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5 System Control

5.1Introduction

From a system control perspective, power transfer from a Power Transmitter to a Power Receiver comprises four phases, namely selection, ping, identification & configuration, and power transfer. Figure 5-1 illustrates the relation between the phases. The solid arrows indicate transitions, which the Power Transmitter initiates; and the dash-dotted arrows indicate transitions that the Power Receiver initiates. By definition, if the Power Transmitter is not applying a Power Signal, the sytem is in the selection phase. This means that a transition from any of the other phases to the selection phase involves the Power Transmitter removing the Power Signal.

Power Transfer Contract violation unexpected Packet

time-out

power transfer

Power transfer complete

Figure 5-1: Power transfer phases

The main activity in each of these phases is the following:

selection In this phase, the Power Transmitter typically monitors the Interface Surface for the placement and removal of objects. The Power Transmitter may use a variety of methods for this purpose. See Annex B for some examples. If the Power Transmitter discovers one or more objects, it should attempt to locate those objects—in particular if it supports Free Positioning. In addition, the Power Transmitter may attempt to differentiate between Power Receivers and Foreign Objects —keys, coins, etc. Moreover, the Power Transmitter should attempt to select a Power Receiver for power transfer. If initially the Power Transmitter does not have sufficient information for these purposes, the Power Transmitter may repeatedly proceed to the ping and subsequently to the identification & configuration phases—each time selecting a different Primary Cell—and revert to the selection phase after collecting relevant information. See Annex C for examples. Finally, if the Power Transmitter selects a Primary Cell, which it intends to use for

power transfer to a Power Receiver, the Power Transmitter proceeds to the ping phase—and eventually to the power transfer phase. On the other hand, if the Power Transmitter does not select a Power Receiver for power transfer—and is not actively providing power to a Power Receiver for an extended amount of time—the Power Transmitter should enter a stand-by mode of operation.6 See [Part 2] for performance requirements on such a mode of operation.

ping In this phase, the Power Transmitter executes a Digital Ping, and listens for a response. If the Power Transmitter discovers a Power Receiver, the Power Transmitter may extend the Digital Ping, i.e. maintain the Power Signal at the level of the Digital Ping. This causes the system to proceed to the identification & configuration phase. If the Power Transmitter does not extend the Digital Ping, the system shall revert to the selection phase.

identification & configuration In this phase, the Power Transmitter identifies the selected Power Receiver, and obtains configuration information such as the maximum amount of power that the Power Receiver intends to provide at its output. The Power Transmitter uses this information to create a Power Transfer Contract. This Power Transfer Contract contains limits for several parameters that characterize the power transfer in the power transfer phase. At any time before proceeding to the power transfer phase, the Power Transmitter may decide to terminate the extended Digital Ping—e.g. to discover additional Power Receivers. This reverts the system to the selection phase.

power transfer In this phase, the Power Transmitter continues to provide power to the Power Receiver, adjusting its Primary Cell current in response to control data that it receives from the Power Receiver. Throughout this phase, the Power Transmitter monitors the parameters that are contained in the Power Transfer Contract. A violation of any of the stated limits on any of those parameters causes the Power Transmitter to abort the power transfer—returning the system to the selection phase. Finally, the system may also leave the power transfer phase on request of the Power Receiver. For example, the Power Receiver can request to terminate the power transfer— battery fully charged—reverting the system to the selection phase, or request to renegotiate the Power Transfer Contract—change to trickle charging the battery using a lower maximum amount of power—reverting the system to the identification & configuration phase.

Section 5.2 defines the system control protocols in the ping, identification & configuration, and power transfer phases from a Power Transmitter perspective. Section 5.3 defines the system control protocols in these four phases from a Power Receiver perspective. Note that this version 1.1.1 of the System Description Wireless Power Transfer, Volume I, Part 1, does not define the system control protocol in the selection phase. Further note that—from a power transfer point of view—the Power Receiver remains passive throughout most of the selection phase.

At any time a user can remove a Mobile Device that is receiving power. The Power Transmitter can recognize such an event from a time-out in the communications from the Power Receiver, or from a violation of the Power Transfer Contract. As a result, the Power Transmitter aborts the power transfer and the system reverts to the selection phase.

Throughout the power transfer phase, the Power Transmitter and Power Receiver control the amount of power that is transferred. The Figure 5-2 illustrates a schematic diagram of the power transfer control loop, which basically operates as follows: The Power Receiver selects a desired Control Point—a desired output current and/or voltage, a temperature measured somewhere in the Mobile Device, etc. In addition, the Power Receiver determines its actual Control Point. Note that the Power Receiver may use any approach to determine a Control Point. Moreover, the Power Receiver may change this approach at any time during the power transfer phase. Using the desired Control Point and actual Control Point, the Power Receiver calculates a Control Error Value—for example simply taking the (relative) difference of the two output voltages or currents—such that the result is negative if the Power Receiver requires less power in order to reach its desired Control Point, and positive if the Power Receiver requires more power in order to reach its desired Control Point. Subsequently, the Power Receiver transmits this Control Error Value to the Power Transmitter.

6Note that it is up to the Power Transmitter implementation to determine whether this stand-by mode of operation is part of the selection phase or is separate from the selection phase.

System Description

Wireless Power Transfer

The Power Transmitter uses the Control Error Value and the actual Primary Cell current to determine a new Primary Cell current. After the system stabilizes from the communications of the Control Error Packet, the Power Transmitter has a short time window to control its actual Primary Cell current towards the new Primary Cell current. Within this window, the Power Transmitter reaches a new Operating Point—the amplitude, frequency, and duty cycle of the AC voltage that is applied to the PrimaryCell. Subsequently, the Power Transmitter keeps its Operating Point fixed in order to enable the Power Receiver to communicate additional control and status information. See Section 5.2.3.1 for details.

Figure 5-2: Power transfer control loop

5.2Power Transmitter perspective

Section 5.2.1 defines the protocol that the Power Transmitter shall execute in order to select a Power Receiver for power transfer. This protocol comprises a Digital Ping. Section 5.2.2 defines the protocol that the Power Transmitter shall execute in order to identify the Power Receiver and establish a Power Transfer Contract. This protocol extends the Digital Ping, in order to enable the Power Receiver to communicate the necessary information. Section 5.2.3 defines the protocol that the Power Transmitter shall execute after it has established the Power Transfer Contract. During execution of this protocol, the Power Transmitter controls its Primary Cell current in response to control data that it receives from the Power Receiver.

Many provisions in this Section 5.2 refer to the start and/or the end of a Packet, or the start of a Packet’s preamble. For the purpose of those provisions, the start of a Packet is defined as the instant the Power Transmitter receives the first edge of the start bit of the Packet’s header byte; the end of a Packet is defined as the instant the Power Transmitter receives the second edge of the stop bit of the Packet’s checksum byte; and the start of a Packet’s preamble is defined as the instant the Power transmitter receives the first edge of the first preamble bit.

If the Base Station can take its input power from a USB Micro-B or Micro-AB receptacle, the Power Transmitter can potentially not provide the requested amount of power to a Power Receiver. If a Power Receiver has made at most three unsuccessful attempts to initiate and maintain power transfer—e.g. has terminated the power transfer three times in a row with an End Power Transfer Packet containing an End Power Transfer Code of 0x01 (Charge Complete), 0x07 (Reconfigure), or 0x08 (No Response)—the Power Transmitter shall refrain from entering the power transfer phase until the Power Receiver has been removed from the Interface Surface of the Base Station.

5.2.1Ping phase

In the ping phase, the Power Transmitter shall execute a Digital Ping. This Digital Ping shall proceed as follows, with conditions appearing earlier in this list take precedence over conditions appearing later:

The Power Transmitter shall apply a Power Signal at the Operating Point defined for the particular Power Transmitter design (see Section 3).

Primary Cell current amplitude reaches 50% of the stable level, the Power Transmitter shall remove the Power Signal (i.e. reduce the Primary Cell current to zero) within . See Figure 5-3(a).

If the Power Transmitter correctly receives a Signal Strength Packet, the Power Transmitter may proceed to the identification & configuration phase of the power transfer, maintaining the Power

Signal at the Operating Point as defined for the particular Power Transmitter design. See Figure 5-3(b). If the Power Transmitter does not proceed to the identification & configuration phase, the

If the Power Transmitter does not correctly receive (see Section 6.2.4) the first Packet within the

If the Power Transmitter correctly receives any other Packet than a Signal Strength Packet, and in

particular if the Power Transmitter receives an End Power Transfer Packet, the Power

If the Power Transmitter does not proceed to the identification & configuration phase, the Power Transmitter shall revert to the selection phase.

Note that the thick line in Figure 5-3 represents the amplitude of the Power Signal, which is zero at the left-hand side of the diagrams. The dashed line represents possible communications from the Power Receiver, which the Power Transmitter shall ignore—as follows from the above conditions.

System Description

Wireless Power Transfer

Figure 5-3: Power Transmitter timing in the ping phase

Table 5-1: Power Transmitter timing in the ping phase