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O+P Fluidtechnik 6/2016

O+P Fluidtechnik 6/2016


SIMULATION FORSCHUNG UND ENTWICKLUNG PEER REVIEWED DEVELOPMENT OF A BOOM ENERGY RECUPERATION SYSTEM (BERS) Dipl.-Ing. Markus Gärtner, Univ.-Prof. Dr.-Ing. Hubertus Murrenhoff Energy recuperation systems in construction machines are becoming more and more important due to increasing fuel costs and more stringent emission standards. In this contribution, work on a boom energy recuperation system is demonstrated. This paper focusses on the experimental analysis of the hydraulic machine, the development of a control algorithm and the analysis of its performance. 74 O+P – Ölhydraulik und Pneumatik 6/2016

SIMULATION 1 INTRODUCTION This article presents the advance on the boom energy recuperation system for hybrid excavators, short BERS. The development was achieved in cooperation with Doosan Infracore Co. Ltd., one of the biggest construction equipment manufacturers worldwide. A systematic analysis of energy recuperation systems, which was developed by Kang [Kan10] and Sgro [Sgr10] is the basis of the presented work. The BERS recuperates the potential and kinetic energy that becomes free during boom lowering. Therefore a hydraulic machine, an asynchronous machine and supercaps are used. In Figure 1–1 the BERS collaborating with the main hydraulic system is depicted. The boom energy recuperation system basically consists of a hydraulic pump-motor-unit (a), which is driven by an electric machine (b). During boom rising mode the electric machine works as a motor, during boom lowering mode the hydraulic motor drives the electric machine (b) which works as a generator. The pump-motorcombination always has the same direction of rotation, however the directional control valve (1) switches the pressurised lines of the hydraulic unit (a). During boom lowering the recuperated energy is stored in supercaps (2). Subsequently this energy is used for raising the boom. In case of high power demand the BERS is supported by the main hydraulic system (3), which is driven by the internal combustion engine (ICE). In the following it is focused upon the combination of the electric and hydraulic machine. In order to reach a high efficiency and to reduce pressure ripples the commutation process of the hydraulic unit was optimised. The expertise of former works that were released in several publications ([Iva00], [Jan97], [Jar97], [Mur14], [Pet92]) is the basis for the optimisation process, which is described below. The influence of the electrical inverter and the supercaps respective to total efficiency, endurance and cyclic behaviour is not considered in this work. This system boundary is depicted in Figure 1–1 as a grey dot-dashed line. Considering the given operating points, the standard valve plate design was analysed and a simulation model, a kinematical model and an analytical description of valve plate geometry were built up. In the simulation with DSHplus the following design parameters are varied: pre-compression angle, relief grooves, and the volumes for pre- and de-compression. A hydraulic unit with a standard valve plate and another with an improved valve plate are tested afterwards in the laboratory. The findings from the measurements lead to an enhanced simulation model of the BERS. The development and analysis of an algorithm to control the system is topic of the second part of this paper. 01–1 BERS collaborating with the main hydraulic system 02–1 Efficiency difference between improved pump and standard pump 2 EXPERIMENTAL STUDY For the BERS a hydraulic unit that fits to the system requirements, i.e. pressure and speed range was developed in the previous project stage. Aim was the optimisation of the valve plate. Efficiency measurements are carried out to compare the standard unit and the unit with improved valve plate design. Both units are tested in pump and motor mode. For this purpose a test bench according to ISO 4409 was built and used to validate the simulation results. 2.1 EFFICIENCY RESULTS For an easy comparison of both units a chart with the efficiency difference is practicable. Figure 2–1 depicts such a chart. The x-axis shows the pressure difference, the y-axis the efficiency difference. The different lines refer to varying rotational speeds. The difference Δη is defined as follows: Δη=η #2 – η #1 eq. 2 1 02–02 Response time of control With following convention (for pump and motor mode): #1 … standard unit #2 … unit with improved valve plate In general it can be seen, that the unit with improved valve plate design has a higher efficiency than the standard unit for pressures of 150 bar and above. Beneath 100 bar the efficiency of the improved unit is slightly lower. These findings correspond to the results of former simulations with DSHplus with an acceptable margin. 2.2 RESPONSE TIME OF CONTROL In Figure 2–2 the response time of a test pump is plotted. The reference signal of the solenoid consists of a rectangular function between 0 and 100 %. O+P – Ölhydraulik und Pneumatik 6/2016 75


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