World Academy of Science, Engineering and Technology International Journal of Mechanical and Mechatronics Engineering Vol:15, No:11, 2021 Abstract—In a task to assist NASA in analyzing the dynamic forces caused by operational countermeasures of an astronaut’s exercise platform impacting the spacecraft, an active proportional- integral-derivative controller commanding a linear actuator is proposed in a vibration isolation system to regulate the movement of the exercise platform. Computer simulation shows promising results that most exciter forces can be reduced or even eliminated. This paper emphasizes on parameter uncertainties, variations and exciter force variations. Drift and variations of system parameters in the vibration isolation system for astronaut’s exercise platform are analyzed. An active controlled scheme is applied with the goals to reduce the platform displacement and to minimize the force being transmitted to the spacecraft structure. The controller must be robust enough to accommodate the wide variations of system parameters and exciter forces. Computer simulation for the vibration isolation system was performed via MATLAB/Simulink and Trick. The simulation results demonstrate the achievement of force reduction with small platform displacement under wide ranges of variations in system parameters. Keywords—Control, counterweight, isolation, vibration. I. INTRODUCTION T is extremely important for astronauts to have sufficient exercise during space missions [1], [2]. Without a special isolation device, exercise activities inevitably produce excited forces that are transmitted to the spacecraft and may cause operation difficulty. In an effort to minimize the transmitted forces, the use of vibration isolation systems (VIS) has been studied in a microgravity environment [3], [4]. Proportional-integral-derivative (PID) controllers have been used to direct the motion control in various applications [5]-[8]. A one-dimensional (1D) active controlled VIS was developed and published by [9]. As shown in Fig. 1 [9], a Simulink diagram of a 1D VIS for astronaut’s exercise platform uses a discrete PID controller to command a DC motor and a lead screw to push and pull a counterweight which injects actuator force to the system. The platform displacement is the primary controlled variable with the goal to minimize its movement around the initial location. The PID control algorithms calculate a voltage command to drive a DC motor with inductance, L a , resistance, Ra, back electromotive constant, Kb, motor torque constant K m , motor moment of inertia, J l . A saturation function is imbedded in the PID controller so that the voltage command to the DC motor would not exceed motor’s physical limitations. The motor output is restricted by a dead zone dynamic function to accommodate friction loss; motor torque then converted to actuator force through a leadscrew. The actuator force will be added to exciter force and two passive forces: spring force and damping force that drives the motion of the exercise platform. A sensor is used to feed platform displacement back to the PID controller that completes the loop. The connecting structure between exercise platform and spacecraft structure is modeled as a passive control unit which includes a spring and a damper. Therefore, the force transmitted to the spacecraft equates the sum of spring force and damping force. When the motion of the platform is restricted, the transmitted force to the wall of spacecraft is also confined. In this study, uncertainties and variations of system parameters are defined and included in the control loop. Controller gains and other system parameters are tuned under the consideration of the uncertainties and variations. A series of computer simulation for several ranges of uncertainties and variations was conducted. The results show excellent reduction of force being transmitted to the spacecraft structure while maintaining small platform displacement and acceleration. II. PARAMETER DRIFT AND VARIATION “Drift” represents constants and parameters in a dynamic system that change their values over time. In the VIS, the main factor causing drift is the operating temperature. “Variation” stands for the change of input conditions. For example, exciter force changes from one exerciser to another exerciser on the exercise platform, or when a different exercise is being performed on the platform; even the same individual doing the same exercise could change intensity and frequencies over time. A. Drift of Motor Constants Motor parameters are considered to be “constants” when they are operated in an ideal situation. These constants will drift when the environment changes. The largest parameter drift in a DC motor is the armature resistance, R a , as the function of temperature  ௔ ሺሻ =  ௔ ሺሻ ∗ 1 +  ௖௢௡ௗ௨௖௧௢௥ ൫ ௙ − ௜ ൯൧ [10]. As an example, ࢻfor copper is 0.004. S. B. Lin is with the Department of Mechanical Engineering at Prairie View A&M University, Prairie View, Texas 77446 USA (corresponding author, phone: 936-261-9958; e-mail: shlin@pvamu.edu). Simulation with Uncertainties of Active Controlled Vibration Isolation System for Astronaut’s Exercise Platform Shield B. Lin, Ziraguen O. Williams I 471 International Scholarly and Scientific Research & Innovation 15(11) 2021 ISNI:0000000091950263 Open Science Index, Mechanical and Mechatronics Engineering Vol:15, No:11, 2021 publications.waset.org/10012326/pdf