Over the last decade, a number of hybrid architectures have been proposed with the main goal of minimizing energy consumption of off-highway vehicles. One of the architecture subsets which has progressively gained attention is hydraulic hybrids for earth-moving equipment. Among these architectures, hydraulic hybrids with secondary-controlled drives have proven to be a reliable, implementable, and highly efficient alternative with the potential for up to 50% engine downsizing when applied to excavator truck-loading cycles. Multi-input multi-output (MIMO) robust linear control strategies have been developed by the authors' group with notable improvements on the control of the state of charge of the high pressure accumulator. Nonetheless, the challenge remains to improve the actuator position and velocity tracking. Recent developments on adaptive robust control (ARC) algorithms for displacement-controlled actuation systems by the authors' group have opened the door for the application of more advanced control strategies for hydraulic hybrid systems. This paper presents the first time synthesis of an ARC strategy for the motion control of the secondary-controlled hydraulic hybrid system. Similar to work previously developed by the authors' group, the controller is formulated to compensate for uncertain parameters through online parameter adaptation. Additionally, its structure allows for the inclusion of un-modeled nonlinearities such as friction, nonlinear friction of low frequency content and external loads and disturbances. Transient performance, and tracking accuracy are also guaranteed in the presence of both parametric uncertainties and uncertain nonlinearities and asymptotic tracking is achieved in the presence of parametric uncertainties. To evaluate the synthesized ARC controller, a prototype excavator is utilized and the measurement results are compared with those of a PI and an H∞ controller.
