Connection of strain gauges with bridge configurations for load cells
Strain gauges are the basis of load cell technology, as they convert mechanical force into measurable electrical signals. Interface defines the strain gauge connection in our basic Load Cell Field Guide.
The interconnection of strain gauges in a Wheatstone bridge—in the form of quarter, half, or full bridges—is a critical design decision that affects performance, cost, and application suitability.
When thousands of strain gauges are required to perform a large-scale test, the quarter-bridge configuration is a necessity for cost control. The single active bridge leg (a strain gauge) and the other three inactive legs are fixed resistors, simulating a full bridge. All other parameters being equal, the performance of a full bridge is twice that of a half bridge and four times that of a quarter bridge.
This overview provides an engineer's perspective on these configurations, drawing on established principles in the field.
The basic principle: stretch to resistance
As highlighted in the Interface Technical Manual, the basis of strain gauge technology is the principle that the electrical resistance of a strain gauge changes proportionally to the applied strain. This relationship is quantified by the strain factor (GF): RΔR=GF⋅ϵ
This small change in resistance is accurately measured using a Wheatstone bridge circuit, the standard method for converting this electrical change into a usable output voltage.
Adaptation of bridge configurations to application needs
Interface's experience in manufacturing standard and custom load cells for nearly 58 years demonstrates that selecting a bridge configuration is a strategic decision that requires balancing initial cost with the desired sensitivity and the need to mitigate environmental factors and external mechanical stresses.
Quarter bridge: simplification for special cases
A single active strain gauge is integrated with three fixed resistors to complete the Wheatstone bridge. The bridge's output voltage directly reflects the resistance change of this single active sensor. Common load cell design practices recommend that the quarter bridge provides the most economical output configuration due to the single active sensing element. Its simplicity can be advantageous in cost-sensitive applications where accuracy and stability are less important.
While quarter bridges are inexpensive, they offer the lowest sensitivity, making the signal less susceptible to noise. They are also susceptible to temperature fluctuations, which can lead to zero-point drift. Compensation for bending or torsional loads is minimal.
TIP: Quarter bridges are typically used for simple weighing devices without high precision, basic stress analysis under controlled conditions, and as teaching aids to illustrate strain measurement principles. The Interface guide recommends limiting this configuration to applications where environmental stability and load purity are less critical.
Half bridge: Improved performance through strain gauge pairing
Two active strain gauges, strategically placed on the load sensing element, are paired with two fixed resistors in the bridge circuit. The output voltage is proportional to the combined resistance changes of the two active strain gauges. These strain gauges are often oriented to experience opposite strain under load, effectively doubling the output compared to a quarter bridge.
Interface's load cell engineers emphasize that the half-bridge offers a significant performance leap. The use of two active strain gauges provides greater sensitivity and the potential for inherent first-order temperature compensation, provided both strain gauges are subject to similar temperature fluctuations. Careful strain gauge placement can reduce sensitivity to off-center loads, an important consideration in load cell design.
Installation is more complex than with a quarter bridge. Adequate temperature compensation depends on matched strain gage properties and a uniform temperature distribution. The ability to reject certain external loads depends largely on the precise position and alignment of the two active strain gages on the sensing element.
TIP: Integrated load cells where the system design allows for special mounting and signal processing for improved stability, force measurement in predominantly uniaxial loading scenarios where some bending may occur, and custom sensor designs that seek improved sensitivity to a quarter bridge.
Full Bridge: The pinnacle of performance and reward
Four active strain gauges form all four arms of the Wheatstone bridge circuit. The output voltage is proportional to the sum of the resistance changes of all four strain gauges, maximizing sensitivity. Typically, two strain gauges are used to measure the tensile stress and two to measure the compressive stress under the applied load.
Interface's extensive experience makes the full bridge the best configuration for high-precision and high-stability load cells. Its inherent compensation for temperature effects on zero and span, as well as its ability to achieve minimal sensitivity to off-center loads and bending moments through strategic strain gage placement, make it the standard for demanding applications. This configuration offers the highest sensitivity and linearity.
These devices are considered high-quality and worth a long-term investment because they use four active strain gauges and have a more complicated installation procedure. Their superior performance often justifies this investment for applications requiring reliable and precise measurements.
TIP: Industrial weighing systems, precision force measurement in materials testing and calibration , aerospace and automotive testing where complex loads and temperature fluctuations are standard, high-performance scales for legal metrology applications, and critical process control systems. Interface's load cell offering in these areas predominantly utilizes full-bridge configurations to ensure optimal performance.
Bridges and instrument connections – the most important technical findings
Interface's many years of experience in load cell design and manufacturing makes it clear that the choice of strain gauge bridge configuration is a fundamental engineering decision based on the specific requirements of the application.
Factors such as the required accuracy, the anticipated operating environment, potential extraneous loads, and budget constraints must be carefully considered. While quarter bridges offer initial cost savings, the improved performance and inherent compensation capabilities of half- and especially full-bridge configurations often provide the most reliable and accurate long-term solutions for demanding force and weight measurement applications.
Understanding these trade-offs, which is highlighted in resources such as Interface's Load Cell Field Guide , is crucial for engineers across various disciplines.