Note: Descriptions are shown in the official language in which they were submitted.
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Description
Payload Monitor
Technical Field
This invention relates generally to an
apparatus for accurately determining the payload
carried by a work vehicle, and more particularly, to an
apparatus which correlates suspension strut pressures
and suspension geometry into an accurate indication of
work vehicle payload.
Background Art
In the field of off-highway trucks used, for
example, in mininy operations, it is desirable that an
accurate record be kept of the quantity of material
removed from the mining site. This information can be
used to calculate mine and truck productivity as well
as aid in forecas-ting profitability and work schedules.
Previously, mine operators have been forced to
use traditional fixed scales to obtain the accuracy
considered necessary to make the payload information
useful. The fixed scales are typically located along
the truck haul route such that a minimum amount of time
is expended in weighing the trucks. However, as the
mining operation progresses, the truck haul routes will
necessarily be altered which results in a major capital
expenditure of periodically relocatiny the fixed scales
to more advantageous locations. Further, while the
truck is being weighed, it cannot be performing useEul
work, and thus, the weighing operation intended to
measure truck productivity is a negative inEluence on
truck productivity. An alternate method for
determininy mine productivity simply involves
establishing a standard payload and applying this
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standard to the number oE completed truck loads.
Obviously, this method reduces unproductive truck time,
but the resultant inaccuracy is significant. It is
desirable to have a weighing scheme which is route
independent and yet accurate.
Prior on board payload weighing apparatus are
notoriously inaccurate under actual loading
conditions. Heretofore, these on board systems have
relied upon calibration without consideration to the
effects of load distribution caused by material
placement and unlevel loading conditions. It would be
advantageous for an on board payload monitor to
accurately detect load substantially independent of
load position and underfoot conditions. An important
benefit to accurate payload monitoring is that the
possibility of overloading a truck is greatly reduced
and ,consequentlyr extreme tire wear normally
associated with overloaded trucks is minimized.
Furtherl in prior monitoring systems actual
recording of a completed loading cycle is a process
normally delegated to the vehicle operator~ The
interaction between the monitor and the operator is
typically of a simple nature requiriny the operator to
push a button and cause the monitor to record the
current weight as a valid payload. The possibility of
error is introduced by this system through inadvertent
or deliberate multiple recordings or conversely missed
recordings. It would be advantageous for an on board
payload monitor to automatically record each completed
loading cycle. Preferably, the monitor would be
capable of automatically recording each loading cycle
without benefit of additional sensors.
The present invention is directed to over-
coming one or more of the problems as set forth above.
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Visclosure of_the Invention
In accordance with one aspect of the present
invention, an apparatus measures and indicates the
weight of a payload carried by a work vehicle. The
work vehicle has at least one front and rear strut
disposed in supporting relation to a load carrying
portion of the work vehicle. A means separately senses
the pressures of the front and rear struts and delivers
signals respectively responsive to the magnitude of the
front and rear strut pressures. A means modifies the
front and rear strut pressure signals by applying
respective unique correction Eactors thereto, summing
the resultant modified signals, and delivering a
control signal responsive to the magnitude of the sum
of the modified signals. A means receives the control
signal and delivers an indication of the magnitude of
the work vehicle payload in response to the magnitude
of the control signal.
Prior payload monitors have systematically
ignored the effects of load distribution caused by
material placement and underfoot conditions. These
pri~r monitors are typically inaccurate under less than
ideal conditions such as loading on a grade or loading
with a wheel racked. The present apparatus compensates
for these inaccuracies without the need for additional
sensors.
Brief Description of The Drawings
Fig, 1 illustrates a diayrammatic view of an
off-highway truck and the location of critical
suspension components;
Fig. 2 illustrates a block diagram oE the
payload monitor;
Fig. 3a illustrates a portion of one
embodiment of the software flowchart for i~plementing
the payload monitor;
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Fig. 3b illustrates a portion of one
embodiment of the software flowchart Eor implementing
the payload monitor;
Fig. 3c illustrates a portion of one
embodiment of the software flowchart for implementing
the payload monitor; and
Fig. 3d illustrates a portion of one
embodiment of the soEtware flowchart for implementing
the payload monitor.
Best Mode For Carrying Out The Invention
Referring now to the drawings, wherein a
preferred embodiment of the present apparatus 10 is
shown, Fig. 1 illustrates a work vehicle 12 which can
be, for example, an off-highway truck 14. The truck
has at least one front and rear strut 16,18 disposed in
supporting relation to a load carrying portion 20 of
the work vehicle. The preferred embodiment has two
front and two rear struts 16,18 which are the gas-over-
liquid type commonly known in the industry and not
described herein. It is sufficient in the understanding
of the instant apparatus to recognize that the pressure
of the fluid is indicative of the magnitude of the load
applied to the strut 16,180 The load carrying portion
25 20 includes a vehicular frame 22 and a dump body 24.
The dump body 24 is connected to the frame 22 by pivot
pin 26 and hydraulic cylinder 28 such that the contents
of the dump body 24 can be removed by controllably
pressurizing the cylinder 28 to effect pivotal movement
of the dump body 24 about the pivot pin 26. In thetransport mode the cylinder 28 is not pressurized and
the weight oE the dump body is transferred to the frame
through the pivot pin 26 and a support pad 30 fixed to
the frame 22.
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The work vehicle 12 Eurther includes a ground
engaging portion 32 and a suspension means 34 for
supporting said load carrying portion 20 in a manner to
provide damped oscillatory motion between the ground
engaging portion 32 and the load carrying portion 20.
The suspension means 34 includes a rear axle housing 36
and an A-frame moment arm 38. The A-frame moment arm
38 has a first end portion 40 pivotally connected to
the vehicular frame 22 by a socket 42 and a second end
portion 44 fixedly connected to the rear axle housing
36. The first end portion 40 of the A-frame moment arm
38 is a king bolt arrangement, substantially spherical
in shape and retained from lateral movement by the
socket 42. The rear strut 18 has a first end portion
46 pivotally connected to the vehicular frame 22 and a
second end portion 48 pivotally connected to the second
end portion 44 of the A-frame moment arm 38.
During loading of the truck, as the payload
increases~ the load carrying portion 2~ will be
displaced in a direction toward the ground engaging
portion 32. The rear strut 18 begins to collapse while
the A-frame moment arm 38 pivots about first end
portion 40. A distance L2 is defined to be the
distance between the first end portion 40 pivot point
and the second end portion 44 pivot point of the arm
38. Therefore, it can be shown that the rear strut
pressure differential is a function of the suspension
means 34. Moreover, the rear strut pressure
differential can be related to the reaction force ~
between a work surEace and the ground engaging portion
32. A force S experienced by the rear strut 18 can be
detersnined by measuring the internal pressure of the
strut 18, subtracting the rear strut pressure
corresponding to an unloaded truck, and multiplying the
differential pressure by the area of the strut 18. The
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reaction Eorce R is proportional to the payload oE ~he
vehicle 12 and can be assumed to act through the center
of the rear a~le housing 36 such that a summation of
the moments about the pivot point of the king bolt
would derive the following equation:
~eqn. 1.1) R = S * L2/L3
where the horizontal distance between the first end
10 portion 40 pivot point and the center of the rear axle
housing 36 is defined to be L3.
Similarly, the Eront strut 16 will be
compressed as the load increases; however, the frollt
strut is connected directly between the frame 22 and a
15 front axle housing 50. A more straightforward
relationship exists here in that a force F experienced
by the front strut 16 can be determined by measuring
the internal pressure of the strut 16, subtracting the
front strut pressure corresponding to an unloaded
20 truck, and multiplying the pressure by the area of the
strut 16. The reaction Eorce F between the ground
engaging portion 32 and the work surEace i5
substantially equivalent to the force F experienced by
the front strut 16.
The apparatus 10 is shown in Fig. 1 to
illustrate the relationship between the work vehicle 12
and the location of the apparatus 10. A more detailed
block diagram of the apparatus 10 is shown in Fiy. 2
and diagraMmatically illustrates a means 52 which
30 separately senses the pressures of the front and rear
struts 16,18 and delivers signals respectively
responsive to the rnagnitude oE the front and rear strut
pressures. The means 52 includes a plurality of
pressure sensors 54,56,58,60 of the type commercially
35 available from Dynisco as part number PT306. The
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pressure sensors 54,56,58,6~ are respectively
associated with the two front struts 16 and the two
rear struts 18. Each of the pressure sensors
54,56,58,60 delivers an analog signal proportional to
the magnitude of the pressure of the respective strut
16,18 to respective analog to digital converters (A/D)
62,64,66,68. The A/D's 62,64,66,68 can be of the type
commercially available from Analog Devices as part
number AD574A. Other types of A/D converters have been
contemplated by the inventor and the choice of the
particular A/D disclosed herein is simply a matter of
designer discretion. The selection of a device which
provides an analog to frequency output is particularly
well suited to the digital microprocessor environment
disclosed herein; however, other similar devices could
be easily substituted without departing from the spirit
of the invention.
A Motorola programmable interface array (PIA)
70 receives the digital frequencies output by the A/D
20 converters 62,64,66,68 and delivers these signals to a
microprocessor 72 under software control. In the
preferred embodiment, the microprocessor 72 is part
number 6809 provided by the Motorola Corp. The
microprocessor 72 is programmed to have a means which
modifies the front and rear strut pressure signals by
applying respective unique correction factors thereto,
summing the resultant modified signals, and delivering
a control signal responsive to the magnitude of the sum
of said modified signals. More specifically, the
correction factor of the rear pressure signal is a
function of the suspension means 34. Preferably, the
rear strut pressure correction factor is a function of
the length of the A-frame moment arm 38 and the
horizontal distance between the center of the rear axle
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housing 36 and the first end portion 40 of the A-frame
moment arm 38. The rear strut pressure correction
factor includes L3/L2.
The appara~us also includes a means 7~ which
receives the control signal and delivers an indication
oE the magnitude of the work vehicle payload in
response to the magnitude oE the control signal. The
indicating means 7~ has a first means 7~ which delivers
a first signal in response to the magnitude of the
control signal being greater than a Eirst preselected
magnitude and a second means which delivers a second
signal in response to the magnitude of the control
signal being greater than a second preselected
magnitude. The indicating means 74 includes a second
PIA 78 connected through a driver circuit 8U to a pair
of individually energizable incandescent lamps 82,~4.
The indicating means 74 also includes a portion of the
software program of the microprocessor 72 which is
disclosed in greater detail in conjunction with the
flowchart of Fig. 3d.
Fig. 3a shows a flowchart representation of
PMINIT2 TSK program which is an initialization routine
performed only during power up of the payload monitor.
The initialization routine is responsible to move all
constants and variables resident in memory into working
RAM. The variables "OLD PAYLOAD" and "OLD VALID
PAYLOAD" are also respectively moved from EEPROM into
the RAM variables "PREVIOUS PAYLOAD" and "P~EVIOUS
VALID PAYLOADn.
The ~EIGH2 TSK program is illustrated ViA
flowchart beginning in Fig. 3b and is primarily
configured to calculate the current total payload,
determine if the calculated payload is correct, and
record only those calculated payloads which reflect a
completed loading process. The program begins by
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checking to determine if a new EEPROM has been placed
in the apparatus. The EEPROM is used to store the
value of each actual valid payload without the need for
continuously powering the memory, and as is well-known
in the art, the EEPROM may only be written to a limited
number of times which necessitates the need to
periodically replace the EEPROM, A memory address
pointer is read and compared to the highest valid
memory location. If the pointer is indicating a memory
location greater than the highest valid memory, then
the program assumes that all of the usable memory
locations have been accessed. The pointer is
subsequently reset to the first valid memory location
and the initial flag is set. Further, the last reading
of the pressure sensors 54,56,58,60 is stored as the
INITIAL PRESSURE" variables and the variable "LOAD ~"
is incremented. Should the pointer indicate a valid
memory location, then control passes directly to
increment the variable "LOAD #~ which is merely a
sequential indication of each of the completed loads.
The software routine then checks to determine
if the calibration switch 85 has been actuated. The
operator will typically actuate the calibrate switch 86
at the beginning of a shift when the vehicle is known
to be empty. When the calibrate switch is actuated~
the variables "INITIAL PRESSURE" are replaced with the
last reading of the pressure sensors iE the pressures
are within a preselected acceptable range. Thus,
"INITIAL PRESSURE" contains the value of the pressure
signals for an unloaded truck. The program pauses
until the calibration switch 86 has been released, at
which point the program calculates the payload by
computing the pressure differential ~etween ~INITIAL
PRESSURE" ancl the last reading of the pressure
siynals. If the truck is empty~ then the pressure
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differential will be zero and, consequently, the
payload will not have changed from the callibration
reading. The pressure differential and the calculated
payload will remain unchanged until such time as the
truck begins to receive material. The actual total
payload is calculated Erom a series of three equations
where the front and rear payload components are
determined separately and then summed to determine
total payloadO The correction factors illustrated
herein are L3/L2 * strut area Eor the rear payload and
simply strut area for the front payload. Additionally,
offsets have been added to both the front and rear
payloads to compensate for offsets introduced by
Eriction in the struts lfi,l8.
The flowchart of Fig. 3c illustrates a method
for determining the validity oE the computed total
payload. The computed total payload is recorded as an
indication of actual current payload in response to the
magnitude of the computed payload changing by less than
a preselected magnitude during a first preselected
duration of time. The program once again checks the
status of the memory and if the EEPROM is new, the
program stores the computed total payload as the
variable ~INITIAL PAYLOAD~. This process is performed
during only the first iteration of the program aEter a
new EEPROM has been loaded because there is no
meaningful value stored in the variable "INITIAL
PAYLOADn. After the first iteration, the "INITIAL
PAYLOAD" is always the previous computed total
payload. The computed total payload is compared to
"INITIAL PAYLOAD" for an indication that the payload
has not changed since the last pressure reading was
taken. More specificallyr the truck is now in a stable
period and there is, therefor, an assumption of
validity as to the value oE the total payload. The
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program then ~onitors the pressure signal for a period
of 5 seconds to ensure that the total payload was
computed using pressure signals taken during a stable
period. Should the pressure signals change by greater
than a preselected tolerance, then the program exits to
the beginning o the WEIG~2 TSK in an attempt to record
only a total payload which was computed from pressure
readings taken during a stable period either between
individual bucket loads or at the completion of the
loading cycle and prior to roadiny. The status of the
EEPROM is checked again to avoid recording the ~irst
iteration pressure readings as an indication of the
completion of an actual loading operation. If the
EEPROM is new, then the computed total payload is
stored as the variable "INITIAL VALID PAYLOADn.
Subsequently, for either a new or an old EEPROM, the
computed total payload is stored as the variable "VALID
PAY LOAD n .
Program control proceeds to the section of the
software which controls the queing lights for a loading
vehicle operator (see Fig. 4d3. The total payload is
compared to 90% of the rated load and the 90% lamp is
illuminated to indicate to the operator of a loading
vehicle that the truck has been filled to at least 90%
of capacity. Similarly, the 100% lamp is illuminated
in response to the total payload reaching at least 100
of capacity~ Use of the queing lights enables the
loader operator to accurately Eill the truck within a
prescribed tolerance of rated load.
The software is capable oE determining the
status of the truck relative to the three conditions of
loading, dumping, or roading. The "VALID PAYLOAD~ is
compared to the "INIrrIAL VALID PA~LOAD~ to determine if
the payload has been increased by a bucket load, dumped
by the truck operator, or no change has occurred since
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the last pressure reading was taken. If ~VALID
PAYLOAD" is greater than ~INITIAL VALID PAYLOAD" by a
preselected tolerance, then the proyram assumes that an
additional bucket load of material has been added to
the truck and the "PAYLOAD CHANGE FLAG" is set.
Changes within the prescribed tolerance are considered
to be inconsequential and can be ignored. Detecting a
durnping condition involves determininy iE the "VALID
PAYLOAD" is less than the "I~ITIAL VALID PAYLOAD" by
the sarne prescribed tolerance. If neither loading nor
dumping is detected, then control transEers to the
beyinning of the ~EIG~2 TSK and the process repeats.
Either dumping or loading results in "VALID PAYLOAD"
being stored as n INITIAL VALID PAYLOAD~, and n INITIAL
PAYLOAD" being set to the value of the total payload
after which control also transfers to the beginning of
WEIGH2 TSK. Both of these transfers place the most
recent accurate value of actual payload in both
"INITIAL VALID PAYI.OAD" and "INITIAL PAYLOAD" to ensure
that the next iteration of the software routine
compares the next computed total payload to the last
known accurate payload.
Referring now to the ability of the software
routine to detect roading of the truck, the actual
portion of the program designed to detect roading is
illustrated in ~ig. 3c. The routine begins where a
decision on the strut pressure stability of the vehicle
is made and proceeds in response to the vehicle strut
pressure beiny unstable. At this point, the variable
"VALID PAYLOAD" contains a value which is guaranteed to
be an actual payload rneasurement taken during a stable
pressure period of the loading operation. The control
does not ~know" if the value in "VALID PAYLOAD" is
representative of a completed loading cycle or is an
interim value taken between bucket loads. The software
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includes a means which stores the maynitude of the
actual current payload as a completed loading cycle in
response to the absence of the computed total payload
being recorded as an actual indication of current
payload for a second preselected duration of time. In
the preferred embodiment, the second duration of time
is selected to be approximately 30 seconds. Thus, the
routine monitors the total payload to determine if the
strut pressure is unstable for at least 30 seconds with
no stable pressure periods of at least 5 seconds.
Specifically, this is accomplished by a software
counter which increments toward a target value
representative of the 30 second interval. The routine
which determines 5 seconds of pressure stability is
running simultaneously and acts to reset the soEtware
counter each time a 5 second period of pressure
stability is detected. Consequently, the software
counter will only reach the target value if there are
no 5 second periods o~ pressure stability within a 30
second interval. An extended period of pressure
instability is assumed to indicate a roading condition,
for example, after the vehicle is loaded the operator
typically drives the truck to a dump site ~hich will
result in unstable pressure readings due to the shock
absorbing action of the struts 16,18 over the work
surface. The presence of roading is used to indicate
that the loading process is completed and the present
value of "VALID PAYLOAD" is indicative of the actual
payload removed from the loading site.
A check must be included to preclude the
possibility of multiple recordings of a single load.
For example, if the haul route includes multiple
extended stops and roading periods, then it is possible
that after each of the multiple roadings the program
would attempt to record the total payload as separate
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complete loading cycles. This possibility is negated
by use o~ the ~PAYLOAD CHAN~E ELAGn . If the flag has
not been set, then control transfers to the beginning
of WEIG~2 TSK and the load is not recorded. The flag
is set, as discussed earlier, in response to the
payload increasing by a tolerance roughly equivalent to
one bucket load of material and reset after the Eirst
roading operation of that particular loading cycle.
Subsequent roading operations will not result in the
payload being recorded unless the payload has increased
signiEicantly and set the "PAYLO~D C~ANGE FLAG". If a
roading operation is detected and the Elag is set, then
the ~INITIAL PAYLOAD~ and "INITIAL VALID PAYLOAD"
variables are updated, nLOAD~n, strut pressures, and
total payload are stored in EEPROM, and the "PAYLOAD
CHANGE FLAG~ ls cleared. This information can be
recalled at the end of a shift to provide an indication
of the quantity of material removed from the work site
and the productivity of a particular truck.
Industrial Applicability
In the overall operation o the apparatus 10,
assume that the off-highway truck 14 is positioned at a
loading site prepared to receive a series of bucket
loads of material from a loader vehicle. The payload
monitor has previously been calibrated and is currently
indicating that the load has not increased. As the
first bucket load of material is dumped into the dump
body 24 of the truck 14, the pressures in the front and
30 rear struts 16,18 begin to oscillate as the frame 22 is
displaced re~ative to the ground engaging portion 32.
Simultaneously, the microprocessor 72 receives the
signals from the pressure sensors 54,56,58,60 at 100
msec intervals and computes instantaneous payloads
based on the magnitudes of these signals. Each
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instantaneous payload is compared to the previous
payload computation and rejected as a valid payload
signal because it differs Erorn the previous payload by
more than the preselected tolerance. This process is
repeated until the damping effect of the struts 16~18
causes the pressure fluctuations to cease and the truck
14 stabilizes. After the truck 14 has been in the
stable pressure condition for at least 5 seconds, then
the computed total payload is stored as an actual
indication of truck payload. Further, bucket loads of
material result in the actual truck payload being
updated after each 5 second period of stability.
However, the payload monitor will not record the actual
truck payload as an indication of a completed loading
cycle until an extended period of strut pressure
instability is detected. For example, after the
loading cycle is completed, the operator of the truck
14 will normally transport the completed load to a dump
site. In the course of roading the vehicle, the strut
pressure signals will vary in magnitude relative to the
rugosity of the road surface. Therefore, the payload
monitor assumes that a 30 second period of unstable
pressure signals is indicative of roading the truck to
a dump site and thus the last valid payload must be a
complete load.
Other aspects, objectsl and advantages of this
invention can be obtained from a study oE the drawings,
the disclosure, and the appended claims.
