Strong magnetic fields (such as motors, electromagnets, high-current busbars, welding machines, and excitation coils) can cause: signal drift, numerical jumps, random zero point floating, and reading jitter. The anti-magnetic capabilities of different sensors vary greatly. Priority should be given to handling them in sequence from selection → hardware wiring → shielding and grounding → installation layout.
I. Selection level (Fundamentally, many on-site wiring modifications that are ineffective are due to the wrong sensor selection)
1.Try to avoid types that are easily disturbed by magnetic fields
2. Clear selection requirements: Anti-electromagnetic interference grade (EMC). For high-magnetic field conditions, industrial-grade versions with fully shielded metal enclosures are preferred.
II. Wiring and Shielding (4-20mA/RS485 Signal)
1.Twisted-pair shielded cables must be used; ordinary cables won't do
Analog quantity 4-20mA: Shielded twisted pair; Digital RS485: Dedicated twisted-pair shielded wire.
The shielding layer is grounded at one end!! Strong magnetic fields must not be grounded at both ends
Grounding at both ends will form a ground loop, and the magnetic field cutting through the cable will generate induced current, causing even greater interference. Generally, the sensor end is suspended, and the shielding layer on the side of the control cabinet is connected to the PE protective ground.
2. Cable routing isolation (very crucial)
Signal cables and power high-current cables should be routed separately, maintaining a distance of ≥30-50cm. The same conduit and the same steel pipe cannot be run through.
If crossing is necessary: Cross vertically at 90°. Do not lay them side by side in parallel. Parallel wiring will be disturbed by magnetic field coupling.
Signal cables should not be placed beside electromagnets, motor power lines or large copper bars.
The cables should be as short as possible and not have extra large coils. A coil is equivalent to a coil. Under a strong magnetic field, it will induce interference voltage.
III. Grounding System
1. The sensor housing and sensor shielding housing should be connected to the protective ground PE. Do not connect to the signal negative ground or the analog ground AGND.
2. Inside the control cabinet: The simulated ground AGND and the power ground PE converge at a single point. Multi-point grounding is strictly prohibited to prevent ground circulating current.
3. Avoid sharing the same grounding stake with the sensor and strong magnetic equipment.
IV. Hardware Filtering processing
Analog output (4-20mA)
1.PLC analog module side: Enable hardware filtering and increase the filtering time (100ms-500ms, depending on the on-site response speed acceptance level).
2. Some can add RC filtering at both ends of the signal (note that it should not affect the response speed); Don't blindly increase filtering, as it will sacrifice dynamic response.
Digital signal (RS485)
1.Terminal resistor: Add a 120Ω terminal resistor at both ends of the bus.
2. Reduce the communication baud rate. The lower the baud rate, the stronger the anti-interference ability. With a lot of interference, the baud rate has dropped.
Note: Software filtering can only "mask jitter", but cannot eliminate the root interference. If the magnetic field is too strong, the value will still drift slowly even after the filter is turned up.
V. Installation of mechanical isolation (Obvious effect on strong magnetic field)
1. Keep the sensor body away from the magnetic field source: electromagnet, excitation coil, high-power motor. When the distance is increased, the magnetic field intensity decays with the square of the distance. When the distance is slightly increased, the interference drops significantly.
2. Magnetic shielding: Use high-permeability steel (silicon steel sheets, permalloy) as a shielding cover to enclose the sensor
⚠ Magnetic shielding ≠ ordinary iron sheet shell! Ordinary thin iron sheets are almost ineffective against low-frequency strong magnetic fields; high-permeability materials should be used. The shielding cover must be grounded.
The magnetic ring/magnetic float of the magnetostrictive sensor should be kept away from the direct radiation area of the magnet and electromagnet.
VI. Tips for Quick On-site Judgment
1. Unplug the sensor signal line from the module and check if the module reading jumps
Still jumping → Module, grounding, cabinet interference;
No jump → Interference comes from external cables/the sensor body is affected by the magnetic field.
2. When the handheld sensor is taken away from the strong magnetic field of the device and powered on to check the reading: If the signal is normal after being taken away, it indicates that it is caused by an external magnetic field.
The priority of the simplest processing (practical sequence)
Keep the sensor, cable and strong magnetic source at a distance. Separate the cable from the power line and cross them vertically
Check the single-end grounding of the shielding layer to eliminate ground loops
Enable module filtering and reduce the communication baud rate
Add a high-permeability shielding cover
Replace the anti-magnetic sensor