Environmental restrictions and economic pressures have spurred greater interest in hybrid and electric off-road mobile machinery. According to researchers Tatiana Minav, Matti Pietola and Shuzhong Zhang of Aalto University in Espoo, Finland, in addition to well-established methods for energy saving in hydraulic systems, a promising concept is zonal or decentralized hydraulics—an approach first pioneered in the aircraft industry.
In a presentation at The 15th Scandinavian International Conference on Fluid Power, held this June in Linköping, Sweden, they described a fully decentralized system that can increase productivity, minimize energy consumption and ensure robust performance in mobile machines operating in various environments.
In a conventional excavator, said the researchers, hydraulic power corresponding to the maximum workload is always supplied from a pump and excessive power dissipates as heat. It is often difficult to reduce input power, even during low-workload operation, because combined controls of actuators require distributed flows. In addition, potential and kinetic energies during lowering and slewing braking also dissipate as heat.
Multi-actuator construction machinery is often equipped with load-sensing (LS) systems. In LS systems, the pump supply pressure is adjusted to match the highest load pressure plus a constant-pressure offset. When multiple actuators operate simultaneously, power losses in lower-load actuators are particularly significant. Sizeable energy losses during the working cycle could be avoided by removing the control valves in a typical digging cycle, and a portion of the energy could be recovered.
The researchers noted that hybrid systems have been developed for construction machinery, such as series and parallel hybrid hydraulic excavators. Hybrid systems minimize the power peaks normally required from the engine, which significantly reduces fuel consumption, prolongs engine life, and lowers maintenance costs. Examples include a Komatsu 20-ton parallel hybrid excavator, the Hitachi ZH200 and the Logset 12H GTE hybrid forestry harvester. Other trends in the industry include valveless or displacement control and the common-pressure-rail method.
In contrast, the work of the Aalto engineers centers on decentralized hydraulics using direct-driven hydraulics (DDH). In a fully decentralized system, the hydraulic pumps are disconnected from the engine and replaced within hydraulic power packs distributed throughout the system.
DDH combines the best properties of traditional hydraulics and electric intelligence and offers benefits such as: easy electrification of construction machinery; higher efficiency compared to conventional machines; fewer potential leak points; smooth and precise movements; controlled power-on-demand; and energy regeneration.
In this research, a 1-ton class JCB micro excavator was electrified by replacing the 14-kW diesel engine with a 10-kW electric motor and lithium-titanate batteries. A start-stop logic system of electric motor operation improved energy efficiency with conventional valve control.
The aim was to investigate the energy efficiency of an excavator hydraulic system with DDH for typical digging cycles. First, they developed a coupled multi-body dynamics model in Matlab Simulink. To acquire the dynamic response and energy consumption distribution of the actuators, this study constructed a particular model in Matlab/Simulink which integrated the mechanics, hydraulics, electrics and control systems of the micro excavator. Simulation results were subsequently validated by a simplified prototype of the micro excavator.
In the micro excavator, internal gear motors (referred to as pump/motors) were selected for DDH units, with operating mode switching between pumping and motoring. The original valve control system for boom, arm and bucket was replaced with three DDH units respectively. In the DDH unit, a fixed displacement pump/motor with a speed-controlled electric servomotor directly controls the amount of hydraulic oil pumped into and out of the system. The hydraulic pump/motor creates a flow depending on the rotating speed of the servomotor. Additionally, a hydraulic accumulator replaces a conventional tank.
In the simulation, a typical digging cycle represents an excavator digging a load of earth, rotating, releasing the load onto a pile, and then returning to its initial position. Bucket capacity is 20 kg.
A key objective was to calculate the energy efficiency of DDH units and overall efficiency in a typical operating cycle, and to determine the energy recovery potential. Results showed energy efficiencies of the boom DDH, arm DDH, and bucket DDH were 67.5%, 75.4% and 77.5% respectively for the work cycle. The overall efficiency of the front attachment with three DDH units was approximately 73.3%, much higher than the reported 31% efficiency of a typical 5-ton excavator with an LS system. Further, the potential energy recovery for boom DDH was estimated as 1.54 kJ.
For simplicity, simulation results were validated using a standalone DDH power pack representing a bucket or arm cylinder and the same required pump/motor displacement ratio. Measurement results of position, velocity and pressures showed acceptable accuracy, compared with the simulation.
The researchers cautioned that the model entailed a number of simplifications. The model for cylinder friction did not account for temperature and pressure changes. The pump leakage model considers only pressure difference, not speed and temperature variations. The on/off valves are not included in this model, and piping is regarded as static fluid volumes. Likewise, cavitation problems were not investigated. For this initial stage of research, these simplifications are considered acceptable, although they should be accounted for in the future, they said.
System control for DDH and decentralized hydraulics was not the focus of this research and was not investigated in detail. A self-adjusting fuzzy PID control handled speed control for the electric motor, using a fixed PID control as current control and cylinder position control in response to load–torque disturbances from the hydraulics. Because decentralized hydraulics did not use a proportional control valve, system damping is low compared to conventional systems, which leads to some oscillations. Therefore, the control strategy of the DDH can be improved in the future.
Energy regeneration with DDH could offer additional benefits. For instance, potential energy recovery for the boom DDH can be transformed into electrical power. In this study, the inverter for the DDH was directly powered from the electrical grid, and energy storage, inverter and power electronics were not considered in this stage of research. Energy could be stored using a battery or supercapacitor, although that requires additional components and affects costs, efficiency and maintenance.
Nonetheless, this research has shown that efficiency gains, typical cycle control and potential energy recovery by DDH are feasible in micro excavators. Therefore, the decentralized approach could be recommended for future construction-equipment designs.