Control Strategies for a Dual AC Motor Pump System in Aircraft Hydraulic Power Packages

Abstract

Hydraulic power packages (HPP) integrate electric motor-driven pumps (EMP) and hydraulic equipment to supply (on demand) hydraulic power to specific functions. To achieve a high operational availability two redundant EMPs are installed per HPP. To produce maximum output power, the EMPs need to operate in parallel. In the first part of this paper a baseline pressure control strategy is developed that enables the parallel operating mode. It has to cover the performance requirements and it is crucial to avoid stability issues of today’s aircraft multi-pump hydraulic systems, induced by slightly differing pump characteristics. A central pressure controller, which calculates the total (cumulated) control effort and allocates it evenly to both EMPs, is selected as the basic controller structure. A loopshaping approach, where the requirements are mapped on target loop shapes, is applied. The control design is verified by nonlinear simulation and by experiments using representative aircraft prototype EMPs. Their slightly differing characteristics are utilized for an implicit proof of robustness. The second part of this paper makes use of the low utilization of the EMPs during most parts of the flight to achieve secondary objectives (efficiency, dynamic performance) and to improve the handling of operational constraints (e.g. electric input power limit). A model predictive control allocation (MPCA) algorithm flexibly allocates the total control effort to the EMP units. The allocation is derived from the solution of an optimization problem with the operational limits as constraints. Non-linear simulations of two exemplary scenarios show that the MPCA algorithm minimizes power losses by increasing the utilization of the more efficient unit. In addition, the MPCA algorithm improves the dynamic performance in case of an assymetric performance degradation by prioritizing the unit with better performance capability. The potential for improvement increases with the difference between the units and therefore offers advantages in dynamic allocation in particular for fault-tolerant operation.