In advanced robotic applications such as robotic locomotion, vehicle and flight simulators, and material test devices, there are higher requirements on stiffness, robustness and power ability for the mechanical structure and the actuator. Hence, it is common for such applications to use parallel manipulators and hydraulic actuators, due to their advantages in these aspects over their counterparts of serialmanipulators and electrical actuators. When high-precision motion control is required for such systems, advanced model-based controllers, including feedback linearization and adaptive control, have been proposed in state-of-the-art studies for both hydraulic and parallel mechanical systems. However, the high complexity, nonlinearity and model uncertainty of these systems raise significant challenges for their motion control accuracy. @en

High precision motion control of hydraulic manipulators is challenging due to the highly nonlinear dynamics and model uncertainties typical for hydraulic actuators. This paper addresses the implementation of a novel sensor-based incremental nonlinear dynamic inversion control technique for a high-precision hydraulic force controller in existence of parameter uncertainties. Combined with a widely used force computation outer-loop controller, the proposed motion control structure is implemented on a 6-DOF hexapod hydraulic robot, the Simulation, Motion and Navigation (SIMONA) Research Simulator, TU Delft. The proposed control technique is inherently robust to hydraulic parameter uncertainties. As an important contribution, the robustness against parameter uncertainty is rigorously proven. Stability of the proposed controller is also analyzed. Techniques for solving characteristic implementation issues, such as higher-order valve dynamics and oil transmission effects, are discussed in detail. Motion tracking experiment results on the SIMONA simulator validate the effectiveness of the proposed method in terms of performance and the robustness against parameter uncertainties. Significant control accuracy improvement is demonstrated by comparing with the state-of-the-art motion control implementations.@en

Hydraulic driven manipulators face serious control problems due to the nonlinear system dynamics and model and parametric uncertainties of hydraulic actuators. In this paper, a novel sensor-based Incremental Nonlinear Dynamic Inversion controller is applied to force tracking control of hydraulic actuators of a hexapod flight simulator motion system, which together with an outer-loop motion tracking controller forms a motion control system. Due to the use of feedback of pressure difference derivatives, the proposed technique is not dependent on accurate model and parameters, which makes the controller inherently robust to model uncertainties. Furthermore, The sensor-based control approach is particularly suitable for hydraulic force tracking in existence of an outer-loop controller decoupling hydraulic-mechanic interaction term from the inner-loop dynamics. Simulation results indicate that the novel approach yields better tracking performance and confirm the greater robustness to model and parametric uncertainties compared with a traditional nonlinear dynamic invention approach.@en

In this paper, an online flight envelope protection system is developed and implemented on impaired aircraft with structural damage. The whole protection system is designed to be a closed-loop of several sub-systems, including system identification, damage classification, flight-envelope prediction and fault-tolerant control. Based on the information given by damage classification, the flight envelopes are explicitly retrieved and processed online from the database and fed into the fault-tolerant controller, which makes the protection system adaptive to a wide range of abnormal conditions. Simulation results show that with envelope protection, loss-of-control accidents are more likely to be prevented, since both the controller and pilots are aware of the shrunken flight envelopes after damage and excessive commands are restricted. In this way, the fault-tolerance of the impaired aircraft can be effectively enhanced. @en

In this paper, an online flight envelope protection system is developed and implemented on impaired aircraft with structural damage. The whole protection system is designed to be a closed loop of several subsystems, including system identification, damage classification, flight-envelope prediction, and fault-tolerant control. Based on the information given by damage classification, the flight envelopes are explicitly retrieved, processed online from the database, and fed into the fault-tolerant controller, which makes the protection system adaptive to a wide range of abnormal conditions. Simulation results show that with envelope protection, loss-of-control accidents are more likely to be prevented, since excessive commands to the controller are restricted based on the updated information of the changed flight envelopes. In this way, the fault tolerance of the impaired aircraft can be effectively enhanced.@en

This paper presents a motion control strategy for a hexapod flight simulator based on a novel incremental nonlinear dynamics inversion methodology. By using the feedback of the motion base acceleration measurement in joint space, this strategy is capable of achieving accurate system linearisation in existence of model inaccuracies which will significantly degrade the performance of a typical inverse dynamics approach. The proposed control scheme is not sensitive to model and parametric mismatch, hence is robust to model uncertainties. This feature is very helpful for a nonlinear simulator motion system without accurate model, while high performance is generally required. The robustness feature of this strategy allows the use of simplified model and state set-points, instead of full model and state feedback, to invert the nonlinear dynamics, which will reduce the computation burden. The performance and robustness of the proposed scheme is validated by numerical simulations.@en

This paper offers a brief review of hydraulic Stewart platform related control schemes, for purpose of obtaining a guideline for high performance flight simulator motion system control strategy design. The motion tracking control schemes for a pure Stewart platform for an outer-loop are discussed first, then applicable inner-loop hydraulic actuator control strategies are introduced. Inverse dynamics control, force feedforward control, adaptive and robust control schemes are discussed for the outer-loop mechanical system, while feed- back linearization control, cascade pressure difference control as well as other simple and advanced control schemes are discussed for the inner-loop hydraulic system. Based on an analysis of system requirements of the control system after the review, an novel sensor based Incremental Nonlinear Dynamic Inversion (INDI) controller is introduced to deal with the hydraulic actuator. The robustness of INDI is verified and compared with model- based controller by simulation. Finally the possible application of INDI to outer-loop Stewart platform control is discussed. @en