Walter Schumacher

A parallel x-y manipulator with actuation redundancy for high-speed and active-stiffness applications

A 2-DOF parallel manipulator with actuation redundancy is examined for high-speed and stiffness-controlled operation. Advantages of actuation redundancy are outlined. The kinematics and singularity-free workspace of the manipulator are presented together with a force transmission analysis. Finally, a novel control scheme that guarantees a lower bound of the end-effector stiffness (LBSC) is presented. Simulation results are compared with a traditional control scheme for high-speed applications using the minimal 2-norm of actuator torques.

Control of a fast parallel robot with a redundant chain and gearboxes: experimental results

The control of a prototype 2 DOF parallel redundant manipulator (PA-R-MA) is outlined. The manipulator has a redundant chain and is actuated by 3 servomotors and high-reduction gears, giving it the potential of high accelerations and a uniform force transmission. Link torques are picked up by strain gauges, null-space torques are controlled to avoid backdriving of the gearboxes and to allow the use of torque optimization. A decoupled cartesian motion controller then allows operation with accelerations exceeding 100 m/s/sup 2/ and velocities up to 4 m/s. Experimental results show the feasibility of this high-speed robot concept.

A mixed elastic and rigid-body dynamic model of an actuation redundant parallel robot with high-reduction gears

A 2 DOF parallel redundant manipulator (PA-R-MA) is introduced. The manipulator is redundantly actuated by three servomotors with high-reduction gears. It is equipped with structurally integrated torque sensors. An elastic model of the null-space dynamics is derived needed for feedback control of redundant torques. A cartesian rigid-body dynamic model of the motion DOF is shown to be decoupled from the elastic model. Experimental results show the validity of the models.

On contact models for assembly tasks: Experimental investigation beyond the peg-in-hole problem on the example of force-torque maps

Force guided assembly is attractive but the implementation is challenging when uncertainties are present. In these cases, contact models that guide the assembly process are attractive. This article elaborates the usage of static contact models, which map displacements to force-torque vectors of particular contact situations (force-torque map). This model, a map of discrete points, contains force and torque values that correspond to position errors. The inversion of this map, using forces and torques measured from an assembly attempt, yields correction movements in order to accomplish the assembly iteratively. A hypothesis stating the dependency of the models quality on its injectivity is investigated. This aspect is studied thoroughly in so-called redundancy maps, which reveal regions of considerable ambiguity of the model. Experimental results are presented, which validate the hypothesis about the dependency of the convergence of the assembly process on the ambiguity of the initial position. In addition to the peg-in-hole problem, which has become a standard scenario to validate force guided assembly, the scope of this article also covers force guided assembly of more complex parts. Here, the analysis gives evidence to believe that it is unlikely that the implementation convergences acceptably, which is validated by experimental results.

Distributed coalitions for reliable and stable provision of frequency response reserve

The increase in renewable power generation causes an overall decrease in conventional power generation from large-scale and highly predictable fossil power plants. Aside from market-based provision of active power schedules, these power plants are crucial for the provision of short-term automatic ancillary services such as frequency and voltage control. Substituting these plants for renewable generation units requires the latter to be capable of providing these ancillary services in order to guarantee a reliable and stable power supply. In this paper we present an integrated approach for identifying distributed coalitions of agents representing units capable of providing frequency response reserve. We present a method for calculating the individual droop control parameters of each participating device taking into account opportunity costs, device specific reliabilities (e.g. for photovoltaic or wind installations) as well as the small-signal stability of such a coalition for frequency response reserve.

Integrated Force and Motion Control of Parallel Robots – Part 1: Unconstrained Space

Parallel robots in the context of handling and assembly benefit from their inherent high dynamics. However, their coupled nonlinear system dynamics demand for an effective control system in terms of accuracy, robustness and disturbance rejection. This paper presents a hybrid control scheme that is designed specifically to these features. The hierarchical approach encapsulates the robots dynamics on drive-level and, thus, provides the basis for effective real-time hybrid control. It is interfaced by a fully specified C 2-continuous trajectory, see [1] for detailed design. Force-torque control is derived and an analysis of controller performance during the transition to constrained motion presented. These modules form a unique, integrated control-scheme.

Model-based controller design for antagonistic pairs of fluidic muscles in manipulator motion control

A control structure is presented for manipulators actuated by joints with pairwise antagonistic pneumatic muscles. The used muscles and the resulting behavior of a single manipulator joint featuring antagonistic muscles in a symmetric configuration are characterized. Pneumatic joint actuation results in a hysteretic behavior, it is shown that in this case the hysteresis can be described by a Preisach hysteresis model. The resulting hysteresis model allows the construction of a model-reference following controller, with a model control loop, designed for good tracking performance and a disturbance rejection loop optimized for suppression of disturbances. Experiments confirm the improvement in tracking control as compared to the system solely controlled by a feedback regulator.

On the joint design and hydraulic actuation of octahedron VGT robot manipulators

In this paper, the design for variable geometry truss manipulators, with 3-DOF octahedron-shaped modules and hydraulic actuation is introduced. The main features of the concept are the optimized multiple collocated spherical joints and a structure-integrated supply of the drive fluid for the hydraulic actuators. Based upon the known Spherical Joint Mechanism, design rules are deduced and modified joint elements are presented to provide a larger workspace for a single module. The potential of the resulting module design is demonstrated by the calculation of the workspace, as well as the payload-to-mass ratio. Based upon the presented results, a family of highly maneuverable light-weight hyper-redundant manipulators can be derived.

Redundant Parallel Kinematic Structures and Their Control

The design and experimental verification of a control system for fast parallel robots with actuation redundancy was pioneered with the prototype Pa-R-Ma. It is shown how actuation redundancy can improve the capabilities of parallel robots, and how a control can be implemented. A particular focus has been put on verifying selected redundancy strategies that are well-known from literature, but whose applicability for fast parallel robots had not yet been verified, in particular in conjunction with designs that use gear transmissions rather than gearless servo motors. Undesired stress peaks caused by the inherently overconstrained structure were reduced by a factor of 6. To achieve this, a measurement and feedback control of structural forces was developed and verified. As a result, we can show that the actuation redundant manipulator can be moved at very high accelerations without losing control of the internal stress levels. One investigated objective is anti-backlash control, which resulted in an increase of positioning repeatability by a factor of 7.

Derivation and Calculation of the Dynamics of Elastic Parallel Manipulators

Many algorithms for the modelling and calculation of the dynamics of rigid parallel manipulators already exist, and are based on two approaches: The Newton-Euler method and the Lagrangian principle. For the Newton-Euler method the dynamics equations are generated by the complete analysis of all forces and torques of each rigid body in the robot’s structure (Featherstone & Orin, 2000, Spong & Vidyasagar, 1989). Therefore, the derivation of the equations of motion for complex systems becomes very complicated and laborious. However, due to the fact that all forces are explicitly regarded and analysed, this method supplies a very advanced understanding of the system’s dynamics. The use of the Lagrangian principle is a much more elegant and efficient procedure. A scalar function called the Lagrangian is generated, and describes the entire kinetic, potential and dissipative energy of the system in generalized coordinates. For parallel manipulators, additional equations which describe the closed kinematic loop constraints, still have to be provided. The equation of motion for the parallel structure consists thus of the system of Lagrange and algebraic equations (DAE). The Lagrangian method is very widely used in the area of parallel manipulators (Beyer, 1928, Kock, 2001). In particular, two procedures from this family are established here: Namely the Lagrangian equations of the first type (Kang & Mills, 2002, Miller & Clavel, 1992, Murray et al., 1994, Tsai, 1999) and the Lagrange-D’Alembert formulation (Nakamura, 1991, Nakamura & Ghodoussi, 1989, Park et al., 1999, Stachera, 2006a, Stachera, 2006b, Yiu et al., 2001). The use of generalized coordinates is employed in these procedures. Those being the coordinates of the active joints as well as an additional set of redundant coordinates of the passive non-actuated joints or end-effector coordinates. Active joints are the actuated joints of the machine. In the case of elastic manipulators a set of elastic degrees of freedom (DOF) will be introduced. In these generalized coordinates the energy function will be formulated. Additionally, the closed kinematic loop constraints of the parallel structure must also be considered. In the Lagrangian equations of the first type this is achieved by Lagrange multipliers. Contrary to these equations, for the Lagrange-D’Alembert formulation, the Jacobian matrices of the kinematic constraints parameterised by the nonredundant coordinates are used. The policy with the Jacobian matrix has the great advantage that the well known methods and techniques for the modelling of the manipulator’s chain dynamics, which were already applied to serial elastic robots can be