TECHNICAL PAPER |
Reader Field Pattern: The electromagnetic field in the reading volume is defined by the reader coil geometry, the magnetic environment near the reader coil(s) and Maxwell's equations relating to magnetostatics. The field will generally not be consistent in intensity or orientation, due to all these factors. Static Tag Position and Orientation: For a tag stationary in the reader field at a given position and orientation, a deterministic function of tag activation is associated with the variation of magnetic field strength and orientation of the reader field. The maximum reading distance for a stationary tag in the reader field is a function of the field strength and the tag orientation in the field. This function, though complex, may be integrated over all possible static tag orientations within the reading volume to yield a probability of reading the static tag in the volume for all possible tag orientations. A tag will have the greatest activation distance at optimum orientation to the reader field lines, and less (or no) activation distance as a function of sub-optimal orientation. The threshold of tag activation therefore varies as a direct function of the field strength and as an inverse function of the distance between the tag and the reader. The probability of reading a stationary tag in the reading volume can be computed as a function of the activation distance for all tag orientations and the probability of tag orientation in the given direction throughout the volume (Figure 2).
An "ideal" reader could receive and decode the message in the time period corresponding to the maximum speed per trajectory. Above this speed, the probability for obtaining a reading is zero. A tag can also move through the reading volume with varying orientation, thereby varying its relative signal strength or even going through periods of de-activation on its way. For all speeds below the maximum speed, the probability of data transaction increases dependent on tag speed, orientation, trajectory, reader signal-to-noise ratio and other factors. The probability that a tag will be readable on account of its trajectory could be computed taking all these factors and all possible trajectories into account (Figure 3).
Even in systems that utilize "anti-collision" methods, multiple tags in the field will increase the amount of time necessary for completed data transactions with all the tags. Therefore another probability function for multiple tags in the reading volume simultaneously may be computed by: the trajectory of each tag, the number of tags in the volume, and the nature of the anti-collision algorithm (Figure 4). Noise Sources: Electromagnetic noise sources in the vicinity of the reader will decrease the probability of a successful data transaction. If the tag outputs a good signal in the presence of noise, the probability of the reader receiving erroneous information along with the tag signal increases according to a function of the noise intensity and frequency spectrum as related to the signal processing characteristics of the reader.
Understanding the diverse and interactive aspects of RFID technology, particularly in dynamic systems (tag and reader in relative motion), will enhance the possibility of optimizing system and product designs for specific applications. Systems and products may be optimized for such qualities as: maximum reading distance, maximum reading volume, minimum system power output, non-directional tag reading characteristics, minimum tag size, minimum data transaction time, most reliable (or secure) data transaction, maximum number of tags simultaneously in reader field, and others. Compliance with de facto and legal transmission and protocol standards limits design flexibility but provides the opportunities for interoperability of systems and wider markets. Optimizing systems and products for multiple objectives requires careful judgement regarding the design trade-offs, and the continuous challenge of improving the "state of the art" in RFID. References [1] US Patent 4,333,072 "Identification Device" Michael Beigel, June 1, 1982 [2] Objective Measurements for RFID System Performance , Michael Beigel, January 1993 http://www.rapidttp.com/transponder/beigel.html [3] RFID Design Guide, Microchip Technology, 1997 [4] Remote Control and Identification Systems Design Guide, TEMIC Semiconductors, August 1997 [5] Schuermann, J., Meier, H., TIRIS - Leader in Radio Frequency Identification Technology, Texas Instruments Technical Journal, TITJ Vol.10, No. 6, Nov. 1993, pp. 2-14 [Continued...] |
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