**Foreword**
Analog sensors are widely used in various fields, from industrial and agricultural applications to defense, daily life, education, and scientific research. However, one of the main challenges in designing and using analog sensors is maximizing measurement accuracy. Interference can significantly impact the reliability and precision of these devices.
Several factors contribute to this issue. For example, large energy-consuming devices on-site, especially high-power inductive loads, can cause voltage spikes that reach hundreds or even thousands of volts. Power supply fluctuations, such as those seen in steel plants where voltages vary between 160V and 310V, can lead to overvoltage conditions. These issues may persist for minutes, hours, or even days. Additionally, signal lines often share the same multi-core cables, leading to interference, especially when they run alongside AC power lines in long conduits. Poorly designed switches or relays can also introduce noise into the system. Environmental factors like electromagnetic interference, weather changes, lightning, and even variations in the Earth’s magnetic field can disrupt sensor performance.
Moreover, environmental factors such as temperature and humidity changes can alter circuit parameters, while corrosive gases, dust, rain, and insects can affect the sensor's long-term reliability. Analog sensors typically output small signals, which require careful amplification, processing, and filtering to ensure accurate conversion into standardized signals (e.g., 1–5 VDC or 4–20 mA). This process demands attention to anti-interference measures that are not always evident from the circuit diagram. Understanding the sources and modes of interference is crucial to designing systems that operate reliably under real-world conditions.
**Sources of Interference**
Interference can come from multiple sources. Static induction, for instance, occurs due to parasitic capacitance between components, allowing charges to transfer through capacitive coupling. Electromagnetic induction arises when a changing current in one circuit induces a voltage in another through magnetic fields. Leakage currents can occur due to poor insulation, especially in harsh environments, causing unwanted interference. Radio frequency interference from large equipment or thyristor rectifiers can also degrade performance. Other types of interference include mechanical, thermal, and chemical disturbances caused by poor working conditions.
**Types of Interference**
Interference can be classified into normal mode and common mode. Normal mode interference affects both signal lines equally, often due to surrounding magnetic fields. Common mode interference involves signals flowing through both lines and ground, and it can be converted into normal mode interference due to line imbalances. Long-term interference, such as 50 Hz power line noise, is continuous and easier to detect, while transient interference occurs during equipment operation, such as switching or lightning events.
**Common Interference Phenomena**
In practical applications, common issues include irregular motor rotation, unexpected jumps in digital displays, random errors in sensor outputs, and stable but incorrect readings. Devices sharing the same power source may also experience malfunctions. Signal transmission and power supply channels are particularly vulnerable to interference, with long-distance transmission leading to delays, distortions, and signal loss.
**Anti-Interference Measures**
To mitigate these issues, several strategies can be employed. Power supply design should include hardware filters, isolation transformers, and surge suppressors to reduce voltage spikes. Software techniques such as averaging, median filtering, and limiting can help eliminate noise. Signal transmission can benefit from shielded twisted-pair cables and optocouplers to isolate sensitive parts of the system. Grounding must be carefully managed to avoid ground loops and potential differences. Additional measures include voltage regulation, differential amplification, and software compensation for environmental changes.
**Conclusion**
Anti-interference is a complex but essential aspect of sensor design. Multiple factors can contribute to interference, so a comprehensive approach involving hardware, software, and proper installation is necessary. Continuous improvement in shielding, power supply stability, grounding, and protection ensures reliable and accurate sensor performance in real-world conditions.
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