Commit 7b241062 authored by U-RAJESH-SIS\rajesh's avatar U-RAJESH-SIS\rajesh

edits to section 6

parent 23aa5a27
\section{Micro benchmarks}
\section{System Level Measurements}
\label{sec:benchmark}
In this section, we present micro benchmarks detailing how various
components of the system work.
In this section, we show how \name works under different conditions.
\subsection{Experiment Setup \& Calibration}
\raj{still figuring out what to do here. we need to describe the base setup
and then extend it for the user study}
We setup our experiment in a meeting room, as shown in
Figure~\ref{fig:exprsetup}, to simulate an office environment.
The antenna sets are placed at the 2 corners of the table (Figure
\ref{fig:exprsetup}). Ideally, the antenna array should be placed as shown
in Figure \ref{fig:exprsetupideal}, so that the AoA at the receiver and
transmitter are the same. However, even we use widely separated channels,
the interference is too strong, and the receiver cannot receive packets. In
the setup shown in Figure \ref{fig:exprsetup}, there is still interference
and it increases the packet drop rate, so we have to transmit 10 ping
packets once triggered instead of only 1 packet. After estimation of the
AoA of ping packets, we then use triangulation to estimate the direction to
the transmitter antennas and then change the beam direction accordingly.
The distance between transmitter and receiver is 1m, the table width is
XXXm, and length is XXXm. Because the performance of AoA estimation
algorithm at extreme angle (0\degree or 180\degree) is poor, the receiver is
tilted about 35\degree to increase the overlapped coverage between
transmitter and receiver.
Because of interference between transmitter and receiver, we set the
transmission gain of 35 instead of a maximum of 63. Given that at maximum
power (gain of 63), each antenna can transmit 20dBm theoretically.
When we measure harvested power at fixed positions on the table, the energ
is much more than the power consumption of the wearable, so we want to test
evaluate how the wearing of the device affect the harvested energy.
All our experiments were conducted in a meeting room using the setup shown
in Figure~\ref{fig:exprsetup}. This simulates an office environment and the
transmitting and receiving antenna arrays were placed at the 2 corners of
the table. Ideally, the antenna arrays should be place together, as shown
in Figure \ref{fig:exprsetupideal}, so that the AoA at both array is
similar. However, in the 802.11b spectrum, even when using widely separated
channels, the interference is too strong, and the receiver is unable to
receive packets when placed next to the transmitter. Even with the array
separated, as shown in Figure \ref{fig:exprsetup}, we still experience
interference and packet drops. To overcome this, we transmit 10 ping
packets from the wearable device, instead of just 1, when triggered.
We estimate the AoA of these ping packets and then use triangulation to
estimate the direction to the transmitter antennas and then adjust the beam
direction accordingly. The distance between the transmitter and receiver
array is 1m, the table width is XXXm, and length is XXXm. To avoid poor AoA
estimation errors caused by extreme angles (0\degree or 180\degree), we
tilted the receiver by about 35\degree to increase the overlapped coverage
between the transmitter and receiver. The gain of the transmitter and
receiver was set to 35 out of 63 (max theoretical power of 20dBm at 63 gain)
to reduce inteference.
%When we measure harvested power at fixed positions on the table, the energ
%is much more than the power consumption of the wearable, so we want to test
%evaluate how the wearing of the device affect the harvested energy.
%show microscopic energy characteristics of our system.
......@@ -43,7 +42,7 @@ evaluate how the wearing of the device affect the harvested energy.
%is placed at some fixed positions without a user wearing it.
\subsection{Harvested Energy without User Presence}
\subsection{Baseline Harvested Energy}
\begin{figure}
\centering
......@@ -52,19 +51,18 @@ evaluate how the wearing of the device affect the harvested energy.
\label{fig:nouser}
\end{figure}
We put one of our devices on the at three different locations on a table.
The AP is placed at a corner of the same table. The three position L1, L2
and L3 is 1.91, 1.32 and 1.17 meter from the transmitter with an angle of
45\degree, 28\degree and 0\degree with respect to the transmitter's antenna
array respectively. We measure the voltage of the super capacitor before
and after the experiment (each session for 15 minutes). The device
(microcontroller, accelerometer and RF module) consume the energy, thus
decreases the voltage level at the capacitor. On contrast, the energy
harvester store the energy in the super capacitor, thus increases the
voltage level. So if the WiFi-based harvested power is higher than the
power consumed by the device, the voltage level at the capacitor should
rise, otherwise, it should drop. From the voltage level, we compute the
energy stored in the capacitor using the following equation:
Our first experiment was to determine how much harvested energy \anon could
produce. To do this, we placed our wearable device at three different
locations on the table. The three positions L1, L2 and L3 were 1.91, 1.32
and 1.17 meter from the transmitter with an angle of 45\degree, 28\degree
and 0\degree with respect to the transmitter's antenna array respectively.
We measured the voltage of the wearable's super capacitor before and after
activating \anon for 15 minutes. The wearable's components
(microcontroller, accelerometer and RF module) consumes energy while the
energy harvester stores energy in the super capacitor. Thus, if the
harvested power is higher than the consumed power, the voltage level at the
capacitor should rise; otherwise, it should drop. With the capacitor's
voltage level, we compute the energy stored using the following equation:
\begin{equation}
U = \frac{1}{2}CV^2
......@@ -72,7 +70,23 @@ U = \frac{1}{2}CV^2
\begin{equation}
P = \frac{\Delta U}{T}
\end{equation}
C is the capacitance of the super capacitor in Farad (we use a 0.47F capacitor), V is the voltage in Volt, U is the stored energy in Joule, P is the power in Watt, T is the time to charge/discharge the capacitor with an amount of $\Delta U$. A positive residual power means that the energy harvested from the WiFi beam is higher than the energy consumed by the device, and thus the capacitor is charged.
C is the capacitance of the super capacitor in Farad (we use a 0.47F
capacitor), V is the voltage in Volt, U is the stored energy in Joule, P is
the power in Watt, T is the time to charge/discharge the capacitor with an
amount of $\Delta U$. A positive residual power means that the energy
harvested from the WiFi beam is higher than the energy consumed by the
device, and thus the capacitor is charging.
Figure~\ref{fig:nouser} shows the results of this experiment. In all 3
positions, \anon was able to deliver enough power to operate the wearable
device and charge the capacitor (residual power is higher than 0) with
positions L2 and L3 having the highest charging potential.
% These results
%show that \anon can deliver sufficient power to operate a reasonably
%complicated sensordevice.
\subsection{Effect of Duty Cycle}
......@@ -83,7 +97,21 @@ C is the capacitance of the super capacitor in Farad (we use a 0.47F capacitor),
\label{fig:dutycycle}
\end{figure}
Although we can use the AP as a wireless charger, which transfers energy to the device all the time. We are keen to see if it is possible to be part of a commercial AP in the future. With real AP, the main function is to transmit and receive data. However, it not always fully loaded, and thus can exploit free time to transfer power to nearby device. We place a device at the edge of the table in the experiment room (See Figure \ref{fig:exprsetup}). The position is XXX meter from the AP. We set the AP to transfer charging signal at different duty cycle from 20\% to 100\%. The voltage level of the super capacitor is then measured before and after each session. The residual energy is reported in Figure \ref{fig:dutycycle}. It is quite clear that the residual power is approximately linear with the duty cycle. Moreover, with only 20\% time, the AP can transmit about 50 \micro W more than the required power for the wearable to work.
Although we can use the AP as a wireless charger, which transfers energy to
the device all the time. We are keen to see if it is possible to be part of
a commercial AP in the future. With real AP, the main function is to
transmit and receive data. However, it not always fully loaded, and thus
can exploit free time to transfer power to nearby device. We place a device
at the edge of the table in the experiment room (See Figure
\ref{fig:exprsetup}). The position is XXX meter from the AP. We set the AP
to transfer charging signal at different duty cycle from 20\% to 100\%. The
voltage level of the super capacitor is then measured before and after each
session. The residual energy is reported in Figure \ref{fig:dutycycle}. It
is quite clear that the residual power is approximately linear with the duty
cycle. Moreover, with only 20\% time, the AP can transmit about 50 \micro W
more than the required power for the wearable to work.
\subsection{Effect of Number of Antennas}
\begin{figure}
......
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