Experimental investigation of the coefficient of sliding friction in rock at normal stress and sliding velocity of typical earthquakes (10-100 MPa and 0.01-1 m/s) is necessary to develop velocity-dependent constitutive relations useful for earthquake rupture simulations. The velocity dependence of rock friction is best explored in parametric studies of interfacial friction by imposing step-like changes in velocity and measuring the frictional force, but such experiments are technically challenging. We present a testing-machine incorporating a prototype loading system designed to achieve high accelerations (up to 100 g) in test samples at earthquake conditions. A 3-degree-offreedom model representing dynamics and vibrations of the machine is developed to assess machine-sample interactions and the capability to achieve step-changes in velocity. In addition, the prototype loading-system is instrumented with several sensors and operated in order to validate model analyses. Several preliminary experiments on test-samples that display different behaviors were conducted with the prototype system to document machine-sample interactions. The results show that for an impulse-type loading condition associated with dramatic weakening of test-samples, undesirable vibrations of the system can be significant. The dynamic model is modified to identify and treat the source of vibrations accurately, and is used to improve the design of the ultimate loading-system to minimize vibrations and best emulate load paths appropriate to earthquake slip. Finally, a set of sliding friction experiments on rock is presented and the weakening micro-mechanism is investigated. The results suggest that flash weakening at asperity contact points can be a dominant weakening mechanism in earthquake slip.

In this paper, a novel design has been presented for a spatial translational cable-driven robot. The original design of this robot was introduced by Bosscher et al. (2005, “A concept for rapidly-deployable cable robot search and rescue systems,” DETC2005: ASME International Design Engineering Technical Conferences and Computers and Information in Engineering Conference, Long Beach, CA). In this paper, a structure has been proposed to enhance the workspace of the robot. This has been done by using more cables in the mechanism. However, no extra actuator has been added to the robot. Additionally, a method has been proposed to obtain the workspace of the presented robot by calculating the available moment (AM) set in each point of space. The results imply that the workspace of the robot has been enhanced significantly by using the new design.

Object handling is one of the most important applications of cable-suspended robots, which can be obtained by use of a gripper as its end-effector. In this paper, a novel cable-driven multi-finger gripper assembled on a cable-suspended robot has been presented. Using lock/unlock mechanisms, the under-actuated finger mechanism has been designed to have a human like motion. A cable-suspended robot structure with 3 position degrees of freedom is also proposed by employing active/passive cables in such a way that makes it capable of resisting external moments, while it may be simplified to a spatial point-mass cable robot during positioning operation. Furthermore, the robot workspace has been investigated and by considering both lower and upper cable tension limits, a formulation for obtaining the force-feasible workspace is presented and the influence of the minimum tension limit on the workspace is discussed. Finally the moment-resisting capability of the proposed robot has been investigated and by considering several cases, its moment-resisting region is compared to an analogous robot.

Cable-driven parallel robots have several outstanding characteristics that make them unique in many robotic applications. Since cables can only pull, one of the most important issues associated with these robots is obtaining their workspace. In this paper a spatial translational cable-driven robot with active/passive cables is considered and its workspace is investigated from several points of views. First the moment resisting capability of the robot is discussed and the effects of some robot’s parameters on the workspace are studied. Then, both force-feasibility and moment-resisting capability of the robot are considered to find the region where the end-effector may exert the required force-set and resist an external moment simultaneously. Furthermore, the wrench-feasibility of the redundant cable-driven robot is studied and finally a method of obtaining non-fluctuating positive tensions in all cables is proposed by using a particle swarm optimization approach.