U.S. patent application number 12/665026 was filed with the patent office on 2010-07-22 for load monitoring system.
Invention is credited to Frank Roger Bowden.
Application Number | 20100181143 12/665026 |
Document ID | / |
Family ID | 38352600 |
Filed Date | 2010-07-22 |
United States Patent
Application |
20100181143 |
Kind Code |
A1 |
Bowden; Frank Roger |
July 22, 2010 |
LOAD MONITORING SYSTEM
Abstract
A load monitoring system for a mobile work platform includes a
load cell (12) having at least one strain gauge (16) and a sensing
circuit (14) connected to receive a strain signal from the strain
gauge. The sensing circuit (14) includes a first sensing device
(24) for sensing the strain in the strain gauge (16), and a second
sensing device (26) for sensing a negative shift in the strain
signal.
Inventors: |
Bowden; Frank Roger;
(Totternhoe, GB) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET, FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
38352600 |
Appl. No.: |
12/665026 |
Filed: |
June 20, 2008 |
PCT Filed: |
June 20, 2008 |
PCT NO: |
PCT/GB08/02110 |
371 Date: |
January 21, 2010 |
Current U.S.
Class: |
182/112 ;
177/25.11; 73/862.627 |
Current CPC
Class: |
B66F 17/006 20130101;
B66F 11/046 20130101; G01G 3/145 20130101 |
Class at
Publication: |
182/112 ;
73/862.627; 177/25.11 |
International
Class: |
B66F 11/04 20060101
B66F011/04; B66F 17/00 20060101 B66F017/00; G01G 19/08 20060101
G01G019/08; G01L 1/22 20060101 G01L001/22 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 21, 2007 |
GB |
0711960.5 |
Claims
1. A load monitoring system for a mobile work platform, the system
including a load cell having at least one strain gauge and a
sensing circuit connected to receive a strain signal from the
strain gauge, the sensing circuit including a first sensing device
for sensing the load on the load cell and a second sensing device
for sensing a negative shift in the unloaded strain signal.
2. A load monitoring system according to claim 1, wherein the
strain gauge is connected in a bridge circuit configuration.
3. A load monitoring system according to claim 1, wherein the
strain gauge is connected to an amplifier that amplifies the strain
signal.
4. A load monitoring system according to claim 1, wherein the first
sensing device is a first comparator.
5. A load monitoring system according to claim 4, wherein the first
comparator has a first input terminal connected to receive a load
signal that represents the strain in the strain gauge, a second
input terminal connected to receive a load reference signal, and an
output terminal for an overload signal that is generated when the
load signal exceeds the load reference signal.
6. A load monitoring system according to claim 5, wherein the
output signal of the first comparator is connected to a safety
device that is activated by said overload signal.
7. A load monitoring system according to claim 1, wherein the
second sensing device is a second comparator.
8. A load monitoring system according to claim 7, wherein the
second comparator has a first input terminal connected to receive a
load signal that represents the strain in the strain gauge, a
second input terminal connected to receive a second reference
signal, and an output terminal for a negative shift signal that is
generated when the load signal is less than the second reference
signal.
9. A load monitoring system according to claim 8, wherein the
output signal of the second comparator is connected to a safety
device that is activated by said negative shift signal.
10. A load monitoring system according to claim 1, wherein the
system includes at least two strain gauges, each of which is
connected to a respective load sensing circuit, wherein the load
sensing circuits operate in parallel.
11. (canceled)
12. A mobile work platform that includes an operator platform, a
base and a lift mechanism for lifting the operator platform
relative to the base, and a load monitoring system according to
claim 1 for monitoring the load on the operator platform.
13. A mobile work platform according to claim 12, wherein the load
cell of the load monitoring system is connected between the lift
mechanism and the operator platform.
14. (canceled)
Description
[0001] The present invention relates to a load monitoring system
for a mobile work platform and to a mobile work platform having a
load monitoring system.
[0002] Mobile work platforms typically include a cage or platform
that is designed to receive one or more human operators. The cage
is mounted on a lift mechanism, such as a hydraulic boom or a
scissor lift mechanism, that allows its height to be adjusted. The
mobile work platform also includes a wheeled or tracked chassis,
which allows it to be moved easily to a desired location. Various
types of mobile work platform are available, including
self-propelled, self-drive, trailer and vehicle-mounted
platforms.
[0003] A load monitoring system is generally included to provide
cage overload protection. Cage overload protection is required on
mobile elevating work platforms as defined by European standard
EN280:2001 Amendment 1. European standard EN954 is cited by EN 280
and describes the requirements for safety switches in electrical
systems providing overload protection. EN954 Category 3 requires
that common mode faults resulting from damage to the protection
system are taken into account, if the probability of such a fault
occurring is significant.
[0004] One such fault may occur if the mechanical structure of the
load protection system is damaged, causing the zero position of the
load cell to shift in an unsafe direction. The resulting load
measurement may then be inaccurate. There is currently no
requirement for on-going calibration of load monitoring systems. It
is thus possible for damage to the load monitoring system or the
structure on which it is mounted to go undetected. This may allow
the cage to be overloaded without triggering the protection system,
causing the work platform to become unstable.
[0005] It is an object of the present invention to provide a mobile
work platform and a load monitoring system for a mobile work
platform that mitigate at least some of the aforesaid
disadvantages. In particular, but not exclusively, the invention
seeks to provide a detector within the load monitoring system for
detecting any unsafe damage to the load monitoring system that may
result in an unstable work platform.
[0006] According to one aspect of the present invention there is
provided a load monitoring system for a mobile work platform, the
system including a load cell having at least one strain gauge and a
sensing circuit connected to receive a strain signal from the
strain gauge, the sensing circuit including a first sensing device
for sensing the load on the load cell and a second sensing device
for sensing a negative shift in the unloaded strain signal.
[0007] The unloaded strain signal is the strain signal provided by
the strain gauge when the cage of the mobile work platform is
empty. A shift in the unloaded strain signal may occur when, for
example, the load cell is damaged causing the zero position of the
load cell to shift. A negative shift in the unloaded strain signal
causes the load cell to sense less than the full weight of the
cage, with the result that when the cage is empty the strain gauge
indicates a negative load value.
[0008] The provision of a second sensing device that senses a
negative shift in the unloaded strain signal ensures that any
damage to the load cell that would cause it to detect less than the
actual load in the cage is automatically detected. This prevents
the load monitoring system from giving a false reading and ensures
that the cage is not inadvertently overloaded. The safety of the
mobile work platform is thus improved and the risk of accidents is
reduced.
[0009] The strain gauge is preferably connected in a bridge circuit
configuration, which preferably provides compensation for
temperature variations.
[0010] Advantageously, the strain gauge is connected to an
amplifier that is constructed and arranged to amplify the strain
signal.
[0011] The first sensing device may be a first comparator.
Advantageously, the first comparator has a first input terminal
connected to receive a load signal that represents the strain in
the strain gauge, a second input terminal connected to receive a
load reference signal, and an output terminal for an overload
signal that is generated when the load signal exceeds the load
reference signal. The load reference signal may be adjustable,
allowing the required load limit to be selected.
[0012] Advantageously, the output signal of the first comparator is
connected to a safety device that is constructed and arranged for
activation by said overload signal, thereby informing the operator
of a potentially hazardous overload situation and allowing remedial
action to be taken. The safety device may for example be an alarm
and/or an automatic cut-out device.
[0013] The second sensing device may be a second comparator.
Advantageously, the second comparator has a first input terminal
connected to receive a load signal that represents the strain in
the strain gauge, a second input terminal connected to receive a
second reference signal, and an output terminal for a negative
shift signal that is generated when the load signal is less than
the second reference signal. The second reference signal may be
adjustable, allowing an acceptable limit of negative shift in the
unloaded strain signal to be selected.
[0014] Advantageously, the output signal of the second comparator
is connected to a safety device that is constructed and arranged
for activation by said negative shift signal. Again, this informs
the operator of the potentially hazardous situation, allowing
suitable remedial action such as repairing or replacing the damaged
load cell to be taken. The safety device may for example be an
alarm and/or an automatic cut-out device.
[0015] The system preferably includes at least two strain gauges,
each being connected to a respective load sensing circuit, wherein
the load sensing circuits are constructed and arranged to operate
in parallel. This ensures that the monitoring system operates
correctly even if one of the circuits is faulty, thus increasing
safety.
[0016] According to a further aspect of the invention there is
provided a mobile work platform that includes an operator platform,
a base and a lift mechanism for lifting the operator platform
relative to the base, and a load monitoring system according to any
one of the preceding statements of invention for monitoring the
load on the operator platform.
[0017] Advantageously, the load cell of the load monitoring system
is connected between the lift mechanism and the operator
platform.
[0018] An embodiment of the invention will now be described by way
of example, with reference to the accompanying drawings, in
which:
[0019] FIG. 1 is a front elevation of a mobile work platform
according to an embodiment of the invention, in various operating
configurations;
[0020] FIGS. 2a and 2b are schematic side views of a mobile work
platform load, illustrating in FIG. 2a a normal operating condition
and in FIG. 2b a possible failure condition; and
[0021] FIG. 3 is a circuit diagram of a load monitoring system
according to an embodiment of the invention.
[0022] FIG. 1 shows a typical mobile work platform according to an
embodiment of the invention, which includes a wheeled base unit 2,
a hydraulically operated lift mechanism comprising a boom 4 and a
lifting structure 5, and a platform (or cage) 6 for a human
operator 8. The boom 4, which is shown here in various operating
configurations, may be retracted and folded onto the base unit 2
for transportation or storage. Movement of the boom 4 is controlled
by various hydraulic cylinders 10, which are connected by hydraulic
hoses (not shown) to a hydraulic drive system. Hydraulic motors may
also be provided for driving the wheels. The components shown in
FIG. 1 are all conventional and will not therefore be described in
detail. It should be understood that the mobile work platform may
take various alternative forms.
[0023] As shown schematically in FIGS. 2a and 2b, the cage 6 is
connected to the boom 4 via a load cell 12. The load cell 12 is
conventional in design and comprises an aluminium block that acts
as a cantilever spring and carries one or more strain gauges,
preferably of the resistive foil type. The strain gauges are
connected to a load monitoring system 14, for example as
illustrated in FIG. 3.
[0024] During normal use as shown in FIG. 2a, the load cell 12
supports a downwards load mg equal to the mass m of the cage and
its contents multiplied by gravity g. It therefore experiences a
positive moment M+ (shown as clockwise in the drawing). The strain
in the load cell produced by this moment is measured by the strain
gauges and used to calculate the load in the cage. If the load
exceeds a predetermined limit, the load monitoring system 14 sounds
an alarm to warn the user and/or activates a cut-out device to
prevent further operation of the platform.
[0025] Under certain circumstances, it is possible for the cage to
experience a force that results in a negative moment being applied
to the load cell 12. For example, as illustrated in FIG. 2b, if the
mobile work platform is accidentally pushed or driven backwards
into an obstruction, a force F may be applied to the cage 6,
producing a negative moment M- on the load cell 12. If this
negative moment exceeds the elastic limit of the load cell it may
be permanently deformed. This may affect the zero condition of the
strain gauges, such that they read zero only when the cage 6 is
partially loaded. As a result, it may be possible to overload the
cage without activating the safety system, creating a potentially
dangerous situation.
[0026] For example, if the strain gauge experiences a shift in the
zero position that causes it to indicate a load of -X kg when the
cage is unloaded, then the cage overload alarm will not be
activated until the cage carries a payload of (X+Y) kg, where Y kg
is the normal safe working limit. By design, the machine may
sustain a cage load of S. Y kg, where S is a design safety factor.
The machine may therefore become unstable if (X+Y)>S. Y.
[0027] In the present invention, the load monitoring system 14 is
designed to detect a negative strain signal resulting from a
negative shift in the zero position of the strain gauge (that is, a
shift that causes the strain gauge to indicate less than the true
load carried in the cage). Such a shift may result for example from
an excessive negative moment being applied to the system. Upon
detecting a negative strain, the system is designed to activate an
alarm system to warn the operator of the mobile work platform that
there is a fault in the load detection system. A positive shift in
the zero position (which causes the strain gauges to indicate more
than the true cage load) is not detected, since this does not
adversely affect the safety of the work platform.
[0028] The monitoring circuit of the load monitoring system 14 is
illustrated in FIG. 3. In accordance with European safety
legislation, the circuit has two channels 14,14', which are
connected to separate strain gauges 16,16' and operate in parallel
to ensure that failure of one channel or one strain gauge does not
compromise the safety of the platform. Only one channel (channel 2)
will be described in detail.
[0029] The other channel (channel 1) may be similar to channel 2,
although compliance with legislation only requires that it detects
an overload condition. It is not essential that channel 1 also
detects a negative strain signal. Optionally, channel 1 may also be
arranged to detect a pre-overload condition and provide an advisory
signal when, for example, the load reaches 90% of the maximum safe
working load. Such an arrangement is conventional and so will not
be described in further detail.
[0030] The strain gauge 16 has a conventional full bridge
configuration with four active legs. A constant voltage supply
17,17' is connected to a pair of input connections 18,18' at
opposite corners of the bridge, while a pair of output connections
20,20' at the other corners of the bridge are connected to the
inputs of an instrumentation amplifier 22, which detects the
voltage across the output connections of the bridge. This bridge
output voltage is determined by the strain in the load cell 12.
[0031] The output of the amplifier 22 is connected to the positive
input terminal of a first comparator 24 and the negative input
terminal of a second comparator 26. The negative input terminal of
the first comparator 24 is connected to a first potentiometer 28,
which is connected in series with a fixed resistor 29. The positive
input terminal of the second comparator 26 is connected to a second
potentiometer 30, which is connected in series with a second fixed
resistor 31. The potential dividers created by the first
potentiometer 28 and the first resistor 29, and by the second
potentiometer 30 and the second resistor 31, are connected in
parallel across the input terminals 18,18' of the strain gauge 16
and thus measure the voltage across the strain gauge bridge. Thus
the measuring system is ratiometric and has first order
independence from supply voltage variations. The first
potentiometer 28 may be adjusted to control the input voltage at
the negative input terminal of the first comparator 24, which
represents the cage overload reference value. The second
potentiometer 30 may be adjusted to control the input voltage at
the positive input terminal of the second comparator 26, which
represents the zero shift reference value. The output of the first
comparator 24 is connected to an overload alarm device 32 and the
output of the second comparator 26 is connected to zero shift alarm
34.
[0032] To calibrate the circuit for use, the second potentiometer
30 is adjusted while the cage is empty to set the zero shift
reference. A known weight equal to the safe working limit of the
platform is then placed in the cage 6 and the first potentiometer
28 is adjusted to set the cage overload reference.
[0033] During operation, the first comparator 24 compares the
output voltage of the instrumentation amplifier 22 with the
overload reference voltage provided by the first potentiometer 28.
As the strain on the load cell 12 increases, the output voltage of
the instrumentation amplifier 22 also increases. Providing that the
amplifier output voltage is less than the reference voltage, the
overload alarm 32 will remain silent. However, if the amplifier
output voltage exceeds the reference voltage, the output state of
the comparator 24 will change, triggering the overload alarm 32 and
indicating that the cage is overloaded.
[0034] The second comparator 26 compares the output voltage of the
instrumentation amplifier 22 with the zero shift reference voltage
provided by the second potentiometer 30. Normally, under all
positive load conditions, the amplifier output voltage is greater
than the zero shift reference voltage and the zero shift alarm 34
remains silent. However, if the load cell 12 has suffered damage
from a negative load causing deformation of the load cell, the
amplifier output voltage for an unloaded cage will reduce to below
the zero shift reference voltage provided by the second
potentiometer 30. Should this occur, the output state of the second
comparator 26 will change, triggering the zero shift alarm 34 and
indicating that the load cell is operating incorrectly.
[0035] If either the cage overload limit (the supposed positive
voltage) or the zero shift monitoring voltage (the supposed
negative-going voltage) are outside the acceptable ranges, one of
the comparators will change state, so generating an alarm. The
monitoring system is able to detect both overloading of the cage
and a situation in which, for example, an excessive negative load
has been applied to the cage, causing deformation of the load cell
and a negative shift in the zero position of the load monitoring
system. This results in a more stable elevating work platform and
improved safety. The system is not however designed to prevent an
incorrect cage load measurement.
[0036] Various modifications of the load detection system are of
course possible. For example, instead of providing an alarm, the
system may be designed to actuate a cut-off device preventing
operation of the mobile work platform, or a device that limits the
boom operating speed to half of its normal value or that restricts
its direction of travel. Alternatively, both an alarm and a cut-off
device may be provided, possibly with different trigger points so
that the alarm is activated at a lower level than the cut-off
device. The detection circuit may also be constructed and arranged
to use digital electronic devices as well as or instead of analogue
circuit components.
* * * * *