Send Orders for Reprints to [email protected] 1532 The Open Automation and Control Systems Journal, 2014, 6, 1532-1540 Open Access Modeling and Simulation of Simplified Aid for EVA Rescue Using Virtual Prototype Technology Jian Wen1,*, Junguo Zhang1, Lin Gao1 and Xinlei Li2 1School of Technology, Beijing Forestry University, Beijing, China 2Department of Computer Science and Engineering, The Chinese University of Hong Kong, Hong Kong SAR, China Abstract: The Simplified Aid for EVA Rescue (SAFER) System uses 24 GN2 thrusters to achieve six degree-of freedom maneuvering control and Automated Attitude Hold (AAH) control. It is a typical hybrid control system and difficult to model. In this article, we applied the virtual prototyping technology; and used MATLAB/Simmechanics for modeling and simulation of the SAFER system. The implementation model of hybrid control system for SAFER is demonstrated, in- cluding kinematic and dynamic model, and logical status for Automated Attitude Control (AAH). The model of SAFER is modeled by 3D modeling software, and then imported in Simmechanics as a virtual machine. Together with other Sim- ulink module such as STATEFLOW, Look-up Table block, etc, the virtual SAFER hybrid control system can be modeled and simulated in Simulink. The result shows the validation possibilities of such a combined tool. Simulation result verifies the effectiveness of the method. Keywords: Virtual Prototype Technology, SAFER, Automated Attitude Control, Bang-Bang Control, Simmechanics. 1. INTRODUCTION control. It is a typical hybrid system, the command logical statues, selected thruster valve state, change discretely, while The astronaut extravehicular activity (EVA) is the key the physical vectors like angular velocity or acceleration technology of manned space flight. Because of the influence change continuously over time. of complex extravehicular environment and weightlessness, the astronaut in extravehicular activity faced with many As an aerospace product, the process of development for problems such as movement difficulty, attitude hold, pin MMU is influenced by many problems, such as experimental reorientation, and even have drifted away from the spacecraft. conditions, experimental cost and safety problem. It is very Manned Maneuvering Unit (MMU) is to ensure the effec- difficult to test the MMU in the ground applying the physical tive scheme for astronauts EVA [1-3]. prototype [9-11]. The technology of MMU in the United States, Russia and In this article, we applied the virtual prototyping technol- other developed countries is well developed. Some of them ogy, used Formal language MATLAB/SIMU-LINK for have been through space flight experiment verification, such modeling and simulation of SAFER. In this method, as MMU, SAFER in USA, YMK in Russia, and HERMESS SIMMECHANICS was applied for generated 3D animation in European Space Agency. As compared with other devel- makes the system dynamics visualization. This method can oped countries, China’s MMU technology lags fairly far overcome the problem of real effect of weightlessness condi- behind [4-5]. tion could not be simulated on the ground. It can be used to simulate and validate SAFER hybrid system including con- Modern control applications usually execute more and trol subsystem, dynamic and kinetic model. The result shows more complex control logic, and the complexity is increased the validation possibilities of such a combined tool. by the fact that control software interacts with a physical environment through different actors and sensors. Such sys- tems are called Hybrid systems due to the hybrid evolution 2. THE SIMPLIFIED AID FOR EVA RESCUE (SAFER) SYSTEM of their state: One part of the state or variable changes dis- cretely, the other part changes continuously over time [6-8]. The following SAFER system model in this article is based on, and partly copied from, the NASA guide book[12], The Simplified Aid for EVA Rescue (SAFER) System and can be found in the Similar literatures, like[13,14], uses 24 GN2 thrusters to achieve six degree-of freedom ma- neuvering control and Automated Attitude Hold (AAH) which describes a cut-down version of a real SAFER system. 2.1. Overview of SAFER System The simplified Aid for EVA Rescue (SAFER) is a small, self-contained, backpack propulsion system enabling 1874-4443/14 2014 Bentham Open Modeling and Simulation of Simplified Aid The Open Automation and Control Systems Journal, 2014, Volume 6 1533 Fore(+X) Up(-Z) Pitch Down (-Pitch) Left(-Y) Right(+Y) Pitch Up (+Pitch) Aft(-X) Down(+Z) Fig. (1). Hand Controller Module of SAFER. 19-D2R 20-D2F 6-F2 11-R2R 12-R2F 17-D1R 18-D1F 2-B2 5-F1 pitch roll 1-B1 X 9-L1R Y yall 10-L1F 8-F4 15-R4R 16-R4F Z 7-F3 4-B4 23-U4R24-U4F 13-L3R 14-L3F 3-B3 22-U3F 21-U3R Fig. (2). SAFER Thrusters and Axes. free-flying mobility for a crewmember engaged in extrave- mands that make the thrusters selection logic rather compli- hicular activity (EVA). It is intended for self-rescuing on cated. Translation commands issued from the HCM are pri- Space Shuttle missions, as well as during Space Station con- oritized with the priority X first, Y second, and Z third. struction and operation, in case a crewmember got separated When rotation and translation commands are present simul- from the shuttle or station during an EVA. taneously from the HCM, rotations take higher priority and translations are suppressed. Moreover, rotational commands SAFER attached to the underside of the Extravehicular make the corresponding rotation axes of the AAH remain off Mobility Unit (EMU) primary life support subsystem back- until the AAH is reinitialized. However, if rotational com- pack and is controlled by a single hand controller module mands are present at the time when the AAH is initiated, the (HCM), shown in Fig. (1). The HCM is a four-axis mecha- corresponding hand controller axes are subsequently ignored, nism with three rotary axes and one transverse axis using a certain hand controller grip. The HCM can operate in two until the AAH is deactivated. modes, selected via a switch, either in translation mode, or in Fig. (2). illustrates the thruster layout, designations, and rotation mode. The arrow in Fig. (1). shows the rotation directions of forces. SAFER uses 24 GN2 thrusters to mode commands. There are various priorities among com- achieve six degree-of freedom manoeuvring control. After 1534 The Open Automation and Control Systems Journal, 2014, Volume 6 Wen et al. Mech Config 3 S PS 4 S PS 2 S PS 1 S PS C 3-B3 4-B4 2-B2 1-B1 World Frame Solver fx fx fx fx 14 S PS fy F 16 S PS fy F 12 S PS fy F 10 S PS fy F W f(x)=0 14-L3F fz 16-R4F fz 12-R2F fz 10-L1F fz A Thrust Force B Thrust Force C Thrust Force D Thrust Force 22 S PS 24 S PS 20 S PS 18 S PS 22-U3F 24-U4F 20-D2F 18-D1F B F B F B F B F A B C D Transform Transform Transform Transform 1 B F world Frame Free float Joint R Solid B F B F B F B F 2 Safer Frame E F G H Transform Transform Transform Transform 7 S PS 8 S PS 6 S PS 5 S PS 7-F3 8-F4 6-F2 5-F1 fx fx fx fx 13 S PS fy F 15 S PS fy F 11 S PS fy F 9 S PS fy F 13-L3R fz 15-R4R fz 11-R2R fz 9-L1R fz E Thrust Force F Thrust Force G Thrust Force H Thrust Force 21 S PS 23 S PS 19 S PS 17 S PS 21-U3R 23-U4R 19-D2R 17-D1R Fig. (4). Parts of the Scheme to SAFER Body. passing through the regular valve, GN2 is routed to four system. Given the Euler angles φ , θ and ψ that denote the thruster manifolds, each containing six electric-solenoid deviation of the absolute x , y and z axis, the angular ve- thruster valves. Thruster valves open when commanded by the avionics subsystem. When a valve opens, GN2 is re- locities vector can be calculated according to the following leased and expanded through the thruster’s conical nozzle to formula. provide force. A total of 24 thrusters is provided, with four & & ! = D (" )# D ($)#($!,0,%!) + (0,0,"! ) (3) thrusters pointing in each of the ±X , ±Y and ±Z directions. 3 1 ⎛1 0 0 ⎞ 2.2. Model of SAFER System ⎜ ⎟ D1 = ⎜0 cosθ sinθ ⎟ (4) For SAFER, translation and rotation equations from me- ⎜ ⎟ chanics are sufficient for modelling the motion of a crew- ⎝0 − sinθ cosθ ⎠ member using the propulsion system in the body’s own co- ordination system. Also absolute coordinates have to be de- # cos! sin! 0 & termined to visualize the SAFER movement. In order to % ( D = "sin! cos! 0 (5) combine translation and rotation in a single model of motion, 2 % ( % ( coordinate transformation are necessary. All the mathematics $ 0 0 1 ' needed can be found in the standard literature of mechanics. The translation of a crewmember wearing SAFER is de- Where D1 and D2 are rotation matrices that turn the co- scribed by Newton’ second law of expressed by ordinate system by a given angle. Solving equations with given thruster forces result in SAFER’s position x(t) and F = mv! = p! (1) the angular velocity Ω(t) used for AAH. Where F , m , v and p denote force vector, mass, ve- locity vector and impulse vector. 3. SAFER HYBRID SYSTEM MODEL IN MATLAB The rotation is modelled by equation known as the Eu- ler’s equations of the motion for the rotation of a rigid body, 3.1.