Patent Publication Number: US-2023146627-A1

Title: Robot control method and robot

Description:
The present application is based on, and claims priority from JP Application Serial Number 2021-181064, filed Nov. 5, 2021, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a method for controlling a robot and a robot. 
     2. Related Art 
     For example, JP-A-7-178689 describes a robot having a first arm, a second arm, a piezoelectric actuator for rotating the second arm with respect to the first arm, and an inclination angle sensor disposed at a tip end section of the second arm. In the robot, deflection and torsion of the second arm are measured based on output of the inclination angle sensor, and a drive signal of the piezoelectric actuator is corrected based on the measured value, thereby suppressing a positional shift of the second arm. 
     In the robot described in JP-A-7-178689, since the inclination angle sensor is disposed on the second arm, the positional shift of the second arm may be suppressed with high accuracy. However, the positional shift of an end effector connected to the tip end section of the second arm cannot be suppressed with high accuracy. 
     SUMMARY 
     A robot control method of the present disclosure, for a robot including a first member, a second member connected to the first member, a drive device configured to rotate or slide the second member with respect to the first member, and an end effector connected to the second member, wherein posture of the end effector is changed by drive of the drive device, the robot control method includes detecting, based on an output signal from an inertial sensor disposed on the end effector, a gravity influence amount indicating a degree of influence of gravity received by the end effector, determining, based on the detected gravity influence amount, a drive algorithm for the drive device from among a plurality of drive modes, and driving the drive device by the determined drive algorithm. 
     A robot of the present disclosure includes a first member, a second member connected to the first member, a drive device configured to rotate or slide the second member with respect to the first member, an end effector connected to the second member, and a control device that controls drive of the drive device, wherein the control device detects, based on an output signal from an inertial sensor disposed on the end effector, a gravity influence amount indicating a degree of influence of gravity received by the end effector, determines, based on the detected gravity influence amount, a drive algorithm for the drive device from among a plurality of drive modes, and driving the drive device by the determined drive algorithm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram showing a robot according to a first embodiment. 
         FIG.  2    is a diagram showing a piezoelectric drive device. 
         FIG.  3    is a diagram showing a drive signal of a piezoelectric actuator. 
         FIG.  4    is a diagram showing vibration state of the piezoelectric actuator. 
         FIG.  5    is a diagram showing vibration state of the piezoelectric actuator. 
         FIG.  6    is a block diagram showing configuration of a control device. 
         FIG.  7    is a view showing a posture which is hardly influenced by gravity. 
         FIG.  8    is a view showing a posture easily influenced by gravity. 
         FIG.  9    is a diagram showing a posture easily influenced by gravity. 
         FIG.  10    is a diagram showing a first drive mode. 
         FIG.  11    is a diagram showing the first drive mode. 
         FIG.  12    is a diagram showing the first drive mode. 
         FIG.  13    is a flowchart showing a method for controlling the robot. 
         FIG.  14    is a diagram illustrating a drive example in which the gravity influence amount is “low”. 
         FIG.  15    is a diagram illustrating a drive example in which the gravity influence amount is “medium”. 
         FIG.  16    is a diagram illustrating a drive example in which the gravity influence amount is “high”. 
         FIG.  17    is a diagram showing a second drive mode used in a method for controlling a robot according to a second embodiment. 
         FIG.  18    is a flowchart showing the method for controlling the robot. 
         FIG.  19    is a diagram showing a robot according to a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, a method for controlling a robot and a robot according to the disclosure will be described in detail based on an embodiment illustrated in the accompanying drawings. 
     First Embodiment 
       FIG.  1    is a diagram showing a robot according to a first embodiment. FIG.  2  is a diagram showing a piezoelectric drive device.  FIG.  3    is a diagram showing a drive signal of a piezoelectric actuator.  FIGS.  4  and  5    are diagrams showing vibration state of the piezoelectric actuator.  FIG.  6    is a block diagram showing configuration of a control device.  FIG.  7    is a view showing a posture which is hardly influenced by gravity.  FIGS.  8  and  9    are views each showing posture easily influenced by gravity.  FIGS.  10  and  12    are diagrams showing a first drive mode.  FIG.  13    is a flowchart showing a method for controlling the robot.  FIG.  14    is a diagram illustrating a drive example in which the gravity influence amount is “low”.  FIG.  15    is a diagram illustrating a drive example in which the gravity influence amount is “medium”.  FIG.  16    is a diagram illustrating a drive example in which the gravity influence amount is “high”. 
     A robot  1  shown in  FIG.  1    is a horizontal articulated robot (scalar robot). The use of the robot  1  is not particularly limited, and examples thereof include feeding, removing, transporting, and assembling of objects such as a precision apparatus or components constituting this. 
     The robot  1  includes a base  10  fixed to a floor or the like, a robot arm RA connected to the base  10 , and an end effector  15  connected to the robot arm RA. The robot arm RA includes a first arm  11  connected to the base  10 , a second arm  12  connected to the first arm  11 , a third arm  13  connected to the second arm  12 , and a fourth arm  14  connected to the third arm  13 , and an end effector  15  is connected to the fourth arm  14 . 
     Further, the first arm  11  moves in the direction of a first linear motion axis Jr 1  with respect to the base  10 , and rotates around a first rotation axis Jθ 1 , which is parallel to the first linear motion axis Jr 1 . The second arm  12  moves in a second linear motion axis Jr 2  direction orthogonal to the first linear motion axis Jr 1  with respect to the first arm  11 , and rotates around the second rotation axis Jθ 2 , which is parallel to the first rotation axis Jθ 1 . The third arm  13  rotates with respect to the second arm  12  around a third rotation axis Jθ 3 , which is orthogonal to the second rotation axis Jθ 2 . The fourth arm  14  rotates with respect to the third arm  13  around a fourth rotation axis Jθ 4 , which is orthogonal to the third rotation axis Jθ 3 . The robot  1  sets the end effector  15  in a target position and a target posture by a combination of movements around these four rotation axes Jθ 1 , Jθ 2 , Jθ 3 , and Jθ 4  and movements in the two linear motion axes Jr 1  and Jr 2  directions. 
     The first arm  11  includes a first linear motion section  111  that is connected to the base  10  and that moves with respect to the base  10  in the direction of the first linear motion axis Jr 1 , and a first rotation section  112  that is connected to the first linear motion section  111  and that rotates with respect to the first linear motion section  111  around the first rotation axis Jθ 1 . Note that in the present embodiment, each of the first linear motion axis Jr 1  and the first rotation axis Jθ 1  extends along the vertical direction. However, the directions of each of these axes Jr 1  and Jθ 1  are not particularly limited. 
     The second arm  12  includes an elongated second linear motion section  121  that is connected to the first rotation section  112  and that moves in the second linear motion axis Jr 2  direction with respect to the first rotation section  112 , and the second rotation section  122  that is connected to a tip end section of the second linear motion section  121  and that rotates around the second rotation axis Jθ 2  with respect to the second linear motion section  121 . Note that the second linear motion axis Jr 2  is orthogonal to the first rotation axis Jθ 1  and rotates around the first rotation axis Jθ 1  as the first rotation section  112  rotates around the first rotation axis Jθ 1 . The second rotation axis Jθ 2  is parallel to the first rotation axis Jθ 1 , and a separation distance D between the second rotation axis Jθ 2  and the first rotation axis Jθ 1  changes as the second linear motion section  121  moves in the direction of the second linear motion axis Jr 2 . 
     The third arm  13  includes an arm section  131  connected to the second rotation section  122 , and the third rotation section  132  connected to the arm section  131  so as to be rotatable around the third rotation axis Jθ 3 . The arm section  131  has a substantial L-shape that is bent at a substantially right angle in the middle thereof, the second rotation section  122  is connected to a base end section thereof, and the third rotation section  132  is connected to a tip end section thereof. Note that the third rotation axis Jθ 3  is orthogonal to the second rotation axis Jθ 2  and rotates around the second rotation axis Jθ 2  as the second rotation section  122  rotates around the second rotation axis Jθ 2 . 
     The fourth arm  14  includes an arm section  141  as a first member connected to the third rotation section  132 , and the fourth rotation section  142  as a second member connected to the arm section  141  so as to be rotatable around the fourth rotation axis Jθ 4 . The arm section  141  has a substantial L-shape that is bent at a substantially right angle in the middle thereof, the third rotation section  132  is connected to a base end section thereof, and the fourth rotation section  142  is connected to a tip end section thereof. Note that the fourth rotation axis Jθ 4  is orthogonal to the third rotation axis Jθ 3  and rotates around the third rotation axis Jθ 3  as the third rotation section  132  rotates around the third rotation axis Jθ 3 . 
     Although the robot arm RA has been described above, the configuration of the robot arm RA is not particularly limited. For example, it may be a six-axis robot arm having six rotation axes. 
     The end effector  15  is connected to the fourth rotation section  142 . The end effector  15  is a mechanism for causing the robot  1  to execute a predetermined work, for example, may have any configuration such as a mechanism for gripping the work W, a mechanism for sucking the work W, and a mechanism for applying an adhesive or the like to the work W. The configuration shown in the drawing includes a base section  150  connected to the fourth rotation section  142  and a pair of claw sections  151 ,  152  connected to the base section  150 , and grips and releases the work W by opening and closing the pair of claw sections  151 ,  152 . 
     The robot  1  further includes an inertial sensor  4  disposed on the base section  150  of the end effector  15 . The robot  1  detects the posture of the end effector  15  based on an output signal of the inertial sensor  4 . In the present embodiment, an acceleration sensor  41  is used as the inertial sensor  4 . In addition, the acceleration sensor  41  is a three-axis acceleration sensor capable of independently detecting acceleration in each axial direction of an X axis, a Y axis, and a Z axis, which are orthogonal to each other. This makes it possible to accurately detect the posture of the end effector  15 . However, the inertial sensor  4  is not particularly limited as long as it can detect the posture of the end effector  15 , and may be, for example, an angular velocity sensor. 
     The robot  1  further includes a piezoelectric drive device  2 A that moves the first linear motion section  111  in the first linear motion axis Jr 1  direction with respect to the base  10 , a piezoelectric drive device  2 B that rotates the first rotation section  112  rotates around the first rotation axis Jθ 1  with respect to the first linear motion section  111 , a piezoelectric drive device  2 C that moves the second linear motion section  121  in the second linear motion axis Jr 2  with respect to the first rotation section  112 , a piezoelectric drive device  2 D that rotates the second rotation section  122  rotates around the second rotation axis Jθ 2  with respect to the second linear motion section  121 , a piezoelectric drive device  2 E that rotates the third rotation section  132  rotates around the third rotation axis Jθ 3  with respect to the second rotation section  122 , a piezoelectric drive device  2 F that rotates the fourth rotation section  142  rotates around the fourth rotation axis Jθ 4  with respect to the third rotation section  132 , a piezoelectric drive device  2 G that drives the pair of claw sections  151 ,  152  to open and close, and a control device  3  that independently controls each of these piezoelectric drive devices  2 A to  2 G. 
     From among the piezoelectric drive devices  2 A to  2 G, the drive of at least the piezoelectric drive device  2 F, which controls the drive of the fourth rotation section  142  located at the most tip end side of the robot arm RA, controls the drive by using the following characteristic control method. Therefore, hereinafter, for convenience of description, the piezoelectric drive device  2 F will be described as a representative, and description of the other piezoelectric drive devices  2 A to  2 E, and  2 G will be omitted. 
     As shown in  FIG.  2   , the piezoelectric drive device  2 F as a drive device is a rotary type piezoelectric drive device. The piezoelectric drive device  2 F includes a piezoelectric actuator  21 , a rotor  22  as a driven body that receives drive force from the piezoelectric actuator  21  and that rotates around the fourth rotation axis Jθ 4 , a biasing member  23  that presses the piezoelectric actuator  21  against the rotor  22 , and an encoder  24  that detects rotation amount of the rotor  22 . The piezoelectric actuator  21  is fixed to the arm section  141  via the biasing member  23 , and the rotor  22  is fixed to the fourth rotation section  142 . Therefore, when the piezoelectric actuator  21  is driven, the fourth rotation section  142  rotates around the fourth rotation axis Jθ 4  with respect to the arm section  141 . As described above, according to the rotary type piezoelectric drive device, a device suitable for rotationally moving the fourth rotation section  142  is obtained. Note that the piezoelectric actuator  21  may be fixed to the fourth rotation section  142  via the biasing member  23 , and the rotor  22  may be fixed to the arm section  141 . 
     According to the piezoelectric drive device  2 F, the driving force from the piezoelectric actuator  21  is directly transmitted to the rotor  22 . Therefore, a relay mechanism that relays and transmits the driving force is not needed, and the device can be simplified and miniaturized. In addition, a decrease in movement accuracy due to backlash or insufficient rigidity, which is a problem in a relay mechanism such as a decelerator, is substantially eliminated and the robot  1  has excellent drive accuracy. The same rotary type piezoelectric drive device as the piezoelectric drive device  2 F is used for the piezoelectric drive devices  2 B,  2 D,  2 E, and  2 G, and a linear movement type (linear type) piezoelectric drive device, in which a linear moving slider is used instead of the rotor  22 , is used as the piezoelectric drive devices  2 A and  2 C. 
     The piezoelectric actuator  21  includes a vibration section  211 , a support section  212  that supports the vibration section  211 , a beam section  213  that connects the vibration section  211  and the support section  212 , and a protrusion-shaped transmission section  214  that is disposed at a tip end section of the vibration section  211  and that transmits vibration of the vibration section  211  to the rotor  22 . 
     The vibration section  211  has a plate-shape, and has a rectangular-shape with the vertical direction of the paper surface in  FIG.  2    as a longitudinal side. The vibration section  211  includes piezoelectric elements  21 A to  21 F for drive and piezoelectric element  21 G for detection that detects vibration of the vibration section  211 . The piezoelectric elements  21 C and  21 D are disposed side by side in a longitudinal direction in a central portion of the vibration section  211 . The piezoelectric elements  21 A and  21 B are disposed side by side in the longitudinal direction on one side of the piezoelectric elements  21 C and  21 D, and the piezoelectric elements  21 E and  21 F are arranged side by side in the longitudinal direction on the other side. Each of the piezoelectric elements  21 A to  21 F expands and contracts in the longitudinal direction of the vibration section  211  when energized. 
     The piezoelectric element  21 G for detection is disposed between the piezoelectric elements  21 C and  21 D. The piezoelectric element  21 G receives an external force corresponding to vibration of the vibration section  211  and outputs a detection signal corresponding to the received external force. Therefore, a vibration state of the vibration section  211  can be detected based on the detection signal output from the piezoelectric element  21 G. 
     The transmission section  214  is provided at the tip end section of the vibration section  211 , and the tip end thereof is in contact with the rotor  22 . Therefore, vibration of the vibration section  211  is transmitted to the rotor  22  through the transmission section  214 . The support section  212  is a portion that supports the vibration section  211 , and has a U-shape that surrounds both sides and the base end side of the vibration section  211 . In addition, the beam section  213  connects the vibration section  211  and the support section  212  in a state in which vibration of the vibration section  211  is allowed. 
     The biasing member  23  biases the piezoelectric actuator  21  toward the rotor  22  and presses the transmission section  214  against the rotor  22 . Accordingly, vibration of the vibration section  211  is efficiently transmitted to the rotor  22  via the transmission section  214 . When the piezoelectric drive device  2 F is not driven, a brake is applied to the rotor  22 , and the posture of the fourth rotation section  142  is maintained. The biasing member  23  includes a holding section  231  that holds the support section  212 , a base  232  that is fixed to the arm section  141 , and spring groups  233  and  234  that connect the holding section  231  and the base  232 . The biasing member  23  is fixed in a state in which the spring groups  233  and  234  are deformed, and presses the piezoelectric actuator  21  against the rotor  22  using a restoring force of the spring groups  233  and  234 . 
     Such a piezoelectric drive device  2 F is driven as follows. For example, when a drive signal V 1  illustrated in  FIG.  3    is applied to the piezoelectric elements  21 A and  21 F, a drive signal V 2  is applied to the piezoelectric elements  21 C and  21 D, and a drive signal V 3  is applied to the piezoelectric elements  21 B and  21 E, as illustrated in  FIG.  4   , the vibration section  211  makes a bending vibration that bends in the short side direction while performing a longitudinal vibration that expands and contracts in the longitudinal direction, and these vibrations are combined so that the tip end of the transmission section  214  performs an elliptical motion that draws an elliptical trajectory counterclockwise as indicated by the arrow A 1 . As a result, the rotor  22  is moved and rotates clockwise as indicated by an arrow B 1 . On the other hand, when the drive signals V 1  and V 3  are switched, that is, when the drive signal V 1  is applied to the piezoelectric elements  21 B and  21 E and the drive signal V 3  is applied to the piezoelectric elements  21 A and  21 F, as shown in  FIG.  5   , the tip end of the transmission section  214  performs an elliptical motion that draws an elliptical trajectory clockwise as indicated by the arrow A 2 , and the rotor  22  rotates counterclockwise as indicated by an arrow B 2 . 
     Note that, from among the longitudinal vibration and the bending vibration of the vibration section  211 , which are the basis of the elliptical motion of the transmission section  214 , the longitudinal vibration is excited by applying of the drive signal V 2  to the piezoelectric elements  21 C and  21 D, and the bending vibration is excited by applying of the drive signals V 1  and V 3  to the piezoelectric elements  21 A,  21 B,  21 E, and  21 F. 
     The control device  3  is constituted by, for example, a computer, and includes a processor that processes information, a memory that is communicably connected to the processor, and an external interface. In addition, the memory stores a program executable by the processor, and the processor reads and executes the program stored in the memory. Such a control device  3  receives a command from a host computer (not shown) and independently controls the drive of each of the piezoelectric drive devices  2 A to  2 G so that the end effector  15  becomes the target position and the target posture based on the command. 
     As shown in  FIG.  6   , the control device  3  includes control sections  3 A,  3 B,  3 C,  3 D,  3 E,  3 F, and  3 G that control the piezoelectric drive devices  2 A,  2 B,  2 C,  2 D,  2 E,  2 F, and  2 G, and in particular, the control section  3 F, which controls drive of the piezoelectric drive device  2 F, includes a drive signal generation section  31 , a drive algorithm selection section  32 , and a posture detection section  33  that detects posture or the like of the end effector  15 . The posture detection section  33  detects the posture of the end effector  15  based on the output signal of the acceleration sensor  41 . In addition, the posture detection section  33  detects, based on at least the detected posture, a degree of influence (hereinafter, a gravity influence amount) of gravity G received by the piezoelectric drive device  2 F. Note that, in this embodiment, in addition to the detected posture, the gravity influence amount is detected in consideration of the weight of the end effector  15  and the work W gripped by the end effector  15 . Accordingly, it is possible to detect a more accurate gravity influence amount. Note that a method of detecting the gravity influence amount is not particularly limited as long as the method is based on an output signal of the acceleration sensor  41 . 
     The drive algorithm selection section  32  selects a drive algorithm of the piezoelectric actuator  21  based on the detection result of the posture detection section  33 , that is, the posture and the gravity influence amount of the end effector  15 , and on the target position of the end effector  15 . The drive algorithm is selected from a first drive mode Dm 1  and a second drive mode Dm 2 , as described below. The drive signal generation section  31  generates the drive signals V 1 , V 2 , and V 3  based on the drive algorithm selected by the drive algorithm selection section  32  and a command from the host computer (not shown), and applies the generated drive signals V 1 , V 2 , and V 3  to the piezoelectric actuator  21 . According to such the method, since the actual rotation amount and rotation direction detected by the encoder  24  are fed back, the movement of the end effector  15  can be accurately controlled. 
     The configuration of the robot  1  has been briefly described above. Next, a method of controlling the piezoelectric drive device  2 F will be described. In the control method of the piezoelectric drive device  2 F, an optimal drive algorithm is selected based on the posture and the gravity influence amount of the end effector  15  and on the target position of the end effector  15 , and drive of the piezoelectric drive device  2 F is controlled by using the selected drive algorithm. Accordingly, it is possible to reduce an influence of gravity G on the piezoelectric drive device  2 F as much as possible and to accurately control the minute movement of the robot  1 . 
     Before describing the control method, a case where the piezoelectric drive device  2 F is not likely to be influenced by gravity G and a case where the piezoelectric drive device  2 F is easily influenced by gravity G will be briefly described.  FIG.  7    shows an example which is hardly influenced by gravity G, and  FIG.  8    shows an example which is easily influenced by gravity G. 
     In  FIG.  7   , the fourth rotation axis Jθ 4  is along the vertical direction V. In this case, in the operation of rotating the end effector  15  around the fourth rotation axis Jθ 4 , the gravity influence amount received by the piezoelectric drive device  2 F is constant regardless of the current position and the target position of the end effector  15 . On the other hand, in  FIG.  8   , the fourth rotation axis Jθ 4  is along the horizontal direction H. In this case, the gravity influence amount received by the piezoelectric drive device  2 F varies depending on the posture of the end effector  15 , for example, when the end effector  15  is oriented in the vertical direction as indicated by solid line, the gravity influence amount is smallest, and when the end effector  15  is oriented in the horizontal direction as indicated by chain line, the gravity influence amount is largest. 
     In addition, in the example of  FIG.  8   , the gravity influence amount received by the piezoelectric drive device  2 F also varies depending on the rotation direction of the end effector  15 . For example, as shown by one dot chain line in  FIG.  9   , when it is desired to rotate the end effector  15  upward (counterclockwise) by 90° from the current position, gravity G resists the driving force of the piezoelectric drive device  2 F, and conversely, when it is desired to rotate the end effector  15  downward (clockwise) by 90° from the current position, gravity G is added to the driving force of the piezoelectric drive device  2 F. As described above, in the example of  FIG.  8   , the gravity influence amount varies depending on the current position or the target position of the end effector  15 , and accordingly, the drive of the piezoelectric drive device  2 F is likely to become unstable. Therefore, it becomes difficult to accurately perform the minute movement control of the end effector  15 . 
     As described above, there is a concern that the position accuracy of the end effector  15  may decrease due to the gravity influence amount. Therefore, as described above, in the present embodiment, an optimal drive algorithm is selected based on the posture and the gravity influence amount of the end effector  15  and on the target position of the end effector  15 , and the drive of the piezoelectric drive device  2 F is controlled by using the selected drive algorithm. 
     Next, the drive algorithm previously set for the robot  1  will be described. In the present embodiment, as the drive algorithm, as shown in  FIGS.  10  to  12   , a first drive mode Dm 1  is set to increase separation amplitude W 2 , which is the amplitude of the longitudinal vibration, while keeping feed amplitude W 1 , which is the amplitude of the bending vibration, constant. Further, a plurality of modes Dm 11 , Dm 12 , and Dm 13 , each having a different feed amplitude W 1 , is set as the first drive mode Dm 1 . That is, in the present embodiment, three drive modes are set as the drive algorithm. 
     According to the first drive mode Dm 1 , it is easy to generate a minimum necessary drive force. Therefore, a sudden large movement of the rotor  22 , due to an excessive driving force, is unlikely to occur and the stopping accuracy is also good. On the other hand, since the driving force increases little by little in order to generate the minimum necessary driving force, the driving force is easily influenced by gravity G at the initial stage of the drive start. Therefore, in the present embodiment, by selecting a drive mode having a driving force that is optimal with respect to a degree of the influence of gravity G, that is, that can realize an appropriate amount of movement without yielding over to gravity G, a drive method that is less likely to be influenced by gravity G and is excellent in stopping accuracy is provided. 
     However, the drive mode set as the drive algorithm is not particularly limited, and it is sufficient that least two different drive modes are set. 
     In this embodiment, the feed amplitude W 1  is controlled by the voltage values of the drive signals V 1  and V 3 , and the separation amplitude W 2  is controlled by the voltage value of the drive signal V 2 . This facilitates control of the amplitude W 1  and W 2 . However, a method of controlling the amplitudes W 1  and W 2  is not limited thereto, and, for example, it may be controlled by the frequency or phase of the drive signals V 1 , V 2 , and V 3 . Further, as will be understood from the following description, “making the feed amplitude W 1  constant” means a state in which the voltage values of the drive signals V 1 , V 3  for controlling the bending vibration are made constant, and the actual amplitudes are not necessarily constant. Further, the above mentioned term “constant” includes the meaning of, for example, when a minute change, which may occur due to the configuration of the circuit, occurs, in addition to when there is no change with time. 
     In addition, as illustrated in  FIGS.  10  to  12   , in the first drive mode Dm 1 , the longitudinal vibration is excited after the bending vibration is excited in the piezoelectric actuator  21 . As a result, the first drive mode Dm 1  is less influenced by gravity G. Specifically, in a state in which the bending vibration is excited in the piezoelectric actuator  21 , the transmission section  214  is kept in a state pressed against the rotor  22  by the biasing member  23 . Therefore, bending deformation of the vibration section  211  is not allowed, and the bending vibration does not actually occur in the vibration section  211 . For example, in the case of a vehicle, this state corresponds to a state of strongly stepping on the brake while stepping on the accelerator, so as not to start movement of the vehicle. In this state, when the longitudinal vibration is excited in the piezoelectric actuator  21 , the transmission section  214  is separated from the rotor  22  by the longitudinal vibration, and at the same time, the suppressed the bending vibration is released and generates an elliptical motion of the transmission section  214 . That is, since the time lag from the separation of the transmission section  214  from the rotor  22  to the generation of the driving force is very short (substantially 0), the rotor  22  does not become free and is less likely to be influenced by gravity G. 
     On the other hand, when the bending vibration is excited after the longitudinal vibration is excited, the transmission section  214  is separated from the rotor  22  before the force for sending out the rotor  22  is generated. For example, in the case of a vehicle, this state corresponds to a state in which the brake is released without stepping on the accelerator, that is, a neutral state. Therefore, there is a concern that the rotor  22  becomes free and unintentionally moves due to the influence of gravity G, and the accuracy of the minute movement of the end effector  15  decreases. 
     The method of controlling the piezoelectric drive device  2 F will be described below with reference to  FIG.  13   , but this control is executed by the control section  3 F of the control device  3 . In the method of controlling the piezoelectric drive device  2 F, first, as step S 1 , the posture of the end effector  15  is detected based on the output signal of the acceleration sensor  41 . Next, as step S 2 , the gravity influence amount is detected based on the posture or the like detected in the step S 1 . Note that in the present embodiment, since three drive modes Dm 11 , Dm 12 , and Dm 13  are set as the drive algorithm, the gravity influence amount is classified into three levels of “low”, “medium”, and “high” in accordance with thereof. 
     Next, as step S 3 , it is determined whether the gravity influence amount detected in step S 2  is classified into “low”, “medium”, or “high”. Next, as step S 4 , one of the drive modes Dm 11 , Dm 12 , and Dm 13  is selected based on the classification of the gravity influence amount determined in step S 3  and on the rotation direction (clockwise/counterclockwise) of the end effector  15 , and is set as the drive algorithm. For example, when the gravity influence amount is “low”, such as when the end effector  15  is to be rotated counterclockwise from the 9 o&#39;clock position as shown in  FIG.  14   , the drive mode Dm 11  with the smallest driving force is selected, when the gravity influence amount is “medium”, such as when the end effector  15  is to be rotated counterclockwise from the half past 4 o&#39;clock position as shown in  FIG.  15   , the drive mode Dm 12  with the middle driving force is selected, and when the gravity influence amount is “high”, such as when the end effector  15  is to be rotated around the counterclockwise from the 3 o&#39;clock position as shown in  FIG.  16   , the drive mode Dm 13  with the largest driving force is selected. 
     Next, as step S 5 , the piezoelectric actuator  21  are driven by the set drive algorithm, and the end effector  15  is moved toward the target position. Next, as step S 6 , it is determined whether the end effector  15  has reached the target position. If the determination result is “not reached”, the process returns to step S 1 , and steps S 1  to S 6  are repeated until the determination result is “reached”. Accordingly, since it is possible to switch the drive mode in real time based on the posture of the end effector  15  and the gravity influence amount which changes from moment to moment, excellent minute movement accuracy is possible. When the determination result is “reached”, as step S 7 , the drive of the piezoelectric drive device  2 F is stopped. As a result, the movement of the end effector  15  to the target position ends normally. According to such a control method, it becomes less likely to be influenced by gravity G, and it is possible to effectively suppress positional shift of the end effector  15 . 
     The robot  1  and the robot control method  1  according to the present embodiment have been described above. As described above, in the control method for the robot  1  that includes the arm section  141  as the first member, the fourth rotation section  142  as the second member connected to the arm section  141 , the piezoelectric drive device  2 F as the drive device configured to rotate or slide the fourth rotation section  142  with respect to the arm section  141 , and the end effector  15  connected to the fourth rotation section  142 , wherein the posture of the end effector  15  is changed by drive the piezoelectric drive device  2 F, the robot control method  1  includes detecting, based on the output signal from the inertial sensor  4  disposed on the end effector  15 , the gravity influence amount indicating the degree of the influence of gravity G on the end effector  15 , determining, based on the detected gravity influence amount, the drive algorithm for the piezoelectric drive device  2 F from among the plurality of drive modes Dm 11 , Dm 12 , and Dm 13 , and driving the piezoelectric drive device  2 F by the determined drive algorithm. This makes drive of the piezoelectric drive device  2 F less likely to be influenced by gravity G, and it is possible to effectively suppress the positional shift of the end effector  15 . 
     Further, as described above, the drive algorithm is determined based on the gravity influence amount and moving direction of the end effector  15  to the target position. This makes it possible to determine a more optimal drive algorithm. 
     As described above, the piezoelectric drive device  2 F rotates the fourth rotation section  142  around the fourth rotation axis Jθ 4  as the rotation axis, and, in a plan view along the fourth rotation axis Jθ 4 , a centroid of the end effector  15  is separated from the fourth rotation axis Jθ 4 . Accordingly, the drive of the piezoelectric drive device  2 F becomes likely to be influenced by gravity G, and the effect of the control method described above is more remarkably exhibited. 
     As described above, the drive device includes the vibration section  211  disposed on one of the arm section  141  and the fourth rotation section  142  and including the piezoelectric elements  21 A to  21 F, the rotor  22  as a driven body disposed on the other of the arm section  141  and the fourth rotation section  142 , and the transmission section  214  configured to transmit vibration of the vibration section  211  to the rotor  22  and the drive device is the piezoelectric drive device  2 F that, by energization to the piezoelectric elements  21 A to  21 F, vibrates the vibration section  211  in a combination of longitudinal vibration and bending vibration to cause the transmission section  214  to perform the elliptical motion, and to move the rotor  22  by the elliptical motion. According to the piezoelectric drive device  2 F, the driving force from the piezoelectric actuator  21  is directly transmitted to the rotor  22 . Therefore, a relay mechanism that relays and transmits the driving force is not needed, and the device can be simplified and miniaturized. In addition, a decrease in movement accuracy due to backlash or insufficient rigidity, which is a problem in a relay mechanism such as a decelerator, is substantially eliminated and the robot  1  has excellent drive accuracy. 
     Further, as described above, the drive mode includes a first drive mode Dm 1  in which the separation amplitude W 2 , which is the amplitude of the longitudinal vibration, is increased while the feed amplitude W 1 , which is the amplitude of the bending vibration, is kept constant and a plurality of different feed amplitudes W 1  are set in the first drive mode Dm 1 . According to such the first drive mode Dm 1 , the minute movement control of the fourth rotation section  142  becomes easy. 
     Further, as described above, in the first drive mode Dm 1 , the bending vibration is excited and then the longitudinal vibration is excited. Accordingly, the influence of gravity G can be further reduced, and the positional shift of the end effector  15  can be more effectively suppressed. 
     As described above, the robot  1  includes the arm section  141  as a first member, the fourth rotation section  142  as a second member connected to the arm section  141 , the piezoelectric drive device  2 F as a drive device configured to rotate or slide the fourth rotation section  142  with respect to the arm section  141 , the end effector  15  connected to the fourth rotation section  142 , and the control device  3  that controls drive of the piezoelectric drive device  2 F. Also, the control device  3  detects, based on the output signal from the inertial sensor  4  disposed on the end effector  15 , the gravity influence amount indicating the degree of the influence of gravity G received by the end effector  15 , determines, based on the detected gravity influence amount, the drive algorithm for the piezoelectric drive device  2 F from among a plurality of drive modes Dm 11 , Dm 12 , and Dm 13 , and drives the piezoelectric drive device  2 F the determined drive algorithm. This makes drive of the piezoelectric drive device  2 F less likely to be influenced by gravity G, and it is possible to effectively suppress the positional shift of the end effector  15 . 
     Second Embodiment 
       FIG.  17    is a diagram showing the second drive mode used in a method for controlling a robot according to a second embodiment.  FIG.  18    is a flowchart showing the method for controlling the robot. 
     The robot  1  of the present embodiment is the same as the robot  1  of the first embodiment described above except that the drive mode included in the drive algorithm is different. Therefore, in the following description, the present embodiment will be described with a focus on differences from the first embodiment described above, and the description of similar matters will be omitted. In the drawings of the present embodiment, the same components as those of the above described embodiment are denoted by the same reference numerals. 
     In the present embodiment, as the drive algorithm, two modes are set, the first drive mode Dm 1  illustrated in  FIG.  10    of the first embodiment described above, and a second drive mode Dm 2  in which the feed amplitude W 1  is increased while the separation amplitude W 2  is increased as illustrated in  FIG.  17   . 
     In the first drive mode Dm 1 , the feed amplitude W 1  is kept constant and only the separation amplitude W 2  is gradually increased. Therefore, it is easy to generate the minimum necessary driving force. Therefore, a sudden large movement of the rotor  22 , due to an excessive driving force, is unlikely to occur and the stopping accuracy is also good. On the other hand, since the driving force is increased little by little in order to generate the minimum necessary driving force, the driving force is easily influenced by gravity G at the initial stage of starting the driving at the initial stage of the drive start. In contrast, in the second drive mode Dm 2 , both the feed amplitude W 1  and the separation amplitude W 2  are gradually increased. For this reason, although it is easily influenced by gravity G immediately after drive, since the degree of increase in the driving force is higher than that in the first drive mode Dm 1 , afterward it is less susceptible to the influence of gravity G than is the first drive mode Dm 1 . On the other hand, since the pace of increase of the feed amplitude W 1  is faster than that of the first drive mode Dm 1 , the stopping accuracy may be reduced due to excessive driving force depending on the rotational speed of the rotor  22  or the like. In this manner, the first drive mode Dm 1  and the second drive mode Dm 2 , which have opposite characteristics, are switched in accordance with the gravity influence amount, so that the influence of gravity G is less likely to occur. 
     Hereinafter, the method of controlling the piezoelectric drive device  2 F will be described with reference to  FIG.  18   . In the method of controlling the piezoelectric drive device  2 F, first, as step S 1 , the posture of the end effector  15  is detected based on the output signal of the acceleration sensor  41 . Next, as step S 2 , the gravity influence amount is detected based on the posture or the like detected in the step S 1 . Note that in this embodiment, since two drive modes Dm 1  and Dm 2  are set as the drive algorithm, the gravity influence amount is classified into two stages of “low” and “high” accordingly. 
     Next, as step S 3 , it is determined whether the gravity influence amount detected in step S 2  is classified into “low” or “high”. Next, as step S 4 , one of the first drive mode Dm 1  and the second drive mode Dm 2  is selected based on the classification of the gravity influence amount determined in step S 3  and the rotation direction (clockwise/counterclockwise) of the end effector  15 , and is set as the drive algorithm. Specifically, when the gravity influence amount is “low”, the first drive mode Dm 1  with small driving force and high minute movement accuracy is selected, and when the gravity influence amount is “high”, the second drive mode Dm 2  with large driving force is selected. 
     Next, as step S 5 , the piezoelectric actuator  21  are driven by the set drive algorithm, and the end effector  15  is moved toward the target position. Next, as step S 6 , it is determined whether the end effector  15  has reached the target position. If the determination result is “not reached”, the process returns to step S 1 , and steps S 1  to S 6  are repeated until the determination result is “reached”. Accordingly, since it is possible to switch the drive mode in real time based on the posture of the end effector  15  and the gravity influence amount which changes from moment to moment, excellent minute movement accuracy is possible. When the determination result is “reached”, as step S 7 , the drive of the piezoelectric drive device  2 F is stopped. As a result, the movement of the end effector  15  to the target position ends normally. According to such a control method, it becomes less likely to be influenced by gravity G, and it is possible to effectively suppress positional shift of the end effector  15 . 
     As described above, the robot control method  1  according to the present embodiment has, as the drive mode, the first drive mode Dm 1  that increases the separation amplitude W 2 , which is the amplitude of the longitudinal vibration, while keeping the feed amplitude W 1 , which is the amplitude of the bending vibration, constant and the second drive mode Dm 2  that increases both the feed amplitude W 1  and the separation amplitude W 2 . In the first drive mode Dm 1 , it is easy to generate the minimum necessary drive force, but the drive force is increased little by little, so that the first drive mode SL is easily influenced by gravity G at the initial stage of the driving start. In contrast to this, the second drive mode Dm 2  is hardly influenced by gravity G, but the stopping accuracy may be reduced due to excessive driving force. In this manner, by setting the first drive mode Dm 1 , which is easily influenced by gravity G but has high fine movement accuracy, and the second drive mode Dm 2 , which is hardly influenced by gravity G but has inferior fine movement accuracy, it is possible to more suitably perform selection of the drive mode according to the gravity influence amount. Therefore, it is possible to effectively suppress the positional shift of the end effector  15 . 
     According to the second embodiment as described above, the same effects as those of the first embodiment described above can be exhibited. 
     Third Embodiment 
       FIG.  19    is a diagram showing a robot according to a third embodiment. 
     The robot  1  of the present embodiment is the same as the robot  1  of the first embodiment described above except that the configuration of the robot arm RA is different. Therefore, in the following description, the present embodiment will be described with a focus on differences from the first embodiment described above, and the description of similar matters will be omitted. In the drawings of the present embodiment, the same components as those of the above described embodiment are denoted by the same reference numerals. 
     In addition to the first embodiment, in this embodiment, the robot arm RA further includes a stage ST as the second member connected to the fourth rotation section  142  as the first member, and a piezoelectric drive device  2 H as a drive device for moving the stage ST with respect to the fourth rotation section  142  in the direction of the third linear motion axis Jr 3 , which is orthogonal to the fourth rotation axis Jθ 4 . The end effector  15  is disposed on the stage ST. In this case, the piezoelectric drive device  2 H is not influenced by gravity G when the third linear motion axis Jr 3  is horizontally oriented, but is influenced by gravity G when the third linear motion axis Jr 3  is inclined with respect to the horizontal direction, particularly when the third linear motion axis Jr 3  is vertically oriented. Therefore, by applying the control method of the piezoelectric drive device  2 F described in the above described embodiments to the control of the piezoelectric drive device  2 H in the present embodiment, the drive of the piezoelectric drive device  2 H is hardly influenced by gravity G, and the positional shift of the end effector  15  by gravity G can be effectively suppressed. 
     According to the third embodiment as described above, the same effects as those of the first embodiment can be achieved. 
     Although the robot control method and the robot according to the disclosure have been described above based on the illustrated embodiments, the disclosure is not limited thereto, and the configuration of each part can be replaced with an arbitrary configuration having the same function. In addition, other arbitrary components may be added to the present disclosure. 
     In addition, in the above described embodiments, the configuration in which the piezoelectric drive device is used as the drive device has been described, but the disclosure is not limited thereto, and a drive device other than the piezoelectric drive device, for example, a drive device in which an electromagnetic motor and a decelerator are combined may be used.