f calibration, or even mechanical damage to the sensing element

Wiki Article

A load cell converts a force such as tension, compression, pressure, or torque into an electrical signal that can be measured and standardized. It is a force transducer. As the force applied to the load cell increases, the electrical signal changes proportionally. The most common types of load cell are pneumatic, hydraulic, and strain gauges.

Strain gauge load cell
Strain gauge load cells are the kind most often found in industrial settings. It is ideal as it is highly accurate, versatile, and cost-effective. Structurally, a load cell has a metal body to which strain gauges have been secured. The body is usually made of aluminum, alloy steel, or stainless steel which makes it very sturdy but also minimally elastic. This elasticity gives rise to the term "spring element", referring to the body of the load cell. When force is exerted on the load cell, the spring element is slightly deformed, and unless overloaded, always returns to its original shape. As the spring element deforms, the strain gauges also change shape. The resulting alteration to the resistance in the strain gauges can be measured as voltage. The change in voltage is proportional to the amount of force applied to the cell, thus the amount of force can be calculated from the load cell's output.

Strain Gauges

A strain gauge is constructed of very fine wire, or foil, set up in a grid pattern and attached to a flexible backing. When the shape of the strain gauge is altered, a change in its electrical resistance occurs. The wire or foil in the strain gauge is arranged in a way that, when force is applied in one direction, a linear change in resistance results. Tension force stretches a strain gauge, causing it to get thinner and longer, resulting in an increase in resistance. Compression force does the opposite. The strain gauge compresses, becomes thicker and shorter, and resistance decreases. The strain gauge is attached to a flexible backing enabling it to be easily applied to a load cell, mirroring the minute changes to be measured.

Since the change in resistance measured by a single strain gauge is extremely small, it is difficult to accurately measure changes. Increasing the number of strain gauges applied collectively magnifies these small changes into something more measurable. A set of 4 strain gauges set in a specific circuit is an application of a Wheatstone bridge.

Common types of load cells
There are several types of strain gauge load cells:[2]

Single Point load cells; used in small to medium platform scales with platform sizes of 200x200mm up to 1200x1200 mm.
Planar Beam load cells; used in low profile solutions where space is limited, like medical scales and retail scales.
Bending Beam load cells; used in pallet, platform and small hopper scales.
Shear Beam load cells; used in low profile scale and process applications, available in capacities from 100kg up to 50t.
Dual Shear Beam load cells; used in truck scales, tank and hopper applications.
S-type load cells; used in tension applications where you will find static and dynamic loads.
Compression load cells; used in truck scales, large platform scales, weighbridges and hopper scales.
Ring Torsion load cells; used in high accuracy hoppers, silo's, platforms and pallet scales.
Spoke Type load cells; used in low profile, high precision application. High forces varying from 1t-500t.
Onboard load cells; used for onboard weighing systems on trucks, tractors and other vehicles.
Loadpins; used in applications for measuring dynamic, static or hoisting forces.
Weighpads; portable weighpads for the weighing of cars and the measure the center of gravity of planes.
Specials; all kind of special sensors.
Pneumatic load cell
The pneumatic 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 to 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.

Hydraulic load cell
The hydraulic load cell uses a conventional piston and cylinder arrangement with the piston 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 results in an increase of oil pressure. This pressure is then transmitted to a hydraulic pressure gauge via a high pressure hose.[3] The gauge's Bourdon tube senses the pressure and registers it on the dial. Because this sensor has no electrical components, it is ideal for use in hazardous areas.[4] Typical hydraulic load cell applications include tank, bin, and hopper weighing.[5] By example, a hydraulic load cell is immune to transient voltages (lightning) so these type of load cells might be a more effective device in outdoor environments. This technology is more expensive than other types of load cells. It is a more costly technology and thus cannot effectively compete on a cost of purchase basis.[6]

Other types
Vibrating load cell
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.

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. The piezoelectric effect is dynamic, that is, the electrical output of a gauge is an impulse function and is not static. The voltage output is only useful when the strain is changing and does not measure static values.

However, depending on conditioning system used, "quasi static" operation can be done. Using a charge amplifier with a long time constant allows accurate measurement lasting many minutes for small loads up to many hours for large loads. Another advantage of Piezoelectric load cells conditioned with a charge amplifier is the wide measuring range that can be achieved. Users can choose a load cell with a range of hundred of kilonewtons and use it for measuring few newtons of force with the same signal-to-noise ratio; again this is possible only with the use of a charge amplifier for conditioning.

loadcell

Common issues
Mechanical mounting: the cells have to be properly mounted. All the load force has to go through the part of the load cell where its deformation is sensed. Friction may induce offset or hysteresis. Wrong mounting may result in the cell reporting forces along undesired axis, which still may somewhat correlate to the sensed load, confusing the technician.
Overload: Within its rating, the load cell deforms elastically and returns to its shape after being unloaded. If subjected to loads above its maximum rating, the material of the load cell may plastically deform; this may result in a signal offset, loss of linearity, difficulty with or impossibility of calibration, or even mechanical damage to the sensing element (e.g. delamination, rupture).

load cell

Wiring issues: the wires to the cell may develop high resistance, e.g. due to corrosion. Alternatively, parallel current paths can be formed by ingress of moisture. In both cases the signal develops offset (unless all wires are affected equally) and accuracy is lost.
Electrical damage: the load cells can be damaged by induced or conducted current. Lightning hitting the construction, or arc welding performed near the cells,[7] can overstress the fine resistors of the strain gauges and cause their damage or destruction. For welding nearby, it is suggested to disconnect the load cell and short all its pins to the ground, nearby the cell itself. High voltages can break through the insulation between the substrate and the strain gauges.
Nonlinearity: at the low end of their scale, the load cells tend to be nonlinear. This becomes important for cells sensing very large ranges, or with large surplus of load capability to withstand temporary overloads or shocks (e.g. the rope clamps). More points may be needed for the calibration curve.
load cell
Particularity of application: A load cell that is not well suited to the specific magnitude and type of pressure will have poor accuracy, resolution, and reliability.