Kinematic Optimization of a Parallel Manipulator 5R 2-dof Driven by Pneumatic Cylinders

In industrial field, the need to ensure high levels of production enhancing working precision requires the development of ever more efficient robots. The behaviour of a serial or parallel manipulator can be investigated through its kinetostatic performances [1] such as repeatability, stiffness, maximum force or velocity. They all depend on the kinematics structure of the system, on its configuration inside the working space and on the kind of drive system used to operate the robot. The manipulator may have singular configurations in which the performances in some directions are extremely poor while in others are extremely good. Conversely the manipulator may have configurations where the performances are identical in all directions. This behaviour can be described through the concept of isotropy [2], [3]. Naturally, the design of an isotropic machine is desirable because it assures homogeneous performances in all the directions in terms of accuracy, repeatability, stiffness, maximum force and velocity [4]. This paper deals with the optimization of a parallel kinematics machine (PKM) from the isotropy point of view examining the case of a five-bar linkage manipulator which is a widely used mechanism in industrial applications like pick-and-place, assembly and positioning. First the geometrical dimensions of the system are optimized to achieve the best performance in term of isotropy inside a given workspace. This process consists in optimizing some indices related to the Jacobian matrix [5], [6] of the system (an example is shown in figure 1), representing the kinematical relationships between the end-effector and the actuated joints. In this contest a simple geometric condition to guarantee isotropy will be demonstrated. Secondly, the performance of the system are investigated when the drive system changes from rotational motors to linear actuators. The resulting PKM is characterized by different performance due to a different behaviour of the actuators which are no more independent by the position of the manipulator. Differences between the two manipulators are shown, evaluating how the drive system can affect the kinetostatic properties of the robot.