Motion planning for redundant manipulators in uncertain environments based on tactile feedback

Abstract

The exploitation of new fields of application in addition to traditional industrial production for robot manipulators (e.g. agriculture, human areas) requires extensions to the sensor as well as to the planning capabilities. Motion planning solely based on visual information performs poorly in cluttered environments since contacts with obstacles might be inevitable and thus a distinction between hard and soft objects has to be made. In our contribution we present a novel intrinsic tactile sensing module mounted on a multipurpose 9 DOF agricultural manipulator. With its innovative sensor arrangement we consider it to be a low-cost, easily manageable and efficient solution with a reasonable abstraction layer in comparison to complex torque sensing or tactile skins. The sensor provides information about the resulting force and torque. In the second part of our paper, the tactile information is used for minimizing contact forces while pursuing the end-effector tasks as long as reasonable. Hence, we present robust and efficient extensions to Resolved Motion Rate Control for real-time application. We introduce a general formulation providing control inputs in task-space, joint-space and nullspace. Thus, we design a suitable controller by feedback linearization and feed-forward terms. Results from real-world experiments show the potential of our approach. A discussion of the different control schemes completes the paper.

Adaptive motion control in uncertain environments using tactile feedback

“…This section briefly recapitulates the control framework, which has been introduced in our previous work [1].…”

“…In [1] we showed the transformation of the motion equation into the space along the contact force F p (in the following referred to as contact space) and derived the scalar equatioṅ…”

“…The proposed control framework allows several variants of tactile feedback controllers: An obvious approach is to choose controllers, which only use one system input u {n,j,t} . A second order null space feedback controller u n has already been presented in our previous contribution [1]. The design for task-space u t and joint-space u j results in the following general control law:…”

“…All controllers contain the desired velocity of the contact point p in form of a feedforward term and depend on the scaling factors k {n,j,t} (see (16) to (19)), the expected stiffness c and the parameters of the target dynamics T and d. Control law extensions, e.g. limitation of the null space controller output, apply as presented in our previous publication [1]. In the following, we present the joint-space only controller with second order target system dynamics.…”

“…It shall be enabled to interact with unmodeled environments in a deterministic manner while always respecting admissible contact force limits. In our previous publication [1] we introduced a general tactile planning framework and conducted preliminary experiments using only a small subset of control options. Our new contribution extends this framework significantly and implements capabilities that only have been indicated before.…”