Power consumption and operating time are important factors in battery powered RFID readers. We set out to test and evaluate different power saving options.
Author: Toni Heijari
Less power consumption, more operating time
for battery powered readers
When we talk about battery powered RFID readers, power
consumption and operating time are very important parameters. This
is especially true for UHF RFID readers, as UHF RFID readers
consume relatively large amounts of power when operating at full RF
This article covers three different approaches on how to decrease
power consumption. These tests were carried out using Nordic ID
Stix USB RFID reader.
There are three different approaches for decreasing power
consumption of the RFID reader we tested. All three are good ways
to save power as long as the right approach is chosen for the right
use case. The first approach is to decrease the actual RF power
level in which the reader is operating. The down side to that
approach is that you will lose the maximum read range. The second
option is to use integrated power save modes. This decreases power
consumption when reading is on, but there are no tags in the
reader's field. The third option is to implement the "duty cycle"
into the host application or to use the RF-off time parameter
inside the firmware in order to apply the duty cycle functionality.
Note that these three approaches can be used simultaneously. We
shall now look further into these options one by one.
1. REDUCING THE RF POWER LEVEL
Reducing the RF power level is a very straight forward way to
reduce the power consumption. Nevertheless, it will mean that the
reader is not working at maximum read range. This is not always an
efficient way to decrease power consumption, because as you cut the
RF power to half, the power consumption is not decreased by half.
It will not be even close to that. This is because the RF power
amplifiers are usually tuned to provide maximum efficiency when the
maximum output power is used and the efficiency drops greatly with
lower RF power levels.
Figure 1 shows the average power consumption with different RF
power levels. Note that the test was carried out with tags in the
reader's field, to ensure that RF test results are not impacted by
the reader's power save mode (see next section for definition). The
graph illustrates that when the RF power level is set to the
minimum value, the power consumption has dropped to half.
- Easy way to save power if read range is not an issue
- Not an efficient way to gain power savings
Figure 1. Measured power consumption
with different RF power levels.
2. Power save modes
In our test the battery operated RFID reader has integrated
power save modes, which can be used as a power saving option. The
power save mode operates in a way that when there are no tags in
the reader's field, the reading is stopped for the selected time
period. Our test device has three different period length options
available. The shortest period is 100ms, the second option is 500ms
and the longest is 1000ms. Immediately when a tag enters the
reader's field, the reading is switched back on and continues
without any breaks.
Using this method, the reading performance is not decreased at
all. Nevertheless, this will obviously save power only in
situations, where there are no tags in the reader's
Figure 2 illustrates the measured power consumption with the
three different period length options. It is apparent that the
power consumption is not decreased when tag(s) are in the reader's
field. Once there are no tags in reading range, the power
consumption is decreased enormously. This will have a big effect on
battery life in cases where reading is ON most of the time but
there are no tags in the reader's reading range. Some possible use
cases might be access control applications.
- Very efficient way to reduce power consumption without reducing
the read range
- No effect to read performance
- Saves power only when there is no tag(s) in the reader's
Figure 2. Measured power consumption
in different power save modes.
3. Applying the duty cycle
For the reader these tests were carried out with, there are two
different approaches to applying duty cycles. The first one is to
apply duty cycles in the application layer. In practice this means
that the host application will have off times between separate
reads. The other and more efficient way, is to use an RF off time
parameter. We selected the RF off time parameter
approach for duty cycle testing.
What the RF off time parameter does, is that it will turn off
the transmission and power down the RF power amplifier for the
selected time period. This will decrease the average power
consumption regardless of whether there are tags in the reader's
field or not. It does not affect the maximum read distance but will
lower the read speed of the reader. Nevertheless in many cases the
lower read speed should not pose a problem. Figure 3 illustrates
measured power consumptions with different RF off time parameters
and with two RF power levels. This test was carried out with tags
in the reader's field, to ensure that duty cycle results are not
impacted by the device's power save mode.
- Decreases the power consumption without decreasing the maximum
read range although there are tags in the reader's field.
- The read speed is decreased
Figure 3. Power consumption with
different RF off time parameters.
Figure 4 shows the effect of RF off time parameters on reading
speed. Reduced read speed is in many cases not an issue.
Figure 4. Read speed with different
RF off time parameters.
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