You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
TECHNICAL FIELD 
     The invention relates to electrical motors. More particularly, the invention relates to linear motor actuators for sliding panels. 
     BACKGROUND 
     Actuators are used to automatically open and close doors in subway cars, passenger trains, supermarket entrances, elevators, etc. Examples of such actuators are basically pneumatic cylinders, ball screws coupled to an electric motor, straps coupled to an electric motor or a linear motor that moves the door, opening and closing it, and that is able to detect an obstruction of the door. 
     One drawback associated with these actuators is that they need maintenance, adjustments and lubrication. There is also a problem associated with obstruction detection, which is normally achieved using a sensitive edge and which also needs maintenance and adjustments. In the case of a linear motor actuator, one other problem is the high power consumption. 
     SUMMARY 
     One aspect of the invention provides a method of controlling an electrical motor for minimizing its power consumption. The motor has a first element with magnets of alternating polarities, and a second element with electrical conductor coils, the first and the second elements being mounted for relative motion to one another. The method comprises the steps of: energizing the coils with an alternating current to produce a movement of said first and said second elements relative to one another, said alternating current having an amplitude, a frequency and a phase; sensing a physical quantity representative of a phase shift between said magnets and said current in said coils; substantially maintaining said phase shift to an optimum phase shift by varying at least one of said amplitude, said frequency and said phase; and varying said amplitude such that a minimum amplitude is provided and a power consumption of said electrical motor is minimized. 
     Another aspect of the invention provides an electrical motor controller for minimizing power consumption in an electrical motor having a first element with magnets of alternating polarities, and a second element with electrical conductor coils. The first and the second elements being mounted for relative motion to one another. The controller comprising: a current source for energizing the coils with an alternating current to produce a movement of the first and the second elements relative to one another, the alternating current having an amplitude, a frequency and a phase; a sensor for sensing a physical quantity representative of a phase shift between the magnets and the current in the coils; and a current source controller for substantially maintaining the phase shift to an optimum phase shift, the current source controller having an amplitude controller for varying the amplitude such that a minimum amplitude is provided. 
     Another aspect of the invention provides an alternating current electrical motor with reduced power consumption. The motor comprises: a first element having magnets disposed with alternating polarities along the motion direction of the motor; a second element mounted to the first element for relative motion to one another and having electrical conductor coils disposed along the motion direction; a current source for energizing the coils with an alternating current to produce a movement of the first and the second elements relative to one another, the alternating current having a phase, a frequency and a variable amplitude; a sensor for sensing a physical quantity representative of a phase shift between the magnets and the current in the coils; and a current source controller for substantially maintaining the phase shift to an optimum phase shift, the current source controller having an amplitude controller for varying the amplitude such that a minimum amplitude is provided. 
     Another aspect of the invention provides a method for detecting an obstruction in an electrical motor. The motor has a first element with magnets of alternating polarities, and a second element with electrical conductor coils. The first and the second elements are mounted for relative motion to one another. The method comprises the steps of: energizing the coils with an alternating current to produce a movement of the first and the second elements relative to one another; sensing a physical quantity representative of a phase shift between the magnets and the current in the coils; and detecting a presence of an obstruction condition when a value of the phase shift is greater than a phase shift limit value and a value of the amplitude of the current is greater than an amplitude limit value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
         FIG. 1  is a perspective view of a linear motor door actuator according to an embodiment of the invention; 
         FIG. 2  is an elevation view of the linear motor door actuator of  FIG. 1 , wherein magnets in the mobile section are shown; 
         FIG. 3  is a cross-sectional view of the linear motor door actuator, taken along the plane  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a graph showing a relation between the magnets position and the current phase in the coils in the linear motor door actuator of  FIG. 1 ; 
         FIG. 5  is a block diagram illustrating a control scheme of the linear motor door actuator of  FIG. 1 , according to an embodiment of the invention and wherein the current frequency is varied to control the velocity of the motor in an open loop and the current amplitude is varied to control the phase shift between the magnet position and the current in the coils; 
         FIG. 6  is a block diagram illustrating a control scheme of the linear motor door actuator of  FIG. 1 , according to another embodiment of the invention and wherein the current amplitude is varied to control the velocity of the motor and the current phase is varied to control the phase shift between the magnet position and the current in the coils; 
         FIG. 7  is a block diagram illustrating a control scheme of the linear motor door actuator of  FIG. 1 , according to another embodiment of the invention and wherein the current frequency is varied to control the velocity of the motor in a closed loop and the current amplitude is varied to control the phase shift between the magnet position and the current in the coils; 
         FIG. 8  is a block diagram illustrating a control scheme of the linear motor door actuator of  FIG. 1 , according to another embodiment of the invention and wherein the current amplitude and the current phase are varied using a multivariable controller, to control the velocity of the motor and the phase shift between the magnet position and the current in the coils; 
         FIG. 9  is a flow chart illustrating a method of controlling an alternating current electric motor, e.g. the linear motor door actuator of  FIG. 1 , for minimizing its power consumption; and 
         FIG. 10  is a flow chart illustrating a method for detecting an obstruction in an alternating current electrical motor, e.g. the linear motor door actuator of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  to  FIG. 3  illustrate a linear motor door actuator  10  having a fixed section  12  with electrical conductor coils  16  and a mobile section  14  with magnets  18  mounted to the fixed section  12  for relative motion to one another. The mobile section  14  has a mechanical connector  20 , a pin for instance, through which a door, a window or any structure to be actuated may be connected to the mobile section  14 . The fixed section  12  has rails  22  to guide the mobile section  14  as it moves along the fixed section  12 . 
     The fixed section  12  is composed of coils  16  placed side-by-side, disposed along the motion direction of the actuator  10  and driven with three-phase current. The first coil is connected in series with the fourth, the seventh and the tenth coils and so on by multiple of three, and corresponds to phase A. The coil in position two is connected with the coils in positions five, eight, eleven and so on, and corresponds to phase B, while the coil in position three is connected in series with the coil in position six, nine, twelve and so on, and corresponds to phase C. 
     As best illustrated in  FIG. 2 , the mobile section  14  of this embodiment has four pairs of facing magnets  18  (4 of the magnets not shown) disposed along the motion direction of the actuator  10 . The two magnets of each pairs being separated by a gap through which passes the coils  16  of the fixed section  12 . Polarities of the magnets alternate along the motion direction of the actuator  10  and the magnets of each pair faces with opposite polarities. That is, the magnets  18  of each pair has the same magnetic orientation, i.e. the south side of one magnet faces the north side of the magnet in front, while a north side faces the south side of the facing magnet. On each side of the fixed section  14 , the magnets  18  are mounted on a steel plate  30  (see  FIG. 3 ) to contain the magnetic field. 
     To move the mobile section  14 , a three-phase current source powers the three-phase coils  16 . The speed of the actuator is controlled by the frequency of the three-phase current. Higher frequencies correspond to higher speeds. 
     Also shown in  FIG. 1  to  FIG. 3 , a position sensor  24  is enclosed in one of the rails  22  and senses the position of the mobile section  14  with respect to the fixed section  12  using a positioning magnet  28  located on the mobile section  14 . As will be discussed later on, the sensed position of the mobile section  14  is used to maintain the phase shift θ between the magnets  18  and the three-phase current in the coils  16  while the actuator  10  is in operation. The amplitude and the frequency of the three-phase current are controlled with feedback of the position sensor  24  in order to minimize power consumption needed to slide the door. Specifically, the amplitude and the frequency of the current in the coils  16  are varied to maintain the phase shift θ between the magnets  18  and the current in the coils  16  to an optimum phase shift value, thereby providing minimum power consumption for proper operation of the actuator  10 . 
     In this embodiment, a Temposonics® LK magnetostriction position sensor  24  is used to sense the position of the mobile section  14  with respect to the fixed section  12  but any other sensor could be alternatively used. Other non contact sensors that could be used includes Hall effect sensors distributed along the fixed section  12 . In order to maximize the force produced in the actuator  10 , the coils  16  are made from flat wires. With flat wires, coils  16  are easier to make and very little space is needed to connect the center of each coil  16  but any other wires could also be used. The maximum Lorentz force to be generated in the actuator is proportional to the number of coil turns and to the amplitude of the current. The flat wire also maximizes the quantity of copper in the available space. Flat wire in this embodiment provides the lowest resistance for a given number of turns. 
     The linear motor door actuator  10  is activated by Lorentz force which exerts a force on a charged particle that passes through a magnetic field. In the linear motor door actuator  10 , the charged particles corresponds to electrons that pass through the coils  16  and the magnetic field is created by the magnets  18  in the mobile section  14 . Since the coils  16  are fixed, the Lorentz force is reflected, in this case, in a translational force on the magnets  18 , which provides the translational motion of the mobile section  14  relative to the fixed section  12 . The produced mechanical force is transmitted to the door through the mechanical connector  20  attached to the mobile section  14  and to the door. The produced force is a function of the current flowing through the coils  16 , the direction of the current and the position of the coils  16  in reference to the magnets  18 . In other words, the produced force is a function of the phase shift θ between magnets  18  and the three-phase current in the coils  16 . 
     The graph of  FIG. 4  shows the relation between the position of the magnets  18  with reference to the fixed section  12  and the current in the coils  16  when the Lorentz force is at its minimum, i.e. the position the magnets  18  are attracted to. This position corresponds to a reference phase shift position, i.e. a zero phase shift between the magnets  18  and the current in the coils  16 . In this case, a 360° phase shift between the magnets  18  and the current in the coils  16  corresponds to the distance between two consecutive coils  16  to the same electrical phase, e.g. the distance between the first and the fourth coil. As an example, at a position  100  of the magnets  18 , the Lorentz force is minimized when the current in electrical phase A is zero, the current in electrical phase B is 86% of the current amplitude and the current in electrical phase C is negative and 86% of the amplitude. 
     The translational force on the mobile section  14  increases when the phase shift θ increases until it reaches maximum force corresponding to a 90° phase shift. Passed 90°, the force decreases to reach again a minimum when the phase shift θ is 180°. For phase shift a higher than 180°, the direction of the force is inverted. Passed 180°, the force increases to a maximum at 270° and decreases again to a minimum at 360°, which corresponds to 0°. 
     In this embodiment, the zero phase shift position along with the corresponding position sensed by the position sensor  24  is defined during an initialization procedure of the actuator  10 . For this procedure, one arbitrary magnet position is chosen, e.g. position  100 . The coils are powered with direct current such that the current intensity ratio between the electrical phases A, B and C corresponds to a minimum Lorentz force at the chosen position, as can be read on graph of  FIG. 4 . In this case, current in phase A is null and current in phase B and C are of the same intensity, current in phase B being positive and current in phase C being negative. The position of the magnets  18  is then sensed using the position sensor  24  and corresponds to a zero phase shift. In order to minimize the initialization errors, a maximum current is provided to the coils, the current ratio remaining as defined above, and no external force is applied to the actuator  10 . In this case, the defined current conditions are applied for about five seconds before the position is sensed. 
     In order to operate the actuator  10 , coils  16  are energized with three-phase current and the mobile section  14  moves along the fixed section  12 . The greater is the amplitude of the current, the more external resistance is required to produce a phase shift θ between the magnets  18  and the three-phase coils  16 . In order to minimize power consumption of the actuator  10  in operation, the current amplitude must provide just the right amount of power such that the phase shift θ corresponds to the maximum Lorentz force, i.e. the maximum translational force. This optimal phase shift corresponds to 90°. 
     Numerous control schemes may be used to carry out the invention.  FIG. 5  illustrates a control scheme according to an embodiment of the invention and wherein the frequency of the current in the coils is varied to control the velocity of the motor in a open loop and the amplitude of the current is varied to control the phase shift θ between the magnet position and the current in the coils. According to this embodiment, a current source  42  energizes the coils of the actuator  10  with an alternating current  44 , i.e. a three-phase current, having an amplitude, a frequency and a phase φ to produce a movement of the mobile section relative to the fixed section. The current source  42  receives an amplitude signal  45  and a frequency signal  47  to vary the amplitude and the frequency of the current  44 . The resulting position  46  of the mobile section is sensed using the position sensor  24  and the phase shift θ is determined using the phase converter  41  according to the sensed position  46  and the phase φ of the current  44  in the coils. In order to minimize the power consumption of the actuator  10 , the phase shift θ is substantially maintained to the phase shift set point θ c , i.e. the optimum phase shift of 90°, using an amplitude controller  40 . The amplitude controller  40  varies the amplitude of the current  44  produced by the current source  42  (by varying the amplitude signal  45 ), with feedback on the phase shift θ. If the phase shift θ exceeds the set point θ c  (i.e. 90°), the amplitude of the current is increased and if the phase shift e is lower than the set point θ c  (i.e. 90°), the amplitude of the current is decreased such that the phase shift θ is substantially maintained to the set point θ c . The velocity of the actuator  10  is controlled by varying the frequency of the current  44  (by varying the frequency signal  47 ) using an open loop frequency controller  48 , i.e. with no feedback on the actual velocity of the actuator  10 . Higher frequencies correspond to higher speeds. The frequency controller  48  can include filtering capabilities such that the frequency of the current  44  does not change abruptly, which could result in a loss of synchronism in the actuator  10 . A source controller  49  comprises the frequency controller  48  and the amplitude controller  40 . 
       FIG. 6  illustrates another possible control scheme. In this embodiment, the amplitude of the current is varied to control the velocity of the motor and the phase φ of the current is varied to control the phase shift between the magnet position and the current in the coils. According to this embodiment, a current source  42  energizes the coils of the actuator  10  with a current  44  having an amplitude, a frequency and a phase φ. The current source  42  receives an amplitude signal  45  and a phase signal  51  to vary the amplitude and the phase of the current  44 . The resulting position  50  and velocity  54  of the mobile section are sensed using the position sensor  24  and the required phase φ of the current  44  for maintaining the phase shift θ to the phase shift set point θ c  (i.e. the optimum phase shift of 90°) is determined using the phase controller  52 . The phase controller  52  adjusts the phase of the current  44  to the required phase φ by varying the phase signal  51 . Each position  50  of the mobile section corresponds to an given current phase φ for a given phase shift set point θ c  (i.e. the optimal phase shift of 90°). The velocity of the actuator  10  is controlled by varying the amplitude of the current  44  (by varying the amplitude signal  45 ) using an amplitude controller  56  with feedback on the sensed velocity  54 . When accelerating the actuator  10 , the amplitude controller  56  increases the current amplitude and, consequently, the phase shift θ tends to decrease. In response to the decreased phase shift θ, the phase controller  52  varies the current phase such that the phase shift θ is maintained to the phase shift set point θ c  (i.e. 90°). When the set point θ c  is the optimal phase shift of 90°, the force of the actuator  10  is optimized and the current amplitude is minimized for a given instructed velocity. Continuously varying the current phase φ actually results in varying the frequency of the current such that higher velocities correspond to higher frequencies. In this control scheme, the current is indirectly varied as a function of the phase shift θ, through feedback on the sensed position  50  and velocity  54 , such that a minimum amplitude is provided to follow the instructed velocity. The power consumption of the actuator  10  is thus optimized. A source controller  57  comprises the phase controller  52  and the amplitude controller  56 . 
       FIG. 7  illustrates another possible control scheme. In this embodiment, the frequency of the current is varied to control the velocity of the motor and the amplitude of the current is varied to control the phase shift θ between the magnet position and the current in the coils. As opposed to the embodiment of  FIG. 5 , the velocity of the motor is controlled with a feedback loop. According to this embodiment, a current source  42  energizes the coils of the actuator  10  with a current  44  having an amplitude, a frequency and a phase φ. The current source  42  receives an amplitude signal  45  and a frequency signal  47  to vary the amplitude and the frequency of the current  44 . The resulting position  46  and velocity  54  of the mobile section are sensed using the position sensor  24  and, as in the embodiment of  FIG. 5 , the phase shift θ is determined using phase converter  41  according to the sensed position  46  and the phase φ of the current  44  in the coils. In order to minimize the power consumption of the actuator  10 , the phase shift θ is substantially maintained to the phase shift set point θ c , i.e. the optimum phase shift of 90°, using an amplitude controller  40 . The amplitude controller  40  varies the amplitude of the current  44  produced by the current source  42  (by varying the amplitude signal  45 ) with feedback on the phase shift θ (determined using the sensed position  46  and the phase φ). If the phase shift θ exceeds the set point θ c  (i.e. 90°), the amplitude of the current is increased and if the phase shift θ is lower than the set point θ c  (i.e. 90°), the amplitude of the current is decreased such that the phase shift θ is substantially maintained to the set point θ c . The velocity of the actuator  10  is controlled by varying the frequency of the current  44  (by varying the frequency signal  47 ) using a frequency controller  58  with feedback on the sensed velocity  54  of the actuator  10 . The velocity of the actuator  10  is increased by increasing the frequency and it is decreased by decreasing the frequency. The frequency controller  48  can include filtering capabilities such that the frequency of the current  44  does not change abruptly, which could result in a loss of synchronism in the actuator  10 . A source controller  49  comprises the frequency controller  58  and the amplitude controller  40 . 
       FIG. 8  illustrates another possible control scheme. In this embodiment, a multivariable source controller is used to vary the phase φ and the amplitude of the current for maintaining the phase shift θ to the phase shift set point θ c  and for controlling the velocity of the actuator  10 . According to this embodiment, a current source  42  energizes the coils of the actuator  10  with a current  44  having an amplitude, a frequency and a phase φ. The current source  42  receives an amplitude signal  45  and a phase signal  51  to vary the amplitude and the phase of the current  44 . The resulting position  50  and velocity  54  of the mobile section are sensed using the position sensor  24 . The multivariable source controller  62  varies the phase and the amplitude of the current  44  (by varying the amplitude signal  45  and the phase signal  51 ) with feedback on the sensed position  50  and velocity  54 , for maintaining the phase shift θ to the phase shift set point θ c  (i.e. the optimum phase shift of 90°) and for controlling the velocity of the actuator  10 . The phase shift θ is thus maintained to the phase shift set point θ c  while the velocity  54  corresponds to the instructed velocity. 
     In the embodiments depicted in  FIG. 5  to  FIG. 8 , the position of the mobile section is the sensed physical quantity representative of the phase shift θ between the magnets and the current in the coils because the phase shift θ is determined according to the sensed position  46  and the phase φ of the current  44  in the coils. Alternatively, the phase shift θ may be directly sensed in the actuator  10  and may provide the sensed physical quantity. 
     Though the present invention is not limited to the integration of this feature, an obstruction of the actuated structure can advantageously be detected. Accordingly, in reference to the control schemes of  FIG. 5 ,  FIG. 6 ,  FIG. 7  or  FIG. 8 , the amplitude of the current is limited to a given limit. If the amplitude of the current reaches this limit when the phase shift θ exceeds the optimal phase shift of 90°, an obstruction unit (not shown) determine that there is an obstruction of the door, the window or the other actuated structure. In response to an obstruction detection, the direction of movement of the actuator  10  can be reversed in order to release the obstruction. In this specific embodiment, the door reopens, waits a few seconds and closes again. One skilled in the art will understand that other possible actions could occur in response to an obstruction detection. 
       FIG. 9  illustrates a method of controlling an electrical motor, e.g. the alternator, for minimizing its power consumption. In step  102 , the coils are energized with an alternating current, e.g. three-phase current, to produce a movement of the mobile section and the fixed section relative to one another. In step  104 , a physical quantity is sensed in the motor. The physical quantity is representative of a phase shift θ between the magnets and the current in the coils. Examples of suitable physical quantities are the position of the mobile section relative to the fixed section and the phase shift θ between the magnets and the current in the coils. In step  106 , the phase shift θ is substantially maintained to an optimum phase shift, e.g. 90°. Finally, in step  108 , the amplitude of the current is varied such that a minimum amplitude is provided and a power consumption of the motor is minimized. Furthermore, in one embodiment, a velocity set point is received and the velocity of the motor is sensed. The amplitude of the current is varied to control the velocity according to the velocity set point and the phase shift θ is maintained by varying the phase of the current. 
       FIG. 10  illustrates a method for detecting an obstruction in an electrical motor, e.g. the actuator. In step  152 , the coils are energized with alternating current to produce a movement of the mobile section and the fixed section relative to one another. In step  154 , a phase shift between the magnets and the current in the coils  16  is sensed. Finally, in step  156 , a presence of an obstruction condition is detected when a value of the phase shift is greater than a phase shift limit value and a value of the current amplitude is greater than an amplitude limit value. 
     One skilled in the art will understand that the presently described embodiments do not limit the invention to linear motor door activators. The presented teachings could be applied as well to a rotary motor having a rotor with permanent magnets and a stator with electrical conductor coils. The electrical motor could alternatively be used to actuate a turntable used for industrial applications for example, or for driving a vehicle. 
     While in the some of the presented embodiments, the coils are powered with three-phase current, it should be appreciated that the present invention could be applied as well to a single phase motor or to any multiple phase motor. An alternating current may be a single phase current or a multiple phase current. 
     The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.

Summary:
The present invention relates to an alternating current electrical motor having a first element with magnets of alternating polarities, and a second element with electrical conductor coils, the first and the second elements being mounted for relative motion to one another. A controller for the electrical motor comprising: a current source for energizing the coils with an alternating current to produce a movement of the first and the second elements relative to one another; a sensor for sensing a phase shift between the magnets and the current in the coils; and a current source controller for varying an amplitude of the current to substantially regulate the phase shift to an optimum phase shift value, thereby providing a minimum power consumption for proper operation of the electrical motor.