Hierarchical Constraint-Based Singularity Avoidance for Multi-Axis Robots

A generalized reflexive approach for Hierarchical Constraint-Based Singularity Avoidance (HCB-SA) is proposed and demonstrated for the multi-axis robot control of a six Degrees Of Freedom (DOF) hybrid manipulator system. The concept utilizes a dynamic adaptation of virtual constraints by introducing virtual damped actuation on the velocity control level and an anisotropic reflexive trajectory deflection depending on the robot constraints on the position control level. Redundant or low priority DOFs can be used to minimize the pose error in the more important DOFs reflexively without calculating new trajectories. Accordingly, the end-effector can be safely controlled in the vicinity of singularities and all constraints in the task and joint space can be surely hold. Furthermore, the proposed cascaded feedback control with the generalized HCB-SA algorithm is able to react on the presence of external disturbances which is validated using software-in-the-loop simulation on the real-time control system. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The anionic ring-opening polymerization and copolymerization of cyclic carbonates

Cyclic carbonates are eligible to ring-opening polymerization using a wide variety of initiators such as carbanionic or alcoholate species as well as initiators known to be effective for the ring-opening polymerization of lactones and for the group transfer polymerization of vinyl monomers. Depending on the catalyst, high molecular weight polymers may be obtained in high yields (kinetically controlled regime) or a ring-chain equilibrium is observed upon end-biting, back-biting and transesterification reactions (thermodynamically controlled regime). The polymerizability of the cyclic carbonates is strongly dependent on their structure. Five-membered cycles generally cannot be polymerized, whereas six-membered cycles can be polymerized and copolymerized in an ideal manner. The polymerizability of higher cyclics, in particular when containing aromatic ring systems, is highly dependent on the substitution pattern of the aromatics. Since the active species in the polymerization of aliphatic cyclic carbonates was disclosed to be of alcoholate type, a copolymerization with ϵ-caprolactone is easily achieved, the reactivity of the cyclic carbonate, however, being by far larger than that of the lactone. On the other hand, the copolymerization with pivalolactone exerts a different behaviour, since the active species of the growing pivalolactone chain after a few steps assumes the character of a carboxylate anion which is unable to promote the ring-opening polymerization of cyclic carbonates. Since carbanionic species may be used as initiators for the ring-opening polymerization of cyclic carbonates, polystyryl, polybutadienyl, and polyisoprenyl anions may be used as initiators to achieve the corresponding block copolymers. To obtain block copolymers with poly(methyl methacrylate) blocks a group transfer polymerization of the respective acrylate has to be performed, followed by the polymerization of the cyclic carbonate. The latter, however, rather proceeds by a metal- free anionic process than by a group transfer process. The ring-opening polymerization and copolymerization of cyclic carbonates allows the preparation of a broad variety of new polymers with remarkable properties.