a wireless battery-less computer mouse with super capacitor energy buffer

A Wireless Battery-less Computer Mouse with Super

Capacitor Energy Buffer

A. P. Hu, Senior Member IEEE, I. L. W. Kwan, C. Tan, Y. Li

The Department of Electrical and Computer Engineering

The University of Auckland, New Zealand

Abstract-This paper proposes a novel wireless computer mouse power supply system which runs off a standard USB port and eliminates the use of batteries completely. The system uses a soft switched current-fed push-pull converter for DC to AC conversion. The AC current, which flows through a disk coil embedded into a mouse-pad, provides a time varying magnetic field that transfer power to a power pick-up coil located within the mouse, through magnetic induction. This power, which was tuned-up and regulated by an advanced voltage control method- dynamic detuning control, has proven to be able to provide sufficient power to drive a wireless mouse. Furthermore, an energy buffer system is added which backs up and allocates the induced power to allow mouse operation and energy storage to occur in a coordinated way. This system allows the mouse to sustain in operation even when it is moved away from the mouse-pad. A minimum duration of 2 minutes was achieved when the mouse was constantly active.

I. I NTRODUCTION

Inductive power transfer (IPT) systems have been developed and found many industrial applications such as materials handling systems and electric vehicles [1-3]. They utilise the principle of electromagnetic induction to achieve power transfer without galvanic contacts [4]. The basic principle of IPT can be used in many other situations where contactless power supplies are required.

Although a lot of research work has been undertaken on modeling and design of IPT systems for high power applications at Watts to kilo Watts level [5-12], not much has been reported on design of wireless power systems at very low power level. This research is about the application of IPT in computer mice with power consumption at mW level. The input power is strictly limited by a standard USB power capacity, and the circuit size has to be very small. Cutting the tail off a computer mouse makes it much more convenient to use. However, most existing commercial wireless computer mice are battery powered. The batteries require regular replacement, which creates three obvious problems: the inconvenience of battery replacement, the long-term battery replacement costs, and the environmental concerns associated with battery disposal. Therefore, it would be ideal if the batteries can be eliminated. An additional advantage of a wireless mouse without batteries is it would be lighter in weight. This paper proposes a wireless battery-less computer mice solution, and presents the design and practical implementation of a wireless computer mouse prototype with a super capacitor energy buffer.

II. O VER V IEW OF THE P ROPOSED IPT C OMPUTER M ICE

A.System Configuration

The configuration of the proposed IPT wireless computer mouse is shown below in Fig. 1. As can be seen, the IPT primary circuit and the wireless receiver module were located together to form a single integrated mouse-pad solution. The mouse-pad was then connected to a standard USB port, which provided power and the communication channel with the computer. This architecture was built on top of a commercial battery powered wireless computer mouse, with the original RF hardware replaced by a pair of Nordic nRF24E1 RF transceiver such that the receiver could be integrated with the IPT primary module. The primary track was formed to cover the area of a standard mouse-pad, providing a magnetic field distribution over it. Finally, the pick-up and the wireless transmitter are located within the

battery compartment of the wireless mouse.

Fig. 1: The proposed configuration of a wireless computer mouse.

B.Design Objectives /Requirements

The design objective is to produce sufficient power at the pick-up for the load, which is the wireless computer mouse. According to the specification of the original wireless computer mouse, the power demand is shown in Table I.

Table I: Estimated power consumption of wireless computer mouse Active State Stand-by State

3.3 V x 50mA = 165mW 3.3 V x 33mA = 108.9mW

However, this does not take into account of the wireless receiver module, the losses in the power converter, primary track and the rectifier. This is one of the major design

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challenges, considering the available power of a standard USB is only 100mA for low power mode, and 500mA for high power mode at 5V. To make the system actually work, the power converter for high frequency magnetic field generation, as well as the power pick-up circuit has to be very efficient. With respect to this challenge, the appropriate converter, track material and secondary voltage regulation has been selected and designed. These will be discussed in the preceding section.

Fig. 2: Design Flowchart.

The design and implementation of an IPT system is a complex process involving the juggling of many parameters. The steps as shown in Fig. 2 were adopted to provide a rational flow of the project work.

III. P RIMARY I MPLEMENTATION

A.Resonant Converter

As described in section I, resonant converters are more power efficient due to its ability to achieve ZVS. Traditionally, controllers and zero point detectors were required to:

?Assist the set-up of the oscillation.

?Control the commutation of the switches on the two branches of the resonant converter, such that the

oscillation can be sustained and ZVS achieved.

However, a novel self-sustained current-fed push-pull resonant converter with no additional controllers [4] has been developed for this project. This largely reduced the complexity, power budget stress and the over all cost of the system. The structure of the current-fed push-pull resonant

converter is shown in Fig. 3.

Fig. 3: The self-sustained current-fed push-pull resonant converter.

As can be seen, the circuit comprises a current-fed push-

pull inverter with parallel compensation. The exception is that each switch obtains its switch driving signal form the input of the alternative switch. Since the switching is fully controlled by the resonance, the switching instances will occur at the zero crossing and ZVS is achieved. Since the direction of the current is reversed when the switches exchange its on-off state, the current is also in phase with the voltage. This means zero phase angle (ZPA) operation is also achieved, hence only real power P is delivered to the resonant tank.

Self-starting and ZVS cannot be achieved if the primary quality factor is below a critical value. The primary quality factor is defined as:

P

P

P R

L

Q0

ω

=(1)

Where ?0is the nominal (oscillating) frequency, L P is the primary track inductance and R P is the track resistance. According to [13] and [14], the minimum Q P for self-starting

and ZVS is 2.54 and 1.86 respectively.

The current-fed push-pull resonant converter was constructed, and tested. It was able to start up and shut down automatically without any problem. The resultant voltage waveforms are shown in Fig. 4. It c an be seen c learly that

the voltage a c ross one swit c h rises smoothly from zero voltage after the voltage a c ross the other swit c h rea c hes zero. The final resonant voltage a c ross the tuning c apa c itor

is an ex c ellent sinusoidal voltage sour c e, whi c h is used to drive the tra c k loop.

B.Primary Track Configuration

The physical configuration of the primary track determines the distribution of the magnetic field, which has a great effect on the power transfer efficiency. Equation 2 below describes the relationship between V OC, magnetic

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field strength and distribution:

⊥=B OC NBA V 0ω (2)

.

Fig. 4: Voltage waveforms over switches (1&2) and tuning capacitor (3)

Where N is the number of turns of the pick-up coil, A is the area enclosed by the pick-up coil and B is the magnetic flux density component that cuts perpendicularly through A. From this equation it can be seen the primary track configuration should have a field distribution such that it is cutting perpendicular through the area enclosed by the pick-up, at any location on the mouse-pad.

Two configurations, the spiral and the disk coil configuration, have been considered. The analysis of each is

shown below:

Fig. 5: The spiral coil configuration.

For the spiral configuration, the flux in the vertical

direction is cancelled out by neighboring wires.

Consequently, the magnetic field flows within the horizontal plane, or, along the surface of the mouse-pad. The flow

direction in the horizontal plane is shown by the arrows

below in Fig.5 (a).

With this field orientation, the pick-up will have to be

rotated on different parts of the spiral coil to achieve maximum power transfer, as shown in Fig.6 (b). Another

possibility is to use two pick-ups. However, both of these

options are not practical considering the limited space

available in a wireless-mouse.

Fig. 6: The magnetic field distribution of a spiral coil distribution.

Fig. 7: The disk coil configuration.

In contrast, a disk coil configuration, as shown in Fig.7, has a simpler magnetic field distribution, as shown in Fig.8. By having the enclosed plane of the pick-up perpendicular to the vertical axis, the angle between the pick-up and the field direction would be more optimal on different parts of a disk

coil. Hence the disk coil was adopted.

Fig. 8: The magnetic field distribution of the disk coil configuration.

Multi-strands Litz wires were used to form the primary track, as it has a lower AC resistance which helps to reduce the AC power losses. After practical experiments, it has been

found that the maximum power requirement of 165mW has

been met in most locations on the mouse pad.

IV. S ECONDARY P ICK -UP I MPLEMENTATION

So far, most blocks of the IPT system have been implemented. However, it has been found at certain locations the open circuit voltage V OC was lower than the

requirement of 3.3 volts. As discussed in section II-B, parallel pick-up compensation can increase V OC by a factor of Q S .Hence parallel tuning was adopted. However, this

introduced another problem: a parallel compensated pick-up behaves like a current source and the supplied voltage

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changes with the load. There is a potential danger of the tuned voltage exceeding the maximum voltage of 3.6 volts. For this reason, DDC (dynamic detuning control), a smart voltage regulation method has been developed to regulate the output voltage to be constant. A.

Principle of DDC

Fig. 9: The DDC circuit diagram.

A simplified DDC circuit is shown below in Fig. 9. In this circuit C t is the detuning capacitor and C f is a voltage stabilising capacitor. When the load voltage V L exceed a limit threshold, the capacitor switching control circuit switches on S, which completes the connection of C t ,changing the over-all tuning capacitance to (C S +C t ). This changes the amount of tuned-pickup current output, and hence V L .

There are two possible tuning schemes: under-tuning and over-tuning. In under-tuning the combined value of C t and C S is equal to the resonance capacitance, which gives the maximum tuned pick-up output current as shown in Fig.10. In this application, this maximum point should provide a V L of 3.3 volts. For over-tuning C S is set to the resonance capacitance, while C t is set such that (C S +C t ) will reduce the tuned pick-up output current to an appropriate level. In a nutshell, the under-tuning scheme attempts to switch on more frequently to provide the power needed under heavy loads, where as the over-tuning scheme switches on when

there is excess power.

Fig. 10: Relationship between tuning capacitance and parallel

compensated pick-up output current.

This control mechanism is much more efficient than Zener diodes or linear regulators, in which unused power is

dissipated as heat. This disadvantage is not present in DDC, as it regulated by detuning the power pick-up.

B.Performance of the DDC circuit

The performance of the DDC circuit was tested with both the tuning schemes. It was discovered for this application the under tuning scheme produced less power (140mW), as the DDC circuit was constantly switching to provide sufficient power to the load, and power was lost in this constant switching process. In contrast, the over-tuning scheme switched only when there was excess power. Consequently the switching loss was much smaller and a power output of 193mW was obtained.

The waveform of V L ,which is the voltage across the wireless computer mouse input,and the switching signals for the over-tuning scheme is shown below in Fig. 11. It can be seen that V L

was successfully controlled at about 3.3V.

Fig. 11: V L and switching signal with over-tuning DDC.

V. P ROTOTYPE P ERFORMANCE

The developed prototype circuit was tested with a standard USB power supply. It has found that the IPT power supply can successfully delivered the required power at all locations within the disk coil.

Finally the total power consumed from the USB power supply is recorded in Table II. It can be seen that the maximum power consumption under active state was within the low-power mode limit of the USB. Hence, it was concluded a successful prototype capable of supplying the required power from a USB power supply was achieved.

Table II: Power consumed from the USB power supply. VI. E NERGY B UFFER S YSTEM

An intrinsic limitation of the IPT technology is the limited power transfer range between the primary and the pick-up. Consequently, the movement of the IPT wireless computer mouse was limited within the mouse-pad. This would be

Operating Condition

Current Drawn

(mA)

Power Consumption

(mW)

Active State 96 480 Stand-by State 85 425 Mouse away from pad

57

285

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inconvenient in situations where the mouse is required to move away from the mouse-pad, for examples, when it is used for power point presentations or computer gaming. This

intrinsic disadvantage however, could be remedied by installing an energy buffer in the mouse.

The objective of the energy-buffer is to reserve excess

energy to sustain computer mouse operation when the power

supply is not available. The following design objectives were

set:

?The buffer must be charged relatively quickly, and

provide enough power for a suitable duration. In other

words, the energy buffer must have high power

density and suitable energy density. The suitable

candidate identified was super capacitors, which have

a very high power density and superior energy density

compared to the traditional capacitors.

?The charging of the energy buffer must have minimum interference to mouse operation. In other

words, a suitable power management circuit must be

implemented to distribute the power between the buffer and the mouse.

?The power transfer capability of the IPT system must be boosted such that enough power would be available to support buffer charging and mouse operation.The super capacitor was selected based on its capacitance to provide a suitable balance between the required charge time and sustain duration, so as to achieve a smooth operation. The IPT system was enhanced by increasing the operating frequency, improving the primary track configuration to provide a stronger magnetic field. The required V OC was increased to 10 volts to supply the super capacitors, in order to provide a larger discharge profile. Consequently a 3.0 volt step-down regulator was used before the load.

A.Power Management Circuit

Fig. 13 illustrates the DC part of the secondary circuit after the pick-up tuning. The energy buffer is highlighted in

orange, and the power management circuit in yellow.

Fig. 12: Circuit diagram showing the power management circuit and the energy

buffer in the secondary circuit.

The power management circuit is consisted of a simple comparator and BJT switch. When power is induced in the pick-up, the temporary capacitor and the load is supplied

first. When the voltage across the temporary capacitor (V Ctemp ) reaches a certain threshold, the comparator circuit

will drive the BJT on, completing the energy buffer’s connection and enable charging. During this time the load is

momentarily supplied by the temporary capacitor. The charging of the energy buffer stops as soon as V Ctemp drops below the threshold. This cycle repeats until the energy buffer becomes fully charged. As such, the power is appropriately distributed between the energy buffer and the computer mouse. B.Result

It has been discovered that the enhanced IPT system produced sufficient power to handle both the real time mouse operation and the energy buffer storage. After the temporary capacitor was charged to the preset threshold, sufficient power was still available to charge the energy

buffer capacitors and at the same time allow mouse operation. However, if both C temp and the energy buffer were charged simultaneously, it would take a very long duration

for V L to reach 3.3 volts. Hence it was concluded the power management circuit was successful in distributing the

induced power between the mouse and the energy buffer appropriately. Fig. 13 shows V L (top waveform) has been kept constantly at 3.3 volts, while the energy buffer is

steadily charged (bottom waveform).

Fig. 13: Waveform showing simultaneous charging of energy buffer and mouse

operation.

Finally, the integrated IPT and energy buffer system was tested on a commercial wireless mouse (originally battery powered). When the final prototype was placed on the mouse-pad location with the strongest field strength, it took 0.96 seconds for V L to reach to 3.0 volts, and 21 seconds to charge both C temp (1Farad) and the energy buffer (1Farad) fully. When fully charged, the mouse could be sustained for 2 minutes under constant active-state and 9 minutes under constant stand-by state. This would be very useful for short duration off-pad operation of the mouse, such as the situation in computer gaming.

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VII. C ONCLUSION

This paper has proposed a complete wireless/battery-less

IPT computer mouse solution. A working prototype driven

by a standard USB power source have been successfully

designed and tested. The self-sustained current-fed push-pull

resonant converter and the DDC (Dynamic Detuning

Control) circuit were proven to be simple but very effective.

Furthermore, an energy buffer system has been developed

for a minimum 2 minutes off-pad option, which further

enhanced the usability of the proposed mouse system.

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