Magnetic Geartooth Sensing The need to sense speed and position of ferrous gears occurs in numerous industries. The ability to convert the repetitive passing teeth to an electrical impulse has been sought for many decades. Purely mechanical systems have been used with the attendant issue of wear and failure limiting its use to low speed and low duty cycle applications. Hall effect geartooth sensing makes use of the Hall element to sense the variation in flux found in the airgap between a magnet and passing ferrous gearteeth. A modern approach is to convert the signal from the Hall element to a digital value and then perform signal processing to create a digital output from that effort. In the case of the Melexis geartooth sensing scheme, each time the signal changes direction a counter is reset. If the signal level changes beyond the preset magnitude from the positive or negative peak the output level is changed. This creates a digital zero speed peak detection speed sensor. It is immune to orientation requirements and can follow the gear speed down to the cessation of motion. It will detect the first edge of the next tooth immediately after power on. The digital signal processing does introduce an uncertainty from quantization that is greater at larger speeds. Extremely demanding timing requirements like those found in crank position sensors may suffer from the loss of accuracy at high speeds. Animation above shows the Melexis MLX90217 geartooth sensor operation.
Gear Tooth Sensor Magnetics In order to detect the passing gear teeth with a Hall effect sensor it is necessary to provide a source of magnetic energy. The simple way to do this is to arrange a permanent magnet such that the axis of magnetization is pointing toward to surface of the gear teeth. As a tooth moves across the surface of the magnet the flux will become attracted to the lower reluctance path provided by the ferrous steel structure. When this occurs the flux density measured by the Hall element between the face of the sensor and the gear tooth increases. Many schemes have been developed and some patented that use the various attributes of the vector flux field and its changing nature to create zero speed Hall effect gear tooth sensors. Melexis has chosen to work with digital signal processing schemes and in this way minimize the magnetic circuit manipulation required of the end user. Put simply, by applying silicon "smarts" the magnetic subtlety and "sleight of hand" is nearly eliminated.
