Abstract-A hydraulic hybrid passenger vehicle using a Hydro-mechanical Transmission (HMT) or power-split architecture is being developed as a testbed within the Center for Compact and Efficient Fluid Power. In this paper, the design and experimental implementation of a three-level hierarchical control approach for this vehicle with a second generation hardware are presented. This control strategy segregates the tasks of the drive-train into three layers that respectively 1) manages the accumulator energy storage (high level); 2) performs vehicle level optimization (mid-level); and 3) attains the desired vehicle operating condition (low level). Different high level energy management strategies can be employed without affecting the mid and low level controllers. Two "high level" energy management strategies have been implemented and experimentally tested initially, a continuously variable transmission (CVT) strategy used as a baseline for comparison, and a rule based hybrid strategy. Results illustrate that the mid and low level power-train control satisfy the driver's demand and the efficiency is dependent on the energy management used.
Lagrange multiplier approach is a computationally efficient method for computing optimal energy management strategy for a hydraulic hybrid vehicle under the assumption that the accumulator dynamics can be ignored and only the net use of storage energy is considered. Although it provides a close estimate to the fuel economy compared to that obtained using dynamic programming, the resulting control strategy does not respect the physical limits of the storage capacity of the hydraulic accumulator. Thus, the synthesized control strategy is not feasible for actual driving. This article investigates the basic Lagrange multiplier approach for real-time control and proposes modifications so that the storage capacity is respected. It is shown that the Lagrange multiplier can be interpreted as an equivalent loss factor which turns out to be the marginal loss associated with the discharge of stored energy. The two proposed modifications are as follows: (1) a moving horizon approach and (2) making the Lagrange multiplier a function of the current state of charge. Both methods are successful in maintaining the accumulator state of charge within limits with modest effect on fuel economy (3%–5% lower).
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