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About Load Cells
Load cells are measuring devices that monitor and
gauge forces of compression, tension and shear. Known by other names—such
as load control, force sensor and transducer—these load cells are
used widely in mechanical testing, ongoing system monitoring and as components
in devices such as industrial
scales. The measuring of levelness and
tension of these devices is the main purpose of load cells. The information
that load cells monitor is then signaled to a recorder or other computerized
data collection system. Load cells can utilize analog or digital technology
for the recording and transferring of information. When load cells are
used to measure any variance in certain ongoing systems, the load cells
can sound an alarm or shut down the system itself until the discrepancy
is corrected.
Load cells can vary greatly in size and shape depending on type of use.
The two basic components of a load cell are the sensing element and circuit.
The sensing element is most often a strain gauge, which is comprised
of coil; the circuit is the connection of these gauges throughout the
load cell. Load cell outputs include analog voltage, analog current,
analog frequency, switch or alarm, serial and parallel. The most basic
designs consist of four gauges, which make up the measuring circuit.
More complex and detailed cells can have up to thirty gauges as part
of the measuring circuit. The more gauges inside the load cell, the more
sensitive the cell is in recording and monitoring variance in measurement.
Many types of industries benefit from the accuracy of load cells. For
example, they are used in warehouse environments where pallets of inventory
are shuffled around often, and the accurate weight of the pallet is crucial
for the filling and accepting of orders. Another example of the use of
load cells is in the testing of bridge building materials such as beams
for their tension strength. Load cells are essential components in many
calibration
systems, as well. Load cells are commonly used in series
with a standard hydraulic actuator in various testing applications. Load
cells are also used for fatigue testing material specimens in a precise,
controlled manner.
Several pieces of information are needed in order to determine the proper
load cell for the application. In figuring out the right capacity of
a load cell, the maximum force value, the dynamics of the system (i.e.
frequency response), the effect that placing the transducer in the force
path will have and the maximum extraneous loads that the load cell will
see are determining factors. Evaluate the system in which the load cell
will be placed. In other words, will the load cell be in the primary
load path or will it see the forces indirectly, and are there any physical
constraints that should be met for size and mounting? Also, know what
accuracy is required and what environmental elements the load cell will
be subjected to that may cause special problems.
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Types of Load Cells
- or sensors have an internal reference chamber sealed at vacuum or
near
vacuum and typically
provide increasing output voltage for increases in pressure.
- have low profile construction for integration into restricted
areas.
- may feature an all stainless steel design and
are hermetically sealed for washdown and wet area. Canister
load cells
are utilized in both single and multi-weighing applications.
- are used for measurement of a straight line
pushing force along a single axis. This pushing force is
often denoted as negative force.
- utilize digital technology, as opposed to the
more commonly used analog voltage, to measure tension,
compression and shear.
- are devices inside a load cell that measure and monitor and compression and tension.
- is a common synonym for load cell.
- are force-balance devices, measuring weight as a change in pressure
of the internal filling fluid.
Typical hydraulic load cell applications include tank, bin and
hopper weighing.
- are used as part of a larger system that constantly
measures force and torque.
- communicate to
a computer how heavy the load is.
- are compression and tension/compression load
cells often used in weighing and in-line force
monitoring.
- are designed to fit into tight areas.
- are the most accurate scales, as they take
readings from more than one point of the
scale.
- provide accurate readings regardless
of the positioning of the load on the platform.
- operate on the force-balance principle. These
devices use multiple dampener chambers
to provide higher accuracy than a hydraulic device and contain
no fluids
that might contaminate
the
process should the diaphragm rupture.
- measure pressure in a fluid network.
- are S-shaped and provide superior side load rejection
and an output if under tension or compression.
- are devices that measures strain when stress is applied.
- are often attached to a bending beam to measure a deformation
in which planes of material slide
with respect to one another.
- contain four piezoresistors located within
the face of a thin, chemically-etched
silicon diaphragm. The diaphragm flexes with changes in pressure,
causing
a stress or strain in
the diaphragm and the buried resistors;
the resistor values are
proportionally related to the stress
applied and produce
an electrical output.
- convert the load acting on them into electrical
signals. The gauges themselves
are bonded onto a beam or structural member that is deformed when
weight
is applied.
- are used for measuring the pulling apart or positive
force along a single axis.
- measure the torque transferred along the driveline
axis at the place where the sensor
is positioned.
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