differential
electrical circuit, known as an electrical bridge.
[0037] In FIG. 4A, transducers 43, 45 each comprises two sets of
strain elements R.sub.T, R.sub.C arranged in a ninety degree
chevron pattern, which is a standard configuration for strain
gauges used to measure strain on the surface of a member under
load, and are mounted relative to a neutral axis of the sensing
member. In the presence of a torsional stress, each transducer 43,
45 will be subject to a stress .sub.A, .sub.B respectively, such as
shown in FIG. 4A. The actual direction of the stresses will vary
depending on the position of the load with respect to locations A
and B. For any loading situation, however, one set of strain
elements of transducer 202 (e.g. R.sub.TA) will be in tension while
the other set of strain elements (e.g. R.sub.CA) is in compression.
The strain elements R.sub.TB, R.sub.CB comprising transducer 45 are
similarly stressed. The strain elements of both transducers are
coupled in a Wheatstone bridge, such as shown in FIG. 4B, where
similarly strained elements are located on opposite legs of the
bridge. The gauges are powered by approximately 7 mA of current.
The differential output signal V.sub.O is characterized by 1 V O =
V D ( R C B R TA + R C B - R T B R C A + R T B ) Eqn . 1
[0038] However, since R.sub.CB and R.sub.TB are oppositely
strained, the difference signal actually represents a summation of
the torsional stresses sensed at both transducers 43, 45, namely
.tau..sub.A+.tau..sub.B.
[0039] FIG. 5A shows an embodiment using left and right transducers
57, 59 which have a simpler construction. Here, each transducer
comprises only a single set of strain elements, rather than the
chevron pattern of FIG. 4A. FIG. 5B shows a voltage divider circuit
used for such transducers. In this case, the output signal is
defined by the voltage divider equation which characterizes the
circuit. Unlike the bridge circuit of FIG. 4B, the circuit of FIG.
5B provides an output signal that is directly proportional to the
torsional stress experienced by a single transducer, namely 2 V O =
V D ( R C B R TA + R C B ) Eqn . 2
[0040] This embodiment produces a smaller output signal than the
embodiment of FIG. 4A. However, a differential signal can be
generated by using the circuit of FIG. 5C which comprises a pair of
fixed resistances R used in conjunction with R.sub.TA and R.sub.CB
in a bridge configuration. The equation which characterizes this
circuit is 3 V O = V D ( R 2 - R T B R C A + R T B ) Eqn . 3
[0041] In all equations, the output signal, V.sub.O, is
proportional to force on the seat or weight. By calibration, the
constant of proportionality may be determined so that true weight
is known. This alternate embodiment, however, lacks the sensitivity
afforded by that shown in FIG. 4A. The embodiment of FIG. 5A,
nonetheless, offers the advantage of being simpler and less costly
to manufacture, and therefore under the right circumstances may be
preferable over the embodiment of FIG. 4A.
[0042] FIG. 6 illustrates a preferred embodiment of the apparatus
of the present invention in which the glide rail 61 is connected to
automotive seat 63. The glide rail slides over a guide rail 65,
being separated by bearings which transfer the load from the seat,
through the glide rail and to the bearings and thence, the guide
rail. The guide rail is mounted to a fixed foot 67 through a
torsional sensing load cell 69 of the present invention. The load
cell has the configuration previously described with reference to
FIG. 3, with one arm of a tuning fork shaped load cell connected to
the fixed rail and the other arm connected to the fixed foot 71.
The relationship between the glide and guide rails has been
reversed compared to the embodiment of FIG. 3.
[0043] The torsional sensing load cell has a pair of bolts 73, one
of which holds one arm of the cell to the fixed foot, while another
bolt secures the second arm to the fixed rail.
[0044] In the side view of FIG. 7, an overlapping fixed flange 73
is seen having a leg 75 protruding downwardly for contact with
automotive upholstery or matting and partially shielding the
torsion cell from accidental damage and contact with objects. The
forward load cell 69 is seen spaced apart from the rearward load
cell 77 which also has an overlapping fixed flange 79 which
partially shields the rearward load cell. Glide rail 61 may be seen
above guide rail 65, with support from fixed foot 71. The fixed
flange 79 also makes contact with automotive upholstery or matting.
The position for four load cells is apparent because left and right
side views of the seat support structure would be identical.
[0045] The transducers used in the apparatus of the present
invention may be manufactured in-situ, on the load cell. In FIG. 8A
the torsional sensing load cell 21 is seen to have a generally
tuning fork shape with bridge section 37 having a flat top which is
coated with an electrically insulative epoxy or epoxy-glass layer
81 which is several millimeters thick. Next, using photomask and
etching processes, a desired circuit pattern 83 is deposited as one
or more layers having a thickness of only a few millimeters upon
the insulative layer, as shown in FIG. 8B. The desired strain gauge
transducers 85 with chevron elements at 90 degrees are also
deposited in the same manner as microcircuits are placed on small
circuit boards in electronics fabrication. A terminating header 87
is deposited to make contact with a cable which carries away
electrical signal from the transducers. Next, an electrically
insulative cover layer 89 is disposed over the circuit pattern, as
shown in FIG. 8C. The insulative layer closely adheres to the
circuit pattern and is typically epoxy. Lastly, a tough encapsulant
shell 91 is formed over the entire bridge section. The encapsulant
is selected from known potting materials or may be a thick shell of
epoxy as shown in FIG. 8D.
[0046] With reference to FIG. 9, load cell 101 is seen to be a
metallic block, preferably steel, but also aluminum or titanium.
The block is split by a narrow slot 103 resembling a saw cut which
defines two independent linear arms 105 and 107. A first mounting
hole, not seen, allows a bolt 111 to fasten the load cell to a
fixed riser 113. Bolt 107 is secured to linear arm 107 but does not
contact linear arm 105. One or more mass relief holes 108 may be
provided. Torsion between the linear arms is measured by
transducers 117 and 119 which operate in the same manner as the
transducers described above. An automotive seat, not shown, carries
a glide rail, also not shown. The glide rail transfers force to the
fixed guide rail 121 which is connected to the fixed riser 113 by
means of a bolt 123 which extends through hole 125 in arm 105. The
bolt also passes through hole 125 in the fixed riser 113 where it
is held in place by a first nut 127. A second nut 129 secures bolt
123 after it passes through the guide rail 121.
[0047] In FIGS. 10A-10C, as well as FIG. 11, riser 113 may be seen
mounting the load cell 101 with a horizontal bolt 111 passing
through both the riser and into the load cell 101. The vertical
bolt 123 secures the load cell to guide rail 121 which is held
fixed relative to riser 113. A glide rail 131 is a C-shaped member
carrying an automotive seat 133 which slides over the guide rail
121. Weight is transferred from seat 133 to the glide rail 131 and
then to the guide rail 121. Force is then transmitted by means of
bolt 123 through the fixed riser 113 and to one arm of load cell
101. In FIG. 10C, the load cell 101 is seen having bolt 123
connected to one arm of the load cell. Bolt 111 is seen connected
to the other arm.
[0048] With reference to FIGS. 12 and 13, automotive seat 133 rests
on glide rails 131 connected to guide rails without any upright
risers 113. In FIG. 12, the load cells are seen to be located at
the corners 141, 143 of the seat. Load cells are also located at
the two opposite corners, not seen. By placing cells at the corners
of the seat, the weight on the automotive seat may be determined in
a manner which indicates how much force is on the rearward part of
the seat and how much force is on the forward portion. This would
give an indication of how to deploy an air bag. If most of a
person's weight appears at the forward edge of a seat, less
deployment force should be used than for a person whose weight is
evenly distributed on the seat or whose weight is mostly toward the
rear of the seat.
[0049] In FIG. 13, the glide rail having the load cell mounted
above is seen to be mounted over guide rail 121 which is fixed to
an automotive floor 173. No riser is used. The fixed guide rail 121
allows the glide rail 131 to move over it by means of a motor and a
third rail, not shown, but described above. An automotive seat 133
mounts the cell 101 by means of a bolt 175 which extends into one
of the linear arms of the cell. A bolt 179 passes through the other
linear arm of the load cell and is secured to the top of glide rail
131. A pair of nuts on either side of the glide rail top retain
bolt 179 in place. In this manner the load on an automotive seat
may be measured.
[0050] In FIG. 14, the glide rail 151 is seen to have load cell 153
integrated into the rail. A first linear arm 155 of the cell is
separated from a second linear arm 157 by a spiral slot 159 so that
one arm supporting the weight of an automotive seat through hole
161 may transmit force to the linear arm 157, across bridge 163 and
to the other linear arm 155 which is held fixed to glide rail 151.
On the other hand, linear arm 157 is not fixed but is free to move
and deliver torsional forces to bridge 163, measured by transducers
165 and 167. The integrated load cell of FIG. 14 operates in the
same manner as the load cell of FIG. 9.
[0051] Turn now to FIGS. 15-18 for a discussion in connection with
another embodiment of the present invention. FIG. 15 shows a basic
stress-strain diagram for a material. Position A on the diagram
represents a point where there is no stress and no strain applied
to the material. The shape of the curve from point A to point B is
a substantially straight line defined by the elastic modulus, E, of
the material. This is rhe elastic portion of the curve and the
relationship between the stress, .sigma., and the strain,
.epsilon., is given by the following: 4 = E Eqn . 4
[0052] If the material is stressed beyond the point B, the
stress-strain relationship is no longer linear. The material begins
to yield. Point B is referred to as the elastic limit. The stress
level at point B is known as the yield stress, or yield point. A
continued application of stress to the material will cause failure
at a point E.
[0053] Stress past the yield point B to a point C will result in a
new strain curve, as the material will not relax to its original
condition. This new strain-stress curve is represented by the line
C-D. The offset is indicated as .epsilon..sub.0. This effect
manifests itself in the load cell electrically as a D.C. offset.
Consequently, the load cell produces an output and appears to be
loaded, even though it is not.
[0054] In an automobile, it is desirable to have high sensitivity
in the measurement range. Signal processing costs are reduced and
accuracy is increased when a sensitive signal can be made
available. This means that for a given load, the strain should be
as high as possible. A "safe limit" can be established well below
the yield stress point B as shown in FIG. 15.
[0055] With reference to FIG. 16, a problem with auto seats is that
a very high overload can occur in a rapid acceleration of
deceleration scenario, such as an automobile collision. For
example, FIG. 16 shows a 200 pound passenger subjected to an 8 g
load at a distance of 18 inches measured from the floor. This
creates a 2400 foot-pound couple on seat attachment points 1502,
assuming a spacing of fourteen inches.
[0056] Referring now to FIG. 17, another particular illustrative
embodiment of a load cell 1702 in accordance with the present
invention is shown. The load cell is generally of the form
disclosed in forgoing paragraphs. In the particular embodiment
shown in FIG. 17, the load cell is comprised of steel. The beam
members 1732A, 1732B of the load cell are lengthened in the
X-direction. Tuning holes T.sub.1, T.sub.2, are made in each beam
member to increase the electrical sensitivity of the load cell.
This is achieved by decreasing the resistance of the load cell to
torsion.
[0057] Mounting holes M.sub.1, M.sub.2, are provided to mount the
load cell. The lengthening of the first beam member 1732A
accommodates a clearance opening 1722 formed through the first beam
member. Similarly, the lengthening of the second beam member 1732B
accommodates placement of a stop pin 1724 that is press fit into a
blind hole formed in the second beam member. The clearance opening
1722 has an opening sufficient to allow the stop pin to extend into
and be disposed within the clearance opening. As can be seen in the
figure, a certain clearance distance `d` is provided between the
stop pin and the clearance opening.
[0058] In a particular embodiment, a pilot hole is drilled through
beam member 1732A, but not all the way through beam member 1732B to
form blind hole in 1732B. A round stop pin is press fit into the
blind hole in beam member 1732B. The pilot hole formed in beam
member 1732A is enlarged somewhat to yield a clearance hole 1722
that is substantially concentric (i.e., in registration) with the
stop pin 1724. In a particular embodiment, the clearance distance
`d` might be on the order of five thousandths of an inch. Because
of this registration, the pin stop can provide an overload accuracy
of about 5% of total load; or about 50 pounds on a load cell rated
for 1000 pounds. In the particular embodiment shown in the figure,
the stop is symmetrical and thus provides a bi-directional
performance.
[0059] A first view in FIG. 17 shows the load cell 1702 in an
unloaded condition. The second view in FIG. 17 shows the load cell
in a loaded condition, where a load F is applied to one beam member
1732B. As can be seen, the beam member deflects in response to the
applied force. The resulting torsional stress is sensed and
reported, as discussed above.
[0060] When the load F exceeds the safe limit of the material
comprising the load cell, as indicated in the figure, the stop pin
1724 contacts a side of the clearance opening 1722, thus preventing
further deflection of the beam member 1732B. This arrangement
prevents the material comprising the load cell from exceeding its
elastic limit, thus allowing the load cell to return to its
original condition when the load is removed.
[0061] FIG. 18 shows a load vs. strain curve typical of the load
cell shown in FIG. 17. The load cell of the particular embodiment
shown in the figure has a very high proportional output over its
linear range of about .+-.600 pounds, or 1200 .mu.strain. Beyond
this range of loading, the stop pins 1724 engage and overload of
about .+-.4200 pounds is achieved using a steel stop pin about
one-quarter inch in diameter. Beyond this range of loading, failure
occurs, where the stop pin is physically compromised; e.g. by
shearing.
[0062] It can be seen how this is advantageous in an automobile
environment. Under a quick acceleration or deceleration condition
such as a collision, the sudden high magnitude force applied to the
load cell will not destroy the unit. It is clear that the disclosed
load cell can be configured in other applications where the unit is
subject to sudden forces that would otherwise cause damage but for
the stop pin arrangement.
[0063] Although the embodiments of the invention have been
described with reference to an automotive seat, the disclosed load
cell is not limited to its use in automotive applications. It is
clear that the torsional sensing load cell of the present invention
could be used with other seats or other types of loads. One of the
advantages of the present invention is that the manner in which the
load cell is loaded is not relevant. In the automotive example, a
seat occupant may have his or her feet on the dashboard, increasing
the seat load, or may be reclining so that seat loading is reduced.
The present invention is capable of accurately reporting actual
seat load.
[0064] As mentioned above, the amount of weight on forward cells
can be compared with the amount of weight on rearward cells to
determine whether a seated occupant is seated at the forward edge
of a car seat or is seated toward the center of the car seat. This
information is used to control the amount of gas flowing into an
air bag to protect an occupant seated at the forward edge of a seat
or to protect an occupant of low weight, such as a child.
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