Several sensors for detecting the rotor position are available, e.g. incremental indicators or resolvers. Unfortunately they are too expensive for usage in a low cost drive. On the other hand, there is no need for high resolution position sensors in this brushless DC motor design, because the commutation of this motor needs only one pulse every (el.). This low resolution can be achieved by using an optical sensor, detecting marks on the rotor shaft or by placing a hall sensor in the motor windings, detecting the rotor stray field. Optical sensors must not be used in textile machines because of the dust, resulting from the textile fibers. Hall sensors in the motor windings are difficult to mount - and therefore quite expensive. They are sensitive to distortion, e.g. the one caused by PWM.
In any case, explicit position sensors need additional wiring which increases the risk of failure as well as the costs of mounting.