Patent Publication Number: US-2023158912-A1

Title: An apparatus for and method of guiding an electric vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is the U.S. National Phase of PCT/EP2021/059498, filed on 13 Apr. 2021, which claims priority to German Patent Application No. 10 2020 110 220.8, filed on 14 Apr. 2020, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Field 
     An apparatus for use in guiding an electric vehicle to a position in which a magnetic coil on the vehicle is placed relative to a supply magnetic coil at a charging location such that energy can be transferred between the vehicle coil and the supply coil; a pad module for use with another pad module to transfer power magnetically in an electric vehicle charging system; and a method of guiding an electric vehicle to a position in which a vehicle magnetic coil is placed relative to a ground magnetic coil at a charging location such that energy can be transferred between the vehicle coil and the ground coil. 
     Related Art 
     Wireless power transfer techniques are increasingly being used to transfer power from electrical power sources to a wide range of devices from small hand-held consumer electronics devices, such as mobile phones and tablets requiring a few watts of power, to electric vehicles requiring kilowatts of power. In addition to the convenience of not having to plug in a device to power it or recharge its batteries, the absence of wires and cables makes for tidier desks and parking spaces, while reducing clutter, trip and shock hazards. There are several ways in which power may be transferred wirelessly, including capacitive coupling and inductive coupling, both of which offer advantages over resistive (i.e., wired) coupling for the purpose of supplying power to a device. 
     Wireless power transfer systems can be designed to deliver anything from fractions of a watt, a few watts to many kilowatts from a power source across a gap to a device or load. Typically, the gap is an air gap between magnetic coils, although other techniques include the delivery of power between plates of a capacitor. And they can be designed to operate at fixed or variable frequencies which can be useful for varying load conditions. 
     The energy thus delivered may be used for, example, to power an electronics circuit or to power a device. This includes powering a consumer device such as a cell phone or tablet. It also includes driving an electric motor in an electric vehicle and charging batteries in the circuit or vehicle. Powering a cell phone or charging its battery requires a few watts, whereas powering the motor in an electric vehicle or charging the battery requires several kilowatts. The larger the battery, circuit or motor, or the faster the battery is required to charge, the greater the power that must be transferred across the air gap. 
     Wireless power transfer techniques have developed in different fields of technology which has resulted in different terms being used to describe essentially the same thing. Such terms as ‘magnetic coupling,’ ‘magnetic induction,’ ‘inductive power transfer,’ ‘inductive charging’ and ‘resonant inductive power transfer’ are common. Although there may be minor differences, these terms are generally used broadly and interchangeably to refer to systems that transfer power from a source across an air gap to a load by way of a magnetic field. The term ‘inductive charging system’ or ICS will be used herein to identify this kind of system. 
     In a similar vein, various terms are used to refer to different elements of an inductive charging system (ICS). Essentially an ICS comprises equipment associated with the power supply and equipment associated with the device. The power supply equipment comprise circuitry that converts energy from the power supply into a form suitable for driving a coil. Similarly, the device equipment converts energy induced in a coil by the magnetic field into a form suitable for powering the device or charging batteries in the device. 
     Inductive charging systems can be used to charge batteries in electric vehicles. Drivers can park their cars over charging equipment on the ground, which couples magnetically with a charging equipment on the vehicle to transfer energy to the battery. For autonomous vehicles inductive charging eliminates the need to manually connect the vehicle to a supply once it has driven to a charging location. When needed, power from the vehicle&#39;s battery may be fed back into the power source, e.g., the power network in a smart home or a utility company&#39;s power grid. 
     In an inductive charging system (ICS) for use with electric vehicles the power supply equipment has various names including ground assembly, ground pad and ground pad module (GPM) which is connectable to a main power supply. The device equipment is variously known as the vehicle assembly, the vehicle pad or the car pad module (CPM) and is mountable in a vehicle such as a car to provide energy to charge the vehicle&#39;s battery. The naming in many situations depends on what language is adopted by a given manufacturing company. Naturally, an electric vehicle ICS is capable of working with diverse vehicles, including cars and heavier road vehicles, including trucks, buses and trams, and is not limited to use with cars, be they road-going or otherwise. The terms ‘ground pad module’ (GPM) and ‘car pad module’ (CPM) will be used herein to identify the two parts of an ICS for electric vehicles. 
     Other terms that vary between different implementations of an ICS include ‘magnetic coils’, ‘induction coils’ and ‘antennas.’ These terms too are used loosely and essentially interchangeably to describe the parts of an inductive charging system that transfer energy across the air gap. Although nothing should turn on the use of these different terms, it is worth noting for the sake of accuracy that the elements are actually coils rather than antennas. This is because at typical operating frequencies the elements transfer energy in the near field where only the magnetic field is present. 
     Antennas are designed with the electromagnetic field in mind, which forms once the radiated energy passes from beyond the near field to the far field. Where the near field ends and the far field begins depends on characteristics of the transmitting device (e.g., coil or antenna). For wireless power transfer applications an exact definition is usually unnecessary because the size of the air gap and the frequencies that the system operates places it firmly in the near field. Nevertheless, the aforementioned ‘magnetic coils,’ induction coils&#39; and ‘antennas’ are similarly used interchangeably by those active in designing wireless power transfer devices and systems. 
     Inductive charging systems may use magnetic coils either alone or coupled with other tuned or tuneable elements. In electric vehicle power transfer applications, a ground pad module may contain a coil in combination with associated driving electronics or it may contain the coil with some or all the associated electronics being provided in a separate enclosure. Either way, the coil in the ground pad module is used to transmit power via a magnetic field. Similarly, a car pad module may contain a coil in combination with associated control electronics or it may contain the coil with some or all the associated electronics being provided in a separate enclosure. Either way, the coil in the car pad module is used to receive power via a magnetic field. 
     One area of focus in the design of inductive charging systems is accurate alignment of the ground coil and the vehicle coil. Poor alignment reduces efficiency of the energy transfer. When the system is operational, the magnetic field can transfer large amounts of energy across the space between the ground pad module and the vehicle pad module. Even a small domestic system in use generates a magnetic field capable of transferring 2 to 3 kW of energy between the pads. Other systems operate at much higher power levels. An inefficient link between the ground and vehicle pads leads to stray currents in conducting elements of the system. The energy therefrom is converted to heat. So not only is energy lost, but it also causes heating, which is plainly undesirable. 
     With this in mind, effort has been put into designing inductive charging systems that are able to assist an operator (e.g., a driver of a vehicle) in positioning the vehicle coil over the ground coil to optimise power transfer and minimize losses and the problems associated therewith. 
     SUMMARY 
     As defined in the claims, the invention provides an apparatus for use in guiding an electric vehicle to a position in which a magnetic coil on the vehicle is placed relative to a supply magnetic coil at a charging location such that energy can be transferred between the vehicle coil and the supply coil; a pad module for use with another pad module to transfer power magnetically in an electric vehicle charging system; and a method of guiding an electric vehicle to a position in which a vehicle magnetic coil is placed relative to a ground magnetic coil at a charging location such that energy can be transferred between the vehicle coil and the ground coil. 
     An apparatus for and method of determining the relative position of a ground coil and a vehicle coils enables an electric vehicle to be guided to a position at which energy can be transferred between the ground and vehicle coils. Position signaling devices are mountable relative to the ground and vehicle coils. At least one signaling device is associated with one of the coils and two or more signaling devices are associated with the other of the coils. Each signaling device is able to transmit, or receive, or transmit and receive signals for or from other ones of the signaling devices. Signals are processed to obtain time-related information from the positioning signals. The information is obtained using at least two positioning protocols from a set of protocols including time of flight, two-way ranging, time difference of arrival and phase difference of arrival. This enables determination of distances between signaling devices and thus the relative position of the ground and vehicle coils. 
     The invention and features thereof are set forth with particularity in the claims and together with advantages thereof may become clearer to those possessed of the appropriate skill from consideration of the following detailed description given by way of example with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram of an inductive power transfer system; 
         FIG.  2    is a block diagram showing elements of a positioning apparatus for use with an inductive power transfer system; 
         FIG.  3    illustrates one way of using signals between a source and two receivers in determining position; 
         FIG.  4    identifies coordinates and times of flight between two signalling devices; 
         FIG.  5    illustrates a potential uncertainty in calculated position; 
         FIG.  6    illustrates a way of using signals from three sources to a receiver in determining position; 
         FIG.  7    illustrates another way of using signals between a source and two receivers in determining position; 
         FIG.  8    shows cars relative to parking bays; 
         FIG.  9    is a graph showing longer range accuracy; and 
         FIG.  10    is a graph showing shorter range accuracy. 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to  FIG.  1    of the accompanying drawings, an inductive charging system (ICS)  10  for use in charging a battery in an electric vehicle. The ICS  10  comprises a ground pad module (GPM)  12  containing a ground or supply coil  14  and driving circuitry  16  couplable to an electrical power supply  18 . Power signals from the supply  18  are conditioned by the driving circuitry  16  to put them in suitable form for application to the supply coil  14 . The coil  14  is driven by this application of the power signals—represented as current I and voltage V—to generate a magnetic field  20 . 
     The power supply  18  may be a domestic voltage supply at, e.g.,  110   v  or  220   v . Such domestic installations would be limited to 2-3 kW, meaning that the charging of a battery will typically take several hours. A larger power supply such as a poly- (plural) phase supply at 415 volts or higher enables charging to be completed more quickly. Larger powers—and therefore faster charging—may be provided in commercial or industrial implementations. 
     The size and form of the ground pad module  12  depends on the technical requirements of the system. The supply coil  14  is depicted as a circle in  FIG.  1   , but it could be any polygonal or elliptical shape. The coil  14  may be configured as a solenoid, arranged in a double-D configuration or any other of the widely available coil topologies. The exact form is determined by the technical requirements of the system and on routine design choices. To give an idea of size, the ground pad module  12  is typically around 600 mm in width. The height of the GPM  12  should be as low as possible to avoid the GPM causing a snagging hazard with the underside of a vehicle or a tripping hazard. 
     The inductive charging system  10  also comprises a car pad module (CPM)  22 . In use, the CPM  22  is placed on a car (not shown) at a location determined by design considerations, e.g., under the chassis or floor pan. When the car is driven over the GPM  12  and the CPM  22  positioned thereover, energy can be transferred from the GPM to the CPM. The magnetic field  20  is converted by a car or vehicle coil  24  into power signals—represented as current I and voltage V—that are conditioned by driving circuitry  26  to put them in a form suitable to deliver energy to, and thus charge, a battery  28 . 
     As with the ground pad module (GPM)  12 , the size and form of the CPM  22  is governed by technical requirements and choices made when designing the inductive charging system  10 . The car pad module (CPM)  22  is similarly provided in a package. Again, the exact form of the vehicle coil  24  and thus the shape and size of the CPM  22  is determined in the most part by technical requirements and design choices. The coil  24  need not be circular as depicted and, indeed, it need not even have the same form or topology as the coil  14  in the ground equipment  12 . Space in a vehicle is limited. The aim usually is to make the CPM  22  as small as possible, e.g., around 300 mm in width. 
     A controller  30  serves to control operation of the GPM  12  and CPM  22 . Although shown as a single unit in  FIG.  1   , the controller  30  may be wholly in the GPM  12  or the CPM  22  or have elements placed in, the GPM  12  and the CPM  22 . Among other things, the controller  30  provides a way for the two parts  12 ,  22  of the system  10  to communicate with each other, as represented by the dashed lines  31   a  and  31   b  in the drawing. The communication medium may be any one of the widely available wireless protocols, including Wi-Fi and Bluetooth. 
     In use, information is transferred between the GPM  12  and the CPM  22 , for example, to control operation, e.g., to control generation of the magnetic field  20  to optimise power transfer and minimise losses. Safety features (not shown) may be included in the system  10  that send signals to the controller  30  indicating when power transfer may begin or end or when it must be interrupted because of a hazard situation such as a foreign or living object entering the magnetic field  20 . 
     One convenience of an ICS  10  is that the battery  28  on a vehicle may be charged simply by parking the vehicle over the ground pad  12  in a position where the car pad module  22  and the ground pad module  12  are aligned. Vehicle guidance and alignment equipment is provided to assist the driver in correctly positioning the vehicle, and thus the car pad module  22 , relative to the ground pad module  12 . 
     The inductive charging system  10  includes a positioning apparatus  40  or subsystem which is illustrated in  FIG.  2    of the accompanying drawings. The positioning apparatus  40  is provided with a number of position signalling devices  32 ,  33  and  34  that transmit or receive or transmit and receive signals that enable relative positions between the GPM  12  and CPM  22  to be determined. Although  FIG.  2    shows two signalling devices  32 ,  33  on the ground pad module  12  and one on the car pad module  22 , it will be appreciated that the signalling devices could be placed the other way around, i.e., one on the GPM  12  and two on the CPM  22 . 
     Operation of the signalling devices,  32 ,  33 ,  34  is controlled by the positioning apparatus  40 . The positioning apparatus  40  is shown separately from the inductive charging system (ICS)  10  for the sake of convenience of explanation. As with the ICS controller  30  shown in  FIG.  1   , the positioning apparatus  40  may be included in or alongside the ICS system  10 . It may be placed in the ground pad module GPM  12  or the car pad module CPM  22  or distributed between the GPM and the CPM. Indeed, the functionality of the positioning apparatus  40  could be integrated into the ICS controller  30  either partially or wholly. 
     In early adoption of wireless vehicle charging, it is expected that automobile companies will supply an entire charging system, i.e., both GPM and CPM, to customers. As the market develops, supply of the GPM may be vested with third parties, with a desire to make the CPM as reliable, light and as cheap as possible. In future, a greater proportion of the system control, including the positioning apparatus  40  of  FIG.  2   , may therefore be placed in the GPM. Naturally, some of the positioning apparatus will remain in the CPM or the vehicle or both. Exactly how much is a matter of choice depending on constraints and practicalities in the design specification. 
     The positioning apparatus  40  comprises a process controller  42  which, at a high level, serves two main tasks represented by a signal processor  44  and a positioning controller  46 . At the high level, the signal processor  44  processes transmitted and received signals  47 ,  48  associated with each of the signalling devices  32 ,  33 ,  34  to extract time related information, and thus distance data, therefrom. And, again at the high level, the positioning controller  46  uses information and data from the signal processor  44  to determine position relative to a coordinate system or frame of reference. 
     These tasks could be provided separately. But in practice, because the operations are interrelated, it will usually make engineering and commercial sense to provide them as a unit as shown. Indeed, the processing and positioning functionality of the process controller  42  may be shared between the GPM  12  and the CPM  22  or replicated in both the GPM and the CPM. 
     The process controller  42  communicates with the signalling devices  32 ,  33 ,  34  associated with the GPM coil  14  and the CPM coil  24  via communication channels  41   a ,  41   b . The channels  41   a ,  41   b  serve to transfer data between parts of the positioning apparatus  40 . This is similar to the way in which the controller  30  of the system  10  of  FIG.  1    controls coupling of the GPM  12  and the CPM  22 . It follows that the channels  41   a ,  41   b , could be included in or made part of the channels  31   a ,  31   b  associated with the controller  30 . Indeed, some or all of the functionality of the process controller  42  could be included in or provided by the ICS controller  30 . The distribution of the various elements of the positioning apparatus  40  is a matter of design choice governed by the requirements and practicalities of a given implementation. 
     The process controller  42  determines the position of the coils relative to a coordinate system (x, y)  49 . The position of the coordinate system (x, y)  49  is arbitrary in that it can be defined at any location. In practice it is convenient for the coordinate system to be centred on one or other of the coils  14 ,  24 , with the position of the other coil being defined relative thereto. Usually the coordinate system  49  will be defined relative to the stationary coil in the GPM. For example, the origin (O, O) of the coordinate system can be conveniently located at the centre of the CPM  22  with the x-axis aligned to the driving direction of the vehicle (not shown). 
     The signalling devices  32 ,  33 ,  34  are shown placed at arbitrary positions on the coils  14  and  24 . In practice, the position of the supply and vehicle coils  14 ,  24  relative to each other will be calculated at a known location on each coil, e.g., their centres. But in a given system it may not be possible to place the signalling devices  32 ,  33 ,  34  exactly at the centres of the coils  14 ,  24 . Other equipment may take priority in being located there. 
     Indeed, there is no reason why the signalling devices have to be placed in or on the ground pad module  12  or car pad module  22 . They could be placed at the edges of a parking space and at arbitrary positions on a vehicle. As long as the offsets from the centre of a coil to each signalling device  32 ,  33 ,  34  is known, accurate position data can be calculated. 
     The signalling devices associated with a fixed element or position are sometimes called anchors and those associated with a moving element or position are called tags. Anchors and tags do essentially the same thing, namely, transmit, receive or transmit and receive signals in a manner that enables the position(s) of tag(s) to be determined relative to the position(s) of anchors. ‘Anchor’ and ‘tag’ are convenient labels that identify whether the signalling unit is associated with a fixed position (e.g., the GPM  12 ) or a movable or moving position (e.g. the CPM  22 ) respectively. 
     In the ICS  10  of  FIG.  1    and the positioning apparatus  40  of  FIG.  2   , the signalling devices  32 ,  33  are positioned on the GPM  12  relative to the coil  14 . Likewise, the signalling device  34  is positioned on the CPM  22  relative to the coil  24 . Since the GPM  12  is deployed on the ground it is easier to consider the signalling devices  32 ,  33  to be associated with a fixed position. Similarly, since the CPM  22  is deployed on a vehicle, it is easier to consider the signalling device  34  to be associated with a moving position. But the mathematics works equally well with the signalling device  34  associated with the ground or the GPM  12  and the signalling devices  32 ,  33  on the vehicle or CPM  22 . 
     It is common practice for the signals transmitted between the signalling devices in a positioning system to be ultra-wideband (UWB) signals. Processing these UWB signals gives very narrow pulses providing high time resolution, which in turn results in greater accuracy in the calculated position of the tags. Moreover, in UWB delayed pulses are narrow, which means they do not overlap and do not cancel each other. It therefore also gives high robustness against multipath. Suitable signalling devices that deploy UWB signals are available. 
     There are several ranging techniques in which signals transmitted between the signalling devices  32 ,  33  and  34  can be processed to determine the relative positions of the devices  32 ,  33  in the GPM  12  and the device  34  in the CPM  22 . These are generally known as positioning protocols. 
     One approach or positioning protocol is known as time of flight (ToF) or time of arrival (ToA). Time of arrival is the simplest and most common ranging technique, most notable used in the Global Positioning System (GPS). This method is based on knowing the exact time that a signal was sent from a transmitting signalling device, the exact time the signal arrives at a receiving signalling device, and the speed at which the signal travels (essentially the speed of light for radio signals). 
     Time of arrival, as applied to the positioning apparatus  40 , is shown in  FIG.  3   . A signal is transmitted from the device  34  and received by the devices  32  and  33 . Operation of the devices is synchronised by the process controller  42 . Knowing the times at which the signal is transmitted by device  34  and arrives at the device  32  and at the device  33  enables the path lengths to be determined between devices  34  and  32  and between devices  34  and  33 . And that gives enough information to calculate the relative positions. 
       FIG.  4    shows coordinates (x 0 , y 0 ) and (x, y) associated with a transmitting device  32  and a receiving device  34  separated by a distance d. It also shows times t s  and t a  associated with the synchronised sending (transmission) and arrival (reception) of the signal between the devices  32 ,  34 . Note that in  FIG.  4    the direction of travel of the signal is reversed as compared to that in  FIG.  3   . This has been done to make the point that the decision to designate the device  34  a transmitter and the devices  32  and  33  receivers or vice versa is simply a design choice. Similarly, the relative positions may be expressed as being relative to the ground pad module (GPM)  12  of  FIG.  1    or as being relative to the car pad module (CPM)  22 . 
     Referring to  FIG.  4   , the distance d from the signalling device  32  at reference position (x 0 , y 0 ) to the position of a target signalling device  34  at position (x, y) can be calculated using the simple equation: 
         d=C ( t   a   −t   s ) 
     where t s  is the time the signal was sent, t a  is the time of arrival of the signal and c is the speed of the radio signal, i.e., the speed of light. 
     Using this distance d, the location of the target signalling device  34  can be determined. In two dimensions this yields a circle with the equation: 
         d =√{square root over (( x   0   −x ) 2 +( y   0   −y ) 2 )}
 
     A single signal between two signalling devices  32 ,  34  only gives sufficient information to determine that the device  34  is on the circumference of a circle at a distance d from the coordinate (x 0 , y 0 ) i.e., the position of the device  32 . Another signalling device  33  will give similar information relative to the position of that second signalling device  33 . 
     As shown in  FIG.  5   , processing of a signal between the signalling device  32  and the signalling device  34  enables the system to determine that the signalling device  34  is positioned somewhere on the circumference of a circle  36  centred on the signalling device  32 . Similarly, the processing of a signal between the signalling device  33  and the signalling device  34  enables the system to determine that the signalling device  34  is positioned somewhere on the circumference of a circle  37  centred on the signalling device  33 . The device  34  is located where these lines  36 ,  37  cross. 
     Thus, one drawback of using the combination of one-plus-two signalling devices as shown in  FIGS.  3 ,  5  and  6    is ambiguity of position. The calculations involve the above quadratic equations and that gives two possible solutions for the position of the device  34 . As shown in  FIG.  5   , the device  34  may be at a position represented by the box  34 ′ or at a position represented by the box  34 ″. Only one solution  34 ′ or  34 ″ is correct. 
     Sometimes the surrounding environment makes it clear that one solution is not viable. For example, the position  34 ′ may be in an area that does not correspond to a parking space or is, say, behind a wall. But in other circumstances, say a public parking facility, there will not be enough information to determine whether the car is to the left or right of the devices  32 ,  33  and thus to the ground pad module  12 . Left and right are relative terms here. The position could equally be in front or behind depending on the orientation of the ground pad module and the vehicle. 
     In these circumstances, the system  40  may be designed to eliminate the incorrect calculated position. Where that is not possible, further information is required to ensure accurate determination of the relative positions. 
     In  FIG.  6    a third signalling device  38  is added to the ground pad module  12  (GPM—not shown in  FIG.  6   ) to give further distance information with respect to the signalling device  34  on the car pad module  22  (CPM—not shown in  FIG.  6   ). As can be seen from the diagram, the additional signalling device  38  enables calculation of the position of the device  34  as lying on the circumference of a circle  39  centred on the device  38 . The point where the circumferences of the three circles coincide is unambiguously the position at which the signalling device  34  is located. 
     Further signalling devices may be included in the system to increase the accuracy of positioning. A four-plus-one or a four-plus-two combination will provide greater accuracy than a system based on a three-plus-one or three-plus-two combination of signalling devices. But there is also an increase in cost and weight of the system. Indeed, for some protocols a larger number of devices may be required to obtain sufficient data for the position calculation. The trade-off here, of course, is that more elements increase cost and add weight to the GPM or CPM. The number of signalling devices and their location on the GPM or CPM is therefore a design choice governed by such engineering constraints as cost, weight and accuracy. 
     Another positioning protocol, known as two-way ranging (TWR), is shown in  FIG.  7   . A signal is transmitted by the device  34 . The signal is received by the devices  32 ,  33  which transmit signals back to the device  34 . The time taken for the signals to travel to the devices  32 ,  33  and back to the device  34  is proportional to the distance between the devices  34  and  32  and the devices  34  and  33 . This approach is useful where synchronisation of clocks is not possible. 
     This two-way transmission approach enables compensation for differences between clocks in the transmitting device and the receiving device  32 ,  33 ,  34 . Naturally, internal delays in the devices, e.g., between the device receiving the signal and transmitting a signal in reply, are also taken into account in determining times and thus calculating distances. Once the distances have been determined, it is a simple matter to calculate the relative positions. 
     Time difference of arrival (TDoA) is like time of arrival (ToA) in that it relies on times of arrival of signals. Time difference of arrival (TDoA) is more versatile than ToA in that it does not require the time that the signal was sent from a target signalling device  34  (see  FIG.  4   ). TDoA only needs to know the speed at which the signal travels (i.e., the speed of light for radio) and the times the signal was received by the signalling devices  32 ,  33 . 
     The process controller  42  synchronises operation of the signalling devices  32 ,  33  in time such that the difference in time of arrival can be determined. The difference in arrival time is used to calculate the difference in distances between the target signalling device  34  (i.e., the device whose position is to be determined) and the two reference signalling devices  32 ,  33  (i.e., the devices whose positions are known). 
     This difference in distance  11   d  is calculated using the equation: 
       Δ d=c (Δ t )
 
     where Δt is the difference in arrival times at the two other signalling devices  32 ,  33 . This leads to the equation: 
       Δ d =√{square root over (( x   1   −x ) 2 +( y   1   −y ) 2 )}+√{square root over (( x   2   −x ) 2 +( y   2   −y ) 2 )}
 
     where (x 1 , y 1 ) and (x 2 , y 2 ) are the known positions of the signalling devices  32 ,  33  and (x, y) is the position of the signalling device  34 . 
     Other positioning protocols suitable for use in the calculation of relative positions of the ground pad module (GPM)  12  of  FIG.  1    and the car pad module (CPM)  22  include phase difference of arrival (PDoA). In a PDoA system two receiving signalling devices are placed close to each other, within half a wavelength of the transmitted signal, in order to avoid ambiguity associated with a 180-degree (±π/2) phase shift that would otherwise occur. PDoA processing by the process controller  42  results in data representing an angle. Data from several receivers can be used in triangulation calculations to determine position. 
     In addition to that described with reference to  FIGS.  4 ,  5  and  6   , position can be calculated from time information using other positioning protocols and mathematical approaches. Orthogonal (x, y) coordinates ( 49 —see  FIG.  2   ) could be replaced by polar (r, Φ) coordinates. The mathematical approach chosen depends in varying degrees on the positioning protocol being used. The mathematics is documented elsewhere and, in the interest of brevity, will not be described in any detail herein. 
     Proprietary signalling devices are available that use a two-way ranging approach called ‘asymmetric double-sided two-way ranging’, which uses various techniques to cancel errors and other inaccuracies. Devices using this asymmetric approach are suitable for use in the system  40 . 
       FIG.  8    shows cars  51 ,  52 ,  53  relative to parking spaces  54 ,  55 ,  56 . A ground pad module GPM  12  is placed in the parking space  54  at a position convenient for the car  51  to park with its car pad module CPM  22  positioned such that the coils (not shown) in the CPM  22  and the GPM  12  are aligned. Four positions signalling devices  58   a - 58   d  are provided in the vicinity of the parking space  54 , e.g., around the parking space and signals are transmitted between those signalling devices  58   a - 58   d  and signalling devices  34   a ,  34   b  at the CPM  22  on the car  51 . 
     Plainly, the signalling devices  58   a - 58   d  are not located at the GPM  12 . But the positions of the GPM  12  and the signalling devices are static and their spatial relationship is therefore known. The positions of the signalling devices  58   a - 58   d  can therefore be defined by offsets I, J, K, L relative to, say, the centre of the coil in the CPM  12 . The position of the GPM  12  can be readily determined from the spatial relationship and is thus also known. 
     As car  51  approaches parking space  54 , positioning signals are transmitted between the devices  58   a - 58   d  associated with the parking space  54  and the devices  34   a ,  34   b  associated with the CPM  22  on the car  51 . The position of the devices  34   a ,  34   b , and thus that of the CPM  22  and indeed the car  51 , is calculated by processing the signals in the manner described above. The position is calculated repeatedly to provide guidance information to the vehicle as it moves toward the parking space. 
     The ground pad module  12  in parking space  55  includes signalling devices  32 ,  33  which work with similar devices on the car pad module  22  in the car  52  as the car enters the parking space. This arrangement works in the same manner as already described with reference to  FIGS.  1  and  2   . 
     The car  53 , already parked in the space  56 , has position signalling devices  31   a - 31   d  located at different positions on its body. Each of the devices  31   a - 31   d  is offset from the CPM  22  by an offset vector i, j, k, l. The position of the CPM  22  is therefore known and is taken into account when guiding the car  53  into the space  56 . 
     Positioning systems such as the one described herein give very accurate position information down to a few centimetres when the static signalling devices  32 ,  33  (a.k.a. anchors) enclose an area (e.g., area  59  in  FIG.  8   ) and the mobile signalling devices  34  (a.k.a. tags) are within the defined area. 
     Positional accuracy can suffer where the signalling devices are proximate. This can be a problem where the signalling devices are placed close together on the ground pad module  12  and/or close together on the car pad module  22 , as is the case of parking space  55  and car  52  in  FIG.  8   . In this situation the GPM signalling devices (anchors) will usually be less than 600 mm apart—the typical width of a GPM—and the CPM signalling devices (tags) less than 300 mm apart—the typical width of a CPM. 
     Where possible, the placement of anchors  58   a - 58   d  around the parking space  54  (see  FIG.  8   ) will help to mitigate this problem. Similarly, the placement of tags  31   a - 31   d  around the car  56  in  FIG.  8    will serve to improve the accuracy as compared to that with signalling devices clustered on the CPM. Such placement is, of course, dependent on the constraints of the specific implementation. 
     Positional accuracy can also fall off quickly with distance when the signalling devices or tags on the car are outside the defined area  59 . This is shown in  FIGS.  9  and  10    in which graphs show several lines representing different levels of accuracy or tolerance. These graphs were generated during simulations and are close to experimental results obtained subsequently. Each graph  70 ,  74  has a horizontal axis representing distance (D) in metres between the ground pad module  12  and the car pad module  22  (See  FIG.  1   ) in meters. The vertical axis represents accuracy (A) in metres of the calculated distance. 
     In  FIG.  9   , line  72  represents positioning to an arbitrary level of accuracy deemed—for the purpose of illustration—to be acceptable for a system of a given design. When a vehicle is outside a parking space, e.g., car  51  in  FIG.  8   , a tolerance or accuracy of approximately 8-10% may well be acceptable. As the vehicle gets closer to and then enters a parking space, e.g., car  52  in  FIG.  8   , greater accuracy is required as the car pad unit moves into alignment with the ground pad unit. 
     The same line  72  is shown in the graph  74  to a larger scale in  FIG.  10   . The line is given the designation  72 ′ merely to distinguish graph  74  from graph  70 . The line is the same in both. In  FIG.  10   , the graph  74  is shown over a smaller distance of zero to one meter compared to the zero to 14 m scale of graph  70  in  FIG.  9   . For the purpose of illustration here the positioning system is required to have a tolerance of 2-3 cm, i.e., the calculated position is within 2-3 cm of the actual position from a distance between the GPM and the CPM of less than 0.5 m. 
     In addition to the line  72 ,  72 ′ representing a desired tolerance or accuracy, both graphs  70  and  74  show two lines  76 ,  76 ′ and  77 ,  77 ′ representing the accuracy of a system in which time difference of arrival (TDoA) is deployed as the positioning protocol. Here the signalling devices are radio-based and have antennas with slightly different along-axis and along-diagonal characteristics. Line  76 ,  76 ′ shows the along-axis accuracy and line  77 ,  77 ′ the along-diagonal accuracy. Both lines  76 ′ and  77 ′ are close to and below the desired tolerance line  72 ′ in the short range of less than ˜0.4 m and are acceptable. Line  72 ′ changes abruptly from diagonal to horizontal at 0.5 m because of the way this tolerance line  72 ′ has been defined. The line  76 ′ is slightly above the abrupt change in line  72 ′. This is merely an artefact from the abrupt definition that makes very little difference in practice. 
     As can be seen in both  FIGS.  9  and  10   , the TDoA lines  76 ,  76 ′ and  77 ,  77 ′ rapidly rise above the required tolerance line  72 ,  72 ′ at distances over 0.5 m. This means that, while TDoA is able to give acceptable accuracy when the vehicle reaches the parking space and the tags are inside, it is unable to deliver the desired tolerance at further distances outside the parking space. 
     However, TDoA also offers the advantage of being able to provide angular information. This angular information remains useful even outside the parking space. The angular information can still be used in the positioning calculations. Of course, the angular information is also useful when it comes to aligning the vehicle  51 ,  52 ,  53  in a parking space  54 ,  55 ,  56  (see  FIG.  8   ). 
     Although not shown in  FIG.  9    or  FIG.  10   , it should be noted that the phase difference of arrival (PDoA) protocol similarly gives good angular information which is useful for both distance and final alignment of the car inside the space. PDoA may therefore be used as well as or instead of TDoA. 
     The graphs of  FIGS.  9  and  10    also show a line  78 ,  78 ′ representing the accuracy of a two-way ranging (TWR) or time of flight (ToF) arrangement. As can be seen in  FIG.  10   , the accuracy of this ToF positioning protocol is similar to that of the TDoA protocol below ˜0.5 m. It is, however, significantly better—and well within the desired tolerance—at distances greater than ˜0.5 m. ToF is therefore suitable for use in guiding a car or other vehicle over several metres toward a parking space. 
     One way of implementing the two approaches would be to use TWR or ToF outside the parking space and then change to TDoA or PDoA to guide the vehicle as it enters a space, e.g., as shown for car  52  and space  55 . However, using a combination of two positioning protocols both outside and inside the parking space gives greater accuracy than using only one. Put simply, TWR is good for range and TDoA good for angle. Together they provide greater accuracy than individually, especially, but not only, outside the space. Implementing two different protocols does not require additional hardware. The same signalling devices can be used for both. 
     In the foregoing, the signalling operations performed by the positioning system are assumed to be performed on radio signals transmitted, received, or transmitted and received by the signalling devices  32 ,  33 ,  34 . The use of radio signal is common, but it is not the only approach. 
     Other suitable signalling devices include ultrasonic time of flight sensors which send out an ultrasonic pulse, and then listen for echoes returning from targets in the sensor&#39;s field-of-view. By calculating the distance based on the time of flight (ToF) and speed of sound, the sensor can determine the distance of an object relative to a device. 
     Another approach would be to use beacons as the transmitting signalling units to send out signals to multiple receivers which are arranged to measure the power therein. The powers of the received signals would be compared across different receivers, and the receiver “hearing” the beacon with the highest power level considered to be closest to the beacon. 
     Modern cars are being designed increasingly to include more signalling devices, sensors and monitoring sub-systems to control many routine operations associated with the running and driving of the vehicle. In the interest of saving cost and depending on the equipment to be supplied on a given model of car, it may be possible to use some of the other equipment to assist in the aligning process. Suitable existing sensors provided for other subsystems may be deployed in the above-described determination of distance. For example, data from accelerometers or inertial sensors provided on the vehicle may be fed into the process controller  42  ( FIG.  2   ) to provide supplementary angular information. 
     Modern vehicles usually comprise a user interface, i.e., a monitor in the dashboard that displays useful vehicle information while stationary and driving. The positioning apparatus may be configured to output position data to the user interface for display of location information that assists the driver in parking the vehicle so that the charging system coils are aligned. Data from the coil-positioning system may be displayed to provide a visual aid to parking. Or the data may be provided to a self-parking system to include coil alignment in its functionality. For cars that include a self-parking feature, the positioning apparatus output data may be fed to the self-parking equipment to assist that equipment in guiding the car to the correct charging position in, e.g., a garage or parking space. 
     Having described by reference to the accompanying drawings an apparatus for use in guiding an electric vehicle to a position in which a magnetic coil on the vehicle is placed relative to a supply magnetic coil at a charging location such that energy can be transferred between the vehicle coil and the supply coil; a pad module for use with another pad module to transfer power magnetically in an electric vehicle charging system; and a method of guiding an electric vehicle to a position in which a vehicle magnetic coil is placed relative to a ground magnetic coil at a charging location such that energy can be transferred between the vehicle coil and the ground coil, it is to be understood that the same have been described by way of example only and that modifications and variations such as will occur to those possessed of appropriate knowledge and skills may be made without departure from the spirit and scope of the invention as set forth in the accompanying claims and equivalents thereof.