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O+P Fluidtechnik 5/2017

O+P Fluidtechnik 5/2017

CONTROLS AND REGULATIONS

CONTROLS AND REGULATIONS FORSCHUNG UND ENTWICKLUNG PEER REVIEWED NOVEL HYDRAULIC HYBRID SYSTEM FOR EXCAVATORS Ken Sugimura, Hubertus Murrenhoff Deutsche Kurzfassung: In diesem Beitrag wird ein hydromechanisch gesteuertes Ventilsystem für Bagger mit verschiedenen Konstantdrucknetzen vorgestellt. Durch den Einsatz von nach-geschalteten Druckwaagen gelingt die druckgesteuerte Umschaltung der Rücklaufseite ohne elektronische Steuerung. 80 O+P Fluidtechnik 5/2017

CONTROLS AND REGULATIONS 1 INTRODUCTION point operation of the ICE is used. The rotation speed of the ICE is decreased to the low rotation speed where the maximum energy efficiency In recent years, a lot of excellent researches about energy saving of hydraulic systems for hydraulic excavators have been conducted. Load Sensing Systems (LS-system) are one of the most efficient hydraulic systems, as the pumps generate only the required flow, and the pump output pressure is controlled to be a certain amount higher than the maximum load pressure. There are two major throttling losses in LS-system. The first one is the losses across the metering edges consisting of meter-in and -out. The second loss is occurring in the pressure compensators, while two or more actuators with different load pressure levels are powered by one pump simultaneously. To eliminate these throttling losses, displacement control system proposed by Ivantysynova is very effective [1]. In this system, the same number of the variable displacement units as the actuators are installed instead of the control valves. The operating speed of the actuator is controlled by individual displacement of the unit, and therefore no throttling losses occur. Another possibility to eliminate the throttling losses is a system using hydraulic transformers [2]. A disadvantage of these systems is its high costs, and to overcome this issue, some ideas were proposed to decrease the number of the expensive components [3, 4]. When an excavator lowers its boom or decelerates the rotating upper structure, it is possible to recover the potential and kinetic energy. Some hybrid machines recovering only the rotary kinetic energy are already on the market [5]. The hydraulic swing motor is replaced to the electric one, and brake energy during deceleration is stored in a super capacitor. The stored electric energy is reused to drive the electric swing motor or to assist the ICE. There is another approach, for example, the rotary kinetic energy is stored in hydraulic accumulators in the CAT336E [6]. Although more energy can be recovered when the boom is lowered than when the swing motor is decelerated, energy recover from the boom has not been applied to commercial models yet. Because the mass of the front end attachment and the area of the boom cylinder produce lower pressure levels than the normally required pressure level to drive the actuators, at least an additional variable displacement unit is needed to adapt the pressure level for reuse. It is also possible that the energy during exists. STEAM has two constant pressure rails with accu- mulators and the main pump is used only to charge them without any relationships with the load conditions. Therefore the pump displacement can be regulated to produce the efficiently optimal braking torque of the ICE. The constant pressure system, on the other hand, cannot adjust the pressure level to the load pressure level, and for example when the load pressure is considerably lower than the constant pressure level, the throttling losses becom quite large and the benefit given by the efficient operation of the ICE is lost. To overcome this negative aspect, the hydraulic system of STEAM is well devised. The hydraulic system mainly consists of three pressure rails of high, middle and tank pressures, and six switching valves and electrically controlled valves to control the flow rate and direction are installed for each linear actuator. Three of the six switching valves connect the bottom side of the cylinder and each pressure rail, and another three valves connect the rod side and each pressure rail in parallel. These switching valves generate nine combinations producing different cylinder forces and the one of them, which decreases as much of throttling losses as possible, is chosen automatically. Moreover, some combinations allow the energy recuperations while the load on the actuators is assistive. However this superior idea is realized by a lot of pressure sensors, displacement sensors for linear actuators, electric signals of the joysticks, the electric controller and the complex algorithm, making the system complex and expensive. Moreover, technical challenges of robustness and behaviours during a failure situation remain in the hydraulic excavators which are used in a harsh environment. In the compact excavators, there is a traditional abhorrence against positive usage of such a complexity. Therefore the purpose of this study is set to achieve the basic idea of STEAM without using electronic control as much as possible. As the first step, the electrically controlled switching valves are replaced by hydro-mechanically controlled switching valves, the electronic controller and all sensors are removed and furthermore the number of switching valves is reduced to make the system simple. This paper begins by introducing the system configuration. Then the boom lowering is directly used to assist the ICE with torque [7], however an additional variable displacement unit is necessary to convert the hydraulic energy into mechanical rotate energy. To consider not only the energy efficiency of the hydraulic system but the whole efficiency of the hydraulic excavators, it is obvious that the energy efficiency of the ICE has the most effect on the energy consumption, because more than 60 % of the fuel energy is lost. IFAS at RWTH Aachen proposed a system, called STEAM, which increases the efficiency of the whole excavator [8, 9, 10]. This is a constant pressure system, and to minimize the losses in the ICE 02-1 Configuration of hybrid system O+P Fluidtechnik 5/2017 81

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