Design, Control, and Dynamic Simulation of Securing and ...The HGAR near vertical position. The...

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:20 No:02 100

203302-5858-IJMME-IJENS © April 2020 IJENS I J E N S

Design, Control, and Dynamic Simulation of

Securing and Transformation Mechanisms for a

Hybrid Ground Aerial Robot Maha Salman*1, Ahmed Sameh*2, Mohamed Fanni*2, Shigeki Sugano3, and Abdelfatah M.Mohamed*

*Mechatronics and Robotics Engineering Department

School of Innovative Design Engineering, Egypt-Japan University of Science and Technology, E-JUST New

Borg El Arab, Alexandria, PO Box:21934, Egypt 1 On leave: Power Electronics Dep., Electronic Research Institute, Cairo, Egypt.

2 On leave: Pord. Eng. Mechanical Design Dep., Faculty of Engineering, Mansoura University, Egypt. 3 Department of Modern Mechanical Engineering, Faculty of Science and Engineering Waseda University, Japan. Email: maha.salman, ahmed.ismaeil, mohamed.fanni, abdelfatah.mohamed@ ejust.edu.eg

[email protected]

Abstract— A hybrid ground aerial robot (HGAR) has been

developed to combine both capabilities of aerial robots and

ground mobile robots to overcome the limitations of each single

type. This research introduces a new securing mechanism and

improves also the propeller-thruster transformation mechanism

for the HGAR. The securing mechanism is designed to be light

and to give high stability, and low power consumption for both

flying and ground motion modes. In the developed

transformation mechanism, the robot uses the propellers which

are already installed for the aerial mission as actuators to

transform between the flight and ground-motion

configurations. In contrast to the previous design, no need here

to additional position controller to avoid propellers’ collision or

springs to start the switch to the ground configuration. The

propellers are controlled by the Feedback-Linearization which

is combined with Robust-Internal Compensator to achieve the

controller robustness. The HGAR is virtually built and

dynamically modeled using ADAMS® software then connected

with MATLAB/Simulink® to test the proposed mechanisms and

the proposed controller. The results indicate a satisfactory

performance of the proposed mechanisms and controller.

Index Term— Hybrid Ground Aerial Robot; Securing

Mechanism; transformation mechanism; feedback

linearization; Internal Compensator;

I. INTRODUCTION

Nowadays, mobile Robots have seen fast developments due

to their abilities to support the human daily life and substitute

humans in the hazardous area [1], [2]. There are many types

of the mobile robots such as flying robots (FRs) and mobile

ground robots (MGRs). Recently, FRs research has become

an interesting topic because the application fields are spread

rapidly from military to civilian applications. Also, the

capabilities of FRs are expanded and improved through

adding manipulators. Various manipulators types were

developed for example a gripper only, a multi-link robotic

arm,….. etc [3]–[9].

The main advantages of FRs over other types of mobile

robots are the ability to fly with high speed over ground

obstacles and arrive at unreachable areas such as areas after a

natural disaster. The main disadvantage is the limited

interaction with the environment. MGRs have a great

importance in different industries and services as they can

work indoor and outdoor. The main advantage is the large

payload and effective interaction with the environment [10]–

[13]. The main disadvantage is the limited speed and the

limited ability of navigating rough unstructured trains.

However, the scientific research are expanding for each kind

of these two mobile robots separately [3]–[14]. In [19], there

was a promising idea of merging FR with MGR. The robot

which merges these two kind of mobile robots is called

Hybrid ground aerial robot. The hybrid robot has the merits

of both flying robot and mobile ground robot. Via flying

mode, it can reach to remote areas with a high speed which is

not allowed for MGRs then makes a landing and interacts

with the environment via the ground motion mode. Flying

motion is based on a quadcopter which has Vertical Take Off

and Landing (VTOL). The ground motion is based on the

wheeled differential platform [13]. The robot is equipped

with 3 degrees of freedom manipulator. Flying and ground

motion configurations of the HGAR are shown in Fig.1 and

Fig.2 respectively. The transformation mechanism was

introduced to transform the robot from the flying

configuration to the ground motion configuration by raising

the arms to vertical position to reduce the space occupied by

the robot. Two solenoids are inserted to each propeller arm to

secure the arms in the desired position. Some drawbacks were

found such as low stability for the arm at the secured position,



high power consumption, and induced vibration during the

ground motion [20]. Also, the propellers need a complex

control technique to keep the blades in vertical direction after

transforming process, to prevent the adjacent propellers from

collision. Accordingly, each two opposite propeller arms will

be transformed in the same time while the other propeller

arms are fixed in their position.

The main contribution of this paper are as follows:

A new securing mechanism is designed to precisely

secure the arm in both flying mode (Horizontal

position) and the ground motion mode (Near vertical

position). The new design achieves important

characteristics compared to the previous technique

such as low power consumption, low vibration and

high stiffness during flying and ground motion

modes.

A simple modification is applied on the mechanical

design of the HGAR to overcome the propellers

collision and to return the arm to flying mode

without new mechanisms or complex controller.

The transformation mechanism uses the thrust force

to move the arm to the ground motion configuration

but uses the weight of the arm mainly to return it

again to the horizontal position to start flying.

A robust controller is designed to control the

propeller-arms motion during flying/ground

transformation.

Fig. 1. Hybrid robot in flying configuration [19].

Fig. 2. Hybrid robot in Ground configuration [19].

II. SECURING AND TRANSFORMATION

MECHANISMS

The main objective of the securing mechanism is to precisely

lock the moving arm at the desired positions such as securing

the arm at horizontal position during flying mode and

securing it at a position near vertical during ground motion

mode as shown in Fig.3. The near vertical position is

proposed here, instead of the vertical position proposed

previously [19], [20], for two reasons. The first reason is to

avoid collision of the propellers which is possible if the arms

are in pure vertical direction. The previous design used

position controller to force the blades of each propeller to be

vertical and hence avoid the possible collision [20]. We need

no position controller here since the blades will never collide

with each other at the near vertical position where the arms

make 82o with the horizontal plane. The second reason is to

avoid the use of springs to start the transformation from

flying configuration to ground configuration which was the

approach in [20]. Since the arms here are not in vertical

position, the gravity force plays the role of springs and starts

the downward motion of the arm, as soon as the securing

mechanism is released. The main objective needed in this

design is to be light weight. Light weight is desirable for

flying robots. Also, the suggested design must provide an

accurate locking with high stability and rigidity during both

configurations. The batteries are very important as known in

this type of robots as it affects the flying time. So, the

suggested design must consume small power. This is

achieved by proposing a design that consumes power only

during of locking or releasing the arm and not during

flying/ground configurations as in the previous design [20].



Fig. 3. The HGAR near vertical position.

The proposed securing mechanism consists mainly of four

parts: the sliding frame, spring, small rolling wheel, and

solenoid. Actually, there are two securing mechanisms for

each arm. One for securing the arm in horizontal position

during flying configuration as shown in Fig.4. The other one

for securing the arm in a position near vertical with angle

about 82o with respect to the horizontal position during the

ground motion configuration as shown in Fig.3. This angle is

selected accurately to use the gravity force due to the weight

of the arm as a driving force to return it back to the horizontal

position during transformation to flying mode without adding

any mechanisms. The thrust force of the propeller is

controlled to ensure that the transformation to flying

configuration is accomplished in a smooth way without

impacts. Also, the 82o angle is chosen to ensure that there will

be no collisions between the propellers’ blades.

The two securing mechanisms are similar. Fig.4 shows the

securing mechanism of the flying configuration in locking

mode. In such locking mode the small wheel which is

attached to the arm is prevented to move upward by the

sliding frame which is pushed to the left by the action of the

spring. On the other hand, the arm is prevented to move

further downward by the base which is attached to the robot

platform. The near vertical securing mechanism is shown in

Fig.5 in locking mode. In such locking mode, the small wheel

which is attached to the U-shaped part of the arm is prevented

to move backward by the sliding frame. Also, the arm can not

move forward by the robot base.

The transformation from flying to ground configuration is

started by powering the solenoid shown in Fig.4 which pulls

the sliding frame to the right compressing the spring and

releasing the small wheel and hence the propeller arm. Then

the propeller is rotating and produces controlled thrust force

which rotates the arm around its horizontal axis to transform

it from the horizontal position to the near vertical position as

shown in Fig.6. Meanwhile, the solenoid of the flying

configuration is unpowered and the solenoid of the walking

configuration is powered, pulling the sliding frame to the

right (see Fig.5). This movement opens the slot in the base

allowing the U-shaped part attached to the arm to go through

the slot.

Fig. 4. Horizontal securing mechanism.

Fig. 5. Near vertical securing mechanism.

Then the solenoid of the walking configuration is unpowered

releasing the compressed spring which pushes the sliding

frame to the left such that the small wheel attached to the U-

shaped part rests on the sliding frame locking the arm and

hence the arm in this near vertical position. The propeller is

then stopped and the transformation is ended. The process is

reversed in the transformation for walking configuration to

flying configuration.

The stiffness of the securing mechanism spring is calculated

such that the displacement of the spring during flying will not

exceed 0.5 mm which is enough to avoid releasing the

securing mechanism with large safety factor. The maximum

force acting on the spring equals the mass of the sliding frame

and the solenoid plunger multiplied by the maximum

acceleration. The maximum acceleration during flying equals

the maximum total thrust multiplied by sin 𝜑𝑚𝑎𝑥 (𝜃𝑚𝑎𝑥) and



divided by the total mass of the robot. The maximum

acceleration during ground motion is much smaller than that

of flying, so it will not considered in designing the spring.

According to the design, the spring should have a

displacement not less than 4 mm to release the securing

mechanism. It is found that a standard spring with the

parameters shown in Table I satisfies the required

specification. The solenoid is chosen such that it can produce

a force not less than the spring stiffness multiplied by 4 mm

with a stroke not less than 4 mm. The push pull solenoid 191

satisfies the required specification.

The stress analysis and deformation are checked for the

designed securing mechanisms. The material of the arm,

sliding bar, and the base is aluminum alloy. The pin wheels

were modeled with fiber plastic. The maximum forces that

are applied on the securing mechanisms are 2.2 Kg.

According to Fig.7, the maximum stress in the horizontal

secure mechanism is 32.6 MPa and the maximum

deformation is 0.114 mm. The maximum stress in the near

vertical secure mechanism is 13.8 MPa and the maximum

deformation is 0.0259 mm. While the aluminum yield

strength is 41.8 MP. The securing Mechanism is considered

safe design based on the stress analysis. In section IV, a

simulation using ADAMS/ MATLABS Simulink is

presented to check the validity of the securing mechanism

during the operation of the robot.

Table I

THE SPRING PARAMETERS

Outer Diameter 3.175 mm Inner Diameter 2.667 mm

Free Length 12.7 mm Solid Height 8.128 mm

Wire Diameter 0.254 mm Solid Total Coils 31.000

Rate 0.057 N/mm

Max. Load 0.261 N

Max. Deflection 4.572 mm Material Type MW- Music Wire

The end type is C.

Fig. 6. Flying/ground transformation idea.

Fig. 7. Stress analysis for the horizontal secure mechanism.

Fig. 8. Stress analysis for the near vertical secure mechanism.

III. DYNAMIC MODELING AND CONTROL

For the transformation process, the secure mechanism is

released which allows the arm to rotate around its pivot axis.

The propeller thrust makes a moment around the pivot axis

and hence controls the rotation of the propeller arm. By using

Newton’s law, the equation of motion which governs the

arm’s motion is presented as follows:

𝐽𝑎 �̈� = 𝑇 ∗ 𝐿 − (𝑊𝑚 ∗ 𝐿 + 𝑊𝑎 ∗𝐿

2) cos 𝑞 − 𝑐�̇� (1)



Where 𝐽𝑎 is the moment of inertia of the arm around its pivot,

𝑇 is the thrust force, 𝐿 is the total length of the arm, 𝑊𝑎 and

𝑊𝑚 are the arm and BLDC motor weights, 𝑐 is the damping

coefficient, and 𝑞, �̇� and �̈� are the arm angle, the arm angular

velocity, and the arm angular acceleration respectively. It is

desired to control the propeller-arms to reach the desired

angle while the vehicle can be exposed to external

disturbances. The Model-Disturbance compensating

approaches [17], [18] are very effecting for adding robustness

to the control scheme because they are able to estimate the

disturbances and cancel their effect in the compensating loop.

The proposed control system is based on the feedback

linearization approach technique (FBL) with an internal

compensator. The FBL technique [15], [16] converts the

nonlinear system into a totally or partially linearized system.

The FBL technique is sensitive to the uncertainties in system

parameters and disturbances. Therefore, the internal

compensator is used to compensate system uncertainties,

modeling errors, and external disturbances. The FBL

controller is designed in the RIC framework as shown in

Fig.9. The proposed controller design can be divided into two

parts. The first part is the design of the FBL controller law

without considering the disturbances. So, the FBL controller

law is designed as follows:

𝑢𝑓𝑏𝑙 = �̂� 𝑎𝑞 + 𝐶 ̂�̇� + 𝑔 (2)

𝑎𝑞 = 𝑞�̈� + 𝑘𝑝𝑓(𝑞𝑑 − 𝑞) + 𝑘𝑑𝑓(�̇�𝑑 − 𝑞) (3)

Where �̂� is an estimation of the arm moment of inertia 𝐽𝑎,

𝑔 is an estimation of the gravity term 𝑔 = (𝑊𝑚 ∗ 𝐿 + 𝑊𝑎 ∗

𝐿/2) 𝑐𝑜𝑠(𝑞), 𝐶 ̂ is an estimation of the damping coefficient

c, 𝑞𝑑 , �̇�𝑑 , and �̈�𝑑 are the desired arm angle, desired arm

angular velocity, and desired arm angular acceleration

respectively, and [𝑘𝑝𝑓 , 𝑘𝑑𝑓 ] are the feedback gains which

are chosen based on the required closed-loop performance.

Then, the Internal Compensator in Fig. 9 can be designed

where the virtual reference model, 𝑃𝑚, is chosen as a simple

double integrator system which can be stabilized easily by the

linear feedback term of the FBL control law.

Fig. 9. The proposed controller in the RIC construction.

The internal controller, 𝐾(𝑠), is simply chosen as a PD

controller as following:

𝑢𝑟𝑖𝑐 = 𝑘𝑝𝑟(𝑞𝑟 − 𝑞) + 𝑘𝑑𝑟(�̇�𝑟 − 𝑞) (4)

It is to be noted that the use of feedback linearization does not

require extensive computations as in robotic systems since

we have here single degree of freedom system.

IV. SIMULATION AND DISCUSSION

A. Flying/Ground motion test

In this simulation study, a 3D CAD model of the robot with

modified parts is virtually built. Then, the model is imported

into ADAMS® as shown in Fig.10 to perform an accurate

dynamic simulation with realistic results. All parts

characteristics such as material, mass, and inertia are

established. The joints between parts are identified where

three types of joints are used in this model Revolute joint,

Fixed joint, and Translational joint. Also, the contact forces

between bodies are specified with impact method. The initial

robot position is (0, 0, 0.168 𝑚) with respect to global frame

and the gravity is in the negative 𝑍-Direction. The aggregate

mass of the robot is 4.2 Kg. Then, the MATLAB/ADAMS

co-simulation is performed with a 0.005 communication

interval to test the proposed mechanisms and controllers.

The suggested scenario is as follows: Firstly, the robot is

flying from home position to another position then it returns

back to the ground. Then, the transformation process will be

started to switch the robot configuration. Finally, the robot

will move on the ground, while the manipulator is controlled

to be in home position during this scenario. The robot is

derived by three loops namely; flying control, transformation

process control, and ground motion control. The switching

between the control loops is managed by three factors time,

the robot position, and the angles of the propeller-arms. The

flying scenario is started when the propeller arms are in

horizontal position and the time is over zero, t > 0. The simple

PID controller is used for the flying motion and also for the

joints of the manipulator. The quentic trajectories are chosen

for the position 𝑋, 𝑌, and 𝑍. While the desired roll and pitch

angles, (𝜑, 𝜃), are calculated from the desired (𝑋, 𝑌)

position. The desired yaw, 𝜓 , is zero. Fig.11 presents the

simulated position of the robot with the desired position.

Fig. 10. The hybrid robot in ADAMS.



The results of the flying controller are satisfactory even the

robot is asymmetry. Because there is no aerial manipulation,

the manipulator is controlled to be in the home position. The

transformation process will be started when the robot is on

the ground floor and 𝑡 > 10.5. The flying/ground

transformation scenario will be as follows: the sliding frames

of the horizontal securing mechanisms are moved to release

the propeller-arms and make them free to rotate. Then, the

four propeller-arms are rotated around their pivot axes at the

same time from horizontal position, flying configuration, to

the near vertical position, ground mode configuration. The

thrust force which is generated by the propeller is controlled

via the control law in section III. In order to examine the

robustness of the designed controller, a step disturbance is

introduced on the second arm, at 𝑡 = 12 𝑠, a continuous

sinusoidal disturbance is applied to the fourth arm, and also

the parameter uncertainty disturbance is considered for the

first arm where the attached manipulator parameters are not

included in the controller design. The angles of the propeller-

arms during flying/ground transformation process are shown

in Fig. 12.

Fig. 11. The flying scenario.

Fig. 12. The flying/ground transformation process.

The position of the robot is recorded for (𝑋, 𝑌, 𝜓 ) as shown

in Fig.13 to detect the effect of the transformation process on

the robot stability and position. In Fig.13, the maximum

displacement is occurred in the 𝑋-direction, 10 mm, and the

maximum rotation is less than 0.2 degree because of the robot

asymmetry. The simulation results is prove the feasibility of

the transformation mechanism and also the robustness of the

proposed controller. During ground motion, the motorized

wheels are driven by applying a constant torque. So the robot

will move in the 𝑋-direction only. The position and

orientation of the robot are shown in Fig.14 and Fig.15 for

the whole scenario. The propeller-arms’ angles are shown in

Fig.16 for the whole scenario. The spring deformation can be

measured to be sure that the sliding bar attached to this spring

is not moving.

Fig. 13. The Robot position during the transformation process.

As shown in Fig.17, spring deformation has values only at

the locking and releasing the arms and then there is no any

deformation during securing the arm which confirms the

stability of the sliding bar without any vibrations. So, the

simulation results proves the stability of the securing /

transformation mechanisms. The dynamic simulation of this

scenario is presented in Fig.18.

Fig. 14. The position of the robot during the suggested scenario.

Fig. 15. The Robot orientation during the whole scenario.



Fig. 16. The angles of the propeller-arms during the whole scenario.

Fig. 17. Spring deformation during the scenario of the robot motion.

Fig. 18. Dynamic simulation of the flying/ground motion scenario.

V. CONCLUSION

A new securing / transformation mechanisms are proposed

and investigated. They provide solutions to the appeared

problems in the HGAR with minimum number of actuators.

Description and design of the securing / transformation

mechanisms are presented. During flying / Ground

transformation, a robust control scheme has been designed

based on FBL combined with internal compensation, where

the propeller-arms track the desired trajectories in spite of the

disturbed environment. Proof that the propeller-thrust can be

used as a precise actuator is done through dynamic

simulation. Furthermore, the dynamic Simulations prove the

feasibility of the proposed mechanisms with a satisfactory

results. In a future work, the securing mechanisms will be

manufactured and implemented in the real prototype. Also,

the transformation process will be experimentally tested by

implementing the designed controller.

ACKNOWLEDGMENT

The first author is supported by a scholarship from the

Ministry of Higher Education of the Government of Egypt

which is gratefully acknowledged. Also, the first author takes

the permission from Electronic Research Institute to study in

E-JUST.

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