in the zero cross detectors 32 33 the time between zero crossings is used to measure the period t of the sensor output voltage signal the signal frequency freq 1 t the shaft speed in rpm is then freq 60 n where n is the number of sensor pole pairs 6 for the pmm shown in fig1 and 60 is the conversion factor from seconds to minutes the signal period t may be measured by two methods suitable for low and high rotation speeds respectively at low speed the period may be expressed as a multiple of a control cycle time tc the number y of control cycles between successive sensor zero crossings is counted and the period t is then t tcy this method gives acceptable resolution provided that the period t is long compared to tc at higher speeds when t approximates tc the resolution is too coarse for an accurate rotation speed measurement in a further embodiment a processor capture module is used at higher rotation speeds to measure the period t the capture module is a hardware counter timer included in a processor as part of the vehicle control unit typically the processor has a 16 bit counter that will overflow if used at low speeds as shown in fig4 in this embodiment independent speed sensing is provided for each of the two stator windings with independent outputs 15 17 for an increased level of fault tolerance in applications where the rotation direction must be sensed in addition to the rotation speed the two independent outputs 15 17 are.