3.1. Experimental Setup Devices
In this report, we have used Powercast Technology Company (Pittsburgh-PA-USA)-provided devices for our research and practical testbeds. There are four components of the Powercast energy harvesting model: an energy transmitter, a sensor board, an evaluation board, and antennas. The Powercast technology company introduces RF-based energy harvesting WCD to replenish the sensor devices’ energy. The Powercast Technology Company has provided energy harvesting devices that power IoT devices since 2003. It provides a temperature scanning system, a wireless charging grip for the Nintendo joy-con, power spot, a UHF RFID retail price tag, and development kits. The development kits are used for research and powering IoT devices, which consist of evaluation boards (P1110, P2110), antennas, RF field-detecting light sticks, and sensors. We are using the P2110-EVAL-01 development kit for our research work, which is designed for extremely low-power IoT devices. Our focus is to study the data received from dipole and patch antennas. We aim to find the relationship between distance, RSSI, recharge time of the capacitor, angle impact on the packet transmission time and energy harvesting, and the routing of a packet to the access point. The following devices are used in our testbeds:
The RF Powercast transmitter omits both data and power in the form of RF signals with a unique ID and 915 MHz frequency. The output power (Pt) is 3 w EIRP with a beam pattern of 60° in vertical polarization, and the frequency range is 915 MHz. The distance for permanent installations of the TX91501B transmitter is eight feet or more above floor level. The Powercast Company provides the transmitter, which is covered in a black box with fixed output power and settings. The user cannot make changes to the transmitter.
- 2.
Wireless Sensor Board
The board can measure and transmit light, temperature, humidity data, and external inputs. The sensor board is connected with the evaluation board through a 10-pin connector to obtain the energy from the evaluation board for the transmission of data. We can set the ID of the sensor nodes from 0 to 7 by using ID SELECT switches. The sensor board has a PICkit connector, through which the PICkit programmer can be connected.
- 3.
P2110 Evaluation Board
The evaluation board has the responsibility of energy harvesting. The board contains the functionality of energy storage JP1 (C3, C4, and C5 jumpers), a 10-pin connector (J2) for wireless sensor board connection, a rectifier to convert the RF energy into DC, an SMA connector for an antenna or RF input (J1), and a visual LED indicator. The sensor board obtains the harvested energy from the evaluation board.
- 4.
Powercast Antennas
The Powercast development kit comes with two types of antennas: dipole and patch. These antennas are connected to the evaluation board through an SMA connector for the antenna (J1). The dipole antenna has the RF connector at the bottom, and the patch antenna has the RF connector in the middle. The dipole antenna is flat, omnidirectional, and vertically polarized, and the gain power is 1.0 dBi with a 360-degree reception beam pattern. The patch antenna is two-layered, directional, and vertically polarized, and the gain power is 6.1 dBi with a 120-degree reception beam pattern.
- 5.
Vehicle for Transmitter
The energy transmitter is equipped with a vehicle to make it mobile, and then it moves in a circle to charge the devices.
3.2. Experimental Models
In our testbed model, we are using the Powercast P2110-EVAL-01 development kit, which is used for energy harvesting. The TX 91501-3W-ID power transmitter (transmitter) transmits the power; the P2110 Evaluation Board P2110-EVB (receiver) receives this power; the Wireless Sensor Board WSN-EVAL-01 plugin with the P2110 Evaluation Board P2110-EVB sends the sensed data to the access point; and the hyper terminal is used to show the sensed data on a computer/laptop screen. The TX 91501-3W-ID transmitter is responsible for transmitting the energy signal to the P2110 Evaluation Board P2110-EVB. The evaluation board obtains the energy signal, converts it to DC, and recharges the super capacitor. Then, the harvested energy is used for sending data by the wireless sensor board. In our previous experiment (fixed charger and fixed boards), if the charger went farthest from the evaluation board, then it degraded the sensing and communication processes. So, we need a technique that can cover the farthest and nearest problems. For the solution to this problem, we are going to equip the energy transmitter with a moving toy to make it a mobile charger. The movement will occur in such a way that it can keep in mind our constraints, not going away from the maximum transmission range of the energy transmitter and not crossing a 60-degree area. The sixty-degree area has the beam pattern of the transmitter, which has 60° widths and 60° heights. For simplicity, we used 60° areas.
To fulfill these constraints, we are going to present our solution. Our solution is based on two sub-problems: an optimal charging tour to find optimal positions for a charger and discovering the charging area or points that are under the eye of the charger from any point when the charger moves.
The charger moves in circular form with a constant velocity in an anti-clockwise manner and completes its one tour in Tk time.
- 2.
Discovering the charging area or points.
As we know, the charger transmits the energy in a fixed directional, i.e., 60°. We need a fixed area that has points that come under the direction of the energy transmission whenever the charger moves; otherwise, the charging device will never receive energy when the charger moves, and it will go to a dead position. For this purpose, we are going to find the fixed area or points for the charging device to obtain energy from every position of the charger during movement. Since we have two antennas, a patch and a dipole, with different receiving patterns, the dipole is omnidirectional, vertically polarized, and its energy pattern is 360°, while the patch is directional, vertically polarized, and its energy pattern is 122°. For simplicity, we are using 360 coverage areas for the dipole and 122 coverage areas for the patch antenna. The Powercast technology company used the Friis equation for energy calculation. It has an online calculator (in the form of .xls) [
28] through which we can obtain the received energy. For example, if the distance is 1 m, then the received energy is 8.11; at 2 m, it is 2.030; and so on.
where
Pt is the transmitted power,
Pr is the received power,
Gt is the transmitter antenna gain power,
Gr is the receiver antenna received power,
d is the T-R separation distance, and
λ is the wavelength. Gain power is based on the aperture of the antenna. Keeping in mind the polarization loss in power transfer, signal power should be rectified and converted to electrical energy before it can be used.
where
Lp represents polarization loss,
η is rectifier efficiency, and
β is a parameter to adjust the Friis free space equation for short distance transmission.
Now we can describe our model as follows: Let
Mk be the set of chargers, and
vk be the set of nodes; then, we give the charging model based on Equation (10) as follows:
Where ||vk – Mk|| represent the distance between the device vk and charger Mk, Pt represents the transmission power of the charger Mk, .
For a directional charging model, the charger and devices are equipped with directional antennas, so the angle of the charger and devices will be kept in mind. Let
be the directional vector of the charger (i.e., charger angle θ
mk) and
be the directional vector of the device (device angle θ
vk) then equation 11 can be written for the directional model as follows:
Let
Mkr be the maximum transmitter range of the energy transmitter and
vkr be the maximum transmission range of the device. Since the energy transmitter has a 0 to 60° angle, the dipole has a 0 to 360° angle, and the patch has a 0 to 122 degree range then Equation (12) can be written for a dipole antenna as follows:
Equation (12) can be written for a patch antenna as follows:
The maximum transmission power received at nod v
k from charger
Mk can be calculated as follows: from Equation (11), we now calculate the nearest-farthest problem as:
In our model, we changed the position of the charger according to different distances from the energy transmitter, but the position of the energy transmitter remains constant. In our experiment, we have checked the received power and the incoming data time for one to three-meter distances in outdoor as well as indoor environments.
Currently, we have two sensors and one charger, so we can create fixed points for sensors when the charger is moving.
The Powercast transmitter directionally transmits the energy up to a 60-degree area. The evaluation board receives this signal and converts it to DC. If the evaluation board is not lying in the direction of the transmitter, it cannot harvest energy. Consequently, it halts all processes and leads to a dead situation. Next, the Powercast Company provides a transmitter range of up to 80 feet [
29], which means that the farthest node can be placed 80 feet away from the transmitter. So, the maximum range is from 0 to 80 feet. Here, we can define two problems for the energy transmitter: distance and range. If the distance is 80 feet above then the receiver antenna (device) cannot receive energy, and if the evaluation board took place outside of a 60-degree area, then the receiver antenna cannot recieve the energy. We need a solution that meets the above problems efficiently and makes the network nearly perpetual.
3.3. Testbeds Studies
In this section, we are going to study experimental testbed setups. Our experimental setup is based indoors.
3.3.1. Testbeds Model
In our testbed model, we are using the P2110-EVAL-01 energy harvesting development kit, which contains components. We used a small vehicle toy for the transmitter to make it mobile, and it starts working by blinking the blue light. Moreover, we used this toy to make the sensor mobile. After starting the TX 91501 transmitter, we connected the antenna to the evaluation board. After the evaluation board, we plug in the wireless sensor board into the evaluation board. Next, we installed the hyper-terminal on the laptop and connected the access point to the laptop through a USB cable. After the installation of the hyper-terminal emulator, we open it and start the step-by-step installation process. In the installation process, we gave the name Powercast and clicked the ”ok” button. Another dialogue appeared that required region, area code, and connection port. We entered Pakistan, 46000, and COM16 and clicked the ”ok” button. Another screen opened some basic information such as bit rate, parity, and flow control. We only set the bit rate to 19200 and clicked the “ok” button. A blank screen appeared, but after clicking switch PB1 on the access point board, the emulator started working and showed the built-in message of the Powercast Company. We have to make points on the ground in circular form with the help of scotch tape. The circular points are separated from one another by a distance of one foot and are identified by numbers. So, we draw two circular points one foot away from each other. The circular shape is used for moving the charger, and the circular points are used for getting data from these specific points. The charger is equipped with a vehicle toy and uses a small wire to make it movable. The charger moves in an anti-clockwise direction, and the charger is equipped on the toy vehicle so that it cannot lose the 60-degree direction. Our observation is based on both antennas, patches, and dipoles.
3.3.2. Indoor Experimental Setup and Observations
We have conducted our experiment according to our solution discussed in
Section 2. We can state our problem and solution as follows:
3.3.3. Problem Identification
Powercast energy transmitters disseminate energy in a directional form up to 60 degrees, while the dipole and patch antennae receive this energy at 360 and 120 degrees. In our previous experiment, we used static charger position and dynamic sensing device position. We have used 1 to 5 feet distance for devices to check the energy harvesting and data sending processes. Moreover, we have conducted the 1 to 3-m distance experiment and observed that when the sensor device goes away from the charger, it degrades the energy harvesting and consequently the data sending process. This phenomenon is called the nearest-farthest charger problem, and this is the proof that received signal strength is inversely proportional to the square of transmission distance [
30].
The next problem is related to Powercast devices. The energy transmitter transmits in a directional form and not in an omnidirectional form. Next, the dipole antenna has a 360-degree receiving angle, but the patch antenna has a 122-degree receiving angle. So, we cannot shrink the distance to a point where all devices can perform their specific task efficiently. For example, if the charger was omnidirectional, then the usage of distance was nil, and we would put the charging devices near the charger.
Powercast evaluation board/charger devices are zero-battery devices, and during our previous experimental setup, it has been observed that these zero-battery devices required continuous energy for life. If at any stage or at any time the energy transmitter direction changes, then the evaluation board goes to die.
So, we need such a technique that can overcome the nearest farthest problem.
3.3.4. Solution
The above problems are not new ones, and the researchers tried to solve these problems. A mobile charger is a good solution for charging devices. We also use the mobile charger for charging the devices, but our problems and solutions are different from those of these researchers. Let me explain that there are omnidirectional and directional phenomena. If a charger device is omnidirectional, then the charging devices are directional, and vice versa. So, the researchers solved these problems in general terms, but we are dealing with directional-directional phenomena with dipole antennas. So, we need a technique for a directional device that can solve the above three problems. For this purpose, our solution can be divided into two phases: the first is the charger movement phase to find the optimal charging tour to find optimal positions for the charger, and the second is the optimal area or points for charging/sensor/evaluation board devices.
When the charger is at point 1, the theta is 60 degrees, and the maximum distance is dependent on the charger to send the signal, as Powercast 35903 has a range of 80 feet [
29], so when the charger transmits the energy in a circular form, as shown in
Figure 3, the points are 1 foot from each other. This means that the lines are uniform, and one foot away, they intersect the circle. Considering two straight lines, a
1x + b
1y + c
1 and a
2x + b
2y + c
2, we will use the following equation:
3.3.5. Optimal Charging Tour to Find the Optimal Position for a Single Charger
Figure 3 shows our charger placement strategy, in which the charger is moving in a circular way and provides energy to the evaluation board for scavenging. The charger moves by taking the transmitter angle into account so that the eye of the charger stays in the optimal place or point.
3.3.6. Optimal Points/Area for Sensor Devices
When the charger is at point 1, the theta is 60 degrees, and the maximum distance is dependent on the charger to send the signal, as Powercast 35903 has a range of 80 feet, so when the charger transmits the energy in a circular form, as shown in
Figure 3, the points are 1 foot from each other. This means that the lines are uniform, and one foot away, they intersect the circle (see
Figure 4).
At point 2 when the transmitter transmits the energy, it intersects the previous lines at any point, as shown in
Figure 4, and we want to find the intersection points.
Let lines intersect each other at points ps1 and ps2 then we can find the location of intersection points by the intersection formula. In the end, we can obtain the desired intersection points. The area between these intersection points is called the optimal area, and the points are optimal points to place the charging devices.
We have presented two algorithms for our solution. Algorithm 1 is designed by keeping the dipole antenna range in mind, and Algorithm 2 is designed by keeping the patch antenna range in mind. Our main purpose is to find the optimal charger placement points and optimal points for sensor placement by keeping in mind the constraints of the transmitter at 60 degrees. IP refers to intersection points by inscribed angles (IA), while 120° + Ɛ is the angle of the patch antenna with some area of the charger because we have observed that the patch range is 120 degrees but also that it can take some benefits from the charger range to sense a large area.
Algorithm 1: Charger Placement Strategy for Dipole antenna. |
Input | Set of Sensor vk ={1,2,3..}, Charger Points Mkp = {Mk1 Mk2, Mk3,…,3600}, Distance dk = {1,2, Mkr}, Tx value |
Output | Optimal Points/area for charger tour, IA, θ, IP |
|
- 2.
IP = ϕ, θ = ϕ, AT = ϕ
|
- 3.
If Pki ≠ ϕ then
|
- 4.
Select the Pk1 point for mobile charger.
|
- 5.
AT ← An inscribed triangle start from point Pk1 with dk = 1ft
|
- 6.
Θ = as Equation (13)
|
- 7.
Start Moving of charger from point Pk1 to Pki
|
- 8.
Add AT
|
- 9.
Θ = as Equation (13)
|
- 10.
IP = as Equation (16)
|
- 11.
Repeat
|
- 12.
Select IP
|
- 13.
Until Pik ≤ 360
|
- 14.
Return IP, Pki
|
- 15.
end If
|
Algorithm 2: Charger Placement Strategy for Patch Antenna. |
Input | Set of Sensor vk ={1,2,3..}, Charger Points Mkp = {Mk1 Mk2, Mk3,…,1220}, Distance dk = {1,2, Mkr}, Tx value |
Output | Optimal Points/area for charger tour, IA, θ, IP |
|
- 2.
IP = ϕ, θ = ϕ, AT = ϕ
|
- 3.
If Pki ≠ ϕ then
|
- 4.
Select the Pk1 point for mobile charger.
|
- 5.
AT ← An inscribed triangle start from point Pk1 with dk = 1ft
|
- 6.
Θ = as Equation (14)
|
- 7.
Start Moving of charger from point Pk1 to Pki
|
- 8.
Add AT
|
- 9.
Θ = as Equation (14)
|
- 10.
IP = as Equation (16)
|
- 11.
Repeat
|
- 12.
Select IP
|
- 13.
Until Pik ≤ 1200 + ε
|
- 14.
Return IP, Pki
|
- 15.
end If
|
3.3.7. Experimental Setup with a Patch Antenna
The patch antenna has 122-degree reception. From the experiment, we have observed that the path antenna worked properly within the 122 + ε (where ε represents some benefits from the charger’s angle) direction and the charging devices would receive the energy, but when the charger moved in another direction, the charging device went into a dead position or data sending and the energy harvesting process degraded to zero. We have obtained the experimental data and shown them in
Table 1. According to
Figure 3, it is clear from the table that when the charger moves from 0° to 250°, it degrades the energy harvesting and data sending processes accordingly, but when the charger moves to 240°, the energy harvesting and data sending processes gain power. Consequently, from these experimental data, we can say that the area or points shown in
Figure 5 are the optimal points/area for placing the sensors, and the points 340° to 100° are the optimal charger tour points or path to provide unstoppable energy to the sensors. For more clarification, see
Table 2. We have also added some figures (
Figure 6,
Figure 7,
Figure 8,
Figure 9,
Figure 10 and
Figure 11) to better understand the patch antenna sensed data.
3.3.8. Experimental Setup with Dipole Antenna
The dipole antenna has a 360-degree reception angle to receive the energy from the charger, so there is no restriction on direction for the sensor device as the patch antenna has a 122-degree restriction. When the charger moves, the sensor receives the energy in all directions. We have collected the receiving data and shown them in
Table 3 for observation. Consequently, these experimental data in
Table 3 are the proof that the area or points shown in
Figure 12 are the optimal points/area for placing the sensors, and the circular direction is the optimal charger tour point or path to provide unstoppable energy to the sensors.
3.3.9. Analysis of Data in Dipole Antenna
The dipole antenna can receive data from all sides because it has 360-degree reception antennas. however when the sensor devices become under the charger device, then the time differential (dT) between received packets increases, and the sensor devices gain sophisticated power for data transmission.
3.3.10. Analysis of RSSI of Patch Antenna in a Mobile Environment
We have equipped our charger with a moving vehicle to make it mobile, and the sensor with a path antenna was put on the ground for experimentation. The sensor send data successfully to the access point, and we used
Table 4 to present all the sensed data from sensors for 100 to 360 degrees.
3.3.11. RSSI of Patch Antenna during Circular Tour of the Charger
In this section, we are going to present the charger movement on a circular and obtain the RSSI of the patch antenna. We used 100 degrees as the first point and moved to 360. The RSSI is good from 100 to 280 degrees but starts degrading after 280 (see
Table 5).