Loads Simulator System for Testing and Qualification of Flight Actuators

The flight actuation system plays important role in the accurate guidance of the flight vehicles. The actuators driving the control surfaces are aerodynamically loaded during flight. The design, testing and selection process of the flight actuators play important role to ensure the stable and safe flight. Since a reliable flight actuation system can ensure appropriate guidance, the importance of qualification process cannot be neglected. Qualification of the actuators through field trials is a very costly and time-consuming process. The testing process using real flights takes more time and is costly. For ground testing, aerodynamic loading systems are used. The aerodynamic loading system is ground-based hardware in the loop (HWIL) simulator that can be used for exerting aerodynamic loads on actuation system of flight vehicles in real-time experiment. The actuation system under test is directly connected to the loading motor through a stiff shaft and the aerodynamics loading is applied in real time according to the flight trajectory generated by a flight computer. This chapter is preliminarily focused on the basic working principle, mathematical modelling and torque/force control system design of the load simulator system. As a case study, the response and dynamics of the electrical aerodynamic loading system are analysed using mathematical modelling concept. The dynamic model is discussed and adaptive fuzzy sliding mode control techniques are introduced to ensure the high-performance torque loop of the aerodynamic loading systems.


Introduction
In a flight control system, aerodynamic loads are introduced during flight at the control surfaces or fins of flight vehicle as a function of air density, fin angle, etc. The fin control system must respond to these loads to maintain the accuracy of flight control system [1][2][3]. Since load simulators are used for the qualification of flight actuation system, the control performance of load simulators is vital [4,5]. Before the invention of the motorized load simulators, the mechanical springs or torsion bars were utilized for the testing process. Conventional testing has several disadvantages, for example, the loading devices are not adaptable to varying loads. In modern ages, the testing and qualification of flight actuators are done through a hardware in the loop simulator system. In order to reduce cost and time, hardware in the loop simulators are utilized in which a loading motor is used as the aerodynamic loading device [6][7][8][9]. There are three types of load simulators depending on the type of applications and loading requirements [2][3][4][5][6]. Electro-hydraulic load simulators (EHLS) are widely used as hardware-in-theloop-simulation (HILS) system in flight control applications that can simulate the air load executed on flight actuation systems. The EHLS can simulate rapid and large torque/force loads with big volume. Some advantages and disadvantages are listed. The hydraulic load simulators move fast and have rapid steering. The natural frequency of the electro-hydraulic servo valve is generally above 100 HZ, so the hydraulic load simulator's frequency response is very fast and smooth. The low-speed performance of electro-hydraulic actuator is good. The load simulators can be operated in wide speed range. On other hand, due to big size and volume, maintenance cost is high. Temperature effects can cause degradation of performance. At low temperature the friction loss is high, and at high temperatures the leakage phenomenon is significant. Leakage of hydraulic fluid or oil is a major drawback. It causes the environmental pollution. This can easily cause a fire. The oil contamination causes system blockage and system failures. Another type is the pneumatic load simulator that is a typical hardware in loop (HIL) system used to apply medium-range torque/force loads on the actuator under test. The pneumatic load simulator can track the fast deployed load with compressibility. Pneumatic systems are often used instead because pneumatic pressure is usually cheaper to obtain, especially since most industrial facilities already have compressed air available. The pneumatic load simulators use air or any other compressed gas to transmit power. It is hard to obtain smooth performance due to large compressibility factor, especially at low speeds. Similarly, temperature changes can degrade the performance of pneumatic load simulators. The pneumatic load simulators are smaller in size and volume as compared to hydraulic load simulators, so the maintenance cost is lower. The electrical load simulator is an important ground-based hardware tester simulator used for qualification of flight actuation system, such as the aircraft control surfaces, ship steering system, robotics arm, undercarriage of the plane and high-speed elevator system. The electrical load simulators are used to apply medium and small loads on actuators under test. Some advantages of electrical load simulator are its small size and volume, low installation and maintenance cost, less sensitive to temperature changes as compared to hydraulic and pneumatic load simulators and high performance. There are no environmental hazards associated with electrical load simulators.

Working of load simulator system
Laboratory testing and qualification of flight actuators play vital role for ensuring the design of reliable flight control with cost-effective solutions. The hardware diagram given in Figure 1 is the minimal setup that is used in the laboratory testing of the flight actuators. The flight actuator is directly connected to the loading motor through a stiff shaft. The direct mechanical connection allows the application of the loading torque generated at the output shaft of loading motor in a close loop system. The Autopilot controller is used to set the reference position trajectory of the flight actuator. An important block of the system is the aerodynamic load calculator system, and the inputs to this block are the reference position command of the actuator system, air speed, Mach number and angle of attack. The output of the load calculator block serves as reference command to the loading motor. Thus, the torque load loop is closed using the feedback signal from torque sensor. The advantage of using the proposed setup provides the designer a freedom for applying real-time loads simulated in the laboratory with minimum cost.

Actuator testing: a case study
The flight actuator under test is a brushless DC motor. A permanent magnet synchronous motor (PMSM) is used as the loading motor. For simplicity, the load calculator is used in proportional mode. In proportional mode, the output of the load calculator is directly pro- portional to the reference command of the flight actuator. Let θ a represents the reference command of flight actuator, then the output of the load calculator T r is Cθ a where "C" represents the proportionality constant. The parameter T r is set as reference command of the torque loading motor. From Figure 2, the reference command signal of the flight actuator θ a is constructed from a real test scenario of a flight vehicle.
Here, [θ m θ a ] represents the angular position of loading motor and actuator, K s is the total stiffness of torque sensor and connecting shaft. To achieve largest torque operation and to eliminate coupling effect between speed and currents, we set d-axis reference current i d ′ equal to zero. Considering the effect of PWM driver and the current feedback, Eq. (2) can be written as given in [4]. For simplicity, we set L sq = L and R s = R Here, k i is the current controller gain. Current feedback is assumed to be unity. Assuming that load and friction torque are zero and taking Laplace transform of Eq.
From the numerator of Eq. (5), it is concluded that extra torque is caused by the effect ofθ a , θ a and θ a . If the reference input command of loading torque motor is zero, i.e.u = 0, then Eq. (5) is reduced to the following simplified relation From Eq. (6), it is concluded that extra torque is acting on the loading torque motor even if the reference input command u is zero. Extra torque is a function of the acceleration and velocity components of the actuators movement. After some simplifications, the state equation representation of electrical load simulator is written as In Eq. (7) the parameters are defined as In Eq. (7) the nonlinear friction is represented using LuGre model, which is written as In Eq. (8) the parameter g(v) is the Stribeck effect, v s is the Stribeck velocity, f c is coulomb friction, f s is static friction, z is the average bristle defection,a 0 is the stiffness of the bristles, a 1 is the damping term and a 2 is the viscous friction coefficient. Now to realistically apply the loading torque on flight actuator, a feedback torque control system plays vital role. In this study, adaptive fuzzy sliding mode control system is used to formulate the torque control system.

Adaptive fuzzy sliding mode control for electrical load simulator system
Sliding mode is a robust control method which has been widely applied to many nonlinear systems [10][11][12][13][14][15]. This section provides an overview of derivations of torque control system for electrical load simulator's system. Let T L be the output load torque and T r be the desired torque signal, we define tracking error vector as Assuming that the nominal parameters of the system are known, then by combining Eqs. (7), (9) and (10) yields The control law is given by From Eq. (12), it can be analyzed that the total control effort u is the sum of three terms
Using Eq. (19) the following adaptive law is derived

Loading motor torque tracking performance
The testing of actuator is performed under the loading torque T r = 16θ a where "C = 10". From Figure 3, it is concluded that the output torque applied by the loading motor is exactly the same as the reference loading torque.