The present invention relates to an electric alternating current, ‘AC’, synchronous motor and to a system comprising the same. The invention further relates to a method of manufacturing an electric AC synchronous motor.

Electric AC synchronous motors are known in the art. These motors comprise a primary part provided with a coil array and a secondary part provided with a magnetic array. The coil array comprises a plurality of regularly spaced coils and the magnetic array comprises a plurality of regularly spaced magnetic units. The magnetic units are typically formed by permanent magnets.

The coils and the magnetic units are oppositely arranged. Moreover, the coil array can be energized using an AC current or voltage to cause a relative motion between the primary part and secondary part in an actuation direction. More in particular, a magnetic force is generated as a result of the current carried by the coil array and the static magnetic field generated by the magnetic array. For example, in ironless motors the magnetic force may comprise the Lorentz force. By fixing one of primary part and the secondary part, a relative motion between these parts can be generated. For example, the primary part can be fixed relative to a mounting frame or other support structure on or to which the motor is mounted. In such case, the primary part can be referred to as the stator whereas the secondary part can be referred to as the rotor. Alternatively, the secondary part can be fixed relative to a mounting frame or other support structure on or to which the motor is mounted. In such case, the secondary part can be referred to as the stator whereas the primary part can be referred to as the rotor. It is noted that the present invention relates to both embodiments.

The magnetic units are configured to generate a magnetic field that is oriented in a first direction that is substantially perpendicular to the actuation direction. Moreover, the first direction and actuation direction define a transverse direction that is perpendicular to both the first direction and actuation direction. The transverse direction defines a top and bottom surface of the magnetic array. More in particular, the top and bottom surfaces are separated from each other in the transverse direction. Additionally or alternatively, the top and bottom surfaces extend substantially perpendicular to the transverse direction.

Within the context of the present invention, a magnetic field is oriented in the first direction when the magnetic field is either parallel or anti-parallel to a vector describing the first direction.

In some applications it is important to know the relative offset between the primary part and the secondary part. Within the context of the present invention, the relative offset represents the displacement between the primary part and secondary part relative to a predefined positioning of these parts. This predefined positioning may correspond to a positioning wherein both the primary part and secondary part have a predefined absolute position. Alternatively, the predefined positioning may describe how an individual magnetic unit and opposing coil are positioned with respect to each other whereas the absolute positions of the magnetic unit and opposing coil may be unknown.

To determine the relative offset, the electric AC synchronous motor can be provided with a sensor unit for measuring a magnetic field component. When the measured magnetic field component at the measurement position varies sinusoidally as a function of the displacement between the primary part and secondary part along the actuation direction, the relative offset can be determined.

Figure 1 illustrates how the relative offset can be determined between a magnetic array and a coil array in an AC synchronous ironless motor, although the method can be equally applied to iron core motors. The magnetic array comprises a plurality of permanent magnets 1. The orientation of magnets 1 , corresponding to the direction between the poles of the magnet and indicated by arrows 2, switches between adjacent magnets 1. The coil array comprises a plurality of coils 3, of which only one is shown in figure 1.

Typically, each coil 3 is provided with current carrying segments. In figure 1, such segments may be arranged on the outer edges. As the current carried by these segments runs in opposite directions and because adjacent magnets 1 have an opposite orientation, the generated Lorentz force in both segments adds up. Consequently, when coil 3 is energized, a relative motion between the primary part and secondary part will be generated.

Figure 1 illustrates two measurement positions Ml, M2, at which the magnetic field component that is parallel to lines A-D is measured. At position Ml, the magnetic field component, referred to as HI, varies sinusoidally with the displacement d between the primary and secondary parts, i.e. HI = cos (d/S x 2p). At position M2, the magnetic field component, referred to as H2, also varies sinusoidally with the displacement d albeit with a 90 degrees offset, i.e. H2 = sin (d/S x 2p). This allows displacement d to be determined using d = arctan (H2/H1) x S / 2p. The relative offset d can only take on values between -1/4S and +1/4S, with S being the center-center distance between two magnets having the same orientation. By monitoring H1/H2 versus time, for example using Hall sensors, the direction of the relative movement between the primary part and secondary part can be determined.

Figures 2A and 2B illustrate a known AC synchronous rotational motor. The motor comprises a coil array 30 that is provided with a ring shaped support 31 in which planar coils are arranged perpendicular to the radial direction. Ring shaped support 31 typically comprises magnetic material to confine the magnetic field. The motor also comprises a magnetic array 10. This array is provided with a ring shaped support 11 to which permanent magnets 1 are mounted. Permanent magnets 1 are configured to generate a magnetic field in the radial direction. This direction corresponds to the abovementioned first direction.

In this known motor, permanent magnets 1 extend beyond the coil array as illustrated in the cross sectional view in figure 2B. This allows a Hall sensor unit to be placed such that it is able to measure the magnetic field component in the radial direction. The location of such sensor is indicated by arrow 40.

A drawback of the known system is related to the space occupied in the height direction of the motor. As can be seen in figures 2 A and 2B, additional height is needed to measure the radial component of the magnetic field. In addition to additional height, the known system also suffers from the additional costs and weight associated with the extension of permanent magnets 1.

DE 102013018277 A1 discloses an electric AC synchronous motor as defined by the preamble of claim 1. In this motor, the permanent magnets of the magnetic array each comprise at least two regions that differ in the extent of magnetic anisotropy. More in particular, a second region having a lower degree of magnetic anisotropy extends above the coils of the coil array and a first region having a higher degree of magnetic anisotropy is positioned opposite to the coils. A sensor is used that is arranged above the permanent magnets to measure the magnetic field generated by the second region to thereby determine a displacement between the stator and rotor.

A drawback of the known motor described above is related to the occupied space that is associated with the second region extending above, or below, the coil array. This occupied space is not directly related to the generation of force between the stator and rotor. Secondly, manufacturing permanent magnets with distinct regions as described above can be costly.

An object of the present invention is to provide an electric synchronous motor in which the abovementioned problems do not occur or at least to a lesser extent.

According to the present invention, this object is achieved with the electric synchronous motor as defined in claim 1 that is characterized in that the magnetic units each comprise a respective permanent magnet having a magnetization that is oriented in the first direction and that is substantially uniform when viewed along the transverse direction, wherein said component is a component of the magnetic field that is substantially parallel to the transverse direction or substantially parallel to the actuation direction. Furthermore, the sensor unit is positioned at a position at which a magnitude of the measured component displays a substantially sinusoidal behavior as a function of the displacement between the primary part and secondary part along said actuation direction.

The Applicant has found that it is possible to perform a useful measurement of the magnetic field to enable the displacement between the primary and secondary parts to be determined. This has been achieved by using the sensor unit to measure a component of the magnetic field that is substantially parallel to the transverse direction or substantially parallel to the actuation direction and by carefully arranging the sensor unit at a position at which a magnitude of the measured component displays a substantially sinusoidal behavior as a function of the displacement between the primary part and secondary part along said actuation direction. It should be noted that the measured component does not display sinusoidal behavior at every position close to the permanent magnets. However, the Applicant has found that so-called sweet spots can be identified, for example by measurement or simulation, at which the desired behavior can be observed. This allows the displacement to be determined without relying on protruding magnets having a difference in magnetization along the transverse direction.

The electric AC synchronous motor may comprise a further sensor unit configured for measuring a magnetic field generated by the coil array. The measurement of the further sensor unit can be used to remove distortion, caused by the magnetic field generated by the coil array, in the component of the magnetic field that is measured by the sensor unit. This further sensor unit can be positioned offset from the coil array in the transverse direction. Preferably, the further sensor unit is positioned closer to the coil array than to the magnetic array. Similarly, the sensor unit is preferably positioned closer to the magnetic array than to the coil array.

In an embodiment, a height of the coils along the transverse direction is substantially identical to a height of the permanent magnets along that direction. Hence, the present invention does not rely on protruding magnets to allow the displacement between stator and rotor to be determined.

Instead of a single sensor unit for measuring the component of the magnetic field generated by the magnetic array that is substantially parallel to the transverse direction or substantially parallel to the actuation direction, multiple of such sensor units can be used to determine such component at different positions. These sensor units can be spaced apart on a curve that runs parallel to the actuation direction. The various measurements obtained from the different sensors can be combined to determine the relative displacement between stator and rotor. Similarly, the electric AC synchronous motor may additionally or alternatively comprise a plurality of further sensor units that are also spaced apart on a curve that runs parallel to the actuation direction. Measurements from all or part of the further sensor units can be used for removing the abovementioned distortion.

The present invention may relate to a linear motor, wherein the magnetic array and the coil array are linearly elongated and wherein the actuation direction corresponds to a linear motion. Additionally, the present invention may also relate to a rotational motor wherein the magnetic array and the coil array are arranged along respective circles and wherein the actuation direction corresponds to a rotational motion. In an embodiment, coils that are adjacently arranged in the actuation direction can be separately energized. The coil array may be formed by first, second and third coils that are adjacently arranged, wherein the first coils are electrically connected to each other, wherein the second coils are electrically connected to each other, and wherein the third coils are electrically connected to each other, wherein the first, second and third coils each correspond to a respective phase are configured to be energized separately. Any coil will encounter positions where the net generated force acting on that coil has no component in the actuation direction. Using second and third coils will ensure that this does not happen to the motor as a whole. Moreover, the invention is not limited to this three-phase configuration but other configurations using more or less phase may equally be possible.

During normal operation, the polarity of the currents through the coils is switched. This results in a ‘moving’ magnetic field. The motion of the magnetic array and the magnetic field associated therewith tracks this moving magnetic field.

The primary part may comprise a yoke and a plurality of teeth extending from the yoke. Typically, the yoke and teeth are made from electrical steel. For a rotational motor, the yoke may have a cylindrical shape around the secondary part. More in particular, the teeth may extend radially inwardly. For a linear motor, the yoke has an elongated shape and the teeth extend towards the secondary part.

Each of the first, second, and third coils may be wound around a single respective tooth among the plurality of teeth. Alternatively, at least one of the first, second, and third coils can be wound around multiple preferably adjacent teeth among the plurality of teeth. For at least one tooth among the plurality of teeth, a first coil and a second coil can be at least partially wound around said at least one tooth. Hence, in this case, a first and second coil can be wound at least partially around the same tooth.

Respective further sensor units may be positioned for each of the respective phases. For example, each further sensor unit may be configured to measure the magnetic field associated with a particular phase.

Generally, magnetic units that are adjacently arranged in the actuation direction have opposite orientations with respect to their magnetic poles. The orientation of the permanent magnet corresponds to the magnetization vector, which points between the magnetic poles of the magnet.

The magnetic units may have their poles aligned with the first direction. Moreover, each of the coils may comprise windings that are wound in a winding plane that extends substantially perpendicular to the first direction.

The magnetic units are preferably all identical and the separation between adjacent magnetic units is preferably constant. Similarly, the separation between adjacent coils in the coil array is preferably constant. The sensor unit and/or further sensor unit may comprise a Hall sensor and/or a magneto resistive sensor. However, the present invention does not exclude other sensors for measuring the relevant magnetic field component. The sensor unit and/or further sensor unit may be fixedly attached and/or positioned to the coil array.

The present invention also provides an electric AC synchronous motor system. This system comprises the electric AC synchronous motor as defined above and a controller that is configured to determine a relative offset between the magnetic array and the coil array along the actuation direction in dependence of the component(s) of the magnetic field measured by the sensor unit. For example, a component of the magnetic field may be measured at two spaced apart positions as illustrated in figure 1 using Hall sensors. In this case, the ratio of the components may be used by the controller.

The controller can be configured to determine the relative offset between the magnet array and the coil array along the actuation direction in dependence of the component of the magnetic field measured by the sensor unit(s) and the magnetic field measured by the further sensor unit(s). More in particular, the controller can be configured to reduce distortion of the component of the magnetic field measured by the sensor unit(s) caused by the coil array using the magnetic field measured by the further sensor unit(s).

The controller may be configured to reduce distortion of the component of the magnetic field measured by a given sensor unit among the sensor unit(s) using the magnetic field measured by a selection of one or more further sensor units among the further sensor units. Such selection can be based on a distance between the further sensor units and the given sensor unit. For example, only further sensor unit(s) that are closest to the given sensor unit will be taken into account.

In case of a rotational motor, the controller can be configured to reduce distortion of the component of the magnetic field measured by a given sensor unit among the sensor unit(s) using the magnetic field measured by a selection of one or more further sensor units among the further sensor units. Such selection can be based on a similarity in azimuthal position of the further sensor units and the azimuthal position of said given sensor unit. Here, it is important to note that for a rotational motor, the arrangement of coils typically repeats in azimuthal direction. For example, the coil array may comprise 12 coils for each phase of a three-phase system. Furthermore, these coils are alternately arranged for example using the sequence coil phase 1, coil phase 2, coil phase 3, coil phase 1, coil phase 2, etc. In this case, positioning a further sensor unit at a given azimuthal angle al will be substantially identical to positioning a further sensor unit at an azimuthal angle al + n x 30 with n an integer. It is preferred to use one or more further sensor units for correcting the distortion encountered by a given sensor unit using those further sensor units of which the azimuthal position is most similar to that of the given sensor unit. In the example above, if the azimuthal position of the given sensor unit is 25 degrees relative to the fixed arrangement of the coil array, further sensor units may be used having an azimuthal position of 25 + nx 30, with n an integer.

The distortion for a given sensor unit can be corrected by calculating or estimating an effect of the coil array on the component of the magnetic field at the position of the given sensor unit, and subtracting this effect from a value of the component of the magnetic field measured by the given sensor unit. For example, a magnetic field that is generated by the coil array at the position of the given sensor can be estimated or calculated based on the magnetic field(s) measured by the further sensor unit(s). This estimated or calculated field can be used to reduce the effect the coil array has on the component of the magnetic field that is measured by the given sensor unit.

The sensor unit(s) is/are preferably placed as far away from the coil array as is feasible in order to minimize distortion by the field generated by the coils. The further sensor unit(s) should be positioned so as to make a representative measurement of the magnetic field caused by one of the coils. Therefore, the further sensor unit(s) is/are positioned as far away as is feasible from the magnetic array. Specifically, if the coil array is equipped with a flux-guiding member, this member will also attract the flux of the magnets of the magnetic array into itself, further reducing the flux sensed at the further sensor unit.

From first principles of motor actuation, it follows that both rotor and stator produce an oscillating magnetic field that varies substantially sinusoidally along a path parallel to the actuation direction. Usually, when assembling rotor and stator of the motor together, the field is no longer sinusoidal. In order to generate force or torque optimally, the two fields must be identical in spatial frequency, and spaced 90° apart in phase. This principle can be used to anticipate the distortion caused by the coils of the coil array at the position of the sensor unit(s), which is/are located substantially near the magnetic array.

This system may further comprise a power source for energizing the coils of the coil array, wherein the controller is configured for controlling the actuator in dependence of a currently determined relative offset. As described in conjunction with figure 1 , determining the relative offset should be distinguished from determining an absolute positioning of the primary and secondary parts. However, the controller may be configured for controlling the power source in dependence of a currently determined relative offset and in dependence of previously determined relative offsets. By accumulating the determined relative offsets the position of the secondary part relative to the primary part and the direction of the relative motion can be traced in time. Consequently, a predefined amount of motion can be achieved. For example, with a linear motor, the secondary part can be moved relative to the primary part over a well-controlled distance.

The present invention further provides a method for manufacturing the electric synchronous motor as described above. The method comprises the steps of: providing a primary part that comprises a coil array; providing a secondary part that comprises a magnetic array, said coil array comprising a plurality of regularly spaced coils and said magnetic array comprising a plurality of regularly spaced magnetic units, wherein the coils and the magnetic units are oppositely arranged; providing a sensor unit for measuring a magnetic field component; wherein the coil array can be energized to cause a relative motion between the primary part and secondary part in an actuation direction; wherein the magnetic units are configured to generate a magnetic field that is oriented in a first direction that is substantially perpendicular to the actuation direction, said first direction and said actuation direction defining a transverse direction that is perpendicular to both the first direction and actuation direction, said magnetic array having a top and bottom surface that are separated from each other in said transverse direction, and the magnetic units each comprising a respective permanent magnet having a magnetization that is oriented in the first direction and that is substantially uniform when viewed along the transverse direction.

The method further comprises the steps of measuring, calculating or simulating a component of the magnetic field generated by the magnetic array at different positions above and/or below the magnetic array, said positions differing with respect to a distance to the top or bottom surface of the magnetic array and/or with respect to a displacement in the first direction, wherein said component is a component of the magnetic field that is substantially parallel to the transverse direction or substantially parallel to the actuation direction. The method further comprises finding at least one position among said different positions at which a deviation from a sinusoidal behavior of the behavior of a magnitude of the measured or simulated component as a function of the displacement between the primary part and secondary part along said actuation direction is below a given threshold, and by positioning the sensor unit above or below the magnetic array such that said one or more positions correspond to the at least one found position with respect to the distance to the top or bottom surface of the magnetic array and with respect to a displacement in the first direction.

It is preferred to find a single position using simulation and to use this position as the or one of the positions for which the sensor unit measures a magnetic field component. Other positions for which the sensor unit measures the magnetic field component, if required, can be found by displacing the single found position along a curve that corresponds to the actuation direction. The separation between these positions should preferably be ¼ of the smallest center- center distance between magnets having the same orientation. At these positions additional sensor units can be arranged.

Next, the invention will be described in more detail by referring to the appended figures, wherein: Figure 1 illustrates a method for determining the relative offset between the primary part and the secondary part of a known electric synchronous motor;

Figure 2A illustrates a perspective view of a known electric synchronous motor and figure 2B illustrates a corresponding cross section;

Figures 3A and 3B illustrate an exploded view and a schematic view of an electric synchronous motor according to the present invention;

Figure 4A and 4B illustrate a secondary part of the motor in figure 3A in cross-sectional view and a top view, respectively;

Figure 5 illustrates a detailed view of the determination of a sweet spot in accordance with the present invention;

Figure 6 illustrates an exploded view of a further embodiment of an electric AC synchronous motor according to the present invention; and

Figure 7 illustrates a detailed view of the determination of a sweet spot for the motor of figure 6 in accordance with the present invention.

Figure 3A presents an exploded view of an embodiment of an electric synchronous motor according to the present invention. This motor is a rotational motor and it comprises a secondary part comprising a magnetic array 110. This array comprises a ring-shaped support 112 on which permanent magnets 1 are mounted, see the cross-sectional and top view of the secondary part illustrated in figures 4A and 4B, respectively. The motor further comprises a primary part comprising a coil array 130. This array comprises a plurality of coils. More in particular, the plurality of coils can be divided into a group of interconnected first coils 131, a group of interconnected second coils 132, and a group of interconnected third coils 133. These groups can be separately energized by a power source 160 that is controlled by a controller 150, see the schematic view of the motor in figure 3B. Electrical connection between power source 160 and coil array 130 is realized via feed lines 135.

The motor also comprises a ring-shaped Hall sensor unit 140 that is capable of measuring a component of the magnetic field at several positions along a circle that corresponds to the rotational motion of secondary part. The measurement information is fed to controller 150 via a feed line 141. This allows controller 150 to control the relative motion between the primary and secondary parts in a known manner.

Hall sensor unit 140 can be fixedly connected to primary part. By doing so, Hall sensor unit 140 will be positioned above magnetic array 110 instead of being positioned next to the magnetic array in the radial direction, as is the case for the known motor in figure 2A.

In figure 3A, the first direction corresponds to the radial direction, the transverse direction corresponds to the axial direction, and the actuation direction corresponds to the circumferential direction. As can be seen by comparing with the known motor of figure 2A, the height of permanent magnets 1 of the motor in figure 3A can be reduced. This is made possible because the applicant has found that this placement of Hall sensor unit 140 still offers the possibility to measure a magnetic field component related to permanent magnets 1 that is sufficiently strong to be measured and which displays a sinusoidal behavior as a function of displacement between the primary part and the secondary part.

The placement of Hall sensor unit 140 relative to magnetic array 110 relies on finding sweet spots close to magnetic array 110 at which positions a magnetic field component displays the desired sinusoidal behavior. These spots are normally found by performing electromagnetic simulations on the motor. Results of such simulations are shown in situ in figure 4A and in more detail in figure 5.

In figure 5, a pattern 180 can be seen that illustrates residuals for different positions relative to magnetic array 110 in the radial direction (corresponding to the horizontal axis) and in the axial direction (corresponding to the vertical axis). Here, a residual can be computed by summing the squared differences between the ideal sinusoidal behavior and the measured behavior. A sweet spot 190 can then be found at which the residuals are minimal.

The present invention does not exclude other methods to determine the residuals, for instance using Fourier analysis.

Hall sensor unit 140 typically comprises a plurality of Hall sensors that are configured to measure a magnetic field component at a particular position relative to magnetic array 110. The Hall sensors are distributed along the circumferential direction but share the same positioning relative to magnetic array 110 in terms of distances in the first direction and transverse direction.

According to the present invention, the Hall sensor unit should be positioned such that its Hall sensors are configured to measure the magnetic field component at the sweet spot. This component preferably comprises the axial component (as shown in figure 5) but may equally comprise the radial component or circumferential component, as long as the selected component displays the desired sinusoidal behavior. The applicant has found that the circumferential component, i.e. along the actuation direction, and the axial direction, i.e. along the transverse direction are particularly well suited for this purpose.

Figure 6 illustrates a cross-sectional view of a further embodiment of an electric AC synchronous motor according to the present invention. The primary and secondary parts of this motor are similar to those of the motor shown in figure 3A. The motor of figure 6 comprises a plurality of Hall sensor units M1-M4, illustrated as black dots in figure 6, and which are mounted above permanent magnets 1 similar to Hall sensor unit 140 in figure 3 A. The motor in figure 6 further comprises a plurality of further Hall sensor units Cl, C2, C3, which are mounted above coils 131, 132, 133 of coil array 130. Coils 131, 132, 133 are mounted around respective teeth 135 that extend radially from a cylindrical yoke 134. Teeth 135 are closed by respective pole shoes 136.

Sensor units M1-M4 and further sensor units C1-C3 are used to determine the angular position f of the secondary part relative to the primary part. More in particular, the measurements obtained using further sensor units C1-C3 are used to correct the measurements obtained using sensor units M1-M4.

Hereinafter, it is assumed that figure 6 illustrates the situation for f=0 and that the motor comprises m permanent magnets and n coils 131. Typically, when the secondary part is moving, sensor units M1-M4 will observe a magnetic flux density that varies sinusoidally as a function of f. More in particular, the signal will have an angular frequency equal to m/2 x dcp(t)/dt. However, in addition to the magnetic flux density generated by permanent magnets 1 , magnetic flux density is also sensed that is generated by coils 131, 132, 133. This latter contribution presents a disturbance for determining the angular position f. Hereinafter, a method will be described by which the effects of this disturbance can be mitigated. This method will be described assuming a configuration that uses p Hall sensor units Mk (k=l ...p) and q further Hall sensor units Ci (i=l...q).

In general, the measurement obtained by sensor Mk can be described as: which can be re-written as:

M<P = V _M,k - åZWtVc i EQ. 2 wherein V _{M k} is the voltage measurement of sensor unit Mk, wherein V _C is the voltage measurement of sensor unit Ci, wherein f _k (cp) describes the contribution of all permanent magnets 1 on the measurement of sensor unit Mk, and wherein a _{k l} is a geometric factor that describes how the magnetic flux density generated inside coil i, which is reflected in the measurement by sensor unit Ci, affects the voltage measurement of sensor unit Mk. Put differently, each coil 131, 132, 133, is driven by a given current. Geometric factor a _{k l} describes how the magnetic flux density associated with the current through coil i, on top of which sensor unit Ci is mounted, impacts the measurements of sensor unit Mk.

Now referring to figure 6, which illustrates an example for p=4, q=3, it can be noted that sensor Ml will substantially only be affected by the coils on top of which sensor units Cl and C2 are mounted. Similarly, sensor unit M4 will substantially only be affected by the coils on top of which sensor units C2 and C3 are mounted. Therefore, for a given set of sensor units, it is preferred if a further sensor unit is mounted above each coil that may substantially affect a measurement of those sensor units.

Typically, coil array 130 is fed using three different phases, e.g. phases I, II, III. In this case, coil array 130 can be divided into a first group of coils 131 that are substantially simultaneously fed using phase I, a second group of coils 132 that are substantially simultaneously fed using phase II, and a third group of coils 133 that are substantially simultaneously fed using phase III. All the coils 131, 132, 133 in a given group perform substantially identically. This means that a sensor does not need to be placed above a coil that is in the vicinity of the sensor units MI MA For example, in figure 6, sensor Cl can also be arranged above a coil far remote from sensor Ml provided that said coil belongs to the same group. However, geometric factor a _{k l} should be determined using the (original) coil that is in the vicinity of sensor Cl.

Adjacent permanent magnets 1 have their magnetic polarization oriented oppositely. Furthermore, in figure 6, sensors Mk are arranged such that the odd numbered sensors are arranged at an angular position in between two permanent magnets 1 , and the even numbered sensors are arranged directly opposite to a permanent magnet 1. Using this configuration, the contribution of permanent magnets 1 to the measured magnetic flux density for adjacent odd numbered sensors will be opposite, i.e. f _k (cp) = -/ _fe+2 (<p)·

Furthermore, in figure 6, f _k (cp) will be proportional to sin(cp) for odd numbered sensors and proportional to cos(cp) for even numbered sensors.

Now referring to the embodiment shown in figure 6, to determine the angular position, equation 2 will be used to determine functions f _k (cp) as a function of measurements V _{M k} and V _C . Thereafter, a first component will be determined using:

PI = /i(<p) - /sOz = sin f EQ. 3 and a second component using:

P2 = f 2 (p) — 0/0 = A cos f EQ. 4 wherein A is a constant. The angular position can then be determined using: f = tan ^_1 (Pl/P2) EQ. 5

From equations 3 and 4, it may be appreciated that the abovementioned method equally applies to a system using two sensors. However, by employing more sensor units, an improved accuracy can be obtained. It is further noted that the present method is not limited to the particular type of coil winding that is used. For example, coils belonging to different phases may extend over multiple teeth, and a single tooth may comprise windings belonging to different coils of different phases.

It should be apparent that the abovementioned method can be extended to an arbitrary number of sensor units and further sensor units. However, it is preferred to use two or more sensor units, more preferably an even number of sensor units, e.g. 4 or 6, and to use at least as two further sensor units. Moreover, the description above mentions a voltage measurement for characterizing the magnetic field or magnetic flux density. The present invention does not exclude other parameters provided that these parameters allow equations 3, 4, and 5 to be used for determining the angular position.

Figure 7 illustrates a detailed view of the determination of a sweet spot for the motor of figure 6 in accordance with the present invention. The sweet spot is determined in a similar manner as explained when referring to figures 4A and 4B. However, in this case, the residuals are not computed for the component(s) measured by the Hall sensor unit(s) but for the corrected component(s), for example as determined using Equations 2-5. Furthermore, for this determination, the position of the further Hall sensor units is fixed. However, the determination of a respective sweet spot can be performed for various configurations of the further Hall sensor units, e.g. for different positional arrangements. Then, the best position for both the Hall sensor units and the further Hall sensor units can be determined by selecting the best spot among the sweet spots found.

In the manner described above, a pattern 280 can be generated using the residuals that are associated with the corrected measurements of the Hall sensor unit(s) and a sweet spot 290 can be identified.

The Applicant has found that by removing the distortion associated with the magnetic field generated by the coil array, a more sinusoidal behavior at a given position of the Hall sensor unit can be obtained. This improvement can be used to arrange Hall sensor unit at a position different from the sweet spot position without losing the possibility to perform accurate measurements on the mutual displacement between rotor and stator. For example, the Hall sensor unit could be positioned closer to the magnetic array allowing a more compact form of the motor.

It should be noted that other sensors for measuring one or more field components may equally be used, such as magneto-resistive sensors. These latter sensors can be able to determine the relative offset using the magnetic field component at a single position. Moreover, the skilled person readily understands that, although the present invention has been described using detailed embodiments thereof, the scope of the present invention is not limited to these embodiments. Instead, various modifications can be made without departing from the scope that is defined by the appended claims. For example, figures 3-7 relate to rotational motors. However, the skilled person will readily understand that the invention is equally applicable to linear motors.

In addition, other aspects of the present invention can be described using the following numbered clauses:

Clause 1: An electric alternating current, ‘AC’, synchronous motor, comprising: a primary part provided with a coil array that comprises a plurality of regularly spaced coils; a secondary part provided with a magnetic array that comprises a plurality of regularly spaced magnetic units, wherein the coils and the magnetic units are oppositely arranged; a sensor unit for measuring a magnetic field component; wherein the coil array can be energized to cause a relative motion between the primary part and secondary part in an actuation direction; wherein the magnetic units are configured to generate a magnetic field that is oriented in a first direction that is substantially perpendicular to the actuation direction, said first direction and said actuation direction defining a transverse direction that is perpendicular to both the first direction and actuation direction, said magnetic array having a top and bottom surface that are separated from each other in said transverse direction; wherein the sensor unit is mounted above or below the top surface or bottom surface, respectively, of the magnetic array, and wherein the sensor unit is configured to measure a component of the magnetic field generated by the magnetic array; wherein the magnetic units each comprise a respective permanent magnet having a magnetization that is oriented in the first direction and that is substantially uniform when viewed along the transverse direction, wherein said component is a component of the magnetic field that is substantially parallel to the transverse direction or substantially parallel to the actuation direction and wherein the sensor unit is positioned at a position at which a magnitude of the measured component displays a substantially sinusoidal behavior as a function of the displacement between the primary part and secondary part along said actuation direction.

Clause 2: The electric AC synchronous motor according to clause 1, comprising a further sensor unit configured for measuring a magnetic field generated by the coil array.

Clause 3: The electric AC synchronous motor according to clause 2, wherein the further sensor unit is positioned offset from the coil array in the transverse direction.

Clause 4: The electric AC synchronous motor according to clause 3, wherein the further sensor unit positioned closer to the coil array than to the magnetic array, and wherein the sensor unit is positioned closer to the magnetic array than to the coil array. Clause 5: The electric AC synchronous motor according to any of the previous clauses, wherein a height of the coils along the transverse direction is substantially identical to a height of the permanent magnets along that direction.

Clause 6: The electric AC synchronous motor according to any of the previous clauses, comprising of a plurality of said sensor units and/or a plurality of said further sensor units, which are spaced apart on a curve that runs parallel to the actuation direction.

Clause 7: The electric AC synchronous motor according to any of the previous clauses, wherein the magnetic array and the coil array are linearly elongated and wherein the actuation direction corresponds to a linear motion.

Clause 8: The electric AC synchronous motor according to any of the clauses 1-6, wherein the magnetic array and the coil array are arranged along respective circles and wherein the actuation direction corresponds to a rotational motion.

Clause 9: The electric AC synchronous motor according to any of the previous clauses, wherein coils that are adjacently arranged in the actuation direction can preferably be separately energized.

Clause 10: The electric AC synchronous motor according to clause 9, wherein the coil array is formed by first, second and third coils that are adjacently arranged, wherein the first coils are electrically connected to each other, wherein the second coils are electrically connected to each other, and wherein the third coils are electrically connected to each other, wherein the first, second, and third coils each correspond to a respective phase and are configured to be energized separately.

Clause 11: The electric AC synchronous motor according to clause 10, wherein the primary part comprises a yoke and a plurality of teeth extending from the yoke.

Clause 12: The electric AC synchronous motor according to clause 11, wherein each of the first, second, and third coils is wound around a single respective tooth among the plurality of teeth.

Clause 13: The electric AC synchronous motor according to clause 11, wherein at least one of the first, second, and third coils is wound around multiple preferably adjacent teeth among the plurality of teeth.

Clause 14: The electric AC synchronous motor according to clause 13, wherein for at least one tooth among the plurality of teeth, a first coil and a second coil are at least partially wound around said at least one tooth.

Clause 15: The electric AC synchronous motor according to any of the clauses 10-14, in so far as depending on clause 6, wherein the electric AC synchronous motor comprises a respective further sensor unit for each of the respective phases.

Clause 16: The electric AC synchronous motor according to any of the previous clauses, wherein magnetic units that are adjacently arranged in the actuation direction have opposite orientations with respect to their magnetic poles. Clause 17: The electric AC synchronous motor according to any of the previous clauses, wherein the magnetic units have their poles aligned with the first direction, and wherein each of the coils comprises windings that are wound in a winding plane that extends substantially perpendicular to said first direction.

Clause 18: The electric AC synchronous motor according to any of the previous clauses, wherein the magnetic units are all identical and wherein the separation between adjacent magnetic units is constant, and wherein the separation between adjacent coils in the coil array is constant.

Clause 19: The electric AC synchronous motor according to any of the previous clauses, wherein the sensor unit and/or further sensor unit comprises a Hall sensor and/or a magneto resistive sensor.

Clause 20: The electric AC synchronous motor according to any of the previous clauses, wherein the sensor unit and/or further sensor unit are fixedly attached and/or positioned to the coil array.

Clause 21: An electric AC synchronous motor system, comprising: the electric synchronous motor according to any of the previous clauses; a controller configured to determine a relative offset between the magnet array and the coil array along the actuation direction in dependence of the component of the magnetic field measured by the sensor unit.

Clause 22: The electric AC synchronous motor system according to clause 21 in so far as depending on clause 2, wherein the controller is configured to determine the relative offset between the magnet array and the coil array along the actuation direction in dependence of the component of the magnetic field measured by the sensor unit(s) and the magnetic field measured by the further sensor unit(s).

Clause 23: The electric AC synchronous motor system according to clause 22, wherein the controller is configured to reduce distortion of the component of the magnetic field measured by the sensor unit(s) caused by the coil array using the magnetic field measured by the further sensor unit(s).

Clause 24: The electric AC synchronous motor system according to clause 23, wherein the controller is configured to reduce distortion of the component of the magnetic field measured by a given sensor unit among the sensor unit(s) using the magnetic field measured by a selection of one or more further sensor units among the further sensor units, said selection being based on a distance between the further sensor units and said given sensor unit.

Clause 25: The electric AC synchronous motor system according to clause 23, in so far as depending on clause 8, wherein the controller is configured to reduce distortion of the component of the magnetic field measured by a given sensor unit among the sensor unit(s) using the magnetic field measured by a selection of one or more further sensor units among the further sensor units, said selection being based on a similarity in azimuthal position of the further sensor units and the azimuthal position of said given sensor unit.

Clause 26: The electric AC synchronous motor system according to any of the clauses 23-

25, wherein the distortion for a given sensor unit is corrected by: calculating or estimating an effect of the coil array on the component of the magnetic field at the position of said given sensor unit; and subtracting said effect from a value of the component of the magnetic field measured by said given sensor unit.

Clause 27: The electric AC synchronous motor system according to any of the clauses 21-

26, further comprising a power source for energizing the coils of the coil array, wherein the controller is configured for controlling the power source in dependence of a currently determined relative offset.

Clause 28:The electric AC synchronous motor system according to clause 27, wherein the controller is configured for controlling the power source in dependence of a currently determined relative offset and in dependence of previously determined relative offsets.

Clause 29: A method for manufacturing the electric AC synchronous motor according to any of the clauses 1-20, comprising: providing a primary part that comprises a coil array; providing a secondary part that comprises a magnetic array, said coil array comprising a plurality of regularly spaced coils and said magnetic array comprising a plurality of regularly spaced magnetic units, wherein the coils and the magnetic units are oppositely arranged; providing a sensor unit for measuring a magnetic field component; wherein the coil array can be energized to cause a relative motion between the primary part and secondary part in an actuation direction; wherein the magnetic units are configured to generate a magnetic field that is oriented in a first direction that is substantially perpendicular to the actuation direction, said first direction and said actuation direction defining a transverse direction that is perpendicular to both the first direction and actuation direction, said magnetic array having a top and bottom surface that are separated from each other in said transverse direction, the magnetic units each comprising a respective permanent magnet having a substantially uniform magnetization that is oriented in the first direction when viewed along the transverse direction; measuring, calculating or simulating a component of the magnetic field generated by the magnetic array at different positions above and/or below the magnetic array, said positions differing with respect to a distance to the top or bottom surface of the magnetic array and/or with respect to a displacement in the first direction, wherein said component is a component of the magnetic field that is substantially parallel to the transverse direction or substantially parallel to the actuation direction; finding at least one position among said different positions at which a deviation from a sinusoidal behavior of the behavior of a magnitude of the measured or simulated component as a function of the displacement between the primary part and secondary part along said actuation direction is below a given threshold; and positioning the sensor unit above or below the magnetic array such that said one or more positions correspond to the at least one found position with respect to the distance to the top or bottom surface of the magnetic array and with respect to a displacement in the first direction. Clause 30: The method according to clause 29 and clause 6, comprising finding multiple positions among said different positions at which a deviation from a sinusoidal behavior of the behavior of a magnitude of the measured or simulated component as a function of the displacement between the primary part and secondary part along said actuation direction is below a given threshold, and positioning a respective sensor unit at different positions among the positions found, wherein the multiple positions are found by displacing a single found position along a curve that corresponds to the actuation direction.