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The presence of semiconductors in automobiles is increasing geometrically, in part due to automotive systems developers' desire for electronic motor control to address consumer requirements for safer, more efficient cars. Part of this growth in automotive electronics is spurred by replacement of mechanically actuated automotive systems, such as power steering and fuel and water pumps, with systems using electric motor technology.
In the field of 3-phase electric motor control, the majority of control techniques require the use of some position sensors to determine the required rotor position. In order to reduce the cost of the total system, low-resolution position sensors such as Hall sensors are used. On the other hand, the inherent errors due to the use of this type of low-cost sensor reduce the control system performance, and in some applications, this is not acceptable. But with a proper Hall-sensor management strategy, design engineers can reduce the cost of the global system using low-resolution position sensors without affecting the control system performance.
The task
Rotor position sensors are directly mounted on the motor stator and detect the rotor magnetic field generating a voltage signal proportional to the field intensity, as shown in Fig. 1 here.
Magnetic field intensity decreases with the increasing gap between the permanent magnet synchronous machine's (PMSM) magnet poles and the Hall sensor's sensitive element as well as the voltage generated from the sensor. The voltage signal from the Hall sensors has a periodicity in a mechanical rotation that depends on the number of pole pairs and so the mechanical resolution of the single sensor improves with the number of polar pairs while the electric resolution does not vary.
The output of the Hall sensors is analogical, but by means of a Schmitt trigger, the signal becomes digital, and so the information on the rotor position is given by the rising and falling edges of this digital signal.
Typically, an electric motor is equipped with three Hall sensors, put at an angular distance of 120 degrees between each other. Thus 120-degree spacing is preferred over 60-degree spacing since unpowered or unconnected sensors produce 111 or 000 codes, which are discarded as illegal. The rising and falling edges of the Hall sensors and the relative electrical angle are shown in the Fig. 2 below.
Fig. 2: This signal pattern is generated by the three Hall sensors spaced 120 degrees apart.
In the same figure it can be noticed that a rising or a falling edge is at every 60 electric degrees between each other. This resolution is not enough to carry out complex control strategies whose goal is to improve the control system performance in terms of reduced torque ripple produced by the motor or total harmonic distortion (THD) of currents on the motor's phases. So, theoretically, the rotor angular position is known only in correspondence of edges, and the problem arises of the reconstruction of the rotor position between two consecutive edges.
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