Determining the position of the rotor in an electric motor

#The present invention relates to a method and a device for determining the position of the rotor in an electric motor.

#A precise knowledge of the position of the rotor is of crucial importance for the operation of a brushless electric motor whose speed is determined by the frequency of a supplied alternating voltage in order to be able to start such a motor without using an auxiliary motor and operate it at arbitrarily definable speeds. A conventional technique for determining the rotor position involves attaching a plurality of Hall sensors in the proximity of a tacho disk equipped with magnets which is affixed on the shaft of the motor, which sensors each display the passage of a north or south pole of a magnet of the tacho disk by a signal pulse or a level change of a continuous signal. The number of magnet poles of this type of tacho wheel is generally the same as that of the rotor.

#The energy supply of the motor is generally accomplished by means of an electronic AC/DC inverter which electrically generates a sequence of six switching states at an angular spacing of 60° for each 360° rotation of the magnetic field in the motor (corresponding to a rotation of the shaft of 360°/n for a motor with n pairs of poles). The sequence of switching states generates a rotating magnetic field in the motor which continues to turn electrically from one switching state to the next in steps of 60°. When the rotor turns further through 360°/n in accordance with a complete rotation of the magnetic field, a single Hall sensor only delivers two pulses. However, since six switching states alternate in the course of a rotation of the magnetic field, six switching pulses are required per complete rotation of the magnetic field in order to trigger the transition from one switching state to the next. In order to generate these switching pulses respectively in the correct phase, it is known to arrange a total of three Hall sensors respectively in a solid angle of 120°/n in relation to one another on the tacho disk. These three sensors deliver switching pulses for switching the AC/DC inverter from one state to the next at angular distances of the shaft of 120°/n.

#Further features and advantages of the invention are obtained from the following description of the exemplary embodiments with reference to the appended figures. In the figures:

#FIG. 1 is a highly schematic diagram of a device according to the invention for detecting the position of the rotor and an electric motor controlled using the device;

#FIG. 2 is a schematic circuit diagram of an AC/DC inverter used in the device in FIG. 1;

#In the block diagram in FIG. 1, 1 designates a brushless DC motor whose rotor has n=4 pairs of poles. The DC motor 1 i supplied by an AC/DC inverter 7 whose switches are brought by control circuit 6 in a cyclically alternating manner into six different states each corresponding to magnetic fields in the motor 1 electrically phase-shifted with respect to one another by 60°. A Hall sensor 2 is disposed in the neighbourhood of the rotor to detect the field of each individual pole of the rotor which passes thereby. The Hall sensor 2 delivers an output signal which has an ascending flank in each case when passing a first type of pole and a descending flank when passing the other type of pole. The frequency f of the output signal of the Hall sensor 2 is thus n times the rotational frequency of the motor 1.

#The output signal of the Hall sensor 2 is applied to a first input of a phase comparator 3 whose second input is supplied with a comparison signal whose formation will be explained. The phase comparator 3 can be formed, for example, by an electronic counter which begins to count pulses of a clock signal whenever a descending signal flank arrives from the Hall sensor, the frequency of this signal being a multiple of the frequency f, until a descending signal flank is received at the second signal input and outputs the counter result as the measured value for a phase difference between the two signals. The output of the phase comparator 3 smoothed by a low-pas filter 4 controls a voltage-controlled oscillator 5. In the steady-state mode, this oscillator delivers the comparison signal with the frequency f.

#As a result of a first embodiment, the comparison signal has a form which allows the phase to be deduced from the signal level at any time, for example, a saw-tooth, triangular or sinusoidal shape. The control circuit 6 receives the comparison signal of the oscillator 5 and compares it with a series of reference levels, which correspond for example to the phases 0°, 60°, 120°, . . . , 300° in order to thus detect the corresponding phases and change the state of the switches of the AC/DC inverter 7 at these phases.