Load cell

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".

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 V_\text{EX} is a known constant, and output voltage V_\text{o} is variable depending on the shape of the strain gauges. If all resistors are balanced, meaning R1/R2 = R4/R3 , then V_\text{o} is zero. If the resistance in even one of the resistors changes, then V_\text{o} likewise changes. The change in V_\text{o} can be measured and interpreted using Ohm's law. Ohm's law states that the current ( I , measured in amperes) running through a conductor between two points is directly proportional to the voltage V across the two points. Resistance ( R , measured in ohms) is introduced as the constant in this relationship, independent of the current. Ohm's law is expressed in the equation I = V/R . When applied to the 4 legs of the Wheatstone bridge circuit, the resulting equation is V_\text{o} = \left(\frac{R3}{R3 + R4} - \frac{R2}{R1 + R2}\right) V_\text{EX}. 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 V_\text{o} is measured. From the resulting data, V_\text{o} can be easily determined using the equation above. ===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, ==Uses== Load cells are used in several types of measuring instruments such as laboratory balances, industrial scales, platform scales and universal testing machines. From 1993 the British Antarctic Survey installed load cells in glass fibre nests to weigh albatross chicks. Load cells are used in a wide variety of items such as the seven-post shaker which is often used to set up race cars. Another common use is within sim racing, where the advantage of a load cell over a potentiometer is that simulated braking force is based on the user’s force on the pedal, rather than the position of the pedal. Load cells are also widely used in food processing, warehousing, logistics, and general manufacturing for applications such as batching, filling, pallet weighing, shipping verification, and process monitoring. ==Load cells weighing performances== Load cells are commonly used to measure weight in an industrial environment. They can be installed on hoppers, reactors, etc., to control their weight capacity, which is often of critical importance for an industrial process. Some performance characteristics of the load cells must be defined and specified to make sure they will cope with the expected service. Among those design characteristics are: Combined error Minimum verification interval Resolution ==Load cell specifications== The electrical, physical, and environmental specifications of a load cell help to determine which applications it is appropriate for. Full-scale output (FSO): Electronic output expressed in mV/V. Measured at full scale. Combined error: percent of the full scale output that represents the maximum deviation from the straight line drawn between no load and load at rated capacity. Often measured during decreasing and increasing loads. Non-linearity: The maximum deviation of the calibration curve from a straight line drawn between the rated capacity and zero load. Measured on increasing load and expressed as percent of full scale output. Hysteresis: Maximum difference between load cell output signals for the same applied load. The first measurement can be obtained by decreasing the load from rated output and the second by increasing the load from zero. Repeatability: Maximum difference between output measurements for repeated loads under identical conditions. Measured in percent of rated output. Zero balance (offset): Output reading of the load cell with rated excitation under no load. The deviation in output between a true zero measurement and a real load cell under zero load expressed as a percentage of full scale output. Compensated temperature range: The temperature range over which a load cell is compensated so that it can ensure zero balance & rated output within specified limits. Expressed as °F or °C. Operating temperature range: Temperature range extremes in which a load cell can operate without permanent, adverse effects on any of its performance characteristics. Expressed as °F or °C. Temperature effect on output: Modification of output readings caused by load cell temperature. Expressed as percent of full scale output per degree of °F or °C. Temperature effect on zero: Change in zero balance caused by ambient temperature changes. Expressed as percent of full scale output per degree of °F or °C. Input resistance: Input resistance of the load cell's bridge circuit. Measured at the positive & negative excitation leads with no load applied. Measured in ohms. Output resistance: Output resistance of the load cell's bridge circuit. Measured at the positive & negative excitation leads with no load applied. Measured in ohms. Insulation resistance: The resistance measured along pathways between the: bridge circuit and transducer element, bridge circuit and the cable shield, and the transducer element and the cable shield. Typically measured at fifty volts under standard test conditions. Recommended excitation: Maximum recommended excitation voltage of the transducer for it to operate within its specifications. Expressed in VDC. Cable length: Length of the standard cable for which the load cell is calibrated. Cable length affects how the load cell is calibrated. Safe overload: The maximum load that can be applied to a load cell without causing permanent effects to its performance specifications. Measured as a percent of full scale output. Ultimate overload: Maximum load that can be withstood without causing structural failure. Material: Substance that comprises the spring element of the load cell. == See also== Spring scale Stretch sensor ==References== ===Standards=== ASTM E4 – Practices for Force Verification of Testing Machines ASTM E74 – Practice for Calibration of Force Measuring Instruments for Verifying the Force Indication of Testing Machines NTEP – National Conference on Weights and Measures (Certificate of Conformance)

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