Optimal Power Management of Hydraulic Hybrid Mobile Machines—Part II: Machine Implementation and Measurements

Abstract

The problem of achieving maximum system efficiency through nearoptimal supervisory control (or system power management) in mobile offhighway machines is a theoretically challenging problem. It has been tackled for the first time in this work for displacementcontrolled (DC) hydraulic hybrid multiactuator machines such as excavators, through a twopart publication. In Part I, the theoretical aspects of this problem were outlined, supported by simulations of the theoretically optimal supervisory control (relying on dynamic programming) as well as a novel, implementable rulebased supervisory control strategy (designed to replicate theoretically optimal results). In Part II of the publication, the world's first prototype hydraulic hybrid excavator using throttleless DC actuation is described, together with machine implementation of the novel supervisory control strategy proposed in Part I. The design choice, or set of component sizes implemented on the prototype, was driven by an optimal sizing study that was previously done. Measurement results from implementation of two different supervisory control strategies are also presented and discussed—the first, a conservative, suboptimal strategy that commanded a constant engine speed and proved that drastic engine downsizing can be performed in excavator and similar applications. The second strategy implemented was the novel, nearoptimal rulebased strategy (or the “minimumspeedâ€‌ strategy) proposed in Part I that exploited all available system degreesoffreedom, by commanding the minimumrequired engine speeds (to meet DC actuator flow requirements) at every instant in time. While the actual engine was not downsized on the prototype excavator, both the singlepoint and minimumspeed strategies showed that for the aggressive, digging cycles that such machines are typically used for, the DC hydraulic hybrid architecture enables engine operation at or near 50% of maximum engine power without loss of productivity. As described in Part I, actually downsizing the engine by 50% with use of the nearoptimal, minimumspeed strategy will enable significant gains in efficiency (in terms of grams of fuel consumed) over standard valvecontrolled architectures (55%) as well as DC nonhybrid architectures (25%) in cyclical operation.