Robot and method of controlling the same

This Nonprovisional application claims priority under 35 U.S.C. § 119 on Patent Application No. 2022-033903 filed in Japan on Mar. 4, 2022, the entire contents of which are hereby incorporated by reference.

The present invention relates to a robot including a robot arm and a robot hand. The present invention also relates to a method of controlling such a robot.

Conventionally, direct teaching has been used as one of teaching methods of teaching motions to robots. The direct teaching is a method of teaching motions to robots by having an operator who performs the teaching directly move a robot hand or a robot arm thereof. One document that discloses the direct teaching is, for example, Patent Literature 1.

Patent Literature

Patent Literature 1

Japanese Patent Application Publication, Tokukai, No. 2019-018340

Technical Problem

A robot for work disclosed in Patent Literature 1 performs impedance control (motion input and force output-type control) of a robot arm in the direct teaching. Thus, the operator is required to apply a relatively greater force when directly moving a robot arm or a robot hand. Further, because of the influence of the inertia of rinks, it is difficult for the operator to move the robot arm satisfactory. This makes it difficult to directly teach precise motions to robots.

An aspect of the present invention has been made in view of such circumstances, and an object thereof is to provide a robot capable of being subjected to direct teaching of a more precise motion with a smaller force.

Solution to Problem

A robot in accordance with an aspect of the present invention is a robot including a robot arm, a robot hand attached to the robot arm, a force sensor configured to detect a force acting on the robot hand, and a control device. The control device is a device that performs admittance control (force input and motion output-type control) to determine a position of the robot hand in accordance with the force detected by the force sensor, provides, to the robot arm, an instruction to move the robot hand to the determined position, and records, as teaching data, the instruction provided to the robot arm.

Further, a method of controlling a robot in accordance with an aspect of the present invention is a method of controlling a robot including a robot arm, a robot hand attached to the robot arm, and a force sensor configured to detect a force acting on the robot hand, and the method includes a controlling step. The controlling step is a step of: performing admittance control (force input and motion output-type control) to determine a position of the robot hand in accordance with the force detected by the force sensor; providing, to the robot arm, an instruction to move the robot hand to the determined position; and recording, as teaching data, the instruction provided to the robot arm.

Advantageous Effects of Invention

According to an aspect of the present invention, it is possible to provide a robot capable of being subjected to direct teaching of a more precise motion with a smaller force.

FIG. 1 is a schematic view illustrating a configuration of a robot in accordance with an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a function of a control device included in the robot illustrated in FIG. 1 .

FIG. 3 is graphs illustrating temporal changes in force detected by a force sensor, manipulated variable inputted into a manipulating device, position of a robot hand, and force sensitivity, in a case in which an admittance control section has no force sensitivity reduction function.

FIG. 4 is graphs illustrating temporal changes in force detected by a force sensor, manipulated variable inputted into a manipulating device, position of a robot hand, and force sensitivity, in a case in which an admittance control section has a force sensitivity reduction function.

A robot 1 in accordance with an embodiment of the present invention will be described with reference to FIG. 1 . FIG. 1 is a schematic view illustrating a configuration of the robot 1 in accordance with the present embodiment.

The robot 1 is a robot that is capable of being subjected to direct teaching of any motion. According to the present embodiment, a motion directly taught to the robot 1 is assumed to be a press-fitting motion for press-fitting a second member W 2 (e.g., metal pin) into an opening or a recess of a first member W 1 (e.g., metal collar). However, the motion directly taught to the robot 1 is not particularly limited thereto, and may be any motion.

As illustrated in FIG. 1 , the robot 1 includes a robot hand 11 , a robot arm 12 , a force sensor 13 , and a manipulating device 14 , and a control device 15 .

The robot hand 11 is a mechanism for grasping a workpiece (the second member W 2 , in the present embodiment). According to the present embodiment, a gripper is used as the robot hand 11 . The robot arm 12 is a mechanism for moving the robot hand 11 . According to the present embodiment, a vertical articulated arm is used as the robot arm 12 .

The robot hand 11 is attached to the tip of the robot arm 12 via the force sensor 13 . The force sensor 13 is a sensor configured to detect forces in three directions (x-, y-, and z-axis directions) acting on the robot hand 11 , in particular, forces in the three directions applied by the user to move the robot hand 11 in the direct teaching. According to the present embodiment, a six-axis force sensor is used as the force sensor 13 . However, any sensor that is capable of detecting forces in the three directions may be used as the force sensor 13 .

Further, the manipulating device 14 is attached to or near the tip of the robot arm 12 . The manipulating device 14 is a device that receives an operation that specifies a velocity in one direction (z-axis direction) of the robot hand 11 , that is, a velocity in one direction along which the user intends to move the robot hand 11 in the direct teaching. According to the present embodiment, a joystick is used as the manipulating device 14 . However, any device that is capable of receiving an operation specifying a velocity in one direction may be used as the manipulating device 14 .

The control device 15 determines a position of the robot hand 11 in accordance with the forces detected by the force sensor 13 and a manipulated variable inputted into the manipulating device 14 (the inclination of the joystick, in the present embodiment), and the control device 15 provides, to the robot arm 12 , an instruction to move the robot hand 11 to the determined position. Then, the control device 15 records the instruction provided to the robot arm as teaching data. By doing this, teaching is performed. According to the present embodiment, a personal computer (PC) is used as the control device 15 . However, any device that is capable of providing below-mentioned function of the control device 15 may be used as the control device 15 .

The control device 15 operates in either a first operation mode or a second operation mode. Here, the first operation mode is an operation mode in which admittance control (force input and motion output-type control) is performed to determine positions x, y, and z in the three directions of the robot hand 11 in accordance with forces F x , F y , and F z in the three directions detected by the force sensor 13 . The second operation mode is a mode in which (i) admittance control is performed to determine positions x and y in two directions of the robot hand 11 in accordance with forces F x and F y in the two directions detected by the force sensor 13 , and (ii) velocity control is performed to determine position z in one direction of the robot hand 11 in accordance with manipulated variable θ inputted into the manipulating device 14 . The following will describe a function of the control device 15 .

The function of the control device 15 included in the robot 1 will be described with reference to FIG. 2 . FIG. 2 is a functional block diagram illustrating the function of the control device 15 .

The control device 15 includes an admittance control section 151 , a multiplying section 152 , an integrating section 153 , and a mode switch 154 . To the control device 15 , inputted are forces F x , F y , and F z in the three directions detected by the force sensor 13 and manipulated variable θ inputted into the manipulating device 14 (e.g., the inclination of the joystick). Each of forces F x , F y , and F z may be raw data of forces as detected by the force sensor 13 , or alternatively, may be data obtained by shaping, in the dead zone, forces detected by the force sensor 13 .

The admittance control section 151 performs, in the first operation mode, admittance control to determine positions x, y, and z in the three directions of the robot hand 11 in accordance with forces F x , F y , and F z in the three directions detected by the force sensor 13 . Specifically, the admittance control section 151 determines positions x, y, and z in the three directions of the robot hand 11 by solving (integrating) the following equation of motion (1).

Herein, x′ is first-order derivative dx/dt (velocity) of position x, and x″ is second-order derivative d 2 x/dt 2 (acceleration) of position x. Further, y′ is first-order derivative dy/dt (velocity) of position y, and y″ is second-order derivative d 2 x/dt 2 (acceleration) of position y. Further, z′ is first-order derivative dz/dt (velocity) of position z, and z″ is second-order derivative d 2 x/dt 2 (acceleration) of position z. Each of m x , m y , and m x is a constant representing a virtual mass (e.g., 10 kg). Each of c x , c y , and c x is a constant representing a virtual coefficient of viscosity (e.g., 100 kg/s). Each of α x , α y , and α z is a constant indicating force sensitivity (e.g., 1). Position x may be calculated by solving an equation of motion other than that mentioned above, and such an equation may be, for example, x′=α x F x . The same applies to position y and position z.

Further, the admittance control section 151 performs, in the second operation mode, admittance control to determine positions x and y in two directions of the robot hand 11 in accordance with the forces F x and F y in the two directions detected by the force sensor 13 . Specifically, the admittance control section 151 determines positions x and y in the two directions of the robot hand 11 by solving (integrating) the following equation of motion (2a).

Herein, x′ is first-order derivative dx/dt (velocity) of position x, and x″ is second-order derivative d 2 x/dt 2 (acceleration) of position x. Further, y′ is first-order derivative dy/dt (velocity) of position y, and y″ is second-order derivative d 2 y/dt 2 (acceleration) of position y. Further, z′ is first-order derivative dz/dt (velocity) of position z. Each of m x and m y is a constant representing a virtual mass (e.g., 10 kg). Each of c x and c y is a constant representing a virtual coefficient of viscosity (e.g., 200 kg/s). Each of α x and α y is a constant indicating force sensitivity (e.g., 1). Position x may be calculated by solving an equation of motion other than that mentioned above, and such an equation may be, for example, x′=α x F x . The same applies to position y.

The multiplying section 152 and the integrating section 153 perform, in the second operation mode, velocity control to determine position z in the remaining one direction of the robot hand 11 in accordance with manipulated variable θ inputted into the manipulating device 14 . Specifically, the multiplying section 152 and the integrating section 153 determine position z in the remaining one direction of the robot hand 11 by solving (integrating) the following equation of motion (2b).

z′=Kθ   (2b)

Herein, K is a conversion factor for converting manipulated variable θ into velocity z′ (e.g., 0.0005). Position z may be calculated by solving an equation of motion other than that mentioned above, and such an equation may be, for example, z″=−(c/m z )z′+(α z /m z )θ.

The mode switch 154 is a switch configured to switch the operation modes in accordance with manipulated variable θ inputted into the manipulating device 14 . Specifically, when θ=0, that is, z′=0, the mode switch 154 switches the operation mode to the first operation mode, whereas, when θ≠0, that is, z′≠0, the mode switch 154 switches the operation mode to the second operation mode.

With the foregoing configuration, it is possible to determine, in the first operation mode, by performing admittance control, positions x, y, and z in the three directions of the robot hand 11 in accordance with forces F x , F y , and F z in the three directions detected by the force sensor 13 . That is, in the first operation mode, it is possible to offer power assistance to a user in moving the robot hand 11 by the control of the robot arm 12 performed by the control device 15 . This enables the user to perform direct teaching more precisely with a smaller force.

However, there may be a case in which a reaction force in the z-axis direction is exerted by the workpiece on the robot hand 11 , in addition to a user's force applied to move the robot hand 11 . For example, during press-fitting operation, a reaction force in the z-axis direction is exerted by the second member W 2 on the robot hand 11 , in addition to a user's force applied to move the robot hand 11 . In this case, it is difficult, in the first operation mode in which the admittance control is performed in the three directions, to move the robot hand 11 to a position desired by the user.

In contrast, with the above configuration, it is possible, in the second operation mode, to (i) determine, by performing admittance control, positions x and y in two directions of the robot hand 11 in accordance with forces F x and F y in the two directions detected by the force sensor 13 , and (ii) determine, by performing velocity control, position z in one direction of the robot hand 11 in accordance with manipulated variable θ inputted into the manipulating device 14 . Thus, even in a case in which a reaction force in the z-axis direction is exerted by the workpiece on the robot hand 11 , it is possible to easily move the robot hand 11 to a position desired by the user by switching the operation modes from the first operation mode to the second operation mode.

It should be noted that the switching from the first operation mode to the second operation mode occurs in response to a situation in which manipulated variable θ of the manipulating device 14 changes from 0. For example, in a case in which a joystick is used as the manipulating device 14 , the switching from the first operation mode to the second operation mode occurs when the user touches the joystick and the joystick starts inclining. Thus, it is possible to cause transition to a control that uses the manipulating device 14 by a natural action, without making the user aware of the switching of the operation modes.

In addition, the control device 15 includes a safety switch 155 . The safety switch 155 is a switch configured to set velocity z′ of the robot hand 11 to zero (to stop the motion in the z-axis direction) in the second operation mode, in accordance with forces F x , F y , and F z detected by the force sensor 13 and position z of the robot hand 11 . Specifically, assuming that z0 is a predetermined constant, when (i) z>z0 and (ii) F x ≠0, F y ≠0, or F z ≠0, velocity z′ of the robot hand is set to zero. Herein, for example, z0 is a position of the robot hand 11 at the time when the height of the lower face of the second member W 2 and the height of the upper face of the first member W 1 are the same.

With the foregoing configuration, when a force acts on the robot hand 11 in a case of z>z0, velocity z′ in the z-axis direction of the robot hand 11 becomes zero regardless of manipulated variable θ of the manipulating device 14 . Thus, for example, in a case in which a user's hand is caught between the first member W 1 and the second member W 2 , it is possible to reduce the probability that the user is injured. It should be noted that, in a case of z≤z0, even when a force acts on the robot hand 11 , velocity z′ in the z-axis direction of the robot hand 11 is Kθ. Thus, even when a reaction force in the z-axis direction is exerted by the second member W 2 on the robot hand 11 , it is possible to complete the press-fitting operation of the second member W 2 .

It is preferable that the admittance control section 151 has, in the second operation mode, a function of reducing force sensitivities α x and α y in accordance with the numbers of change n x and n y of sign of forces F x and F y acting on the robot hand 11 . Specifically, it is preferable to have a function of reducing force sensitivities α x and α y in accordance with the following formulas (3). In the following formulas (3), r is a constant that is greater than 1, which represents the reduction ratio.

When the fitting of the first member W 1 and the second member W 2 is performed, chattering may occur during the press-fitting operation. As used herein, “chattering” refers to movement that repeats the following motions: (1) a side face of the second member W 2 comes into contact with an inner wall of the opening of the first member W 1 on a side of the positive direction of x-axis, so that force F x in the negative direction of x-axis acts on the side face, (2) the second member W 2 moves in the negative direction of x-axis, (3) the side face of the second member W 2 comes into contact with an inner wall of the opening of the first member W 1 in the negative direction of x-axis, so that force F x in the positive direction of x-axis acts on the side face, and (4) the second member W 2 moves in the positive direction of x-axis. In contrast, with the above configuration, it is possible to reduce force sensitivity α x each time the sign of force F x changes, so that it is possible to suppress the occurrence of chattering.

FIG. 3 is graphs illustrating temporal changes in force F x , F y , and F z detected by the force sensor 13 , manipulated variable θ inputted into the manipulating device 14 , positions x, y, and z of the robot hand 11 , and force sensitivities α x and α y , in a case in which the admittance control section 151 has no force sensitivity reduction function. The graphs show that forces F x , F y , and F z and position y wiggle, that is, chatter, because force sensitivities α x and α y are constant.

FIG. 4 is graphs illustrating temporal changes in force F x , F y , and F z detected by the force sensor 13 , manipulated variable θ inputted into the manipulating device 14 , positions x, y, and z of the robot hand 11 , and force sensitivities α x and α y , in a case in which the admittance control section 151 has the force sensitivity reduction function. The graphs reveal that, because force sensitivities α x and α y is reduced suddenly, chattering, which occurs when the control device 15 has no force sensitivity reduction function, is suppressed.

The present invention is not limited to the foregoing embodiments, but can be altered by a skilled person in the art within the scope of the claims. The present invention also encompasses, in its technical scope, any embodiment derived by combining technical means disclosed in the foregoing embodiments.

1 Robot

11 Robot hand

12 Robot arm

13 Force sensor

14 Manipulating device

15 Control device