Seven-Axis Robot Control With Non-Intersecting Joint Axes
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Solution Overview
Problem
Seven-axis articulated robots with non-intersecting rotational axes of proximal end side joints face challenges in precise control, particularly in maintaining a constant elbow angle during inverse transformation, leading to reduced degrees of freedom and imprecision in motion planning.
Innovation Solution
A control method that calculates and maintains a constant ratio of the joint angle of the middle joint within a defined range, using the joint angle of the distal end side joint as a reference, to ensure smooth operation similar to a human arm, by setting the joint angle of the distal end side joint to 0° or 180° and using these values to determine possible joint angles and their ratios for inverse transformation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If inverse transformation is performed under a constraint condition that the elbow angle is kept constant, then smooth motion similar to human arm motion is realized, but the control becomes difficult when rotational axes of proximal end side joints do not intersect at one point
Solution Approach 1:
The invention changes the constraint parameter from a fixed elbow angle to a dynamically calculated elbow angle that varies based on the joint configuration. By calculating the elbow angle θe as a function of the proximal joint angles (θ1, θ2, θ7) rather than keeping it constant, the control system adapts to non-intersecting rotational axes while maintaining smooth human-like motion. This parameter transformation resolves the contradiction by making the constraint condition flexible rather than rigid.
2Manufacturing precision
If the elbow angle is defined based on intersecting rotational axes, then inverse transformation can be performed, but the method becomes inapplicable when rotational axes do not intersect at one point
Solution Approach 1:
Instead of defining the elbow angle based on the intersection point of rotational axes (which doesn't exist in non-intersecting mechanisms), the invention inverts the approach by defining the elbow angle through the relationship between joint angles and wrist position. The elbow angle is calculated as θe = atan2(θ2, θ1) - atan2(θ7, θ2), which derives the angle from the joint configuration rather than from geometric intersection. This inversion makes the method applicable to both intersecting and non-intersecting rotational axis configurations.
3Adaptability or versatility
If a seven-axis articulated robot is configured with non-intersecting rotational axes to increase working range, then versatility is improved, but control precision and smoothness are compromised
Solution Approach 1:
The invention applies dynamics by making the elbow angle constraint dynamic rather than static. The elbow angle θe is calculated in real-time based on the current joint angles and wrist position, allowing the constraint to adapt as the robot moves through its workspace. This dynamic calculation ensures that even with non-intersecting rotational axes that provide extended working range, the motion remains precise and smooth by continuously adjusting the elbow angle to maintain the inverse transformation relationship.
Data Source
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AI summary
Provided is a method of controlling 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 rotational axes of three proximal end side joints of the robot do not intersect with each other at one point. The method includes performing inverse transformation using, as a constraint condition, such a joint angle of a middle joint among the three joints as to cause an assumed elbow angle to be constant in a case where a rotational axis of the middle joint is assumed as a shoulder.