Patent Publication Number: US-2023150112-A1

Title: Robot system

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This is a National Stage Entry into the United States Patent and Trademark Office from International Patent Application No. PCT/JP2021/019187, filed on May 20, 2021, which claims priority to Japanese Patent Application No. 2020-089282, filed on May 22, 2020, the entire contents of both of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates to a robot system. 
     BACKGROUND OF THE INVENTION 
     Wheeled mobile robots having manipulators mounted on wheeled platforms are known (for example, see Japanese Unexamined Patent Application, Publication No. 2006-150567). 
     Japanese Unexamined Patent Application, Publication No. 2006-150567 describes that a target change in ZMP is set on the basis of an actual ZMP determined on the basis of the behavior of a platform-mounted robot and the ZMP limit value at which the standing state of the platform-mounted robot becomes unstable. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present disclosure, there is provided a robot system including a robot and a controller that controls the robot, in which the robot includes a wheeled platform and a manipulator mounted on the wheeled platform, the manipulator includes a sensor that detects a force or a moment that acts on at least one joint, and the controller controls at least one of the manipulator and the wheeled platform on the basis of the force or moment detected by the sensor so that a moment acting on the wheeled platform does not exceed a tip-over moment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view illustrating a robot system according to one embodiment of the present disclosure. 
         FIG.  2    is a block diagram illustrating the robot system illustrated in  FIG.  1   . 
         FIG.  3    is a plan view of a wheeled mobile robot in the robot system illustrated in  FIG.  1   . 
         FIG.  4    is a graph illustrating one example of the relationship between the tip-over moment and the angle of a rotary barrel of the wheeled mobile robot in the robot system illustrated in  FIG.  1   . 
         FIG.  5    is a side view illustrating moments acting on individual parts when the angle θ 1  of the rotary barrel of the wheeled mobile robot illustrated in  FIG.  3    is 0°. 
         FIG.  6    is a front view illustrating moments acting on individual parts when the angle θ 1  of the rotary barrel of the wheeled mobile robot illustrated in  FIG.  3    is 90°. 
     
    
    
     DESCRIPTION OF EMBODIMENT(S) OF THE INVENTION 
     A robot system  1  according to one embodiment of the present disclosure will now be described with reference to the drawings. 
     As illustrated in  FIGS.  1  and  2   , the robot system  1  of this embodiment includes a wheeled mobile robot (robot)  2  and a controller  3  that controls the wheeled mobile robot  2 . The controller  3  is built into the wheeled mobile robot  2 , for example. 
     The wheeled mobile robot  2  includes an automated or hand-pushed wheeled platform  4  that can travel on a road surface, and a manipulator  5  mounted on the wheeled platform  4 . 
     The wheeled platform  4  is, for example, a four-wheeled vehicle having a top surface having the manipulator  5  mounted thereon and a stage  6  that holds workpieces and the like within the motion range of the manipulator  5 . In the drawings, reference sign  14  denotes wheels of the wheeled platform  4 . 
     The manipulator  5  includes a base  7  fixed to the upper surface of the wheeled platform  4 , and a rotary barrel  8  rotatably supported about a vertical first axis J 1  with respect to the base  7 . The manipulator  5  is also includes a first arm  9  rotatably supported about a horizontal second axis J 2  with respect to the rotary barrel  8 , a second arm  10  rotatably supported about a third axis J 3  parallel to the second axis J 2  with respect to the first arm  9 , and a three-axis wrist unit  11  at a tip of the second arm  10 . The manipulator  5  is not limited to the vertical articulated type, and may be of any type such as a horizontal articulated type or a linear motion type. 
     As illustrated in  FIG.  1   , a torque sensor (sensor)  12  that detects the moment about the second axis J 2  is installed in the first arm  9  in the vicinity of the second axis J 2 . The torque sensor  12  that detects the moment about the second axis J 2  can directly detect the moment acting on the wheeled platform  4  from the manipulator  5 . 
     A hand  13  that grips the workpiece (not illustrated) is attached to the tip of the wrist unit  11 . 
     The controller  3  includes a storage unit  16  that stores a program etc., and a control unit  17  that controls the manipulator  5  according to the program stored in the storage unit  16 . The storage unit  16  is constituted of a memory, and the control unit  17  is constituted of a processor and a memory. 
     The controller  3  receives the moment about the second axis J 2  detected by the torque sensor  12  and controls the manipulator  5  such that the detected moment does not exceed the tip-over moment Ma at each of the positions of the manipulator  5  in operation relative to the wheeled platform  4 . 
     The tip-over moment Ma is a moment that acts on the wheeled platform  4  from the manipulator  5 , and is a limit value at which the wheeled platform  4  tips over. The tip-over moment Ma is stored in the storage unit  16  in advance in accordance with the angle θ 1  of the rotary barrel  8  about the first axis J 1  with respect to the base  7 . 
     For example, as illustrated in  FIGS.  1  and  3   , when the wheeled platform  4  has four wheels  14  and the first axis J 1  lies at the center between left and right wheels and at a position closer to the front wheels  14  than the rear wheels  14 , the tip-over moment Ma changes according to the angle θ 1 , as illustrated in  FIG.  4   . 
     In addition, for example, as illustrated in  FIG.  5   , when the rotary barrel  8  of the manipulator  5  lies at the origin position (θ 1 =0°) to face the forward traveling direction of the wheeled platform  4  relative to the base  7 , the conditions for the static tip-over of the wheeled platform  4  are as expressed by the following formula (1). 
         L 1 ×W 1 &lt;L 2 ×W 2   (1)
 
     where 
     W 1  represents the mass of the wheeled platform  4 , 
     L 1  represents the distance from the center of the front wheels of the wheeled platform  4  to the center of gravity of the wheeled platform  4 , 
     W 2  represents the mass of the manipulator  5 , the hand  13 , and the workpiece, and 
     L 2  represents the distance from the center of the front wheels of the wheeled platform  4  to the center of gravity of the manipulator  5 , the hand  13 , and the workpiece in total. 
     The masses W 1  and W 2  and the distance L 1  are known values. 
     The distance L 2 , which is determined by the orientation of the manipulator  5 , can be calculated from the following formula (2) by using the moment M about the first axis J 1  detected by the torque sensor  12 . 
         M =( L 2 +L 3) ×W 2   (2)
 
     where L 3  represents the distance from the center of the front wheels of the wheeled platform  4  to the first axis J 1 , and is a known value. 
     Rearranging formula (1) by using formula (2) gives the following formulae (3) and (4): 
         L 1 ×W 1&lt;( M/W 2 −L 3) ×W 2   (3)
 
         M&gt;L 1 ×W 1 +L 3 ×W 2 =Ma (θ1)   (4)
 
     where 
     Ma(θ 1 ) represents the tip-over moment, which changes according to the angle θ 1 . 
     In other words, when the moment M on the left side of formula (4) exceeds the tip-over moment Ma( 01 ) on the right side, the wheeled platform  4  tips over. 
     The controller  3  calculates the differential moment, which is the difference between the moment M detected by the torque sensor  12  and the tip-over moment Ma, for each of the positions of the manipulator  5 . 
     Generally, the manipulator  5  is designed to operate throughout its motion range without causing tip-over of the wheeled platform  4  since the tip-over conditions expressed in formula (4) will not be satisfied as long as the manipulator  5  grips a workpiece having a rated mass or less. However, there may be cases where the rated mass of the workpiece to be handled needs to be increased even if this requires limiting the motion range of the manipulator  5 , and tip-over of the wheeled platform  4  must be considered in such cases. 
     The situations where the wheeled platform  4  tips over are as follows. 
     First, for example, when the first arm  9  and the second arm  10  are extended in the forward travelling direction of the wheeled platform  4  while the hand  13  is gripping a heavy workpiece, the conditions approach the tip-over conditions expressed in formula (4), eventually reaching a zero differential moment at some time point and thereby causing the wheeled platform  4  to tip over. In order to avoid the tip-over in this case, the controller  3  de-actuates the manipulator  5  before the tip-over conditions expressed in formula (4) are satisfied. 
     Second, when the manipulator  5  moving in a direction approaching the tip-over conditions expressed in formula (4) is decelerated to make a stop, a counterforce torque equal to the deceleration torque is added, and this rapidly increases the moment acting on the wheeled platform  4 , thereby satisfying the tip-over conditions expressed in formula (4). 
     In order to avoid the tip-over in this case, the controller  3  decelerates the motion of the manipulator  5  when the differential moment calculated at each of the positions of the manipulator  5  in operation relative to the wheeled platform  4  reaches a predetermined value. 
     The largest deceleration torque is generated in the event of an emergency stop during operation of the manipulator  5 ; thus, the predetermined value is preferably set to a value equal to or higher than the deceleration torque needed for the emergency stop. 
     The deceleration torque needed for the emergency stop differs depending on the motion speed of the manipulator  5 , and thus is calculated on the basis of the motion speed at each position. 
     When the manipulator  5  is traveling in a direction approaching the tip-over conditions expressed in formula (4), decreasing the speed at a deceleration smaller than that for the emergency stop causes the conditions to further approach the tip-over conditions; however, decreasing the speed decreases the deceleration torque needed for the emergency stop. Thus, the manipulator  5  can be operated while maintaining the state in which the tip-over conditions expressed in formula (4) are not satisfied, and even if an emergency stop command is output, the wheeled platform  4  does not tip over. 
     Third, the tip-over conditions expressed in formula (4) may be satisfied by a moment that dynamically occurs due to a counterforce generated by the manipulator  5  accelerating on any one axis. 
     For example, at the position illustrated in  FIG.  5   , even when the tip-over conditions expressed in formula (4) are not satisfied, accelerating the first arm  9  counterclockwise about the second axis J 2  may cause a clockwise counterforce torque to act on the wheeled platform  4 . As a result, the tip-over conditions expressed in formula (4) become satisfied, and the wheeled platform  4  may tip-over. In such a case, the controller  3  limits the magnitude of the counterclockwise acceleration of the first arm  9 . 
     As described above, according to the robot system  1  of this embodiment, the moment acting on the wheeled platform  4  is detected by the torque sensor  12  installed in the manipulator  5 , and the controller  3  controls the manipulator  5  to remain in a state where the detected moment does not exceed the tip-over moment Ma. This provides an advantage in that the manipulator  5  can be operated without causing the wheeled platform  4  to tip over while the wheeled platform  4  is at a stop. 
     Note that, in this embodiment, an example in which the tip-over of the wheeled platform  4  is prevented by adjusting the motion speed or the acceleration/deceleration speed without changing the operation locus of the manipulator  5  on the basis of the operation command is described. Alternatively, the tip-over of the wheeled platform  4  may be prevented by changing the operation locus of the manipulator  5  on the basis of the operation command. 
     For example, as described above, the tip-over moment Ma of the wheeled platform  4  changes according to the angle θ 1  of the rotary barrel  8  about the first axis J 1 . In the case illustrated in  FIG.  4   , the tip-over moment Ma is the smallest at an angle θ 1 =0°, and thus, the possibility of the tip-over is higher in the case illustrated in  FIG.  5    where θ 1 =0° than in the case illustrated in  FIG.  6    where θ 1 =90°. Thus, the rotary barrel  8  may be rotated to an angle θ 1  that yields a larger tip-over moment Ma when the moment detected by the torque sensor  12  at each of the positions of the operating manipulator  5  is close to the tip-over moment Ma at the angle θ 1  at that point in time. 
     In addition, for example, when the wheeled platform  4  is autonomous and when the first arm  9  is rotating clockwise in  FIG.  5   , the controller  3  may control the wheeled platform  4  to accelerate in the forward direction if deceleration of the first arm  9  causes the wheeled platform  4  to tip over forward. In this manner, the moment that acts on the wheeled platform  4  from the manipulator  5  can be reduced, and thus tip-over of the wheeled platform  4  can be prevented. 
     In addition, when the first arm  9  is rotating clockwise in  FIG.  5    and decelerating the first arm  9  would cause the wheeled platform  4  to tip over forward, the second arm  10  may be accelerated clockwise about the third axis J 3  with respect to the first arm  9 . 
     In this case also, the moment acting on the wheeled platform  4  from the manipulator  5  can be reduced by bringing the position of the center of gravity of the manipulator  5  close to the second axis J 2 , and the moment can be reduced by generating a counterforce torque in the first arm  9  by accelerating the second arm  10 . 
     When the rotary barrel  8  is rotated or the second arm  10  is rotated, this operation is not the operation of the manipulator  5  on the basis of the motion command, and thus it is necessary to ensure that no interference occurs between the wheeled mobile robot  2  and the peripheral equipment. 
     Furthermore, in this embodiment, the torque sensor  12  that detects the moment about the second axis J 2  is provided in the first arm  9 ; alternatively, the torque sensor  12  may be built into the rotary barrel  8  or the base  7 , or may be disposed between the wheeled platform  4  and the base  7 . Alternatively, a force sensor or torque sensor such as an acceleration sensor that detects the force or moment acting on at least one joint may be employed. 
     In this embodiment, the case in which the manipulator  5  is controlled to prevent tip-over of the wheeled platform  4  when the manipulator  5  is actuated while the wheeled platform  4  is at a stop is described. Alternatively, these features may be applied to cases where the manipulator  5  is actuated while the wheeled platform  4  is moving. In such a case, the moment acting on the wheeled platform  4  due to the acceleration/deceleration speed of the wheeled platform  4  may be additionally considered.