Analysis of electro-hydraulic lifting system's energy efﬁciency with direct electric drive pump control Tatiana A. Minav ⁎, Lasse I.E. Laurila, Juha J. Pyrhönen LUT Energy, Lappeenranta University of Technology, P.O. Box 20, 53851 Lappeenranta, Finland abstract article info Article history: Accepted 6 November 2012 Available online 12 December 2012 Keywords: Potential energy recovery system (PERS) Forklift truck Mobile work machine Electric drives Energy-saving ratio Modelling Energy efﬁciency has become a major research issue in all ﬁelds of engineering. Opportunities of utilizing electric servo drives in the control of hydraulic lifting systems directly by an electric-servomotor-driven hydraulic machine and enabling energy recovery in them are studied. In this paper, the modelling and testing of an electro-hydraulic lifting and lowering system and a forklift with two lifting zones are carried out and analyzed. The efﬁciencies and energy-saving ratios of the experimental drive system are determined for the free lift and the second lift zones of a modiﬁed industrial forklift with different speeds and payloads. The maximum achieved energy-saving value in the Potential Energy Recovery System (PERS) was 50% for a maximum tested payload of 1000 kg and a maximum fork velocity of 0.5 m/s for the second lift zone. The efﬁciencies and energy consumptions of the experimental drive system and its components are determined. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Combating environmental problems and meeting challenges related to energy efﬁciency and energy saving in mobile working machines are highly topical issues [1–6]. It is easy to understand that energy saving is, in particular, very important in mobile machine applications operated purely by an electric accumulator. When ener- gy consumption is reduced by improving the energy efﬁciency of a machine, harmful emissions will also be reduced somewhere in the energy chain. Ways to improve the energy efﬁciency are now studied widely owing to tightening emissions standards [7] set to limit the global warming. The two main recoverable forms of energy in work- ing machines are the kinetic and potential energy [8–13]. Commercial application of kinetic energy regeneration systems (KERS) is well known [14,15]. In the ﬁeld of mobile machines, there are well-known types of energy regeneration systems (ER) based on a hydraulic accumulator [8,13,16], a battery or a combination with a super capacitor [5,17,18], or a ﬂywheel system [19]. The idea of using potential energy for regeneration is not new. It has already been implemented for instance in lifts [16], excavators [20], cranes [9,21] and forklifts [12,22]. In mobile working machines, both of these energy forms provide an opportunity to save signiﬁcant amounts of energy and thereby provide an opportunity to operate the machine signiﬁcantly longer with the original energy source [23,24]. As an example of a mobile machine, an industrial forklift is used. For energy-saving purposes, the forklift was modiﬁed to allow potential energy recovery to re-evaluate the concept proposed for instance in [12]. By this concept minimizing throttling losses happens by switching to on/off (digital) valves in the system and the energy-saving ratio of forklift trucks can be increased by a considerable amount by implementing energy re- covery during lowering. Lifting functions of the telescopic mast and the traction of the ma- chine consume most of the energy. In the case of other functions of the machine, there is no energy to be recovered, and also the kinetic energy levels are fairly low for signiﬁcant energy recovery. Hence, this paper concentrates on the opportunities of recovering energy from the telescopic lifting mast. The properties of its different lift zones are observed from the perspective of energy recovery. This is done by using different fork speeds and payloads in the potential energy recovery system (PERS). The paper is organized as follows: Section 2 describes the test setup arrangements. Section 3 explains the construction of the Matlab/Simulink model. Detailed information about the equations ap- plied in the study is given. Section 4 presents the simulation results, and Section 5 veriﬁes the results by measurements. Section 6 provides discussion and analysis of the results. Section 7 concludes the paper. 2. Overview of the test setup The general structure of the hydraulic and electric systems for testing of the forklift with PERS is illustrated in Fig. 1. The lift Automation in Construction 30 (2013) 144–150 Abbreviations: PERS, potential energy recovery system; DC, direct current; DTC, di- rect torque control; SC, super capacitor; KERS, kinetic energy regeneration system; ER, energy regeneration system. ⁎ Corresponding author. Tel.: +358 408372249; fax: +358 56216799. E-mail address: Tatiana.minav@lut.ﬁ (T.A. Minav). 0926-5805/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.autcon.2012.11.009 Contents lists available at SciVerse ScienceDirect Automation in Construction journal homepage: www.elsevier.com/locate/autcon