# A Supervised Neural Network Control for Magnetorheological Damper in an Aircraft Landing Gear
## 2 Drop Test Environment of MR Damper Landing Gear  
The most common way to evaluate the performance of landing gears is to perform drop tests. When an aircraft touches down on the ground, a large amount of kinetic energy due to the aircraft's descent speed must be effectively dissipated through the landing gears. At this moment, the damper of a landing gear reciprocates several times after being compressed in a very short time, and thus a significant impact is transmitted to the aircraft structure above the landing gear. Therefore, for the aircraft's structural safety and passengers' convenience, the damper must be designed and controlled so that the strut force and stroke, i.e., the compression distance of the damper's piston, appear at appropriate levels. Figure 1 shows a schematic diagram of the drop test environment for the MR damper landing used in this study. The landing gear is fixed solidly to a carriage that can move smoothly in the vertical direction by four slider guides. The carriage is lifted by using an electric lift winch. The electric lift winch pulls the cable that connects with the carriage by a hook. When the carriage is released out of the hook, the landing gear falls down to the bottom, where a load cell sensor is installed to measure the ground reaction force \(F_{T}\) of the landing gear. The mass of the aircraft that the landing gear supports can be simulated and adjusted by changing the weight of the flat plates placed on the carriage. The sink speed \(v\), i.e., the descent speed of the aircraft at touchdown, can also be changed by adjusting the height \(H\) at which the landing gear is released from the bottom. Figure 1: Structure of drop test apparatus. Figure 2 shows the details of the sensors and signal processing components of the drop test environment. The drop test environment has six sensors for monitoring the state variables and for computing control commands: a load cell measuring the ground reaction force, a laser sensor for the displacement \(z_{1}\) of the sprung mass \(m_{1}\) (including the plate weights, carriage, and damper cylinder), a position sensor for the stroke \(s\) of the damper, two pressure sensors for the damper's upper and lower oil chambers, and an accelerometer for measuring the vertical acceleration of the sprung mass. Since the stroke is the compression distance of the damper's piston, it can be expressed as the difference between the sprung mass displacement \(z_{1}\) and unsprung mass (including piston and tire) displacement \(z_{2}\), and the \(z_{2}\) can be computed from the sensors for \(s\) and \(z_{1}\). The data acquisition is achieved via a dSPACE mini box that connects to a host PC running MATLAB Simulink. The controllers for the MR damper are developed in Simulink models to claim the control signal, which then provides the electrical current to the MR damper's coil. Figure 3 shows the structure of the landing gear equipped with the MR damper in this work. The MR damper consists of a piston connecting to a wheel assembly and a cylinder fixed to the carriage. Two wheels are set up symmetrically to the landing gear to reduce the impact of the friction force on the bearings. The internal spaces of the cylinder and piston, named the upper and lower chambers, are filled with MR fluid (LORD MRF-140 CG), which changes its microscopic property subject to a magnetic field. The LORD MRF-140 CG has many features and benefits that are suitable for landing gear, such as fast response time, dynamic yield strength, temperature resistance, hard settling resistance, and non-abrasiveness [18]. The cylinder has an MR core at its top. The core has multiple coils placed near the annular passage (Figure 4), where the major portion of the damping force is generated. As the damper is compressed or stretched, the cylinder rises or falls, and a flow toward the lower chamber or the upper chamber is formed through the annular passage.