Reference no: EM131199791

In brushless motors, commutation is achieved by switching the stator phases at the correct rotor positions (e.g., at the points of intersection of the static torque curves corresponding to the phases, for achieving maximum average static torque). We have noted that the switching points can be determined by measuring the rotor position using an incremental encoder. Incremental encoders are delicate, costly, cannot operate at high temperatures, and will increase the size and cost of the motor package. Also, precise mounting is required for proper operation. The generated signal may be subjected to electromagnetic interference (EMI) depending on the means of signal transmission. Since we only need to know the switching points (i.e., continuous measurement of rotor position is not necessary) and since these points are uniquely determined by the stator magnetic field distribution, a simpler and cost effective alternative to an encoder for detecting the switching points would be to use Hall-effect sensors. Specifically, Hall-effect sensors are located at switching points around the stator (a sensor ring) and a magnet assembly is located around the rotor (in fact, the magnetic poles of the rotor can serve this purpose without needing an additional set of poles). As the rotor rotates, a magnetic pole on the rotor will trigger an appropriate Hall-effect sensor, thereby generating a switching signal (pulse) for commutation at the proper rotor position. A microelectronic switching circuit (or switching transistor) is actuated by the corresponding pulse. Since Hall-effect sensors have several disadvantages, such as hysteresis (and associated nonsymmetry of the sensor signal), low operating temperature ratings (e.g., 125°C), thermal drift problems, and noise due to stray magnetic fields and EMI, it may be more desirable to use fiber optic sensors for brushless commutation. Describe how the fiber-optic method of motor commutation works.