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O+P Fluidtechnik 5/2017

O+P Fluidtechnik 5/2017


CONTROLS AND REGULATIONS FORSCHUNG UND ENTWICKLUNG PEER REVIEWED 03-1 Simplified configuration of LS-system former is most probably larger than a standard transformer using two displacement units, however the unit costs are lower and its performance and robustness are superior to the transformer with displacement units. The intensification ratio is defined by the area ratio between the piston and rod sides. The drive direction of the transformer is changed by the solenoid valve SV5 when the piston reaches its stroke end as shown in Figure 02-5. 2.4 VALVE SYSTEM FOR ROTARY ACTUATOR Because of the large inertia of the upper structure including the front end attachment, the rotation speed seldom reaches the rated rotation speed and typical swing motions consist of acceleration and braking, as shown in Figure 02-6. While the proportional valve is operated to accelerate the swing motor, the return side pressure does not overpass any pressure rail, and therefore the return port of the proportional valve is directly connected to the tank. On the other hand, if the proportional valve is returned to its neutral position to decelerate the rotating upper structure, the return side pressure increases above a suitable pressure level for energy recovery. This pressure can be adjusted by the braking valves, 03-2 90˚ dig-dump cycle and in the hybrid system the kinetic energy of the upper structure can be recuperated in MP. To ensure the swing motor can always be operated preferentially, a pressure reducing valve from HP to MP is installed. The valve avoids that the pressure level of MP becomes considerably low. 3 SIMULATION 3.1 SIMULATION CONDITIONS To validate the hybrid system, a 18 ton hydraulic excavator with a 1.0 m 3 bucket was modeled and simulations were conducted to analyze the energy consumption. As a reference system, a hydraulic excavator operated by a load-sensing system with one variable displacement pump is used, as shown in Figure 03-1 [12]. In this simulation, the pump swash plate of the LS-System is controlled to ensure the pump pressure is 15 bar higher than the maximum load pressure and the pressure difference across the control valves are also kept constant at 15 bar by the springs of the pressure compensator. Although the conventional LS-Systems use flow regeneration circuits in the arm and boom cylinders resulting in higher cylinder operating speeds and lower power consumption, they are not considered in this simulation model. The pump rotation speed is set 1800 min -1 which is higher than that of the hybrid system, however the losses caused by the auxiliary components are assumed same as the hybrid system in the simulation. Parameters of both systems are shown in Table 3-1 and the efficiency maps of ICE and pump introduced in previous section are used in the simulation. Although the pump displacement is larger in the LS-System than in the hybrid system, it is assumed that their efficiency maps are the same. The set pressures of the braking valves are 220 bar for both systems. The maximum flow rate of all switching valves in the new circuit is 400 l/min at 10 bar. The accumulator volumes of HP and MP are 60 l each, and their pre-charge pressures are 280 bar and 160 bar respectively. The pressure increase ratio of the linear transformer is 1.65 and the maximum operating speed is 0.5 m/s. The operating pressure of the reducing valve connecting HP to MP is set at 180 bar. The rotation speed of ICE is almost constant during the simulation. The 90˚ dig-dump cycle, used in the simulation, is shown in Figure 03-2. This is obtained from the IFAS’s test measurements of a real excavator operated by an expert driver. The bucket reaches approximately 1.2 m below the ground. The cycle begins with a digging motion using the arm and bucket (1). After the bucket is fully loaded with soil, the boom lifts the front end attachment and at the same time the upper structure is rotated 90˚. Before the upper structure completely stops, the arm is slightly extended to move the bucket above the dump truck (2). After the bucket is over the dump truck, the bucket releases the soil (3). The upper structure is returned to the original angle without stopping at the truck during the dumping motion. The final phase is to return to the original posi- 86 O+P Fluidtechnik 5/2017

CONTROLS AND REGULATIONS 03-3 Displacements and loads of actuators during dig and dump 03-4 Operating points of ICE and main pump for the hybrid system O+P Fluidtechnik 5/2017 87


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