Dae-Oen Lee

Active Vibration Isolation Demonstration System Using the Piezoelectric Unimorph With Mechanically pre-Stressed Substrate

In this article, a pre-stressed piezoelectric actuator made at room temperature using a mechanically pre-stressed substrate is applied as an actuator for an active vibration isolation system. The fabricated piezoelectric actuator, called PUMPS (piezoelectric unimorph with mechanically pre-stressed substrate), is a kind of unimorph actuators in which actuation stroke level is enhanced by displacement amplification mechanism that converts longitudinal piezoelectric in-plane deformation to large bending/pumping motion. Despite its much simpler fabrication process, PUMPS actuation performance is comparable to that of conventional pre-stressed piezoelectric unimorphs due to its special pre-stress level inside piezoelectric layer. In applying PUMPS as an actuator for active vibration control, multilayer stack configuration using three PUMPS actuators was adapted in order to obtain higher actuation force because unimorph actuators are displacement amplified actuators in which displacement amplification is attained with a sacrifice of the actuation force. Preliminary vibration tests were performed to check the performance of PUMPS as actuators for active vibration control in a lab environment. Two feedback control schemes, the positive position feedback and negative velocity feedback, were applied for active vibration control and about 10 dB vibration reduction was achieved near the resonant frequency region. With the promising results obtained in the preliminary vibration test, PUMPS actuators in multilayer stack configuration were used to develop an integrated active vibration isolation demonstration system. The purpose of the developed system is to demonstrate the need for vibration isolation system to remove degrading effects of jitter and improve the performance of optical payloads in satellites such as its observation and image acquisition abilities. The test results with the demonstration system show that severe blur observed in the image taken without the vibration control is reduced visibly in the image taken during the operation of the active vibration isolation system.

Development of Integrated Simulation Tool for Jitter Analysis

Pointing stability of high precision observation satellites must satisfy the stringent requirements to perform at a designed level. As even a small vibrational disturbance can result in severe degradation of the optical performance, the effects of inorbit vibrational environment on the performance of optical payload must be predicted and analyzed in the design phase in order to ensure that the requirements imposed on the payload are fully met. In this paper, an integrated framework for the evaluation of the performance of optical payloads is developed. The developed simulation tool comprises of the reaction wheel induced disturbance model, state space model of a structure in modal form and Cassegrain reflector model. The performance degradation of the optical system due to jitter is expressed by using modulation transfer function (MTF) and image simulation. Moreover, vibration isolator model is also added to show the effectiveness of using a vibration isolator for the elimination of the effects of jitter in the acquisition of an image.

Development of vibration isolation platform for low amplitude vibration

The performance of high precision payloads on board a satellite is extremely sensitive to vibration. Although vibration environment of a satellite on orbit is very gentle compared to the launch environment, even a low amplitude vibration disturbances generated by reaction wheel assembly, cryocoolers, etc may cause serious problems in performing tasks such as capturing high resolution images. The most commonly taken approach to protect sensitive payloads from performance degrading vibration is application of vibration isolator. In this paper, development of vibration isolation platform for low amplitude vibration is discussed. Firstly, single axis vibration isolator is developed by adapting three parameter model using bellows and viscous fluid. The isolation performance of the developed single axis isolator is evaluated by measuring force transmissibility. The measured transmissibility shows that both the low Q-factor (about 2) and the high roll-off rate (about -40 dB/dec) are achieved with the developed isolator. Then, six single axis isolators are combined to form Stewart platform in cubic configuration to provide multi-axis vibration isolation. The isolation performance of the developed multi-axis isolator is evaluated using a simple prototype reaction wheel model in which wheel imbalance is the major source of vibration. The transmitted force without vibration isolator is measured and compared with the transmitted force with vibration isolator. More than 20 dB reduction of the X and Y direction (radial direction of flywheel) disturbance is observed for rotating wheel speed of 100 Hz and higher.

A measurement method for piezoelectric material properties under longitudinal compressive stress?-a compression test method for thin piezoelectric materials

We introduce a new compression test method for piezoelectric materials to investigate changes in piezoelectric properties under the compressive stress condition. Until now, compression tests of piezoelectric materials have been generally conducted using bulky piezoelectric ceramics and pressure block. The conventional method using the pressure block for thin piezoelectric patches, which are used in unimorph or bimorph actuators, is prone to unwanted bending and buckling. In addition, due to the constrained boundaries at both ends, the observed piezoelectric behavior contains boundary effects. In order to avoid these problems, the proposed method employs two guide plates with initial longitudinal tensile stress. By removing the tensile stress after bonding a piezoelectric material between the guide layers, longitudinal compressive stress is induced in the piezoelectric layer. Using the compression test specimens, two important properties, which govern the actuation performance of the piezoelectric material, the piezoelectric strain coefficients and the elastic modulus, are measured to evaluate the effects of applied electric fields and re-poling. The results show that the piezoelectric strain coefficient d31 increases and the elastic modulus decreases when high voltage is applied to PZT5A, and the compression in the longitudinal direction decreases the piezoelectric strain coefficient d31 but does not affect the elastic modulus. We also found that the re-poling of the piezoelectric material increases the elastic modulus, but the piezoelectric strain coefficient d31 is not changed much (slightly increased) by re-poling.

Development of Multi-DOF Active Microvibration Emulator

Recently, some components and payload systems installed in satellites are exposed to various disturbance sources, such as the reaction wheel assembly, the control moment gyro, coolers, and others. Because there is low damping in space, the continuous microvibration causes the degradation of the performance of various payload systems. Therefore, the development of a practical isolation system that shields against microvibration are very important and the author is on the way to developing the microvibration isolation system for the improvement on the performance of the optical payload. In order to develop appropriate microvibration isolation device for a specific payload, it is necessary to understand vibration characteristics of the main disturbance sources; modeling and analysis of disturbance sources including reaction wheel assembly and control moment gyros have been studied by many researchers. However, there are practical difficulties to obtain and perform an experiment with real flight model (FM) reaction wheel assembly and control moment gyros because of expensive price and security reasons. Generally, the disturbance characteristics of a prototype of the reaction wheel assembly or control moment gyros are significantly different from those of FM ones even when the reaction wheel type, size and wheel speed are the same.Therefore, in order to facilitate the microvibration isolation experiment during the satellite development process, this paper proposes a microvibration emulator that could generate the real disturbance spectrums of FMs. Note that the disturbance profiles are quite complex, consisting of several higher harmonics, and also changing for varying operational wheel speeds. The disturbance characteristics of FMs are typically measured in advance. First an analytical model for the RWA is presented and the development procedure for the emulator is also described. The performance of the first prototype emulator is demonstrated.Copyright

Active Vibration Isolation System Using the Piezoelectric Unimorph With Mechanically Pre-Stressed Substrate

In this paper, a pre-stressed piezoelectric unimorph made by a new fabrication method in room temperature, and an active vibration isolation system using the pre-stressed unimorph actuators are introduced. The fabricated piezoelectric unimorph, called PUMPS (piezoelectric unimorph with mechanically pre-stressed substrate), is an actuator in which actuation level is enhanced by displacement amplification mechanism that converts piezoelectric extension and contraction to large bending/pumping motion without sacrificing the actuation force. Preliminary vibration tests were performed to check the performance of PUMPS as actuators for active vibration control in a lab environment. Two feedback control schemes, the positive position feedback (PPF) and negative velocity feedback (NVF), were applied for active vibration control. Using a smart vibration isolation system with improved load capacity obtained by stacking pre-stressed piezoelectric unimorph actuators, about 10dB vibration reduction of the system was achieved near the resonant frequency region. With the preliminary vibration test results showing promising performance of PUMPS actuator in active vibration control, an integrated active vibration isolation system composed of PUMPS actuators is developed. The developed system contains compact analogue circuits and a sensor for PUMPS actuation and control, and power is supplied by Li-Polymer battery which means the system is completely standalone and portable. In addition, an integrated jitter isolation demonstration system was developed to demonstrate the degrading effect of jitter and the effectiveness of the developed integrated active vibration isolation system in improving the performance of optical payloads. Comparison of image qualities taken before and after the operation of vibration control system indicates that effective suppression of vibration disturbances can be achieved using the developed vibration isolation system with PUMPS actuators.Copyright