Imitation learning-based framework for learning 6-D linear compliant motions

TL;DR

This paper introduces a learning from demonstration framework for 6-D linear compliant motions, enabling robots to learn contact-guided tasks without prior knowledge, improving adaptability in assembly and uncertain environments.

Contribution

The method learns to identify desired motion directions and compliance axes from demonstrations, leveraging mechanical gradients for contact-rich tasks without prior information.

Findings

Successfully learned and reproduced compliant motions on a KUKA robot.

Reduced need for precise localization in assembly tasks.

Effectively utilized environmental contact cues for motion guidance.

Abstract

We present a novel method for learning from demonstration 6-D tasks that can be modeled as a sequence of linear motions and compliances. The focus of this paper is the learning of a single linear primitive, many of which can be sequenced to perform more complex tasks. The presented method learns from demonstrations only, without any prior information, how to take advantage of mechanical gradients in in-contact tasks, such as assembly, both for translations and rotations. The method assumes there exists a desired linear direction in 6-D which, if followed by the manipulator, leads the robot's end-effector to the goal area shown in the demonstration, either in free space or by leveraging contact through compliance. First, demonstrations are gathered where the teacher explicitly shows the robot how the mechanical gradients can be used as guidance towards the goal. From the demonstrations, a set of directions is computed which would result in the observed motion at each timestep during a demonstration of a single primitive. By observing which direction is included in all these sets, we find a single desired direction which can reproduce the demonstrated motion. Finding the number of compliant axes and their directions in both rotation and translation is based on the assumption that in the presence of a desired direction of motion, all other observed motion is caused by the contact force of the environment, signalling the need for compliance. We evaluate the method on a KUKA LWR4+ robot with test setups imitating typical tasks where a human would use compliance to cope with positional uncertainty. Results show that the method can successfully learn and reproduce compliant motions by taking advantage of the geometry of the task, therefore reducing the need for localization accuracy.