Developing a Low-Cost Autonomous Blimp with a ReducedNumber of Actuators

Safwan Ul Ferdousa, Alireza Mohammadi*a, and Sridhar Lakshmanana

aDepartment of Electrical & Computer Engineering, University of Michigan-Dearborn,Dearborn, MI 48128, USA

ABSTRACT

Miniature blimps will have numerous applications in future smart cities. This paper presents the design of anautonomous blimp that can be autonomously operated and controlled. In order to be able to operate over longperiods of time, the blimp design employs a novel actuation mechanism with only one servomotor and two DCmotors. Experiments are carried out to demonstrate the capabilities of the constructed autonomous blimp.

Keywords: Autonomuos blimps, unmanned aerial vehicles (UAV), autonomous robots.

1. INTRODUCTION

Unmanned aerial vehicles (UAVs) will play a crucial role in smart city applications. Due to their sharp andpowerful propellers, conventional UAVs such as quadrotors are not inherently safe while operating in the vicinityof humans. Furthermore, since conventional UAVs can only offer relatively short flight times (∼45 minutes),they are not good candidates for operating over extended periods of time. A safer autonomous flying robotthat can operate over longer time intervals would be more beneficial in smart cities. Autonomous blimps, whichare a special class of UAVs, can provide an effective platform for combining the capabilities of hot air balloonswith those of airplanes into one aerial vehicle. Being capable of performing relatively complex maneuvers andbeing more energy efficient in comparison with other classes of UAVs make autonomous blimps an ideal candi-date for smart city applications such as traffic management,1 pollution monitoring,2 mobile sensor networks,3

human-robot interaction,4,5 networked robotics,6,7 and localization and mapping,8,9 amongst others. Moreover,autonomous blimps, which are inherently cushioned, cause minimal harm to humans during collisions.

Table 1. An overview of the various types of autonomous blimps in the literature.

Number of actuators

Ko et al.,10 D’Souza et al.11 2 servomotors

Liu et al.,12 Saiki et al.13 2 DC motors, 1 servomotor

Kawano et al.,14 Wang et al.15 3 DC motors

Watanabe et al.,16 Fukushima et al.17 4 DC motors

Oh et al.,18 Takaya et al.19 6 DC motors

Table 1 provides an overview of various types of autonomous blimps in the literature. In this paper, wepropose a novel miniature blimp actuation mechanism to achieve autonomous operations over long periods oftime (∼2 hours). Our philosophy in designing the blimp has been based on the insight that the attitude of thesensing units such as the mounted cameras can be separated from the attitude of the blimp, where the relationshipbetween them is continually adjustable. For instance, the orientation of the gimbaled cameras20 on UAVs can becontinuously controlled in relation with the UAV attitude and the target. Consequently, our designed miniature

* Send correspondence to A. Mohammadi.A. Mohammadi: E-mail: amohmmad@umich.edu, S. Ul Ferdous: E-mail: sferdous@umich.edu, S. Lakshmanan: Email:lakshman@umich.edu

autonomous blimp merely utilizes two coreless DC motors and one low-power micro servomotor in order toachieve three principal motion primitives (see Figure 1), namely, moving along the body Xb axis, moving alongthe body Zb axis (changing altitude/elevation), and rotating about the body Zb axis (altering the yaw angle).These three main motion primitives can be combined to allow the blimp to move in 3D space. Our blimp designphilosophy and the combination of a large capacity lightweight battery as well as low-current consuming sensorsand actuators makes the designed miniature blimp capable of continuous operation for up to two hours.

The rest of the paper is organized as follows. In Section 2, we present the physical design of the blimp,the gondola structure, the electronics of the blimp, and the embedded control system of the blimp. Next, inSection 3, we present the experimental results. Finally, in Section 4, we conclude the paper with some finalremarks and future research directions.

2. BLIMP DESIGN

In this section we present the physical design of the blimp, the gondola structure, the electronics of the blimp,and the embedded control system of the blimp.

2.1 Physical Design

Our blimp has two main modules, a gondola and an envelope (see Figure 1). In order to improve the stabilityof blimp operation and dampening the undesired rolling and pitching motions, we have designed the blimp sothat the center of gravity is located below the center of buoyancy. Our design tries to address the challengingtradeoffs that exist between structural stability requirements, restrictions on the number of actuators, limitedpayload capabilities, and maneuverability in outdoor environments. Figure 1 depicts the overall structure of theblimp.

Figure 1. The overall structure of the blimp. The blimp has two main modules, a gondola and an envelope. The blimpcan achieve three principal motion primitives, namely, moving along the body Xb axis, changing altitude, and alteringthe yaw angle. These three main motion primitives can be combined to allow the blimp to move in 3D space.

2.2 Gondola Structure

In order to optimize the topology of the gondola and to reduce its mass without sacrificing strength and dura-bility, the mechanical design underwent finite element analysis (FEA) and computational fluid dynamics (CFD)calculations to optimize both the weight distribution and the structure of the gondola. Both FEA and CFDcalculations were carried out using SolidWorks inbuilt modules. The FEA calculations demonstrate that thegondola can withstand the weight of heavier components such as the battery and the servomotor while beingcapable of housing additional sensors and small circuitry. Topology optimization allowed us to visually inspectand reconstruct the model in order to minimize the amount of material and the weight of the gondola, whilerespecting the deformation and stress constraints. Using an iterative topology optimization procedure, we were

able to achieve a weight reduction of about 30%. Finally, the gondola structure is optimized in a way to makethe assembly process easier.

The CFD analysis provides information on the drag and lift forces. In order to determine the proper typesof motors and propellers for the blimp, the air lift and drag forces that are acting on the blimp should bedetermined. Using the SolidWorks Flow Simulation, it was determined that the drag force acting on the blimpis approximately equal to 0.3 N. Therefore, each DC motor must be capable of providing a thrust force of atleast 0.15 N. The total weight of the 3D printed gondola, which is made from standard PLA material with 1.75mm diameter filament, is 120.53 grams (plus an additional 26.73 grams for the protection case). Figures 3 and 2depict the stress/strain deformation contours of the gondola, the final CAD model, and the 3D-printed gondola,respectively.

Figure 2. The stress/strain deformation contours of the gondola.

Figure 3. The final CAD model of the gondola and the 3D-printed gondola.

The steering of the blimp is achieved through a micro servomotor and two coreless DC motors. The microservomotor is mounted in the gondola while the two coreless DC motors are mounted on each side of the gondolato control the yaw and thrust. Furthermore, a shaft attaches the two coreless DC motors via gears to the microservomotor in a way that each DC motor can be rotated of up to 180 degrees (see Figure 4). This actuatorplacement topology enables the blimp to create thrust in the direction of the Xb axis as well as the inertial ZI

axis, and rotation about the Zb axis (see Figure 1), using the same DC motors. The servo can align the shaftat any angle from 0 to 180 degrees and thus enables the blimp to perform complex maneuvers. Figure 4 depictsthe actuator placement topology and the achievable motion primitives.

2.3 Electronics and the Embedded Control System

The electronics components and sensors used in the miniature blimp are chosen to require very low currentswhile providing complete functionality (see Table 2). The Li-Po battery is chosen to be lightweight (20 grams)while having a large capacity (2000 mAh). This choice of battery enables the blimp to operate for up to 2 hourscontinuously. An inertial measurement unit (IMU) sensor and an infrared proximity sensor are used to measurethe orientation of the blimp and the local position of the blimp, respectively. An averaging filter is employedto smooth the measurements of the IR proximity sensor. The employed IMU sensor, i.e., BNO055, is a 9-DOFsensor that provides accurate Euler angle measurements at a frequency of 100 Hz. The block diagram of theactuation and control system of the blimp is depicted in 5.

3. EXPERIMENTAL RESULTS

In this section, we present our preliminary experimental results to demonstrate the realization of the blimpprimitive motions. In the design of the control laws, we have assumed that the roll and pitch angles as well as

Figure 4. The actuator placement topology. A shaft attaches the two coreless DC motors via gears to the microservomotor in a way that each DC motor can be rotated of up to 180 degrees. The blimp can move along the body Xb

axis and the inertial frame ZI axis. It can also rotate about the body Zb axis.

Figure 5. The block diagram of the actuation and control system of the blimp.

the roll and pitch angular velocities, and the side-slipping velocity of the blimp are negligible during the steadystate and transient phases of the blimp flight. These assumptions are justified because the restoring forces due tobuoyancy and gravity damp out pitch and roll motions in an effective manner. Furthermore, the air drag causesthe side-slipping velocity of the blimp to get attenuated quickly during its forward motion, since the blimp has alarge envelope and is lightweight. As a proof-of-concept study, we have used simple PD control laws and tunedthem using the Ziegler-Nichols tuning method∗ to control the primitive motions of the blimp.

Primitive motion 1 (moving along the Xb axis). After the tuning procedure, the PD controller gains wereset to Kp = 5.0 and Kd = 0.8. The IR proximity sensor was used to determine the position of the blimp withrespect to a wall. Intermittent disturbances were applied to the blimp to evaluate the performance of the blimpactuation and control system in controlling the position of the blimp along the body-fixed Xb axis. Figure 6depicts the time profile of the position of the blimp with respect to the wall. The uppermost row in Figure 9shows the snapshot of the realized primitive motion along the Xb axis.

Primitive motion 2 (moving along the ZI axis). After the tuning procedure, the PD controller gains wereset to Kp = 3.2 and Kd = 0.5. It is remarked that reversal rotation of propellers generates lower downwardthrust since their blades are primarily designed for generating upward thrust. The IR proximity sensor was usedto determine the position of the blimp with respect to the floor. Intermittent disturbances were applied to theblimp to evaluate the performance of the blimp actuation and control system in controlling the position of theblimp along the inertial ZI axis. Figure 7 depicts the time profile of the position of the blimp with respect tothe floor. The middle row in Figure 9 shows the snapshot of the realized primitive motion along the Zb axis.

∗We remark that there are more robust methods for tuning the gains of PID controllers such as the robust loop shapingextension to the Ziegler-Nichols method (see the work by Astrom and Hagglund21 for further details).

Table 2. Electronic components used in the miniature autonomous blimp.

Type/Vendor Max. current consumption

Microprocessor Arduino Nano 3, ATmega328P 50 mA

Coreless DC motors 8520 Coreless DC Motors, 3.7V 800 mA

DC motor driver H-bridge L9110S 50 µA

IR proximity sensor Sharp GP2Y0A21YK 50 mA

Bluetooth module HC-05 Arduino Bluetooth module 50 mA

IMU sensor Adafruit BNO055 Absolute Orientation Sensor 9DOF 30 mA

Micro servomotor Micro Servo SG90 800 mA

Power source 2000 mAh 3.7V 802748 rechargeable Li-Po Battery –

Figure 6. Controlling the position of the blimp along the Xb axis. The control system makes the blimp to move towardsthe desired setpoint in the presence of intermittent disturbances.

Primitive motion 3 (rotating about the Zb axis). After the tuning procedure, the PD controller gains wereset to Kp = 2.3 and Kd = 0.4. The IMU sensor was used to determine the yaw angle of the blimp. Intermittentdisturbances were applied to the blimp to evaluate the performance of the blimp actuation and control system incontrolling the yaw orientation of the blimp about the Zb axis. Figure 8 depicts the time profile of the angularposition of the blimp. The lowermost row in Figure 9 shows the snapshot of the realized primitive rotation aboutthe Zb axis.

Energy consumption tests. In order to measure the energy consumption of the autonomous blimp, thefollowing consecutive tests were carried out. In the first test, the blimp starts moving in a straight line (fromt = 0 to t = 100) and comes to complete rest (from t = 100 to t = 120). Next, the blimp starts changing itsaltitude both upwards and downwards (from t = 120 to t = 170). In the final test, the power consumption ismeasured while the blimp is changing its yaw angle (from t = 170 to t = 220). Figure 10 depicts the time profileof the energy consumption of the blimp.

Figure 7. Controlling the altitude of the blimp along the ZI axis. The control system makes the blimp to move towardsthe desired setpoint in the presence of intermittent disturbances.

Figure 8. Controlling the orientation of the blimp around the Zb axis. The control system makes the blimp to rotatetowards the desired yaw orientation in the presence of intermittent disturbances.

4. CONCLUDING REMARKS

In this paper, we proposed a novel miniature blimp structure to achieve autonomous operations over long periodsof time. In order to optimize the topology of the gondola and to reduce its mass without sacrificing strengthand durability, our blimp mechanical design has undergone finite element analysis (FEA) and computationalfluid dynamics (CFD) calculations to optimize both the weight distribution and the structure of the gondola.Our miniature blimp only uses one micro servomotor and two coreless DC motors in a way that it can move in3D space by a proper combination of its achievable motion primitives. The current autonomous blimp platformcan be employed to test more sophisticated control algorithms in the presence of wind gusts and other typesof disturbances. By installing more advanced types of sensors in the blimp gondola, it is also possible to

Figure 9. Snapshot of the primitive motions of the blimp experiments. The uppermost, the middle, and the lowermostrows depict the primitive motion along the Xb axis, motion along the ZI axis, and rotation about the Zb axis, respectively.

Figure 10. Energy consumption of the blimp. In the first test, the blimp starts moving in a straight line (from t = 0to t = 100) and comes to complete rest (from t = 100 to t = 120). Next, the blimp starts changing its altitude bothupwards and downwards (from t = 120 to t = 170). In the final test, the power consumption is measured while the blimpis rotating about the yaw axis (from t = 170 to t = 220).

further investigate the potentials of this platform in smart city applications such as pollution monitoring, trafficmanagement, and localization and mapping.

ACKNOWLEDGMENTS

This work was supported by the Office of Research and Sponsored Programs (ORSP) and the College of Engi-neering and Computer Science (CECS), University of Michigan-Dearborn.

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