CA2245662A1 - Seat weight sensor having fluid filled bladder - Google Patents
Seat weight sensor having fluid filled bladder Download PDFInfo
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- CA2245662A1 CA2245662A1 CA 2245662 CA2245662A CA2245662A1 CA 2245662 A1 CA2245662 A1 CA 2245662A1 CA 2245662 CA2245662 CA 2245662 CA 2245662 A CA2245662 A CA 2245662A CA 2245662 A1 CA2245662 A1 CA 2245662A1
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Abstract
A seat weight sensor (10) incorporates a fluid filled bladder (12) placed in series with the load path in the seat (3), whereby a load applied to and distributed across the bladder (12) increases the pressure of the fluid therein. The pressure of the fluid is measured by a pressure sensor (20) and is substantially proportional to the magnitude of the applied load, and substantially inversely proportional to the supported area of the bladder (12). The output signal is substantially linear with respect to weight.
Preferably, the amount of fluid in the bladder should be less than the capacity of the bladder when the bladder is unloaded. The seat weight sensor (10) is incorporated into an accupant restraint system (7) for controlling the safety restraint system responsive to the weight of the occupant (5).
Preferably, the amount of fluid in the bladder should be less than the capacity of the bladder when the bladder is unloaded. The seat weight sensor (10) is incorporated into an accupant restraint system (7) for controlling the safety restraint system responsive to the weight of the occupant (5).
Description
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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-WO 98126961 . . PCT/US97/23~63 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.
CA 0224~662 1998-08-0~
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 CA 0224~662 1998-08-0~
.. , WO 98/26961 PCTr~JSg7/23S63 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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-WO 98/26961 PCTtUS97/23563 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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-WO 98/26961 . PCTIUS97/2~563 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.
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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.
WO 98n6961 . PCT/US97/23563 , 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.
WO 98/26961 . PCT/US97/23563 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.
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 CA 0224~662 1998-08-0~
.
WO 9812~i961 PCTIUS97123563 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 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.
CA 0224~662 l998-08-0~
~ .
-WO 98/26961 . PCT/US97/23563 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, CA 02245662 l99X-08-05 WO 98/26961 . PCT/US97/23S63 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 CA 02245662 l998-08-05 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.
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
CA 0224~662 l998-08-0~
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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-WO 98126961 . . PCT/US97/23~63 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.
CA 0224~662 1998-08-0~
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 CA 0224~662 1998-08-0~
.. , WO 98/26961 PCTr~JSg7/23S63 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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-WO 98/26961 PCTtUS97/23563 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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-WO 98/26961 . PCTIUS97/2~563 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.
CA 0224~662 1998 - 08 - o~
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.
WO 98n6961 . PCT/US97/23563 , 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.
WO 98/26961 . PCT/US97/23563 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.
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 CA 0224~662 1998-08-0~
.
WO 9812~i961 PCTIUS97123563 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 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.
CA 0224~662 l998-08-0~
~ .
-WO 98/26961 . PCT/US97/23563 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, CA 02245662 l99X-08-05 WO 98/26961 . PCT/US97/23S63 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 CA 02245662 l998-08-05 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.
Claims (19)
1. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto, comprising:
a. a bladder mounted beneath the cushion of the seat and supported by the base of the seat, whereby said bladder is constructed from a flexible material;
b. a fluid contained by said bladder;
c. a pressure sensor operably coupled to said bladder for generating a signal responsive to the pressure of said fluid within said bladder; and d. a signal processor for measuring the weight of the occupant from said signal for generating a control signal for controlling the safety restraint system responsive to said weight measurement.
a. a bladder mounted beneath the cushion of the seat and supported by the base of the seat, whereby said bladder is constructed from a flexible material;
b. a fluid contained by said bladder;
c. a pressure sensor operably coupled to said bladder for generating a signal responsive to the pressure of said fluid within said bladder; and d. a signal processor for measuring the weight of the occupant from said signal for generating a control signal for controlling the safety restraint system responsive to said weight measurement.
2. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 1, wherein said bladder further comprises a plurality of sheets of coated fabric sealably connected to one another at a periphery so as to form an inflatable confinement within said periphery whereby said sheets of coated fabric are further connected to one another at one or more locations within said periphery so as to create a plurality of fluid containing zones within said inflatable confinement which are in fluid communication with one another.
3. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 2, wherein said fluid containing zones are non-uniformly distributed within said periphery.
4. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 3, wherein said sheets of coated fabric are further connected to one another along one or more closed paths within said periphery.
5. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 1, further comprising a sheet of semi-rigid material proximate a load hearing surface of said bladder.
6. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 1, further comprising a coating on the outside of at least one load bearing surface of said fluid filled bladder.
7. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 1, whereby said fluid is a gas.
8. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 1, whereby said fluid is a liquid.
9. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 1, whereby for substantially no external load applied to said fluid filled bladder the volume of said fluid in said fluid filled bladder is less that the capacity of said fluid filled bladder.
10. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 1, wherein said pressure sensor is responsive to the absolute pressure of said fluid within said fluid filled bladder.
11. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 1, wherein said pressure sensor is responsive to the differential pressure of said fluid within said fluid filled bladder relative to local atmospheric pressure.
12. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 1, wherein said pressure sensor is responsive to the strain in the surface of said bladder.
13. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 1, wherein said pressure sensor is internally integrated within said bladder.
14. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 1, wherein said pressure sensor is isolated from said fluid by the surface of said fluid filled bladder.
15. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 14, further comprising a restraint located on the inside of said fluid filled bladder proximate said pressure sensor, whereby said restraint prevents said fluid filled bladder from collapsing proximate said pressure sensor.
16. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 1, wherein said bladder further comprises a plurality of sheets of coated fabric sealably connected to one another at a periphery so as to form an inflatable confinement whereby said sheets of coated fabric are further connected to one another at one or more locations within said periphery so as to create a plurality of fluid containing zones which are in fluid communication with one another, further comprising a coating on the outside of at least one load bearing surface of said fluid filled bladder, whereby for substantially no external load applied to said fluid filled bladder the volume of said fluid in said fluid filled bladder is less that the capacity of said fluid filled bladder,, wherein said pressure sensor is responsive to the differential pressure of said fluid within said fluid filled bladder relative to local atmospheric pressure, wherein said pressure sensor is isolated from said fluid by the surface of said fluid filled bladder, further comprising a restraint located on the inside of said fluid filled bladder proximate said pressure sensor, whereby said restraint prevents said fluid filled bladder from collapsing proximate said pressure sensor.
17. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 16, wherein said fluid containing zones are non-uniformly distributed within said periphery.
18. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 16, wherein said sheets of coated fabric are further connected to one another along one or more closed paths within said periphery.
19. A system for measuring the weight of an occupant in a vehicle seat and for controlling a safety restraint system responsive thereto as recited in claim 17, wherein said sheets of coated fabric are further connected to one another along one or more closed paths within said periphery.
Applications Claiming Priority (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US993,701 | 1992-12-21 | ||
| US3238096P | 1996-12-19 | 1996-12-19 | |
| US60/032,380 | 1996-12-19 | ||
| US08/993,701 US5957491A (en) | 1996-12-19 | 1997-12-18 | Seat weight sensor having fluid filled bladder |
| PCT/US1997/023563 WO1998026961A1 (en) | 1996-12-19 | 1997-12-19 | Seat weight sensor having fluid filled bladder |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2245662A1 true CA2245662A1 (en) | 1998-06-25 |
Family
ID=29424317
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2245662 Abandoned CA2245662A1 (en) | 1996-12-19 | 1997-12-19 | Seat weight sensor having fluid filled bladder |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2245662A1 (en) |
-
1997
- 1997-12-19 CA CA 2245662 patent/CA2245662A1/en not_active Abandoned
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