Causes of Interference and Solutions
Implementing monitoring and control during the production process requires a variety of instruments, control systems, and actuators. Signal transmission among these components encompasses both very weak signals—on the order of millivolts or milliamperes—and high‑level signals reaching tens of volts, or even thousands of volts and hundreds of amperes.
2025-05-23
In the production process, monitoring and control require a variety of instruments, control systems, and actuators. Signal transmission among these components encompasses both very weak signals—on the order of millivolts or milliamperes—and high‑level signals reaching tens of volts, or even thousands of volts and hundreds of amperes; it also includes low‑frequency DC signals as well as high‑frequency pulse signals. After integrating such systems, it is often observed that signal interference between instruments and equipment can lead to system instability. In addition to inherent performance limitations of individual instruments and devices—such as poor electromagnetic interference immunity—the primary causes of this issue are as follows.
“Ground Loop” Effects
For various reasons, a system connection may exhibit multiple grounding points, which can result in potential differences between the signal reference nodes of different devices, thereby forming a “ground loop” that causes distortion during signal transmission (as shown in Figure 1).
To address signal distortion caused by “ground loops,” theoretical and practical analyses indicate three possible solutions:
The first type : All field devices are left ungrounded, so the process loop has only a single grounding point and no closed ground path. While this approach appears straightforward, it is often difficult to implement in practice, as some equipment must be grounded to ensure measurement accuracy or safeguard personnel. Additionally, certain devices may develop new ground paths due to insulation degradation caused by prolonged corrosion, mechanical wear, or environmental factors.
The second type : The goal is to ensure that the potentials at the two grounding points are equal (as shown in Figure 1, V1 = V2); however, since the resistance of a grounding point is influenced by numerous factors—including geological conditions and climate variations—this approach is, in practice, impossible to achieve perfectly.
The third type : By employing signal isolation in each process loop, the loop can be broken without compromising the normal transmission of process signals, thereby effectively resolving ground-loop issues (as shown in Figure 2).
Electromagnetic interference, signal leakage effects
In industrial process monitoring systems, unstable measurement signals are frequently encountered. One cause is electromagnetic interference (as shown in Figure 3). Another possibility is the intrusion of high-frequency signals. For example, when a current‑signal output controls a variable‑frequency drive, high‑frequency noise from the drive can couple into the signal, often resulting in erratic or abnormal operation of both the drive and the associated valves (as shown in Figure 4).
To address electromagnetic interference and the impact of high-frequency signal leakage, based on experimental experience, adding an appropriate Signal Isolator between the signal connections of two devices is one of the effective methods. Diagrams are shown in Figures 5, 6, and 7.
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