Low Mobility Cable Robot with Application to Robotic Warehousing

Cable-based robots consist of a rigid mobile platform connected via flexible links (cables, wires, tendons) to a surrounding static platform. The use of cables simplifies the mechanical structure and reduces the inertia, allowing the mobile platform to reach high motion acceleration in large workspaces. These attributes give, in principle, an advantage over conventional robots used for industrial applications, such as the pick and place of objects inside factories or similar exterior large workspaces. However, unique cable properties involve new theoretical and technical challenges: all cables must be in tension to avoid collapse of the mobile platform. In addition, positive tensions applied to cables may affect the overall stiffness, that is, cable stretch might result in unacceptable oscillations of the mobile platform. Fully constrained cable-based robots can be distinguished from other types of cable-based robots because the motion and force generation of the mobile platform is accomplished by controlling both the cable lengths and the positive cable tensions. Fully constrained cable-based robots depend on actuator redundancy, that is, the addition of one or more actuated cables than end-effector degrees of freedom. Redundancy has proved to be beneficial to expand the workspace, remove some types of singularities, increase the overall stiffness, and support high payloads in several proposed cable-based robot designs. Nevertheless, this strategy demands the development of efficient controller designs for real-time applications. This research deals with the design and control of a fully constrained cable-based parallel manipulator for large-scale high-speed warehousing applications. To accomplish the design of the robot, a well-ordered procedure to analyze the cable tensions, stiffness and workspace will be presented to obtain an optimum structure. Then, the control problem will be investigated to deal with the redundancy solution and all-positive cable tension condition. The proposed control method will be evaluated through simulation and experimentation in a prototype manufactured for testing

Integrated Trajectory-Tracking and Vibration Control of Kinematically-Constrained Warehousing Cable Robots

With the explosion of e-commerce in recent years, there is a strong desire for automated material handling solutions including warehousing robots. Cable driven parallel robots (CDPRs) are a relatively new concept which has yet to be explored for high-speed pick-&-place applications in the industry. Compared to rigid-link parallel robots, a CDPR possesses significant advantages including: large workspace, low moving inertia, high-speed motion, low power consumption, and incurring minimal maintenance cost. On the other hand, the main disadvantages of the CDPRs are the cable’s unilateral force exerting capability and low rigidity which is resulting in undesired vibrations of their moving platform. Kinematically-constrained CDPRs (KC-CDPRs) include a special class of CDPRs which provide a considerably higher level of stiffness in undesired degrees of freedom (DOFs) via connecting a set of constrained cables to the same actuator. Nevertheless, undesired vibrations of the moving platform are still their main problem which request more attention and investigation. Dynamic modeling, stiffness optimization, vibration and trajectory-tracking control, and stiffness-based trajectory-planning of redundant KC-CDPRs are studied in this thesis. As a new technique, we separate the moving platform’s vibration equations from its desired (nominal) equations of motion. The obtained vibration model forms a linear parametric variable (LPV) dynamic system which is based for the following contributions: 1) Proposing a new tension optimization approach to minimize undesired perturbations under external disturbances in a desired direction; and demonstrating the effectiveness of kinematically-constrained actuation method in vibration attenuation of CDPRs in undesired DOFs. 2) Providing the opportunity of using a wide class of well-established robust and optimal LPV-based control methods, such as H∞ control techniques, for trajectory-tracking control of CDPRs to minimize the effect of disturbances on the robot operation; and showing the effectiveness of kinematically-constrained actuation method in control design simplification of such robots. 3) Proposing the concept of stiffness-based trajectory-planning to find the stiffness-optimum geometry of trajectories for KC-CDPRs; and designing a time-optimal zero-to-zero continuous-jerk motion to track such trajectories. All the proposed concepts are developed for a generic KC-CDPR and verified via numerical analysis and experimental tests of a real planar warehousing KC-CDPR