Overapproximative arm occupancy prediction for human-robot co-existence built from archetypal movements

Abstract

Predicting the occupancy of a human in real time is of great interest in human-robot coexistence for obtaining regions that a robot should avoid in safe motion planning. The human body is composed of joints and links, suiting approximation by a kinematic chain, but the control strategy of the human is completely unknown, meaning the potential occupancy grows very fast and it is difficult to compute tightly in real time. As such, most previous work considers only specific, known, or probable movements, and usually does not account for a range of human dimensions. Focusing on the human arm, we analyze archetypal movements performed by test subjects to create a dynamic model. Motion-capture data of subjects are fitted, for modeling purposes, to two abstractions: a 4-degree of freedom (DOF) model and a 3-DOF model, to obtain dynamic parameters. We validate our approach on movements from a publicly available database. The prediction is shown to be computationally fast, and reachable sets of the abstraction are shown to enclose all possible future occupancies of the arm for different subjects, tightly but overapproximatively. The 3-DOF model has advantages over the 4-DOF in terms of speed, though the 4-DOF model is tighter at smaller time horizons. Such an overapproximative representation is intended for certifiable safety-guaranteed collision avoidance algorithms for robots. Note to Practitioners-Motivated by the need to keep humans safe when working alongside robots, our earlier work proposes a method of trajectory planning where the robot certifies each movement as safe before it performs it. For this to prove that unsafe collisions cannot occur, an overapproximative prediction of the human is needed, meaning that no possible future position of the human is outside the predicted region, or reachable occupancy. However, making this prediction both small enough (so that it does not include unreachable regions) and fast enough for real-time use is not straightforward. We find the limits of human motion by asking a range of test subjects to perform movements as fast as possible. We calculate the reachable occupancies based on these limits and show that our predictions are indeed overapproximative, fast, and not wasteful of volume. One can then use the aforementioned approach to guarantee safety; future challenges are reliably sensing the human's pose and implementing our approach on an industrial robot.

Calculating human reachable occupancy for guaranteed collision-free planning

“…The JS approach (Fig. 2, upper part) is first described in [11]. The model we use here has an extra degree of freedom, and we also consider conversion to task space, so we present these novelties in Sec.…”

“…In [6] the authors use a 4-DOF arm model with a unicycle model of human torso movement to estimate human occupancy at a future time. In [11] the authors identify difficulties in a 4-DOF overapproximative model which arise from the need to avoid singularities in the parameterisation, and propose a 3-DOF model to alleviate these; while simpler, a 3-DOF model suffers from large overapproximation at low prediction horizons. We address the difficulty of workspace singularities here as well as the heretofore unsolved problem of accounting for a moving base coordinate system.…”

“…However, if the upper arm moves such that it aligns with the axis of joint 1, the inverse kinematics is singular. As discussed in [11], we avoid this by defining the joint 1 axis along the clavicle.…”

“…This method has been also used in [6] and [11] to define the occupancy of a human arm modelled as a kinematic chain. Due to space limitations we do not describe the method here but refer the reader to [22].…”

“…In previous work [11], [12], we propose a joint space approach to constructing such an occupancy, using a kinematic model of the arm and a dynamic model built from observed extreme movement. This occupancy encloses relevant arm movements, whereas a model based on the 1.6ms −1 maximum upper body speed given in the ISO Standard [13] did not.…”