Planar Cable-suspended Haptic Interface: Design for Wrench Exertion

Cable-suspended robots and haptics interfaces are appealing because of their structural simplicity, high stiffness, and high exerted wrench-to-weight ratio. A major drawback is that cables cannot push but can only exert tension. Therefore, actuation redundancy is required; even so, certain configurations and wrenches will fail since they would require one or more cables to push. The objective of this paper is to present the best design for a planar cable-suspended haptic interface with regard to largest workspace with general wrench exertion in light of this cable tension problem. There are infinite designs with infinite wrenches to apply; therefore, the best and worst designs are found by extensive computer simulation given a reasonable quantification of the problem parameters. INTRODUCTION A haptic interface is a device which can exert wrench (force/moment) and/or tactile feedback to the human from virtual reality and/or remote environments. The current paper focuses on wrench feedback. The Cable-Suspended Haptic Interface (CHSI) studied in this paper is an extension of two recently-developed technologies in cable-suspended robots (CSRs) and stringed haptic interfaces. An early CSR is the Robocrane developed by NIST for use in shipping ports (Albus, et. al., 1993). This device is similar to an upside-down six-dof Stewart platform (Stewart, 1966), with six cables instead of hydraulic-cylinder legs. In this system, gravity is an implicit actuator which ensures cable tension is maintained at all times. Another CSR is Charlotte, developed by McDonnell-Douglas (Campbell, et. al., 1992) for use on International Space Station. Charlotte is a rectangular box driven in-parallel by eight cables, with eight tensioning motors mounted on-board (one on each corner). Four stringed haptic interfaces have been built and tested, the Texas 9-string (Lindemann and Tesar, 1989), the SPIDAR (Ishii and Sato, 1994), the 7-cable master (Kawamura and Ito, 1993), and the 4-cable planar CSHI (Fig. 1, Williams, 1998). Cable-suspended robots and haptic interfaces can be made lighter, stiffer, safer, and more economical than traditional serial robots and haptic interfaces since their primary structure consists of lightweight, high load-bearing cables. One major disadvantage is that cables can only exert tension and cannot push on the single moving link. All of the devices discussed above are designed with actuation redundancy, i.e. more cables than wrenchexerting degrees-of-freedom (except for the Robocrane, with tensioning by gravity) in attempt to avoid configurations where certain wrenches require an impossible compression force in one or more cables. Despite actuation redundancy, there exist subspaces in the kinematic workspace where some cables can lose tension. Roberts et al. (1997) developed an algorithm for CSRs to predict if all cables are under tension in a given configuration while supporting the robot weight. None of these previous papers have presented CSR or CSHI design for optimal wrench exertion. Figure 1. Four-Cable Planar CSHI Prototype The objective of the current paper is to present the best design for the 4-cable planar CSHI considering general wrench exertion in general configurations. This work equally applies to CSRs which exert general wrenches on their environment (not just supporting the robot weight). This paper begins with a description of CSHIs, followed by CSHI statics modeling and tension optimization, and then design for wrench exertion. CABLE-SUSPENDED HAPTIC INTERFACE (CSHI) This section describes the 4-cable planar CSHI and the 8-cable spatial CSHI, including a brief discussion on CSHI kinematics. The design focus in this paper is the planar case. Planar and Spatial CSHIs The CSHI consists of a hand-grip supported in-parallel by n-cables controlled by n-independent tensioning actuators; Fig. 2 shows the 4cable planar case (Fig. 2a shows crossed cables and Fig. 2b shows non-crossed cables) and Fig. 3 shows the 8-cable spatial case (Williams, 1998). Each cable actuator system includes a torque motor, cable reel, tensioning mechanism, plus cable length and force sensors. For 3-dof planar operation, there must be at least 3 cables and for 6dof spatial operation, there must be at least 6 cables. Since cables can