Determining The Optimum Transmitting Power - Balluff BIS U-4A7-082-01C-07-S4 Betriebsanleitung

BIS U-4A7-082- _1C-07-S4
Industrial RFID system - Read/write head
5
Application planning (continued)
5.9

Determining the optimum transmitting power

Data carriers require a certain amount of energy. With
passive RFID data carriers, this energy must be taken from
the electromagnetic field provided by the UHF RFID read/
write head or its antenna. The current and voltage supplied
to the data carrier is directly dependent on the field
strength. Only when the field strength exceeds a certain
threshold does the data carrier start to operate.
The value of the electric field strength that is high enough
to supply the data carrier with sufficient energy is called the
response field strength. Since the field strength cannot be
readily measured, the transmitting power or antenna power
is used as a measure to describe the field strength.
Fig. 5-11 describes the response behavior of a passive
UHF RFID data carrier as a function of the set transmitting
power and is only intended to clarify the relationship. The
transmitting power actually required depends, among
other things, on the data carrier used, the distance to the
read/write head and the environment.
Antenna power ERP in dBm
Fig. 5-11: Response field strength – Detections depending on
transmitting power
In the setup of an RFID application, the value of the
response field strength is significant, but ultimately the
required transmitting power must be set. To determine the
required transmitting power, it is first set to a low value and
then gradually increased, with an attempt being made to
address the data carrier in each case. A response from the
data carrier is received when the characteristic response
field strength is reached or exceeded by the set
transmitting power. At the limit, the reliability may still
fluctuate; with higher power and thus field strength, it
should then be possible to read out the data carrier in a
stable manner. With this bottom-up approach, the
optimum transmitting power is approached from below. At
each power level, multiple attempts (e.g. 100) should be
made to estimate the reliability.
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If the lowest power at which the data carrier is detected
100 % of the time were now to be used permanently, the
smallest changes in the ambient conditions could result in
the read operation not being reliable, e.g. due to the
following factors:
– Offset of the data carriers in front of the antenna
– Fluctuations of the electrical parameters of the data
carriers
– Different materials in containers or in the vicinity
– Persons or vehicles moving in the vicinity
Therefore, the transmitting power should be increased by a
safety margin of approx. 2 to 5 dBm. If several data
carriers are to be detected, the safety margin should be
added to the power from which all tags are successfully
detected.
However, the transmitting power should not be set too
high, as this can have a negative effect on the function of
the RFID system (see chapter 5.8 on page 16). In
addition, more distant data carriers whose detection is not
intended can also be detected in this way. The optimum
transmitting power always has a distance of several dBm
to the power at which unintentional detections take place
(see Fig. 5-12).
Antenna power ERP in dBm
Fig. 5-12: Operating range of transmitting power – Detections
depending on transmitting power
The AutoSetup function of the read/write head (see
chapter 8.3.2 on page 36) attempts to find an optimum
setting in the application for the detection of a single data
carrier in the detection range of the read/write head (single
tag operation only). If a minimum safety margin for the data
carrier to be detected and a sufficient distance to any other
data carriers was able to be determined in the process,
AutoSetup is completed successfully and the setting for
the transmitting power is adjusted automatically. In
addition, the function also determines to what extent the
power may have to be increased in order to also perform
write operations. If the AutoSetup is unsuccessful or if a
large number of data carriers are to be detected, manual
setting with the bottom-up procedure described above
can be used to achieve an optimum setting.
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