Note: Descriptions are shown in the official language in which they were submitted.
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SEAT WEIGHT SE~ISOR HAVING FLU~D F~LLED BLADDER
CROSS-REFERENCE TO RELATED APPLICATIONS
The instant application incorporates and converts prior U.S. Provisional Application
Serial No. 60/032,380 filed December 19, 19g6.
TECHNICAL ART
The instant invention generally relates to sensors and systems for measuring weight and
more particularly to a weight sensor for measuring the weight of occ--p~nt~ and other
objects in a motor vehicle seat such as useful det~rmining occupant seating conditions for
controlling a vehcile safety restraint sytsem.
BACKGROUND OF T~E INVENTION
A vehicle may contain automatic safety restraint actuators that are activated
responsive to a vehicle crash for purposes of mitigating occupant injury. Examples of such
e~Lldill~ actuators include air bags, seat belt pretensioners, and deployable knee bolsters.
One objective of an automatic safety restraint system is to mitigate occupant injury,
5 thereby not ç~ ing more injury with the automatic restraint system than would be caused
by the crash had the automatic restraint system not been activated. Notwith~t~nclin~ the
protective benefit ofthese automatic safety le~LldillL actuators, there is generally both a risk
and a cost associated with the deployment thereof. Generally, it is desirable to only
activate automatic safety IC.,Ildi~lL actuators when needed to mitiE~te injury because of the
20 expense of replacing the associated components of the safety reskaint system, and because
of the potential for such activations to harm OC~;LI~ 1'; This is particularly true of air bag
restraint systems, wherein occ--p~nt~ too close to the air bag at the time of deployment -
i.e. out-of-position occ11p~nt~ -- are vulnerable to injury or death from the deploying air bag
even when the associated vehicle crash is relatively mild. Moreover, occupants who are of
25 small stature or with weak constitution, such as children, small adults or people with frail
bones are particularly vulnerable to inJury in~ ed by the air bag inflator. Furthermore,
I
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infants properly secured in a normally positioned rear facing infant seat (RFIS) in
proxirnity to a front seat passenger-side air bag are also wlnerable to injury or death from
the deploying air bag because of the close proximity of the infant seat's rear surface to the
air bag inflator module.
Air bag inflators are ~P~i~nP~l with a given ~ di~ll capacity, as for example, the
capacity to protect an unbelted norrnally seated fiftieth percentile oCcnr~nt when subjected
to a 30 MPH barrier equivalent crash, which results in associated energy and power levels
which can be injurious to out-of-position occupants. While relatively infrequent, cases of
injury or death caused by air bag inflators in crashes for which the oc~1.p~ would have
otherwise survived relatively unharmed have provided the impetus to reduce or elimin~t~?
the potential for air bag inflators to injure the occ~1p~ntc which they are intended to protect.
One technique for mitig~tin~ injury to OCC~dllL~ by the air bag inflator is to reduce
the power and energy levels of the associated air bag inflator, for exarnple by reducing the
amount of gas generant in the air bag inflator, or the inflation rate thereof. This reduces the
risk of harm to oc~-"~ by the air bag inflator while simn1t~neously reducing the restraint
capacity of the air bag inflator, which places occupants a greater risk for injury when
exposed to higher severity crashes.
Another technique for mitip;atin~ injury to occupants by the air bag inflator is to
control the rate of inflation rate or the capacity of the inflator responsive to a measure of
the severity of the crash. ~owever, the risk of injury to such occupants would not be
mitig,qte~l under the conditions of higher crash severity when the inflator is intentionally
made aggressive in order to provide sufficient restraint for normally positioned occupants.
Yet another technique for mitip;;lting injury to occ1-p~nt~ by the air bag inflator is to
control the activation of the air bag inflator responsive to the presence, position, and size of
the occupant, or to the severity of the crash. For example, the air bag inflator can be
disabled if the oc~ weight is below a given threshold. Moreover, the inflation
capacity can be acijusted by controlling the number of inflation stages of a multi-stage
inflator that are activated. Furthermore, the inflation power can be adjusted by controlling
the time delay between the firings of respective stages of a multi-stage inflator.
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One measure of le~dillL capacity of an air bag in~lator is the amount of occupant
kinetic energy that can be absorbed by the associated air bag system, whereby when the
occupant collides with the gas filled air bag, the kinetic energy of the occupant is converted
to potential energy via the pressurization of the air bag, and this potential energy is
5 ~liccirat~d by venting pr(~ssun7~-~l gases from the air bag. As a vehicle in a crash is
decelerated, the velocity of an unrestrained occur~nt relative to the vehicle increases.
Preferably, the occup:~.nt le~ process is commenced early in the crash event so as to
limit the amount of occnr~n~ kinetic energy that must be absorbed and thereby minimi7P
the associàted lc~ dhll forces and accelerations of and loads within the occ~p~nt If the
0 occnr~nt were a simple inertial mass without friction relative to the vehicle, the kinetic
energy of the occupant would be given by 1/2 M-V2, where M is the mass of the occ~lpS-nt
and V is the occupant velocity relative to the vehicle. If a real occupant were represented
by an interconn~ c~e~l set of bodies, some of which have friction relative to the vehicle, each
body of which may have differing velocities relative the vehicle, the above equation would
5 apply to the motion of the center of gravity of the occupant. Regardless of the
representation, occupants of larger mass will have a larger kinetic energy for the same
velocity relative to the vehicle. Therefore, an occ~ L weight sensor is useful in an air
bag system with variable restraint capacity to enable the restraint capacity to be
~refe~ ially adapted to the weight, or mass, of the occupant.
Except for some cases of oblique or side-impact crashes, it is generally desirable to
not activate an automatic safety restraint actuator if an associated occupant is not present
because of the otherwise nnn~cec~ry costs and inconveniences associated with thereplacement of a deployed air bag inflation system. Occupant presence can be ~ietecterl by
a seat weight sensor adapted to provide either a continuous measure of occupant weight or
2s to provide a binary indication if the occupant weight is either above or below a specified
weight threshold.
Known seat weight sensors comprise one or more pads employing force
sensitive resistive (FSR) filrns. These arrangements are typically used as weight threshold
systems to disable a passenger air bag when the seat is empty. Load cells attached to the seat
mounting posts have also been used in research applications. Meçh~nicm~ that use string
based potentiometers to measure downward seat displacement have also been investigated.
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Such known arrangements suffer from several drawbacks. First, variable r~ci~t~nce
force sensors have limited sensitivity and in some situations are not sensitive enough to put
directly under a seat pad while still achieving the desired response. Second, the threshold
weight system provides only very limited information. For example, such arrangements
5 provide no indication as to the size of an occ~lr~nt Third, the reC;~t~nre values of known
variable force resistor change with temperature, and are subject to drift over time with a
constant load on the sensor.
Furtherrnore, other known sensing arran~mellL~ do not otherwise provide suitableresults. For example, the use of load cells is prohibitively expensive for large-scale
10 commercial applications. Strain gauges of any type may be impractical because of the
difficulty in applying them to the strained m~teri~Tl Finally, mech~Tnic~l string potentiometer
based weight sensors are complex, and subj ect to failure from stretching of the string.
The prior art also teaches the use of seat weight sensors outside the automotiveenvironment, for example as a means for riic~hling the activation of either a boat or an
5 in-in~tn ~I m~TChin~ if the operator is not properly seated, or for weighing a person seated on an
exercise bike. These devices employ pneumatic bladders located in the~ seat, whereby the
pressure within the bladder is used to either activate a threshold switch or to provide a
continuous indication of occl-p~Tnt weight.
One problem with prior art pnt?lTm~tic sensors, particularly when applied to the20 automotive environment, is their sensitivity to environmental conditions, particularly to
ambient temperature and pressure. A seat weight sensor in an automotive environment must
function reliably and accurately over a wide range of temp~laLu~t;s and pressures which can
cause significant errors.
The prior art also teaches the use of hydraulic load cells, wherein the weight to be
25 measured acts upon a piston element of known area, whereby the measured weight is found
by multiplying a measured ~leS:~Ul~ times the known area. One problem with hydraulic load
cells in the automotive environment, particularly in a seat, is that the effects of load cell
f rienT~tic)n on hydraulic head can introduce load mea~ulelllenterrors.
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SUMMARY OF THE INVENTION
The instant invention overcomes the above-noted problems by providing a seat weight
sensor which incorporates a fluid filled bladder placed in series with the load path in the seat,
whereby a load applied to and distributed across the bladder increases the ~ ul~ of the fluid
s therein. The ~les~ of the fluid is measured by a pressure sensor and is ~.b~ y
proportional to the m~gnitll(le of the applied load, and subst~ntizll1y inversely ~ ollional to
the supported area of the bladder. The instant invention also incorporates a means for
distributing the applied load across the area of the fluid filled bladder so as to prevent a
concentrated load from compressing the top and bottom ellrfa~es of the bladder against one
0 another and thereby cleaLillg an alternate load path which does not cause an associated
pressurization of the fluid. The output signal is substantially linear with respect to weight
provided that 1 ) the weight is distributed over a sufficient area so that the bladder does not
bottom out, 2) the height of the bladder is sufficiently small relative to the base ~imen~ions
so that the effect of loading on the support area is relatively small. Preferably, the amount
5 of fluid in the bladder should be less than the capacity of the bladder when the bladder is
unloaded. Otherwise, the fluid in the bladder can be pressurized by increasing temperature
or decreasing ambient prcs~ e which results in associated load measurement errors.
The bladder may incorporate either a liquid or a gas as the sensing fluid. A gaseous
sensing fluid is prone to expansion and contraction resulting from changes in ambient
zo temperature and pressure relative to the conditions under which the bladder was initially
filled. A gaseous fluid is also more prone to leakage and to localized collapse of the top
and bottom surfaces of the bladder under the influence of a concentrated load. When
located in the seat under a cushion, the cushion can provide an effective distribution of the
loads applied to the seat. A sheet of semi-rigid material can also be used to distribute load
25 to the bladder, particularly the reaction forces from the seat springs if the bladder is located
thereon.
The bladder may incorporate int~.rn~l seams which secure the top and bottom surfaces
of the bladder to one another within the periphery of the bladder without disrupting the
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fluid communication within the bladder. These searns prevent the bladder from bulging in
the center when the fluid ~nr1.c due to temperature or ~ ule effects. Such bulging is
detrimental to seating comfort. The searns also assist reducing the overall thickness of the
bladder and in conserving the n~cec~ry amount of sensing fluid, which reduces cost when
s liquids such as silicone based fluids area used. The int~rn~l seams are also effective for
modifying the sensitivity of the bladder. For example, a bladder may be more sensitive to
central loads than to distal loads as might result when a portion of the applied load is
carried by a portion of the seat cushion which is not in series with the load bladder load
path. In this case, selective zones within the bladder, for example near the center, may be
l o isolated from the sensing fluid by a closed path searn such that a load applied there~o is not
sensed by the fluid within the bladder.
The bladder may be constructed from several sheets of fabric, such as nylon, coated
with a sealably weldable coating, such a polyu~Lh~le which can be RF (radio frequency)
welded. A coating can be applied to the outside of the bladder to increase the membrane
1~ stiffness thereof and thereby facilitate the distribution of applied loads.
The instant invention integrates ~l~s:iule over the entire loading area of the seat, thereby
producing a co~ L~ output signal that is relatively in~en~itive to the associated load
distribution. The instant invention is relatively flexible, and when installed under the seat
cushion does not interfere with seating comfort. Furtherrnore, this in~t~ t;on is relatively
20 easy, thereby minimi7in~ the impact on the overall m~n~lf~rtnring process of the seat/vehicle.
Accordingly, one object of the instant invention is to provide an improved seat
weight sensor which provides a con~i~tent and accurate measure of the seat loading
independent of the location of the source of weight on the seat.
A further object of the instant invention is to provide an improved seat weight
25 sensor which provides a con.~i~t~ and accurate measure of the seat loading independent of
the size and distribution of the source of weight on the seat.
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A yet further object of the instant invention is to provide an improved seat weight
sensor which provides a consistent and accurate measure of the seat loading independent of
the arnount of weight on the seat.
A yet further object of the instant invention is to provide an improved seat weight
5 sensor which operates under a wide range of ambient temperature and plC~S~iwc conditions.
A yet further object of the instant invention is to provide an improved seat weight
sensor which can distinguish between a rear facing infant seat, for which an air bag system
is preferably not deployed, and other occ~r~ntc for which an air bag system is preferably
deployed in the event of a crash of sufficient severity.
o A yet further object of the instant invention is to provide an improved seat weight
sensor which can be incorporated into an intelligent safety restraint system for which the
preferable mode of the activation of a controllable OCCu~lallt restraint system is dependent
upon the weight of the occupant.
A yet further object of the instant invention is to provide an improved seat weight
sensor which does not i-~ r~,lc with occupant comfort.
A yet further object of the instant invention is to provide an improved seat weight
sensor which is insensitive to the orientation of the seat.
A yet further object of the instant invention is to provide an improved seat weight
sensor which is inexpensive to produce.
In accordance with these objectives, one feature of the instant invention is a fluid filled
bladder mounted in the base of the seat.
Another feature of the instant invention is a ~lC:i:iUlC sensor operably coupled to the
fluid filled bladder for me~ rin~ the ~ ;llIC therein.
,~
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Yet another feature of the instant invention is a differential pl~:S~iUle sensor operably
coupled to the fluid filled bladder for measuring the ~ Ul~ therein relative to local
atrnospheric pl~,S:~ULt:.
Yet another feature of the instant invention is the incorporation of a gas as the fluid in
s the fluid filled bladder.
Yet another feature of the instant invention is the incorporation of a liquid as the fluid
in the fluid filled bladder.
Yet another feature of the instant invention is the incorporation of a means fordistributing load across the load bearing surface of the bladder.
~o Yet another feature of the instant invention is that the volume of fluid in the fluid filled
bladder is such that the volume of the bladder in an unloaded state is less than the
maximum volume of the bladder over the range of environm~nt~l operating conditions.
The specific Çe~Lul~,s of the instant invention provide a nurnber of associated
advantages. One advantage of the instant invention with respect to the prior art is that the
1S fluid filled bladder is responsive to loads over a large area of the seat without regards to the
distribution or arnount of loading.
Another advantage of the instant invention is that the output signal is inherently
relatively linear which simplifies signal analysis.
Yet another advantage of the instant invention is that the seat weight sensor thereof can
20 enable a rear facing infant seat for which the air bag system is preferably not deployed to
be distinguished from an occu~ for which the air bag system is preferably deployed.
Yet another advantage of the instant invention is that the seat weight sensor thereof is
sufficiently robust, reliable and accurate to enable associated occupant weight dependent
control of a controllable oc.;up~lL restraint system.
--8--
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Yet another advantage of the instant invention is that the seat weight sensor thereof is
relatively inexpensive to produce.
Accordingly, the instant invention provides an improved seat weight sensor which is
relatively insensitive to the effects of ambient temperature and ~ UI~,; which is simple in
5 construction and relatively robust and reliable in operation, which can be readily
incorporated into an automotive seat without hll~,.rtl-ng with occupant comfort; and which
can be produced relatively inexpensively.
The instant invention will be more fully understood after reading the following detailed
description of the ~.~r~ d embodiment with reference to the accompanying drawings.
0 While this description will illustrate the application of the instant invention in an
automotive safety le~Lldillt system, it will be understood by one with ordinary skill in the
art that the instant invention can also be applied to other systems for weighing objects.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the incol~ol~lion of the instant invention in a vehicle seat.
FIG. 2 illustrates the bladder of one embodiment of the instant invention in an
unloaded condition together with several alternate pressure sensors connected to an
associated signal processor which determines weight from measured pressure.
FIG. 3 illustrates the instant invention responsive to one possible load distribution.
FIG. 4 illustrates the instant invention responsive to a second possible load
distribution.
FIG. 5 illustrates one environment of the instant invention.
FIG. 6 illustrates a second embodiment of the instant invention constructed fromsealably interconnected sheets of flexible material.
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,
FIG. 7 illustrates a cross section of the Fig. 6 embodiment illl-~tr~ting several fluid
filled zones within the associated bladder, and also i~ etrating an associated dead-zone.
FIG. 8 ill~lctr~t~s a means for distributing the support load from seat springs across the
base of the fluid filled bladder of the instant invention.
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DETAIL~D DESC~IPTION OF THE PR~FERRED EMBODIMENT(S)
Referring to Fig. 5, a seat 3 in a motor vehicle 1 incorporates a l~dr~Jslatic
'' seat weight sensor 10 mounted in the seat base 40. The hydrostatic seat weight sensor 10
comprises bladder 12 and a differential pressure sensor 20 for measuring the difference in
plC~ iUlC between the bladder 12 and the atmosphere 25. The bladder 12 is sandwiched
betweenthe seat frame 46 below and the seat ~..Chi~' foam 44 above. The bladder is filled
with a fluid, either gaseous or liquid.
In operation, an oCcnr~nt 5 seated on the base 40 of seat 3 causes the ~les~ulc inside
a bladder 12 to increase such that that product of the differential pressure, as sensed by
0 differential p, ~ ur~, sensor 20, multiplied times the area of the base 17 of the bladder 12 is
substantially equal to the total weight distributed by the seat cushion foam 44 over the top 19
of the bladder 12. The pressure signal output 22 from differential pr.i,sur~ sensor 20 is
operably coupled to an cle~lru~.ic control module 50 which converts the p~ .U~ e signal
output 22 to a measure of occllr~nt weight using known analog, digital, or rnicroprocessor
ch~uiL~ y and software. A crash sensor 60 is also operably coupled to the electronic control
module 50. Responsive to a crash det~cte-l by the crash sensor 60, and further responsive to
the sensed weight of the occupant as transforrned from the pressure signal output 22, the
electronic control module 50 generates a signal 80 which is operably coupled to one or more
initiators 90 of one or more gas generators 100 mounted in an air bag inflator module 110,
thereby controlling the activation of the air bag inflator module assembly 7 so as to inflate
the air bag 120 as nt~cess:~ry to protect the occupant 5 from injury which might otherwise be
caused by the crash. The electrical power n~ce~ . y to carry out tnese operations is provided
by a source of power 70, preferably the vehicle battery.
Referring to Fig. 1, the seat cushion 44 acts to distribute the load from the oc.u~a~t 5
2s across the top l~ad bearing surface of the bladder 12, thereby causing an increase in pressure
of the fluid within the bladder 12 thereby supporting the top load bearing surface of the
bladder 12. By distributing the load across the top load bearing surface, the seat cushion acts
, to prevent concentrated loads applied to the seat from causing the top and bottom surfaces of
the bladder 12 from coll~psing against one another, thereby creating an alternate path for
load which would not cause an associated increase in l~res~ulc of the fluid.
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Referring to Fig. 2, the ~ i'7Ule of the nnl~ l bladder 12 is given by P0, which is
substantially equal to the local atmospheric plt;S~ . The pressure in the bladder 12 may
be ~lteT~teTy sensed by an absolute pressure sensor 20, or by one or more strain sensors 20
incorporated in or attached to the surface of the bladder 12. The signal from the pressure
5 sensor 20 is operably coupled to a signal processor 50 which measures the weight W of the
applied load the~eLu~
Referring to Fig. 3, a load of weight W supported by the bladder 12 causes the
pressure of the fluid therein to increase by an arnount ~P, such that the weight W is given by
W=~P A (1)
lO where A is the effective area of the bottorn load bearing surface of the bladder 12. If the
bladder 12 is fully supported by the seat base 46, then the effective area A is substantially
the sarne as the area of the base of the bladder 12. This is true regardless of the area and
distribution of loading on the top load bearing surface of the bladder 12 as is illustrated in
Fig. 4, so long as the loading on the top load bearing surface of the b}adder 12 is sufficiently
5 distributed so that the top surface and bottom surfaces of the bladder 12 are not collapsed
upon one another within the periphery of the bladder 12.
Note that the area A of the bladder which rests upon the ~u~o~Lil1g surface remains
approximately equal regardless of the distribution of the weight which is applied to the
bladder 12. In other words two different objects each with weight, W, but with different
20 distributions of weight -- one concenLldted and one whose weight is more spread out -- will
each register the same increase in pressure, ~P. Two equal weights, each with weight W,
both register the same increase in pressure. The pressure increase resulting from an
applied weight, ~P, is in~lep~nf~f~nt of the shape of the applied weight, W, as long as the
contact area between the bottom of the bladder 12 and the supporting surface remains
25 con~t~nt
The bladder 12 is preferably omy partially filled with fluid with an amount such that
relatively high ambient ~ a~ s or relatively low ambient plc:S~ eS do not cause the
fluid of the unloaded bladder 12 within the seat to become p~e~ ized relative to local
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atmospheric ples:,u~. For the bladder 12 mounted within a seat 3 as illustrated in Fig. 1,
with no applied load, the ~t;s~ule of the fluid within the bladder 12 will generally higher
than the local atrnospheric pressure by an amount corresponding to the weight of the top
surface of the bladder 12 and to the hydrostatic pressure of the fluid ~,vithin the bladder 12
5 relative to the location of the pressure sensor 20. Typically, these increment~l
components of p,~ e are negligible relative to the range of loads to be measured.
The bladder 12 is preferably designed so that the contact area at the bottom load
bearing surface of the bladder 12 remains relatively constant over 1 ) the expected range of
applied weights, W, and weight distributions which may result from various sizes and
lo positions of objects, and 2) the expected range of ambient temperature and pressure
conditions.
For situations where it is not possible to design the bladder 12 so as to prevent
significant variation in the contact area at the bottom load bearing surface of the bladder
12, then the differential ~l~s~ e of the fluid within the bladder 12 may not by itself
accurately indicate the applied weight. A significant variation in the contact area will
result in an ambiguity between the increase in fluid pressure relative to the increase in
intern~l tension along the surface of the bladder. In this case, piezoresistive film can be
added to the surface of the bladder in order to measure surface tension. At the upper
surface of the bladder, the weight, W, is supported by an increase in pressure, dP, as well
20 as an increase in the surface tension of the bladder 12. When surface tension is known,
this information can be used to resolve the pressure/tension ambiguity and then accurately
estim~te the weight W of the applied load.
Referring to Figs. 6 and 7, the bladder 12 of a hydrostatic seat weight sensor 10 is
constructed from two sheets of a flexible material which is coated with a material which can
25 be sealably welded. The sheets of flexible material are f1rst placed with sealably weldable
coating sides adjacent one another, and are sealed to one another a welded seam 602 along a
periphery 601 so as to form an infiatable confinement. The sheets of material are also
welded to one another at a plurality of seams 602 at locations 603 within the periphery 601
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so as to forrn a plurality of zones in fluid co"~ ,ic~fion with one another within the
inflatable confinement. The bladder 12 is partially filled with a fluid which is distributed
amongst the various zones. The arnount of fluid and nurnber of zones is such that the top and
bottom surfaces of the b~ er 12 do not collapse against one another responsive to an
5 applied load. The pressure within the bladder 12 is sensed by a pressure sensor 20 ~ h~1
to the outside surface of the bladder 12, whereby a change the ~ s~uc of the bladder applies
a force to a first clc~Lr~de 612 of the pressure sensor 20 c~l~cing this electrode to deforrn or
move relative to a second electrode 614, thereby ch~nging the capacitance between the
electrodes 612 and 614. The second electrode is secured to the p~ e sensor housing 616
0 which is ~t~cht?d to the surface of the bladder. A restraint 702 is located on the inside of
the bladder 12 proximate the pressure sensor so as to prevent the top and bottom surfaces
of the bladder 12 from collapsing against one another proximate the pressure sensor 20.
One problem which can occur with a hydrostatic seat weight sensor 10 is reduced
sensitivity to loads which are distributed in the seat towards the periphery of the bladder. This
s problem can be mitig~tf~d by providing a non-uniform distribution of fluid filled zones 603
within the peripher,v of the bladder. Furthermore, one or more dead zones 604 may be
formed within the bladder, each by a seam 602 which defines a closed path, whereby the
portion of load applied to the seat cushion 44 in the region of the dead zone 604 is either
supported by the adjacent fluid filled zones, or is transferred to the seat base 46 without
20 increasing the pressure of the fluid within the bladder 12.
In an exemplary system in accordance with Figs. 6 and 7, the top surface of the bladder
12 is constructed from 200 denier nylon fabric which is coated with polyurethane, and the
bottom surface of the bladder 12 is constructed from 840 denier nylon fabric which is also
coated with pol~eLhalle. The seams are formed by welding the polyurethane coatings of the
25 S~aldL~ sheets together using an RF welding process. The outside of t~e bottom surface is
also coated with polyurethane so as to distribute lof~li7P~I loads across the bottom surface of
the bladder 12. The bladder 12 is filled with a silicone fluid.
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Referring to Fig. 6, a load distributor 802 constructed comprising sheet of semi-rigid
m~t~rizll can be interposed between the bottom surface of the bladder 12 and the top of the
seat suspension springs 47 so as to distribute the support loads from the seat base 46 across
the bottom load bearing surface of the bladder 12 thereby preventing the top and bottom
5 surfaces of the bladder 12 from col ~ ~psing against one another proximate the springs 47.
A gaseous fluid may also be incol~oldLed into the bladder 12. The gas-filled bladder is
preferably only partially filled to allow for gaseous expansion due to variations in arnbient
tt;lll~tia~ule and pressure, such that over the possible range of environment~l operating
conditions the volume of the unloaded gas-filled bladder generally does not exceed the
I o design capacity thereof. Moreover, under these conditions, the associated absolute
pressure in the bladder would not exceed arnbient pressure.
Under the action of a distributed load, the volume of the bladder 12 decreases until the
pressure therein is sufficiently great to support the load. For a bladder 12 having a design
shape of a rectangular slab having a height and two base rlimen~ions, as the height
5 decreases under the action of the load, the base dimensions increase, thereby increasing the
base area of the bladder 12. The weight of the distributed load is then given by the product
of the base area of the bladder times the difference in pressure inside and outside the
bladder. Even if the loading on the top of the seat is relatively 10~'~1i7~ the associated
weight is given by the dirr~;lell~ial pressure acting on the base area of the bladder, l~nming
20 the base of the bladder is fully supported and that that top surface of the bladder is not
locally colnplessed against the bottom surface.
For a bladder 12 with a square profile, having a height h and a base dimension S, the
effects of load on the support area A of the bladder, and upon the associated differential
pressure DP, relative to atmospheric pl~,;7~ule Patm, are illustrated below:
From the ideal gas law, with P=absolute fluid pressure within the bladder, V---bladder
volume, n=number of moles of gas, R=universal gas constant, and Tatm the temperature
of gas within the bladder,
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P- V= n ~ R- Tatm (2)
The pressure within the bladder is given by,
P= Patrn _ DP (3)
Assuming the bladder is filled to a volume Vfill at a Le~ ,eldLu~e Tfill, the associated
5 pressure Pfill is given from the ideal gas law as,
Pfil}n-R TfI
Vfill (4)
The fill volume may also be expressed in terms of the design height hO and base
fiimen~ion SO of the bladder, and the fraction of the desing volume Vmax which is filled,
Vmax=S02 ~ hO (S)
lo Vfill = a Vmax (6)
The volume of the bladder, Vdpc, is then given in terms of the fill conditions, the local
atmospheric condtions, and the measured pressure differential DP as,
Tatm Pfill
Vdpc=Vfill ~
Tfill, Patm DP ~ '
In this analysis, the surface area "a" of the bladder is assumed to remain constant
15 under loading, whereby under the inflll~nce of an applied load, the base area increases as
the height decreases. This surface area is given by,
a=2- s2 _ 2-S-h, (8)
Solving for the base fiimçn~ion S in terms of the height h and surface area " a" gives
--16--
CA 02245662 l998-08-05
WO 98/26961 PCT/US97/23563
S- h+ l~ 2 ,~12 h2~ a (9)
. The base area A and volume V of the bladder are then given by,
A=S
(10)
A= ~ h .~r2 ~12 h + a) (113
V~A-h (12)
~2
V--h ~ h2 ~ h (13)
By equating equations (7) and (13) and solving for the bladder height, assuming that
a>>2-h2 (14)
the height of the bladder in terms of the surface area, fill conditions, ambientlo conditions, and measureed pl~ ult; differential are then given by:
h---2 (~ ~ 2 ~ ~a - 8 Vdpc-"~a¦ (15)
The measured ples:~-ue differential is related to the m~gnit~ e of the applied load and
bladder surface area by,
W DP
A (16)
CA 0224~662 1998-08-05
WO 98/26961 . PCT/US97/2:~S63
The above equations may be solved implicitly to ~ietermine the sensitivity of the weight
measurement error to the geometry of the bladder, the fill conditions, and the ambient
conditions.
Generally, the sensitivity of the gas filled bladder to ambient t~ Lul~ and
5 ~les~ue is decreased with decreasing amounts of gas in the bladder, and with decreasing
bladder thickness for the same base ~imen~ions of the bladder. However, as the bladder is
made thinner in overall height, and the amount of gas is re(lllce~1, the bladder becomes
more susceptible to bottoming-out under the influence of localized loads applied to the
seat.
o One of ordinary skill in the art will appreciate that while the bladder of the instant
invention is illustrated herein using a rectangular shape, the particular shape of the bladder
is not considered to be limiting to the instant invention.
While specific embof1in~ent~ have been described in detail, those with ol-lill~ y skill
in the art will appreciate that various modifications and alternatives to those details could
be developed in light ofthe overall te~-~hing~ ofthe disclosure. Accordingly, the particular
arrangements disclosed are meant to be illustrative only and not limiting as to the scope of
the invention, which is to be given the full breadth of the appended claims and any and all
equivalents thereof.
