Hierarchichal Current Control Design For Electromagnetic Aircraft Launching System

This paper describes an approach to hierarchichal modular control for an electromagnetic aircraft launching system for the aircraft carrier of the future. The purposes are to analyze the possibility of improving the control performance using efficient evaluation of switching conditions, and to explore the different options for the basic control design and the way in which these designs will be tested using the Virtual Test Bed (VTB) Software. After the descripition of the application of deadbeat control, in conjunction with predictive algorithms, to the Control, back electro-magnetic force (emf) generated by FEA (Finite Element Analysis) is discussed. The non-ideal shape of the back emf produces torque ripple in the control. It is shown that this ripple can be easily compensated for by the new control scheme proposed. The new control scheme is tested under two different conditions: twophase control (or DC brushless) and three-phase control (AC brushless). Introduction Modern ship designs are increasingly oriented towards the use of electricity to distribute, control, and deliver energy for all on-board needs. In some significant cases, including traction, adoption in military applications is rather slow, because of the comparatively low achievable power, energy and torque, per unit volume and per unit mass, of electro-mechanical energy conversion systems. However, the benefits of controllability, robustness, reliability, damage management, operational availability, reduced manning, etc. are undeniable. Hence there is a high level of interest in electromagnetic aircraft launching systems for aircraft carriers. The goal is to design and test an electromagnetic system to replace steam catapults as the energy source for launching aircraft off Navy carriers. This paper will concentrate on the control aspect of this kind of system. In the first section an introduction is given to the basic machine design for a linear motor dedicate to this kind of application. The second section discusses the modular control implemented in VTB. The third section focuses on the back emf analysis using FEA and on the problem of torque ripple. In the final section a new scheme is introduced to control a permanent magnet motor as a Brushless AC to compensate for this torque ripple. The simulation platform used for the project is the VTB Software [1]. Other details of the linear motor design project can be found in [2]. Basic Machine Format In this research project, the majority of effort so far has been directed to a permanent magnet version of the machine. It consists of a stator, which is approximately 110 meters long and a shuttle that is 3 meters long. The schematic of stator and shuttle is shown in Fig. 1. Fig. 1: EMALS stator and shuttle (not to scale) The stator hosts a multiphase winding structure, while the magnets are located in the shuttle. The stator windings are organized in segments. The detailed machine design and power electronics switching matrix design can be found in [2]. Control Design For an aircraft launching system the speed loop is not actually significant; the main concern is the current loop. Considering the final goal of the aircraft take-off, what we want to achieve is maximum constant acceleration for the first part and then a much higher deceleration after the take-off to stop the shuttle. The control design will employ two controllers for the two H Bridge sets, to implement the logic to alternatively switch the sections controlled by the respective bridge set, which is shown in the figure below. Fig. 2: Overall Switching Matrix Fig. 3: The H-bridge, thyristor winding connection One top-level controller will control these two controllers. Considering the high power involved in the design, we want to optimize the commutation frequency and the duty cycle. To achieve performance of the torque control, a fast current control scheme must be incorporated. Different options are currently under consideration and are part of a simulation activity performed within the VTB environment. One possible approach is based on the deadbeat control in conjunction with predictive algorithms. A good combination of the two approaches can give important improvements in the control performance. We have the possibility of limiting the sampling rate and fixing the switching frequency to a constant value. In any given sampling interval, we can bring the present sample value (at the beginning of the switching period) to the reference value at the end of the sampling time by defining a point at the interval t+ from the current sample value, such that the final sample value will be equal to the reference value, as shown in Fig. 4 below. Fig. 4: The deadbeat control using t Using the predictive variable structure control given by [3], we can find a value of t+ for the Permanent Magnet Brushless motors, which is shown below, T+