A load cell is a transducer that is used to create an electrical signal whose magnitude
is directly proportional to the force being measured.
The various types of load cells include hydraulic load cells, pneumatic load cells and strain
gauge load cells.
Strain gauge load cell Through a mechanical arrangement, the force
being sensed deforms a strain gauge.
The strain gauge measures the deformation as a change in electrical resistance, which
is a measure of the strain and hence the applied forces.
A load cell usually consists of four strain gauges in a Wheatstone bridge configuration.
Load cells of one strain gauge or two strain gauges are also available.
The electrical signal output is typically in the order of a few millivolts and requires
amplification by an instrumentation amplifier before it can be used.
The output of the transducer can be scaled to calculate the force applied to the transducer.
Strain gauge load cells are the most common in industry.
These load cells are particularly stiff, have very good resonance values, and tend to have
long life cycles in application.
Strain gauge load cells work on the principle that the strain gauge deformscontracts when
the material of the load cells deforms appropriately.
These values are extremely small and are relational to the stress and/or strain that the material
load cell is undergoing at the time.
The change in resistance of the strain gauge provides an electrical value change that is
calibrated to the load placed on the load cell.
Strain gauge load cells convert the load acting on them into electrical signals.
The gauges themselves are bonded onto a beam or structural member that deforms when weight
is applied.
In most cases, four strain gauges are used to obtain maximum sensitivity and temperature
compensation.
Two of the gauges are usually in tension, and two in compression, and are wired with
compensation adjustments When weight is applied, the strain changes the electrical resistance
of the gauges in proportion to the load.
Other load cells are fading into obscurity, as strain gauge load cells continue to increase
their accuracy and lower their unit costs.
Piezoelectric load cell Piezoelectric load cells work on the same
principle of deformation as the strain gauge load cells, but a voltage output is generated
by the basic piezoelectric material - proportional to the deformation of load cell.
Useful for dynamic/frequent measurements of force.
Most applications for piezo-based load cells are in the dynamic loading conditions, where
strain gauge load cells can fail with high dynamic loading cycles.
Hydraulic load cell The cell uses conventional piston and cylinder
arrangement.
The piston is placed in a thin elastic diaphragm.
The piston doesn't actually come in contact with the load cell.
Mechanical stops are placed to prevent over strain of the diaphragm when the loads exceed
certain limit.
The load cell is completely filled with oil.
When the load is applied on the piston, the movement of the piston and the diaphragm arrangement
in an increase of oil pressure which in turn produces a change in the pressure on a Bourdon
tube connected with the load cells.
Because this sensor has no electrical components, it is ideal for use in hazardous areas.
Typical hydraulic load cell applications include t resulttank, bin and hopper weighing.
By example, a hydraulic load cell is immune to transient voltages so these type of load
cells might be a more effective device in outdoor environments.
Pneumatic load cell The load cell is designed to automatically
regulate the balancing pressure.
Air pressure is applied to one end of the diaphragm and it escapes through the nozzle
placed at the bottom of the load cell.
A pressure gauge is attached with the load cell to measure the pressure inside the cell.
The deflection of the diaphragm affects the airflow through the nozzle as well as the
pressure inside the chamber.
Other types Other types include vibrating wire load cells,
which are useful in geomechanical applications due to low amounts of drift, and capacitive
load cells where the capacitance of a capacitor changes as the load presses the two plates
of a capacitor closer together.
Ringing Every load cell is subject to "ringing" when
subjected to abrupt load changes.
This stems from the spring-like behavior of load cells.
In order to measure the loads, they have to deform.
As such, a load cell of finite stiffness must have spring-like behavior, exhibiting vibrations
at its natural frequency.
An oscillating data pattern can be the result of ringing.
Ringing can be suppressed in a limited fashion by passive means.
Alternatively, a control system can use an actuator to actively damp out the ringing
of a load cell.
This method offers better performance at a cost of significant increase in complexity.
Uses Load cells are used in several types of measuring
instruments such as universal testing machines.
From 1993 the British Antarctic Survey installed load cells in glass fibre nests to weigh albatross
chicks.
See also Loadpin
References
Standards ASTM E4 - Practices for Force Verification
of Testing Machines ASTM E74 - Practice for Calibration of Force
Measuring Instruments for Verifying the Force Indication of Testing Machines