Surgical tool system with brushless, sensorless motor

A surgical tool assembly including:

• a handpiece adapted to receive a cutting accessory, said handpiece including a brushless, sensorless DC motor, said motor having:
• three windings that are adapted to be selectively tied between a power source and ground so as to allow a commutation current to flow through a selected two of said windings;
• and a magnetized rotor shaft positioned between said windings so as to be rotated in response to commutation current flow through said windings and to cause a back electromotive signal to develop across the one selected winding through which the commutation current is not flowing, said rotor shaft being configured so that the cutting accessory can be coupled thereto;
• a plurality of conductors over which the commutation current is applied to said windings of said motor of said handpiece and over which said back electromotive signals developed across said winding are present, each said conductor serving as a bi-directional conductor over which the commutation current is applied to the one said winding and said back electromotive signals developed across the one said winding are present for measurement;
• and
• a control unit connected to said conductors that are connected to said windings of said motor of said handpiece so that said control unit is able to apply the commutation current to said windings and to receive said back electromotive signals therefrom, said control unit including:
• an input processor including a speed control switch that allows entry of an operated-selected varying tool speed and a torque input unit for allowing user entry of a maximum torque, said input processor configured to generate a varying speed-set signal representative of the operator-selected tool speed entered through said speed control switch and a torque-limit signal representative of the user-entered maximum torque;
• a power source and a ground;
• a switch unit connected to said conductors, to said power source and to said ground for selectively tying said motor windings to said power source and said ground so as to apply the commutation current through said windings, said switch unit configured to selectively connect each said motor winding to said power source or to said ground based on switch control signals applied thereto;
• and
• a motor control assembly connected to receive said varying speed-set signal, said torque-limit signal and said back electromotive signals and to generate said switch control signals, said motor control assembly being configured to generate said switch control signals based on said varying speed-set signal, said torque-limit signal and the speed of said motor as indicated by said back electromotive signals, wherein said motor control assembly is configured to generate said switch control signals that results in the application of commutation current to said windings of said motor that causes said rotor shaft of said motor to rotate at a shaft speed substantially equal to the tool speed represented by said varying speed-set signal as the torque loads to which the cutting accessory is exposed to vary as long as said motor does not exceed a torque indicated by said torque-limit signal.