A Jacobian-based Redundant Control Strategy for the 7-DOF WAM

The mapping between the Cartesian space and joint space of robot manipulators has long been a difficult task for redundant robots. Two main methods are used in the classical approach. One is by using direct kinematic inversion in the position regime; the other is to use Jacobian Transformation in the velocity regime. However, for a redundant robot, a non-squared Jacobian matrix is resulted when mapping between the two spaces. This results in using appropriate optimization algorithms to compute along with the Jacobian matrix. Taking the second approach, the Jacobian matrix for a redundant robot will be non-square. One approach to obtain a solution is to use pseudo inverse, this approach is however computational intensive. This paper presents a pragmatic approach by which a joint of a 7-DOF Whole Arm Manipulator (WAM) is initially fixed to facilitate the computation of the squared Jacobian matrix. Based on this approach, appropriate optimization strategies that are outlined in the paper, can then be applied to determine the optimal value of the 'fixed' joint in real time. Experiments are performed to verify the viability of this approach, and the results established that a robust and flexible, Cartesian trajectory planning framework can be achieved for general redundant manipulators.published_or_final_versio

Relay Control of a Morphing Tensegrity Structure with Distributed Pneumatic Actuation

It is believed that structures and actuation systems should be tightly integrated together in the future to create lightweight dynamic machines. This requires actuators to be distributed through the structure. A tensegrity structure is a very promising candidate for this future integration due to its potentially excellent stiffness and strength-to-weight ratio, and the inherent advantage of being a multi-element structure into which actuators can be embedded. In this paper, an antagonistic multi-axis control of a tensegrity structure is achieved, using a dead band controller. The controller is studied by the describing function technique, and a condition to guarantee stability is derived. The stability condition is illustrated with simulation and experimental results, and is used as a general rule to achieve stable control of the structure

An investigation into the velocity-dependence of the coefficient of friction between concrete and maraging steel

This work investigates the velocity-dependent coefficient of friction between concrete and 300 Maraging steel over short displacements. A modified torsional Hopkinson bar is utilized for rotating thin-walled steel rings in contact with a concrete disk under static precompression. Rotational velocity is varied between tests to determine the velocity dependence of the friction coefficient. Normal force is varied between certain tests to determine the pressure dependence of the friction coefficient between the concrete and steel. Three different types of concrete are tested to deduce any composition effect on the friction coefficient. Dry and greased conditions’ impact on the friction coefficient are also evaluated. Lastly, the displacement dependence (fade) is considered for the concrete with regards to the steel. Discussion of the usefulness of this data in modeling and experimentation of impact between concrete and steel is disclosed

New concepts for parallel kinematic mechanisms using fluid actuation

Belgium Herbarium image of Meise Botanic Garden

Distributed actuation and control of a morphing tensegrity structure

Structures and actuation systems need to be closely integrated together in the future to create faster, more efficient, lightweight dynamic machines. Such actuated structures would be used for morphing aircraft wings, lightweight actuated space structures, or in robotics. This approach requires actuators to be distributed through the structure. A tensegrity structure is a very promising candidate for this future integration due to its potentially excellent stiffness and strength-to-weight ratio, and the inherent advantage of being a multi-element structure into which actuators can be embedded. This paper presents methods for analysis of the structure geometry, for closed-loop motion control, and includes experimental results for a structure actuated by lightweight pneumatic muscles. In a practical morphing tensegrity structure, it cannot be assumed that tension and compression members always meet at a point. Thus, a form-finding method has been developed to find stable geometries and determine stiffness properties for tensegrity structures with nodes of finite dimension. An antagonistic multi-axis control scheme has been developed for the shape position and motion control. In the experimental actuated tensegrity system presented the pneumatic muscles are controlled by on-off valves, for which a dead-band switching controller is designed based on a new stability criterion. The experimental system demonstrates accurate control of shape change while maintaining a desired level of internal preload in a stiff structure, showing considerable promise for future lightweight dynamic machines

Development of an actively compliant underwater manipulator

Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1988This thesis describes the design, construction, and evaluation of an actively compliant underwater manipulator for installation on the underwater remotely operated vehicle (ROV) JASON. The goal of this work has been to produce a high fidelity force-controllable manipulator exhibiting no backlash, low stiction/friction, high backdriveability, wide dynamic range, and possessing a large work envelope. By reducing the inherent dynamic nonlineari ties, a wide range of joint compliances can realistically be achieved. This feature is important when implementing various force control schemes, particularly impedance control. In addition, a mechanically "clean" transmission reduces the need for sensors and allows the user to rely on integral motor sensors to provide torque, position, and velocity information. A three axis manipulator rated to full ocean depth was built. Each of the revolute joints is driven by a DC brushless sensorimotor working through a multi-stage cable/pulley transmission. The manipulator mechanism and wiring is fully enclosed by cast aluminum housings filled with mineral oil. Mineral oil functions to pressure compensate and lubricate the system. Exterior surfaces of the manipulator are smooth and continuous, and were designed to act as work surfaces. Joints one and two have a 240° range of motion, while joint three can rotate 380°. The manipulator transmissions are modeled and predictions of manipulator stiffness, dynamic range, payload capacity, and hysteresis are compared with the results of tests conducted on the actual system. Operation of the cable/pulley transmissions are evaluated and suggestions for improvements are given