**Foreword**
Analog sensors are widely used in various fields, ranging from industrial and agricultural applications to defense construction, everyday life, education, and scientific research. However, a key challenge in the design and use of these sensors is achieving maximum measurement accuracy.
Numerous disturbances can affect the precision of analog sensor readings. For example, large energy-consuming devices on-site often cause voltage spikes—sometimes reaching hundreds or even thousands of volts—especially when high-power inductive loads start or stop. Fluctuations in power supply voltage, such as those seen in steel plants where the voltage may vary between 160V and 310V, can reach up to 35% of the rated voltage. These unstable power conditions can last for minutes, hours, or even days. Additionally, signal lines that are bundled together, especially when sharing the same multi-core cable with AC power lines, can introduce interference. Poor performance of multi-way switches or relays can also lead to signal distortion. Electromagnetic interference, weather conditions, lightning, and even changes in the Earth's magnetic field can disrupt the normal operation of sensors.
Environmental factors like temperature and humidity changes can alter circuit parameters, while corrosive gases, acid and alkali salts, wind, sand, rain, and insects can reduce sensor reliability. Analog sensors typically output small signals, which require careful amplification, processing, and filtering to ensure accurate conversion into standard signals (e.g., 1–5 VDC or 4–20 mA). This process demands attention to anti-interference measures that may not be evident from the circuit diagram alone. Understanding the sources and modes of interference is essential for designing circuits that either eliminate or prevent interference, ensuring optimal sensor performance.
**Sources of Interference**
(1) **Static Induction**
This occurs due to parasitic capacitance between two components, allowing charge transfer via capacitive coupling.
(2) **Electromagnetic Induction**
When current changes in one circuit, it induces a voltage in another through mutual inductance, such as in transformers or parallel wires.
(3) **Leakage Current Sensing**
Poor insulation in electronic components or enclosures can increase leakage current, especially in harsh environments, leading to interference.
(4) **Radio Frequency Interference (RFI)**
Generated by the startup or shutdown of large equipment or harmonic distortions, such as from thyristor rectifiers.
(5) **Other Interference**
Mechanical, thermal, and chemical interferences can also impact sensor performance in poor working environments.
**Types of Interference**
(1) **Normal Mode Interference**
Interference that appears on both signal lines, often caused by alternating magnetic fields.
(2) **Common Mode Interference**
Interference that flows through both lines relative to ground, often due to grounding issues or unbalanced lines.
(3) **Long-Term Interference**
Persistent interference, such as 50 Hz power line noise, which is continuous and measurable.
(4) **Transient Interference**
Sudden interference, such as during switching operations or lightning strikes.
Interference can be categorized into: (a) local production (like unwanted thermocouples), (b) internal coupling (ground path issues), and (c) external generation (power line interference).
**Interference Phenomena**
In practice, common interference issues include irregular motor rotation, jumping digital displays, inconsistent sensor outputs, and unstable readings. Devices sharing the same power source may also malfunction. Signal transmission and power supply channels are major sources of interference, with long-line transmission causing delays, distortions, and noise.
**Anti-Interference Measures**
**Power Supply Design**
The most serious threat to sensor operation is power grid spikes. Devices like electric welders and motors can generate such spikes. Hardware solutions include using spectrum equalization filters, isolation transformers, and varistors. Software methods, such as time-domain filtering, can also suppress periodic interference. A watchdog timer can help reset the system if a "flying program" occurs. Separating drive and control power supplies, using noise filters, and employing isolation transformers can further reduce interference.
**Signal Transmission Design**
Optocouplers isolate control systems from external spikes, improving signal-to-noise ratios. Twisted-pair shielded cables, along with differential signaling, can reduce electromagnetic interference over long distances.
**Local Error Elimination**
Careful material selection and soldering techniques are crucial in low-level measurements. Thermoelectric potentials from solder joints or connectors must be minimized to maintain accuracy. Using low-temperature solder and avoiding unnecessary connections can improve stability and reduce noise.
**Grounding Issues**
Proper grounding is essential to minimize ground noise. In multi-power systems, grounding should be centralized and isolated to avoid potential differences. Voltage and current signals should be chosen based on transmission distance, with current signals being more immune to interference.
**Software Filtering**
Digital filtering techniques, such as averaging, median filtering, limiting, and inertial filtering, can effectively remove various types of interference, including low-frequency noise.
**Additional Anti-Interference Technologies**
Voltage regulation, differential amplifiers, and software compensation techniques can further enhance sensor performance by stabilizing power, reducing common-mode interference, and correcting for environmental effects.
**Summary**
Anti-interference is a complex but essential aspect of sensor design. Multiple factors can contribute to interference, so proactive measures and thorough analysis during debugging are necessary. By continuously refining shielding, power supply immunity, grounding practices, and protection strategies, sensor reliability and stability can be significantly improved.
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