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<article xmlns:xlink="http://www.w3.org/1999/xlink">
  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>Drone path planning method to reduce energy consumption</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Takuto Maejima</string-name>
          <xref ref-type="aff" rid="aff2">2</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Xiangbo Kong</string-name>
          <email>kong@pu-toyama.ac.jp</email>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Tomoyasu Shimada</string-name>
          <xref ref-type="aff" rid="aff2">2</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Hiroki Nishikawa</string-name>
          <email>nishikawa.hiroki@ist.osaka-u.ac.jp</email>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Hiroyuki Tomiyama</string-name>
          <xref ref-type="aff" rid="aff2">2</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Department of Intelligent Robotics, Factory of Engineering, Toyama Prefectural University</institution>
        </aff>
        <aff id="aff1">
          <label>1</label>
          <institution>Graduate School of Information Science and Technology, Osaka University</institution>
        </aff>
        <aff id="aff2">
          <label>2</label>
          <institution>Graduate School of Science and Engineering, Ritsumeikan University</institution>
        </aff>
      </contrib-group>
      <fpage>127</fpage>
      <lpage>137</lpage>
      <abstract>
        <p>In this study, two methods are proposed to reduce the energy consumption of drones. The first method is path planning using depth images, which combines the path planning algorithm of the previous study with the algorithm for speed control in the direction of travel. The second method inputs depth images into deep reinforcement learning to learn speed control and collision avoidance. This method also uses the algorithms of previous studies for path planning. As it is difficult to fly drones freely and conduct experiments due to the severe punishment of drone flight laws in recent years, experiments are conducted using a drone simulator called AirSim. Also, since energy consumption cannot be directly obtained on AirSim, we propose a method for calculating energy consumption on AirSim. The experimental results show that the time required for the arrival of the destination was shortened and the energy consumption was drastically reduced in both proposed methods compared with the previous study. Specifically, the path planning method using speed control in the traveling direction reduced the arrival time by about 39 seconds and the energy consumption by about 42% compared with the previous study. Also, the path planning method using deep reinforcement learning reduced the arrival time by about 47 seconds and the energy consumption by about 30% compared with the previous study. In addition, the collision rate was improved by about 2.8% in the path planning method using speed control in the traveling direction and by about 3.8% in the path planning method using deep reinforcement learning compared with the previous study.</p>
      </abstract>
      <kwd-group>
        <kwd>1 drone</kwd>
        <kwd>depth image</kwd>
        <kwd>path planning</kwd>
        <kwd>AirSim</kwd>
        <kwd>deep reinforcement learning</kwd>
        <kwd>energy consumption</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction</title>
      <p>
        In recent years, drones have attracted strong interest all over the world, and the market size is
growing year by year. Drones are expected to be used in a variety of scenes by developments (e.g.
package delivery, survey of disaster-affected areas, aerial photography). However, drones have some
problems, such as not being able to carry heavy loads due to weight restrictions, not flying on optimal
paths, and not colliding with each other [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ][
        <xref ref-type="bibr" rid="ref2">2</xref>
        ]. In addition, most of the current mainstream drones are
battery-powered, which means they need to fly with limited power. Moreover, in recent years, it has
become difficult to fly drones freely and conduct experiments due to the severe punishment of drone
flight laws [
        <xref ref-type="bibr" rid="ref3">3</xref>
        ].
      </p>
      <p>Therefore, this study was conducted for the following two purposes. The first is the use of drone
simulators to conduct experiments freely since it is difficult to conduct sufficient experiments in the
real world due to strict legal regulations. In the experiment, a simulator named AirSim was used. The
second is to fly to the destination with a low collision rate and low energy consumption. In the
experiment, to reduce the time it takes to reach the destination and to reduce energy consumption, path
planning using speed control in the traveling direction of the drone is carried out. In addition, we thought
that collision avoidance with minimal action would also reduce energy consumption, so we used deep
reinforcement learning to control the speed in the traveling direction and avoid collisions.</p>
      <p>The structure of this paper is as follows. Section 2 describes the relevant research of this study.
Section 3 describes an algorithm for speed control in the traveling direction, which we devised to solve
the problems of the previous study, and an algorithm for computing energy consumption on AirSim. In
Section 4, the environment and experimental results of deep reinforcement learning and learning of
speed control and collision avoidance using depth images are described. In Section 5, the summary of
this paper and future issues are described.</p>
    </sec>
    <sec id="sec-2">
      <title>2. Related study</title>
      <p>
        This section describes the relevant research of this study. In paper [
        <xref ref-type="bibr" rid="ref4">4</xref>
        ], LiDAR was used to study
obstacle detection and collision avoidance of drones. Although LiDAR has high performance, it is not
suitable for installation on small drones due to its high cost and weight. In paper [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ], a depth image
acquired from a drone is divided into 289 sections to determine the safest direction from each of the
sections to avoid a drone collision. However, in paper [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ], experiments are conducted using actual
drones, but the experimental environment is simple and not suitable for conducting experiments on
collision avoidance, which is a problem that the experimental environment is insufficient. Study [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ]
proposes a path planning method for drones by improving the collision avoidance method in paper [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ].
In paper [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ], they used a drone simulator named AirSim for experiments and a map suitable for
experiments distributed for simulation. Several literature studies are showing the effectiveness of
AirSim, and in paper [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ], they conducted a comparative experiment between flight in real space and
flight on AirSim and showed that the flight characteristics of a drone on AirSim are close to those in
real space. Also, in paper [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ], it was shown that AirSim is suitable for experiments such as deep learning
because it can acquire data and images without delay. Therefore, in paper [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ], it can be said that the
problem of experimenting in an insufficient environment in paper [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ] has been solved. However, in
paper [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ], there is a problem in that the speed of the drone in the traveling direction is slow and constant,
resulting in poor power efficiency and large energy consumption required to reach the destination.
      </p>
      <p>
        In addition, in recent years, reinforcement learning and deep reinforcement learning have been
increasingly used in drone research to improve the performance of autonomous flight. In paper [
        <xref ref-type="bibr" rid="ref9">9</xref>
        ],
AirSim and reinforcement learning were used to learn how to land a drone safely, and the learned model
was transferred to a real vehicle to accomplish the task in a real vehicle with little learning on the
simulator. It was shown that the cost and time required for real machine learning could be drastically
reduced by advanced learning on the simulator. However, study [
        <xref ref-type="bibr" rid="ref9">9</xref>
        ] has a problem that is not realistic
because the learning content of reinforcement learning of drones is only landing and does not carry out
complicated tasks such as collision avoidance. In paper [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ], they used deep reinforcement learning to
learn how to plan a path to a destination while avoiding collisions on an AirSim. However, problems in
paper [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ] are that the environment for experiments is insufficient because the destination is set in a
straight line of the drone's camera and the number of obstacles between the initial point and the
destination is small. The approach of [
        <xref ref-type="bibr" rid="ref11">11</xref>
        ] proposes a path planning algorithm that performs collision
avoidance while saving power based on deep reinforcement learning. In the experiment, they can say
that the problems of paper [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ] are solved because the simulator is used in an environment where there
are many obstacles and the destination is not in a straight line. Furthermore, the learned model achieved
the task not only in the learned environment but also in the unknown environment without significantly
reducing the collision rate. This shows that in drone collision avoidance, the deep reinforcement
learning-based algorithm can accomplish the task regardless of the environment. However, there is a
problem in paper [
        <xref ref-type="bibr" rid="ref11">11</xref>
        ] that learning is performed by setting the movement speed of drones at a constant
speed. However, papers [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ], [
        <xref ref-type="bibr" rid="ref11">11</xref>
        ] and [
        <xref ref-type="bibr" rid="ref12">12</xref>
        ] have a problem in that they do not take advantage of the
vertical movement that is a characteristic feature of drones because the flight altitude is fixed.
      </p>
      <p>To address these issues, we propose a path planning method for drones with low energy
consumption in this paper.</p>
    </sec>
    <sec id="sec-3">
      <title>3. Path planning method using direction-of-travel speed control</title>
      <p>This section describes a path planning method using speed control in the direction of a drone's travel,
which aims to reduce power consumption by reducing the time it takes to reach a destination.</p>
    </sec>
    <sec id="sec-4">
      <title>Previous study</title>
      <p>
        In this section, we describe collision avoidance and path planning algorithms using depth images in
paper [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ]. Collision avoidance is divided into two algorithms: a direction-specifying algorithm that
determines the direction a drone flies from depth images and an algorithm that performs speed control
during flight.
      </p>
      <p>
        First, we describe a direction-specifying algorithm. we acquire a depth image of 256 × 144 pixels
from AirSim. Then, we divide the depth image into 289 sections, each 17 by 17 in length and width, as
in paper [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ]. Each section is 69 × 42 pixels in size, and the sections overlap by shifting 11 pixels to the
right and 6 pixels down. Also, each section is assigned a number and managed in a coordinate format
such that the top left section is (0, 0), the center section is (8, 8), and the bottom right section is (16,
16). After dividing the sections, the pixel values of each section are calculated, and if the average of the
pixel values of the center section is greater than the set threshold, the center section is set as the largest
section, otherwise, the section with the largest pixel value is selected from the sections other than the
center, and that section is set as the largest section. Finally, collision avoidance is realized if the speed
is controlled so that the selected maximum section becomes the central section in the next process. Then,
an algorithm for speed control is described. In the algorithm for speed control, speed control is
performed using PID control so that the maximum section selected in the algorithm for collision
avoidance moves to the position of the center section.
      </p>
      <p>Finally, an algorithm for path planning to a destination is described. First, the rotation angle of the
drone is calculated from the coordinates of the current drone and the coordinates of the destination.
Then, when the drone is not performing collision avoidance, that is, when the input speed on the Y and
Z axes is 0 when the direction in which the drone's camera is facing is taken as the X axis, and when
the calculated rotation angle is larger than the threshold defined as the allowable angle deviation, the
drone can be redirected toward the destination by rotating the drone by that rotation angle.</p>
      <p>In this study, to avoid a collision between a drone and an object as much as possible, the speed of
the drone in the direction of travel is fixed at a low speed, and since the speed remains low even in a
situation where collision avoidance is not performed, the time required to arrive at the destination is
long and the energy consumption is large. Therefore, we propose a method to improve energy
consumption, which is a problem, without lowering the collision rate by referring to the previous study.
3.2.</p>
    </sec>
    <sec id="sec-5">
      <title>Proposed method</title>
      <p>
        In this section, we describe an algorithm for speed control in the traveling direction, which was
inspired by a paper [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ], and an algorithm for calculating power consumption on AirSim.
      </p>
    </sec>
    <sec id="sec-6">
      <title>3.2.1. Algorithm for speed control in the direction of travel</title>
      <p>In this section, we describe an algorithm for controlling the speed of a drone in its traveling direction.
In the AirSim environment, the coordinate axes relative to the drone are shown in Fig. 3.1, and in this
study, the velocity of the drone in the X-axis direction is vx, the velocity in the Y-axis direction is vy,
and the velocity in the Z-axis direction is vz. Therefore, the velocity control in the traveling direction
in this experiment is to control vx.</p>
      <p>First, when the uppermost left coordinate of the acquired depth image is (x, y) = (0, 0), a 21 × 69
rectangular section such as Fig. 3.2 with (x, y) = (88, 48) as the upper left corner is cut out to make vx
_ section.</p>
      <p>Next, we prepare a variable avg _ vx _ section that serves as a threshold for controlling vx and store
the average of the pixel values in vx _ section in avg _ vx _ section = 50 if the drone is moving to avoid
obstacles, or in avg _ vx _ section if it is not. Then, by preparing limit _ vx, which is a variable for
setting the upper limit of vx, limit _ vx = 9.0 if the value of avg _ vx _ section is 200 or more, 6.0 if it
is 150 or more but less than 200, and 3.0 otherwise. Finally, it adds 0.01 to vx when vx is less than limit
_ vx and subtracts 0.01 when vx is greater than limit _ vx. The above flow can be expressed as Eq. (3.1)
and Eq. (3.2).</p>
      <p>9.0 (
= {6.0 (200 &gt; 
3.0 (150 &gt; 
(3.1)
(3.2)
This reduces the large inclination of the drone due to a sudden change in speed while controlling the
drone to be fast when there are no obstacles in the direction of travel and slow when there are obstacles.</p>
    </sec>
    <sec id="sec-7">
      <title>3.2.2. Algorithm for energy consumption calculation</title>
      <p>In this section, we describe an algorithm for calculating the energy consumption of drones. In
calculating the power consumption of a drone, we divide the calculation into two parts: the power
consumed by the motor that turns the propeller required for the drone to fly, and the power consumed
other than the motor.</p>
      <p>First, we explain how to calculate power consumption in the motor. Since AirSim cannot obtain the
values of the motor voltage and the current flowing there, which are necessary for calculating power
consumption, we obtain power consumption from
motor power. The motor power P' [W] can be
expressed by Eq. (3.3), where the motor angular velocity is denoted by ω [rad/s], and the motor torque
is denoted by T [N · m].</p>
      <p>′ =</p>
      <p>The output per unit time of a motor is called power, and to obtain this power, electrical power must
be supplied to the motor input. However, not all of the supplied power is converted into output power,
as some of the power is lost as heat energy and other losses. The ratio of the supplied power to the
output power is known as motor efficiency. Considering the motor efficiency as n, the power
consumption taking into account the motor efficiency can be calculated as P_motor [W], and the power
consumption is obtained by Eq. (3.4).</p>
      <p>
        Next, the calculation method of power consumption other than the motor is explained. Drones also
have cameras that capture depth images [
        <xref ref-type="bibr" rid="ref13">13</xref>
        ] and small boards [
        <xref ref-type="bibr" rid="ref14">14</xref>
        ] in addition to motors. In experiments,
the power consumption of these devices was determined as a single constant, and the power
consumption was calculated by multiplying the flight time of the drone by this constant. Power
consumption per unit time other than the motor is expressed as q [W] and the drone flight time is
expressed as t [s] and the total drone energy consumption in one flight is expressed as P [Ws], energy
consumption is obtained by Eq. (3.5).
      </p>
      <p>_
=  ′

1
 = (</p>
      <p>+  ) 

1
(3.3)
(3.4)
(3.5)
3.3.</p>
    </sec>
    <sec id="sec-8">
      <title>Comparative experiment</title>
      <p>
        This section describes the contents, experimental results, and discussion of the comparison with the
previous study [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ]. In this experiment, we compare the average energy consumption, the collision rate,
and the average arrival time at the destination for each method at 500 randomly selected points in the
range of 100≦| x |, | y |≦200. Also, we use a distribution map called Blocks [
        <xref ref-type="bibr" rid="ref15">15</xref>
        ] where many static
obstacles of various shapes are placed as shown in Fig. 3.3. The motor efficiency is n = 0.8 and the
Power consumption per unit time other than the motor is q = 10.5. In this experiment, power
consumption was determined based on the specifications of the NVIDIA Jetson Orin Nano [
        <xref ref-type="bibr" rid="ref14">14</xref>
        ] as the
single-board computer and the Intel RealSense™ Depth Camera D400 series [
        <xref ref-type="bibr" rid="ref13">13</xref>
        ] as the depth sensor
camera. In this method, the power consumption is set to the minimum specified in the documentation
(7W) plus the power consumption of the depth sensor camera (3.5W).
⚫
⚫
⚫
⚫
⚫
      </p>
      <p>CPU: Intel Core i7-9750H
GPU: NVIDIA GeForce GTX 1650
RAM: 16GB
AirSim v1.8.1</p>
      <p>Unreal Engine 4.25.4</p>
      <sec id="sec-8-1">
        <title>The experimental results are shown in Table 3.1. Table 3.1 Experimental results Average energy consumption [Ws]</title>
      </sec>
    </sec>
    <sec id="sec-9">
      <title>4. Drone speed control and collision avoidance using deep reinforcement learning</title>
      <p>
        This section describes drone speed control and collision avoidance using depth images and deep
reinforcement learning. The proposed method described in Section 3 aims to achieve more optimal
speed control and collision avoidance by using deep reinforcement learning because the speed is
constant at low speed during collision avoidance. Also, the path planning to the destination of the drone
is based on the route planning algorithm proposed in the previous study [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ].
4.1.
      </p>
    </sec>
    <sec id="sec-10">
      <title>Learning environment</title>
      <p>This section describes the environment, agents, rewards, and hyperparameters of deep reinforcement
learning set up in the experiment.</p>
    </sec>
    <sec id="sec-11">
      <title>4.1.1. Environment and Agent</title>
      <p>
        First, the environment is described. The algorithm of deep reinforcement learning uses Proximal
Policy Optimization (PPO) [
        <xref ref-type="bibr" rid="ref16">16</xref>
        ] of Stable Baselines 3[
        <xref ref-type="bibr" rid="ref17">17</xref>
        ]. The policy network uses CNN because depth
images are used for input. For learning, a map called Blocks [
        <xref ref-type="bibr" rid="ref15">15</xref>
        ] is used, where the initial point of the
drone is (x, y, z) = (0,0, -9), and the initial velocity is set to (vx, vy, vz) = (0, 0, 0), where vx is the
velocity in the X-axis of the drone, vy is the velocity in the Y-axis, and vz is the velocity in the Z-axis.
      </p>
      <p>Next, the agent is described. The agent is a drone that can be operated by the API provided by AirSim,
and the following seven actions can be taken by the agent.
⚫ 
⚫ 
⚫ 
⚫ 
⚫ 
⚫ 
⚫ 
=  + 0.25,  =  , 
=  − 0.25,  =  , 
=   ,  =  + 0.25, 
=  ,  =  − 0.25, 
=  ,  =  ,  = 
=  ,  =  ,  = 
=  ,  =  ,  = 
4.1.2. Reward
(4.1)
(4.2)
(4.3)</p>
      <p>In this section, we describe the rewards set in learning. The ideal control of drones that we want to
realize by reinforcement learning is a movement that increases the traveling speed as much as possible
while not colliding with obstacles.</p>
      <p>Next, the reward set in the experiment is explained. The reward consists of two parts: reward _ speed
and reward _ state. The reward _ speed is a reward for the forward velocity vx of the drone. When the
vx of the current drone is between 3 ~ 10 m/s, the reward is vx multiplied by 0.05, and when vx is 0 ~
3 m/s, the reward is -0.05, otherwise, the reward is -0.1. Therefore, when the speed of vx is fast, the
reward is given positively, and when the speed is slower than a certain value, the reward is given
negatively. The reward _ speed is expressed as Eq. (4.1).</p>
      <p>0.05 (3 ≤ 
= { −0.05 (0 ≤ 
−0.1
≤ 10)
≤ 3)</p>
      <p>Next, the reward _ state is explained. The purpose of this reward is to keep the motion during
collision avoidance as small as possible by keeping the non-traveling velocities vy and vz as zero as
possible during the flight. If vx of the traveling speed is greater than 0 and the other speeds vy and vz
are 0, the reward is 0.05, otherwise 0. The reward _ state is expressed as Eq. (4.2).</p>
      <p>The total reward _ speed and reward _ state are given as a reward, but if a drone strikes, the reward
is -10, and done = 1, which is the judgment for ending an episode. Therefore, if the reward is expressed
as a reward, the reward looks like equation (4.3).</p>
      <p />
      <p>= {
_
+  _ (If the drone has not collided)
−10 (In case of drone collisions)</p>
      <p>Next, Other exceptional rewards and termination conditions are explained. If there is a drone
within a 1-meter radius of the destination, it is determined that the drone has reached the destination
and done = 1. This is because, if the arrival judgment is made only in the coordinates of the
destination, it is necessary to pass exactly in the coordinates of the destination for the timing of the
arrival judgment, but the judgment is severe for a drone with high speed, and reinforcement learning
is also difficult to progress. In addition, if the drone somehow goes below the ground level, because
the behavior is impossible in reality, reward = -10 is set and done = 1. In addition, if the drone is more
than 400 m away from the initial point, if the current height of the drone exceeds 150 m, or if the
flying time during one flight exceeds 1000 seconds, the learning efficiency is poor even if the drone is
allowed to continue learning, so done = 1.</p>
    </sec>
    <sec id="sec-12">
      <title>4.1.3. Hyperparameters</title>
      <p>
        In this section, we list the hyperparameters. the parameters used in the experiment are shown in
Table 4.1. The names of the parameters used in the experiment are based on Stable Baselines 3 [
        <xref ref-type="bibr" rid="ref17">17</xref>
        ].
      </p>
    </sec>
    <sec id="sec-13">
      <title>Comparative experiment</title>
      <p>
        In this section, we compare the model learned by deep reinforcement learning with other methods.
The comparison methods are the path planning method using speed control in the traveling direction
proposed in Section 3 and the previous study [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ] introduced in the 3.1 section.
      </p>
      <p>
        In this experiment, we compare the average energy consumption, the collision rate, and the average
arrival time at the destination for each method at 500 randomly selected points in the range of 100≦| x
|, | y |≦200. Also, we use Blocks [
        <xref ref-type="bibr" rid="ref15">15</xref>
        ]. We also set the motor efficiency at n = 0.8, q = 10.5 for power
consumption other than the motor using the method without deep reinforcement learning, and q = 18.5
for power consumption other than the motor using the method with deep reinforcement learning. In this
experiment, power consumption was determined based on the specifications of the NVIDIA Jetson Orin
Nano [
        <xref ref-type="bibr" rid="ref14">14</xref>
        ] as the single-board computer and the Intel RealSense™ Depth Camera D400 series [
        <xref ref-type="bibr" rid="ref13">13</xref>
        ] as
the depth sensor camera. The reason for setting the power consumption to 10.5W for the method that
does not utilize deep reinforcement learning and 18.5W for the method that utilizes deep reinforcement
learning is that it is believed that employing deep reinforcement learning increases the power
consumption of the single-board computer installed in the drone. In the method that does not utilize
deep reinforcement learning, the power consumption is set to the minimum specified in the
documentation (7W) plus the power consumption of the depth sensor camera (3.5W). In the method
that utilizes deep reinforcement learning, the power consumption is set to the maximum specified in the
documentation (15W) plus the power consumption of the depth sensor camera (3.5W).
      </p>
      <p>The experimental environment is as follows.
⚫
⚫
⚫
⚫
⚫
⚫
⚫</p>
      <p>
        CPU: Intel Core i7-9750H
GPU: NVIDIA GeForce GTX 1650
RAM: 16GB
AirSim v1.8.1
Unreal Engine v4.25.4
Stable Baselines 3 v1.6.2
OpenAI Gym v0.21.0 [
        <xref ref-type="bibr" rid="ref18">18</xref>
        ]
      </p>
      <sec id="sec-13-1">
        <title>The experimental results are shown in Table 4.2.</title>
      </sec>
      <sec id="sec-13-2">
        <title>Average arrival time</title>
        <p>[s]</p>
        <p>From the experimental results in Table 4.2, the proposed method using deep reinforcement learning
was able to improve all 3 items of energy consumption, collision rate, and average arrival time
compared to the previous study. Specifically, the path planning method using speed control in the
traveling direction reduced the average arrival time by about 39 seconds and the average energy
consumption by about 42% compared with the previous study. The path planning method using deep
reinforcement learning also reduced the average arrival time by about 47 seconds and the average
energy consumption by about 30%. The collision rate was improved by about 2.8% in the path planning
method using speed control in the traveling direction and by about 3.8% in the path planning method
using deep reinforcement learning compared with the previous study.</p>
        <p>The method using deep reinforcement learning results in a shorter average arrival time but larger
energy consumption than the path planning method using speed control in the traveling direction. There
are two possible reasons why the energy consumption of the method using deep reinforcement learning
is larger than the path planning method using speed control in the traveling direction. The first reason,
we believe, is that when deep reinforcement learning is used, power consumption per unit of time is
large because more power consumption is put on the board than when it is not used. However, in an
environment where the distance to the destination is longer than that of the present experiment or where
there are many obstacles, the difference in the average arrival time between the method using deep
reinforcement learning and the method using speed control in the traveling direction becomes larger,
and the difference in power consumption of the board by using deep reinforcement learning is
considered to have less impact.</p>
        <p>The second reason, we believe, is that the lack of learning time has prevented us from minimizing
collision avoidance, and even when there are no obstacles in front of us and we can go straight, the vy
and vz values may not be zero. We believe this can be improved by further increasing the total number
of learning steps. We also believe that by making the positive reward of the reward _ state larger than
the current one, we can perform optimized path planning with minimal collision avoidance than the
present model. However, if the reward in the reward _ state is increased, receiving a positive reward in
the reward _ state will be prioritized over avoiding a collision, which may lead to a higher collision rate,
so the balance with a negative reward in a collision must also be adjusted. In terms of collision rate, the
proposed method using deep reinforcement learning showed the lowest rate compared to the previous
study and the proposed method using speed control in the traveling direction. In this regard as well, we
believe that the collision rate can be further reduced by increasing the total number of learning steps,
which we have previously described as an improvement measure.</p>
        <p>The improvement measures described so far are based on hyperparameters and rewards, but by
giving not only depth images but also the current location and destination of energy consumption and
drones as feature values to the input of deep reinforcement learning, the number of power consumption
other than motors will increase further, but we think that energy consumption will be able to reduce it
more by allowing path planning considering energy consumption without making a detour when
avoiding a collision.</p>
      </sec>
    </sec>
    <sec id="sec-14">
      <title>5. Conclusion</title>
      <p>
        In this paper, we proposed a path planning method for drones to reduce energy consumption. In this
study, we focus on the energy consumption of the path planning method of drones in the previous study
[
        <xref ref-type="bibr" rid="ref6">6</xref>
        ], and show two proposed methods for its improvement. In the path planning method using speed
control in the traveling direction proposed in Section 3, speed control in the traveling direction using
depth images was added to the path planning method of the drone to improve the problems of the
previous study [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ]. Also, because AirSim cannot directly acquire energy consumption, the power
consumption of the drone was divided into the power consumed by the motor and the power consumed
other than the motor in this experiment. power consumption of the motor was calculated from the power
of the motor, and power consumption other than the motor was set to a certain value to calculate the
power consumption and energy consumption of the drone. The method using deep reinforcement
learning proposed in Section 4 uses depth imaging and deep reinforcement learning to control the speed
of the drone and avoid collisions. Comparative experiments between the previous study and the two
proposed methods show that both of the proposed methods can improve energy consumption compared
to the previous study. Specifically, the path planning method using speed control in the traveling
direction reduced the arrival time by about 39 seconds and energy consumption by about 42% compared
with the previous study. The path planning method using deep reinforcement learning also reduced the
arrival time by about 47 seconds and energy consumption by about 30% compared with the previous
study. In terms of collision rate, the path planning method using speed control in the traveling direction
improved by about 2.8% and the path planning method using deep reinforcement learning improved by
about 3.8% compared with the previous study. Comparing the two proposed methods, energy
consumption was smaller in the path planning method using speed control in the traveling direction,
but the collision rate was smaller in the path planning method using deep reinforcement learning.
However, regarding energy consumption, we think that the method using deep reinforcement learning
still has room for improvement.
      </p>
      <p>As a future research issue, we plan to experiment by giving not only depth images but also
information on the current location and destination of the drone and energy consumption as feature
quantities to the input of deep reinforcement learning. We think that this will enable us to carry out path
planning considering energy consumption. In this experiment, we focused on the power consumption
drone motor and other than the motor, so in future experiments, we think it is necessary to think about
a more accurate calculation method of the power consumption on AirSim, to conduct experiments in
an environment considering dynamic obstacles, and to conduct experiments using a real vehicle in the
real space.</p>
    </sec>
    <sec id="sec-15">
      <title>Acknowledgments</title>
      <p>This work is partly commissioned by NEDO (Project Number JPNP22006).
Papers</p>
    </sec>
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