Fault-tolerant electric drive for an aircraft nose wheel steering actuator

This study describes the design and testing of a dual-lane electric drive for a prototype, electromechanically actuated, nose wheel steering system for a commercial aircraft. The drive features two independent motor controllers, each operating onehalf of a dual three-phase motor, resulting in an actuator capable of full performance following an electrical fault. An isolated communications link between controllers allows parameter consolidation to identify faults and to synchronise outputs, ensuring even load sharing. A selection of results is presented from motor dynamometer performance analysis and from fully loaded output tests, performed on a hydraulic load rig at Airbus, UK. 1 Electromechanical nose wheel steering The More Electric Aircraft concept involves the migration of all hydraulic, pneumatic and mechanical ‘secondary’ power systems (e.g. surface actuation, air conditioning, wing de-icing) to electrical power, reducing power generation requirements by load sharing [1] and by optimisation of the electrical generation source [2]. In the case of nose wheel steering, replacing the conventional hydraulic system with an electromechanical alternative follows the More Electric philosophy, while reducing the actuator weight and complexity. There are additional safety and flight turnaround benefits of eliminating hydraulic fluid from the landing gear bay; the fluid presents a potential fire risk unless recently applied brakes are allowed time to cool before takeoff and retraction. From a safety perspective, nose wheel steering is of a far lower criticality than flight control surface actuation, as steering is required only when taxiing to and from the runway. The most important requirement is that during landing and take-off, the steering should be free to castor – i.e. the actuator must allow the steering to move freely, thereby allowing the forces between the runway and the moving tyres to centralise the wheels. To provide this ability, a clutch arrangement is built into the new electromechanical actuator, disconnecting the electric motor and gearbox from the nose leg. Aircraft manufacturing requirements specify that the nose wheel steering must remain operational following any single electrical fault. Rather than a safety requirement, this is specified to allow the aircraft to remain operational for a return-to-base to undergo repairs. To accommodate a single failure of the motor windings, power electronic controller, sensors, power supply or IET Electr. Syst. Transp., 2011, Vol. 1, Iss. 3, pp. 117–125 doi: 10.1049/iet-est.2010.0054 control signals, a dual-lane electric drive was developed. Although the tolerance of a single fault can be achieved with two motors and a clutch or speed-summing arrangement [3], a fault-tolerant motor was developed to minimise the mechanical components and mass. A prototype actuator, including motor, gearbox and clutch, was developed by Goodrich Actuation Systems and is shown in Fig. 1. The electric motor and gearbox are located towards the top of the leg, moving the centre of mass nearer the pivot than in a conventional arrangement, reducing the load on the extend/retract actuator. The lower section of the leg rotates, with rotary, variable differential transformers providing position feedback to each of the electric drive control lanes; a requirement as the steering angle cannot be reliably derived by counting motor revolutions because of the decoupling of the clutch. 2 Dual-lane motor drive A dual-lane fault-tolerant motor was developed by Goodrich Actuation Systems. The design is a permanent magnet motor with dual three-phase windings. Full rated torque can be provided by either motor half in the event of a failure. As the motor is ‘fault-tolerant’, one per-unit (p.u.) reactance is present in the windings, allowing the fault current of a short-circuit winding to be limited to 1 p.u. by the controlled application of a terminal short circuit [4]. The coils from each motor lane occupy independent slots, providing thermal isolation, while the two windings sets are interleaved, preventing axial loading on the bearings when operating on one lane [5]. Key motor parameters are listed in Table 1.