Energy Efficient Excavator Implement by Electro-Hydraulic/Mechanical Drive Network

Abstract

The focus on electrification of mobile working machines is increasing in industry as well as in the academic community, and ways to realize both technically and commercially feasible solutions are continuously being pursued. At this point, solutions presented by industry has mainly focused on avoiding internal combustion engines by installing cable or battery fed electric motors powering the main pump(s) which supplies the working hydraulics. However, rotary functions are sought powered directly by electro-mechanical drives, not including hydraulics. In this endeavor a main challenge is the operation of linear actuators that remain controlled by hydraulic control valves. The associated throttle losses necessitates large batteries to be compensated or alternatively results in low machine uptimes, potentially rendering electrified machines commercially infeasible. An obvious approach to avoid throttle losses may be the replacement of valve-controlled linear actuators by electro-mechanical solutions in low to medium force applications, whereas heavy duty working machines subject to large forces such as medium/large excavators may benefit from standalone electro-hydraulic primary controlled/variable-speed drives. Utilization of such solutions will substantially increase the energy efficiency due to absent or at least limited throttle losses, and the electric power sharing and electric energy recuperation capabilities offered by common DC-bus’ and batteries. However, such standalone solutions/drives must be able to meet both the required maximum force and maximum speed, and even thought these maximum quantities seldom are required concurrently, these requirements may render the associated motors and inverters somewhat large. Hence, applying such solutions may lower the battery requirements, but require substantial levels of motor and inverter power to be installed, which again may compromise the commercial feasibility. This paper presents a potentially feasible alternative to these solutions for an excavator implement, in the form of an electro-hydraulic/mechanical drive network. This is applied for actuation of three linear implement functions as well as the rotary swing function. The realization of the electro-hydraulic/mechanical drive network involves chamber short-circuiting and electro-hydraulic variable-speed displacement machines enabling electro-hydraulic power sharing. The proposed drive network is compared to a highly efficient standalone dual motor electro-hydraulic drive solution as well as a separate metering valve drive supplied by a battery fed electro-hydraulic pump. Results demonstrate that, compared to the standalone dual motor electro-hydraulic drive solution, the proposed drive network is realizable with similar energy efficiency and hydraulic displacement but less installed motor power and likely less integration effort, rendering this a more sustainable and cost-efficient solution. Finally, besides being realizable with less installed motor power and hydraulic displacement, the proposed drive network shows substantially improved energy efficiency compared to the separate metering valve drive solution.