External Wiring in Strain Gauge Load Cells

Strain gauge load cells are widely used in various industries for precise force measurement. These devices rely on internal bridge circuitry and require proper external wiring to ensure accurate readings. In this article, we will explore the differences between the four-wire and six-wire configurations in strain gauge load cells, shedding light on their respective advantages and considerations.

The Four-Wire Load Cell Configuration

The four-wire system is a commonly used wiring configuration for strain gauge load cells. It consists of two wires that supply power to the Wheatstone bridge, known as the positive and negative input terminals. The other two wires are the positive and negative signal output terminals of the bridge. This setup is typically found in bending beam load cells.

While textbooks often depict a lumped parameter model of the strain gauge Wheatstone bridge, real-world applications involve wires that have detectable resistance. This resistance can cause voltage drops, which may affect the load cell's calibration. Therefore, it is crucial to ensure that the length of the wires connecting the load cell remains unchanged throughout its usable life. It's worth noting that cutting cables can alter the calibration, while inserting a cable connector in a same-length cable run will not significantly affect the load cell's calibration.

The Six-Wire Load Cell Configuration

In situations where longer cables or a more accurate model of the bridge circuit are required, a six-wire load cell configuration is often preferred. Unlike the four-wire setup, the six-wire configuration accounts for the effects of distributed parameters, including the resistance, length, cross-sectional area, and resistance variation with temperature of the connecting wires. It features the same power supply/excitation and signal output terminal wires as the four-wire setup but includes an additional positive and negative sense wire.

The sense wires connect to the same nodes as the power supply wires and are routed to an amplifier's input port. The output of the amplifier then connects back to the power supply terminals of the load cell, forming a loop. This design allows the amplifier to detect the actual voltage powering the load cell and make necessary adjustments to maintain the desired operating level. By compensating for voltage drops across the cables, which can vary with temperature, the system becomes less sensitive to changes in cabling and ensures accurate readings. Essentially, the six-wire system integrates a simple voltage controller design with the Wheatstone bridge.

Additional Wiring Considerations: Shield Wire

In both the four-wire and six-wire configurations, an additional wire known as the shield wire may be present. The shield wire is not connected to the strain gauge circuitry but rather to the body or structural element of the transducer. Its purpose is to protect the internal circuitry from electromagnetic interference, enhancing the overall performance and reliability of the load cell.

Understanding the different wiring configurations in strain gauge load cells is crucial for selecting the appropriate setup based on specific application requirements. The four-wire configuration is commonly used and can provide accurate results when cable length remains constant. On the other hand, the six-wire configuration offers greater accuracy by compensating for voltage drops and changes in cabling. Additionally, the inclusion of a shield wire helps protect the load cell from electromagnetic interference.

When working with strain gauge load cells, it is essential to consider factors such as cable length, resistance, and environmental conditions to ensure reliable and precise force measurements. By selecting the appropriate wiring configuration and following best practices, you can optimize the performance of strain gauge load cells in your applications.