Design, dynamics and control issues in a parallel link robot crane: A system with unilateral cable constraints

Cables provide a means for developing lightweight manipulators with positioning and orientation capabilities and large payload capacities. Such manipulators find extensive use in hazardous waste handling, construction automation and other material handling applications that are unsafe for humans. This thesis addresses the dynamics and control issues arising due to the unilateral nature of the cables in the context of a lightweight, fast, cable-controlled parallel link crane.; The various control issues associated with the six degrees-of-freedom parallel link crane are identified based on experiments and a simple model. Some design suggestions that simplify the controls task are outlined. The dynamics and control of such systems is studied in detail on an experimental three degrees-of-freedom planar manipulator.; A multibody dynamics framework is used to model the system with time-varying cable constraints. Change in the constraint set is detected by monitoring the lagrange multipliers and the non-holonomic kinematic (cable length) constraints. Linear Complementarity theory is used to determine the new active constraints and the new velocities after an imposition of constraint (impact). The model is validated by comparing simulation results with experiments. An explicit expression for the tension in the cables is derived based on the model and the various sources that can lead to a loss of cable tension are identified.; A control strategy based on Joint Tension Feedback that reduces the Multiple Input Multiple Output non-linear control problem to a Single Input Single output linear control problem has been developed and implemented. With the Joint Tension Feedback controller the cables tend to lose tension during fast trajectory motions. A predictive tension control strategy based on the model developed that would maintain the cables in tension has been proposed. Also "catching" control strategy to recover from a state of loss of cable tension without inducing impulsive forces or inducing slackness in other cables has been developed and experimentally validated.; An integrated approach to the design and control of this system has been adopted. Simple design modifications on the configuration, choice of cables and additional sensors have significantly simplified our control tasks and enhanced performance.