Industrial robot

Details Industrial robot Industrial robot

1999-12-27

2001-10-02

Cuchlinski, Jr., William A. (Department: 3661)

Data processing: generic control systems or specific application

Specific application, apparatus or process

Robot control

C700S193000, C700S245000, C318S016000, C318S568200, C318S568230, C318S560000, C318S640000

Reexamination Certificate

active

06298283

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an industrial robot, specifically to a technique of reducing shock when a robot collides with an obstacle and abnormal load is exerted on axes of the robot.
2. Description of the Related Art
FIG. 7
is a block diagram showing velocity loop control which is performed by servo controller for a servomotor for driving an axis of an industrial robot. A velocity deviation &egr; is obtained by subtracting a velocity feedback signal Vf sent from a velocity detector attached to a servomotor from a velocity command Vc outputted from a position loop control system or directly from a numerical controller. The value obtained by multiplying the integral of velocity deviation &egr; by an integral constant K1 (output of an integrator
100
) and the value obtained by multiplying the velocity deviation &egr; by a proportional constant K2 (output of a proportional device
101
) are added to obtain a torque command Tc. The servomotor is driven in accordance with the torque command Tc. Thus, the servomotor for driving a robot axis is generally drivingly controlled by the velocity loop control including proportional-plus-integral control.
Conventionally, in the control system as described above, when a collision of the robot with an obstacle is detected, each servomotor is drivingly controlled with a velocity command Vc turned to “0” so as to prevent damage such as breakage due to the collision.
In order to detect a collision, it can be adopted a method in which using a disturbance estimating observer
102
for estimating a disturbance torque Td based on a torque command Tc and a fed-back actual velocity Vf, it is determined that a collision has occurred when the estimated disturbance torque exceeds a predetermined value.
When a collision occurs and the velocity command Vc is turned to “0”, a velocity feedback signal for the servomotor having a reversed sign is outputted to the velocity loop control system, and as a result, a torque command having a reversed sign, that is, a torque command which is to reverse the rotation of the servomotor is outputted to reduce shock due to the collision. Actually, the velocity loop system includes the integrator
100
, and the influence of the integrator
100
needs to be taken into account. Here, in order to simplify the explanation, it is supposed that the influence of the integrator
100
is negligible.
FIGS. 6
a
to
6
b
are illustrations for explaining how a robot operates when a collision occurs and each servomotor is drivingly controlled with a velocity command Vc turned to “0”. In
FIGS. 6
a
to
6
b,
reference numeral
20
denotes an obstacle,
21
a hand attached to a wrist of the robot, and
22
an arm of the robot. Reference symbol Ma denotes a servomotor for driving the arm
22
(hereinafter referred to as “arm motor”), and reference symbol Mw denotes a servomotor for driving a wrist axis (hereinafter referred to as “wrist motor”).
Suppose that the arm
22
is driven by the arm motor Ma in a direction indicated by arrow a in
FIG. 6
a.
When an end of the hand
21
collides with the obstacle
20
as shown in
FIG. 6
b,
the arm motor Ma continues producing torque and motor velocity having the same direction as before the collision (the counter-clockwise direction in
FIG. 6
b
) and the hand
21
receives disturbance torque having the opposite direction (the clockwise direction in
FIG. 6
b
) from the obstacle, as indicated in
FIG. 6
b.
When the collision is detected, the velocity command Vc for each motor is turned to “0”. When the velocity command Vc is turned to “0”, only the velocity feedback signal Vf is inputted to the velocity loop, and as a result, the torque command Tc for each motor has a direction opposite to that it had before, as described above. Thus, the torque having a direction opposite to that before, that is, the clockwise direction is produced by the arm motor as indicated in
FIG. 6
c,
and as a result, the arm
22
and hand
21
recede from the obstacle
20
and the robot stops. However, since the hand
21
and arm
22
are pushed by repulsive force due to deflection of the obstacle caused by the collision, the arm motor has also a velocity having a direction such that the arm
22
recedes from the obstacle
20
, that is, the clockwise direction, as indicated in
FIG. 6
c.
Since that velocity is fed back to the velocity loop, the sign of the torque command Tc is reversed and toque having a direction such that the arm collides with the obstacle
20
, that is, the counter-clockwise direction is produced by the arm motor Ma as indicated in
FIG. 6
d.
Thus, a collision may occur again.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an industrial robot capable of receding from an obstacle with which it has collided, without causing a re-collision.
An industrial robot of the present invention comprises servomotors for respectively driving robot axes including wrist axes and servo controllers each having a speed loop to drivingly control each of the servomotors. When a collision of a robot or an end effector mounted on the robot with an obstacle is detected, the servo controllers for servomotors for driving the wrist axes drive the associated servomotors for a predetermined time period in a direction opposite to the direction of torque generated at the time of the collision. Further, when the collision is detected, the servo controllers for servomotors for driving the axes other than the wrist axes may stop the associated servomotors.
According to the present invention, it is determined that a collision has occurred between a robot or an end effector mounted on the robot and an obstacle when a disturbance torque exerted on any of the servomotors exceeds a predetermined threshold value. When a collision is detected in that manner, the servo controllers for servomotors for driving the wrist axes drive the associated servomotors for a predetermined time period in a direction to reduce the disturbance torque. Further, when the collision is detected, the servo controllers for servomotors for driving the axes other than the wrist axes may stop the associated servomotors. The disturbance torque can be estimated by a disturbance estimating observer. In this case, the servo controllers for servomotors for driving the wrist axes issue torque commands to the associated servomotors to generate torque in the same direction with that of the disturbance torque. The absolute value of the torque command may be predetermined or the torque command may be set to the same value with that of the disturbance torque.


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Chen et al., Servo control of flexible beam with inverse-dynamics feedforward and distrubance observer, 1993, IEEE, pp. 702-707.*
Hooper, Motion coordination based on multiple performance criterial with a hyper-redundant serial robot example, 1995, IEEE, pp. 133-138.

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