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Cuttable Electronics – Who Would Have Thought?

A Japanese research engineer has created a thin flexible wireless charging array that can be cut to size with scissors in almost arbitrary ways.

You can do more than just cut this sheet into fun or interesting shapes,” said Ryo Takahashi of the University of Tokyo. “The sheet is thin and flexible so you can mold it around curved surfaces such as bags and clothes. Our idea is anyone could transform various surfaces into wireless charging areas.”

  Another aim was to make the whole sheet available for charging – many wireless schemes only work in some spots, and have dimples or magnets to nudge users into picking a viable spot.

The cuttable bit is dealt with by using a square array of almost touching square power transmitting antennas, and using a fractal H-pattern of RF feeds from the centre of the charging sheet to the various antennas. In this way, within reason, outside bits can be cut away leaving much of the RF distribution system operating – until so much is removed that nothing, not even one of the four adjacent central coils, are left in tact.

In many ways, the more interesting part of Takahashi’s design is that way that almost all of the surface is available for charging.

The receive coil in the object-to-be-charged is a rectangular coil and, if all the transmit coils were always always active, in many positions the receive coil will bridge two or more transmit coils leading to coupling that pushes the system off resonance, considerably reducing power delivered to the receiver.

To get over this, RF power to the transmit coils is time-multiplexed so that no two adjacent transmit coils are ever activated at the same time – which takes four circuits for a rectangular array.

This idea is not unique, but the team took it one stage further to avoid wasting power in Tx coils that would be activated but not near the receiver. Magnets were added to the receiver and local Hall sensors to the Tx array. The Hall sensors turn on (via reed relays, selected over solid-state alternatives) only Tx coils near the receiver – a scheme which needs no data processing and is local within the Tx array – no off-array connections are needed except for dc power to the Hall sensors and relay coils.

The prototype is 400 x 400mm hosting a 4 x 4 array of Tx coils, and is described in the paper ‘A Cuttable Wireless Power Transfer Sheet‘, published on the ACM digital library, and available in full free – and a pleasant read it makes, being a long light paper which covers a lot of ground and ends by describing installing the system in various use concepts including a jacket pocket, which was then built.

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