Closed-loop Measurements Research Articles

Real-Time Operation and Characterization of a High-Performance Time-Based Accelerometer

An accelerometer based on the electrostatic pull-in time of a microstructure is presented in this paper. The device uses a parallel-plate overdamped microstructure, and real-time control operation is performed using a field programmable gate array and high precision digital-to-analog converters. Both open-loop and closed-loop measurements are presented. The low noise is a key feature of this approach, which is limited only by the mechanical–thermal noise of the microstructure used, 2 $\mu \text{g}/\surd $ Hz as shown in the open-loop results (3 $\mu \text{g}/\surd $ Hz in closed-loop operation). The time readout method has extremely high-resolution capabilities. The pull-in time sensitivity can be adjusted up to 1.6 $\mu \text{s}/\mu \text{g}$ , and the electrostatic feedback voltage sensitivity is 61.3 $\text{V}^{2}$ /g. The closed-loop control allows operation of the accelerometer in a much larger range than in open-loop (±500 mg have been achieved) and the linearity is greatly improved ( $\mu \text{g}$ has been measured over 45 h in open loop and 250 $\mu \text{g}$ in closed loop. [2015-0074]

Multivariable State Feedback Control of a 500 000-r/min Self-Bearing Permanent-Magnet Motor

The use of magnetic bearings in electrical drive systems enables very long lifetimes at highest speeds and the operation in high-purity or vacuum environments. The machine prototype presented in this paper overcomes several limitations of previously presented high-speed magnetically levitated electrical drive systems. The design features linear bearing characteristics, which enables applying linear state feedback control without linearization of bearing actuators. A multivariable rotor position control scheme is proposed and implemented on a signal processor. The implemented Kalman filter and the linear state feedback controller proved to perform well in practice, stabilizing the system over the design speed range of the machine with a single set of controller parameters. Closed-loop system transfer function measurements verify the presented system modeling and controller performance. Measurements of the machine spinning at 500 000 r/min verify the functionality of the overall system. To the authors’ knowledge, this is the highest speed achieved by magnetically levitated electrical drive systems so far.

A zero charge-pump mismatch current tracking loop for reference spur reduction in PLLs

The charge-pump (CP) mismatch current is a dominant source of static phase error and reference spur in the nano-meter CMOS PLL implementations due to its worsened channel length modulation effect. This paper presents a charge-pump (CP) mismatch current reduction technique utilizing an adaptive body bias tuning of CP transistors and a zero CP mismatch current tracking PLL architecture for reference spur suppression. A chip prototype of the proposed circuit was implemented in 0.13μm CMOS technology. The frequency synthesizer consumes 8.2mA current from a 1.3V supply voltage and achieves a phase noise of −96.01dBc/Hz @ 1MHz offset from a 2.4GHz RF carrier. The charge-pump measurements using the proposed calibration technique exhibited a mismatch current of less than 0.3μA (0.55%) over the VCO control voltage range of 0.3–1.0V. The closed loop measurements show a minimized static phase error of within ±70ps and a ≃9dB reduction in reference spur level across the PLL output frequency range 2.4–2.5GHz. The presented CP calibration technique compensates for the DC current mismatch and the mismatch due to channel length modulation effect and therefore improves the performance of CP-PLLs in nano-meter CMOS implementations.

Model-Free Controller Tuning Based on DFT Processing: Application to Induction Motor Drives

In this paper, we present a new approach based on discrete Fourier transform (DFT) analysis for controller tuning of the closed-loop system with unknown plant. The DFT analysis is used to process the closed-loop measurements collected online to derive the frequency response of an initial closed-loop system that does not provide a good performance. Based on the closed-loop frequency response data, we propose two methods for tuning PID controller parameters according to some desired closed-loop performance specifications. The proposed approach can be applied online because the closed-loop system does not need to be stopped for data collection. The tuning problem of rotor speed controllers of electric drives, is chosen as an example to experimentally validate our proposed method. Specifically, we are interested here in the control of an induction motor. The availability of high-performance computational and storage facilities greatly simplifies the collection of measured data used for controller tuning. The experimental results presented in this paper demonstrate the efficacy and usefulness of the proposed control design method in many industrial applications.

Combustion Instabilities in a Bifurcating Tube: Open- and Closed-Loop Measurements

Combustion instabilities characterized by large-amplitude self-sustained oscillations are detrimental to gas-turbine and aeroengine systems. To mitigate these unwanted oscillations, a dynamic actuator is generally implemented. In this work, combustion instability in a bifurcating tube with a loudspeaker implemented is investigated. The bifurcating system is different from conventional Rijke-type and T-shaped ones. It can produce nonharmonic “hot” and “cold” oscillating flows, which provides a useful platform to aid experimental research and teaching on combustion instability. To monitor the flow and acoustic fields in the bifurcating branches, two arrays of pressure sensors and an infrared thermal-imaging camera are implemented. When the loudspeaker attached near the open end of the bottom stem is not actuated, the tube is corresponding to an open-loop thermoacoustic system. To gain insights on the distinguishing characteristics of the flow and acoustic fields, two-dimensional numerical simulations and experimental measurement of the open-loop system are conducted first. Comparison is then made between the numerical and experimental results. Good agreement is obtained in terms of mode frequencies, mode shapes, and sound pressure level. Describing function analysis is then conducted to predict the limit-cycle frequency and amplitude. Finally, the loudspeaker is actuated via a multi-input/single-output tuning strategy to minimize the combustion oscillations. It is shown that actuating the loudspeaker results in the combustion system being stabilized via reducing the sound pressure level by approximately 45 dB, even when the fuel flow rate is slightly changed.

A Dynamically Reconfigurable System for Closed-Loop Measurements of Network Traffic

Streaming network traffic measurement and analysis is critical for detecting and preventing any real-time anomalies in the network. The high speeds and complexity of today's networks, coupled with ever evolving threats, necessitate closing of the loop between measurements and their analysis in real time. The ensuing system demands high levels of programmability and processing where streaming measurements adapt to the changing network behavior in a goal-oriented manner. In this work, we exploit the features and requirements of the problem and develop an application-specific FPGA-based closed-loop measurement (CLM) system. We make novel use of fine-grained partial dynamic reconfiguration (PDR) as underlying reprogramming paradigm, performing low-latency just-in-time compiled logic changes in FPGA fabric corresponding to the dynamic measurement requirements. Our innovative dynamically reconfigurable socket offers 3× logic savings over conventional static solutions, while offering much reduced reconfiguration latencies over conventional PDR mechanisms. We integrate multiple sockets in a highly parallel CLM framework and demonstrate its effectiveness in identifying heavy flows in streaming network traffic. The results using an FPGA prototype offer 100 percent detection accuracy while sustaining increasing link speeds.

The effect of the temperature-dependent nonlinearities on the temperature stability of micromechanical resonators

Micromechanical resonators show a discrepancy between the frequency-temperature (f-T) characteristics they have in open-loop and closed-loop measurements, and this discrepancy adversely affects resonator's temperature stability performance. We explain the discrepancy with a model that combines the temperature-dependent quality factor (Q) with the nonlinear amplitude-frequency (A-f) effect; we then experimentally verify the model using two types of double-ended tuning fork resonators. In addition, we present an improved closed-loop system that removes the discrepancy, thus improving the temperature stability.

A Digitally Controlled Switching Regulator With Reduced Conductive EMI Spectra

This paper presents the use of a combined random pulse position modulation (RPPM)/digital pulsewidth modulation (DPWM) digital controller for a buck converter to simultaneously achieve low-conductive electromagnetic interference (EMI) and a fast transient response. An area-efficient RPPM/DPWM controller suitable for advanced system-on-chip integration is proposed and implemented in a field-programmable gate array-based prototype to verify its EMI-suppression capability. The experimental results for a 1.2-V digital buck converter show that the proposed digital switching regulator can switch between RPPM and DPWM modes smoothly and automatically. Closed-loop system measurements demonstrate that, compared to DPWM, up to 17.27 and 14.99 dB power spectra reductions are obtained in the input current and switching node, respectively.

Local and Global Contrast Adaptation in Retinal Ganglion Cells

Retinal ganglion cells react to changes in visual contrast by adjusting their sensitivity and temporal filtering characteristics. This contrast adaptation has primarily been studied under spatially homogeneous stimulation. Yet, ganglion cell receptive fields are often characterized by spatial subfields, providing a substrate for nonlinear spatial processing. This raises the question whether contrast adaptation follows a similar subfield structure or whether it occurs globally over the receptive field even for local stimulation. We therefore recorded ganglion cell activity in isolated salamander retinas while locally changing visual contrast. Ganglion cells showed primarily global adaptation characteristics, with notable exceptions in certain aspects of temporal filtering. Surprisingly, some changes in filtering were most pronounced for locations where contrast did not change. This seemingly paradoxical effect can be explained by a simple computational model, which emphasizes the importance of local nonlinearities in the retina and suggests a reevaluation of previously reported local contrast adaptation.

Performance Optimization of Wind Turbine Rotors with Active Flow Control (Part 1)

This article describes the performance optimization of wind turbine rotors with active flow control. The active Gurney flap concept was tested in the wind tunnel under dynamic AoA variations to simulate unsteady inflow conditions. A high-deflection micro flap was actuated by four digital electric servos with a maximum deflection rate of 360°/sec. A custom code was created to allow dynamic AoA variations of the test wing with simultaneous dynamic force measurements. During the dynamic investigations, various control strategies were tested, starting from standard PID controllers with semi-empirical parameter tuning models to Direct Inverse Controllers with neural network tuning strategies and pure self-learning neural network controllers. The results of the closed-loop measurements using the manually tuned PID controller showed a reduction potential for the dynamic lift loads in the range of 70% as well as a stable controller behavior. The Direct Inverse Controller not only showed a load reduction of 36.8%, but also significant improvement potential with respect to its fine-tuning.

Control of chirped pulse trains: a speedway for free-optimization experiments

Complex phase-only shaping of intense ultrashort laser pulses is applied to generate highly flexible pulse structures with regular envelopes. By incorporating the linear chirp as additional free parameter into the technique of colored pulses, trains of chirped pulses are produced, capable of independent and simultaneous modulation of relative intensity ratio, optical delay, and individual chirp. Such pulses might find applications in multi-parameter scans or closed-loop feedback measurements. For the latter, we demonstrate that with use of these tailored pulse trains, adaptive feedback control experiments quickly converge. They provide near-optimal solutions, already revealing key features of the system under study. Moreover, seeding standard free-optimization routines with these temporary solutions largely accelerates the search for the closest-possible optimum.

Experimental verification for time advancement of negative group delay in RF electronic circuits

An experiment was performed on the signal cancellation loop that utilises a carrier wave (CW) and wideband code division multiple access (WCDMA) signals to show that negative group delays can be used to cancel out positive group delays. According to the closed loop measurements, a 30 dB signal cancellation for a CW signal and a WCDMA 4FA signal were obtained over a 30 MHz bandwidth at the WCDMA downlink band, with a fabricated two-stage planar negative group delay circuit.

Split atmospheric tomography using laser and natural guide stars

Laser guide star (LGS) atmospheric tomography is described in the literature as integrated minimum-variance tomographic wavefront reconstruction from a concatenated wavefront-sensor measurement vector consisting of many high-order, tip/tilt (TT)-removed LGS measurements, supplemented by a few low-order natural guide star (NGS) components essential to estimating the TT and tilt anisoplanatism (TA) modes undetectable by the TT-removed LGS wavefront sensors (WFSs). The practical integration of these NGS WFS measurements into the tomography problem is the main subject of this paper. A split control architecture implementing two separate control loops driven independently by closed-loop LGS and NGS measurements is proposed in this context. Its performance is evaluated in extensive wave optics Monte Carlo simulations for the Thirty Meter Telescope (TMT) LGS multiconjugate adaptive optics (MCAO) system, against the delivered performance of the integrated control architecture. Three iterative algorithms are analyzed for atmospheric tomography in both cases: a previously proposed Fourier domain preconditioned conjugate gradient (FDPCG) algorithm, a simple conjugate gradient (CG) algorithm without preconditioning, and a novel layer-oriented block Gauss-Seidel conjugate gradient algorithm (BGS-CG). Provided that enough iterations are performed, all three algorithms yield essentially identical closed-loop residual RMS wavefront errors for both control architectures, with the caveat that a somewhat smaller number of iterations are required by the CG and BGS-CG algorithms for the split approach. These results demonstrate that the split control approach benefits from (i) a simpler formulation of minimum-variance atmospheric tomography allowing for algorithms with reduced computational complexity and cost (processing requirements), (ii) a simpler, more flexible control of the NGS-controlled modes, and (iii) a reduced coupling between the LGS- and NGS-controlled modes. Computation and memory requirements for all three algorithms are also given for the split control approach for the TMT LGS AO system and appear feasible in relation to the performance specifications of current hardware technology.

Warm-started wavefront reconstruction for adaptive optics

Future extreme adaptive optics (ExAO) systems have been suggested with up to 10(5) sensors and actuators. We analyze the computational speed of iterative reconstruction algorithms for such large systems. We compare a total of 15 different scalable methods, including multigrid, preconditioned conjugate-gradient, and several new variants of these. Simulations on a 128x128 square sensor/actuator geometry using Taylor frozen-flow dynamics are carried out using both open-loop and closed-loop measurements, and algorithms are compared on a basis of the mean squared error and floating-point multiplications required. We also investigate the use of warm starting, where the most recent estimate is used to initialize the iterative scheme. In open-loop estimation or pseudo-open-loop control, warm starting provides a significant computational speedup; almost every algorithm tested converges in one iteration. In a standard closed-loop implementation, using a single iteration per time step, most algorithms give the minimum error even in cold start, and every algorithm gives the minimum error if warm started. The best algorithm is therefore the one with the smallest computational cost per iteration, not necessarily the one with the best quasi-static performance.

Experimental frequency-domain analysis of nonlinear controlled optical storage drives

For nonlinear controlled optical storage drives, a frequency-domain measurement approach is shown to be effective in demonstrating both stability and performance. In an experimental setting based on an industrial optical playback (CD drive) device, the motivation for applying such an approach is illuminated. It is shown that performance can efficiently be assessed using a describing function approach on a model level and using swept-sine measurements on an experimental level. The validity and effectiveness of the approach is extensively shown via closed-loop measurements.

Predicting phase noise in crystal oscillators

In order to predict the phase noise in crystal oscillators an enhanced phase-noise model has been built. With this model, the power spectral densities of phase fluctuations can be computed in different points of the oscillator loop. They are calculated from their correlation functions. The resonator-caused noise as well as the amplifier-caused noise are taken into account and distinguished. To validate this enhanced model, the behavior of a batch of 10 MHz quartz crystal oscillators is observed and analyzed. The tested batch has been chosen in a facility production. Their associated resonators have been selected according to the value of their resonant frequency and their motional resistance. Open-loop and closed-loop measurements are given. The phase noise of the overall oscillator working in closed loop is provided by the usual active method. Theoretical and experimental results are compared and discussed.

Tracking-FCS: Fluorescence correlation spectroscopy of individual particles

We exploit recent advances in single-particle tracking to perform fluorescence correlation spectroscopy on individual fluorescent particles, in contrast to traditional methods that build up statistics over a sequence of many measurements. By rapidly scanning the focus of an excitation laser in a circular pattern, demodulating the measured fluorescence, and feeding these results back to a piezoelectric translation stage, we track the Brownian motion of fluorescent polymer microspheres in aqueous solution in the plane transverse to the laser axis. We discuss the estimation of particle diffusion statistics from closed-loop position measurements, and we present a generalized theory of fluorescence correlation spectroscopy for the case that the motion of a single fluorescent particle is actively tracked by a time-dependent laser intensity. We model the motion of a tracked particle using Ornstein-Uhlenbeck statistics, using a general theory that contains a umber of existing results as specific cases. We find good agreement between our theory and experimental results, and discuss possible future applications of these techniques to passive, single-shot, single-molecule fluorescence measurements with many orders of magnitude in time resolution.

On validating closed-loop behaviour from noisy frequency-response measurements

It is shown how noisy closed-loop frequency-response measurements can be used to obtain pointwise in frequency bounds on the possible difference between the actual closed-loop system and the closed-loop comprising a nominal model of the plant and the stabilising controller. To this end, Vinnicombe's gap metric framework for robustness analysis plays a central role. Indeed, an optimisation problem and corresponding algorithm are proposed for estimating the chordal distance between the frequency responses of the nominal plant model and a plant that is consistent with the closed-loop data and a priori information, when projected onto the Riemann sphere.

Probability density function for frequency response function measurements using periodic signals

Frequency response function (FRF) measurements are often used to characterize linear dynamic systems. Today, uncertainty bounds on FRF measurements still are based on linear approximations, which are valid for sufficiently large input signal-to-noise-ratios (SNR) only. In this paper, exact uncertainty bounds are calculated, which are valid for any input SNR. These bounds are obtained via an analytic expression of the probability density function (pdf) of the FRF measurements. The results are valid for open- and closed-loop measurements, and the theory is illustrated on a real measurement example.

The art of measurement: theory and practice

1. A Philosophical Viewpoint. The Ultimate Measurement. The Mindset of Measuring. Measurement Quality. Attributes of the Measurement Model. Bibliography on Our World Geometry. 2. The Measurement Model. The Optimum Number of Parameters. Various Parameter Attributes. Example: DC Battery Resistance. Example: Control System Loop Gain. Example: Modes of Vibration. Example: GPS Navigation System. Summary. Bibliography on Modeling Examples. 3. Measurements as Random Variables. Prologue on Uncertainty. Introduction to Probability Theory. Probability Theory-Extended. Example: A Noisy Measurement. Example: A Noisy Measurement with Bias. Discrete or Quantized Random Variables. PDFs for Mixed Random Variables. Some Matrices in Probability Theory. Summary. Bibliography on Probability Theory. 4. Estimating Parameter Values. To Measure Is to Estimate. The Concept of Abstract Metric Spaces. Example: Single-Pole Transfer Function. The Least-Squares Estimate. Maximum Likelihood Estimation. Estimation Using Bayes Theorem. Sequential Estimation. The Kalman Filter. General Comments on Estimation Methods. Summary of Estimation Techniques. Bibliography on Estimation Theory. 5. A Few Examples in More Detail. Preview of Four Examples. Measuring the DC Resistance of a Battery. Closed-Loop Control System Measurements. Vibration Modes of Mechanical Structures. GPS Navigation System. Summary of Four Measurement Examples. Appendix A. Review of Basic Mathematical Tools. Preview. The Fourier Transform. The Laplace Transform. Matrix Algebra. Abstract Vector Spaces. Bibliography on Mathematical Tools. Appendix B. Derivation of GPS Navigation Equations. Glossary of Technical Words and Concepts. Index.