Model reduction and composite control for overhead hoist transport system by singular perturbation technique

An innovative Overhead Hoist Transport (OHT) system is proposed and analyzed to transport fragile semi-finished products in factories. A triplet of double-link arm is used to carry the load, in replacement of the cables used conventionally. Unlike conventional OHT, the proposed OHT exhibits superior capability for high-speed transportation, flexible stiffness and is able to account for the inherent auto-sway characteristics and parameters uncertainties of the OHT system. The three-time-scale plant model of the OHT system, including the drive motors, flexible links and rigid links, is developed. By singular perturbation order-reduction technique, the highly nonlinear high-order dynamics of the OHT system can be modeled as a low-order linearized plant so that the synthesis of the feedback controller becomes simpler. The composite control, composed of sliding mode control and input shaping technique, is proposed. The sliding mode control is, as usual, employed to account for the system parameters uncertainties. On the other hand, to suppress the residual vibration, i.e., auto-swaying, the input shaping technique is utilized by implementation of a finite-length sequence of impulses in the appropriate amplitude and time epoch. Finally, the efficacy of the proposal composite control strategy is examined and verified by intensive computer simulations.