Electro-Hydraulic Load Sensing: a Contribution to Increased Efficiency Through Fluid Power on Mobile Machines Antonio Lettini* Marc Havermann** Marco Guidetti* Andrea Fornaciari** * Casappa S.p.A., Lemignano di Collecchio (Parma), Italy ** Walvoil S.p.A., Reggio Emilia, Italy 1 INTRODUCTION In the next years mobile machines like excavators must increase their efficiency and contribute to combustion emission reductions imposed by government legislations. For example, starting from 2012 all mobile machines with a motor power below 130 kw must fulfil EU Stage IIIB or U.S. EPA standard Tier 4 interim or final. An optimization and evolution of hydraulic control systems can contribute significantly to improved energy efficiency on mobile machines. The introduction of first generation hydro-mechanical LS (Load Sensing) systems was a first step into this direction because the pump flow rate was adapted to the actual pressure demand. This technology was followed by second generation systems with flow sharing valves, which allow a proportional distribution of the flow demand to the hydraulic lines even in case of pump saturation. A further improvement regarding energy efficiency and controllability will only be possible by an extensive use of electronic components and control. Recently, new concepts aiming at this direction were proposed, either using modified components with an extensive use of electronics [1] or using completely different solutions with new components [2,3]. The proposed solutions are quite far from traditional LS systems currently used, imply new or quite expensive components and require time to develop and optimize the systems, in terms of performance, stability, controllability. Starting from a second generation hydro-mechanical LS system using components that are reliable and wide-spread on the market, Casappa and Walvoil have developed an
electro-hydraulic LS system. The basic principles of such a system were already described nearly 20 years ago [4], but at that time it was far from practical application on a mobile machine due to state of maturity and cost of electronic components. This has changed today and an electronic control allows the integration of energy-saving strategies and additional functionalities. 2 ELECTRO-HYDRAULIC LOAD SENSING SYSTEM Basically, the electro-hydraulic LS system is an integration of electronic components to a standard hydro-mechanical LS system consisting of a variable displacement piston pump, a flow sharing valve and an electro-hydraulic joystick (Figure 1). One important difference is that the hydraulic connection between pump and the Load Sensing port of the valve is completely removed and replaced by an electric signal. For this purpose, a new electronic pressure control system has been developed for the Casappa MVP pump with a proportional regulator replacing the hydro-mechanical pressure limiter. With this type of control, the pump outlet pressure is proportional to the control current, thanks to a proportional relief valve acting on the pilot signal of the hydro-mechanical LS regulator. Thus, the Load Sensing function is realized by an electronic control unit (Walvoil CED100X, Figure 2) reading the instantaneous measurement of two pressure transducers, the first one on the pump outlet line and the second one on the valve LS port. The CED100X output current signal controls the proportional valve regulating the pump outlet pressure according to the instantaneous LS pressure.
Figure 1: Components of a second generation hydro-mechanical LS system: variable displacement piston pump (Casappa MVP), flow sharing valve (Walvoil DPX), electro-hydraulic joystick (Walvoil SVM) Figure 2: Electronic control unit Walvoil CED 100X with 4 analog/4 digital inputs and 2 outputs Additionally, an angular transducer measuring the instantaneous position of the pump swashplate was integrated to determine the pump torque. In this way a torque control of the hydraulic system is possible, allowing an optimal use of the available engine power. The Casappa MVP PECA pump (Pressure Electronic Control and swashplate Angular sensor) is shown in Figure 3.
Figure 3: Casappa MVP pump with Pressure Electronic Control and swashplate Angular sensor (PECA) and corresponding hydraulic circuit. The flexibility of electronics, for example in changing the LS setting according to the instantaneous working condition, is so added to the advantages of the standard hydraulic control. Furthermore, the control valve is strictly a traditional valve, and the only needed modification is a pressure transducer on the LS port. In the presented system the following functionalities have been implemented: LS electronic control, torque electronic control, LS setting variable with engine speed, torque setting variable with engine speed, fast mode activation by electric switch, fine tuning of the fast mode by potentiometer. 3 FIELD TESTS In order to compare the electro-hydraulic LS system with a standard hydro-mechanical LS-system, an extensive test campaign was conducted on a Kubota KX 161-3 midiexcavator, see Figure 4. The original pump, valve and commands were first all replaced by components described in Figure 1: a Casappa MVP48.54 pump with hydromechanical LS regulator and torque control, a Walvoil DPX100 flow sharing valve, Walvoil hydraulic pilot valves and joystick. A proper data acquisition system was mounted to monitor the main performances of the machine. In a second step, the same
machine was equipped with the proposed electro-hydraulic LS system, replacing the pump with a MVP48.54 PECA (Figure 5) and installing the Walvoil CED 100X unit with the required transducers. Figure 4: Modified Kubota KX 161-3 Figure 5: Casappa MVP PECA mounted on the machine When tested with the hydro-mechanical LS system, a fixed LS pressure difference setting of 17 bar was found to be an optimal value to reach the maximum required movement speed. For the electro-hydraulic LS system the standard mode LS setting becomes a function of engine speed ranging from 10 to 15 bar, while the fast mode LS setting, activated by the operator with a switch on the joystick, can be fine-tuned by the user; the maximum possible value is 27 bar. This leads to less energy consumption in normal conditions; an increase of machine speed (travel or movements) is possible by activating the fast mode when required. Looking at the power management, the hydro-mechanical (HM) torque limiter was set to a fixed value of 105 Nm to prevent the engine from stalling, while the electro-hydraulic (EH) system has a variable torque ranging from 95 Nm at 1600 rpm to 130 Nm at 2200 rpm (Figure 6).
Figure 6: Torque and flowrate diagram measured on test bench Figure 7: Translation at 2100 rpm and boom up movement: comparison between hydromechanical (left) and electro-hydraulic (right) LS system In Figure 7 the torque limiter intervention is shown: with the combined boom up movement and translation the flowrate is 87 l/min at 150 bar for the electro-hydraulic LS system compared to 81 l/min at 130 bar for the hydro-mechanical system. Therefore, the implemented regulation gives visible advantages: at minimum speed no work operation is critical for the engine, while at maximum speed the available hydraulic power is higher than the purely hydraulic system.
4 CONCLUSIONS An electro-hydraulic control system has been proposed, including a Load-Sensing flow sharing control valve, an electronic control unit, 2 pressure transducers, and a variable displacement piston pump with an electronic pressure control system including an integrated angular transducer for the measurement of the swashplate position. Both electronic Load Sensing and torque control have been tested on a medium-size excavator, showing an excellent behaviour in machine work operations and offering additional functionalities compared to traditional completely hydraulic solutions. In particular a less energy consuming LS setting has been used, while a fast mode can be fine tuned by the machine user and properly activated when needed. The power management has been improved through a variable torque limit depending on the engine speed. This system, realized by using standard components provided by Casappa and Walvoil, combines the reliability of traditional hydraulic components and stability of a hydraulic system wide spread on the market of construction machines, with the advantages of the flexibility, customization and optimization given by electronics. REFERENCES [1] Latour, Ch., Electrohydraulic Flow Matching (EFM) The next generation of Load Sensing controls, MOBILE 2006, International Mobile Hydraulic Conference, Bosch Rexroth Group, Ulm, 2006 [2] Rahmfeld, R., Ivantysynova, M., Energy saving hydraulic displacement controlled linear actuators for industrial applications and mobile machine systems, LDIA Birmingham, UK, pp. 105-110, 2003 [3] Zimmerman, J., Pelosi, M., Williamson, C., Ivantysynova, M., Energy Consumption of an LS Excavator Hydraulic System., ASME International Mechanical Engineering Congress and Exposition, Seattle, WA, USA. IMECE2007-42267, 2007 [4] Backé, W., Feigel, H.-J., Neue Möglichkeiten beim elektrohydraulischen Load- Sensing, O+P Ölhydraulik und Pneumatik 34, Nr. 2, p. 106-114, 1990