Open-Loop and Closed-Loop Control (Electric Motors)

10.7.3
The cost of position sensors and the complexity associated with position estimators have resulted in economical open-loop controllers that sequence the phase coil commutation without any position feedback at low operation frequencies, resulting in low speeds of operation. The greatest advantage of such systems is that a motor with gear reduction can achieve extremely accurate positioning capability with high torque output. An example of such a system is found in automotive cruise control,

FIGURE 10.36 Torque-speed characteristics of the SRM drive.
where the throttle plate is positioned in real time to match the commanded speed set by the driver. The motor operating speed required to achieve this positioning is relatively low; hence, an open-loop system can be used.
In variable-speed drives, the operational speed is high; hence, the use of closed-loop feedback for commutation is mandatory. Position feedback mechanisms using optointerrupters with slotted disks, resolvers, and encoders are commonly employed at present. However, research in position estimators has led to the development of sensorless commutation methods that use the variation in the coil inductance to estimate commutation instants.