Active RFID Real Time Location System by Zoning - IT VIET

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Wednesday, October 26, 2011

Active RFID Real Time Location System by Zoning

Real time location tracking can be done either by triangulation or by zoning. The former method makes use of sophisticated algorithms to locate the RFID tag based on comparing the readings of three or more readers while the latter one is done by dividing areas into zones, allocating a reader to each zone and tuning its gain in order to detect only tags in the said zones.

What is zoning method?

By dividing the area into different zones and installing readers at each zone, tags can be detected when entering a specific zone. With a list of records on which tag being read by which reader, the system is able to identify the location of the tag. This is how the zoning method works for real time location tracking.

Tuning of readers of zoning method:

Tuning of readers during installation takes an important role in zoning method. It allows precise and stable reading of readers in specific zones.

For example, even if a tag is positioned close to the wall and could also be detected by the reader in the adjacent room, the tag is still considered located in the correct zone. The signal from the tag is stronger for the reader in the room but weaker for the reader in the next room to detect. With such feature, zone level locating accuracy is performed excellently.

Using the zoning method for RFID real time location tracking has the following advantages:

  •  High zone level reading accuracy of up to approximately 99% with developed software
  •  Less affected by indoor obstacles like partitions
  •  Suitable for both people and assets locating
  •  Cost effective compared to other RFID real time location tracking methods
  •  Less power is required compared to location tracking with Wi-Fi system
What is triangulation?

The triangulation method compares the readings from three or more readers in an area using sophisticated algorithms. There are two types of triangulation methods, Relative Signal Strength Intensity (RSSI) and Time Difference on Arrival (TDOA).

Principle: Location coordinate system for triangulation

Consider the coordinates system for location. During initial installation, coordinates of three or more readers are recorded. When the tag is inside the area, signal of the tag is received by all the readers in the area, with different signal strength or arrival time of signals. Location of the tag is then measured by solving the set of equations.

Wi-Fi tags method used by some companies is also a kind of RSSI triangulation. Special network access points for reading and transmitting the measurement of the signal strength are used instead of readers in the area.

Applying triangulation for location tracking may have inaccurate results especially in indoor areas. Inaccuracies occur mainly due to:
  •  Variation of sensitivities of commercial readers
  •  Non-identical reflection in the beam passage towards each one of the reader
  •  Direct and reflected beam of radio waves may create destructive interferences when they meet each other
  •  Tag does not emit signal uniformly to a range of 360˚
  •  For indoor applications, presence of partitions reduces intensity of signal compared to a direct path without any obstacles
Other than inaccuracy problem, the major disadvantage of a TDOA system is its high cost due to the following reasons:
  •  The readers must use expensive internal crystal clocks
  •  More than three readers must be used in order to get a reasonably accurate result even in a small area, such as 20m x 20m
  •  Complex tuning procedures may be required for accurate readings
  •  Ultra-wide band may be used to improve the range of detection by using short pulses over a broader spectral range
  •  For indoor applications, at least four synchronized readers must be used to cover approximately 15 rooms
Comparing Zoning and Triangulation

Though accuracy of zoning method when used in outdoor environments is reduced mainly due to the reflection and orientation of tags, in comparison to all the methods that we have reviewed, it is clear beyond any doubt that it is still the most suited approach for indoor applications, where room-level accuracy location tracking is required. Reasons are the following.

• Significantly lower installation and maintenance cost with the same level of performance

• Significantly higher accuracy

• Higher stability of reading

• Simple installation


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