Safer landings start with smarter servo valvesWhen the engineers from Airbus' High Lift Test Centre in Bremen, Germany needed a system to test the landing flap systems for the new Airbus A350 airplane, digital servo valve technology with fast EtherCAT fieldbus and interfaces for different external sensors proved key. The Airbus A350 has two landing flaps at each wing that are stressed by high aerodynamic forces. These forces need to be simulated during the test.
The motion control system for testing the landing flaps of the plane was to be mounted in a metal framework. The load frames are connected to six hydraulically-operated servo cylinders and follow the flap motion. Pneumatically-operated plunger cylinders mounted on metal frames would simulate the load. The six hydraulic cylinders are located so that the range of motion and the load forces are spread as equally as possible and each has closed-loop position control.
A standard industrial PC with a sampling rate of 1000Hz, employs an EtherCAT fieldbus to control the 12 axes of motion (6 for each flap) as well as the other actuators. Since both the absolute value and the direction of the aerodynamic forces are changing during the test, the metal framework with the pneumatic cylinders must follow predefined spatial motion. The force and motion profiles are described by coordinates of the plane. From these coordinates the PLC calculates in real time the required cylinder strokes during the motion.
In addition to spatial motion of the metal frame there is another force applied to the system to simulate the load torque to a drive shaft system. The motion for this subsystem required a highly specialised closed loop proportional valve. The testing of the motion of landing flaps is controlled by a combination of rotary drive, drive shafts and gears. The test system has an additional rotary drive to apply load torque to the rotary drive of the plane.
Specialised servo valves
The Project team members for the creation of the test system were Airbus Bremen, Hycom Hydraulic Systems from The Netherlands, the German engineering office IgH and Moog in Germany. Hycom Hydraulic Systems was responsible for designing and building the hydraulic system and IgH for implementing the motion control for the system.
Moog was selected to design and build highly specialised servo valves to fulfil the customer's unique requirements. From the beginning it was clear that the application called for Moog's Digital Valve Technology and special interfaces would be needed. In addition to the high performance hydraulic functionality, the required valves needed specific features and characteristics including an EtherCAT fieldbus interface, analogue inputs for pressure transducers, an interface for an incremental position encoder, a new analogue input for force control by a strain gauge, and special wiring for the 11-pole + PE connector.
The Moog Digital Servo Valve Series D671 with integrated I/O interfaces was determined to be the right solution to provide the closed-loop axis control in the flap test system. This valve, equipped with an EtherCAT fieldbus interface, controls the axis positions via the central PLC. Each valve actually controls two key functions: on one hand it acts as a bus coupler to allow simultaneous reading of cylinder pressures and on the other hand it switches the external safety valves (via the EtherCAT bus) to control clamping/braking of the cylinders or to by-pass the rotary drive.
In order to avoid additional wiring, the Airbus team requested that Moog develop an input on the valve to allow interfacing of a force sensor. This was challenging as an interface for sensors of this kind had never been developed. The solution developed by the Moog team was to implement the connector on the OBE, more specifically a 4-pin female M8x1 connector located at the front of the electronics housing below the analogue inputs X5, X6, X7. This solution greatly increased the flexibility of the digital valves as the number of possible variants that could effectively use this new analogue input was high.
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