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CA 02686523 2009-11-27
File number: 11638-003
Revision: As filed
Date: November 27, 2009
Title of the Invention
[0001 ] Planar Beam Load Cell Assembly
Cross-Reference to Related Applications
[0002] There are no cross-related applications.
Field of the Invention
[0003] The present invention generally relates to weighing apparatus employing
load
cells, and more particularly to weighing apparatus resistant to off-axis
forces produced by
off-center loading and having means to prevent the damage of the load cell due
to an
overload.
Background of the Invention
[0004] It is well known to use load cells to sense loads. Some conventional
load cells
include a block (also known as a load beam) and one or more strain gages
mounted to the
block. Deflection of the block due to an applied load changes the shape of the
strain
gages resulting in a change in the resistance of the strain gages. Generally,
a known input
voltage is applied to the strain gages and an output signal from the strain
gages varies as
the resistance of the strain gages vary to provide a signal indicative of the
load applied to
the load cell. Some conventional load cells include other types of sensors,
such as optical
sensors and capacitive sensors, rather than strain gages, that measure the
size of gaps
between elements of a load cell system. It is desirable, of course, for load
cells to sense
applied loads with a high degree of accuracy and repeatability.
[0005] There is a need to provide a load cell assembly having means to protect
the load
cell from overload and from off-axis charges.
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CA 02686523 2009-11-27
File number: 1 1 63 8-003
Revision: As filed
Date: November 27, 2009
Summary of the Invention
[0006] The aforesaid and other objectives of the present invention are
realized by
generally providing a planar beam load cell assembly comprising a housing
comprising a
substantially horizontal base and at least two sides extending upwardly from
the base; a
planar beam load cell comprising a mounting hole, the planar beam load cell
being
connected to the housing; a load transmitting part having an upper extremity
and a lower
extremity, the load transmitting part extending through the mounting hole of
the planar
beam load cell and wherein there is a gap between the base and the lower
extremity of the
load transmitting part in an uncharged condition and wherein the lower
extremity of the
load transmitting part contacts the base in an overload condition; a load
receiving
element, the load receiving element being located at the upper extremity of
the load
transmitting part; a guide mounted on the load transmitting part, the guide
extending
between the load receiving element and the planar beam load cell; and a spring
assembly
mounted on the load transmitting part, the spring assembly extending between
the guide
and the planar beam load cell wherein the load receiving element transmits the
charge of
a weight to the planar beam load cell through the load transmitting part and
the guide.
[0007] In a preferred embodiment, the load transmitting part is a screw
connected
through the mounting hole to the planar beam load cell. The screw is
preferably installed
so that its head will contact the base in an overload condition.
[0008] The spring is preferably composed by a plurality of Belleville washers,
the spring
being pre-loaded to a load value exceeding the full load capacity of the
planar beam load
cell but less than the rated overload limit of the planar beam load cell.
[0009] In a preferred embodiment, the planar beam load cell assembly further
comprises
an overload plate located between the load receiving element and the guide,
wherein the
overload plate extends over the sides of the housing. There is a gap between
the sides and
the overload plate in an uncharged condition and the overload plate contacts
the sides in
an overload condition or off-axis overload. The overload plate may further
comprise an
extension preventing the overload plate to rotate horizontally.
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CA 02686523 2009-11-27
File number: 11638-003
Revision: As filed
Date: November 27, 2009
[0010] In a further preferred embodiment, the load receiving element is a
loading button,
preferably made from a resilient material such as rubber.
[0011] The load cell according to the present invention preferably includes a
beam or
plate having strain gages mounted on opposite lateral edges of the plate and a
slot or
opening in the plate extending between laterally opposite strain gages. The
load cell is
thus "planar" in form. It may be dimensioned in the lateral direction to
provide much
better resistance to torsional forces than known single beam load cells while
functioning
well as a load cell in other respects.
[0012] The preload of the spring assembly is chosen so as to provide correct
gap
(distance between the head of the screw and the base or between the overload
plate and
the sides of the housing) for the correct amount of overload protection.
[0013] The features of the present invention which are believed to be novel
are set forth
with particularity in the appended claims.
Brief Description of the Drawings
[0014] The above and other objects, features and advantages of the invention
will
become more readily apparent from the following description, reference being
made to
the accompanying drawings in which:
[0015] Figure 1 is an isometric view of an embodiment of the planar beam load
cell
assembly according to the present invention.
[0016] Figure 2 is a top view of the planar beam load cell assembly of FIG. 1.
[0017] Figure 3 is a side view of the planar beam load cell assembly of FIG.
1.
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CA 02686523 2009-11-27
File number: 11638-003
Revision: As filed
Date: November 27, 2009
[00 181 Figure 4 is an exploded view of the planar beam load cell assembly of
FIG. 1.
[0019] Figure 5 is a cross-section view of the planar beam load cell assembly
of FIG. I
along A-A.
[0020] Figure 6 is a cross-section view of the planar beam load cell assembly
of FIG. 1
along B-B.
Detailed Description of the Preferred Embodiment
[0021] A novel planar beam load cell assembly will be described hereinafter.
Although
the invention is described in terms of specific illustrative embodiment(s), it
is to be
understood that the embodiment(s) described herein are by way of example only
and that
the scope of the invention is not intended to be limited thereby.
[0022] The planar beam load cell assembly 100 is a device for general purpose
weighing
applications. The device senses the load or force applied with a high degree
of accuracy.
An arrangement of three or four of these devices can be used to support a
platform or any
suitable assembly onto which items can be placed, the purpose being to measure
the
weight of the items so placed.
[0023] As shown in Figs. 1 to 6, the planar beam load cell assembly 100
comprises a
housing 102 comprising a substantially horizontal base 105 and sides 110 and
115
extending substantially perpendicularly from the base 105. A planar beam load
cell 125 is
mounted by screws 150 on the housing 102. The charge of a weight is
transmitted to the
planar beam load cell 125 through a spring assembly 130 and guide part 135
which is
held in position by a load transmitting part 140, preferably a screw, inserted
from below
the load cell 125. The spring assembly 130 is pre-loaded to a load value
exceeding the
full load capacity of the planar beam load cell 125 but less than the rated
overload limit of
the planar beam load cell 125. Hence, as load 180 increases past the rated
load, the spring
130 will compress and the screw 140 will descend inside the guide 135 until
either the
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CA 02686523 2009-11-27
File number: 11638-003
Revision: As filed
Date: November 27, 2009
screw head 142 contacts the upper surface 107 of the base 105 or until an
overload plate
120 contacts both sides 110 and 115 of the housing 102, at which load the
downward
motion will stop. This prevents further load from reaching the planar beam
load cell 125
and saves it from potential damage as a result. The overload plate 120 is
prevented from
rotating, but is free to move vertically without affecting the accuracy of the
device, by
means of an extension or downturn 170 or pins (not shown) that can contact the
load
sensitive part of the planar beam load cell 125.
[0024] The spring 130 is preferably composed by a plurality of Belleville
springs also
known as cupped spring washer or Belleville washer. It has a slight conical
shape which
gives the washer a spring characteristic. The spring 130 is extending between
the planar
beam load cell 125 and the overload plate 120. It has to be understood however
that
another type of spring may be used with the planar beam load cell assembly
without
departing from the present invention.
[0025] Figs. 1 and 2 show one possible method for preventing rotation of the
horizontal
overload plate 120. The downturn 170, which is partially inserted through an
opening 172
of the planar beam load cell 125, prevents the overload plate to rotate
horizontally. Pins
placed appropriately may also be used to serve this function. The pins may be
inserted
into appropriate openings in the planar beam load cell 125 or of the housing
102.
[0026] The sensing element is a planar beam load cell 125, being manufactured
separately and used as part of the planar beam load cell assembly 100. The
planar beam
load cell 125 is mounted on the housing 102 which provides the rigid mounting
surface
necessary for high accuracy as well as protection from surrounding influences.
The
vertical spring assembly 130 is preferably (but not necessarily) made from a
stack of
several Schnorr disc springs or "Belleville Washers". The Belleville Washers
are aligned
by means of a guide 135 threaded through the spring assembly 130. The screw
140 is
inserted into the mounting hole 127 and into the guide 135. The guide 135 is
preferably a
shoulder washer made from nylon or other suitable material. From below the
planar beam
load cell 125, the screw 140 is inserted such that it extends beyond the top
of the guide
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CA 02686523 2009-11-27
File number: 11638-003
Revision: As filed
Date: November 27, 2009
135 and allows its insertion into a nut 145 or flat top plate (or loading
button) 147. The
loading button or element 147 is preferably assembled onto the top of the
spring
assembly 130 described above in such a way that the platform or weighing
assembly is
supported but some lateral motion is allowed, which is necessary for proper
operation and
accuracy of measurement. Thus, as weight is applied to the platform or weighed
assembly
attached to the loading button 145, the force is measured by the planar beam
load cell
125, creating an electrical signal that can be combined with the signals from
the other
planar beam load cell assembly supporting this same structure and used to
create a weight
display on a digital weight indicator or similar external device.
[0027] Fig. 5 shows the planar beam load cell assembly with no load or preload
applied
on it, the spring 130 is thus in an uncompressed state. There is a gap 160
between the
surface 107 of the base 105 and the head 142 of the screw 140.
[0028] Figs. 3 and 6 show the planar beam load cell assembly in an overload
condition.
When excessive pressure is applied through the loading button 147 to the top
of the screw
140, the spring assembly 130 is compressed, allowing the gap 160 between the
head 142
of the screw 140 and the upper surface 107 of the base 105 to close. The
excess force is
thus taken up by the base 105, which protects the planar beam load cell 125
from any
additional force. The downward movement of the screw 140 due to the overload
creates a
gap 162 between the planar beam load cell 125 and the head 142 of the screw
140. The
overload plate 120, if used, has dimensions that allow it to overlap the sides
110 and 115
of the housing 102 in such a way that when an off-axis force or off-axis
overload is
applied to the loading button (or load receiving element) 147, causing the
overload plate
120 to tilt to one side, the gaps between the overload plate 120 and the sides
110 and 115
of the housing 102 close, protecting the planar beam load cell 125 from
excessive off-axis
forces. This latter protection may be omitted to reduce cost, in which case
the screw 140
is inserted into a nut or simply into the loading button 147.
[0029] A planar beam load cell is a load sensing device fabricated from a flat
piece of
suitable metal with cut-outs inserted so as to form parallel beam structures.
When a force
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CA 02686523 2009-11-27
File number: 11638-003
Revision: As filed
Date: November 27, 2009
is applied to the appropriate point in the axis at right angles to the flat
metal structure,
stresses are created in the two parallel beams. Strain gauges bonded to these
beams form
a measurement circuit that senses the changes in stress and hence measure the
force
applied to the planar beam load cell.
[0030] The loading button 147 may be a commercially available rubber
"vibration
mount" with an appropriate female threaded side that allows its assembly onto
the screw
140 and an appropriate male or female thread that allows attachment of the
platform or
other device (not shown) to its upper surface. Alternatively it may be a
rubber appliance
foot with raised sides and a well in the center with a hole that allows the
screw 140 to
extend through for fixing. Various other loading buttons may be used for
different
applications and these in no way change the operation or general utility of
the device.
[0031] The housing is preferably made of aluminum (cast aluminum), and the
overload
plate is preferably made from steel. However, it is understood that the part
of the planar
beam load cell assembly can be made of other material such as steel, composite
or other
metal or metal alloys without departing from the present invention.
[0032] While illustrative and presently preferred embodiment(s) of the
invention have
been described in detail hereinabove, it is to be understood that the
inventive concepts
may be otherwise variously embodied and employed and that the appended claims
are
intended to be construed to include such variations except insofar as limited
by the prior
art.
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