Improved shielding and filtering applied to immunity enhancement of underground gas sensors

In order to eliminate false alarms, issued by gas sensors in coal mining, caused by Electromagnetic Interference (EMI), both computer simulation and field measurements were introduced to analyze the underground EMI distribution. A simplified model of a sensor with metal enclosure was established and the effects of shielding properties about the enclosure aperture were studied. Because the haulage motor is the moving EMI source, varying with time, the onsite flameproof measuring instruments cannot accomplish synchronous measurements of electromagnetic field vectors. To simplify the field measurements, two sensors, one with a lead and the other without a lead, were chosen to conduct the contrasting measurements. The EMI current caused by the perforation lead was comparatively strong and therefore nickel zinc ferrite beads were used to cut off the EMI propagation paths. The peak value of the interference current was reduced by 20%–70% with the beads. After switching on the sensor power, the sensors still occasionally gave false alarms when the switch of nearby large-scale electric equipment was operated. A complex EMI filter was used and the EMI attenuated markedly. The running results demonstrated that false alarms had been eliminated. We conclude that the improved shielding and filtering are highly significant in enhancing the immunity of the gas sensor.