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Transponder News

A news service reporting on developments regarding the use of radio based tagging transponder systems for commerce and scientific applications. Covering the RFID technologies, EAS technologies and magnetic coupled techniques.

Multiple article scanning


As the distance over which one needs to read the identity of transponders is increased, so the importance of the reader to handle the multiple article scanning increases. This is primarily as it becomes less likely that with increased range only one transponder will be in the reading zone at the reading instant.

To detect the presence of multiple articles using RFID techniques actually is not simple and introduces many complexities. The central issue is that generally there is only one communications channel between all the transponders and the reader. As the transponders are not able to detect the transmissions from other transponders in the same reader area, often more that one transponder is transmitting at the same time causing a confused message to be received by the reader. A model of this situation would be for a room filled with deaf and blind people, where each calls out its identity number (transponder) while on the stage there is a blind person who has to identify all the people in the room (reader). Generally if the transponders in the reading zone are few in number, then by getting them to transmit at random times, occasionally a transponders transmission will not be interfered with by another transmission from some other transponder and its identity can be determined.

EAS tags handle the need for multiple identification by operating on the principle that tags are disabled when the goods to which they are attached are paid for, thereby meaning that if the receiver detects no transmissions, then no stolen goods are being removed. The procedure is the same as followed by the security personel whether there is one transponder detected or many transponders detected.

Few magnetic transponder systems can handle multiple tag situations. This is generally not serious as the reading range of the reader is short, say typically a few inches, and it is not too difficult to arrange the reading situation so that only one transponder is in the reading zone at one time. The reason for the problems with multiple transponders in the magnetic situation is that the protocol generally lacks a freedom of time dimension. The transponders are energised with a relatively strong field which transfers energy from the reader to the transponder for operating its logic, and the transponder replies immediately the energising pulse is removed, that is before it loses its onboard energy and without the energising field interfering with the weak return (this period is often called the flyback period). As all the transponders respond at exactly the same time, they all interfere with each other causing confused transmissions.
The developments from 1998 onwards of the 13.56MHz frequency range magnetic transponders by a number of manufacturers starts to change this situation as almost all manufacturers have realised the importance of multiple tags and have started providing proprietary anit-collision protocols in this product range. These tags operate similarly to Electric Field tags even though they are in fact magnetic coupled tags by virtue of their simple coil antennas. By having a continuous energising field applied by the reader during the scanning which is used to power the tags, the tags can use time for seperating replies as well as use onboard receivers on the tags for addressing specific tags. These tags generally are read/write and have two way communications between the tags and the reader, although their operating ranges are limited to approximately one meter.

For electric transponders, reception from multiple transponders is possible with increased transponder complexity, generally by using the time domain. As the energising field can be present at the same time as the reader is receiving the transponders transmissions. (Primarily because the energising frequency is slightly different from the reading frequency), this allows the transponder to emit transmissions at random times repeatedly over a long time period, increasing the probability that their transmission will be heard.
If the design can afford the additional cost of onboard receivers and does not need frequency agility, another protocol allows for communication between the reader and the transponders whereby each transponder can be individually addressed using unique identities, and the transponders individually polled. There are a number of similar protocols that can be implimented as soon as the transponder complexity can be increased.

A significant development in this regard happened in South Africa in 1994 when a demonstration of a supermarket trolley being scanned in a couple of seconds was shown to the world press. The significance of this development was that the transponders did not have the additional on board receivers, and in fact did not need unique identity numbers, features that were key to multiple transponder situations for electric coupled tags previously. This technology has yet to appear commercially and should cause dramatic reductions in the price of electric transponders when available.

In March 1998 the scanning trolley came a few leaps closer. The South African company Trolley Scan filed a series of provisional patents that focussed on replacing barcoded identification systems with RFID transponders. Having simplified the design of a transponder down to a very small silicon circuit comprising only a few tens of equivalent gates, a simple antenna system and simple packaging, the manufactured price of transponders has been decreased to levels where such an application is starting to be viable. Trolley Scan have as a demonstration of the robustness of their multiple article protocol provided details of 1000 tags being scanned simultaneously in one operation even with the tags having random geometric orientations.

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