In industrial weighing systems, commercial trade, and scientific research on weighing instruments, the wiring distance between the load cell and the instrument is not a fixed, standardized value. Instead, it requires a comprehensive balance based on factors like signal characteristics, equipment parameters, environmental conditions, and engineering requirements. The core objective is to find a balance between "signal integrity" and "engineering feasibility." This article will analyze the principles of determining the wiring distance, the impact of long-distance wiring on core signals, and optimization methods, providing a reference for system design and construction.
I. The "Scientific Distance" from Load Cell to Instrument: A Core Logic with No Fixed Value
The signal output from the load cell is typically a micro-level analog voltage signal (e.g., mV-level signal). Its full-scale output is generally around $2 text{mV/V}$ to $4 text{mV/V}$. This class of signal is highly susceptible to loss and interference. Therefore, the essence of the "scientific distance" is the maximum acceptable distance where signal attenuation and interference control meet the specified weighing requirements, and it is not a fixed value. Its determination revolves around the following four core dimensions:
1. Fundamental Basis: Load Cell Signal Type and Instrument Reception Capability
The transmission capacity of different signal types varies significantly, which is the primary factor determining the upper limit of the wiring distance. Specifically, they can be divided into two categories:
For example, the commonly used combination of "analog load cell + analog weighing instrument" utilizes a mV-level signal. When the wiring exceeds 10 meters in a common shielded cable, the wire resistance causes signal attenuation. (The resistance of a $10 text{m}$ copper wire is $0.1Omega$. With a sensor excitation current of $50 text{mA}$, the voltage drop across the wire can offset a $0.5 text{mV}$ signal, which is more than 12.5% of the full scale), directly affecting accuracy.
In contrast, sensors using RS485 digital output (e.g., smart weighing modules) and utilizing differential signaling can transmit signals without discernible attenuation for up to 100 meters, and some models can even extend the distance to 1200 meters (requires a repeater).
2. Key Variable: Conductor Specifications and Materials
The cross-sectional area and material of the conductor significantly influence the rate of signal attenuation and are important adjustment factors for the "scientific distance."
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Material
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It is preferable to choose high-purity copper conductors (e.g., oxygen-free copper), as their resistivity (at $20^circ text{C}$ is $0.0172Omega cdot text{mm}^2/text{m}$) is much lower than that of aluminum-alloy conductors (about $0.0283Omega cdot text{mm}^2/text{m}$), which can effectively reduce signal loss.
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Cross-sectional Area
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For conventional analog signal transmission, conductors with a cross-sectional area $ge 0.3 text{mm}^2$ are typically selected. If the required distance is up to $15 text{m}$ to $20 text{m}$, the area should be upgraded to
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