Wheatstone Bridge and Strain Gauge
A Wheatstone bridge, when equipped with a strain gauge, serves as a valuable tool for force measurement. A strain gauge undergoes a change in resistance when subjected to compression or stretching caused by an external force. This change in resistance disrupts the initially balanced Wheatstone bridge, leading to an unbalance and a consequent alteration in the output voltage. The detection of this change, even though the resistance change in strain gauges is minuscule, provides high sensitivity. This sensitivity is exploited in applications such as weight scales, where Wheatstone bridges with strain gauges are commonly employed.
Resistive transducers, especially strain gauges, offer a method to accurately measure extremely small resistance changes, which can be challenging with direct measurements. Strain gauges are particularly effective in converting the stretching or compression of a fine wire into a measurable change in resistance. When the wire stretches due to strain, its resistance increases, and when compressed, the resistance decreases.
In various scales, ranging from those for small parts to those weighing large trucks, strain gauges find application. Typically mounted on a specially designed aluminum block that deforms under the applied weight, the strain gauges are delicate and must be correctly positioned. The entire assembly is often integrated into a single unit known as a load cell. A load cell is a transducer that utilizes strain gauges to convert mechanical force into an electrical signal. Various load cells, differing in shapes and sizes, are available for different applications. An example is an S-shaped load cell used in weighing applications, containing four strain gauges, with two in tension (stretching) and two in compression.
What is a Load cell?
Load cells are commonly connected to a Wheatstone bridge circuit. The bridge configuration, with strain gauges in tension and compression in opposite diagonal legs, is depicted in Figure 1(b). The output from the bridge is digitized, converted for display, or sent to a computer for further processing. The Wheatstone bridge offers accurate measurement of small resistance differences and compensates for temperature variations and wire resistance, contributing to its suitability for instrumentation.
Beyond weight measurement, strain gauges paired with Wheatstone bridges are utilized in various applications, including pressure measurements, displacement, and acceleration measurements. In pressure measurements, strain gauges are affixed to a flexible diaphragm that stretches with applied pressure. The degree of flexing corresponds to the pressure and results in a small, measurable resistance change.
Bridge Circuits and Related Terms
Balanced Bridge: A bridge circuit in a state where there is zero voltage across the bridge, indicating equilibrium.
Bleeder Current: The residual current left after subtracting the total load current from the overall current into the circuit.
Load Current: The current supplied to a load, represents the output current of a circuit.
Loading: The impact on a circuit when an element drawing current is connected across the output terminals.
Maximum Power Transfer: The condition where maximum power transfers from the source to the load, occurring when the load resistance equals the source resistance.
Superposition: A method for analyzing circuits with multiple sources by examining the effects of each source individually and then combining these effects.
Terminal Equivalency: The situation where two circuits produce the same load voltage and load current when the same load resistance is connected to either circuit.
Thevenin’s Theorem: A circuit theorem allowing the reduction of any two-terminal resistive circuit to a single equivalent voltage source in series with an equivalent resistance.
Unbalanced Bridge: A bridge circuit in a state of imbalance, indicated by a voltage across the bridge proportional to the deviation from the balanced state.
Wheatstone Bridge: A 4-legged bridge circuit used for accurately measuring unknown resistances in a balanced state, and for measuring deviations in resistance in an unbalanced state.