The present invention relates to weight measurement. In particular,
the present invention concerns apparatus for determining the weight or force exerted
by an object.
Conventional weighing apparatus use either linear distortion of a
material under stress (eg a spring scale in which distortion of the spring is proportional
to force applied) or distortion of a lump of material measured with a strain gauge
as in the case of a 'load cell'. A disadvantage of these methods is that fatigue
of the spring or of the 'load cell' leads to inaccuracies which increase - with
time. Thermal expansion of materials further reduces accuracy which cannot be
predicted unless a temperature sensing device is used. In very sensitive and accurate
apparatus, changes in the earths magnetic field and/or atmospheric pressure are
additional sources of error.
It is an object of the present invention to alleviate the abovementioned
disadvantages of prior art weighing apparatus. A further object is to provide weighing
apparatus which may significantly reduce errors due to fatigue, changes in atmospheric
pressure, earth's magnetic field and temperature.
According to the present invention there is provided apparatus for
measuring weight of an object comprising:
a weighing platform containing a flowable substance and adapted to receive said
object such that said substance is subject to a pressure related to the weight
of said object;
sensing means coupled to said substance and adapted to provide a signal indicative
of said pressure;
processing means for receiving said signal and adapted to provide an output related
to the weight of said object; and
display means for displaying said output.
The weighing platform preferably contains a substance which is substantially
non-compressible. The substance preferably also is isotropic. The substance may
be subject to pressure which varies according to the force or weight applied to
the substance. The substance may be contained in a non-extensible container. The
container may be flexible. The container may comprise plastics or other material.
In one form, the container may comprise a substantially thin (bath mat thickness)
bag. The bag may be substantially rectangular. The bag or container may be filled
with a substance such as a fluid. The fluid may comprise water.
An object to be weighed or force to be measured may be applied to
the substance in any convenient manner. The bag or container may include a substantially
rigid surface portion. The rigid surface portion may be formed integrally with
the bag or container or it may be formed separately and affixed thereto. The substantially
rigid portion may comprise a weighing plate of known surface area. The plate preferably
is substantially rectangular but may be round or any other convenient shape. The
weighing plate preferably is of sufficient size and shape to receive the whole
of the object to be weighed thereon.
The pressure exerted on the flowable substance may be determined
by reference to the weight or force exerted by the object and the area of the substantially
rigid surface portion. The output pressure P is given by the following equation:
(1) P = + constant
where m is the weight of the object, g is acceleration due to gravity and A is
the surface area of the substantially rigid surface portion or weighing plate.
It may be seen from equation (1) that specific pressure may be obtained for any
weight or force simply by altering the area of the weighing plate.
The pressure sensing means preferably comprises a pressure transducer
or other pressure sensitive device. The pressure sensing means may be coupled to
the flowable substance in any convenient manner. The pressure transducer may comprise
a device whose output resistance varies according to the pressure applied to its
sensing element. In one form, the sensing means may comprise a strain gauge. The
strain gauge may comprise a semi-conductor device utilizing the piezoresistive
effect. Alternatively, the pressure transducer may comprise a device whose output
capacitance changes as pressure is applied.
The processing means may be provided in any suitable manner. Preferably,
the processing means includes a suitably programmed microprocessor or microcomputer.
The processing means may be adapted to calculate the weight or force exerted by
The processing means may include means for zeroing the apparatus
before making a reading. Zeroing may be performed during an initial scan by the
The signal from the pressure sensing means may be input directly
to the processing means or it may be transmitted via infra red, radio, extension
leads or other transmission means.
The processing means may include storage means for storing one or
more previous weight readings. The storage means may comprise a digital memory.
In one form, the storage means may comprise a random access memory. The storage
means preferably is non-volatile.
The processing means may be programmed to operate in one or several
modes. Modes of operation may be selectable via a keyboard interfaced with said
There now follows a discussion of the various modes which may be
implemented in an embodiment of this invention.
By utilizing the relationship of equation (1) the processing means
may be adapted to calculate the weight (m) of an object placed on the weighing
platform. The weight may be displayed in kilograms or pounds. The weight may be
displayed as soon as a load is placed on the weighing platform. In one form the
processing means may be adapted to display weight for 20 seconds after the object
is removed from the weighing platform. The processing means may then revert to
an idle mode.
The idle mode may be entered by the processing means upon power up
i.e. upon connection to a power source. The power source may comprise alkaline
or rechargable batteries. When idle mode is entered the display means may be adapted
to display a figure "8" in all positions for a few seconds. The display may then
be blanked until a (non-numeric) key on the keyboard is depressed or a load is
placed on the weighing platform. If a numeric key is pressed the processing means
may be adapted to display a figure "8" in all positions. This may be used as a
test function. Because the idle mode is a no load condition it may be used to perform
automatic zeroing taking into consideration variations in earth's magnetic field,
atmospheric pressure, gravity and temperature.
An additive mode may be selectable via the keyboard. An appropriate
sign may be displayed on the display means. A first weight may be placed on the
weighing platform and displayed (in kg or lbs) as selected. The first weight may
then be removed from the platform to initiate additive mode. The first weight
may be retained in memory and display may remain unaltered.
A second weight may then be placed on the weighing platform. (The
processing means may be arranged to go into idle mode if this is not done within
say 20 seconds). The display means may now show the sum of the first and second
weights. The processing means may be arranged to return to idle mode if after
key depression no load is put on the weighing platform within say 20 seconds. The
additive mode may be used for weighing luggage etc.
A comparative mode may be selectable via the keyboard. The comparative
mode preferably is selectable by pressing a non-numerical key together with a numerical
key eg. a number from 0 to 9 i.e. a personal code. The number may be used to access
a memory used to store a weight result obtained sometime in the past.
A weight may then be put on the weighing platform. (The processing
means may be arranged to go into idle mode if this is not done within 20 seconds).
The weight may be displayed (kg or lbs) in the following manner. If the (new)
weight is less or equal to the (old) weight stored in memory the processing means
may be adapted to show the new weight for a period of 5 seconds and then the old
weight for 5 seconds. This may be repeated as long as the new weight remains on
the weighing platform. If the new weight is more than the old weight stored in
memory the processing means may be adapted to show the new weight for say 0.5 seconds
and then blank the display for 0.5 seconds. This may be repeated for about 5 seconds.
The old weight may then be displayed continuously for 5 seconds. This sequence
may be repeated as long as the (new) weight remains on the weighing platform.
This mode may be selectable to provide in effect a substractive weighing
function. The mode may be used when say a baby is to be weighed. The mother may
step onto the weighing platform and her weight may be displayed. After say 5 seconds
the display may be zeroed with the mother still on the platform. The baby may then
be handed to the mother and only the baby's weight is then displayed.
The accumulative mode, may include a memory for storing the baby's
weight for future reference. The baby's old weight may be recalled for display.
In one form the new weight may be displayed alternately with the
old weight eg. 5 seconds each for say 20 seconds. Preferably the new weight is
shown in a flashing mode and the old weight in a steady state mode. The new weight
may be then be stored in memory, replacing the old weight. The processing means
may then revert to the idle mode.
The apparatus may include temperature sensing means for sensing ambient
temperature and/or temperature of the substance in the weighing platform. The temperature
sensing means may be adapted to compensate for variations in pressure due to ambient
temperature changes. The temperature sensing means may be connected to the processing
means via a suitable interface. The temperature sensing means may be provided in
any suitable manner eg. thermister, thermocouple, semiconductor junction, etc.
The display means may be adapted to display the weight or force exerted
by the object being weighed. The display means may be provided in any suitable
manner. In one form the display means may comprise a digital display such as liquid
crystal or LED. The apparatus may be adapted to display the weight or force in
any convenient units, for example, kilograms, pounds, etc. The apparatus may include
means for converting the weight or force from one unit to another.
The display means may be physically located on the apparatus or it
may be remote therefrom. For example, if the weighing apparatus of the present
invention is applied to a weighing scale, the display means may in one form be
located substantially at eye level to enable easy reading of the scale.
The apparatus of the present invention may be applied to a wide variety
of loads. Due to its construction the present apparatus may be produced with a
relatively low profile not readily achievable with conventional apparatus. Manufacturing
costs also may be reduced because bulky mechanical devices and load calls may be
eliminated. Apparatus according to the present invention also may be less susceptible
to damage due to rough handling when compared to existing designs. Application
of intelligent electronics may eliminate zeroing of the apparatus. Long term calibration
stability of the apparatus may be exceptional.
By way of example only, an embodiment of the present invention will
now be described with reference to the accompanying drawings wherein:
- Figure 1 shows a block diagram of a weighing apparatus according to the present
- Figure 2 shows a block diagram of one preferred form of weighing apparatus
according to the present invention.
The weighing apparatus of Figure 1 comprises a weighing platform
10 operatively connectable to pressure sensing means 11. The output of pressure
sensing means 11 is applied to processing means 12 via suitable interface means
13. The output of processing means 12 is provided to display means 14 via suitable
interface means 15. Storage means 16 is connectable to processing means 12 for
The preferred embodiment of the apparatus shown in Figure 2 includes
a weighing platform comprising a fluid filled container 20 and a substantially
rigid weighing plate 21. The weighing plate 21 is rectangular with area A.
A pressure transducer 22 is attached to the fluid filled container
20 such that the sensing element of the transducer is in communication with the
substance (fluid) in the container 20.
The output of pressure transducer 22 is connected to microprocessor
23 via interface circuit 24. Interface circuit 24 includes an operational amplifier
and a wheatstone bridge in which the transducer 22 forms one arm of the bridge.
The interface circuit 24 includes an analogue to digital converter. The output
of the operational amplifier is connected to the analogue to digital converter.
The analogue to digital converter alternatively may be implemented in microprocessor
Microprocessor 23 calculates the weight or force exerted by an object
m on weighing plate (area A) 21 by utilizing the relationship of equation (1).
Microprocessor 23 additionally may be adapted to linearise any non-linearity present
in weighing platform 20 or in pressure sensing means 22.
A keyboard 25 having numeric and non-numeric keys is connectable
to microprocessor 23 for controlling the measuring apparatus eg. mode selection,
switch on/switch off etc.
Memory 26 is connectable to microprocessor 23 for storing one or
more weight or force readings for subsequent recall. The output of microprocessor
23 is connectable to a digital display 28 via a suitable interface circuit 27.
Digital display 28 may comprise liquid crystal or LED segment displays. Interface
circuit 27 includes suitable driver/decoder circuits.
the electronics of the present apparatus may be provided in substantially
integrated form to minimise unit costs.
It will be appreciated that various alterations modifications and/or
additions may be introduced into the constructions and arrangements of parts previously
described without departing from the spirit or ambit of the present invention.