Performance Characteristics of Symmetrized In-Flight Tracking Control

A control system to reduce vibation in rolling piston-type compressors in room air conditioners is proposed. To suppress compressor vibration due to torque excitation, a torque control system is applied. This method uses a microcomputer, which makes the motor output torque nearly equal to the load torque variation which is caused by gas compression at any angular position. Theoretical analysis and computer simulation show that the control system, in which the rotational variaion (fluctuation of angular speed or angular acceleration) is caused by the torque difference, is detected during one revolution and is reflected to the motor output torque at the next revolution, due to the cyclic characteristic of the load torque. This shows that repetitive control, can be adopted. Thus, reduction of compressor vibration is possible.

The controlled atmospheric re-entry associated with the precision soft-landing of Reusable Launch Vehicles (RLVs) on Earth is very challenging as it depends on multiple parameters [1]. Over the last decade, the cost-effectiveness of such a technology has been finally demonstrated with the successful recoveries of SpaceX’s Falcon 9 first-stage rocket first [2], then followed by other companies such as the Rocket Lab’s Electron micro-launcher [3]. This breakthrough has been made possible by the development of advanced and robust computational methods able to generate in real time the flight conditions and to command the optimal vehicle's deflections accordingly to achieve a safe pinpoint landing. Indeed, during an Earth atmospheric re-entry, the vehicle is subjected to fast system dynamics changes partly induced by external loads associated with the terrestrial environment (e.g., lift, drag, wind and gusts), but also by the actuation commands to answer the landing constraints satisfaction and the vehicle integrity preservation. All those involve uncertainties and nonlinearities, which lead to vehicle’s instability and therefore give reason why for a highly performant Guidance, Navigation and Control system implementation [4]. More particularly, one of the critical aspects is the design of a robust control strategy capable of counteracting the previously defined disturbances and uncertainties while satisfying the strict accuracy requirements associated with the pinpoint landing [5]. As demonstrated by the current state-of-the-art on control design for launchers [6-7], the classical linear control theory represents a rich heritage with a lot of applications. This choice was motivated by its relative easiness of implementation and the possibility to use gain-scheduling techniques to adapt to nonlinear systems. Nevertheless, these techniques are well-adapted to the control system design of single-input single-output systems, such as for example a reusable rocket using a Thrust Vector Control (TVC) system as the unique actuator. The implementation of multiple-input multiple-output control systems becomes then complex since every channel is addressed in a single-loop fashion. This capability is however required for the future generation of reusable rocket, using also aerodynamic steering based on fins in addition to the TVC system for enhancing control authorities. Moreover, model uncertainties are not accurately considered in the design process, developed only with nominal conditions and stability margin requirements. For all these reasons, it results in an extensive (both in terms of time and cost) Verification and Validation campaign with many iterations and Monte-Carlo analyses to assess the performance and robustness of the control system. To overcome these drawbacks, the H-infinity family of methods, introduced a few years ago [8], provides with a powerful solution for robust control design. It relies on defining the control requirements in the frequency domain in terms of weighting functions and minimising the maximum gain of the resulting weighted system from the exogenous inputs to the outputs to be controlled. Moreover, the control-plant interaction is modelled through a Linear Fractional Transformation (LFT) representing the feedback action. Finally, the structured H-infinity method [9] allows to directly impose a specific control structure – like a Proportional-Integral-Derivative (PID), enabling the re-use of gain-scheduling techniques – and to consider parametric uncertainties for enhanced robustness. This paper studies the synthesis of a robust control system via structured H-infinity for the RLV atmospheric re-entry problem. First, the nonlinear 6-Degree-of-Freedom (6-DoF) RLV re-entry dynamics are simplified into a linear model and then linearised along a reference trajectory to get the nominal LFT of the system, then augmented with parametric uncertainties. The model covers the atmospheric re-entry and vertical landing of a first-stage rocket equipped with a TVC system and steerable planar fins. The controllers are built at different points of the re-entry trajectory, using the structured H-infinity framework through PID-like structures. Weighting functions considering the control objectives and requirements of a realistic RLV re-entry scenario are implemented. A robust stability analysis of the obtained system is performed via classical stability margins and structured singular value. Finally, the controllers are gain-scheduled and validated via Monte-Carlo analyses, using a nonlinear 6-DoF RLV re-entry dynamics simulator equipped with successive convex optimisation guidance. The performance of the resulting guidance and control architecture is compared with the baseline developed in previous works [10], using a TVC system only and classical linear feedback control through gain-scheduled PID controllers. This study lies within the ASCenSIon (Advancing Space Access Capabilities - Reusability and Multiple Satellite Injection) project, an innovative training network funded within H2020.

Modal parameters, i.e., modal frequencies, damping ratios, and mode shapes of a two disk-shaft system are estimated in Multiple Input/Multiple Output (MIMO) scheme. The response at the output degrees of freedom (dof) of the considered structure due to the excitation at the input dofs is estimated from theoretical analysis. The generated theoretical data is used to estimate the Frequency Response Function (FRF) matrix, which relates the output (vibration) at response points with the input at the excitation points in the frequency domain. The corresponding Impulse Response Function (IRF) matrix, which relates response and excitation in the time domain, is obtained by Inverse Fast Fourier Transform (IFFT) of the FRF matrix. The unified Matrix Polynomial Approach (UMPA) is employed in the frequency and time domains with the estimated FRF and IRF matrices, respectively, to estimate the desired modal parameters. The obtained results are compared with results from Theoretical Modal Analysis (TMA).

This paper proposes the theoretical model to analyze the performance degradation of control systems subject to common-source digital upsets. In this paper, a multidimensional hidden Markov model (MDHMM) is used to characterize the correlated upsets and reveals the relationship between complex environments and stochastic random digital upsets injected into the control systems. These digital upsets coming from artificial complex environments are operated on distributed redundant processing controllers. Furthermore, this paper develops the theoretical analysis model for performance degradation of control systems under common-source digital interferences modeled by MDHMM. Theoretical estimations for different redundant configurations are analyzed. Then corresponding simulation verifications for a specific control system are also conducted in details and compared with the theoretical analysis results. These analyses can help to select an optimal redundant design and provide an example for control systems design. This analysis also helps to balance the performance of system, reliability of system, and costs of system design in applications.

Quadrotor is widely used in variable application nowadays. Due to its inherent unstable characteristics, control system to augment the stability is essential for quadrotor operation. To design control system and verify its performance through simulation, accurate dynamic model is required. Quadrotor dynamic model is simply compared with conventional rotorcraft such as helicopter. However, the accurate dynamic model of quadrotor is not easy to develop because of the highly correlated aerodynamic effect of each rotor. In this paper, quadrotor dynamic model is identified from the flight data using frequency domain approach. Flight test of quadrotor is performed in closed loop configuration with stability augmentation system included. Frequency sweep input is applied in each of lateral, longitudinal, yaw and heave axis separately. The bare dynamic model is identified from the flight data of quadrotor responses and thrust measurement through Pulse Width Modulation(PWM) data. The frequency responses of identified model match well with those of flight data, and time responses of identified model for doublet input in each axis are also shown to agree with flight data.

The acquisition of aerodynamic loads on helicopter rotors is fundamental to the study of helicopter performance optimization, structural design, flight control, and other aspects. However, at present, aerodynamic loads on rotors are primarily obtained through theoretical calculations, simulation analysis, and wind tunnel tests, with few reports on flight measurements. This paper proposes a method for obtaining helicopter rotor aerodynamic loads by flapping moment measurements in flight with strain gauge sensors. First, strain gauge sensors are installed at different cross-sectional positions on the rotor blades to measure strain during flight. Then, the strains are incorporated into the blade flapping motion equations to establish the relationship between rotor aerodynamic loads and flapping moment. Finally, the aerodynamic loads on the rotor are calculated by the relationship. This method can provide more accurate load calculation results compared to simulation computations and wind tunnel tests. In this paper, the distribution patterns of rotor aerodynamic loads were investigated, which aligned with theoretical analysis and can offer valuable insights for blade design optimization.

<div class="htmlview paragraph">It is the purpose of this paper to make clear the effect of typical torsional viscous-friction damper on the reduction of the axial, torsional and two directions of lateral vibrations from the analytical and experimental points of view.</div> <div class="htmlview paragraph">At first, a typical type of the torsional viscous-friction damper is fitted to a high-speed diesel engine and the axial, torsional and two directions of lateral vibration displacements at the pulley end are measured in order to investigate the effect of the damper on the reduction of these vibrations in the three dimensional space of the engine shaftings from the experimental point of view.</div> <div class="htmlview paragraph">Next, the characteristics of the axial, torsional and two directions of lateral vibrations of the engine shaftings are investigated by three dimensional analysis of forced vibration by the transfer matrix method, which has been developed by the authors. In the vibration model, the viscous medium part is replaced with a three dimensional model in consideration of the axial, torsional and two directions of lateral vibrations. This method enables the amplitude and phase per order of all the vibrations to be analyzed.</div> <div class="htmlview paragraph">Finally, the effect of the typical torsional viscous-friction damper on the reduction of the vibrations in the three dimensional space is evaluated by comparing the experimental results with the calculated ones. It has been shown that the typical torsional viscous-friction damper has a great effect on the reduction of not only torsional vibration but also axial and two directions of lateral vibrations coupled with torsional vibration. But the reduction effect on the coupled axial vibration is not so much as on the coupled lateral vibrations. Also, it has been clarified that the damper has little effect on the reduction of the uncoupled axial vibration due to its structure.</div>

The three-point controller is one of the simplest controllers used in industry. Most environments with programmable logic controllers and distributed control systems allow such blocks in their design. The three-point controller consists of three switching states, i.e., one input and two outputs, depending on the input signal's positive or negative deviation from the desired value. It allows for an upper and lower limit of deviation where no switching takes place. The three-point step control system extends the step controller with a corrective feedback loop. This paper presents the implementation and parameter evaluation of the three-point step control system. Based on theoretical analysis and simulations, general comments are made on the impact of the controller parameters on the control system's performance. Finally, recommendations for tuning the three-point step controller are formulated.

An experimental and theoretical analysis of a vibration isolation system that uses magnetorheological (MR) fluid-based semi-active isolators is presented. In doing so, a vibration isolator that uses MR fluids is designed, manufactured, and experimentally evaluated. Typically, a Bingham-plastic model, with its accompanying zero speed force step discontinuity, would be used to model an MR isolator. A new nonlinear model, with simple low-speed hysteresis characteristics, is proposed to describe the hysteresis force characteristics of the MR isolator. The damping forces of the MR isolator with different excitation frequency and current input are measured and compared with that resulting from the hysteresis model for the verification of the theoretical analysis. A vibration isolation system with the MR isolator is constructed, and its dynamic equation of motion is derived. A simple skyhook controller is formulated to attenuate the vibration of the system. Controlled performances of the vibration isolation system are experimentally and theoretically evaluated in the frequency and time domains. A key conclusion is that a simple model of the low-speed force vs low-speed velocity hysteresis characteristics is necessary for successful prediction of open- and closed-loop performance in both the time and frequency domains.

The utilization of inverters for the interconnection of distributed generators to the grid requires application of control systems capable of regulating the active and reactive output current, ensuring high power quality levels and achieving relative immunity to grid perturbations. This paper proposes a simple current control scheme, based on the combination of deadbeat and PI control, for a three-phase voltage source inverter connected to the grid via an LCL filter. The control system is analyzed in the frequency domain and an analytical expression for the harmonic content of the output current is derived. Theoretical analysis and computer simulation results validate the stability, fast transient response and robustness of the proposed system to network disturbances and variations in filter parameters.

An enhanced multiple-input multiple-output (MIMO) filtered-x least mean square (FXLMS) algorithm using improved virtual secondary path is proposed as the basis for an active noise control (ANC) system for treating vehicle powertrain noise. This new algorithm is developed to overcome the limitation caused by the frequency-dependent property of the standard FXLMS algorithm and to reduce the variation of convergence speed inherent in multiple-channel cases, in order to improve the overall performance of the control system. In this study, the convergence property of the proposed algorithm is analyzed in the frequency domain in order to yield a better understanding of the physical meaning of the virtual secondary path. In practice, because of the arrangement and sensitivities of the actuators (speakers), transducers (microphones), and physical environment, the magnitude response of the main secondary paths can be very different from each other. This difference will cause difficulty in the overall convergence of the algorithm, which will result in minimal attenuation at some of the channels. The proposed channel equalized (CE) virtual secondary path algorithm is designed to tackle this difficulty by equalizing the mean magnitude level of the main secondary paths and by adjusting other secondary paths correspondingly to keep the coupling effects among the control channels unchanged. The performance of the proposed algorithm is validated by analyzing a two-input two-output active powertrain noise control system.

The blockchain technology and cryptocurrency are now in the centre of the financial market. The raise of the cryptocurrencies represented by Bitcoin have attracted a large group of scholars to analyze the underlying dynamics of their price fluctuations. Intensive debate emerged on the intrinsic features of Bitcoin. In theoretical analysis, we developed the principle of monetary convention to define the concept of monetary consensus, capturing the nature of monetary system, and categorize it into three types: traditional, algorithm and hybrid. Based on the Wavelet Coherence Analysis, we try to analyze Bitcoin price dynamics in both time and frequency domains, comparing Bitcoin with financial assets, economic and financial indexes, and other cryptocurrencies.

A modified Cartesian feedback method called "frequency-offset Cartesian feedback" and based on polyphase difference amplifiers is described that significantly reduces the problems associated with quadrature errors and DC-offsets in classic Cartesian feedback power amplifier control systems.In this method, the reference input and feedback signals are down-converted and compared at a low intermediate frequency (IF) instead of at DC. The polyphase difference amplifiers create a complex control bandwidth centered at this low IF, which is typically offset from DC by 200-1500 kHz. Consequently, the loop gain peak does not overlap DC where voltage offsets, drift, and local oscillator leakage create errors. Moreover, quadrature mismatch errors are significantly attenuated in the control bandwidth. Since the polyphase amplifiers selectively amplify the complex signals characterized by a +90° phase relationship representing positive frequency signals, the control system operates somewhat like single sideband (SSB) modulation. However, the approach still allows the same modulation bandwidth control as classic Cartesian feedback.In this paper, the behavior of the polyphase difference amplifier is described through both the results of simulations, based on a theoretical analysis of their architecture, and experiments. We then describe our first printed circuit board prototype of a frequency-offset Cartesian feedback transmitter and its performance in open and closed loop configuration. This approach should be especially useful in magnetic resonance imaging transmit array systems.

Combined internal model and inferential control of a distillation column via closed-loop identification

These days computer aided simulations are being used to perform hardware in loop (HIL) simulations. This is to ensure the safety and economics, as the real flight test cannot easily be done. Moreover due to the latest controller design techniques, which have further complicated the design implementation, the use of computer is inevitable for control system realization. The HIL simulation in real time validates the theoretical design of attitude control system, the stability of autopilot during the real flight and the consistency between the hardware characteristics and the theoretical analysis. This simulation is carried out in a real time environment and is a major step before the finalization of autopilots and overall control system for the actual flight. This paper presents various steps taken during HIL simulation and the discussion on major test results.