Information can be stored in inexpensive electronic “tag” microchips which can be embedded in physical objects. We have invented a new tag reader technology which allows information to be transferred into or out of these tag microchips through the human body via touch. Our technology has enabled us to create a novel user interface which can recognize when physical icons are touched, and a wearable system that can inventory packages when they are touched.
One of the central ideas in modern windowed computer interfaces is the action of clicking on an icon. Recently researchers have placed tag chips in physical objects on a desk and a tag reader in the surface of a desk, to make “physical icons.”  We have invented a new kind of tag reader which only reads a tag when it is touched by the human user, thereby creating TouchTags. TouchTags preserve the intuitive familiarity of pointing to an icon while liberating the icons from the confines of the computer screen. We find TouchTags useful for applications such as quickly sorting a large list of information records by preference into several smaller “piles” of records.
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There are many emerging applications that use tagged physical objects as part of technology interfaces, using material or RFID (Radio Frequency Identification) tags embedded in things such as keys and cards. The “Speedpass” by Mobile Corporation, for example, allows a user to pay for gasoline by holding their tagged key chain in front of the pump. Such interfaces all use the act of bringing the tag to a reader to evoke the associated behavior. Here we show how the act of touching one tag in a collection of tags near the reader can cause it to be read.
In addition to novel interfaces, TouchTags are useful when used in conjunction with a “wearable” computer. When a human wears a computer on their body, we would often like the computer to be able gather information about physical objects in the environment as the human interacts with them. We have integrated our TouchTag reader into a shoe so that it can be worn by a postal worker. Since the TouchTag reader can couple to a tag through the human body, when the worker touches a package, their wearable computer now knows to provide tracking information about that package.
Retrieving information about a package by touching it
THE TOUCHTAG HARDWARE
We now describe our TouchTag reader, and then explain how we use our novel reader with commercially available RFID tags to create physical icons and a wearable inventory reader which are responsive to touch.
The tag reader we built reads and writes to V4050 RFID tag chips which are available from EM Microelectronics Company. They cost a few tens of cents (US) and contain a thousand bits of nonvolatile memory. They also contain a power harvesting circuit which is usually attached to a loop of wire or printed conductor which allows the tag chip to obtain its operating power from surrounding magnetic fields and communicate wirelessly with a nearby tag reader.
Usually, tags communicate with a reader magnetically, to minimize the influence of screening materials. We have invented a way to power and communicate with the tag chip through the human body by coupling to the tag electrostatically . This is also advantageous for cost because it replaces coils with cheaper flat foil electrodes.
Creating TouchTag Physical Icons
A tag chip can only receive power and information when one of its terminals is coupled to the tag reader, while the other terminal is coupled to ground . We connect a tag chip between a pair of foil electrodes inside a physical icon. Our TouchTag reader powers a larger electrode which can be embedded in a surface such as a table, whiteboard, or shelf. When we simply place the physical icon near the larger reader electrode, nothing happens because the capacitive coupling of the icon electrodes to the larger surface electrode are comparable, so the tag chip experiences only a small electrical gradient. If, however, a person approaches the proximity of one of the icon electrodes by touching the surface of the icon object, this symmetry is broken and typically tens of picofarads of ground return coupling are provided by the human body. This loading draws enough displacement current to power the tag.
Once the tag is activated, it can modulate the coupling between the icon electrodes to in turn modulate the load presented to the reader electrode. Knowledge of the modulation pattern can be used to separate this small change in the current sourced by the drive from environmental loading, thereby recognizing interaction with the tag. Since only the designated tag is excited, a large number of other tagged objects can be in proximity to the reader. And because of the large change in the ground return coupling provided by the body, the relative size of the reader’s electrode to the tag can be significantly greater than is possible with the fill-fraction constraint on magnetic tags.
Creating a TouchTag Wearable
The TouchTag reader we have created, like the tags, is small and cheap. Smaller than a credit card and less than US $20, it is well suited for use in a wearable system. In our system, the human user wears the TouchTag reader in their shoe, and their entire body becomes a tag reader. We have built a prototype system that could be used for inventory control in a warehouse or package delivery application.
We connect an RFID tag chip between a pair of foil electrodes inside a container or package. The package must be placed in proximity to a capacitive ground return such as the floor or furniture with a conductive frame. When the user wearing the reader approaches the proximity of one of the package electrodes by touching the surface of the package, a small displacement current couples through the human body, then through the TouchTag package, and finally down to ground [2,3]. This powers the tag and causes it to be read by the wearable reader. By changing the mode of the reader, the user can also write new information into the TouchTag package by touching it.
We invented a new tag reader technology which couples to RFID tags electrostatically through the human body. Information can therefore be transferred into or out of a tag when a person touches a tagged object.
Special thanks to Rehmi Post, Babak Nivi, Matt Reynolds, Yael Maguire and our other Physics and Media colleagues. We acknowledge the support of the Things That Think Consortium and our other sponsors at the MIT Media Laboratory.
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