Anti-jamming measures for analog sensors - Solutions - Huaqiang

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1. Anti-interference design of the power supply system: The most critical threat to the normal operation of sensors and instruments is voltage spikes from the power grid. These spikes often come from devices like electric welders, large motors, variable frequency drives, relays, ballasts in fluorescent lights, and even electric irons. To counter this, a combination of hardware and software solutions can be applied. (1) Hardware-based methods to suppress spike interference include: - Installing a spectrum-equalization-based interference controller at the input of the AC power supply. This spreads out the energy of peak voltages across different frequencies, reducing their harmful effects. - Adding a super-isolation transformer at the input end, which uses ferromagnetic resonance to suppress spikes. - Using a varistor in parallel with the AC power input. When a spike occurs, the varistor lowers its resistance, protecting the instrument by reducing the voltage it receives. (2) Software-based suppression involves time filtering for periodic interference. By programming the system to control thyristor conduction without instantaneous sampling, the impact of interference can be minimized effectively. (3) A watchdog technique that combines both hardware and software can also help. The CPU periodically resets the timer, preventing system crashes due to unexpected program flow caused by spikes. If a "flying program" occurs, the timer will trigger a reset, ensuring the system returns to normal operation. (4) Separating power supplies—such as isolating motor drive power from control power—can prevent cross-interference between devices. (5) Noise filters are effective in suppressing interference from AC servo drives, helping to reduce various types of disturbances. (6) Isolation transformers are designed to handle high-frequency noise. They use parasitic capacitance between primary and secondary coils rather than mutual inductance, and proper shielding reduces distributed capacitance, improving common-mode interference resistance. (7) High-performance anti-interference power supplies, such as those using spectrum equalization techniques, are especially effective against random interference. These power supplies convert high-peak voltage pulses into low-voltage ones (below TTL levels), while keeping the energy of the pulse constant, thus enhancing the sensor’s immunity. 2. Anti-interference design of signal transmission channels: (1) Optocouplers are used to isolate input and output channels, preventing external spikes from entering the system or directly affecting the servo driver. This is crucial for avoiding the first type of interference. The main advantage of optocouplers is their ability to suppress spikes and noise, significantly improving the signal-to-noise ratio. Even with high voltage interference, the current through the LED remains low, so the interference has minimal impact. (2) For long-distance transmission, twisted-pair shielded cables are commonly used. These cables are less affected by electric and magnetic fields and ground impedance. Compared to coaxial cables, they offer higher bandwidth and better common-mode noise rejection. Differential signaling further enhances anti-interference performance, making them ideal for suppressing multiple types of interference during long-distance transmission. 3. Eliminating local errors: In low-level measurements, attention must be paid to materials used in the signal path. Solder, wires, and connectors can generate thermoelectric potentials, especially when used in pairs. Keeping these pairs at the same temperature helps reduce their impact. Heat shields, heat sinks, and circuit separation are used to minimize thermal gradients. The junctions of standard components, like nickel-chromium-constantan thermocouples, can produce temperature drift of up to 0.2mV/°C, which is significant in precision applications. Op-amps like the OP-27CP have twice the temperature drift of chopper amplifiers like the 7650CPA. While switches, connectors, and relays make component replacement easier, they introduce contact resistance and thermoelectric potential, leading to reduced accuracy and reliability. Therefore, minimizing the use of such components in low-level systems is essential for maintaining high precision and stability.