Power management strategies for hydraulic hybrid multi-actuator mobile machines with DC actuators

Rohit R Hippalgaonkar, Purdue University

Abstract

The objective of this work is to devise and evaluate various implementable power management schemes for a novel hydraulic hybrid architecture (called the displacement-controlled series-parallel (DC S-P) hybrid architecture) for mobile, multi-actuator machine systems, with reduced engine power. Specifically, the power management schemes proposed were applied to a proto-type mini-excavator on which the DC S-P hybrid architecture was implemented as part of this work. Previous work had demonstrated significant improvements in overall machine efficiency through transition from the current, standard load-sensing architecture to throttle-less, DC actuation. Measurements showed 40% fuel savings on a prototype, mini-excavator with DC actuation, over a standard mini-excavator in side-by-side testing. Hybridization (through use of hydraulic accumulators) enables storage and re-use of braking energy recovered from the swing drive (and in general, from the rotary actuators), and drastic engine downsizing. A feasibility study undertaken in simulation showed that with the use of a conservative power management scheme, the DC S-P hydraulic hybrid excavator with 50% reduced engine power, offered efficiency improvements over the prototype DC excavator. This work focuses primarily on implementable power management schemes for such hybrid hydraulic multi-actuator machine systems with reduced engine power. A rule-based strategy was first studied that could exploit all available system degrees of freedom. This showed the potential to obtain higher fuel savings than the preliminary, conservative power management scheme wherein only one system degree of freedom was utilized, for both the parallel-hybrid DC architecture as well as the DC S-P hybrid architecture. Patterns in optimal state trajectories and control histories obtained from dynamic programming were also analyzed and identified for various cycles. The rule-based approach lends itself well to replicate optimal results, while exploiting all system degrees of freedom. However, the rules employed for charging the accumulator through the primary unit vary with the duty cycle, and thus a practical solution entails offering the operator a set of rule-based supervisory controllers to choose from. An instantaneous optimization-based approach was also studied, wherein an equivalent fuel consumption objective is minimized at every instant. The strategy proposed, called the equivalent consumption minimization strategy (ECMS), is a promising cycle-independent approach for near-optimal, implementable power management, and approximating optimal behavior for parts of the duty cycle. Transition from the non-hybrid DC architecture on a prototype excavator toward the DC S-P hybrid hydraulic architecture was also achieved as part of this work. An appropriate motion control scheme for closed-loop speed control of the secondary-controlled swing drive was also designed and implemented on the prototype. Engine load-leveling or power limitation (by up to 50%) was demonstrated in measurement through the use of the single-point strategy in a digging cycle (albeit the engine was not actually downsized on the actual prototype). The minimum-speed strategy was also demonstrated on the prototype, where all system degrees of freedom were exploited (including engine speed variation) while maintaining engine power output near 50% engine power.

Degree

Ph.D.

Advisors

Ivantysynova, Purdue University.

Subject Area

Mechanical engineering

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