The present invention concerns a method for determining the angular position of the permanent magnet rotor of a multiphase electric motor including at least two stator coils. The exact determination of the angular position of the rotor of an electric motor is desired, or even necessary for several applications, in particular in the field of robotics and for devices with rotating tools such as those used in dental care and micro-surgery.
There is known an arrangement of three Hall sensors, which are offset by 120xc2x0 and which provide substantially three sinusoidal curves offset by 120xc2x0, in order to determine the angular position of a permanent magnet rotor. It is also possible to arrange a position encoder on the shaft of the rotor to obtain a certain precision in determining the rotor""s angular position. These arrangements require the addition of sensors which make the device more bulky, which is a major drawback for devices of small dimensions integrating micro-motors.
There is also known a generator providing induced voltage signals whose amplitude is proportional to the rotational speed of the rotor. Thus, for a three-phase permanent magnet rotor, it is possible to stop the power supply of the motor periodically for short intervals in order to extract induced voltage data allowing the position and/or speed of the rotor to be determined. This latter method for determining the angular position of a rotor has a major drawback given that the data can only be obtained when the rotor rotates at a certain speed, for example approximately 1,000 revolutions per minute. Thus, at low speed or for rotations of a relatively small angle, no data can be deduced on the basis of the induced voltage in the motor coils.
The object of the present invention is to overcome the drawbacks of the aforementioned prior art by avoiding an angular position detection device which is bulky and by providing a method for detecting the angular position of a rotor whatever its rotational speed and the angular path travelled.
This object is achieved by a method for determining the angular position of the permanent magnet rotor of a multiphase electric motor including stator coils which are respectively associated with the phases of this motor, each stator coil having a first end and a second end, wherein:
an alternating signal, having a determined frequency, is supplied to the first ends of at least two of said stator coils.
measurement signals in response to said alternating signal are recuperated, via conductor means connected to the second ends of said at least two stator coils, and supplied to electronic processing means, these measurement signals having a periodic amplitude modulation due to a periodic variation in the effective inductance of the corresponding coil as a function of the rotor""s angular position, two measurement signals, coming from two different phases of the motor, thus having envelopes of corresponding amplitude modulations which are phase-shifted.
an extraction from each recuperated measurement signal of said envelope of amplitude modulation is made by said electronic processing means.
at least two of said envelopes which are phase-shifted are use for determining said rotor""s angular position which is a function of said periodic amplitude modulation of each measurement signal.
The object of the present invention is also achieved by a method for determining the angular position of a permanent magnet rotor of a multiphase electric motor including stator coils which are respectively associated with the phases of this motor, each stator coil having a first end and a second end, wherein:
a first alternating signal and a second alternating signal, both having a determined frequency, are simultaneously and respectively supplied to the first ends of two of said stator coils, said second alternating signal being phase-shifted relative to said first altemating signal,
a measurement signal, generated by mixing signals received at the second ends of said two stator coils in response to said first alternating signal and said second alternating sianal, is recuperated, via conductor means connected to these second ends, and supplied to electronic processing means,
said measurement signal is processed by said processing means in order to obtain a periodic resulting signal which corresponds to a periodic phase variation of this measurement signal relative to said first alternating signal due to the periodic variation in the effective inductance of said two stator coils as a function of the rotor""s angular position,
the rotor""s angular position is determined from said periodic resulting signal.
Within the developments having led to the present invention, the inventor has observed that this measuring signal includes data relating to a periodic variation in the effective inductance of the coil in which the alternating signal is supplied as a function of the angular position of the permanent magnet rotor, in particular of bipolar permanent magnet rotor.
The invention also concerns the electronic system for implementing the method according to the invention.
Via appropriate processing of the measuring signal, the method according to the invention allows the rotor""s angular position to be determined with a high level of precision whatever its rotational speed and even when it is not moving. As a result of this method, it is possible to control the motor so as to make relatively small paths with a high level of precision.