Due to their flexibility, low cost and large working volume, 6-axis articulated industrial robots are increasingly being used for drilling, trimming and machining operations, especially in aerospace manufacturing. However, producing high quality components has demonstrated to be difficult, as a result of the inherent problems of robots, including low structural stiffness and low positional accuracy. These limit robotic machining to non-critical components and parts with low accuracy and surface finish requirements. Studies have been carried out to improve robotic machine capability, specifically positioning accuracy and vibration reduction. This study includes the description of the hardware, software and methodologies developed to compensate robot path errors in real time using a single three-degrees-of-freedom (DOF) laser tracker, as well as the experimental results with and without compensation. Performance tests conducted include ballbar dynamic path accuracy test, a series of drilling case studies and a machining test. The results demonstrate major improvements in path accuracy, hole position accuracy and hole quality, as well as increases in accuracy of a machined aluminum part.

Serial-link industrial robots that feature small footprints and large working volumes have been used widely in various applications. However, their low-stiffness-induced vibration hinders application in precision robotic machining processes. In robotic milling, the spindle may experience chatter, leading to unsatisfactory surface finishes. Moreover, the chatter frequency varies, depending on the robot pose and its underlying stiffness and inertia characteristics, which is likely to be outside of the controlled robot system bandwidth. One solution is to use inertial actuators close to the source of excitation on the robot to generate forces that counteract the vibration. This paper advances matters by employing optimised H_∞ control strategies for improved effectiveness and robustness to perform active vibration suppression. The strategies are independent of tool paths and take account of different robot poses, hence the variability of the robot dynamic characteristics. Performance is assessed against that of standard velocity feedback controllers in eccentric mass experiments and milling tests. The experimental results show that within a relatively large work-plane (500 mm × 500 mm), H_∞ controllers greatly improve machining capability and reduce machining errors by up to 85%.

Serial robots have been successful in numerous applications; however, their limited motion accuracy hinders the potential for high precision robotic machining. Among the many sources of error in robot movement, non-linearities at robot joints, namely backlash and stiction/friction, are yet to be thoroughly investigated and addressed. When a robot joint reverses, backlash-induced excitation switches direction and stiction induces localized errors that occur over a short duration. Together, backlash and stiction from joint reversal lead to significant positional errors. This paper closes the research gap by proposing feedforward backlash and stiction compensation with a focus on suppressing errors during joint reversals. The controllers are designed using experimental studies of robot joints and the nominal tool centre point (TCP) trajectories. Based on the robot kinematics, the strategy interpolates the nominal trajectories to ensure accurate identification of joint reversals along the path, and therefore provides optimal error compensation with minimum latency. The performance of the controller is evaluated in single axis tests for suppressing backlash in static conditions and in multi-axis path tests for suppressing backlash and stiction in continuous motions. The experimental results show that the feedforward compensation greatly improves the robot accuracy during joint reversal, reducing errors by more than 70%.

Dielectric elastomers are in a special class of electro-active polymers known for generating expansion in plane and contraction in thickness under voltage application. This paper advances the understanding of a planar contractile dielectric elastomer actuator (cDEA) that is distinct from conventional multi-layer cDEAs but generates comparable contractile strains. Its structure has a rod-constrained rhombus-shaped electrode region, which undergoes simultaneous in-plane contraction and extension during actuation depending on the configuration of the rod-constraining. It is demonstrated that when the planar cDEA is driven by high voltages, off-plane deformation (i.e., wrinkling) in the direction of contraction causes the rod-constrained electrode region to lose tension and extend in the lateral direction, resulting in a significant increase in contraction strain. It also demonstrates that the contraction strain can be increased further by having biased bi-axial pre-strains. By incorporating both effects, the new cDEA generates a maximum contraction strain of 13%, twice that reported previously for planar cDEAs. A modified planar cDEA, having an additional rigid frame to maintain the pre-strain in the lateral direction to contraction was also developed to demonstrate contractile force actuation. Finally, a stability study shows that the planar cDEA has a primary failure mode of electrical breakdown close to the corners of the rod-constrained electrode region. Having inactive regions around the active cell is essential for generating contraction and eliminating buckling of the planar cDEA in the lateral direction.