PLC-based control of a robot manipulator with closed kinematic chain

The paper describes the design and implementation of a robot control system on a hardware platform based on a Programmable Logic Controller (PLC). The controlled robot is a 4 degrees of freedom (DOF) manipulator with a closed kinematic chain, designed for high-performance pick and place applications in a packaging workcell. The control software is fully developed on a commercial PLC system, using its standard programming tools and the multi-tasking features of its operating system. In particular, the paper analyse in detail the drawbacks and the advantages related to the choice of standard PLCs in this kind of applications, compared to the much common choice of specialized hardware or industrial personal computers, with particular emphasis on the computational performances obtained with the proposed control architecture.

I. INTRODUCTION

Robot manipulators are largely used in packaging plants, especially for pick and place operations and box filling. In particular, packaging of small food products, like cookies or candies, requires high performance robots with short cycle times and precise motion control, even if their workspace is relatively small. Such performances can be obtained by means of lightweight parallel-drive or delta-like kinematics, whose advantages in terms of reduced moving masses and inertias are well-known. A typical example of this kind of manipulators is the FlexPicker IRB 340 manufactured by ABB [1], which is in fact a delta robot with 4 Degrees Of Freedom (DOF). A typical workcell based on IRB 340 or similar robots includes a vision system, in order to identify and localize products transported in unordered manner and in a large number on the conveyors.