Note: Descriptions are shown in the official language in which they were submitted.
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<r- .~u~ u~ II~El~TION
Field of the Invention
~his invention relates to syste~s for rneasuriny the
load carried by vehicles and, more particularly~ to a rneasur-
iny system having s~rain sensing devices mounted o~ vehicle
suspension me~ers for measuring deflection of the members
responsive to loading of the vehicle.
~escription of the Prior Art
Weight regulations limiting the loade~ weights of
commercial vehicles are almost universally in effect~ r~hese
`;~ regulations generally specify the maximum load of each vehicle
axle as well as the maxirll~n ~otal load. Fines are usually
levied ayainst operators who are found violating ~hese
reyulations~
To rnaximize profits, vehicle operators normally
load thair vehicles as close a5 possible to the leyal load
limit. In order to do this, the vehicle operator must be
able to accurately measure the vehicle's weight while the
vehicle is being loa~ed.
A device commonly used to weigh vehicles is tlle
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; platform scale. l~he operator drives his vehicle onto a plat-
~ form, and the weight of the vehicle and i~s load, known as the
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gross weight, is measured~ The weiyht of the load is then
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~ determined by subtracting the weight of the unloaded vehicle,
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~; known as the tare weight, from the measured value. The weignt
on éach axle may be determined by moving the vehicle until
only th~ axle to be measured rests on the platform. A serious
disadvantage of platform scales is their fre~uent unavail-
ability when the vehicle is being loaded. Where the vehicle
,~ 30 is not resting on ~he platforrn during loadin~, the opera-tor
must estimate the load placed on his vehicle. Only after the
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~ vehicle has been driven frorn the loading area to the weighing
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area does the operator l~arn the exac-t weigilt of his load. If
the estimate is ~oo Low, the vehicle must return to ~he load-
ing area, where part of ~he load is xemoved. If the estimate
i5 too high, the vehicle rnust eith~r return to the loading
area or make a trip at a reduced profi-t.
The a~orementioned disadvantage o~ platform scales
has been eliminated somewhat by using portable scales placed
under each wheel. Such scales must be carried from place ~o
place by the ve}-icle, and althoug~ the scales are relatively
portabl~i, their weight and bulk reduce the load capaci~y of
~ the vehicle. Further, it is often somewhat difficult and
;~ time-consuming to place these scales beneath the wheels of
~ the vehicle.
-`` llo eliminate the aforementioned problems, on-board:, .;,
vehicle weighing systems have been developed~ In these
sy~tems, strain gauyes are generally secured to structural
members of the vehicle, usually the axles. IIowever, strain
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-i; gauges placed on tandem axles to measure loading are inherently
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; inaacurate. This is because axle bending is a function of how
the tixes meet the vehicle supporting surface. For a given
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load, axle deformation, and hence the measured load, is
~i greater where the vehicle~s weight is concentrated at ~he
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`ii~ outside edges~of the tire. Since the interface between the
`,~ vehicle~s tires and the tire supporting surface is not gener-
~ally well defined, it is impossible to control the measurement
accuracy of these systems.
Other on-board load measuring systerns utilizei load
cells placed on structural members inte~nediate the axles
`~ and the load. However, these system~ aLso have shortcomings.
For example, 'chey are difficult to retrofit on existing
~` vehicles and cannot sense the extra weight added to a vehicle
by dirt and other de~ris collected on the surfaces of the
vehicle suspension beneath the load cells.
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Other on--board load measuring syst~ms measure the
~; relative displacemen~ of ~he spri~y~ or the relativ~ displace-
ent b~tween the vehicle f~ame and a~le~ i-Iowever, the accuracy
of ~hese clevices is ~dversely affected by wear, friction and
debris, and installation thereof yenerally rPcluires relatively
extensive modification o~ the vehicle.
U~ R~ OF TI~E INVENTION
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~ It, is an object of this invention to provide an
- on-board load measuring system which accurately measure,s the
weight placed on the axles o a tandem-wheeled vehicle.
~; It is another object of this invention to perform
this mea~urement at ~ point in th~ chain of load supportiny
structures which is relatively close to the vPhicle supporting
surface.
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Still another object of this invention is to provide
; a measuring system which is effected very little by wear,
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friction or road dirt.
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is a further object of this invention to provide
; a load measuring system which requires minimal mo~ifications
to the vehicle, thus making installation o~ the system
,1 relatively easy.
A still further object of this invention is to
A provide an on-board load measuring system which is compact and
light in weight so that the load capacity of the vehicle is
not materially reduced.
These and other objects of the invention are
ac~omplished by an on board load measuring system installed
" ~ on a vehicle having tandem rear wheels and a suspension system
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of the type including equalizing beams positioned at the
sides of the vehicle, with the midpoint o~ the beams pivot-
ally supporting the vehicle frame and the ends of the equal-
izing beams connected to tandem axles. Since the vehicle is
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supported throuyh tile equali~in~ beams, the equalizing ~eams
bend in response to loa~irlg of the veilicle. The vehicle load
is measured by installing a strain sensing transducer 021 each
equali~ing beam. The transducer pro~ides an electrical indi
cation of equalizer beam deforrnation correspondiny to the
vehicle load. A similar transducer is carried by the front
axle to measure the load supported by the front wheels~
; Each transducer includes a pair of spaced apart
mounts carried by the respective equaliæing beam or front axle
such that deformations thereof change the distance between the
mounts. A deformable member extends between the ~nounts t and
strain sensing means are secured there~o. The s~rain sensing
means provides an electrical signal corresponding to varia-
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`` tions in the relative position between the mounts responsive
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to bending of the respective equalizing beam or front axle.
;~, The electrical load signals are received by a load
indicating circuit which displays in the cab the load, either
on the front wheels or on the rear wheels, with respect to a
' preset maximum load point corresponding to the legal limit~
This permits the operator to precisely determine his available
load capacity as his load approaches the legal limit.
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,~ BRIEF DESCRIP'~ION OF THE DRAWINGS
~I Fig. 1 is an isometric view showing a tandem-wheeled
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~ vehicle on which the inventive load measuring system is
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installed.
~` Fig. 2 is an isometric view of the strain sansing
~ transducer~
;i Fig. 3 is a side elevational view, paxtially in
section, showing the transducer installed on the equalizing
beam!.
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~ Fig~ ~ is a cross-sectional view taken along the
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'~''J line 4- 4 o:E Fig . 3 q
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~i~3. 5 is a scilematic showiny the indicatirly circult
.~ WhiCil receives an~ amplifies t~le -transducer outpu-ts and
provicles a ~isual load indicaticjn.
Fig. 6 is a graph showing the voltage and current
at various points in the circuit of ~iy. 5 plQtted as a
function of strain yauye diffe~ential output voltag~.
: Figs. 7-9 show the installation of the traIlsducer on
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~ an axle.
DET~ILED DESCRIPTION OF THE II~E~TION
The on-board vehicle load measuring system of the
present invention i5 shown in Fig. 1 installed on a ve11icle
having a pair of cross-connected longitudinal chassis frame
1~ members 10, 12 carried at the rear by a tandem.-axle suspension
:: system including fore and aft axle assemblies 22, 2~ havin~
sets 14, 16 and 18, 20 of dual wheels mounted on opposite
-.~ ends thereof. The axle assemblies have forked, dependiny
.'5 hanger brackets 15 fixed at their ends and pivotally connected
:l ~ at 17 in straddling relation to eyes formed at the ends of a
`~ pair of equalizing beams 26, 28 in the manner shown in U.S.
Patent No~ 2,914,349. ~t their longitudinal center, these
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equalizing beams support a pair of saddles 30, 32 mounted on
the ends of a cross-tube 34 in the manner shown in U.S. Patent
No. 3,129,016. A1~o as shown in this latter patent, pairs of
U-bolts 4Q, 42 hold a pair of multi leaf spring units 36, 3æ
seated on the flat upper face of the saddles 30, 32, and
these springs are in turn connected by front and rear spring
hangers 44, ~8 and 46, 50 to the chassis frame member 10, 12.
A typical such equalizing bearn suspension system also includes
front and rear:torque rod assemblies ~not shown~ connected to
the chassis frame and mounted on the center of the tandem
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.l a~les 22, 24 as disclosed in ~he aforesaid patents~
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~ e e~ualizing beails 26, 28 ~eflect responsiv~ to
losading of th~ vehicle so thak ~he amount of deflection or
bending is an indication of the loads placed on the tandem
axles 22, 2~. The on-board load measuring systern ok the
pxesent invention includes a p~ir of s~rain measuring trans-
ducers 52, 54 which are fastened respectively to the equal-
izing beams 260 28 at a poin~ prefexably ~e~ween ~he saddles
30, 32 and the rear tandem axle 24 and situated on the upper
faces of the beams. Pairs of leads 56, 58 carry electrical
indica~ions of equalizing beam deformation generated ~y the
transducers 52, 54 to a load indicator 60 mounted in the cab
62 of the vehicle.
The forward portions of the chassis frame members
10, 12 are supported by steerable front wheels 64, 66. These
'~ wheels 64, 66 are connected to a dead front axle ~8 which is
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',~ suspended by leaf spring units 65, and like the equalizing
bea~s 26, 28, deflects or bends responsive to loading of the
vehicle. A third strain measuring transducer 70 is secured to
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the front axle 68 for producing electrical indications of the
'; 20 bending thereof. A pair of leads 72 carry these electrical
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indications to the load indicator 60.
As will be explained hereinafter, the vehicle
~ operator mS~y monitor on the indicator 60 the load on either
,`,~ the forward or rear wheels of the vehicle. The vehicle load
is not indicated in absolute terms, but rather the vehicle
load is expressed in relation to a preset maximum load point.
This sy~tem gives the operator a more precise indication of
'' ~ the load as the load approaches the legal limit, which is the
"~ zone where the opexator is most concerned with the load on the
,vehicle.
, The transducer 52 is part of the present invention
`'~ and is detailed in Figs. 2 through 4. In these views, it is
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seen that -tile ~ransducer includes a pair of mounts ~0, 82
hav1ncJ coplanar, flat rnounting surfaces 84, 86 and countersunk
througll-bores 88, 90 witll their axes perpendicular -to th~
planP of the mounting surfaces. Extending between the mounts
80~ 82 is a bending bearn ~2 integrally connected to the sides
of ~he mounts 80, 82 through cylindrical members 94~ 96~ which
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are circumferentially grooved. The bending beam 92, cylindri-
cal members 94, 96, and moun~s 80, 82 are machined as a unitary
structure from a single block of metal which is preferably of
the same general type as that from which the equalizing beam
is fabricated on which the transducer 52 is to be mounted so
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that the thermal expansion characteristics o~ the bending beam
and equalizing beam will be similar. A pair of strain gauges
98, 100 are secured to opposite faces of the bending beam 92.
These gauyes are of conventional design and are co~nercially
available through Micromeasurements, Inc. of ROMU1US, Michiyan.
The sizes of the mounts 80, 82 are not critical
except that the mount 82 should be of sufficient size to
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accommodate a connector 102 to provide electrical communication
between the leads 56 from the strain gauges 98, 100. As best
` illustrated in Fig. 3, the connector 102 is secured to the
i~ outer end face of the mount 82 by screws 104. The inside end
`, o~ the connector 102 contains a number of terminals, generally
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indicaked at 106, which are electrically conn~cted to the
strain gauges 98, 100 by leads 108, 110 exten~ing from the
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terminals 106 to the ben~ing beam 92 via thxough-bore 112.
The outer cylindrical surface of the connector 102 is threaded
for receiving a pluy (not shown) containing a number of con-
ductor pins which are received by four holes 114. ~hese holes
11~ are alignqd with conductors which are connected to respec-
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" tive terminals 106 throuyh internal conductors. ~en the plug
;, mates with thq connector 102, the leads S6 connected to the
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pins a~e electri(~ally co~ ecteci to the st:rain cJauc~es 9~J 100
throucJh lea~s 10~, 110, I:erminals 10~ an(l the con~uctors
internal t~ ~ e conn~ctor 102 wllich -te~mina~e in the holes 114.
As best ilLustrated iII E~ig. 3~ -the mount 82 contains
a pai.r of small througil-hores 116, 11~ cor.ullunicatincJ with the
cylindrical recess receiviny the conllector 102. After the
trarlsducer has been ass~mbled, a conventional potting material
i5 injected -throuyh th~ aperture 116 to fill ~he cylindrical
void., Air previously in the void ~scapes via through-l~ore 118.
The bending beam 92 an~ strain gauges 98, 100 are protected
from the external environment by a cylindrical rubber boot 120
which is tightly secured at its ends to the cylindrical mem~ers
94, 96 by bands 122, 124 seated in the circ~nferential yrooves
in the members . Boot 120 is placed in position by radially
stretching it over the mount 80 and pulling it endwise into
shielding position.
The transducer 52 is installed on the equalizing
; beam ~6 by first applying putty 126, 128 to the mounting
surfaces 84, 86, prefera~ly in a U-pattern and fI-pattern,
; 20 respectively, as shown in Fig. 2. rrhe putty 126, 128 is not
applied adjacent the inside edyes of the mounting surfaces 84,
86 since, after the transducer 52 is installed on the equal-
izing beam 26, it is difficult to wipe excess putty therefrom.
` Similarly, the putty is not applied to the mounting surface 86
` ~ adjacent its outside edge since putty at this location might
exer~ a bending force on ~he ~ransducer 52. The putty 126, 128
is not applied for the purpose o~ bonding the transducer 52 to
the e~ualizing beam 26, but instead it is used to provide a
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~lat supporting surface for the transducer 52 on the top of
the equalizing beam which, as illustrated in Fig. 4, is some-
~ what rounded~ When the transducer S2 is forced against the
`~ equalizing beam 26, the put~y 126, 12~ spreads out to form a
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platforrn for the tr~rlsducer. It is preferred that the thermal
expansion characteristics of the putty closely match those of
the equalizing beam and the transducer. Thus, for ~luminum
equalizing beams, an aluminum putty is used, while steel putty
is used for steel equaliziny be~ns. The putty is a well-known,
commercially available product and may be obtained from the
~evcon Company. A release ayent may be applied to the mounting
surfaces 84, ~6 prior to application of the putty to facilitate
subsequent removal of the transducer from the equaliziny beam
with the putty platform left intact for reuse.
As best illustrated in Fig. 3, shallow, spa~ed apart
holes 130, 132 are bored into the equali7ing beam 26 and then
tapped with scre~ threads. These holes are preferably located
in a zone of the beam offset from the longitudinal center
wherein the stress laminate differential under load at the
holes is minimal. Care should be taken to avoid boring too
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~ deeply into the equalizing beam 26 to avoid impairing its
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strength. Screws 134, 136 are inserted into the through-bores
88, 90, and the transducer 52 is torqued against the equalizing
0 beam 26, thereby spreading the putty 126, 128. It is preferred
to locate the transducer 52 on the rear upper surface of the
equalizing beam so as to make it less vulnerable to flying
objects, but this is not essential for performance since nor-
i~ ~ mally a protective cover is mounted over the transducer 52 ~o
protect it from being struck by hard objects.
In its relaxed condition, the bending beam 92 of the
i ~ transducer 52 is precurved, as best illustrated in Fig. 3.
;~ Thus the beam 92 is biased to bend in a predetermined direc-
tion for a given deflection of the equalizing bearn 26. For
example, as additional load is applied to the vehicle, the
equalizing beam 26 undergoes increased bending. The increased
bending causes th~ rnounts 80, 82 to angle toward each other,
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tnereby ~ecreasiny the distance from one mount to the other.
Because the bendir~cJ bea~n 92 i~; precurv~cl, i-ts radius o:E curva-
ture decreases in r~s~onse to ~he decrease in distance between
the mounts 80, ~2~ ~ non-curved beam would also bend in response
to load, but it would not be possible to predict in which direc~
tion the beam would bend. In such an instance, although one
strain gauge would undergo tension and the other strain gaucJe
would undergo compression, it would not be possible to predic~
which gauye would be tensioned and which would be compressed.
~lowever, in the case of the strain sensing transducer 52, the
beam always bends such that the strain gauge 98 undergoes com-
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; pression and the strain gauge 100 undergoes tension. T~le result
is a transducer with pre-defined characteristics~
As indicated in Fiy. 1, a transducer 70 which is
identical to ~he transducer 52 is installed on the front
axle 68. With reference to Figs. 7 -through 9, in preparation
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~ for this installation, a pair of mounting pads 140, 142 are
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initially secured to a spacer 144 by bolts 146, 14~ which are
screwed into vertical threaded bores provided in the top of
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j~ 20 the pads 140, 14~. The holes in the spacer 14~ throuyh which
the bol-ts 146, 148 extend are spaced apart a distance equal to
the spacing between tha through-bores 88, 90 (Fig. 2) when the
transducer is in its relaxed condition. The pads 140, 1~2 are
~?,~ then secured to the front axle 68, preferably by welding at
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~150, 152, and the spacex 144 is removed from the pads by dis-
engaging the bolks 146, 148 therefrom. The trans~ucer 70 is
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;~ then secured to the pads 140, 142 in place of the spacer 144.
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For tAis purpose, bolts 154, 156 are inserted thr~ugh the
bores in the transducer mounts and securely tiyh-tened. Tlle
described installation procedure insures proper bolt align-
ment~ Finally, a U-shaped cover 158 is fastened to the ~nount-
ing pads 140, 142 to protect the tran~ducer 70. For this
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~urpose, botll si~l~s o~ th~ cover 158 co~tain apextures 160
a~apted t~ ~lign Witll threacl~d ):~o:~es 162, 16~ in tII/3 sides of
~: t.he r~ountinc.l pads 14U ~ 1~2 . Cap screws 166 are inser~ed
through washers 16~, 170 ~nd apertures 160 to engaye the
threads in bores 162 ~ 164 . An iderltic:al cover securing
. arrangernent is provided on tlle reverse side of the cover 158.
The load indicator cixcuitry receiving the
transducer outputs and providing a visual indicatiorl of
vehicle load is illustrated in Fig. 5. Each of the strain
yauges in ~he transducers 52, 54, 70 has a nominal resistance
of 350 ohrns and~ in normal operation, this resistance can be
expected to vary less than 1 ohm as the yauges undergo strain.
: The strain gauges 200, 202 in the front transducer 70 form,
along with equal valued, matched resistors ~04, 206, a Wheat-
stone bridge circuit having 8 volts applied -there~o on line
208. The 1 ohm resistance variations of the strain gauges
200, 202 produce a rnaximum voltage variation on output line
210 of approximately 8 millivolts. Since the signal varia-
tions are relatively small, the 8 volt power is well regu-
lated by a conventional voltage regulator ~not shown). The
resistors 204, 206 forming the passive leys of the bridge are
matched to each other and to the strain gauge 200, 202 to
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-~ insure good therrnal stability.
` ~ The differential outputs 210, 212 of the bridge are
applied through resistors 214, 216 to a double-pole, double-
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s throw switch 218 which selectively transmits the outputs to
the remainder of ~he indicator circuit. Strain gauge 200,
being located on the upper surface of transducer 70, undergoes
compression, while strain gauge 202, being located on the
-~ 30 lower surface, undergoes tension. Thus, in response to
~;i increasing loads, the re~istance of strain gauge 200 decreases,
while the resistance of strain gauge 202 increases/ thereby
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increasing the voltacJe Oll output line 210~ The voltacJe on
line 212, however, :i5 a constant d ~olts and serves as a
reference to which ~he ou-tput on line 210 is compared~
The strairl gaucJes 9~, lO0 and 220, ~22 in
transducers 52, 54, respectively~ forrn a second Wheatstone
bridge powered by the r~gulated 8 volt power supply through
line 224. The resistance of compression strain yauges 98,
222 decxeases and the resistance of tension strain gauyes lO0,
220 increases in response to increased loading, thereby
increasing the voltage on output line 226 and decreasing the
voltage on output line 228 (i.e., increasing the differe~tial
output voltage~. The outpu~s 226, 228 are connected to the
double-pole, double-throw switch 218 through resistors 230
232.
As will be explainecl hereinafter, the indicator
circuit indicates vehicle loadiny only when the signal appliecl
thereto, as selected by the double-pole, double-throw switch
218, approaches a predetermined voltage. This voltaye is a
composite of a bridge output voltage and an adjustable bias
voltaye. The bias voltages are applied through lines 234,
236 and resistors 238, 240 to the switch 218. The values of
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the bias voltage are determined by adjusting the wiper posi~
tion o~ potentiometers 242, 244 between 8 volts and ground.
The bias voltages, by determining the vehicle loacl at which
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the indicator circuit responds to the transducers, determine
t~e vehicle load corrPspondiny to a maximum load. The bias
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~;~ voltage is adjusted to preset the maximurn load limit to
correspond to the legal loacl lirnit for the front and rear
' ! wheels.
The outputs from the bridge selected hy the switch
218 are applied to a high gain operational arnpli~ier 250. A
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~ pair o~ diodes 252, 254, arranged in parallel~ anode to
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~athode, are placed acros~ ~he input t~rminal~ to prot~c-t thP
arnplifiex 250 from excessiv~ voltage ina~ver~ently placed on
the input lines. A capacitor 256 is als3 placed across the
inpu-t terminals to reduce the high~frequency response of the
amplifier 250. As explaine~ below~ a non-linear feedback
cixcuit provides the respoIIse curve ~ shown in Fig. 6. When
the differential input to the ampliEier 250 is below a pre-
determined voltaye, the VA output is clamped to approximately
3.3 volts, or a l diode drop below the voltage (approximately
4 volt5) on the inverting input terminal, through diode 258.
As the differential input voltaye increases to VA~ diode 258
becomes back-biased, thereby opening the feedback path and
causing the amplifier gain to greatly increase so that the
output rises to approximately 4.7 volts~ the point whexe diode
260 becomes forward-biased. Conduction through diode 260
closes the feedback path through resistor 262 and capacitor
264, arranged in parallel~ In this condition, the gain of the
arnplifier 250 is approximately equal to the resistance 262
divided by the input resistance 216 or 232 plus the source
resistance of the respective bridge. In one operational
embodiment, this ratio was selected to be approximately 150.
Capacitor 264 is provided to roll off the gain of
the amplifier 250 above a predeterminad frequency 50 that the
indicating circuit does not respond to motion-induced deflec-
tions of the equalizing beams or front axle. A frequency
breakpoint of considerably less than l Hz is generally
desired. The output of arnplifier 250 is applied to another
operation~l arnplifier 266 through diode 260 and a resistor
268, The non-inverting input to arnplifier 266 is connected to
the non-inverting input to amplifier 250 through resistor ~70
The voltage at the non-inverting input to amplifiex 266 is
approxirQately 4 volts. Thus, when the output vol~age of
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amplifler 250 is below 4~7 volts, diod2 ~60 is back-biased,
holding the ~lifferentlal input voltage to amplifier 2~ at
zero ~olts an~ hence holding the output voltaye to approxi-
mately 4 volts~ In this condition, no current flows through
a milliameter 272 conn~cted between the S~liny junction of
amplifier 266 and the output throuyll resistor 274 since the
~ voltage at the s~lm~ y junction is also 4 volts. As the
i output of ~nplifiex 250 rises above 4.7 volts, diode 260
becomes forward biased, causing current to flow into the
summing junction of amplifier 266, there~y linearly reducing
the output voltage of a~plifier 266 as shown in curve B of
- Fig. 6. Current then flows throuyh Ineter 272 in proporti~n to
the increased voltage at the outpu-t of arnplifier 250 as shown
;~ by curve C in Fiy~ 6. ~ shunting resistor 276 is placed in
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parallal with ~he meter 272.
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The differential voltage VA, at which the meter
; starts to deflect, may be varied by adjusting the load limit,
i adjusting po~entidmeters 242, 244. In operation, the system
is calibrated by placing a load on a truck which is measured
by a conventional external vehicle weiyhing device to be the
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legal load limit~ The switch 218 is then placed in its front
transducer reading position,`and the front load limit adjust-
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qi ing potentiometer 242 is set so that the meter 272 corresponds
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to a value indicating the maximum allowable load. The switch
218 ls then placed in its rear load measuring position, and
~i~ potentiometer 244 is adjusted so that the meter 272 indicates
the maximum allowa~le load. When the vehicle is subsequently
loaded, the meter remains undeflected until the vehicle load
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approaches ~he legal limit. As the indicator then moves from
graduation to gxaduation toward the maximum allowable load
position responsive to further loading of the vehicle, the
`t,, ~ opexator can readily detexmine the remaining load capacity.
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~ rom ~he foregoiny description, it is s~en.that the
vehicle load measuriny system o the ~resent invention is
unusually accurate and easy to use. It is readily installed
and, in operation, minimally reduces the vehicle's capacity.
Furthex, its accuracy i5 substantially unaffected by external
conditions, such as the interface between the wheels and the
ground, and it is not effected by environmental factors such
as temperature and moisture.
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