Load Cells

View A Video on Load Cells- A Quick Introduction

Of the three major categories by which load cells measure force, compression and tension are the most commonly employed. Sometimes a load cell will measure an object through both of these applications, rather then one or the other. Both compression and tension loadcells often use strain gauges; strain gauges are very small devices that measure the strain of an object by converting internal deformation into electrical signals, precisely measuring weight, force or strain. Force gauges use strain gauges in push-pull testing and flow measurement. While most load cells measure and test with strain gages, some use piezoelectric sensors, which utilize piezoelectric crystals to measure weight, strain, movement and vibrations. Pressure sensors and force sensors are transducers very similar to load cells that measure pressure, applied force and strain in gas pressure, altitude and liquid pressure. These sensors are often piezoelectric sensors. Many of these sensors, although quite small, are built to support or to hold as much as several tons; miniature load cells are built to provide precise measurements for much smaller applications. Equipment such as force transducers, torque sensors and load sensors all sustain strain gauges or load cells that measure and convert energy. Digital load cell technology is the most popular way to access the information gained through the sensors.

Various types of load cells, pressure sensors and gauges are used in manufacturing, processing and testing industries. Pressure sensors and load cells are used in food processing industries to precisely measure ingredients and to properly distribute the products during packaging. In industrial warehouse environments, where pallets of inventory are shuffled around, load cells are often used to determine the precise weight of loaded pallets, which is crucial for the filling and accepting of orders. Other load cell applications include the testing of bridge building materials such as beams for tension strength, as well as in railcar weighing and truck scales. Load cells are essential components in many calibration systems, as well as for fatigue testing in research and development laboratories. With reading accuracies within 0.25%, load cells, sensors and gauges provide accurate mass, weight and pressure measurement of very small loads to loads of several thousand tons.

After load cells transduce mechanical stress into electrical energy, the information that loadcells monitor is then signaled to a recorder or other computerized data collection system. Analog or digital load cell technology is used for the recording and transferring of information. Digital load cells have become more popular than analog load cells in recent years because they work faster, have a higher accuracy rate and better resolution. 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 the industrial arena they will be utilized in. 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. However, it can also be a piezoelectric sensor that utilizes crystals. The circuit is the connection of these gauges or sensors 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 arrangement of gauges is usually done according to the Wheatstone bridge equation, which was developed in 1833 by Samuel Hunter Christie. It was not until ten years later though that the equations' namesake improved upon it and made it popular. The more gauges inside the load cell, the more sensitive the cell is in recording and monitoring variance in measurement. In calculating capacity of a load cell, factors that must be considered are: 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 handle. When mounting load cells, such factors must be considered: whether the load cell be in the primary load path or whether it will see the forces indirectly; whether there are any physical constraints that should be met for size and mounting; what level of accuracy is required, and what environmental elements the load cell will be subjected to that may cause special problems. These complexities are necessary to have the correct measuring force load cell in place, to ensure the safety and productivity of the industries employing them.
 

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Load Cell Types

• Absolute pressure transducers or sensors have an internal reference chamber sealed at vacuum or near vacuum and typically provide increasing output voltage for increases in pressure.
  
• Bending beam load cells have low profile construction for integration into restricted areas.
  
• Canister load cells 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.
  
• Compression load cells are used for measurement of a straight line pushing force along a single axis. This pushing force is often denoted as negative force.
  
• Digital load cells utilize digital technology, as opposed to the more commonly used analog voltage, to measure tension, compression and shear.
  
• Force gauges are devices inside a load cell that measure and monitor and compression and tension.
• Force sensor is a common synonym for load cell.
  
• Force transducers use a spring element to measure applied force, which is then transmitted to computer systems to be interpreted.
• Hydraulic load cells 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.
  
• Interface load cells are used as part of a larger system that constantly measures force and torque.
  
• Load sensors communicate to a computer how heavy the load is.
  
• Loadcells convert mechanical force through compression, tension or shear into measurements interpreted by electrical signals.
• Low profile load cells are compression and tension/compression load cells often used in weighing and in-line force monitoring.
  
• Miniature load cells are designed to fit into tight areas.
• Multi load cell scales are the most accurate scales, as they take readings from more than one point of the scale.
  
• Platform or single point load cells provide accurate readings regardless of the positioning of the load on the platform.
  
• Pneumatic load cells 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.
  
• Pressure sensors measure pressure in a fluid network.
• S-beam load cells are S-shaped and provide superior side load rejection and an output if under tension or compression.
  
• A strain gage simply measures the strain of particular objects or systems when under stress to desipher their strength and durability.
• Strain gauges are devices that measures strain when stress is applied.
• Shear cells are often attached to a bending beam to measure a deformation in which planes of material slide with respect to one another.
  
• Silicon pressure sensors 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.
  
• 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 is deformed when weight is applied.
  
• Tension load cells are used for measuring the pulling apart or positive force along a single axis.
  
• Torque sensors measure the torque transferred along the driveline axis at the place where the sensor is positioned.

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Load Cell Terms

Axial Load - The load applied to the length of, or parallel to, the primary axis with which it shares a common axis.
 
Calibration - Load cells output comparison against standard test loads.
 
Creep - The output change of load cells that occurs over time while the load cell is under load, while all environmental conditions and other variables have remained constant.
 
Dead Volume - The volume inside the pressure port of force sensors, or transducers, at room temperature and barometric pressure.
 
Deflection - The change of length along the primary axis of load cells involving no-load and rated-load conditions.
 
Diaphragm - The membrane part of force sensors that changes its value under pressure-induced displacement.
 
Drift - An unexpected change in output under constant load conditions.
 
Driveline Shaft - A steel tube with a u-joint at each end of load cells that transfer torque from the output of the transfer case to the axle.
 
Eccentric Load - A load, which is applied parallel to, but not having a common axis with, the primary axis of load cells.
 
Electrical Excitation -The current or voltage that is applied to the input terminals of a transducer.
 
Flush Diaphragm - A sensing device of load cells that is located on the very end of a transducer with no pressure port.
 
Full Scale - The amount produced equivalent to the maximum load for a specific load cell application or test.
 
Full Scale Output - The numerical distinction between the least output and the rated capacity of load cells.
 
Hysteresis - The greatest difference between load cell output readings for the same applied load. One load cell reading is obtained by escalating the load from zero, the other load cell reading by lessening the load from rated output.
 
Input Impedance - The resistance measured across the excitation terminals of a transducer at room temperature at the point where there is no load applied and the load cell output terminals are open-circuited.
 
Load - The force, weight or torque that is applied to the transducer, load cell or force sensors.
 
Load Cell - The round shape of the top surface of load cells, transducers or load sensors where the load is applied.
 
Measured Media - The physical number, property or circumstance that is measured by load cells, such as acceleration, force, mass or torque.
 
Piezoresistance - The change in resistance caused by an applied strain of the load cell diaphragm.
 
Primary Axis - The geometric centerline (axis) along which load cells are designed to be loaded.
 
Pull Plate - An attachment to load cells, which allows tension or compression forces to be directed at the center line of load cells through a threaded center hole.
 
Safe Overage - The maximum pressure or load that may be applied to the transducer, load cells or force sensors without causing permanent damage or a change in the load cell performance specifications.
 
Shear - Force that tends to divide an object along a plane parallel to the opposing stresses within load cells.
 
Strain Measurement - The ratio of the change of the length of a structure when force is applied to it to the dimension of the original length.
 
Zero Balance - The output signal rated excitation of load cells with no load applied, usually expressed in percent of rated output.

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