Background
On assembly lines for permanent-magnet motors, magnetic clasps, and magnetic toys, the most common low-skill failure is “magnet installed backwards” — two magnets look identical to the eye but have opposite polarity, and the finished product fails entirely. One brushless-motor factory’s 2024 data: each backwards-installed magnet incurred 38 RMB in rework, against an annual assembly volume over 10 million pieces — millions of RMB in losses from this one cause alone. A handheld pole gauge is a cheap, one-second N/S verification tool any line operator can use.
How it works
The instrument carries a Hall sensor or MR sensor that detects the magnetic-field direction at a magnet’s surface. As the probe nears the magnet, the Hall element’s output voltage flips with field direction; the built-in comparator translates the voltage sign into an N or S indication — typically red LED for N, green for S, with an audible beep. Response time < 200 ms, sensitivity ample for 5 mT-class fields.
Application notes
Three things to watch on-line: (1) re-calibrate the probe regularly against a reference block, especially in temperature-variable shops; (2) keep the probe within 5 mm of the magnet surface — too far gives false readings; (3) inter-magnet coupling fields can offset single readings, so design a shielded fixture for dense arrays. One inverter-AC motor plant cut backwards-install defects from 1500 PPM to under 3 PPM, paying back the gauges in six months.
Selection guide
Three criteria: measurement range covers 1.5× the strongest expected surface field; a data interface (USB / RS-485) for MES integration — scan barcode and the polarity logs automatically; probe shape matched to the workpiece — Φ2 mm micro-probe for SMD magnets, Φ8 mm standard probe for large motor magnet poles. Horus HPM series spans 0–2 T and supports custom probe geometries.
