**Foreword**
Analog sensors are widely used in various fields, including industry, agriculture, defense, daily life, education, and scientific research. However, one of the key challenges in their design and application is achieving maximum measurement accuracy.
In practice, many disturbances can affect the precision of these sensors. For instance, large energy-consuming devices on-site—especially high-power inductive loads—can cause voltage spikes of hundreds or even thousands of volts when they start or stop. Additionally, pressure fluctuations or overvoltage (such as the voltage at a steel plant fluctuating between 160V and 310V) can reach up to 35% of the rated voltage. These poor power conditions may last for minutes, hours, or even days. Signal lines that are bundled together, especially when sharing multi-core cables with AC power lines, can introduce noise and signal distortion. Poor performance of multi-way switches or keepers can also lead to signal interference. Moreover, electromagnetic, meteorological conditions, lightning, and changes in the Earth's magnetic field can all disrupt sensor operations.
Environmental factors like temperature and humidity changes can alter circuit parameters. Corrosive gases, acid and alkali salts, wind, sand, rain, and even insects can impact sensor reliability. The output of an analog sensor is typically a small signal, which requires careful amplification, processing, shaping, and anti-interference measures. This means accurately amplifying the weak signal to a standardized output (e.g., 1–5 VDC or 4–20 mA) while meeting technical specifications. Designers must pay attention to hidden issues like interference, which are not always visible on circuit diagrams. Understanding the sources and types of interference is crucial to designing circuits that either eliminate or prevent it, ensuring optimal sensor performance.
**Sources, Types, and Phenomena of Interference**
Sensors and instruments face various disturbances during field operation. A tailored approach is essential for each specific situation, making a modular design ideal. By combining basic components with optional modules, systems can better resist interference and improve reliability. Before discussing component selection and system design, it's important to understand the sources and types of interference affecting analog sensors.
**Main Sources of Interference**
1. **Static Induction** – Caused by parasitic capacitance between two branches, leading to charge transfer.
2. **Electromagnetic Induction** – Occurs due to mutual inductance between circuits, causing interference through magnetic fields.
3. **Leakage Current Sensing** – Poor insulation can increase leakage current, especially in harsh environments.
4. **Radio Frequency Interference** – Generated by large equipment, thyristor rectifiers, and other high-harmonic sources.
5. **Other Interference** – Includes mechanical, thermal, and chemical interferences from poor working conditions.
**Types of Interference**
1. **Normal Mode Interference** – Interference present on both signal lines, often caused by strong magnetic fields.
2. **Common Mode Interference** – Interference that appears on both signal lines relative to ground, often due to ground potential differences.
3. **Long-Term Interference** – Persistent interference, such as 50 Hz power line noise.
4. **Transient Interference** – Sudden interference from switching events, lightning, or radio equipment.
Interference can be categorized into three main areas: local production, internal coupling, and external generation.
**Interference Phenomena**
Common issues include irregular motor rotation, unstable digital displays, mismatched sensor outputs, and device malfunctions. Signal transmission and power supply channels are the main sources of interference, with long-line transmission and internal resistance contributing to noise.
**Anti-Interference Measures**
**Power Supply Anti-Interference Design**
Voltage spikes from welding machines, motors, and other devices can be mitigated using hardware filters, isolation transformers, and software techniques like time filtering and watchdogs. Separating power supplies and using noise filters can also reduce interference.
**Signal Transmission Channel Anti-Interference**
Optocouplers can isolate control systems from interference, while twisted-pair shielded cables help suppress electromagnetic noise. Differential signaling further improves signal integrity.
**Local Error Elimination**
Careful material selection and soldering techniques are essential to avoid thermoelectric potentials. Reducing the use of connectors and sockets in low-level circuits helps maintain signal accuracy.
**Grounding Issues**
Proper grounding reduces ground noise. Ground loops should be isolated, and multiple power supplies should be connected to a common point for stability.
**Software Filtering**
Digital filtering techniques like averaging, median filtering, and limiting can effectively remove noise from signals, especially in low-frequency applications.
**Other Anti-Interference Technologies**
Voltage regulation, differential amplifiers, and software compensation help mitigate interference from environmental and electrical sources.
**Summary**
Anti-interference is a complex but essential aspect of sensor design. Multiple factors can contribute to interference, so designers must analyze real-world phenomena, consider circuit principles, and implement shielding, power immunity, and proper grounding techniques. Continuous improvements in these areas enhance sensor reliability and stability.
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