Spacecraft vibration reduction using pulse modulated command input and smart materials

This paper presents a new approach to vibration reduction of flexible spacecraft during attitude control by using pulse width pulse frequency (PWPF) modulator for thruster firing and smart materials for active vibration suppression. The experiment was conducted on the Naval Postgraduate School (NPS)'s Flexible Spacecraft Simulator (FSS), which consists of a central rigid body and an L-shape flexible appendage. A pair of on-off thrusters are used to reorient the FSS. To actively suppress vibrations introduced to the flexible appendage, embedded piezoelectric ceramic patches are used as both sensors and actuators to detect and counter react to the induced vibration. For active vibration suppression using the piezoelectric ceramic patches, Positive Position Feedback (PPF) control targeting at the first two flexible modes of the FSS system is used. Experimental results demonstrate the effectiveness of the control strategy of PWPF modulation for attitude control and PPF for active vibration suppression.

: POSITIVE position feedback (PPF) control was introduced by Goh and Caughey in 1985 to control vibrations of large exible space structures. A PPF controller has several distinguished advantagesas compared to then widely used velocity feedbackcontrol laws. It is insensitive to spillover, where contributions from unmodeled modes affect the control of the modes of interest. As a second-order low-pass lter, a PPF controller rolls off quickly at high frequencies and is well suited to controlling the lower modes of a structure with well separated modes. Because of these advantages, PPF controller along with smart materials, in particular PZT (lead zirconatetitanate) type of piezoelectricmaterial, has been applied to many exible systems to achieve active damping. The design of a PPF controller requires the natural frequency of a structure. In practice, the structuralnatural frequencymay not be known exactlyor itmay varywith time.When the frequencyused in thePPF controller is different from that of the structure, the performance of the PPF control will adversely affected. Despite that PPF control is widely researched in literature, robustness study of PPF control when natural frequency is inexactly known is not reported. This motivates the authors to conduct experimental study of robustness of PPF control in active vibration suppression of a smart exible structure.

This paper presents experimental results of vibration suppression of a flexible structure using a miniaturized digital controller, called modular control patch (MCP). The MCP employs a TI-C30 digital signal processor and was developed by TRW for the United States Air Force for future space vibration control. In this research, the MCP is used to implement different control algorithms for vibration suppression of a cantilevered aluminum beam with piezoceramic sensors and actuators. Positive position feedback (PPF) control, strain rate feedback (SRF) control, and their combinations were implemented. Experiments found that PPF control is most effective for single-mode vibration suppression, and two PPF filters in parallel are most effective for multi-mode vibration suppression. Further experiments demonstrated the robustness of PPF control. PPF can achieve effective vibration suppression when there is a 20% error in modal frequency.

Positive position feedback (PPF) control is widely used in active vibration control offlexible structures. To ensure quick vibration suppression, a large PPF scalar gain is oftenapplied. However, PPF control with a large scalar gain causes initial overshoot, which isundesirable in many situations. In this paper, a fuzzy gain tuner is proposed to tune thegain in the positive position feedback control to reduce the initial overshoot while stillmaintaining quick vibration suppression. The fuzzy system is trained by a batch leastsquares algorithm based on desired input–output data so that the trained fuzzysystem can behave like the training data. A 3.35 m long composite I-beam withpiezoceramic patch sensors and actuators is used to demonstrate the fuzzy PPFcontrol. For comparison purposes, three types of control methods are used in theexperiment: a standard PPF control, a standard PPF control with traditionalfuzzy gain tuning, and a PPF control with batch least squares fuzzy gain tuning.Experimental results clearly demonstrate that PPF control with batch least squares fuzzygain tuning behaves much better than the other two control methods in terms ofsuccessfully reducing the initial overshoot and quickly suppressing vibration.

This paper presents a new method of active vibration control (AVC) of flexible composite cantilever beams. A recursive least mean squares (LMS) based filter is used to perform on-line system identification, updating a positive position feedback (PPF) controller in real-time. With conventional time-invariant PPF control the damping ratio and natural frequency are selected based on the structural response, with the feedback path taking the form of a second-order filter; therefore having performed an off-line system identification of the plant dynamics, this controller is relatively straightforward to implement. Some structures undergo parameter changes over time and actuation in the control sequence can contribute to this change in the system dynamics. PPF control is optimized when an accurate estimation of the natural frequency is available. It is demonstrated that the recursive LMS algorithm can be used to provide a continual on-line estimation of the plant dynamics, for robust control of a slender glass fiber beam. The result is an increase in vibration attenuation, which is robust to variations in the beam properties. It is shown that this controller delivers a high performance settling time when compared with the standard PPF controller for free vibration in the first mode. Robustness of the technique is further demonstrated by altering the fundamental frequency of the beam, via the addition of a lumped mass. The adaptive controller re-tunes itself to this parameter variation, ensuring a high level of vibration suppression is upheld.

This paper is concerned with the implementation of the active vibration suppression controller using analog circuit and microprocessor. The target active vibration controller is the positive position feedback(PPF) controller since it provides a simple algorithm suitable for both analog circuit and digital controllers. In this study, the analog PPF controller is realized using an operational amplifier and the digital PPF controller is realized using a low-cost micro-controller. The circuit diagrams are explained in detail. We then discuss the advantages and disadvantages of both methods from the view of practical implementation. Experimental results show that both implementation methods can be effectively used for the active vibration control but need to be chosen based on the mission objective.

In this study, the Genetic Algorithm (GA) was used to search the 4 modes filter frequencies to be applied to the multiple Positive Position Feedback (PPF) controller. The PPF technique, which makes use of generalized displacement measurements to maximize damping in a specific mode effectively and accomplish vibration suppression was employed as control scheme. However it is necessary to tune the filter frequency of the controller to the structural resonance prior to compensator design. Another disadvantage in the multi mode PPF control is that the lower mode natural frequency is shifted to the low-ward as the effect of the higher mode control by PPF controller. To minimize the effect of frequency shift and search the optimal filter frequencies for the multiple modes PPF controller, the GA was applied. The generally used fitness function of the GA in low modes PPF control, the square sum of sensor voltage output hardly distinguishes the definite difference in higher modes. Instead of it, in this study, the GA with the fitness function as the power spectrum information was performed. As a result of GA algorithm with this fitness function, the optimal filter frequencies of higher modes were found successfully.

This paper presents performance comparison of a novel hybrid control algorithm and a Positive Position Feedback (PPF) control law for vibration suppression with low energy consumption. The proposed hybrid control method combines a bang-bang active controller with a PPF control law and results in a significant reduction of actuation energy requirement due to its ability to switch the actuation voltage source between external power supply and PPF feedback controller. Experimental results show that a single mode vibration control system using this proposed hybrid controller consumes 46% less power compared to the traditional PPF control system to give the same settling time. Experiments also reveal that substantial nonlinearity exists in the piezoelectric constant of the transducer, especially under high voltage actuation. The voltage dependent behavior of electromechanical coupling coefficient is identified empirically and represented by a curve fit expression. A real time adaptive control algorithm is developed to account for the voltage dependent behavior of the coupling coefficient and the simulation results are validated experimentally.

A Positive Position Feedback controller for vibration control of wind turbines

A Compact Piezoelectric Stack Actuator and Its Simulation in Vibration Control

Space Qualification is a vital component of successful spacecraft design. The entire scope of the space qualification process is extremely large. This thesis will focus primarily on the Attitude Control System and Space Environment model, with the intent of furthering the verification of the design for the Joint Australian Engineering Satellite (JAESAT). The Attitude Control System proposed for use aboard JAESAT is a combination of permanent magnets with hysteresis rod damping. This system has proved effective on many previous satellite missions, however analysis must be undertaken to optimise the control provided for the individual design proposed. The Space Environment will affect the operation of many of the subsystems aboard JAESAT. Knowledge of the magnitude of such affects is essential to prevent complications through improved design. To do this requires an accurate model of the Space Environment local to JAESAT. Several models for different environmental characteristics are presented as part of this dissertation. Further detailed modelling was performed using the Space Environment Information System, an online software package provided by the European Space Agency. Some of the many results that have been obtained from the Space Environment Information System are presented in graphical format. A detailed model of the Attitude Control System was constructed using Matlab. The environmental disturbance torques were calculated using the environmental conditions previously modelled. Several simulations were conducted yielding mixed results. Generally, it was found that a permanent magnet strength of 14 A.m2 and spin rate of 0.4 revolutions per minute is sufficient to control JAESAT to within 1° of the magnetic field vector. Numerical simulation of the hysteresis rod damping however, has been shown to be ineffective and additional testing is required to successfully implement the hysteresis effects in the system simulation. An analysis of the model revealed that there were minimal sources of error present in the simulation process. The only significant source of error was due to the numerical integration of the equations associated with the satellite motion. Another possible source of model inaccuracy was due to the variation between the numerically simulated space environment and the actual space environment. This dissimilarity however, will only result in small changes of satellite orientation and will not have any effect on the satellite attitude. In the future, the Space Environment and Attitude Control System model that was constructed should be updated to reflect any changes in satellite design and orbit parameters. Simulations of the final Attitude Control System hardware should also be conducted in order to show that adequate attitude control is maintained throughout a period of several orbits. The hysteresis rods should be tested in order to determine their damping properties, and can then be incorporated in the satellite attitude simulations. Based upon numerous simulation results and research, it was concluded that the Attitude Control System that has been proposed was sufficient to provide the required level of attitude control for JAESAT. The successful verification of the Attitude Control System operation implies that the system is Space Qualified. Exact specifications for the Attitude Control System hardware are dependant upon the satellite orbit and design requirements. The model of the space environment is also sufficiently accurate to use in the verification of other satellite systems.

In this article, active vibration control of a micro-actuator for hard disk drives using self-sensing actuation is presented. The micro-actuators used in this article are a slider type and a suspension type. The self-sensing performance of the installing position is compared and evaluated. An assembly including these micro-actuators may not be able to sense some vibration of suspension modes. Feedback control results of direct velocity feedback and positive position feedback are presented. The feasibility of these self-sensing approaches as a vibration suppression control on the micro-actuator was verified. Using the slider type micro-actuator, the sway mode spike signal in power spectrum density was suppressed by 76% using the direct velocity feedback control, and by 60% using the positive position feedback control. Using the suspension type micro-actuator, the suspension second bending mode spike signal in power spectrum density was reduced by 74% using the direct velocity feedback control, and by 96% using the positive position feedback control. The control scheme using self-sensing signals can partially dampen the suspension modes.

T HIS Note proposes a new strategy for the design of the attitude determination and control system (ADCS) of reconfigurable spacecraft, with a specific application example to so-called nanosatellites. These spacecraft adhere to the CubeSat standard [1], and the natural discretization of the overall volume into cube units makes them a popular platform for research into spacecraft reconfiguration [2]. The proposed design strategy, called location-scheduled control (LSC), seeks to reduce the development time required for the ADCS of a reconfigurable spacecraft conducting scientific research. In traditional ADCS design, mission science requirements levy pointing requirements onto the ADCS, which drive sensor and actuator selection. Once the ADCS hardware has been chosen, an appropriate control algorithm is selected and tested. In contrast, the location-scheduled approach to ADCS design is a two-phase process. In the first phase, a general purpose self-contained ADCS hardware unit is designed by selecting a suite of sensors and actuators that are appropriate for a class of science missions. Controller gains and ADCS location are then optimized in simulation for a number of generic science payloads, such as those depicted in Fig. 1, in which payload cube (P) layouts dictate possible locations for the ADCS (cubes A or B). In the second phase of the process, one of the previously tested configurations is selected based on the needs of the current science mission. Some amount offine gain tuningwill inevitably be required, but if a member of the catalog of spacecraft configurations satisfies the needs of the science payload, the developer need not return to the first phase of the design process. Although the LSC approach is applicable for many reconfigurable spacecraft, thiswork considers a nanosatellite ADCSdesign problem to demonstrate this methodology. The objective of the design problem is to find the optimal combination of ADCS unit location and control law gainvalues for a nanosatellite performing a detumble maneuver in which the spacecraft has high initial angular velocity and arbitrary initial orientation. Using the CubeSat standard, a 10 10 10 cm cube is designated as one unit of volume (abbreviated 1U), and complete spacecraft are created from these cubes. Traditional CubeSats are typically no larger than 3U in volume, and they have largely been passively controlled via permanent magnets [3,4], gravity gradient booms, or aerodynamic stabilization. This Note focuses on spacecraft that are 6U in volume (as in Fig. 1), with overall dimensions of 10 20 30 cm. This form factor creates greater volume for both complex science payloads and attitude control hardware and is currently under development at NASAAmes Research Center. A nonlinear control law introduced in [5] is used to control the nanosatellite using actuators for which the control authority is restricted by a known upper bound.

Modulated–demodulated control: Q control of an AFM microcantilever

This paper is concerned with the determination of multiple positive position feedback controller gains using genetic algorithms. The genetic algorithms have proven its effectiveness in searching optical design parameters without falling into local minima thus rendering globally optimal solutions. The use of genetic algorithms to the controller design have been investigated by many researchers but limited to the PID control, and optimal controls. In this paper, the genetic algorithm is applied to the positive position feedback control which has been successfully used for the active vibration suppression of smart structures. The advantage of the positive position feedback controller is that it can be easily implemented in analog circuits and each mode can be tackled independently. However, the disadvantage is that the natural vibration characteristics should be accurately determined a priori either theoretically or experimentally. In this paper, we develop a new algorithm for the determination of controller gains based on the genetic algorithm. Hence, the traditional positive position feedback controller can be used in adaptive fashion. Experimental results will prove its effectiveness.