Method of controlling seven-axis articulated robot, control program, and robot control device

A non-transitory, computer-readable storage medium storing a computer-executable control program which causes an operation of controlling a seven-axis articulated robot to be executed in a computer, the robot including seven joints sequentially arranged from a proximal end of the robot to a distal end of the robot, the seven joints including rotational axes each causing a next joint to turn and rotational axes each causing a next joint to pivot, such that the rotational axes causing turning and the rotational axes causing pivoting are arranged alternately, the seven-axis articulated robot being configured such that at least one of rotational axes of three proximal end side joints of the robot does not intersect with other rotational axes of the three proximal end side joints at one point,

• the operation comprising:
• detecting a position of the robot;
• calculating a maximum value and a minimum value, which are possible values of a joint angle a middle joint among the three proximal end side and which are values to be taken from an initial state at a start of operation of the robot, by performing inverse transformation while setting a joint angle of a distal end side joint among the three proximal end side joints to 0° (a reference position) or 180° (a maximum displacement position from the reference position);
• calculating, as an index indicating an assumed elbow angle in a case where a rotational axis of the middle joint is assumed as a shoulder, a ratio of the joint angle of the middle joint in the initial state within a range from the maximum value to the minimum value;
• performing inverse transformation using, as a constraint condition, such a joint angle of the middle joint as to cause the calculated ratio to be constant;
• and
• controlling the seven-axis articulated robot to move a tool member to a target position.

A robot control device configured to control a seven-axis articulated robot including seven joints sequentially arranged from a proximal end of the robot to a distal end of the robot, the seven joints including rotational axes each causing a next joint to turn and rotational axes each causing a next joint to pivot, such that the rotational axes causing turning and the rotational axes causing pivoting are arranged alternately, the seven-axis articulated robot being configured such that at least one of rotational axes of three proximal end side joints of the robot does not intersect with other rotational axes of the three proximal end side joints at one point, the robot control device comprising:

• a detector configured to detect a position of the robot:
• a maximum and minimum value calculator configured to calculate a maximum value and a minimum value, which are possible values of a joint angle of a middle joint of the three proximal end side joints and which are values to be taken from an initial state at a start of operation of the robot, by performing inverse transformation while setting a joint angle of a distal end side joint among the three proximal end side joints to 0° (a reference position) or 180° (a maximum displacement position from the reference position);
• a ratio calculator configured to calculate, as an index indicating an assumed elbow angle in a case where a rotational axis of the middle joint is assumed as a shoulder, a ratio of the joint angle of the middle joint in the initial state within a range from the maximum value to the minimum value;
• and
• an inverse transformation unit configured to perform inverse transformation using, as a constraint condition, such a joint angle of the middle joint as to cause the calculated ratio to be constant;
• and
• a controller configured to control the seven-axis articulated robot to move a tool member to a target position.