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O+P Fluidtechnik 6/2016

O+P Fluidtechnik 6/2016


SIMULATION FORSCHUNG UND ENTWICKLUNG PEER REVIEWED In case of varying the reference signal between 20 % and 80 % the swash plate angle shows no overshooting (not depicted).The control is able to reduce the displacement angle faster than it increases the displacement on both units. The knowledge that was achieved by means of the experimental study is used for a more realistic BERS model that uses realistic efficiency look-up tables and actual control behaviour. Hence the measurements are necessary for the development of a control algorithm. 3 DEVELOPMENT OF AN ADVANCED CONTROL ALGORITHM A tailor-made algorithm, called advanced control algorithm (ACA), is necessary due to different ideal operating points of the electric (b) 03–01 Polar diagram for classification of optimisation problems 03–02 Schematic of the advanced control algorithm for BERS 03–03 Calculation of ideal swash plate angle and rotational speed and the hydraulic machine (a), (cf. Figure 2–1). Aim is a characteristic diagram, so that the combination of the electric machine and the hydraulic unit has the best total efficiency for numerous operating points. The influence of the inverter and the supercaps is not considered in the control algorithm. 3.1 THEORETICAL BACKGROUND OF OPTIMISA- TION PROCEDURES As depicted in Figure 3–1 an optimisation problem can be assigned to several categories: number of key areas, order, horizon of optimisation and probability [Bli10]. The two dashed lines show a simple problem (small dashes) and a relative complex problem (long dashes). The optimisation, more precise the advanced control algorithm is plotted as the red dot-dashed line. The problem on hand is quite simple, except from the nonlinearities. Aim of the control algorithm is the optimisation of the total efficiency η tot (η = 1). Furthermore the optimisation problem has a non-linear order (m ≥ 1) due to some limitations and a probability of parameters (p = 1). The boom moves quite slowly and the moving direction changes rarely, so that the problem has static or quasi-static behaviour. 76 O+P – Ölhydraulik und Pneumatik 6/2016

SIMULATION 3.2 IMPLEMENTATION OF THE OPTIMISATION In the BERS two different types of machines, i.e. an electric and a hydraulic machine, with unlike operating points are used. Hence it is not possible to operate both machines in a point of optimal efficiency at the same time. The aim of a new control is to find the trade-off between maximal efficiency of the electric unit and the hydraulic unit. By use of the parameters swash plate angle and rotational speed the BERS is held in an optimal operating point, which depends on the load. A Simulink-model (Figure 3–2) calculates two output parameters (α opt , n opt ) out of the two input parameters pressure (p) and desired boom velocity (ν des ) so that the total efficiency is maximised. The boom velocity is set by the machine operator via joystick. The load on the boom imposes a pressure to the hydraulic system. Following limitations are included: P max T max n max α ν cyl,max Limitations overview Power limit of electric unit Maximum torque of electric unit Maximum rotational speed Swash plate angle range of pump-motor-unit Maximum cylinder velocity The limitations shown in the table with exception of the cylinder velocity are defined by Doosan Infracore. At a rotational speed of n max and α max the hydraulic unit delivers a theoretical volume flow of Q max . This corresponds to a cylinder velocity of ν cyl,max . Volumetric losses lead to an asymmetric boom velocity, which the BERS can reach without the support of the main hydraulic system in pump mode. In motor mode an overrun of volume flow is released to the reservoir via a relief valve (c), (c.f. Figure 1–1). The simulation is run using these limitations. In one simulation step the pressure (Δp) and cylinder velocity (ν cyl ) are kept constant and the swash plate angle varies. The model finds the ideal swash plate angle that leads to the highest total efficiency in this operating point (Figure 3–3). The rotational speed depends on following equations: (for pump mode) eq. 3 1 (for motor mode) eq. 3 2 This step was repeated for the entire range of possible operating points, in this case combinations of cylinder velocity and pressure. 03–04 Optimal swash plate angle 03–05 optimal rotational speed O+P – Ölhydraulik und Pneumatik 6/2016 77


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