**Foreword**
Analog sensors are widely used across various fields, from industrial and agricultural applications to defense, everyday life, education, and scientific research. However, a critical challenge in the design and use of analog sensors is maximizing measurement accuracy.
Numerous disturbances can impact sensor performance. For example, large energy-consuming devices on-site, particularly high-power inductive loads, often cause voltage spikes in the power grid—sometimes reaching hundreds or even thousands of volts. Fluctuations in supply voltage, such as those seen in a steel plant where it ranges between 160V and 310V, can reach up to 35% of the rated voltage. These unstable power conditions may last for minutes, hours, or even days. Additionally, signal lines running alongside multi-core cables can lead to interference, especially when AC power lines share the same conduit with signal lines. Poor performance of multi-way switches or keepers can also introduce noise into the channel. Electromagnetic fields, weather changes, lightning, and variations in Earth’s magnetic field can further disrupt sensor operation.
Environmental factors like temperature and humidity changes can affect circuit parameters, while corrosive gases, acid and alkali salts, wind, sand, rain, and even insects can reduce sensor reliability. Analog sensors typically output small signals, which require careful amplification, processing, and filtering to avoid interference. The goal is to accurately amplify these weak signals to standard levels (e.g., 1VDC–5VDC or 4mADC–20mADC) while meeting technical specifications. This demands attention to anti-interference measures that may not be evident from the circuit diagram alone. Understanding the sources and types of interference is essential for designing circuits that either eliminate or prevent interference, ensuring optimal sensor performance.
**Sources, Types, and Phenomena of Interference**
Sensors and instruments face various interferences during field operations. Addressing each specific situation with tailored solutions is key to effective anti-interference strategies. A modular approach is often adopted, allowing instruments to be equipped with different options based on operational conditions. Before discussing component selection and system design, it's crucial to analyze the sources and types of interference affecting analog sensors.
**Main Sources of Interference**
1. **Static Induction**
Caused by parasitic capacitance between components, leading to charge transfer via capacitive coupling.
2. **Electromagnetic Induction**
Occurs when a change in current in one circuit induces a signal in another through a magnetic field, such as transformer leakage or parallel wiring.
3. **Leakage Current Sensing**
Poor insulation in components or enclosures can increase leakage current, especially in harsh environments, causing interference when it enters the input stage.
4. **Radio Frequency Interference (RFI)**
Generated by large equipment startup, shutdown, or harmonic distortion, such as thyristor rectifier systems.
5. **Other Interference**
Mechanical, thermal, and chemical interferences due to poor working environments can also affect sensor performance.
**Types of Interference**
1. **Normal Mode Interference**
Interference present on both signal lines, often caused by surrounding electromagnetic fields.
2. **Common Mode Interference**
Interference that appears on both signal lines relative to ground, often due to grounding issues or unbalanced lines.
3. **Long-Term Interference**
Persistent interference, such as 50Hz power line noise, which is continuous and relatively easy to detect.
4. **Transient Interference**
Sudden interference from events like switching operations, lightning, or radio equipment.
Interference can generally be categorized into three areas: local production (e.g., thermocouples), internal coupling (e.g., ground paths), and external generation (e.g., power frequency interference).
**Interference Phenomena**
In practical applications, common interference phenomena include:
- Motors rotating irregularly after receiving commands.
- Digital displays jumping when the signal is zero.
- Sensor outputs showing random errors compared to actual values.
- Stable measured parameters producing consistent or periodic output deviations.
- Devices sharing the same power source as an AC servo system failing to operate properly.
There are two main channels for interference in positioning control systems: signal transmission and power supply systems. Signal transmission involves feedback and control signals, and long-line interference is a major issue. Power supplies also contribute to noise, as internal resistance causes interference. AC servo drives themselves can act as strong interference sources, affecting other devices through the power line.
**Anti-Interference Measures**
**Power Supply System Anti-Interference Design**
The most severe threat to sensor and instrument operation is power grid spike interference. Equipment like electric welders, large motors, and relays can generate such spikes. Several methods can suppress them:
1. **Hardware Methods**
- Use spectrum-equalized interference controllers at the power input to spread energy across frequencies.
- Add isolation transformers to suppress spikes using ferromagnetic resonance.
- Install varistors in parallel to reduce voltage surges.
2. **Software Methods**
- Implement time filtering to eliminate periodic interference.
- Use watchdog techniques to reset the system if a "flying program" occurs.
3. **Separate Power Supplies**
Isolate motor drive power from control power to prevent cross-interference.
4. **Noise Filters**
Effectively suppress interference from AC servo drives.
5. **Isolation Transformers**
Reduce common-mode interference by isolating primary and secondary sides.
6. **High-Anti-Interference Power Supplies**
Designed to convert high-peak pulses into low-voltage signals, improving overall immunity.
**Signal Transmission Channel Anti-Jamming Design**
1. **Optocoupler Isolation**
Cut off connections between control systems and servo drives to prevent direct interference.
2. **Twisted-Pair Shielded Cables**
Reduce electromagnetic interference through differential signaling and shielding.
**Local Error Elimination**
Careful material selection in low-level measurements is vital. Thermoelectric potentials from solder joints or connectors can introduce errors. Keeping thermocouple pairs at the same temperature and using high-quality components improves accuracy.
**Grounding Issues**
Proper grounding is essential in low-level amplifier circuits to minimize ground noise. In multi-power systems, grounding all components to a single point helps reduce potential differences and interference.
**Software Filtering**
Intelligent sensors use software filters to remove low-frequency and impulse interferences. Common methods include average value filtering, median filtering, limiting filtering, and inertial filtering.
**Other Anti-Jamming Technologies**
1. **Voltage Regulation**
Regulated power supplies help maintain stable operation despite grid fluctuations.
2. **Common Mode Interference Suppression**
Differential amplifiers reduce the impact of common-mode noise.
3. **Software Compensation**
Adjust for environmental factors like temperature and humidity using pre-defined error curves.
**Summary**
Anti-interference is a complex and practical challenge in sensor design. Multiple factors can cause interference, so proactive measures must be taken during development. Continuous improvement in shielding, power supply immunity, grounding, and protection is essential to enhance sensor reliability and stability.
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