System Identification of a Variable Piston Pump and Design of a Hydraulic Load Circuit

Abstract

This master thesis is divided into two main parts. The first part focuses on the identification and modeling of a Bosch Rexroth variable piston pump in both frequency and time domain. The second part presents a hydraulic load circuit which is mounted inside a test container. The load circuit is used to test the pump in a suitable load situation which represents a small scale Active Heave Compensation (AHC) scenario. The considered pump is named A4VSG, and the internal swash plate angle is controlled with a hydraulically driven control system delivered by Bosch Rexroth. Several frequency response tests have been carried out to identify the swash plate angle dynamics. In the time domain, a stochastic input signal have been used to evaluate and verify the time domain response of the internal pump states. In addition to the traditional frequency response test, a proposed method to identify the swash plate dynamics using a single chirp signal test is presented. The chirp signal is a multi-cycle sine wave with a linearly increasing frequency with time. The chirp test analysis have proven promising results, and should be investigated further in other system identification applications. The two main pump modeling techniques proposed are; a Black box model using a linear transfer function and a non-linear Grey box model, where all the unknown model parameters have been identified through non-linear optimization in MATLAB. The accuracy of the proposed models are quantified using the mean and maximum deviation between measured and simulated response in both time and frequency domain. An accurate Grey box model have been identified through non-linear parameter optimization, and a corresponding SimulationX model is created. The simulation model also includes a simplified ripple flow. The simplified ripple flow is found through a mechanical study of the internal piston movements. To quantify the ripple flow effect on the surrounding hydraulic components, two system simulations have been studied and discussed. The second part of the thesis proposes a hydraulic load circuit using a 4/3-way directional servo valve to control the load pressure across a variable displacement piston motor. The motor is connected to a secondary motor using a mechanical coupling. The secondary motor is placed in a closed hydraulic loop which is controlled using the A4VSG pump described in the first part. The load circuit’s purpose is to apply a load torque at the mechanical coupling as the A4VSG’s output flow controls the rotational velocity of the coupling. The couplings velocity reference signal is supposed to represent an offshore crane’s wire drum motion during AHC. The two control systems controlling the load pressure and the shaft speed respectively, are designed based on linear control theory. The controllers were tuned using MATLAB’s controller tuning toolbox. In order to use this tool, the systems were described using linearized transfer functions. The resulting controllers have been implemented using a National Instruments LabVIEW program. In the end the friction torque in both motors combined have been modeled using a non-linear friction model. This model have been used to represent both the stiction and Coulomb friction in the SimulationX model representing the load circuit. The simulation results are compared with real-life experiments to validate the accuracy of the SimulationX model.