A Wheatstone bridge is a circuit arrangement used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, each containing known resistances. It was invented by Samuel Hunter Christie in 1833 and popularized by Sir Charles Wheatstone in 1843. The basic Wheatstone bridge consists of four resistive arms forming a diamond shape with a voltage source connected across one pair of diagonal ends and a galvanometer connected across the other pair. The galvanometer indicates zero current flow when the bridge is balanced, which occurs when the ratio of the resistances in one pair of arms equals the ratio in the other pair.
In the automation field, Wheatstone bridges find applications in various sensing and measurement tasks, particularly in the following areas:
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Strain Gauge Measurements: Wheatstone bridges are commonly used to measure changes in resistance, such as those produced by strain gauges. Strain gauges are sensors that change resistance when subjected to mechanical strain, allowing them to measure forces, pressures, or loads. By incorporating strain gauges into one or more arms of a Wheatstone bridge, small changes in resistance can be detected and converted into measurable electrical signals, providing accurate measurements of physical quantities.
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Temperature Sensing: Wheatstone bridges can also be used for temperature sensing by incorporating temperature-dependent resistors, such as thermistors or resistance temperature detectors (RTDs), into the bridge circuit. The resistance of these devices changes with temperature, allowing Wheatstone bridges to accurately measure temperature variations.
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Pressure and Force Sensing: Wheatstone bridges can be utilized in pressure sensors and load cells to measure changes in resistance caused by pressure or force applied to a sensing element. This principle is similar to strain gauge measurements, where the Wheatstone bridge detects variations in resistance due to changes in the physical parameter being measured.
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Liquid Level Sensing: Wheatstone bridges can be employed in liquid level sensors by utilizing the change in resistance caused by the presence or absence of a conductive liquid between electrodes. The Wheatstone bridge can detect variations in resistance, indicating changes in the liquid level.
In summary, Wheatstone bridges are fundamental circuits used in automation for precise measurement of various physical parameters such as strain, temperature, pressure, force, and liquid level. They provide a reliable and accurate method for converting changes in resistance into measurable electrical signals, making them valuable tools in many industrial and scientific applications.
