# A High-Precision Active Vibration Isolation Control System: Experimental Study
## 1 Introduction  
In recent decades, technology has made a great jump; hence, the demand for the accuracy of devices is much more intense. For instance, electron microscopy is applied to decipher nanoscale elementary processes by using very small electron beams, some of them around tens of nanometers in size. Therefore, even a modest vibration such as the ground vibration from other working devices can lead to blurring in the image and reduction in the effective spatial resolution [1]. Another significant disturbance is the vibration coming from the running of the devices themselves or from their operators. As a result, the higher the performance of these devices, the higher the restrictions applied to vibrations. However, vibrations are unavoidable, especially in the mechanical realm, as mentioned in the example above. Therefore, without vibration isolation, the objective of such electron microscopy, or in general high-precision devices, cannot be achieved [2; 3]. Passive vibration isolation systems (PVISs) and active vibration isolation systems (AVISs) are the two general methods applied to deal with the effect of vibrations. For a PVIS, the three main components are the top platform, the base platform, and the vibration isolation structure. The top platform carries high-precision devices and provides a working environment for them. The base platform is the supporting platform, usually connected to the ground. The vibration isolation structure usually consists of mechanical elements such as springs and dampers inserted between the two platforms, connecting the two platforms. The vibration transmitted from the ground and the amplified oscillation within the system are eliminated by the springs and dampers, hence stabilizing the top platform. PVISs have the advantage of a straightforward structure, with a very low cost, and no energy consumption. However, PVISs also have disadvantages such as uncontrolled vibration isolation performance, and limited working frequency--they only achieve good performance in the high-frequency zone. Furthermore, in the zone of resonant frequency, the vibrations of the system are amplified, so only a small force can make the system oscillate with the maximum amplitude [4]. Hence, there is a need to develop a new vibration isolation system to overcome the disadvantage of the PVISs--that is the AVIS. The AVIS has the advantage of securing vibration suppression performance in a large frequency range. To achieve this objective, the system utilizes sensors to collect the motion, actuators to regulate the motion, and a properly designed control algorithm. The motion sensor is usually classified into three types: displacement, velocity, and acceleration. Displacement and acceleration sensors are deployed in many studies such as in [5; 6; 7]. The acceleration sensor is easy to install without much emphasis on sensor placement and it has a high-frequency response. On the other hand, the displacement sensor has the upper hand on non-contact measurement [8] but suffers from high cost and susceptibility to environmental influences [9]. The velocity sensor has the advantages of being easy to install, low in cost, and high in sensitivity. Therefore, in this study, geophones are used to collect the motion of the system. Another major component in an AVIS is the actuator. A piezoelectric stack actuator [10] has the advantage of high force generation, quick response, and the least moving parts, but the disadvantages of high cost and being easily affected by the environment, so drift may appear. A pneumatic actuator [8] has a fast response but the generated force is small and the setup is often complex. A giant magnetostrictive actuator [10; 11] has the upper hand in high output force and high displacement in high frequency; however, the high power loss and complex setup are the hold-backs of this actuator. The most popular actuator in AVISs is the linear voice coil motor (VCM) [12; 13]. The VCM has the advantage of simple design, low cost, and good linearity.