Patent Publication Number: US-2020298410-A1

Title: Robot hand controller

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims the benefit of priority from earlier Japanese Patent Application No. 2019-055637 filed Mar. 22, 2019, the description of which is incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a controller for controlling operations of a robot and a hand. 
     Related Art 
     There have been known controllers for controlling a robot arm. For example, in JP 2007-136588 A, a hand is fixed to the distal end of a robot arm via a force sensor so that a target position correction amount is generated based on the output from the force sensor, and the arm is controlled to move to a position obtained by adding the target position correction amount to a target position taught in advance. 
     In the controller disclosed in JP 2007-136588 A, the target position correction amount for the arm is generated based on the output from the force sensor. However, in order to teach an operation of the hand, it is necessary to input information such as a gripping force of the hand. For this reason, there is still a room for improvement in the controller disclosed in JP 2007-136588 A for facilitating the teaching of operations of the robot and the hand. 
     SUMMARY 
     It is thus desired to provide a robot hand controller that facilitates teaching of operations of a robot and a hand thereof. 
     According to a first exemplary embodiment for solving the above problem, a robot hand controller for controlling operations of a robot that is configured to be held with a position and a posture of an arm being changed by a user, and a hand that is mounted on the arm and configured to be held with an open/closed position being changed by a user, the controller includes: a first detecting unit that detects the position and the posture; a second detecting unit that detects the open/closed position; a third detecting unit that detects a gripping force of the hand; a reception unit that receives an operation of recording a state of the arm and the hand; a recording unit that records, as state information in time series, the position and the posture, the open/closed position, and the gripping force, which are detected by the first detecting unit, the second detecting unit, and the third detecting unit, respectively, when the operation is received by the reception unit; and a control unit that controls operations of the arm and the hand so as to reproduce the state information recorded in time series by the recording unit. 
     With this configuration, the robot can be held with the position and posture of the arm being changed by the user. Accordingly, the user can directly grasp and move the arm by direct teaching, can move the arm by operating an operation device, and can hold the position and posture of the arm. The hand is mounted on the arm, and the hand can be held with the open/closed position being changed by the user. Accordingly, the user can directly grasp and open/close the hand by direct teaching, can open/close the hand by operating an operation device, and can hold the open/closed position of the hand. 
     The first detecting unit detects a position and a posture of the arm, the second detecting unit detects an open/closed position of the hand, and the third detecting unit detects a gripping force of the hand. The reception unit receives an operation of recording a state of the arm and the hand. Accordingly, when the user performs an operation of recording a state of the arm and the hand, the operation is received by the reception unit. The recording unit records, as state information in time series, the position and the posture of the arm, the open/closed position of the hand, and the gripping force of the hand detected by the first detecting unit, the second detecting unit, and the third detecting unit, respectively, when the operation is received by the reception unit. 
     For example, the user can record the state information including the gripping force of the hand by causing the robot to perform an operation of gripping the workpiece with the hand and performing an operation of recording the state by using the reception unit. Then, the user can record the state information in time series by causing the arm to perform an operation of moving and performing an operation of recording the state. The control unit controls the operations of the arm and the hand so as to reproduce the state information recorded in time series by the recording unit. Therefore, even a user who does not have special skills can easily teach and reproduce the operations of the robot and the hand without the need of inputting the gripping force or the like of the hand. 
     In a gripping operation by which the workpiece is gripped by the hand, it is necessary to control the open/closed position of the hand and to grip the workpiece with an appropriate force. Whether or not the state information is the gripping operation being performed by the hand can be determined according to whether or not the gripping force is larger than a first force, which is a force for determining the gripping operation being performed by the hand. The first force is a force for determining that the hand is gripping the workpiece when the detected gripping force is larger than the first force. 
     According to a second exemplary embodiment, when the gripping force of the state information is larger than a first force, which is a force for determining a gripping operation being performed by the hand, the control unit reproduces the state information with priority to the gripping force over the open/closed position. Accordingly, in the case where the state information is the gripping operation of the hand, the hand can reliably grip the workpiece with an appropriate force even if the relationship between the workpiece and the open/closed position at the time of reproduction is deviated from the recorded state information. 
     Even in the case where the hand grips the workpiece, it is not necessary to adjust the gripping force before the hand comes into contact with the workpiece. 
     According to a third exemplary embodiment, when the gripping force of the state information is larger than a first force, which is a force for determining a gripping operation being performed by the hand, the control unit operates the hand at a highest speed to a position before the open/closed position by a predetermined distance in reproduction of the state information. Accordingly, even in the case where the hand grips the workpiece, the hand can be operated at a highest speed to a position before the open/closed position by a predetermined distance, the open/closed position being a position at which gripping operation ends, and thus the operation time can be reduced. 
     Even in the case where the open/closed position of the hand is controlled, it is not necessary to adjust the gripping force in a non-gripping operation (movement operation of the hand), by which the workpiece is not gripped by the hand. Whether or not the state information is the non-gripping operation being performed by the hand can be determined according to whether or not the gripping force is smaller than a second force, which is a force for determining the non-gripping operation being performed by the hand. The second force is a force for determining that the hand is not gripping the workpiece when the detected gripping force is smaller than the second force. For example, the second force is a force smaller than the first force, and may be a force slightly larger than 0. 
     According to a fourth exemplary embodiment, when the gripping force of the state information is smaller than a second force, which is a force for determining a non-gripping operation being performed by the hand, the control unit reproduces the open/closed position at a highest speed in reproduction of the state information. Accordingly, when the state information is the non-gripping operation of the hand, the open/closed position can be reproduced at the highest speed, and thus the operation time can be reduced. Further, in teaching of the operation of the hand, there is no need of setting the opening/closing speed of the hand. 
     An appropriate operation of the hand between two consecutive pieces of state information varies depending on the gripping forces of the two consecutive pieces of state information. For example, when first state information is a gripping operation and second state information following the first state information is also a gripping operation, it can be determined that the gripping operation should be maintained between the first state information and the second state information as well. 
     According to a fifth exemplary embodiment, the control unit controls an operation of the hand between two consecutive pieces of the state information based on the gripping forces of the two consecutive pieces of the state information. Accordingly, the operation of the hand between two consecutive pieces of state information can be appropriately controlled based on the gripping forces of the two consecutive pieces of state information. 
     According to a sixth exemplary embodiment, specifically, when the gripping forces of two consecutive pieces of the state information are larger than a first force, which is a force for determining a gripping operation being performed by the hand, the control unit may maintain a gripping operation as an operation of the hand between the two consecutive pieces of the state information. 
     According to a seventh exemplary embodiment, specifically, when the gripping forces of two consecutive pieces of the state information are smaller than a second force, which is a force for determining a non-gripping operation being performed by the hand, the control unit may maintain a non-gripping operation as an operation of the hand between the two consecutive pieces of the state information. 
     There may be a case where first state information is taught that the arm is separated from the workpiece and the hand is not gripping the workpiece, and second state information following the first state information is taught that the arm has moved to the workpiece and the hand is gripping the workpiece. In this case, if the hand which has moved from the position of the arm in the first state information closes before reaching the position of the arm in the second state information, the hand cannot grip the workpiece. 
     According to an eighth exemplary embodiment, when the gripping force in first state information is smaller than a second force, which is a force for determining a non-gripping operation being performed by the hand, and the gripping force in second state information following the first state information is larger than a first force, which is a force for determining a gripping operation being performed by the hand, the control unit reproduces the open/closed position and the gripping force after reproducing the position and the posture in reproduction of the second state information. Accordingly, even if a state in which the arm has moved to the workpiece and the hand is gripping the workpiece is taught after a state in which the arm is separated from the workpiece and the hand is not gripping the workpiece, the hand can grip the workpiece. Therefore, the number of pieces of state information (operation state) that are taught can be reduced, and thus teaching of operations of the robot and the hand can be further easily performed. 
     There may be a case where third state information is taught that the arm has moved to the workpiece and the hand is gripping the workpiece, and fourth state information following the third state information is taught that the arm has moved from the position in the third state information and the hand is not gripping the workpiece. In this case, if the hand which has moved from the position of the arm in the third state information opens before reaching the position of the arm in the fourth state information, the hand drops the workpiece and fails to transfer the workpiece to the position in the fourth state information. 
     According to a ninth exemplary embodiment, when the gripping force of third state information is larger than a first force, which is a force for determining a gripping operation being performed by the hand, and the gripping force of fourth state information following the third state information is smaller than a second force, which is a force for determining a non-gripping operation being performed by the hand, the control unit reproduces the open/closed position and the gripping force after reproducing the position and the posture in reproduction of the fourth state information. Accordingly, even if a state in which the workpiece has been moved and the hand has released the workpiece is taught after a state in which the hand is gripping the workpiece, the hand can be moved without dropping the workpiece. Therefore, the number of pieces of state information that are taught can be reduced, and thus teaching of operations of the robot and the hand can be further easily performed. 
     There may be a case where the hand fails to grip the workpiece depending on a position and an angle of the workpiece. In this case, even if the open/closed position of the hand in the state information is reproduced, the gripping force of the detected hand becomes smaller than the gripping force of the state information. 
     According to a tenth exemplary embodiment, when the gripping force detected by the third detecting unit in reproduction of the open/closed position of the state information is smaller than the gripping force of the state information by more than a predetermined degree, the control unit determines that the hand has failed to grip a workpiece. Accordingly, it can be determined that the hand has failed to grip the workpiece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a schematic view illustrating a robot and a hand. 
         FIG. 2  is a flowchart showing a procedure for teaching operations. 
         FIG. 3  is a schematic view illustrating a mode of moving to a pick position. 
         FIG. 4  is a schematic view illustrating a mode of gripping a workpiece at a pick position. 
         FIG. 5  is a schematic view illustrating a mode of moving to a place position. 
         FIG. 6  is a schematic view illustrating a mode of releasing a workpiece at a place position. 
         FIG. 7  is a flowchart showing a procedure for reproducing the taught operations. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the drawings, an embodiment implemented as a controller for an articulated robot and a hand will be described. 
     As shown in  FIG. 1 , a robot  10  is provided as an industrial robot which is, for example, a 6-axis vertical articulated robot, and includes a base  11  and an arm  10 A. Adjacent links  13  and  15  (partially shown) of the arm  10 A are joined by joints  12 ,  14 , and  16  (partially shown) in sequence in a relatively rotatable manner. The joints  12 ,  14 , and  16  are driven by motors (not shown) corresponding to the respective joint  12 ,  14 , and  16 . 
     A hand  20  is mounted on a distal end of the arm  10 A. The hand  20  has a pair of claws  20 A and  20 B, and performs open and close operations for widening and narrowing the interval between the claws  20 A and  20 B. The claws  20 A and  20 B are driven by a motor (not shown). 
     The base  11  includes therein a recording unit  35  for recording the states of the robot  10  and the hand  20 , and a control unit  36  for controlling the operation of the robot  10  and the hand  20 . The control unit  36  is configured as a computer including components such as a CPU, a ROM, a RAM, and an I/O interface. 
     Specifically, by way of example, the control unit  36  is provided with a computer system CP with a CPU (central processing unit)  36 A, a ROM (read-only memory)  36 B, a RAM (random access memory)  36 C and an input/output interface  36 D which are interconnected mutually communicably via a bus  35 E. Various sensed signals are supplied to the CPU  36 A through the input/output interface  36 D and various commands and drive signals are outputted from the CPU  36 A to the drivers  37  and not-shown other devices via the input/output interface  36 D. The drivers  37  are provided to drive the motors arranged at the respective joints. The CPU  36 A executes various robot control process by reading programs stored in advance in the ROM  36 B and temporarily store the read programs in a specified work area for execution thereof. The CPU  36 A is thus able to function as control means, while the ROM  36 B functions as a non-transitory computer-readable recording medium in which source codes of steps of the preset control programs are previously stored. The RAM  36 C is able to temporarily store data generated or needed during the execution of the programs by the CPU  36 A. The foregoing computer configuration is just an example, and other configurations can be adopted as long as the preset computer programs can be performed. For instance, two or more CPUs can be used for decentralized control or redundant system. In addition, the 
     CPU  36 A is provided as an essential calculation component of the computer system, so that different types of calculation components can be used provided that such calculation components have equivalent calculational functions to the CPU  36 A. The CPU  36 A can be replaced by components called a MPU, a processor, or a calculator. 
     The link  15  of the robot  10  is provided with an open button  17  and a close button  18  for the hand  20 , and a position recording button  19 . The control unit  36  (i.e., the CPU  36 A) performs an operation of opening the hand  20  at a first speed while the user presses the open button  17 . The control unit  36  (i.e., the CPU  36 A) performs an operation of closing the hand  20  at a second speed while the user presses the close button  18 . The second speed may be the same as the first speed, or may be lower than the first speed. The control unit  36  (i.e., the CPU  36 A) holds the hand  20  at an open/closed position at the time when the user stops pressing the open button  17  or the close button  18 . That is, the hand  20  can be held with the open/closed position being changed by the user. The position recording button  19  will be described later. 
     For example, the joints  12 ,  14 , and  16  of the robot  10  are provided with encoders  31 ,  32 , and  33  for detecting a rotation angle of the joints  12 ,  14 , and  16 , respectively. That is, the encoders  31 ,  32 , and  33  (first detecting unit) detect the position and posture of a control point of the arm  10 A (hereinafter, referred to as a “position and posture of the arm  10 A”). The control point is set at the center of the distal end of the arm  10 A. 
     The hand  20  is provided with an encoder  22  (second detecting unit) for detecting an open/closed position of the hand  20 . The hand  20  is further provided with a force sensor  23  for detecting a gripping force of the hand  20 . The force sensor  23  (third detecting unit) may be a current sensor that detects a current flowing in a motor that drives the claws  20 A and  20 B, or a pressure sensor that detects a pressure acting on the claws  20 A and  20 B. 
     The control unit  36  (Le., the CPU  36 A) controls the position and posture of the arm  10 A according to an external force acting on the arm  10 A. The control unit  36  holds the position and posture of the arm  10 A at the time when the external force stops acting on the arm  10 A. That is, the robot  10  can be held with the position and posture of the arm  10 A being changed by the user. In the present embodiment, the user can directly grasp and move the arm  10 A by direct teaching, and can hold the position and posture of the arm  10 A. 
     When the user presses the position recording button  19 , the control unit  36  (Le., the CPU  36 A) causes the recording unit  35  to record the position and posture of the arm  10 A, the open/closed position, gripping force, and gripping state of the hand  20  as the state information based on the detection results from the encoders  31 ,  32 , and  33 , the encoder  22 , and the force sensor  23 . The control unit  36  causes the recording unit  35  to record the state information in time series each time the user presses the position recording button  19 . That is, the position recording button  19  (reception unit) receives an operation of recording the states of the arm  10 A and the hand  20 . When the position recording button  19  receives the operation, the recording unit  35  records the position and posture of the arm  10 A, the open/closed position, gripping force, and gripping state of the hand  20 , which are each detected by the encoders  31 ,  32 , and  33 , the encoder  22 , and the force sensor  23 , as the state information in time series. Accordingly, the user can teach the operations of the arm  10 A and the hand  20  by repeating the movement of the arm  10 A, the opening and closing of the hand  20 , and the position recording. 
     The control unit  36  (i.e., the CPU  36 A) determines the gripping state of the hand  20  based on the gripping force detected by the force sensor  23 . Specifically, when the gripping force detected by the force sensor  23  is larger than a first force, which can be used to determine a gripping operation (gripping state) performed by the hand  20 , it is determined that a gripping operation is performed. The first force is a force for determining that the hand  20  is gripping the workpiece W when the gripping force detected by the force sensor  23  is larger than the first force. On the other hand, when the gripping force detected by the force sensor  23  is smaller than a second force, which can be used to determine a non-gripping operation (non-gripping state) performed by the hand  20 , it is determined that a non-gripping operation is performed. The second force is a force for determining that the hand  20  is not gripping the workpiece W when the gripping force detected by the force sensor  23  is smaller than the second force. For example, the second force is a force smaller than the first force, and may be a force slightly larger than 0. 
     The control unit  36  controls the operations of the arm  10 A and the hand  20  so as to reproduce the state information recorded in time series by the recording unit  35 . Accordingly, the user can cause the control unit  36  to reproduce the taught operations of the arm  10 A and the hand  20 . Thus, the user can perform a task on the workpiece W by using the robot  10 . The robot and hand controller is composed of the encoders  31 ,  32 , and  33  (functionally serving as a first detecting unit), the encoder  22  (functionally serving as a second detecting unit), the force sensor  23  (functionally serving as third detecting unit), the position recording button  19  (functionally serving as a reception unit), the recording unit  35 , and the control unit  36 . 
       FIG. 2  is a flowchart showing a procedure for teaching operations. This series of operations are performed by the user. With reference to  FIGS. 3 to 6 , a procedure for teaching operations of transferring the workpiece W from a work table S 1  to a work table S 2  and releasing the workpiece W on the work table S 2  will be described. 
     First, the arm  10 A is moved to a pick position (position at which the workpiece W is picked) (step S 10 ), Specifically, as shown in  FIG. 3 , the user directly grasps the arm  10 A and moves it to the pick position by direct teaching. When the hand  20  is closed and narrower than the width of the workpiece W, the user presses the open button  17  to open the hand  20  wider than the width of the workpiece W. 
     Subsequently, the position recording button  19  is pressed (step S 11 ). When the position recording button  19  is pressed by the user, the control unit  36  causes the recording unit  35  to record the position and posture of the arm  10 A, the open/closed position, gripping force, and gripping state of the hand  20 , which are each detected by the encoders  31 ,  32 , and  33 , the encoder  22 , and the force sensor  23 , as the state information. In  FIG. 3 , a pick position and a downward posture are recorded as the position and the posture of the arm  10 A, an open/closed position opened wider than the width of the workpiece W is recorded as the open/closed position of the hand  20 , 0 is recorded as the gripping force, and a non-gripping operation is recorded as the gripping state. 
     Subsequently, the workpiece W is gripped by the hand  20  (step S 12 ). Specifically, as shown in  FIG. 4 , the user keeps pressing the close button  18  until the hand  20  grips the workpiece W, and stops pressing the close button  18  when an appropriate gripping force is applied to the workpiece W by the hand  20 . Whether the hand  20  grips the workpiece W with an appropriate force or not may be determined by the user observing the states of the hand  20  and the workpiece W, or by the user observing the displayed gripping force, which is detected by the force sensor  23 . 
     Subsequently, the position recording button  19  is pressed (step 
     S 13 ). When the position recording button  19  is pressed by the user, the control unit  36  causes the recording unit  35  to record the state information at the time when the position recording button  19  is pressed, following the previous state information. In  FIG. 4 , a pick position and a downward posture are recorded as the position and the posture of the arm  10 A, an open/closed position corresponding to the width of the workpiece W is recorded as the open/closed position of the hand  20 , a gripping force with which the workpiece W is gripped is recorded as the gripping force, and a gripping operation is recorded as the gripping state. 
     Subsequently, the arm  10 A is transferred to a place position (position at which the workpiece W is placed) (step S 14 ). Specifically, as shown in  FIG. 5 , the user directly grasps the arm  10 A and moves it to the place position by direct teaching. In so doing, the hand  20  maintains the state of gripping the workpiece W (gripping operation). 
     Subsequently, the position recording button  19  is pressed (step S 15 ). When the position recording button  19  is pressed by the user, the control unit  36  causes the recording unit  35  to record the state information at the time when the position recording button  19  is pressed, following the previous state information. In  FIG. 5 , a place position and a downward posture are recorded as the position and the posture of the arm  10 A, an open/closed position corresponding to the width of the workpiece W is recorded as the open/closed position of the hand  20 , a gripping force with which the workpiece W is gripped is recorded as the gripping force, and a gripping operation is recorded as the gripping state. 
     Subsequently, the workpiece W is released from the hand  20  (step S 16 ). Specifically, as shown in  FIG. 6 , the user keeps pressing the open button  17  until the hand  20  releases the workpiece W, and stops pressing the open button  17  when the hand  20  is opened to an appropriate width. 
     Subsequently, the position recording button  19  is pressed (step S 17 ). When the position recording button  19  is pressed by the user, the control unit  36  causes the recording unit  35  to record the state information at the time when the position recording button  19  is pressed, following the previous state information. In  FIG. 6 , a place position and a downward posture are recorded as the position and the posture of the arm  10 A, an open/closed position opened wider than the width of the workpiece W is recorded as the open/closed position of the hand  20 , 0 is recorded as the gripping force, and a non-gripping operation is recorded as the gripping state. Then, this series of operations are completed (END). 
       FIG. 7  is a flowchart showing a procedure for reproducing the taught operations. This series of operations are performed by the control unit  36 . With reference to  FIGS. 3 to 6 , a procedure for reproducing operations of transferring the workpiece W from the work table S 1  to the work table S 2  and releasing the workpiece W on the work table S 2  will be described. 
     First, the arm  10 A is moved to the recorded pick position, and the hand  20  is opened to the recorded open/closed position by a movement operation (step S 20 ). Specifically, as shown in  FIG. 3 , when the gripping state of the state information is a non-gripping operation, that is, when the gripping force of the state information is smaller than the second force, the open/closed position is reproduced at the highest speed in reproduction of the state information. 
     Specifically, the arm  10 A is on standby at the recorded pick position, and the hand  20  is closed to the recorded open/closed position until the recorded gripping force is reproduced by a gripping operation (step S 21 ). Specifically, as shown in  FIG. 4 , when the gripping state of the state information is a gripping operation, that is, when the gripping force of the state information is larger than the first force, the state information is reproduced with priority to the gripping force over the open/closed position. Specifically, when the gripping force detected by the force sensor  23  (hereinafter, referred to as a “detected gripping force”) becomes equal to the gripping force recorded as the state information (hereinafter, referred to as a “recorded gripping force”) before the open/closed position detected by the encoder  22  (hereinafter, referred to as a “detected open/closed position”) becomes equal to the open/closed position recorded as the state information (hereinafter, referred to as a “recorded open/closed position”), an operation of closing the hand  20  is stopped. 
     Further, when the detected open/closed position becomes equal to the recorded open/closed position but the detected gripping force does not become equal to the recorded gripping force, an operation of closing the hand  20  is continued until the detected gripping force becomes equal to the recorded gripping force. However, when the detected gripping force in reproduction of the recorded open/closed position is smaller than the recorded gripping force by more than a predetermined degree, it is determined that the hand  20  has failed to grip the workpiece W. Examples of the case where the detected gripping force is smaller than the recorded gripping force by more than a predetermined degree include the case where the detected gripping force is less than half of the recorded gripping force, or the case where the detected gripping force is 0. 
     Specifically, the arm  10 A is moved to the recorded place position, and the gripping operation of the hand  20  is maintained until the recorded open/closed position and the gripping force are reproduced (step S 22 ). Specifically, as shown in  FIGS. 4 and 5 , when the gripping states of two consecutive pieces of state information are gripping operations, that is, when the gripping forces of two consecutive pieces of state information are larger than the first force, a gripping operation is maintained as the operation of the hand  20  between the two consecutive pieces of state information. In short, the operation of the hand  20  between two consecutive pieces of state information is controlled based on the gripping forces of the two consecutive pieces of state information. 
     Subsequently, the arm  10 A is on standby at the recorded place position, and the hand  20  is opened to the recorded open/closed position by a movement operation (step S 23 ). Specifically, as shown in  FIG. 6 , when the gripping state of the state information is a non-gripping operation, that is, when the gripping force of the state information is smaller than the second force, the open/closed position is reproduced at the highest speed in reproduction of the state information. Then, this series of operations are completed (END). 
     The present embodiment detailed above has the following advantages. 
     When the position recording button  19  receives a recording operation, the recording unit  35  records the position and posture of the arm  10 A, the open/closed position and gripping force of the hand  20 , which are each detected by the encoders  31 ,  32 , and  33 , the encoder  22 , and the force sensor  23 , as the state information in time series. Accordingly, the user can record the state information including the gripping force of the hand  20  by causing the robot  10  to perform an operation of gripping the workpiece W with the hand  20  and performing an operation of recording the state by pressing the position recording button  19 . 
     Then, the user can record the state information in time series by causing the arm  10 A to perform an operation of moving and performing an operation of recording the state. The control unit  36  controls the operations of the arm  10 A and the hand  20  so as to reproduce the state information recorded in time series by the recording unit  35 . Therefore, even a user who does not have special skills can easily teach and reproduce the operations of the robot  10  and the hand  20  without the need of inputting the gripping force or the like of the hand  20 . 
     In a gripping operation by which the workpiece W is gripped by the hand  20 , it is necessary to control the open/closed position of the hand  20  and to grip the workpiece W with an appropriate force. In this regard, when the gripping force of the state information is larger than the first force, which can be used to determine the gripping operation being performed by the hand  20 , the control unit  36  reproduces the state information with priority to the gripping force over the open/closed position. Accordingly, in the case where the state information is the gripping operation of the hand  20 , the hand  20  can reliably grip the workpiece W with an appropriate force even if the relationship between the workpiece W and the open/closed position at the time of reproduction is deviated from the recorded state information. 
     Even in the case where the open/closed position of the hand  20  is controlled, it is not necessary to adjust the gripping force in a non-gripping operation (movement operation of the hand  20 ), by which the workpiece W is not gripped by the hand  20 . In this regard, when the gripping force of the state information is smaller than the second force, which can be used to determine the non-gripping operation being performed by the hand  20 , the control unit  36  reproduces the open/closed position at the highest speed in reproduction of the state information. Accordingly, when the state information is the non-gripping operation of the hand  20 , the open/closed position can be reproduced at the highest speed, and thus the operation time can be reduced. Further, in teaching of the operation of the hand  20 , there is no need of setting the opening/closing speed of the hand  20 . 
     An appropriate operation of the hand  20  between two consecutive pieces of state information varies depending on the gripping forces of the two consecutive pieces of state information. For example, when first state information is a gripping operation and second state information following the first state information is also a gripping operation, it can be determined that the gripping operation should be maintained between the first state information and the second state information as well. In this regard, the control unit  36  controls the operation of the hand  20  between two consecutive pieces of state information based on the gripping forces of the two consecutive pieces of state information. Specifically, when the gripping forces of two consecutive pieces of state information are larger than the first force, the control unit  36  maintains a gripping operation as the operation of the hand  20  between the two consecutive pieces of state information. Accordingly, the operation of the hand  20  between two consecutive pieces of state information can be appropriately controlled based on the gripping forces of the two consecutive pieces of state information. 
     There may be a case where the hand  20  fails to grip the workpiece W depending on a position and an angle of the workpiece W. In this case, even if the open/closed position of the hand  20  in the state information is reproduced, the gripping force of the detected hand  20  becomes smaller than the gripping force of the state information. In this regard, when the gripping force detected by the force sensor  23  in reproduction of the open/closed position of the state information is smaller than the gripping force of the state information by more than a predetermined degree, the control unit  36  determines that the hand  20  has failed to grip the workpiece W. Accordingly, it can be determined that the hand  20  has failed to grip the workpiece W. 
     The aforementioned embodiment can be implemented with the following modification. The same components as those of the above embodiment are denoted by the same reference signs, and the description thereof will be omitted. 
     The process of determining that the hand  20  has failed to grip the workpiece W can be omitted. 
     The control unit  36  may perform the following control as another example of controlling an operation of the hand  20  between two consecutive pieces of state information based on the gripping forces of the two consecutive pieces of state information. That is, when the gripping forces of two consecutive pieces of state information are smaller than the second force, which is a force for determining a non-gripping operation being performed by the hand  20 , the control unit  36  may maintain a non-gripping operation as the operation of the hand  20  between the two consecutive pieces of state information. 
     A third speed, which is a speed of the hand  20  in a gripping operation, may be set to be lower than the first speed in the operation by the open button  17  and the second speed in the operation by the close button  18 , or may be set to be equal to the first speed and the second speed. 
     When the gripping force of the state information is smaller than the second force, the hand  20  may also be operated at a speed slightly lower than the highest speed, a speed higher than the first speed, a speed higher than the second speed, or a speed higher than the third speed in reproduction of the state information. 
     Even in the case where the hand  20  grips the workpiece W, it is not necessary to adjust the gripping force before the hand  20  comes into contact with the workpiece W. Therefore, when the gripping force of the state information is larger than the first force, the control unit  36  may operate the hand at a highest speed to a position before the open/closed position by a predetermined distance in reproduction of the state information. According to this configuration, even in the case where the hand  20  grips the workpiece W, the hand  20  can be operated at a highest speed to a position before the open/closed position by a predetermined distance, the open/closed position being a position at which gripping operation ends, and thus the operation time can be reduced. In addition, the hand  20  may also be operated at a speed slightly lower than the highest speed, a speed higher than the first speed, a speed higher than the second speed, or a speed higher than the third speed to a position before the open/closed position by a predetermined distance, the open/closed position being a position at which gripping operation ends. 
     There may be a case where first state information is taught that the arm  10 A is separated from the workpiece W and the hand  20  is not gripping the workpiece W, and second state information following the first state information is taught that the arm  10 A has moved to the workpiece W and the hand  20  is gripping the workpiece W. In this case, if the hand  20  which has moved from the position of the arm  10 A in the first state information closes before reaching the position of the arm  10 A in the second state information, the hand  20  cannot grip the workpiece W. 
     Therefore, when the gripping force in first state information is smaller than the second force, and the gripping force in second state information following the first state information is larger than the first force, the control unit  36  reproduces the open/closed position and the gripping force after reproducing the position and the posture in reproduction of the second state information. According to this configuration, even if a state in which the arm  10 A has moved to the workpiece W and the hand  20  is gripping the workpiece W is taught after a state in which the arm  10 A is separated from the workpiece W and the hand  20  is not gripping the workpiece W is taught, the hand  20  can grip the workpiece W. Therefore, the number of pieces of state information (operation state) that are taught can be reduced, and thus teaching of operations of the robot  10  and the hand  20  can be further easily performed. 
     There may be a case where third state information is taught that the arm  10 A has moved to the workpiece W and the hand  20  is gripping the workpiece W, and fourth state information following the third state information is taught that the arm  10 A has moved from the position in the third state information and the hand  20  is not gripping the workpiece W. In this case, if the hand  20  which has moved from the position of the arm  10 A in the third state information opens before reaching the position of the arm  10 A in the fourth state information, the hand  20  drops the workpiece W and fails to transfer the workpiece W to the position in the fourth state information. 
     Therefore, when the gripping force in third state information is larger than the first force, and the gripping force in fourth state information following the third state information is smaller than the second force, the control unit  36  reproduces the open/closed position and the gripping force after reproducing the position and the posture in reproduction of the fourth state information. According to this configuration, even if a state in which the workpiece W has been moved and the hand  20  has released the workpiece W is taught after a state in which the hand  20  is gripping the workpiece W, the hand can be moved without dropping the workpiece W. Therefore, the number of pieces of state information that are taught can be reduced, and thus teaching of operations of the robot  10  and the hand  20  can be further easily performed. 
     The gripping operation is not limited to an operation of gripping a workpiece W from the outside thereof, and may also be an operation of gripping the workpiece W by inserting and expanding a pair of claws  20 A and  20 B in a cavity of the workpiece W. 
     The robot  10  may not be necessarily configured to move the arm  10 A by direct teaching, and the arm  10 A may also be moved by operating a device such as a teaching pendant (operation device). 
     In this case, the teaching pendant may be provided with the open button  17 , the close button  18 , and the position recording button  19 . 
     The robot  10  is not limited to a vertical articulated robot, and may also be a horizontal or other types of articulated robot.