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What is a load cell circuit and how they work in force measurement?
Load Sensor manufactured in US by FUTEK Advanced Sensor Technology (FUTEK), a leading manufacturer producing a huge selection of Force Transducers, utilizing one of the most advanced technologies in the Sensor Industry: Metal foil strain gauge technology. A Force Sensor is defined as a transducer that converts an input mechanical load, weight, tension, compression or pressure into an electrical output signal (load cell definition). Force Sensors are also commonly known as Force Transducer. There are several types of load cells based on size, geometry and capacity.
By definition, load cell (or loadcell) is a type of transducer, specifically a force transducer. It converts an input mechanical force such as load, weight, tension, compression or pressure into another physical variable, in this case, into an electrical output signal that can be measured, converted and standardized. As the force applied to the force sensor increases, the electrical signal changes proportionally.
Force Transducers became an essential element in many industries from Automotive, High precision manufacturing, Aerospace & Defense, Industrial Automation, Medical & Pharmaceuticals and Robotics where reliable and high precision force measurement is paramount. Most recently, with the advancements in Collaborative Robots (Cobots) and Surgical Robotics, many novel force measurement applications are emerging.
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Firstly, we need to understand the underlying physics and material science behind the strain gauge load cell working principle, which is the strain gauge (sometimes referred to as Strain gage). Metal foil strain gage is a sensor whose electrical resistance varies with applied force. In other words, it converts (or transduces) force, pressure (i.e. pressure sensor), tension, compression, torque, weight, etc… into a change in electrical resistance, which can then be measured.
Strain gauges are electrical conductors tightly attached to a film in a zigzag shape. When this film is pulled, it – and the conductors – stretches and elongates. When it is pushed, it is contracted and gets shorter. This change in shape causes the resistance in the electrical conductors to also change. The strain applied in the load cell can be determined based on this principle, as strain gauge resistance increases with applied strain and diminishes with contraction. This same concept is also utilized in weight sensors.
Structurally, a force sensor (not a string potentiometer or string pot sensor) is made of a metal body (also called flexure) to which foil strain gauges are bonded. The sensor body is usually made of aluminum or stainless steel, which gives the sensor two important characteristics: (1) provides the sturdiness to withstand high loads and (2) has the elasticity to minimally deform and return to its original shape when the force is removed.
When force (tension or compression) is applied, the metal body acts as a “spring” and is slightly deformed, and unless it is overloaded, it returns to its original shape. As the flexure deforms, the strain gage also changes its shape and consequently its electrical resistance, which creates a differential voltage variation through a Wheatstone Bridge circuit. Thus, the change in voltage is proportional to the physical force applied to the flexure, which can be calculated via the load cell circuit voltage output.
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These strain gauges are arranged in what is called a Load Cell Signal Conditioner Circuit (aka Load Cell Amplifier). This means that four strain gages are interconnected as a loop circuit and the measuring grid of the force being measured is aligned accordingly.
The strain gauge bridge amplifiers (or strain gauge amplifiers) provide regulated excitation voltage to the load cell amplifier circuit and convert the mv/V output signal into another form of signal that is more useful to the user, for example a 4-20ma load cell analog output or a digital USB load cell output. The signal generated by the strain gage bridge is low strength signal and may not work with other components of the system, such as PLC, data acquisition modules (DAQ), computers, or microprocessors. For some applications, it may be needed a local signal readout, also known as a load cell indicator. Thus, force sensor signal conditioner functions include excitation voltage, noise filtering or attenuation, signal amplification, and output signal conversion.
Furthermore, the change in the amplifier voltage output is calibrated to be linearly proportional to the Newtonian force applied to the flexure, which can be calculated via the load cell circuit voltage equation.
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An important concept regarding force transducers is load cell sensitivity and accuracy. Force Sensor accuracy can be defined as the smallest amount of force that can be applied to the sensor body required to cause a linear and repeatable variation in the voltage output. The higher the load cell accuracy, the better, as it can consistently capture very sensible force variations. In applications like high precision factory automation, surgical robotics, aerospace, load cell linearity is paramount in order to accurately feed the PLC or DAQ control system with the accurate force measurement. Some of our Universal Pancake Load Cells presents Nonlinearity of ±0.1% (of Rated Output) and Nonrepeatability of ±0.05% RO.
Check out below a video on the "Benefits of System Calibration":
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When combined with a draw wire sensor (aka string potentiometer), load cells are the linchpin of modern factory automation.