A load cell converts a force such as tension, compression, pressure, or torque into a signal (electrical, pneumatic or hydraulic pressure, or mechanical displacement indicator) that can be measured and standardized. It is a force transducer. As the force applied to the load cell increases, the signal changes proportionally. The most common types of load cells are pneumatic, hydraulic, and strain gauge types for industrial applications. Typical non-electronic bathroom scales are a widespread example of a mechanical displacement indicator where the applied weight (force) is indicated by measuring the deflection of springs supporting the load platform, technically a "load cell".
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, so 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.
Wheatstone bridge
A Wheatstone bridge is a configuration of four balanced resistors with a known excitation voltage applied.
Excitation voltage is a known constant, and output voltage is variable depending on the shape of the strain gauges. If all resistors are balanced, meaning , then is zero. If the resistance in even one of the resistors changes, then likewise changes. The change in can be measured and interpreted using Ohm's law. Ohm's law states that the current (, measured in amperes) running through a conductor between two points is directly proportional to the voltage across the two points. Resistance (, measured in ohms) is introduced as the constant in this relationship, independent of the current. Ohm's law is expressed in the equation . When applied to the 4 legs of the Wheatstone bridge circuit, the resulting equation is
In a load cell, the resistors are replaced with strain gauges and arranged in alternating tension and compression formation. When force is exerted on the load cell, the structure and resistance of the strain gauges changes, and is measured. From the resulting data, can be easily determined using the equation above.
Common types of strain gauge load cells There are several types of strain gauge load cells:Full information about types of loadcells available on https://www.zemiceurope.com/loadcell Single-point load cells; used in small to medium platform scales with platform sizes of up to . 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 100 kg up to 50 t. Dual shear beam load cells; used in truck scales, tank and hopper applications. S-type load cells; used in tension applications with 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, silos, platforms and pallet scales. Spoke-type load cells; used in low-profile, high-precision application. High forces varying from 1 to 500 t. 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 to measure the center of gravity of planes. Specials; various types of special sensors.
Capacitive load cell The digital capacitive technology is based on a non-contacting ceramic sensor mounted inside the load cell body. As the load cell contains no moving parts and the ceramic sensor is not in contact with the load cell body, the load cell tolerates very high overloads (up to 1000%), sideloads, torsion, and stray welding voltages. This allows for simple installation of the load cells without expensive and complicated mounting kits, stay rods, or overload protection devices, which in turn eliminates the need for maintenance.
The basic difference from strain gauge load cells Capacitive and strain gauge load cells both rely on an elastic element that is deformed by the load to be measured. The material used for the elastic element is normally aluminum or stainless steel for load cells used in corrosive industrial applications. A strain gauge sensor measures the deformation of the elastic element, and the output of the sensor is converted by an electronic circuit to a signal that represents the load. Capacitive strain gauges measure the deformation of the elastic material using the change in capacitance of two plates as the plates move closer to each other.
Capacitive sensors have a high sensitivity compared to strain gauges. Because of the much higher sensitivity, a much lower deformation of the elastic element is needed, and the elastic element of a capacitive load cell is therefore strained around 5 to 10 times lower than the elastic element of a strain gauge load cell. The low strained element combined with the fact that a capacitive sensor is non-contacting, provides the very high shock resistance and overload capability of the capacitive load cell compared to the strain gage load cell. This is an obvious advantage in industrial environments and especially for the lower capacity load cells where the risk of damage because of shocks and overloads is high.
Connectivity of capacitive load cells In a standard analog strain gauge load cell, the power supply and the low-level analog signal are normally conducted through a rather expensive 6-wire cable to the instrumentation where the analog signal is converted to a digital signal. Instead, digital capacitive load cells transmit the digital signal back to the instrumentation which may be placed several hundred meters away without influencing the reading.
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 overstrain of the diaphragm when the loads exceed a 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. 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. Typical hydraulic load cell applications include tank, bin, and hopper weighing. By example, a hydraulic load cell is immune to transient voltages (lightning) so these types 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.
Other types Vibrating load cell
Vibrating wire load cells, which are useful in geomechanical applications due to low amounts of drift,
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.
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). Capacitive load cells compared to strain gauges are more resistant to overloads, due to their contactless measuring principle. 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.