Load Cells & Force Sensors Explained

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A load cell – sometimes referred to as a ‘force’, ‘weigh’ or ‘weight’ cell – is a transducer that converts the load acting on it into a measurable (analogue or digital) output. The output is proportional to the force or load applied. The conversion is achieved by the physical deformation of strain gauges, which are bonded onto the load cell structure. The strain gauges are connected into a Wheatstone Bridge circuit with four strain gauges (full bridge), two gauges (half bridge) or one gauge (quarter bridge). With half and quarter bridges, the bridge is completed using precision monitors. Additional bondable resistors are configured within the bridge circuit to compensate for the effects of temperature on the zero or no-load signal and the sensitivity (output due to applied load, plus a further resistor to enable the bridge to be nulled at no load). The completed Wheatstone Bridge requires a stable DC supply to excite the circuit. This is usually 5Vdc or 10Vdc, but can be any value from 1Vdc up to 18Vdc.As stress is applied to the bonded strain gauges, a resistive change takes place which unbalances the Wheatstone Bridge. This provides an output signal that is linearly proportional to the stress value. The value of this signal, which is also proportional to the excitation voltage, is typically a few millivolts. These low level millivolts signals are compatible with a vast range of bespoke strain gauge instrumentation. These instruments include digital displays, analogue and digital amplifiers. Typical analogue amplifiers will generate a higher level voltage (0-5Vdc, 0-10Vdc) or current (0-20mA, 4-20mA) for onward processing. Digital amplifiers typically provide an RS232, RS422 or RS485 output using either the common ASCII protocol or one of a host of more specialised, industry-specific protocols that have been developed such as Modbus. Both the analogue and digital instrumentation have been reduced in size (miniaturised) sufficiently to enable the item to be fitted within the body of a load cell or force sensor. Load cell designs are either distinguished by the type of output signal they generate (pneumatic, hydraulic or electrical) or by the method in which they detect the load or weight (bending, shear, compression, tension, etc). To cater for the huge variety of uses within R&D and industrial applications, a load cell can take many forms. However, the majority of today’s designs utilise strain gauges as the sensing element, where a foil or semiconductor is used. Foil gauges provide the widest choice of load cell designs and so tend to be the most commonly used. Strain gauge patterns offer measurement of tension, compression and shear forces. Semiconductor strain gauges are available in a smaller range of patterns, but offer the advantages of being smaller and provide large gauge factors, resulting in much larger outputs for the same given stress. Due to these properties, they tend to be used for miniature load cell designs. Proving rings are used for load measurement, using a calibrated metal ring, the movement of which is measured with a precision displacement transducer.