This article was contributed by Joe Flanagan, an engineer at Tacuna Systems with over eight years of experience. Tacuna Systems provides a wide range of force and load measurement equipments, including load cells, for the strain and load industry.
A load cell, also commonly known as a load sensor, is a transducer that is used in force measurement applications. It produces a measurable electrical output whose magnitude is proportional to the mechanical force applied. Most industrial weighing systems are based on strain gauge load cells. Other common types of load cells include pneumatic load cells, hydraulic load cells, piezoelectric load cells, and capacitive load cells. Load cells can also be classified according to direction of loading, shape, precision, air tightness, and so on.
Strain gauge load cells
Strain gauge load cells produce a measurable electrical output when a force is applied. The gauges are usually bonded onto a structural member. As the load is applied, the resulting strain causes the resistance of the gauges to change. This change in electrical resistance is proportional to the force applied. In most weighing applications, four strain gauges are used; two in compression and two in tension.
Capacitive load cells
Capacitive load cells are based on the principle of operation of capacitors. The distance between the plates of a capacitor changes when a force is applied. The resulting change in capacitance is proportional to the applied force. As compared to strain gauge load cells, capacitive load cells are more rugged.
Hydraulic load cells
The pressure of a fluid that is contained in an enclosed space increases when a force is applied. The proportional increase in pressure is detected by a calibrated pressure measuring device. The accuracy of a hydraulic load cell is enhanced by ensuring that it is properly mounted and calibrated. Hydraulic load cells are commonly used in hazardous environments because they do not have electrical components.
Pneumatic load cells
Just like hydraulic load cells, pneumatic load cells use the force-balance principle to measure the magnitude of the applied force. These explosion proof load cells do not contain fluids that can cause contamination. Pneumatic load cells offer higher accuracy and they are suitable for measuring small weights.
Applications of strain gauge load cells in force measurement
The weighing industry is dominated by force measurement devices that utilize strain gauge load cells. In weighing systems, load cells are used in combinations or individually depending on the requirements of the weighing application. A suitable configuration is determined by factoring the characteristics of the load, especially its geometry and size. The accuracy of a weighing system depends on many factors including the accuracy of the load cell, signal interference, load factors, and environmental forces. The combined accuracy of a load cell is determined by the following specifications:
The outputs obtained by decreasing the load applied to a load cell from the maximum (maximum rated capacity) to the minimum (no-load) and by increasing the load from the minimum to the maximum load are different. This difference in output readings is referred to as hysteresis. This specification is usually provided by the manufacturer as a percentage of the load cell’s full range.
Repeatability of a load cell refers to its ability to produce readings with minimum deviation when the same load is applied repeatedly under the same loading conditions. For high accuracy, the maximum difference between the readings, usually referred to as non-repeatability, should be as small as possible.
The output of a loaded transducer changes over time when the load is applied for a long time. This variation in output reading over time is referred to as creep. In some applications such as filling, creep has little or insignificant effect because the load is usually applied for one or two minutes. However, in weighing applications where the load may be applied for a longer period of time, it is critical to consider the creep effect.
When an increasing load is applied to a transducer, its output curve deviates from a straight line. This deviation is referred to as non-linearity. Some of the methods used to correct non-linearity in weighing systems include employing a look up table and using multiple calibration points.
5. Temperature effects on the output of a load cell
The output of a load cell changes with temperature. This variation is usually caused by the combined effect of temperature changes on the gauge factor and the spring sensor material. In most weighing systems, temperature compensation is used to minimize the errors caused by changes in temperature.
6. Temperature effect on the output of an unloaded transducer
Changes in temperature can cause a positive or a negative change in no-load output readings of a load cell. Temperature compensation techniques are used to reduce errors caused by this effect.
7. Changes in atmospheric pressure
This refers to the variation in the output of the load cell when no load is applied. The barometric pressure effect is common in load cells that have bellows. To enhance the accuracy of a weighing system, this effect is corrected by equalizing pressure through venting.
Load cells come in different types and designs to meet the diverse requirements of today’s weighing industry. The accuracy of a weighing system is significantly determined by the characteristics of the load cell used. It is therefore critical to consider various factors when selecting a load cell for your weighing system. Key considerations include accuracy, loading conditions, electrical considerations, mechanical requirements, and environmental considerations.
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