2.11:
Design Example: Strain Gauge Bridge or Wheatstone Bridge
A strain gauge, a transducer that converts mechanical strain into electrical signals, is often used to measure force.
The engineer's role involves designing the Wheatstone bridge circuit, having strain gauges in each arm, and identifying the required amplification for the output voltage. Here, it is assumed that the applied force increases the resistance of two strain gauges while the resistance decreases in the other two. Also, the relationship between the bridge output voltage and the change in resistance is known.
This can be achieved using Thevenin's theorem.
Knowing the input voltage, the currents in the top and bottom branches are first determined. These values are then substituted into Ohm's law to calculate the Thevenin voltage.
The source is removed to calculate the Thevenin resistance, and the equivalent resistance is determined in a simplified form.
From the Thevenin equivalent circuit, the bridge output voltage is calculated using the voltage division rule.
Comparing the obtained relation with the known relation allows the determination of the amplifier gain needed to design the circuit to work within the desired range.
2.11:
Design Example: Strain Gauge Bridge or Wheatstone Bridge
The utilization of strain gauges as transducers for converting mechanical strain into electrical signals is a common practice in various engineering applications. These strain gauges are frequently integrated into Wheatstone bridge circuits to accurately measure parameters such as force or pressure. Within this context, each element within the circuit exhibits a resistance that undergoes subtle variations when subjected to mechanical strain. The primary objective is to convert minuscule variations in small voltage output into a more discernible voltage output, which can be conveniently read using a voltmeter.
In the engineer's capacity, the responsibilities include creating a strain gauge design and determining the necessary amplification to reflect changes in resistance accurately. This relies upon the application of Thevenin's theorem, which establishes the connection between the output voltage of the bridge circuit and variations in resistance.
The procedure starts by calculating the Thevenin voltage, which requires the initial determination of currents within both the upper and lower branches of the circuit. As a result, these computed current values are substituted into Ohm's law to derive the Thevenin voltage. Simultaneously, Thevenin resistance is determined by removing the voltage source in a simplified configuration.
The Thevenin equivalent circuit finds the bridge's output voltage through the voltage division rule. The critical outcome of this analysis enables the precise determination of the amplifier gain required for designing the circuit to operate effectively within the specified operational range.