Estimation of the shaft position on low-cost DC actuators

Low power DC motors are today largely used as actuators in home automation, office automation, medical equipment and automotive fields. Among automotive applications of DC motors, those regarding windows lifters, seat and mirror adjustment or multi-zone air conditioning, require a quite precise position control, leading to the introduction of suitable sensors. Unfortunately, the extra cost of even a simple Hall effect position sensor may result unacceptable on these systems. In this paper a robust but inexpensive approach is described to sensorless estimate the angular shaft position by counting the periodical pulses of the armature current. Such a pulse counting approach is very simple in principle, as a constant amount of current pulses per round are generated in low power DC motors without any specific intervention. However, the frequency of the current pulses widely varies according to the speed, thus requiring a relatively complex and expensive adjustable bandwidth filter to be correctly detected. The proposed approach suitably solves this problem, leading to the implementation of an accurate position sensorless control system even on ultra-low cost microcontrollers equipping several electrically actuated automotive devices.