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Design
Bonded strain-gauge load cells are devices producing an electrical output which
changes in magnitude when a force or weight is applied, and which may be displayed
on a readout instrument or used in a control device. The heart of the load cell
is the bonded-foil strain gauge which is an extremely sensitive device, whose electrical
resistance changes in direct proportion to the applied force.
A load cell comprises an elastic element, normally machined from a single billet
of high tensile steel alloy, precipitation hardening stainless steel, beryllium
copper or other suitable material, heat-treated to optimize thermal and mechanical
properties. The element may take many forms, such as hollow or solid column, cantilever,
diaphragm, shear member, or ring. The design of the element is dependent on the
load range, type of loading and operational requirements. The gauges are bonded
on to the element to measure the strains generated and are usually connected into
a four-arm Wheatstine bridge configuration. On larger elements, to get a rue average
of the strains, often 8,16 or even 32 gauges used. To illustrate the working principle,
a cantilever load cell is shown in Figure 1. Figure 2 shows a bridge circuit diagram
that includes compensation resistors for zero balance and changes of zero and sensitivity
with temperature. To achieve high performance abd stability and to minimize glue
line thickness, the gauges are often installed on flat-sided elements.
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Figure 1. Cantilever
load cell.
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The complete assembly is housed within a protective case with sealing sufficient
to exclude the external environment, but capable of allowing the deformation of
the element to occur when the force is applied. In some cases, restraining diaphragms
minimize the effect of side-loading.
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Figure 2. Load cell
bridge circuit with compensation resistors.
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After assembly, the elements are subjected to a long series of thermal and load
cycling to ensure that remaining "locked-up" stress in the element and bonding are
relieved, so that the units will give excellent long-term zero stability.
Selection and installation
There are five basic types of cell on the market: compression, tension, universal
(both compression and tension), bending and shear. The main factors influencing
the selection of cell type are:
(a) The ease and convenience (and hence the cost) of incorporating a cell into the
weigher structure.
(b) Whether the required rated load and accuracy can be obtained in the type of
cell.
Other considerations include low profile, overload capacity, resistance to side-loads,
environmental protection and a wide operating temperature range.
To retain its performance, a cell should be correctly installed into the weigher
structure. This means the structure of the weigher, such as vessel, bin, hopper
or platform, is the governing factor in the arrangement of the load cells. The supporting
structure is also to be considered since it will carry the full weight of the vessel
and contents, Difficulties caused by mis-application leading to poor performance
and unreliability fall into three main headings:
(a) A non-axial load is applied.
(b) Side-loads are affecting the weight reading.
(c) Free-axial movement of the load is restricted.
Figure 3 shows how normal, non-axial and side-loading affects a column stress member.
Under normal loading conditions (A) the active strain gauges go into equal compression;
however, under non-axial (B) or side-loading (C) conditions, asymmetrical compression
results, causing readout errors.
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Figure 3. Effects
of normal, non-axial and side-loading.
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Examples of correct and incorrect fitments are shown in Figure 4. The support bracket
D is cantilevered out too far and is liable to bend under load. The bracket is applying
a load to the side of the vessel, which itself exaggerates this effect as the vessel
is not strong enough to support it. The beam also deflects under load, rotating
the load cell away from the vertical. The correct example E shows how the errors
can be overcome.
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Figure 4. Examples
of correct and incorrect fitments.
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In weighing installations it is important the there is unimpeded vertical movement
of the weigh vessel. Obviously this is not possible where there are pipe fittings
ir stay rods on the vessel, but the vertical stiffness must be kept within allowable
limits. One of the most satisfactory ways of reducing the spring rates is to fit
flexible couplings in the pipework, preferably in a horizontal mode, and after (for
example) the discharge valve so that they are not subject to varying stiffness due
to varying pressure (see F and G in Figure 4.). Where possible, entry pipes should
be free of contact with the vessel (refer to H and I).
Applications
Load cells have many applications including weight and force measurement, weigh
platforms, process control systems, monorail weighing, beltweighers, aircraftm freight
and baggage weighing and conversion of a mechanical scale to an electronic scale.
Over the past few years, the industrial weighing field has been dominated by load
cells because electrical outputs are ideal for local and remote indication and to
interface with microprocessors and minicomputers.
Key features of load cells are:
(a) Load range 5 N to 40 MN.
(b) Accuracy 0.01 to 1.0 per cent.
(c) Rugged and compact construction.
(d) No moving parts and negligible deflection under load.
(e) Hermetically scaled and thermal compensation.
(f) High resistance to side-loads and withstand overloads.
Calibration
Calibration is a process that involves obtaining and recording the load cell output
while a direct known input is applied in a well-defined environment. The load cell
output is directly compared against a primary or secondary standard of force. A
primary standard of force includes dead-weight machines with force range up to about
500 kN; higher forces are achieved with machines having hydraulic or mechanical
amplification.
A secondary standard of force involves the use of high precision load cells and
proving rings with a calibration standard directly traceable to the National Standard
at the National Physical Laboratory in Teddington, Middlesex, or the equivalet standards
in other countries. The choice of the standards to be used for a particular calibration
depends on the range and the location of the device to be calibrated.
The foregoing has indicated some force-measurement methods. Others are many and
varied and no attempt has been made to cover all types. To simplify the selection
of a method for a particular application, the main parameters of the methods discussed
are summarized in Table shown below.
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Method |
Type of Loading |
Force Range, N |
Accuracy % |
Size |
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Lever balance |
Static |
0.001 to 150k |
Very high |
Bulky & heavy |
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Force-balance |
Static/dynamic |
0.1 to 1k |
Very high |
Bulky & heavy |
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Hydraulic load cell |
Static/dynamic |
5 k to 5 M |
0.25 to 1.0 |
Compact & stiff |
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Spring balance |
Static |
0.1 to 10 k |
Low |
Large & Heavy |
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Proving ring |
Static |
2 k to 2 M |
0.2 to 0.5 |
Compact |
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Piezo-electric transducer |
Dynamic |
5 k to 1 M |
0.5 to 1.5 |
Small |
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Strain-gauge load cell |
Static/dynamic |
5 to 40 M |
0.01 to 1.0 |
Compact & stiff |
Further developments
Advancing technology, improvements in manufacturing techniques and new materials
have permitted increased accuracy and improved design of bonded strain-gauge load
cells since their introduction about 30 years ago. Now the microprocessor is available,
and therefore further design improvements in these devices are expected.
New transducing techniques are being constantly researched; a number of them have
been well studied or are being considered, including gyroscopic force transducers,
fiber optics, microwave cavity resonator, and thin-film transducing techniques.
The thin-film techniques are well documented and therefore are briefly discussed.
Pressure transducers based on vacuum-deposited thin-film gauges are commercially
available and attempts are being made to apply these techniques to load cells. The
advantages of these techniques are as follows:
(a) Very small gauge and high bridge resistance.
(b) Intimate contact between the element and gauge. No hysteresis or creep of a
glue line.
(c) Wide temperature range (- 200°C to + 200°C).
(d) Excellent long-term stability of the bridge.
(e) Suitability for mass production.
The techniques are capital-intensive and are generally suitable for low force ranges.
References
Adams, L. F. Engineering Measurements and Instrumentation, The
English Universities Press, (1975).
Cerni, R. H. and Foster, L. E. Instrumentation for Engineering Measurement, John
Wiley and Sons, (1962)
Mansfield, P. H. Electrical Transducers for Industrial Measurement, Butterworth,
(1973)
Neubert, H. K. P. Instrument Transducers, Clarendon Press, (2nd edition, 1975)
WEIGHTECH 79, Proceedings of the Conference on Weighing and Force Measurement; Hotel
Metropole, Brighton, England 24-26 September 1975
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