The present invention is in the field of control of a synchronous motor with permanent magnets. It relates, more particularly to an electronic device and to a method for controlling the same.
Synchronous motors with permanent magnets, such as stepping motors, hybrid motors or direct current motors with no commutating element, are currently well known and used to replace direct current motors with a commutating element, the latter having a relatively short lifetime because of friction generated on the commutating element itself by the carbon brushes.
In these motors with permanent magnets, an electronic phase switching circuit is necessary to replace the commutating element function. Since this type of motor is synchronous, the rotor speed is the same as that of the rotating stator field. However, when the commutating element is removed and replaced with an electronic circuit, it is necessary to determine the position of the rotor for the control logic to be able to perform the switching at the right time. This is usually achieved with Hall effect probes or optical sensors, which are also called direct sensors.
This type of direct sensors has some drawbacks. First, their costs have a non-negligible impact on the whole cost of the motor. Second, the mounting thereof means that some space has to be especially provided for, not just for the sensors themselves, but also for the related electric connecting means. Therefore, the assembly of such motors is much more complex and time consuming. Finally, the reliability of the system is thereby reduced.
Some existing systems propose to overcome these drawbacks and provide a method and/or a device for controlling a synchronous motor with a permanent magnet with no direct sensors. This is particularly disclosed in the U.S. Pat. No. 6,326,760 which describes a method and a device to determine the speed of the motor by measuring the induced voltages, in at least two phases, when the driving power in said phases is turned off. However, the method described here above has the following main drawback. Since the motor with variable load cannot be reliably started by means of a closed loop working in function of the position, this method requires an open-loop starting algorithm for the motor to reach a speed level that is high enough to: 1) create a motion with sufficient kinetic energy to prevent the motor from being stopped by the load between two steps of the control algorithm and 2) generate induced voltages with a sufficiently high magnitude to allow the rotor position to be determined and thus, the motor to be speed and/or torque controlled.
It is therefore an object of the present invention to provide an electronic device and a method for controlling a synchronous motor with permanent magnets that do not suffer from the disadvantages described above. In particular with the device and method according to the invention, it shall be possible to determine the position of the motor even at near-zero speeds. A near-zero speed means that even when the motor is blocked, for example because the generated torque is too low, successively turning on and off driving currents will bring about an oscillating movement of the rotor around its rest position and thus produce induced voltages, which are high enough to allow to determine the position of the rotor. These and other problems are solved by the device and method as defined in the independent claims. Advantageous embodiments of the invention are given in the dependent claims.
The invention is based on the idea to measure induced voltages with the highest possible sensitivity, to determine the rotor position and/or rotor speed from measured induced voltages and then to enter said determined rotor position and/or rotor speed into a state filter which delivers a filtered rotor position and/or a filtered rotor speed that allow to control the power of each phase of the motor. The invention may be used for any synchronous motor be it a two-phase or multiple-phase, unipolar or bipolar motor controlled with or without pulse-width modulation.
The inventive method for controlling a synchronous permanent magnet multiple-phase motor, comprises the steps of:
The electronic device according to the invention comprises:
Embodiments of the invention are described in greater detail hereinafter relative to the attached schematic drawings.
a-2c show different arrangements of the device according to the invention for controlling a synchronous three-phase motor with permanent magnets.
where Ua and Ub are two voltages proportional to the two components of the rotor position vector in said two-axis system (i.e. √{square root over (Ua2+Ub2)} is proportional to the rotor speed) and V1, V2 and V3 are the measured induced voltages in the three motor phases, respectively. The rotor rotational speed may be advantageously determined by computing the square root of the sum of squares of all measured induced voltages V1 to V3. It should be noted however that two measured induced voltages only are actually required since the third one can be determined using the equation V1+V2+V3=0. Other methods, well known to those skilled in the art, may also be used such as derivation of the determined position or calculation of the change in rotor position as a function of time.
The arrangements shown in
According to the invention the control circuit 4 also comprises a state filter to which are applied the measured motor position and/or the measured motor speed and which delivers a filtered motor position and/or a filtered motor speed. With such a state filter, which advantageously may consist in a Kalman filter, it is possible to filter out noise-corrupted signals and/or disturbances that are always present in actual dynamic systems. Generally speaking, the state filter, or Kalman filter, processes all available measurements, regardless of their precision, to estimate the current value of the variables of interest, with use of any available information about initial conditions of the variables of interest. In the present case, the measured motor position and/or the measured motor speed may be corrupted by system noises or other disturbances, especially when the motor rotational speed is low. The use of Kalman filter, or any equivalent filter, allows to take into account the physical knowledge that when the rotational speed of the motor is very low, the position of the motor can not change substantially over a short period of time. In other words, when the speed is very low, the rotor position may be assumed to remain constant. Unlike the measured data, such as measured motor speed and position, the filtered data are thus suitable information that can be used to control the motor properly.
By way of example for the position of the motor, the Kalman filter may be implemented using the following algorithm:
X=X−1+(a*V*T+b*dP)÷2,
where
Parameters a, b, c and d are coefficients that are adapted to adjust the algorithm to the characteristics of the motor. Filtered rotor position and/or rotor speed delivered by the Kalman filter are applied to the motor driver via the connection 20 to possibly adjust the drive currents sent to the motor.
The algorithm described above is only one of numerous examples that can be used in accordance with the principles of the invention. Besides it shall be understood that the state filter designation will encompass any other filtering circuit that substantially achieves the same function as that described above in relation to the state filter.
Of course this invention is not limited to the preferred embodiments described above, to which variations and improvements may be made, without departing from the scope of protection of the present patent. More particularly, although the invention has been described with reference to a synchronous rotating motor with permanent magnets and three phases, it will be understood to also apply to one-phase rotating motors as well as to synchronous linear motors.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/CH05/00064 | 2/4/2005 | WO | 8/7/2006 |
Number | Date | Country | |
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60542349 | Feb 2004 | US |