There is a general trend in industrial applications to move from motors with brushes, such as DC motors, to synchronous motors. For instance, a growing use of synchronous motors has been witnessed in the semiconductor wafer processing industry where they are used for high precision positioning. The trend towards using brushless motors is motivated by their longer lifetime, lower maintenance costs, and improved cleanliness of operation. For example, motor brushes experience wear and tear, which can result in spreading out small dust particles that can be injurious to semiconductor wafers and other work pieces. Synchronous motors therefore make a more reliable solution since they are not subjected to the wear of brushes.
To eliminate mechanical contacts, current commutations are performed electronically for a brushless or synchronous motor. To perform these commutations, a controller driving a synchronous motor ideally has information on the reference or initial phase of the motor. Putting the control currents in exact phase with the motor yields an optimal use of the motor. A lack of knowledge of the reference phase can make it difficult or impossible to avoid heating and to achieve desired precision control. Therefore, the determination of a reference phase is desired for precise synchronous motor control.
To get information on the reference phase, state of the art solutions require additional undesirable hardware to be added to the motor. For example, Hall effect sensors can be used to give a direct measurement of the magnetic field inside the motor. Resolvers may also be used to get a reference position since they provide absolute position or angle measurements. Finally, by measuring current inside the motor, the back electromotive force created by an electrical circuit moving into a magnetic field can lead to a determination of a reference position. No matter how efficient these solutions are, however, they require additional components to determine the reference phase, and these components, especially Hall effect sensors and resolvers, are quite expensive. Furthermore, adding new components means adding wires, connectors, and redesigning communication protocols and new algorithms to process the information from these dedicated components.
Existing solutions with none of the aforementioned hardware additions are based on the periodic magnetic field inside a motor. One such solution injects a constant reference current to one electrical phase of the motor, thereby making it stabilize where the magnetic field is zero, which may not be a desired position. This solution, which is based upon measuring how far the motor moves to arrive at its in-phase position, subjects the motor to an unknown and uncontrolled movement, which may not be acceptable in some applications. This solution may work provided friction is not too significant, but friction can cause the resting position to differ from the true in-phase position. Moreover, this solution does not meaningfully take advantage of the motor's dynamic behavior. Finally, when using this solution, one can witness erratic position shifts around the equilibrium position before stabilizing. These movements can be as large as the magnetic pitch, which may be a couple of millimeters, and such large oscillations are not acceptable for some applications.