This paper describes a novel transformable land/air robot that is capable of terrestrial locomotion and aerial locomotion. What is unusual about the robot is that it can transform between the two modes of locomotion at will through the transformable mechanism, allowing the robot to overcome large obstacles in their mission environment. The wheel mechanism of the robot is shared by both terrestrial and aerial locomotion, instead of simply adding a quadrotor to a wheeled mobile robot. The objective of this paper is to design the robot and establish the kinematic and dynamic models for the transformable process. Herein, we focus on the design of the driving wheels and transformable mechanism. A series of experiments about the energy analysis and the transformation from aerial locomotion mode to terrestrial locomotion mode were performed with the physical prototype; the experiment results confirmed the validity of our design and the theoretical analysis that are helpful to optimize the key parameters in our design. Moreover, our work can provide a reference for the study of the flying car.
The ability to explore areas that are risky and inaccessible to humans and easily deal with the complex terrains makes quadrotors an excellent choice in applications such as search and military surveillance [
There have been many successful attempts to create robots with terrestrial and aerial locomotion capabilities, and many approaches to combining aerial locomotion and terrestrial locomotion have been proposed [
Inspired by these works, we introduce a novel transformable land/air robot that is capable of terrestrial locomotion and aerial locomotion and enables to transform between the two modes of locomotion at will. It combines wheels with quadrotor, and the wheels are shared by both aerial and terrestrial locomotion, instead of simply adding a quadrotor to a wheeled mobile robot, and therefore avoid increasing weight and design complexity of the robot. The robot is primarily used on the ground, the intent of the aerial locomotion mode is to deal with the large obstacles, across complex terrain. Firstly, we described the outline of the platform’s structure, and detailed the design of the driving wheels and transformable mechanism. Secondly, we established the kinematic and dynamic models for the transformable process. Then the transformation and energy analysis experiments were performed with the physical prototype. Finally, conclusions are drawn in Section
There are three states of the robot in the process of motions. The full sequence is illustrated in Figure
The full sequence of motions.
The robot consists of three parts: running part, mode transformation part, and flying part. The running part realizes the stable running on the ground by adopting the architecture of two-wheel differential drive. The flying part realizes the stable flight by quadrotor mechanism. The mode transformation between the two modes of locomotion is directly driven by servos. Figure
Configuration of the platform.
The control system of the robot is shown in Figure
The control system of the transformable land/air robot.
The design of the driving wheel is very crucial. Figure
Structure of the driving wheel.
In aerial locomotion mode, the wheels are the mounting seat of the motor for flying. The rotors and its driving motors are installed at the center of the wheels, and they are cross distributed to control the flight attitude and position.
The transformable process between aerial locomotion and terrestrial locomotion is driven by the servo to rotate 180°, instead of turning over 90°. Figure
Transformation sequence from terrestrial locomotion mode to aerial locomotion mode.
Structure of the transformable mechanism: (a) driving wheel and (b) driven wheel.
The kinematic and dynamic modeling of aerial locomotion and terrestrial locomotion have been adequately described elsewhere in the literature [
Now the research of kinematic modeling is relatively mature [
Simplified coordinate model of the wheel and the transformable mechanism.
According to the rule of equivalent angle-axis representation and right hand, the frame {A} is fixed to the end of the servo shaft and it is considered the reference frame. The frame {B} is described as initially coincident with {A}. The origin of frame {C} is located in the point O, and the pose of wheel is described with the frame {C}. We describe the pose of the wheel in terms of frame {A}. Therefore, the transformable process can be simplified as the frame {C} rotates counterclockwise around the equivalent axis PQ in the
The translation matrix of the frame {C} relative to frame {B} is given as follows:
The equivalent rotation axis PQ in the
The rotation matrix of the frame {B} relative to frame {A} is given as follows:
According to the homogeneous matrix transformation, the pose of the frame {C} relative to frame {A} is given as follows:
Equation (
Position and attitude in the aerial and terrestrial mode of the wheel.
From the simulation results, the wheel changed from parallel to the ground to perpendicular to the ground; position trajectory was consistent to our expectation. The simulation results confirmed the rationality of the transformable mechanism design and the kinematic modeling.
By the force analysis of the left-rear wheel using the isolation method in the transformable process, as shown in Figure
Force analysis of the transformable process.
According to equation (
Prototype experiments and design model of the robot show that the wheels contact the ground when the servo axis rotates
The direction of the supporting force is opposite to the rotation direction of the gravity, given as
The direction of frictional resistance is opposite to the rotation direction of the wheel, given as
Figure
The geometric relationship analysis of a certain angle in the transformable process.
The arm of frictional resistance is given as follows:
The arm of the supporting force is given as follows:
The dynamic analysis of the transformation from terrestrial locomotion mode to aerial locomotion mode is divided into two steps. Next, we calculate the servo output torque referring to the above equations.
Step 1 is from the aerial locomotion mode to the moment when the wheel contacts the ground. Neglecting the air resistance, the servo only needs to overcome the gravity of the wheel. The output torque is given as follows:
Step 2 is from the wheel contacts the ground to the terrestrial locomotion mode; the servo needs to overcome the gravity of the robot and the frictional resistance. The output torque is given as follows:
The performance of the robot was evaluated in both indoor and outdoor environments, you can see in the accompanying
Representations of the physical prototype state: (a) the terrestrial locomotion mode, (b) the state of mode transformation, and (c) the aerial locomotion mode.
Table
Prototype dimensions and performance capabilities.
Terrestrial mode | Aerial mode | |
---|---|---|
Size (mm) | ||
Rotor diameter (mm) | 304.8 | 304.8 |
Wheel diameter (mm) | 328 | 328 |
Weight (g) | 3500 | 3500 |
Max velocity (m/s) | 1.5 | 2 |
Max cargo load (g) | 1500 | 1500 |
Max flight altitude (m) | \ | 20 |
In order to measure the output torque of the servo, the experiments about the transformation from aerial locomotion mode to terrestrial locomotion mode were performed with the physical prototype on different terrains including tiled floor, brick, and cement. You can find the experimental data in the accompanying materials. The friction coefficients between the resin wheel and the ground measured by dragging the wheel with a load cell on different terrains are
According to the above dynamic models, the dynamic simulation results about the output torque were drawn in MATLAB, as shown in Figure
The dynamic simulation results of the output torque in the transformable process.
From the simulation results, the transformable process was from 0 to 10 s, and it took about 5 seconds when the wheel touched the ground. So we can conclude that the wheel started to touch the ground when the servo shaft rotated about 90 degrees, and the output torque was maximum at this point. The simulation results were helpful for servo selection.
We used digital oscilloscopes to measure and record the current real time, and the servo was powered by a 8.4 V dc power supply. Figure
Experimental current tendencies of the four servos on different terrains.
According to the working principle of the servo and the relationship between the torque and the current, the output torque is calculated as
Calculated the output torque tendencies of a single servo.
Comparing Figures
In order to compare the energy consumption in its terrestrial and aerial locomotion modes, we calculated the operating time and distance of the robot by measuring the current using the wireless galvanometer when the robot is moving at a constant speed.
Table
Experimental data in aerial and terrestrial modes.
Terrestrial mode | Aerial mode | |
---|---|---|
Current (A) | 0.5 | 16.5 |
Discharge capacity (mAh) | 3200 | 3200 |
Velocity (m/s) | 1.5 | 2 |
Calculated data of operating time and distance.
Terrestrial mode | Aerial mode | |
---|---|---|
Operating time (s) | 23040 | 698 |
Operating distance (m) | 34560 | 1396 |
The experimental results show that with the same battery the robot can fly about 23040 s and travels 34560 m and it can run 698 s and travels 1396 m. Therefore, the operating time increased about 33 times in the terrestrial mode, and the distance is still 25 times greater comparing to the aerial mode. Our robot takes account of the operating time and distance.
In this paper, we designed a novel transformable robot that is capable of terrestrial locomotion and aerial locomotion and enables to transform between the two modes at will for overcoming obstacles. Firstly, we explained the outline of the platform and focus on analyzing the design of the driving wheel and transformable mechanism. Then, the kinematic and dynamic models about the transformable process were accurately established, which can provide an important reference to the key parameters design. Finally, a series of experiments about the running, mode transformation, and flying in both indoor and outdoor environments are performed. The experimental results show that the terrestrial locomotion and aerial locomotion function efficiently, and obstacles can be overcomed by flying over or running from the side. Moreover, the transformable experiments confirmed the validity of the kinematic and dynamic models that are important to transformable mechanism design and servo selection. The robot in the terrestrial mode can travel a distance almost 25 times greater and operate about 33 times longer compared to the aerial mode. The robot has a longer operating time and good mobility.
Design optimization is a constant issue and requires a significant attention. In the future, we plan to optimize the transformable mechanism and the current loop of the servo to speed up its response speed, making the transformable process between aerial locomotion mode and terrestrial locomotion mode smoother and more stable. The ongoing research also continues to reduce the weight of the robot and improve both running and flying performance.
The experimental data used to support the findings of this study can be available at
The authors declare that there is no conflict of interest regarding the publication of this paper.
This research work is supported by the Defense Industrial Technology Development Program (JCKY2017602C016).
Supplementary Materials contain a video about the terrestrial locomotion, aerial locomotion, transformation between the two modes, and obstacle climbing. Besides, the experimental data are the current changes of four servos measured by digital oscilloscope in a transformable process.