Unmanned aerial vehicle and a landing guidance method using the same

1. An unmanned aerial vehicle (UAV) comprising:

a main body;

a plurality of motors connected to the main body, each of the plurality of motors having a rotor blade;

a plurality of ultrasonic sensors located at least one of the plurality of motors and the main body, and transmitting and receiving ultrasonic waves to and from a ground surface, and measuring distances from the ground surface;

a gyro sensor disposed at the main body and maintaining the UAV in a horizontal state; and

a controller disposed at the main body, detecting an unevenness of the ground surface based on the distances from the plurality of ultrasonic sensors to the ground surface, generating a control signal whether to land on the ground surface or not in response to the detection of the unevenness, and transmitting the control signal to the plurality of motors.

2. The UAV of claim 1 , wherein the plurality of motors are connected to the main body via connection bars, and the plurality of ultrasonic sensors are located underneath the plurality of motors, the connection bars, and main body.

3. The UAV of claim 1 , further comprising sensor protection cases disposed at the plurality of motors to surround the plurality of ultrasonic sensors.

4. The UAV of claim 3 , wherein the sensor protection cases are made of synthetic rubber or coil springs.

5. The UAV of claim 1 , wherein the plurality of ultrasonic sensors includes a first ultrasonic sensor and a second ultrasonic sensor, the first ultrasonic sensor and the second ultrasonic sensor are spaced apart on the UAV by a sensor distance (DS), the first ultrasonic sensor measures a first distance (D 1 ) from the first ultrasonic sensor to the ground surface, the second ultrasonic sensor measures a second distance (D 2 ) from the second ultrasonic sensor to the ground surface; and

the controller receives distance information including the sensor distance (DS), the first distance (D 1 ), and the second distance (D 2 ), calculates a slope of the ground surface, and determines a landing if the slope of the ground surface is within a predetermined range.

6. The UAV of claim 5 , wherein when the first ultrasonic sensor and the second ultrasonic sensor measures the first and second distances (D 1 and D 2 ) respectively, the UAV is in a horizontal state.

7. The UAV of claim 5 , wherein a perpendicular foot is drawn from the main body onto the ground surface, the main body rotates with respect to the perpendicular foot at a different angle, the first ultrasonic sensor and the second ultrasonic sensor measure the first and second distances (D 1 and D 2 ) for each of the different angle.

8. The UAV of claim 7 , wherein the main body rotates with respect to the perpendicular foot more than 360°.

9. The UAV of claim 1 , wherein each of the motors is a stepping motor or a servo motor.

10. The UAV of claim 1 , further comprising a plurality of temperature sensors located on the main body and measuring internal or external temperature of the UAV.

11. The UAV of claim 1 , wherein the plurality of ultrasonic sensors includes a first ultrasonic sensor and a center ultrasonic sensor, the plurality of motors include a first motor, the first ultrasonic sensor is located underneath the first motor and surrounded by a first sensor protection case, and the center ultrasonic sensor is located underneath the main body,

wherein α refers to a distance from one end of the first sensor protection case to the first ultrasonic sensor, β refers to a distance from one end of the first ultrasonic to the center ultrasonic sensor in a vertical direction, a threshold reference height (TRH) is obtained by the following equation: TRH=α+β, and

wherein if the threshold reference height (TRH) is greater than a height of the ground (HG), the controller generates the control signal to land on the ground surface.

12. The UAV of claim 11 , wherein the first ultrasonic sensor measures a first distance (D 1 ) from the first ultrasonic sensor to the ground surface, the center ultrasonic sensor measures a center distance (CD) from the center ultrasonic sensor to the ground surface; and

the height of the ground (HG) is obtained by the following equation: HG=D 1 −CD+β.

13. A landing guidance method for an unmanned aerial vehicle (UAV) using ultrasonic waves, the landing guidance method comprising:

adjusting a gyro sensor located in a main body to maintain the UAV in a horizontal state;

measuring distances from the ground surface for landing by transmitting and receiving, by a plurality of ultrasonic sensors located in the UAV, ultrasonic waves to and from the ground surface;

detecting an unevenness of the ground surface based on the distances from the plurality of ultrasonic sensors to the ground surface;

generating a control signal whether to land on the ground surface or not in response to the detection of the unevenness; and

transmitting the control signal to a plurality of motors.

14. The landing guidance method of claim 13 , wherein the plurality of ultrasonic sensors includes a first ultrasonic sensor and a second ultrasonic sensor, the first ultrasonic sensor and the second ultrasonic sensor are spaced apart on the UAV by a sensor distance (DS),

wherein the step of detecting the unevenness of the ground surface comprises:

measuring, by the first ultrasonic sensor, a first distance (D 1 ) from the first ultrasonic sensor to the ground surface;

measuring, by the second ultrasonic sensor, a second distance (D 2 ) from the second ultrasonic sensor to the ground surface;

receiving, by the controller, distance information including the sensor distance (DS), the first distance (D 1 ), and the second distance (D 2 ),

calculating the unevenness of the ground surface;

determining a landing if the slope of the ground surface is within a predetermined range; and

generating and transmitting a control signal to the plurality of motors.

15. The landing guidance method of claim 14 , wherein the unevenness of the ground surface is a slope of the ground surface.

16. The landing guidance method of claim 13 , wherein the step of detecting an unevenness of the ground surface comprises:

comparing gradients of points of the ground surface around a reference point with a predetermined convexity threshold when it is determined that the ground surface is convex according to the gradients of the points around the reference point, and determining whether the ground surface is suitable for landing and controlling the control signal corresponding to a determination whether the gradients of points are smaller than the predetermined convexity threshold; or

comparing the gradients of the points of the ground surface around the reference point with a predetermined concavity threshold when it is determined that the ground surface is concave according to the gradients of the points around the reference point, and determining whether the ground surface is suitable for landing and controlling the control signal corresponding to the determination whether the gradients are smaller than the predetermined concavity threshold.

17. The landing guidance method of claim 13 , further comprising:

transmitting and receiving, by the ultrasonic sensors, ultrasonic waves to and from the ground surface while the UAV is descending toward the ground surface;

analyzing the ultrasonic waves received by the ultrasonic sensors and determining, by the controller, whether there is an obstacle on the ground surface toward which the UAV is descending;

adjusting, by the gyro sensor, the UAV in the horizontal state when it is determined that there is an obstacle on the ground surface;

generating, by the controller, a signal to the UAV to rotate a circle for a predetermined number about an axis perpendicular from the main body to the ground surface;

transmitting and receiving, by the controller, ultrasonic waves to and from the ground surface for descending when the UAV rotates multiple times;

analyzing, by the controller, the ultrasonic waves and re-determining whether there is an obstacle on the ground surface; and

re-determining, by the controller, whether the ground surface is suitable for the UAV to land on in reply to a determination whether there is an obstacle on the ground surface.

18. The landing guidance method of claim 13 , further comprising analyzing, by the controller, the ultrasonic waves received from the ground surface and determining a physical property of the ground surface.

19. The landing guidance method of claim 13 , wherein the plurality of ultrasonic sensors includes a first ultrasonic sensor and a center ultrasonic sensor, the plurality of motors include a first motor, the first ultrasonic sensor is located underneath the first motor and surrounded by a first sensor protection case, and the center ultrasonic sensor is located underneath the main body,

wherein α refers to a distance from one end of the first sensor protection case to the first ultrasonic sensor, β refers to a distance from one end of the first ultrasonic to the center ultrasonic sensor in a vertical direction, a threshold reference height (TRH) is obtained by the following equation: TRH=α+β,and

wherein if the threshold reference height (TRH) is greater than a height of the ground (HG), the step of the generating the control signal generates the control signal to land on the ground surface.

20. The landing guidance method of claim 19 , wherein the first ultrasonic sensor measures a first distance (D 1 ) from the first ultrasonic sensor to the ground surface, the center ultrasonic sensor measures a center distance (CD) from the center ultrasonic sensor to the ground surface; and

the height of the ground (HG) is obtained by the following equation: HG=D 1 −CD+β.