Simultaneous Stiffness and Trajectory Optimization for Energy Minimization of Pick-and-Place Tasks of SEA-Actuated Parallel Kinematic Manipulators

TL;DR

This paper presents a method to minimize energy consumption in SEA-actuated parallel kinematic manipulators during pick-and-place tasks by optimizing trajectories and stiffnesses simultaneously, leveraging eigenmotions and dynamic modeling.

Contribution

It introduces a novel simultaneous optimization framework for trajectories and stiffnesses in SEA-driven PKMs to reduce energy use during repetitive tasks.

Findings

Energy consumption is effectively reduced using the proposed optimization.

Eigenmotions can be exploited to minimize energy in SEA-actuated PKMs.

The approach is validated on two robot applications with positive results.

Abstract

A major field of industrial robot applications deals with repetitive tasks that alternate between operating points. For these so-called pick-and-place operations, parallel kinematic manipulators (PKM) are frequently employed. These tasks tend to automatically run for a long period of time and therefore minimizing energy consumption is always of interest. Recent research addresses this topic by the use of elastic elements and particularly series elastic actuators (SEA). This paper explores the possibilities of minimizing energy consumption of SEA actuated PKM performing pick-and-place tasks. The basic idea is to excite eigenmotions that result from the actuator springs and exploit their oscillating characteristics. To this end, a prescribed cyclic pick-and-place operation is analyzed and a dynamic model of SEA driven PKM is derived. Subsequently, an energy minimizing optimal control problem is formulated where operating trajectories as well as SEA stiffnesses are optimized simultaneously. Here, optimizing the actuator stiffness does not account for variable stiffness actuators. It serves as a tool for the design and dimensioning process. The hypothesis on energy reduction is tested on two (parallel) robot applications where redundant actuation is also addressed. The results confirm the validity of this approach.