Design of a redimensionable and economic flexure-based 6-DOF manipulator demonstrator

For their ever increasing operation tolerances in multiple degrees of freedom, collaborating company IMS resorts to industrially available high-end parallel manipulators. These parallel manipulators exhibit several performance benefits over their serial counterparts in terms of overall stiffness, positioning accuracy and payload-to-weight ratio. Parallel manipulators generally outperform their serial counterparts when performing precision based micro-assembly tasks, as the layout enables for higher load carrying capacity and superior stiffness properties. Higher positioning accuracy can be achieved as a result of non-accumulating errors from serial linkages combined with a general stiffer geometry.

Over the last years, several performance issues have come to light during continuous operation in industrial environments. Especially regarding fast and short stroke positioning applications, durability and lifetime of the current manipulator seem limited as a result of mechanical wear. Moreover, with ever increasing throughput and tighter processing times, the dynamic
performance aspects of the manipulator are starting to become a holdup in the development of the automation platform.

Industrial available manipulators commonly consist of conventional mechanical joints (e.g. universal- or ball-and-socket joint). These joints often rely on the principle of contact mechanics in which wear contributes to loss of accuracy and lifetime. Flexure elements become ever more apparent in the field of precision applications. These elements provide relative motion between two bodies by means of elastic deformation. This operating principle allows for high-accurate and predictable behavior. In comparison to conventional joints, numerous advantages are provided in terms of backlash-, wear- and contamination-free operation, limiting overall hysteresis. However, challenges present itself in the form of limited range of motion resulting from stress limits, decrease in support stiffness for larger deflections and shift of the rotational center (pivot shift).

State-of-the-art 6-DOF parallel manipulators in precision positioning applications are presented. This overview mainly focuses on recent developments in combining parallel manipulators and flexure elements. Furthermore, an overview is presented considering the individual components which make up a 6-DOF parallel manipulator and elaborates on the principles, advantages and limitations. Based on the individual components, several concepts are composed, compared and discussed with the stakeholders .

This thesis describes the design process of a flexure-based parallel manipulator demonstrator. The demonstrator incorporates a novel principle combining actuation suspension and kinematic joint functionality. The general design focuses on minimized joint rotations for optimal performance of the implemented flexure joints, avoiding the drawbacks presented above.

The demonstrator is identified according to system identification procedures to better and effectively control the system. Several tests are performed to observe the performance of the manipulator. Bi-directional repeatability testing is performed using close-range capacitive sensors, resulting in a bi-directional repeatability of 1.3 μm can be achieved without payload,
and repeatability of 0.75 μm with 15 N payload on the end-effector.

Moreover, general setpoint measurements show an operational velocity >80 mm/s and acceleration of 500 mm/s. The demonstrator possesses a footprint of 295 mm combined with a height of 250 mm. This design demonstrates that the design choices for using limited stroke leaf spring mechanisms in a parallel manipulator can contribute to achieving a cost-effective design with high repeatability.