# A High-Precision Active Vibration Isolation Control System: Experimental Study
## 1 Introduction
Although the VCM provides a smaller force than other actuators, the VCM is preferable for controlling the proposed AVIS in this study [14]. Finally, a control algorithm regulates these actuators using feedback from the sensors such that the top platform and its carried devices are isolated from the external vibrations. However, the high degree-of-freedom (DoF) nature of AVISs creates a great control challenge. Additionally, to achieve the stabilization objective in six DoFs, an AVIS is usually an over-actuated system, as the number of actuators must be greater than the controlled DoF. This, in turn, seriously contributed to the problem of interactions between individual movements in the 6-DoF AVIS. Nevertheless, the control system design requires not only decoupling the interactions but also satisfying the vibration isolation performance of the system. Modal decomposition is a common method that is applied for this. In [6], the authors proposed a novel design of an AVIS, using Halbach magnet array VCM actuators. The motion sensor used in this research is the acceleration sensor. The modal decomposition technique was applied to decouple the coupled degrees in the system. The designed controller consisted of six single-input-single-output (SISO) proportional-integral-derivative (PID) controllers. The paper demonstrated the resulting isolation performance of the AVIS in the \(z\)-direction with three different payloads. Another study [15] proposed a robust control via frequency-shaped sliding mode control by employing modal decomposition to transfer a multi-input-multi-output (MIMO) control problem into individual SISO control problems. However, the results in both studies were discussed only in the \(z\)-direction; therefore, the interaction isolation performances were not clearly shown in their simulation and experiment results. In [12], the modal decomposition technique was deployed to achieve the decoupling performance where the interactions in the system structure were regarded as zero. Then, six composite nonlinear feedback (CNF) controllers were designed for each input-output pair of the 6-DoF AVIS. The actuators used in that study are VCMs, and the motion sensors are two different types of acceleration sensors. The CNF controllers resulted in a quicker response, better transmissibility in the case of no payload, and smaller overshoot compared with the PID-controlled system, but the interaction isolation performances of the controller were still not investigated. Another method is non-interacting control with a linear matrix inequality (LMI) approach, as shown in [16]. An integral-type servosystem was designed with the feedback matrix obtained via the LMI approach so that the performance of the non-interacting control was satisfied. Furthermore, in [17], a robust \(H_{\infty}\) framework-based controller was proposed and the interactions were treated as disturbances of the AVIS. The simulation results in both studies confirmed the effectiveness of the controller in both the response of the system and the interaction isolation, but no experiments were conducted. In addition to that, a controller with a notch filter was designed in [13], but the full 6-DoF performance was not evaluated yet. In this study, a robust decoupling controller for the 6-DoF AVIS is proposed and evaluated. The control objective is to suppress the external vibrations, isolate the interactions, and, hence, stabilize the carrying system. To meet the stated objectives, a feedforward compensator is proposed to suppress the effect of interactions; then, the mixed-sensitivity \(H_{\infty}\) control framework is applied to design a feedback controller. The structure of the paper is as follows: the configuration of the AVIS and its mathematical model are presented in Section 2. The actuators used in this study are VCMs, and the motion sensor is the geophone. In Section 3, the decoupling process and the controller design are conducted. Continuously, in Section 4, the experimental results are used to verify the effectiveness of the proposed controller.