R. T. Griffiths

Bio: R. T. Griffiths is an academic researcher. The author has contributed to research in topics: Servomechanism. The author has an hindex of 1, co-authored 1 publications receiving 19 citations.

Nonlinear modeling and control of a hydraulic seismic simulator

Abstract: Hydraulic actuators, in combination with servo valves, are used to drive unidirectional seismic simulators. The fluid flow within the hydraulic actuator inherently leads to nonlinear behavior for the system. This paper presents a nonlinear model of the seismic simulator at the University of Notre Dame. The validity of the model is demonstrated experimentally. A control strategy uses a desired earthquake acceleration response to generate specific reference inputs by means of an open loop optimization. A Kalman-filter-based controller is employed. The control strategy is implemented and tested by experiment.

Theoretical analysis of tapered pistons in high speed hydraulic actuators

Abstract: In a high speed hydraulic actuator the usage of seals on the piston surface causes excessive wear on the seals and positional inaccuracies owing to coulomb friction. Hence sealless pistons with sloping surfaces have been tried by some manufacturers of such actuators. Here an attempt is made to analyse systematically the three types of profiles commonly considered for high speed actuators, using Reynolds' differential equation. Analyses are made using an analytical method for one-dimensional flow and using the finite element method for two-dimensional flow. The analyses reveal that single tapered pistons have certain limitations and that double tapered pistons can function successfully under different conditions in hydraulic actuators. It is also shown that friction in these types of pistons is much less than in conventional pistons with seals.

The Modelling of Electrohydraulic Control Valves and its Influence on the Design of Electrohydraulic Drives

Abstract: The modelling of the control valve is shown to be a factor of prime importance in the design of electrohydraulic systems. The process required to model a commercially available valve is described in detail, with equal attention to the production of the experimental data and the identification of the model parameters. The quality of the decisions made in the construction of a model is shown to be most critically evaluated when derivative feedback loops are employed. A study of a system with negative transient acceleration feedback is included to illustrate the effectiveness of a third-order model. It is demonstrated that the use of a digital computer is essential for this type of system design.

Self-tuning position and force control of a hydraulic manipulator

Abstract: Tele-operated hydraulic underwater manipulators are commonly used to perform remote underwater intervention tasks such as weld inspection or mating of connectors. Automation of these tasks to use tele-assistance requires a suitable hybrid position/force control scheme, to specify simultaneously the robot motion and contact forces. Classical linear control does not allow for the highly non-linear and time varying robot dynamics in this situation. Adequate control performance requires more advanced controllers. This paper presents and compares two different advanced hybrid control algorithms. The first is based on a modified Variable Structure Control (VSC-HF) with a virtual environment, and the second uses a multivariable self-tuning adaptive controller. A direct comparison of the two proposed control schemes is performed in simulation, using a model of the dynamics of a hydraulic underwater manipulator (a Slingsby TA9) in contact with a surface. These comparisons look at the performance of the controllers under a wide variety of operating conditions, including different environment stiffnesses, positions of the robot and

Research on feed-forward PIDD 2 control for hydraulic continuous rotation motor electro-hydraulic servo system with long pipeline

Abstract: In the applications of hydraulic continuous rotation motors used in the valve control electro-hydraulic servo system, the long distance between the electro-hydraulic servo valves and the hydraulic motors causes long pipeline effect, which results in a decreased natural frequency and delayed dynamic response of the hydraulic system. This paper presents the mathematical model and characteristic analysis of the dynamic characteristic of the system, as well as the long pipeline effect. A second order differential term is added to the traditional PID control method to decrease/eliminate the load torque fluctuation of the hydraulic motor, which is denoted by PIDD2. Based on the mathematical model and analysis of the system, a control strategy combining PIDD2 and feed-forward of velocity and acceleration is proposed. Both the traditional PID control method and the above-mentioned control method for the electro-hydraulic servo system were simulated in the simulation environment AMESim. The experiment results show that the PIDD2 control with feed-forward of velocity and acceleration will reduce the time delay and stability margin decline of the system response to some extent, and improve the dynamic performance of the system.