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B-1 and Table B-1.

B.2 Capacitance change

This analog ping method is based on a change of the capacitance of an electrode on or near the Interface Surface, due to the placement of an object on the Interface Surface.

This method is particularly suitable for Power Transmitters that use Free Positioning, because it enables implementations that have a very low stand-by power, and yet exhibit an acceptable response time to a user. The reason is that (continuously) scanning the Interface Surface for changes in the arrangement of objects and Power Receivers thereon is a relatively costly operation. In contrast, sensing changes in the capacitance of an electrode can be very cheap (in terms of power requirements). Note that capacitance sensing can proceed with substantial parts of the Base Station powered down.

Power Transmitters designs that are based on an array of Primary Coils can use the array of Primary Coils as the electrode in question. For that purpose, the multiplexer should connect all (or a relevant subset of) Primary Coils in the array to a capacitance sensing unit—and at the same time disconnect the Primary Coils from the driving circuit. Power Transmitter designs that are based on a moving Primary Coil can use the detection coils on the Interface Surface (see Annex C.3) as electrodes.

It is recommended that the capacitance sensing circuit is able to detect changes with a rsolution of 100 fF or better. If the sensed capacitance change exceeds some implementation defined threshold, the Power Transmitter can conclude that an object is place onto or removed from the Interface Surface. In that case, the Power Transmitter should proceed to localize the objects and attempt to identify the Power Receivers on the Interface Surface, e.g. as discussed in Annex C.

Annex C Power Receiver Localization (Informative)

This Annex C discusses several aspects that relate to the discovery of Power Receivers amongst the objects that the Power Transmitter has discovered on its Interface Surface.

C.1 Guided Positioning

In the case of Guided Positioning, discovery and localization of a Power Receiver is straightforward: The Power Transmitter should simply execute a Digital Ping, as defined in Section 5.2.1. If the Power Transmitter receives a Signal Strength Packet or an End Power Transfer Packet, it has discovered and located a Power Receiver. Otherwise, the object is not a Power Receiver.

C.2 Primary Coil array based Free Positioning

In the case of Free Positioning, discovery and localization of a Power Receiver is less straightforward. This Annex C.2 discusses one example approach, which is particularly suited to a Primary Coil array based Power Transmitter. In this approach, the Power Transmitter first discovers and locates the objects that are present on its Interface Surface (e.g. using any of the methods discussed in Annex B). This results in a set of Primary Cells, which represents the locations of potential Power Receivers. For each of the Primary Cells in this set, the Power Transmitter executes a Digital Ping (Section 5.2.1), removing the Power Signal after receipt of a Signal Strength Packet (or an End Power Transfer Packet, or after a time out).15 This yields a new set of Primary Cells, namely those which report a Signal Strength Value that exceeds a certain threshold—which the Power Transmitter chooses. Finally, the Power Transmitter executes an extended Digital Ping (Sections 5.2.1 and 5.2.2) for each of the Primary Cells in this new set in order to identify the discovered Power Receivers. In order to select the most appropriate Primary Cells for power transfer from the set, the Power Transmitter should take the situations discussed in Annex C.2.1, C.2.2, and C.2.3 into account.

C.2.1 A single Power Receiver covering multiple Primary Cells

Figure C-1 shows a situation in which the final set contains 12 Primary Cells. In order to select the most appropriate Primary Cell from this set, the Power Transmitter compares all Basic Device Identifiers that is has obtained. In this case, these are all identical. Accordingly, the Power Transmitter concludes that all Primary Cells in the set correspond to one and the same Power Receiver. Therefore, the Power Transmitter selects the Primary Cell that has the highest Signal Strength Value as the most appropriate Primary Cell to use for power transfer. In the specific example shown in Figure C-1, this could be Primary Cell 2, 3, 4, 5, 8, 9, 10, or 11.

15Note that the Power Transmitter should ensure that after terminating a Digital Ping using a particular Primary Cell, it waits sufficiently long—for example (see Table 5-5 in Section 5.3)—prior to executing a Digital Ping to that same Primary Cell or any of its neighboring Primary Cells. This ensures that any Power Receiver that is present on the Interface Surface at the location of the Primary Cell can return to a well-defined state.

1 2 3 4 5 6

7 8 9 10 11 12

Figure C-1: Single Power Receiver covering multiple Primary Cells

C.2.2 Two Power Receivers covering two adjacent Primary Cells

Figure C-2 shows a situation in which the final set contains 12 Primary Cells—the same set as in the situation discussed in Annex C.2.1. In order to select the most appropriate Primary Cell from this set, the Power Transmitter compares all Basic Device Identifiers that is has obtained. In this case, the Power Transmitter determines that there are two subsets of identical Basic Device Identifiers. Accordingly, the Power Transmitter concludes that it is dealing with two distinct Power Receivers. Therefore, the Power Transmitter selects the most appropriate Primary Cell from each subset. In the specific example shown in Figure C-2, this could be Primary Cell 2, or 8 for the left-hand Power Receiver, and Primary Cell 5, or 11 for the right-hand Power Receiver. Note that due to interference, the Power Transmitter most likely cannot communicate reliably using Primary Cells 3, 4, 9, and 10.

1 2 3 4 5 6

7 8 9 10 11 12

Figure C-2: Two Power Receivers covering two adjacent Primary Cells

C.2.3 Two Power Receivers covering a single Primary Cell

Figure C-3 shows a situation in which the final set contains 2 Primary Cells. Here, the underlying assumption is that the two Power Receivers have widely different response times ( , see Section 5.3.1)

to a Digital Ping. For example, the left-hand Power Receiver responds very fast (close to ( )), whereas

the right-hand Power Receiver responds very slow (close to ( )). This enables the Power Transmitter to receive the Signal Strength Packet from the fast Power Receiver, but not from the slow one. However, the Power Transmitter cannot reliably receive any further communications—from either Power Receiver— due to collisions between transmissions from the two Power Receivers. Accordingly, the Power Transmitter cannot select a Primary Cell for power transfer.

1

2

Figure C-3: Two Power Receivers covering a single Primary Cell

C.3 Moving Primary Coil based Free Positioning

In the case of moving Primary Coil based Free Positioning, typically a special Detection Unit provides discovery and localization of a Power Receiver. This Annex C.3 discusses an example of such a Detection Unit, which makes use of the resonance in the Power Receiver at the detection frequency . In this example Detection Unit, detection coils are printed on the Interface Surface of the Base Station. The top right-hand part of Figure C-4 shows a single rectangular detection coil, which consists of two windings. The width of the detection coil is 22 mm, and its length depends on the size of the Interface Surface. As shown in the bottom part of Figure C-4, a first set of these detection coils is laid out in parallel to cover the entire Interface Surface, in such a way that that the areas of two adjacent detection coils overlap for 60%. A second set of these detection coils is laid out similarly, but orthogonal to the detection coils in the first set.

375 ns

5.0 V

30 mm

3.75 ms

Figure C-4: Detection Coil

Detection of a Power Receiver proceeds as follows: In first instance, the Power Transmitter uses the detection coils as an electrostatic sensor to detect the placement or removal of objects on the Interface Surface; see Annex B.2. Once the Power Transmitter has detected an object, it uses the detection coils to determine the position of that object on the Interface Surface. For this purpose, the Power Transmitter applies a short pulse train to each of the detection coils—one by one. This pulse train consists of 8 pulses, and is shaped to trigger the resonance in the Power Receiver at the frequency . See the top left-hand part of Figure C-4. As a result, a minute amount of energy is transferred to the resonant circuit in the Power Receiver. Immediately after the pulse train terminates, this energy is re-radiated, which the Power Transmitter can detect using the detection coils. By analyzing the responses from each of the detection coils, the Power Transmitter can determine the location of the Power Receiver on the Interface Surface. Subsequently, the Power Transmitter can move its coil underneath the Power Receiver, and can start to transfer power as defined in Section 5. During power transfer, the Power Transmitter can adjust the position of the Primary Coil in order to optimize its coupling to the Secondary Coil, e.g. by maximizing the system efficiency—the Power Transmitter can calculate the system efficiency from its input power and the Actual Power Value contained in the Actual Power Packets, which it receives from the Power Receiver.

An advantage of this detection method is that it is not sensitive to Foreign Objects that do not exhibit a resonance near the detection frequency . The reason is that such objects do not store and re-radiate energy picked up from the pulse train. As a result a Power Transmitter does not need to move the Primary Coil to attempt power transfer to such objects.

Annex D Foreign Object Detection (Normative)

In order to enable a Power Transmitter to monitor the power loss across the interface as one of the possible methods to limit the temperature rise of Foreign Objects (see [Part 2], Section 5), a Power Receiver shall report its Received Power to the Power Transmitter.

The Received Power indicates the total amount of power that is dissipated within the Mobile Device due to the magnetic field produced by the Power Transmitter. The Received Power equals the power that is available from the output of the Power Receiver plus any power that is lost in producing that output power. For example, the power loss includes (but is not limited to) the power loss in the Secondary Coil and series resonant capacitor, the power loss in the Shielding of the Power Receiver, the power loss in the rectifier, the power loss in any post-regulation stage, and the eddy current loss in metal components or contacts within the Power Receiver.

This version 1.1.1 of the System Description Wireless Power Transfer, Volume I, Part 1, does not define any specific method for a Power Receiver to determine the Received Power—but as an example, the Power Receiver could measure the net power provided at its output, and add estimates of any applicable power loss.

reported Received Power is greater than or equal to the Transmitted Power in the case that there is no Foreign Object present on the Interface Surface—because in the latter case, the Received Power equals the Transmitted Power—and as a result, a Power Transmitter is less likely to falsely detect a Foreign Object.

(Informative) In view of the accuracy of the Test Power Transmitter that is used to verify compliance to the above requirement (see [Part 3]), it is recommended that a Power Receiver overestimates the actual Received Power by at least .

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Annex E Mechanical Design Guidelines (Informative)

E.1 Base Station

For the best user experience with respect to wireless power transfer, it is recommended that:

The Base Station Interface Surface extends higher than its surroundings, or has a size of at least

.

The Base Station Interface Surface is marked to indicate the location of its Active Area(s)—e.g. by means of the logo or other visual marking, lighting, etc.

In the case of stand-alone Base Stations, the Active Area is centered within the Base Station Interface Surface.

E.2 Mobile Device

The overall shape and size of a Mobile Device is dictated by its primary application. For example, cell phones, head sets, and digital (still) cameras, all have substantially different form factors. For the best user experience with respect to wireless power transfer, it is recommended that the mechanical design of a Mobile Device follows the guidelines listed below—to the extent possible in relation to the primary application of the Mobile Device:

The Mobile Device Interface Surface is flat.

The Mobile Device Interface Surface is marked to indicate the location of its Active Area—e.g. by means of the logo or other visual marking.

The location of the Active Area is centered within the Mobile Device Interface Surface.

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