Abstract:
To provide a small flying object that is inexpensive and capable of stable flying. In order to solve the above problem, a representative example of the small flying object of the present invention includes an upper rotor that generates thrust by rotating, a lower rotor that is disposed below the upper rotor and rotates coaxially with the upper motor and in the opposite direction to the upper motor, and an inertia balancer that is connected to one of the rotors having a lower rotation speed during hovering among the upper rotor and the lower rotor, and rotates integrally with the one rotor. The inertia balancer compensates a difference between an angular momentum of the one rotor and an angular moment of the other rotor during hovering.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a small flying object that flies by producing thrust with two rotors. 
       BACKGROUND ART 
       [0002]    Among flying objects that fly by producing thrust via the rotation of a rotor, there are some that are constituted with two rotors on the top and the bottom, in which a counterforce generated by the rotation of the rotors is cancelled out by rotating the rotors in mutually opposite directions. For example, PTL 1 listed below discloses a well-known example of such flying object. 
         [0003]    Paragraph [0014] of PTL 1 discloses the following: “The main rotors  14  and  15  are provided coaxially at an upper and a lower level on the rotation shaft  16 . The rotation shaft  16  rotationally drives the lower main rotor  15  and rotatably supports the upper main rotor  14 , and the upper main rotor  14  is rotationally driven by a rotation shaft  19  on the inside of the rotation shaft  16 . The main rotors  14  and  15  rotate in mutually opposite directions. The rotation shafts  16  and  19  rotationally drive the respective rotor blades by a motor within the main body  13 .” Further, Paragraph [0029] of PTL 1 discloses the following: “A yaw axis rate gyro  58  that outputs a command to the main rotor motors  55  and  56 , and a roll/pitch axis rate gyro  59  that transmits a signal to the cyclic pitch servomotor  57  and changes an attack angle of the main rotors are also provided.” 
       CITATION LIST 
     Patent Literature 
       [0004]    PTL 1: JP 2013-512149 W 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    In a flying object having counter-rotating rotors as described above, air whose velocity has been increased by the upper rotor flows into the lower rotor. Thus, if the upper and lower rotors are mirror-image symmetrical, the lower rotor must have a higher rotation speed than the upper rotor in order for the flying object to remain stationary relative to the yaw direction. In this way, there has been a problem in that if rotation speeds are generated by the upper and lower rotors, the angular momentum differs between the upper and lower rotors, and thus a whirling movement due to gyro effects is generated when the flying object operates in the pitch or roll direction, and it becomes difficult to stabilize the posture of the flying object. 
         [0006]    Herein, in the method disclosed in PTL 1, rotors in which the attack angle of the rotor blade can be changed are provided such that they can rotate in mutually opposite directions on the top and bottom of the same axis, and the posture of the flying body is controlled by changing the rotation speed of the upper and lower rotors and the attack angle of the rotor blades. However, in the conventional technology disclosed in PTL 1, rotors in which the attack angle of the rotor blade can be changed must be used for posture control, and such rotors have a complex structure and it is cumbersome to adjust the length of the link mechanism and the like, and this may lead to increased costs. 
         [0007]    Thus, an object of the present invention is to provide a small flying object that is inexpensive and capable of stable flying. 
       Solution to Problem 
       [0008]    To solve the above problem, one of the representative small flying objects of the present invention includes: an upper rotor that generates thrust by rotating; a lower rotor that is disposed below the upper rotor and rotates coaxially with the upper motor and in the opposite direction to the upper motor; and an inertia balancer that is connected to one of the rotors having a lower rotation speed during hovering among the upper rotor and the lower rotor, and rotates integrally with the one rotor, and the inertia balancer compensates a difference between an angular momentum of the one rotor and an angular momentum of the other rotor during hovering. 
       Advantageous Effects of Invention 
       [0009]    According to the invention, a small flying object that is inexpensive and capable of stable flying can be provided. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  is an overall perspective view of a small flying object of Embodiment 1 of the present invention. 
           [0011]      FIG. 2  is a view explaining a control device  11  of Embodiment 1 of the present invention. 
           [0012]      FIG. 3  is a view explaining a control algorithm of Embodiment 1 of the present invention. 
           [0013]      FIG. 4  is a view explaining movement around the rotors of Embodiment 1 of the present invention. 
           [0014]      FIG. 5  illustrates whirling movement of Embodiment 1 of the present invention. 
           [0015]      FIG. 6  is a view explaining angular momentum around the rotors of Embodiment 1 of the present invention. 
           [0016]      FIG. 7  illustrates whirling movement of Embodiment 1 of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0017]      FIG. 1  is an overall perspective view of a small flying object of Embodiment 1 of the present invention. In the following explanations, the direction of travel of the flying object will be referred to as the X axis, the direction of gravity will be referred to as the Z axis, and the axis that is orthogonal to both the X axis and the Z axis will be referred to as the Y axis. Rotation around the X axis will be defined as roll, rotation around the Y axis will be defined as pitch, and rotation around the Z axis will be defined as yaw. 
         [0018]    A small flying object  1  shown in  FIG. 1  includes the following as a thrust generation part for making the small flying object  1  float: an upper rotor  3  having a rotor blade, an upper motor  2  for driving the upper rotor  3 , a lower motor  5  that is driven in a rotation direction opposite to that of the upper motor  2  and is disposed so that its rotation axis is coaxial with that of the upper motor  2 , and a lower rotor  6  that is driven by the lower motor  5  and has a rotor blade. An inertia  12 , which is disposed to rotate integrally and is constituted symmetrically relative to the rotation axis of the upper rotor  3 , is provided to a rotating part of the upper rotor  3  and the upper motor  2 . 
         [0019]    For the purpose of changing the thrust direction of the thrust generation part in the pitch and roll directions in order to perform posture control of the small flying object  1 , the following are also provided: a center gimbal part  4  which has the upper motor  2  at a top part thereof and has the lower motor  5  in the opposite direction; a pitch drive motor  7  that is provided on a bottom end of the center gimbal part  4  and includes an output part so as to be capable of rocking the center gimbal part  4  in the pitch direction; a peripheral gimbal part  8  including the pitch drive motor  7 ; and a roll drive motor  9  a roll drive motor  9  which includes an output part so as to be capable of rocking the peripheral gimbal part  8  in the roll direction. 
         [0020]    The structure supporting the above-described mechanisms is constituted by a main frame  10 , which has an approximately symmetrical shape in the X and Y directions relative to the rotation axis of the upper rotor  3  and the lower rotor  6 , is provided so as to not obstruct the rotation of the upper rotor  3  and the lower rotor  6 , and has a shape that becomes stable when, for example, landing on the ground; and a control device  11  that is provided on a lower part of the main frame  10  so as to reduce the center of gravity of the small flying object  1  as much as possible. The control device  11  occupies the majority of the weight of the small flying object  1 , and in order to enhance the stability of the small flying object  1  in the air, the control device  11  should be installed upon positional adjustment so that the center of gravity of the small flying object  1  is positioned on the rotation axis of the upper rotor  3  and the lower rotor  6 . 
         [0021]    The upper rotor  3  and the lower rotor  6  are driven to rotate in mutually opposite directions to generate thrust vertically downwards and make the small flying object  1  fly. The thrust can be changed by changing the rotation speed of the upper rotor  3  and the lower rotor  6 . By rotating in mutually opposite directions, the anti-torque generated when the upper rotor  3  and the lower rotor  6  generate thrust can be utilized so that the movement in the yaw direction can be controlled. The upper motor  2  and the lower motor  5  that drive the upper rotor  3  and the lower rotor  6  are controlled in terms of rotation speed by the control device  11 . 
         [0022]    The pitch drive motor  7  and the roll drive motor  8  include, for example, a power source such as an electric motor (stepping motor, brushless motor, ultrasonic motor, etc.), a deceleration mechanism, and an angle detector (rotary encoder, potentiometer, etc.) built therein. The pitch drive motor  7  and the roll drive motor  8  are appropriately controlled in terms of rotation angle by the control device  11 . By deflecting the direction of thrust generated by the upper rotor  3  and the lower rotor  6  using the pitch drive motor  7  and the roll drive motor  8 , the posture of the small flying object  1  is stably controlled. 
         [0023]      FIG. 2  illustrates a constitution of the control device  11 . 
         [0024]    The control device  11  includes therein a three-axis posture detection means  20 , a command receiving means  21 , an external environment recognition means  22 , a battery  23 , and a central processing unit  24 . The three-axis posture detection means  20  is a means that can detect an angle and angular velocity in the roll, pitch, and yaw directions such as, for example, a three-axis gyro, and is used for the purpose of obtaining a posture of the small flying object  1 . The command receiving means  21  is a means for receiving an external command, and can receive the command wirelessly or via wires. The external environment recognition means  22  is a sensor that measures the height from the ground of the small flying object  1 , a sensor that measures the distance from surrounding objects, or the like. The battery  23  is a power source of the small flying object  1 , but, for example, the battery  23  can supply power through a signal wire in the case that the command receiving means  21  is wired. The central processing unit  24  appropriately controls the upper motor  2 , the lower motor  5 , the roll drive motor  9 , and the pitch drive motor  7  on the basis of information from the three-axis posture detection means  20 , the command receiving means  21 , and the external environment recognition means  22 . 
         [0025]      FIG. 3  illustrates a yaw direction control algorithm of the small flying object  1  in Embodiment 1. The method of control will be explained below in order. 
         [0026]    A target yaw angular velocity θ Y  and a propeller rotation speed N th  are obtained from the command receiving means  21  (S 11 ). 
         [0027]    A yaw angular velocity G Y  is obtained by the three-axis posture detection means  20  (S 12 ). 
         [0028]    A rotation speed N th +(θ Y −G Y )×K Y  is output to the upper motor, and a rotation speed N th +(θ Y −G Y )×K Y  is output to the lower motor (S 13 ). Herein, with regard to the rotation speed, left rotation is regarded as positive, and K y  is a yaw control gain. 
         [0029]    Subsequently, the process returns to the beginning. The above steps are executed at predetermined time increments. 
         [0030]      FIG. 4  is a view explaining movement around the rotors when the small flying object is stationary relative to the yaw direction during hovering in Embodiment 1. The cross-sections of the upper rotor  3  and the lower rotor  6  during hovering are indicated as an upper rotor cross-section F 22  and a lower rotor cross-section F 26 . Herein, the upper rotor  3  and the lower rotor  6  are configured with blade cross-sections having the same angle of attack and the same profile considering the availability and cost reduction, and the only difference between the upper rotor  3  and the lower rotor  6  is the mirror-image symmetry. 
         [0031]    A velocity when viewed from air on the upper rotor cross-section F 22  is an upper rotor velocity F 24 , an upper rotor attack angle F 23 , and an upper rotor thrust F 20  generated at that time, and the upper rotor anti-torque is F 21 . A velocity when viewed from air on the lower rotor cross-section F 26  is a lower rotor velocity F 28 , a lower rotor attack angle F 29 , a lower rotor thrust F 25 , and a lower rotor anti-torque F 27 . Since air with whose velocity is increased by the upper rotor  3  flows into the lower rotor cross-section F 26 , the air has a velocity F 29 . As a result, the upper rotor attack angle F 29  is smaller than the lower rotor attack angle F 23 . Meanwhile, in order for the small flying object  1  to be stationary relative to the yaw direction, it is necessary for the sizes of the upper rotor anti-torque F 21  and the lower rotor anti-torque F 27  to be equal. Therefore, the lower rotor  6  having a small attack angle must have a higher rotation speed than that of the upper rotor  3 . 
         [0032]    Mainly due to cost restrictions, the upper rotor  3  and the lower rotor  6  are often configured with blade cross-sections having the same angle of attack and the same profile with the only difference being the mirror-image symmetry. Further, for the same reasons, the same motor is often used for both the upper motor  2  and the lower motor  5 . During hovering, in the present embodiment as described above, the lower rotor  6  has a higher rotation speed than the upper rotor  3 . If the total moment of inertia around the Z axis of the upper motor  2  and the upper rotor  3  is I 1 , the total moment of inertia around the Z axis of the lower motor  5  and the lower rotor  6  is I 2 , the rotation speed of the upper rotor  3  is w 1 , and the rotation speed of the lower rotor  6  is w 2 , then the angular momentums of the upper and lower rotors are I 1   w   1  and I 2   w   2  respectively. If the rotation speeds of the upper rotor  3  and the lower rotor  6  are equal, the angular momentums will cancel each other out. However, since the rotation speed of the lower rotor  6  is higher as explained above, a total angular momentum of the upper and lower rotors exists. As explained above, in the small flying object  1  of the present embodiment, the orientation of the thrust of the upper and lower rotors is deflected with the pitch drive motor  7  and the roll drive motor  8  to perform posture control. Thus, a whirling movement is generated over the entire the small flying object  1  due to gyro effects when the rotor thrust is deflected.  FIG. 5  illustrates this whirling movement. Displacement around the pitch and displacement around the roll are generated periodically, and vibrations occur continuously without damping. 
         [0033]    Thus, as shown in  FIG. 6 , the small flying object  1  of Embodiment 1 includes an inertia I 2  configured to rotate integrally with the upper rotor  3 . The moment of inertia of the inertia I 2  is determined as follows. 
         [0034]    If the moment of inertia of the inertia I 2  is I add , then from balance conditions of the angular momentum, 
         [0000]      ( I   1   +I   add ) w   1   =I   2   W   2   Eq. 1
 
         [0000]      Therefore, 
         [0000]        I   add =( I   2   w   2   −I   1   w   1 )/ W   1   Eq. 2
 
         [0035]    With regard to w 1  and w 2  at this time, the rotation speeds during hovering are measured to calculate the moment of inertia I add  of the inertia I 2 . 
         [0036]      FIG. 7  illustrates the movement around the pitch and around the roll after installing the inertia I 2 . By installing the inertia I 2 , the whirling movement is reduced and vibrational behavior converges. 
         [0037]    As explained above, according to the method of the present invention, a small flying object capable of stable posture control can be realized with a minimal structure using low-cost rotors. 
         [0038]    In the present invention, in the small flying object in which posture control is performed by changing in terms of roll and pitch the thrust direction of the thrust generation part having counter-rotating rotors in which the attack angle of the rotor blades is fixed, by imparting an inertial mass to the rotor of the rotors rotating in opposite directions that has a lower rotation speed to balance out the angular momentums of the upper and lower rotors so that the sizes of the angular momentums of the upper and lower rotors become balanced, the angular momentum of the thrust generation part can be brought close to zero, and thereby posture changes due to gyro effects during roll and pitch operations can be reduced. 
         [0039]    Further, in the above-described embodiment, the moment of inertia of the inertia I 2  was calculated and imparted so as to balance the angular momentums during hovering of the upper rotor and the lower rotor. However, for example, the moment of inertia to be imparted to the upper rotor can be calculated by predicting the thrust and rotation speed beforehand by simulation or the like, and thereby added in advance to the moment of inertia of the rotating part of the upper motor  2 . 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  small flying object 
           2  upper motor 
           3  upper rotor 
           4  center gimbal part 
           5  lower motor 
           6  lower rotor 
           7  pitch drive motor 
           8  peripheral gimbal part 
           9  roll drive motor 
           10  main frame 
           11  control device 
           12  inertia