Energy Efficient Excavator Functions Based on Electro-hydraulic Variable-speed Drive Network

Electrification of mobile working machines is subject to increasing focus in both industry and
academia. At this stage, focus has been the replacement of conventional internal combustion engines
with cable or battery fed electric motors driving the main pump(s), and the replacement of rotary
functions with electro-mechanical drive solutions. However, the linear functions remain controlled
by hydraulic control valves resulting in substantial throttle losses, which in turn necessitates large
battery sizes and/or low machine uptimes. Alternatively, the valve-controlled hydraulic cylinders may
be replaced with electro-mechanical solutions in applications with limited forces, whereas heavy duty
working machines such as medium/large excavators may benefit from standalone electro-hydraulic
primary controlled drives, i.e., variable-speed standalone drives. The use of such solutions will
substantially increase efficiency due to the absent/limited throttle control and the ability to share
power through the electric supply/DC-bus. A main drawback is that each axis needs to be designed
to meet both the maximum force and maximum speed, hence in the case of using single motor
standalone drives, each motor and associated inverter needs to be designed to meet both the maximum
force and maximum speed, potentially rendering these somewhat large. Alternatively, dual motor
standalone drives can be applied, enabling power distribution via more motors. However, the use of
numerous motors requires more extensive system integration and potentially large motor power
installations considering industrially available non-specialized components. This paper presents a
novel so-called electro-hydraulic variable-speed drive network, applied for actuation of three linear
functions of an excavator implement. Cylinder chamber short-circuiting’s and electro-hydraulic
variable-speed units constitute a drive network allowing both electric and hydraulic power sharing.
The drive network is realized with Bosch Rexroth A2 displacement units and eLION electric motors
as its core components. Results demonstrate that the proposed drive network is realizable with similar
energy efficiency as a standalone dual motor electro-hydraulic drive solution, but with less motor
power and with fewer motors, displacement units and integration effort, rendering this a more
sustainable and cost-efficient solution. Finally, it is shown that the proposed drive network is superior
in terms of installed displacement, electric motor power and energy efficiency, compared to a separate
metering valve drive supplied by a battery fed electro-hydraulic pump.