This invention relates to apparatus for determining the rotational position of an object, such as a rotor arranged to spin inside a stator of an electric motor.

Background

There exists a need to measure the rotational position of a rotating body accurately. For example in electric motors for vehicles, the angular position of the rotor relative to the stator must be accurately known so that changes to the magnetization state of magnetic portions inside the stator can be applied with the correct timing. Previous attempts to determine the angular position of a rotor relative to a stator involve the use of a resolver, which generates an electrical signal indicative of the rotational position of the rotor. However the output signal suffers from interference from magnetic and electromagnetic fields generated by components within the electric motor.

Aspects of the present invention have been conceived to address the forgoing. Summary

According to an aspect of the invention there is provided apparatus comprising: a rotatably mounted cam; an optical bend sensor comprising at least one deformable optical pathway in operative relation to a profiled portion of the cam such that rotation of the cam causes deformation of the at least one optical pathway, whereby the optical bend sensor is configured to generate sensor output indicative of the extent of curvature of the at least one optical pathway at a particular instant and whether the curvature thereof is increasing or

decreasing; and a controller configured to determine the rotational position of an object which is rotationally fixed relative to the cam, based on the sensor output.

Such apparatus is particularly useful to determine the rotational position of an object located in an environment permeated by a strong magnetic field. An example of such an object is a rotor forming part of an electric motor, whereby the rotor can be caused to spin by interacting with magnetic fields generated by a stator. Using the aforementioned apparatus the rotational position of such a rotor relative to a stator can be determined with increased accuracy and reliability compared to when a resolver is used for this purpose. This is because output generated by the optical bend sensor is less susceptible to interference from magnetic and electromagnetic fields generated by components within an electric motor than output generated by a resolver. With improved knowledge of the rotational position of a rotor relative to a stator, changes to the

magnetization state of magnetic portions inside the stator can thus be applied with improved timing, which increases electric motor efficiency. The profiled portion of the cam may be configured such that the curvature of the at least one deformable optical pathway continuously changes when the cam rotates.

The profiled portion of the cam may be configured such that during a single rotation of the cam, the or each respective deformable optical pathway

transitions from a first configuration to a second configuration having a different curvature to the first configuration and then back to the first configuration.

The profiled portion of the cam may be configured such that for a given rotational speed of the cam, the magnitude of the average rate of change of curvature of the or each respective deformable optical pathway when

transitioning from the first configuration to the second configuration is different to that than when transitioning from the second configuration to the first configuration.

The profiled portion of the cam may be an outer rim of the cam.

The profiled portion of the cam may be a surface through which an axis, about which the cam is arranged to rotate, extends.

The at least one deformable optical pathway may comprise an intrinsic optical fibre sensing element.

At least one coupling member may extend between the profiled portion of the cam and the at least one deformable optical pathway. The at least one coupling member may comprise a recoverably deformable member which engages the profiled portion of the cam.

The at least one coupling member may comprise a push rod arrangement and a recoverably deformable member to which the or each deformable optical pathway is fixed, whereby the pushrod arrangement engages the profiled portion of the cam and is configured to act on the recoverably deformable member thereby causing it and the or each deformable optical pathway fixed thereto to flex.

The cam may be mounted on a shaft of an electric motor to which a rotor of the electric motor is also mounted.

The cam may be arranged to rotate eccentrically about an axis in use.

The optical bend sensor may comprise at least one of a fibre Bragg grating, a long-period grating and a tilted fibre Bragg grating.

The optical bend sensor may comprise at least two deformable optical pathways the effective length of which are related to their respective curvatures and the sensor output may be based on the difference between the effective lengths of the at least two optical pathways.

According to another aspect of the invention there is provided a vehicle comprising an electric motor for propelling the vehicle and apparatus according to any arrangement heretofore mentioned for determining the rotational position of a rotor forming part of the electric motor.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of non-limiting example with reference to the accompanying drawings, in which:

Figs, l to 3 schematically illustrate a first embodiment of the present invention, whereby the cam is shown in three rotational positions;

Fig. 4 is a schematic perspective view of the cam and shaft in Figs, ι to 3;

Fig- 5 schematically illustrates a second embodiment of the present invention; Fig. 6 is a schematic side view of the arrangement in Fig. 5; Fig. 7 schematically illustrates a third embodiment of the present invention; Fig. 8 schematically illustrates a fourth embodiment of the present invention; Fig. 9 is a more detailed illustration of the arrangement in Fig. l; and

Fig. 10 is a top side view of an example sensing element.

Detailed Description

Embodiments of the present invention will now be described in the context of determining the rotational position of a rotor arranged to spin inside a stator of an electric motor. However this is not intended to be limiting and embodiments of the invention could be applied in other contexts, such as to determine the rotational position of other types of objects such as a gear mounted to a shaft.

First Embodiment

With the above proviso in mind, Fig. l shows a cam l mounted on a shaft 3. A rotor of an electric motor is mounted to another part of the shaft 3 such that when the shaft 3 rotates due to torque exerted on it by the rotor, both the rotor and cam 1 will spin at the same rotational speed.

For completeness it is hereby stated that the above mentioned rotor is

configured to interact with magnetic fields generated by a stator. In the present example periodic variations in magnetic field intensity in the vicinity of the rotor will cause it to spin. Since the rotor is fixed to the shaft 3 shown in Fig. 1, rotation of the rotor causes the shaft 3 and cam 1 to spin at the same rotational speed as the rotor.

An optical bend sensor 5 includes an intrinsic optical fibre sensing element 7 (hereafter the "sensing element") and sensing electronics 9 for determining the extent of curvature of the sensing element 7. Specific details of suitable optical bend sensors will be set out later on in this specification.

The outer rim 11 of the cam 1 is profiled such that the cross-section of the cam 1 is substantially oval shaped. The shaft 3 extends through the cross-section of the cam 1 closer to one end thereof than the other. In other words the shaft 3 does not extend though the centre of the cross section of the cam 3, which provides that the cam 3 is arranged to rotate eccentrically about an axis extending along the length of the shaft 3. The sensing element 7 (or cam follower) is fixed in a position in which it engages the rim 11 at all rotational positions of the cam 1. Thus when the cam 1 rotates from the orientation shown in Fig. 1 to the orientation in Fig. 3, it causes the sensing element 7 to flex by an increasing amount. Further rotation of the cam 1 in the same direction returns it back to its original orientation, such that the sensing element 7 returns to its original shape.

From Figs. 1 to 3 it will be apparent that the outer rim 11 is profiled such that the curvature of the sensing element 7 continuously changes while the cam 1 rotates.

The sensing electronics 9 output information indicative of the extent of curvature of the sensing element 7 and whether its curvature is increasing or decreasing at a particular instant. Using this output a controller 13 can determine the rotational position of the cam 1 and details of how this is achieved will now be explained. Initially though it is first mentioned that the controller 13 may take any suitable form, for instance it may be a microcontroller, plural microcontrollers, a processor, or plural processors. When the cam 1 has been rotated 90 degrees anti-clockwise from the orientation in Fig. 1, it will be in the orientation shown in Fig. 2. In this orientation the sensing element 7 engages a location A on the rim 11 and has a curvature unique to that particular rotational position of the cam 1 in addition to the rotational position when the sensing element 7 engages another location on the rim 11 which is the same distance from the shaft 3 as location A. In such rotational positions the sensing element 7 engages one of the locations denoted A and B on the rim 11 of the cam 1, which are each the same distance from the shaft 3.

By determining the extent of curvature of the sensing element 7 at the instant when the cam 1 is in the position shown in Fig. 2, based on output from the sensor electronics 9, the controller 13 can thus determine that the rotational position of the cam 1 is either that in which the sensing element 7 engages location A, or that in which it engages location B. This is because as mentioned, the extent of curvature of the sensing element 7 is the same when it is caused to engage location A or location B as the cam 1 rotates. However to determine which specific rotational position the cam l is in, the controller 13 additionally analyses the information output by the sensor electronics 9 which is indicative of whether the curvature of the sensing element 7 is increasing or decreasing.

While the cam 1 rotates anti-clockwise from the orientation in Fig. 1 to the orientation in Fig. 3 the extent of curvature of the sensing element 7 will gradually increase. Thus at the point when the sensing element 7 engages location A on the rim 11, the rate of change of curvature of the sensing element 7 will be positive because the sensing element 7 becomes increasingly more flexed the further the cam 1 is rotated.

After the cam 1 has reached the orientation shown in Fig. 3 further rotation of the cam 1 in the same direction returns it back to the orientation shown in Fig. 1, and when doing so the extent of curvature of the sensing element 7 will gradually decrease. Thus at the point when the sensing element 7 engages location B on the rim 11, the rate of change of curvature of the sensing element 7 will be negative because the sensing element 7 becomes decreasingly flexed the further the cam 1 is rotated.

Using information indicative of the extent of curvature of the sensing element 7 at a particular instant, and information indicative of whether the curvature of the sensing element 7 is increasing or decreasing at that instant (or in other words, whether the rate of change of curvature is positive or negative) , the controller 13 can determine the specific rotational position of the cam 1. This can be achieved by comparing such information with a lookup table stored in non-volatile memory. Each entry in the lookup table associates a respective rotational position of the cam 1 with two data entries, one corresponding to a specific curvature of the sensing element 7 and the other corresponding to a positive or negative rate of change thereof. A specific rotational position of the cam 1 can thus be identified because each entry in the lookup table has a unique combination of data entries.

Since the rotor of the electric motor is mounted to the same shaft 3 as the cam 1, having determined the specific rotational position of the cam 1 the controller 13 can determine that of the rotor, and thereby the rotational position of the rotor relative to the stator.

The foregoing enables the specific rotational position of a rotor inside an electric motor to be determined with increased accuracy and reliability compared to when a resolver is used for this purpose. This is because output generated by the optical bend sensor 5, which is subsequently processed by the controller 13, is less susceptible to interference from magnetic and electromagnetic fields generated by components within an electric motor due to the sensing element 7 being optical in nature rather than electrical (specific details of which will be set out later on in this description) . Changes to the magnetization state of magnetic portions inside the stator can therefore be applied with improved timing, which increases electric motor efficiency. Second Embodiment

Like in the first embodiment, according to a second embodiment a cam 1 is fixed to a shaft 3 that can rotate about an axis extending along the shaft 3. However in the second embodiment, a surface of the cam 1 through which this axis extends is profiled such that the rotational position of the cam 1 can be determined. Such a surface may be the front or rear face of the cam 1.

Looking at Fig. 5 one side of the cam 1 has a surface profile such that when it is traversed circumferentially (illustrated by the dotted line extending around the upper surface or upper face at a constant radius from the shaft 3) the height of the surface increases between locations A and B and then subsequently decreases between locations B and A; whereby locations A and B are separated by 180 degrees from one another about the shaft 3. As a result the section of the illustrated circumferential path between locations A and B and the section of the illustrated circumferential path between locations B and A are mirror images of each other, the profiled surface thereby exhibiting a symmetrical configuration.

The sensing element 7 of an optical bend sensor 5 is fixed in a position in which it contacts with the profiled surface at all rotational positions of the cam 1. Thus while the cam 1 rotates in the direction shown by the arrow X the extent of curvature of the sensing element 7 will gradually increase as it rides up the ramped surface between locations A and B. As the sensing element 7 rides up the ramped surface the rate of change of its curvature will be positive because the sensing element 7 becomes increasingly more flexed the further it rides up the ramped surface between locations A and B. After the location B has been moved under the sensing element 7, further rotation of the cam 1 in the same direction causes the extent of curvature of the sensing element 7 to gradually decrease as it rides down the ramped surface profile between locations B and A. As the sensing element 7 rides down the ramped surface the rate of change of its curvature will be negative since the sensing element 7 becomes decreasingly flexed the further it rides down the ramped surface towards location A.

Using information output from the optical bend sensor 5 a controller 13 can determine the specific rotational position of the cam 1 at a particular instant in a corresponding manner to that heretofore described.

Such information output from the optical bend sensor 5 is indicative of the extent of curvature of the sensing element 7 at a particular instant and whether at that instant the rate of change of curvature is positive or negative. Like in the previous embodiment, by comparing this information with a lookup table stored in non-volatile memory the controller 13 can determine the specific rotational position of the cam 1 at a particular instant in time. For completeness however it is again stated that each entry in the lookup table associates a respective rotational position of the cam 1 with two data entries, one corresponding to a specific extent of curvature of the sensing element 7 and the other corresponding to a positive or negative rate of change thereof. A specific rotational position of the cam 1 can be identified because each entry in the lookup table has a unique combination of data entries. Since the rotor of an electric motor is mounted to the same shaft 3 as the cam 1, the controller 13 can thus determine the specific rotational position of the rotor and thereby the rotational position of the rotor relative to a stator.

Third Embodiment

In the foregoing embodiments during a single cam rotation the sensing element 7 is caused to gradually transition from a first configuration to a second

configuration having a different curvature to the first configuration, and then transition back to the first configuration. For a given cam rotational speed in such embodiments the sensing element 7 transitions between the first and second configurations at a particular average rate of change of curvature and then transitions between the second and first configurations at a similar, but oppositely directed, average rate of change of curvature; meaning that if the average rate of change of curvature between the first and second configurations is positive for example then the average rate of change of curvature between the second and first configurations is negative. However in other embodiments the magnitude of the average rate of change of curvature of the sensing element 7 when transitioning from the first

configuration to the second configuration is different to that when it is transitioning from the second configuration back to the first configuration. Fig. 7 shows an example of a cam 1 having an outer rim 11 with a suitable profile which is in accordance with this condition. As the cam 1 rotates anti-clockwise the sensing element 7 rides between locations A and B and then subsequently locations B and A as the rim 11 moves under it. The magnitude of the average rate of change of curvature of the sensing element 7 as it rides between locations A and B is different to that experienced when riding between locations B and A.

Like in the first embodiment, the sensing element 7 becomes increasingly bent as it rides between locations A and B, and decreasingly bent when riding between locations B and A. As a result a controller 13 using bend sensor output indicative of the extent of curvature of the sensing element 7, and whether the curvature is increasing or decreasing, can determine the specific rotational position of the cam and thereby a rotor in a corresponding manner to that heretofore described; from which the rotational position of the rotor relative to a stator can be determined.

Fourth Embodiment

Fig. 8 shows another example of a cam 1 which is in accordance with the same condition as the cam depicted in Fig. 7. Specifically that in use, the average rate of change of curvature of the sensing element 7 when caused to transition from a first configuration to a second configuration is different to that when

transitioning from the second configuration back to the first configuration. The cam 1 is fixed to a shaft 3 which can rotate about an axis. A surface of the cam 1 (which may be a front or rear face thereof) through which this axis extends is profiled in accordance with the above condition. Looking again at Fig. 8 the upper surface (or upper face) has a profile such that when it is traversed circumferentially (illustrated by the dotted line extending around the upper surface at a constant radius from the shaft 3) the height of the surface increases between locations A and B at a first positive gradient, and then subsequently decreases at a second, negative, gradient between locations B and A; whereby locations A and B are separated by more than 180 degrees around the shaft 3. In another manner of speaking, traversing the surface around the illustrated dotted line the surface travelled between locations A and B is less steep than that between locations B and A.

Like in the second embodiment the sensing element 7 becomes increasingly bent as it rides up the ramped surface between locations A and B, caused by rotation of the cam 1. It subsequently becomes decreasingly bent when riding between surface locations B and A upon further rotation of the cam 1 in the same direction. As a result a controller 13 using bend sensor output indicative of the extent of curvature of the sensing element 7, and whether the curvature of the sensing element 7 is increasing or decreasing, can determine the specific rotational position of the cam 1 and thereby a rotor in a corresponding manner to that heretofore described; from which the rotational position of the rotor relative to a stator can be determined.

Suitable Optical Bend Sensors

Details of suitable optical bend sensors 5 will now be set out.

To illustrate the particulars of suitable sensors Fig. 9 shows the arrangement depicted in Fig. 1 in more detail, whereby the sensing element 7 is shown to comprise a deformable optical fibre 15 mounted on a recoverably deformable coupling element 17. The recoverably deformable coupling element 17 may be formed of spring steel or a fibre reinforced polymer for instance. A curved lip 19 may be provided at the distal end of the recoverably deformable coupling element 17, which is curved away from the cam 1 to assist the cam 1 in sliding against the coupling element 17 as it spins. In use, rotating the cam 1 causes it to urge against the recoverably deformable coupling element 17 and as a result the extent to which the recoverably deformable coupling element 17 is bent changes as the cam 1 rotates. The deformable optical fibre 15 is caused to flex by a corresponding amount because it is fixed to the recoverably deformable coupling element 17.

By directing optical electromagnetic radiation along the deformable optical fibre 15 and analysing the optical electromagnetic radiation reflected back out of the optical fibre 15 (or transmitted thereby), the sensing electronics 9 are able to generate output indicative of the extent of curvature of the deformable optical fibre 15 at a particular instant and whether its curvature is increasing or decreasing at that instant (or in other words, whether the rate of change of curvature is positive or negative). Having read the foregoing disclosure, various methods of achieving this will be apparent to persons skilled in the art.

For instance the deformable optical fibre 15 could comprise a fibre Bragg grating (FBG), in which case by directing broadband optical electromagnetic radiation along the optical fibre 15 and analysing the power of optical electromagnetic radiation reflected back out, the sensor electronics 9 can determine the degree of bending of the optical fibre 15 because changes in this power are related to strain of the optical fibre 15. An optical fibre comprising a fibre Bragg grating (FBG) for quantifying bend measurements is mentioned in the abstract and

introduction of the paper "Bend Sensor Using an Embedded Etched Fiber Bragg Grating" by Yong-Sen et al published by Wiley Periodicals, Inc. in Microwave Optical Technology Letters 43: 414-417, 2004; the entire contents of this document being incorporated herein by reference.

In other embodiments the deformable optical fibre 15 could comprise a long period grating (LPG), in which the optical fibre 15 forms a loop on the

recoverably deformable coupling element 17 as shown in Fig. 10. By directing broadband optical electromagnetic radiation along the optical fibre 15 and analysing the wavelengths at which optical electromagnetic radiation is mostly attenuated while travelling through it, the sensor electronics 9 can determine the degree of bending of the deformable optical fibre 15 because changes in these wavelengths are related to strain of the optical fibre 15. An optical fibre comprising a long period grating (FPG) which is suitable for determining bend radius is mentioned in the introduction (pages R49 and R50) of the paper "Optical Fibre Long-Period Grating Sensors: Characteristics and Application" by Stepehn W James et al published by Institute of Physics Publishing in

Measurement Science and Technology 14 (2003) R49-R61; the entire contents of this document being incorporated herein by reference.

In other embodiments the deformable optical fibre 15 could comprises a fibre Bragg grating (FBG). By directing broadband optical electromagnetic radiation along the optical fibre 15, and analysing the wavelengths of radiation reflected back out, the sensor electronics 9 can determine the degree of bending of the deformable optical fibre 15 because the peak wavelength of radiation reflected back out changes in accordance with strain of the optical fibre 15. An optical fibre comprising a fibre Bragg grating which is suitable for determining bend radius is mentioned in the introduction (pages R49 and R50) of the paper

"Optical Fibre Long-Period Grating Sensors: Characteristics and Application" by Stepehn W James et al published by Institute of Physics Publishing in

Measurement Science and Technology 14 (2003) R49-R61; the entire contents of this document having already been incorporated herein by reference.

A further suitable optical fibre is described in WO98/59219, the entire contents of which are also incorporated herein by reference. Page 16, line 19 to page 17, line 27 explains how the degree of bending of such an optical fibre which includes at least two parallel but axially offset optical pathways can be

determined. A brief summary however is as follows. The effective lengths of the two optical pathways are related to their respective curvatures. Optical electromagnetic radiation is directed into the optical fibre and is caused to travel along the two optical pathways. By conducting an interferometric analysis of radiation reflected back out of the optical fibre the difference between the optical path lengths of these two optical pathways can be determined, which

corresponds to the degree of bending of the optical fibre. If the optical fibre 15 used to implement the present invention is of the type described in this paragraph, preferably the two optical pathways inside the optical fibre should be located on a line perpendicular to the surface of the recoverably deformable member 17, that is from the frame of reference of the recoverably deformable member 17 the two optical pathways are located one on top of the other. Other suitable intrinsic optical fibre sensors will be apparent to persons skilled in the art in view of the foregoing, for instance those of the type including a tilted fibre Bragg grating (TFBG) as mentioned in the abstract and introduction to the paper "Highly Sensitive Bend Sensor with Hybrid Long-Period and Tilted Fiber Bragg Grating" by Li-Yang Shao et al published in the journal Optics Communications, volume 283, issue 13, 1 ^st July 2010, pages 2690 to 2694; the entire contents of this document being incorporated herein by reference. In each of the heretofore described embodiments, the natural vibration frequency of the sensing element 7, particularly the recoverably deformable (spring-like) coupling element 17 thereof, should be out of the normal rotational frequency range of the cam 1. Furthermore, if the foregoing is implemented in a vehicle (e.g. a land based vehicle such as a car or truck having wheels or a main battle tank having tracks for propelling the tank; or alternatively a water based vehicle or aircraft) then the natural frequency of the sensing element 7, particularly the recoverably deformable (spring-like) coupling element 17 thereof, should be out of the range of vibration frequencies experienced by the vehicle during its normal operation.

It will be appreciated that whilst various aspects and embodiments of the present invention have heretofore been described, the scope of the present invention is not limited to the embodiments set out herein and instead extends to encompass all arrangements, and modifications and alterations thereto, which fall within the spirit and scope of the appended claims.

For instance in some embodiments the recoverably deformable coupling member 17 may not be in direct contact with the cam 1. In such embodiments a push rod arrangement is provided between the cam 1 and the recoverably deformable coupling member 17. When the cam 1 rotates it rides under one end of the push rod arrangement which engages a profiled surface of the cam 1 at all rotational positions thereof. The section of the profiled surface in contact with the push rod arrangement urges against the push rod arrangement and causes it to move. Movement of the push rod arrangement is transferred to the recoverably deformable coupling member 17, due to the other end of the push rod arrangement being in contact therewith, which causes the recoverably

deformable coupling member to flex. Using a push rod arrangement in this manner enables the sensing element 7 (which includes the recoverably deformable coupling member 17) to be located away from the environment in which the cam 1 is located. This is useful if the cam 1 is located in a hostile environment (e.g. a high pressure or high heat environment - specifically an environment which could cause damage to the sensing element 7) or when the cam 1 is located in some place where it would be difficult to position the sensing element 7 and associated sensor electronics 9.

Having read the foregoing, persons skilled in the art will appreciate that the specific profile shapes illustrated in the drawings are merely exemplary and that other specific profile shapes could be used instead in a similar manner. For example the gradient of the steep ramped portion in Fig. 8 could have a variety of values, provided that it is steeper than the other surface portion of the cam.

The controller 13 has been described as receiving bend sensor output indicative of the extent of curvature of a sensing element 7 and whether its curvature is increasing or decreasing (or in other words, whether the rate of change of curvature is positive or negative). Sensor electronics 9 could thus output values corresponding to the degree of curvature of a sensing element 7 at a particular instant, and an indication as to whether at that instant the rate of change of curvature is positive or negative (which could merely involve outputting a binary 1 to indicative a positive rate of change of curvature or a binary o to indicate a negative rate of change of curvature). However the sensor electronics 9 could alternatively output raw data to the controller 13 which uses such raw data to determine the information needed to be compared with the aforementioned lookup table in order to determine the specific rotational position of a cam 1. In the example electric motor described in the foregoing description the rotor is said to be configured to interact with magnetic fields generated by a stator. In particular it was previously said that variations in magnetic field intensity in the vicinity of the rotor will cause it to spin. However different electric motor configurations are known in the art and aspects of the present invention could be used to improve their efficiency also. For example in some electric motors the stator includes permanent magnets and the rotor includes coils of wire through which current can flow, whereby interactions between magnetic fields of the permanent magnets and those generated by current flow cause the rotor to spin. Nevertheless, regardless of the specific electric motor configuration used, determining the rotational position of the rotor relative to the stator to a greater degree of accuracy and reliability is useful to control any aspect of the electric motor that is required to be varied in a controlled manner to cause the rotor to spin. More so, that aspect could be controlled in an improved manner and thereby increase overall electric motor efficiency.