1. Field of the Invention
This invention relates to the control of an electric motor, in particular of a brushless electric motor, whereby the absolute phase position of a rotor in relation to a stator of the electric motor is determined.
In the following, the terms "Rotor" and "Stator" shall apply only to the functional description of the basic elements of an electric motor, which either change their position with respect to an external coordinate system (rotate or move)--the rotor--, or are fixed with respect to an external coordinate system--the stator.
During the start-up of the electric motor, it is desirable to know the exact angular position or phase position of the rotor in relation to the respective stator, in order to be able to generate a suitable torque on the rotor. In known methods or apparatus, this information is obtained by measuring an absolute position. For this purpose, for example, an absolute measuring system is attached to the shaft of the rotor, to which the rotor is attached. The absolute measuring system indicates any given moment the exact angular position of the rotor in relation to the stator.
2. Description of the Related Art
Often, so called resolvers or Hall sensors are employed as absolute measuring systems. The resolvers are, for instance, induction sensors or rotatable transformers. The Hall sensor usually consists of three sensors which are arranged on the shaft and offset with respect to each other by 120.degree.. Such absolute measuring systems are arranged on the electric motor in such fashion that they are able to indicate the absolute phase position of the rotor in relation to the stator under all operating condition--even when the controls for the electric motor are switched off. Their control and operation, however, requires sophisticated and therefore expensive electronic circuits.
In a known control process particularly for synchronous motors having brushes and brushholders, respectively, the absolute phase position is determined from the phase position of the sinusoidal voltage induced at the brushholders. The disadvantage of this process, however, is that it is only functional when the rotor rotates, i.e. when the electric motor is operating. Often, however,--e.g. when electric power is switched on--electric motors must not move at all or move only slightly. It is, for instance, particularly important for an apparatus for the electroerosive processing of work pieces where electric motors control the movement of work table and/or the guide the electrodes, that the exact phase position of the rotor in relation to the stator is known when the electric motor is turned on, and, on the other hand, uncontrolled rotation of the rotor during the start-up is prevented, if at all possible. These requirements become especially obvious in view of the high precision required during the operation of such apparatus. When the electric motors, for instance, control the guide heads for the electrodes and the cutting wire, respectively, even unintentional movements in the range of micrometers can have a significant adverse effect on the processing of the work pieces. Uncontrolled movements of the electric motor can even damage the mechanical device (guide head) or the work pieces to be processed.