Model-based Compensation Research Articles

Optimized EPI for fMRI studies of the orbitofrontal cortex: compensation of susceptibility-induced gradients in the readout direction

Object Most functional magnetic resonance imaging (fMRI) studies record the blood oxygen leveldependent (BOLD) signal using gradient-echo echo-planar imaging (GE EPI). EPI can suffer from substantial BOLD sensitivity loss caused by magnetic field inhomogeneities. Here, BOLD sensitivity losses due to susceptibility- induced gradients in the readout (RO) direction are characterized and a compensation approach is developed.Materials and Methods Based on a theory describing the dropout mechanism, an EPI sequence was optimized for maximal BOLD sensitivity in the orbitofrontal cortex (OFC) using a specific combination of an increased spatial resolution in the RO direction and a reduced echo time. Using measured BOLD sensitivity maps and a breath hold experiment, the model and compensation approach were tested.Results Using typical fMRI EPI parameters, susceptibility-induced gradients in the RO direction caused dropouts in the OFC and the inferior temporal lobe. Optimizing the echo time and spatial resolution effectively reduced the dropout as predicted by the theory.Conclusion The model-based compensation approach effectively reduces BOLD sensitivity losses due to susceptibility-induced gradients in the RO direction. It retains the high temporal resolution of single-shot EPI and can be readily combined with methods for the compensation of susceptibility-induced field gradients in the phase-encoding and through-plane direction.

Geometrical Error Modeling and Compensation Using Neural Networks

This paper describes an approach based on neural networks (NNs) for geometrical error modeling and compensation for precision motion systems. A laser interferometer is used to obtain the systematic error measurements of the geometrical errors, based on which an error model may be constructed and, consequently, a model-based compensation may be incorporated in the motion-control system. NNs are used to approximate the components of geometrical errors, thus dispensing with the conventional lookup table. Apart from serving as a more adequate model due to its inherent nonlinear characteristics, the use of NNs also results in less memory requirements to implement the error compensation for a specified precision compared to the use of lookup table. The adequacy and clear benefits of the proposed approach are illustrated via applications to various configurations of precision-positioning stages, including a single-axis, a gantry, and a complete XY stage

Correction for scatter and septal penetration using convolution subtraction methods and model-based compensation in 123I brain SPECT imaging—a Monte Carlo study

Scatter and septal penetration deteriorate contrast and quantitative accuracy in single photon emission computed tomography (SPECT). In this study four different correction techniques for scatter and septal penetration are evaluated for 123I brain SPECT. One of the methods is a form of model-based compensation which uses the effective source scatter estimation (ESSE) for modelling scatter, and collimator–detector response (CDR) including both geometric and penetration components. The other methods, which operate on the 2D projection images, are convolution scatter subtraction (CSS) and two versions of transmission dependent convolution subtraction (TDCS), one of them proposed by us. This method uses CSS for correction for septal penetration, with a separate kernel, and TDCS for scatter correction. The corrections are evaluated for a dopamine transporter (DAT) study and a study of the regional cerebral blood flow (rCBF), performed with 123I. The images are produced using a recently developed Monte Carlo collimator routine added to the program SIMIND which can include interactions in the collimator. The results show that the method included in the iterative reconstruction is preferable to the other methods and that the new TDCS version gives better results compared with the other 2D methods.

A dynamic parameter compensation method for noisy speech recognition

Model-based compensation techniques have been successfully used for speech recognition in noisy environments. Popular model-based compensation methods such as the Log-Normal PMC and Log-Add PMC generally use approximate compensation for dynamic parameters. Hence their recognition accuracy is degraded at low and very low signal-to-noise ratios. In this paper we use time derivatives of static features to derive a dynamic parameter compensation method (DPCM). In this method, we assume the static features independent of the dynamic features of speech and noise. This assumption helps simplify the procedures of the compensation of delta and delta–delta parameters. The new compensated dynamic model together with any known compensated static model form a new corrupted speech recognition model. Experimental results show that the recognition model using this DPCM scheme gives recognition accuracy better than the original model compensation method for different additive noises at the expense of slight increase in computational complexity.

Model-based compensation for quantitative 123I brain SPECT imaging

Previously we have developed a model-based method that can accurately estimate downscatter contamination from high-energy photons in 123I imaging. In this work we combined the model-based method with iterative reconstruction-based compensations for other image-degrading factors such as attenuation, scatter, the collimator-detector response function (CDRF) and partial volume effects to form a comprehensive method for performing quantitative 123I SPECT image reconstruction. In the model-based downscatter estimation method, photon scatter inside the object was modelled using the effective source scatter estimation (ESSE) technique, including contributions from all the photon emissions. The CDRFs, including the penetration and scatter components due to the high-energy 123I photons, were estimated using Monte Carlo (MC) simulations of point sources in air at various distances from the face of the collimator. The downscatter contamination was then compensated for during the iterative reconstruction by adding the estimated results to the projection steps. The model-based downscatter compensation (MBDC) was evaluated using MC simulated and experimentally acquired projection data. From the MC simulation, we found about 39% of the total counts in the energy window of 123I were attributed to the downscatter contamination, which reduced image contrast and caused a 1.5% to 10% overestimation of activities in various brain structures. Model-based estimates of the downscatter contamination were in good agreement with the simulated data. Compensation using MBDC removed the contamination and improved the image contrast and quantitative accuracy to that of the images obtained from 159 keV photons. The errors in absolute quantitation were reduced to within ±3.5%. The striatal specific binding potential calculated based on the activity ratio to the background was also improved after MBDC. The errors were reduced from −4.5% to −10.93% without compensation to −0.55% to 4.87% after compensation. The model-based method provided accurate downscatter estimation and, when combined with iterative reconstruction-based compensations, accurate quantitation was obtained with minimal loss of precision.

DISTURBANCE REJECTION WITH LTI INTERNAL MODELS FOR PASSIVE NONLINEAR SYSTEMS

First, the internal model principle is applied for plants that are passive nonlinear systems, to solve a disturbance rejection problem (without reference signal), where the exogeneous signal is a finite superposition of sine waves of arbitrary known frequencies. The proposed controller assures that both the state of the plant and the error signal converge to zero.Based on the above result, we solve the tracking and disturbance rejection problem for a class of fully actuated passive mechanical systems. Here, we combine our first result with the ideas behind the Slotine-Li controller. We assume that the reference signal r and its first two derivatives are available to the controller. The internal model-based compensator that we propose causes the tracking error to converge to zero.

スティックスリップ補償と連続軌跡追従制御に基づく超音波アクチュエータ駆動精密ステージの一制御法

This paper presents a friction model-based control of an ultra-precision stage that is equipped with the non-resonant ultrasonic actuator (SPIDER). The SPIDER is expected as an actuator suitable for stages for semiconductor manufacturing with the electron beam which is light source in next generation.At first, the friction characteristics as a function of stage position are experimentally identified. Next, measuring the time variation of the static friction characteristics, the on-line identification method for the control system is proposed. And then, a friction model-based compensation for the PI control system is discussed. In addition to the PI control system, the continuous-path tracking control system with the compensation is designed. Finally, the model-based control method is verified through the positioning experiments.

Position control of a dual-servo stage featuring an electrorheological fluid clutch and piezostack actuator

In this work, a novel type of dual-servo stage system is proposed in order to achieve accurate position control. A bidirectional electrorheological fluid clutch is adopted for a coarse-positioning stage, and a multi-stack piezoceramic actuator associated with a displacement amplifier for a fine-positioning stage. After deriving the dynamic model of the coarse-motion stage, a sliding-mode controller associated with a friction compensator is formulated to achieve robust position control performance. In addition, a model-based feedforward compensator incorporating a conventional proportional-integral-derivative controller is designed to compensate the hysteresis non-linearity of the fine-positioning stage. These controllers are experimentally realized in a decentralized strategy and control responses such as step input response are evaluated to verify the effectiveness of the proposed dual-servo positioning system.

Model Identification of a Micro Air Vehicle in Loitering Flight Based on Attitude Performance Evaluation

This paper presents a model identification of a micro air vehicle in loitering flight, based on the input-output data collected from flight experiments on a homemade 1-m-sized aircraft. A miniature flight-control system, which consists of the onboard and the ground sections, is equipped with a multichannel data logger associated with the data acquisition software. Modeling and performance analysis are carried out, based on the flight-test data using a system identification technique. Two fourth-order autoregressive with exogenous input (ARX) models are identified to present the attitude characteristics of the longitudinal (pitch) and the lateral (roll) control channels, respectively. The validity of the identified model is verified by both time-domain model prediction and frequency-domain spectral analysis. Based on the proposed ARX models, two compensators are further designed using a frequency-domain method, and then added to the closed-loop control systems to improve the transient performance of the pitch- and roll-control channels. Simulations and experiments demonstrate that the flight performance obtained by the proposed ARX model-based compensation control can be improved.

Validation and Evaluation of Model-Based Crosstalk Compensation Method in Simultaneous<tex>$^99rm m$</tex>Tc Stress and<tex>$^201$</tex>Tl Rest Myocardial Perfusion SPECT

Simultaneous acquisition of /sup 99m/Tc stress and /sup 201/Tl rest myocardial perfusion SPECT has several potential advantages, but the image quality is degraded by crosstalk between the Tc and Tl data. We have previously developed a crosstalk model that includes estimates of the downscatter and Pb X-ray for use in crosstalk compensation. In this work, we validated the model by comparing the crosstalk from /sup 99m/Tc to the Tl window calculated using a combination of the SimSET-MCNP Monte Carlo simulation codes. We also evaluated the model-based crosstalk compensation method using both simulated data from the 3-D MCAT phantom and experimental data from a physical phantom with a myocardial defect. In these studies, the Tl distributions were reconstructed from crosstalk contaminated data without crosstalk compensation, with compensation using the model-based crosstalk estimate, and with compensation using the known true crosstalk, and were compared with the Tl distribution reconstructed from uncontaminated Tl data. Results show that the model gave good estimates of both the downscatter photons and Pb X-rays in the simultaneous dual-isotopes myocardial perfusion SPECT. The model-based compensation method provided image quality that was significantly improved as compared to no compensation and was very close to that from the separate acquisition.

Experimental modeling and compensation of pivot nonlinearity in hard disk drives

This paper considers a new model-based friction compensator for servo control of hard disk drives (HDDs). Using measured and simulated responses in both time and frequency domains as a guide, an HDD designer can iteratively improve the friction model of the HDD system and verify the correctness of the HDD measurement through this new model. By feeding back the model-based estimated friction forces into the voice coil motor (VCM), the frequency response between the read/write head position and the VCM current can be linearized. This allows the cancellation scheme to be developed and implemented through model-based compensation. Experiments have confirmed that the cancellation scheme is indeed effective in the frequency range of 0-300 Hz. Furthermore, by using a sequence of square waves as references for track-to-track seeking, the overshooting in the time responses can be compared between servo controls with and without compensators for friction.

Stable Adaptive Fuzzy Control with TSK Fuzzy Friction Estimation for Linear Drive Systems

This paper considers the control of a linear drive system with friction and disturbance compensation. A stable adaptive controller integrated with fuzzy model-based friction estimation and switching-based disturbance compensation is proposed via Lyapunov stability theory. A TSK fuzzy model with local linear friction models is suggested for real-time estimation of its consequent local parameters. The parameters update law is derived based on linear parameterization. In order to compensate for the effects resulting from estimation error and disturbance, a robust switching law is incorporated in the overall stable adaptive control system. Extensive computer simulation results show that the proposed stable adaptive fuzzy control system has very good performances, and is potential for precision positioning and trajectory tracking control of linear drive systems.

GMDH‐based autonomous modeling and compensation for nonlinear friction in a table drive system

AbstractThis paper presents a novel mathematical model‐based compensation algorithm for the nonlinear friction in table drive systems using Group Method of Data Handling (GMDH). In the proposed compensation, the nonlinear friction can be autonomously modeled as a polynomial expression of appropriate control state variables according to the process of GMDH and, as a result, the complicated structural modeling and its parameterization, indispensable in conventional model‐based strategies, can be completely eliminated. In addition, since the proposed GMDH‐based model can easily achieve the generalization ability for table drive condition, the robust compensation for friction can be attained against the change of drive conditions. The proposed algorithm has been verified by experiments using a table drive system of actual machine tools and proved the significant performance improvement in the trajectory control with velocity reversal motion. © 2001 Scripta Technica, Electr Eng Jpn, 136(3): 58–66, 2001

Nonlinear control of an underwater vehicle/manipulator with composite dynamics

This paper deals with the problem of control design of an underwater vehicle/manipulator system composed of a free navigating platform equipped with a robot manipulator. This composite system is driven by actuators and sensors having substantially different bandwidth characteristics due to their nature. Two control laws are proposed. The first is a simplification of the computed torque control law which only requires partial compensation for the slow-subsystem. Feedback compensation is only-needed to overcome the coupling effects from the arm to the basis. The second aims at replacing this partial compensation by a robust nonlinear control that does not depend on the model parameters. The closed-loop performance of this controller is close to that of the model-based compensation. Both control laws are shown to be closed-loop stable in the sense of the perturbation theory. A comparative study between a linear PD controller, a partial model-based compensation, and the nonlinear robust feedback is presented.

Simultaneous technetium-99m/thallium-201 SPECT imaging with model-based compensation for cross-contaminating effects

Simultaneous acquisition of dual-isotope SPECT data offers a number of advantages over separately acquired data; however, simultaneous acquisition can result in cross-contamination between isotopes. In this work we propose and evaluate two frameworks for iterative model-based compensation of cross-contamination in dual-isotope SPECT. The methods were applied to cardiac imaging with technetium-99m-sestamibi and thallium-201, and they were compared with a subtraction-based compensation method using a cross-talk estimate obtained from an auxiliary energy window. Monte Carlo simulations were performed to carefully study aspects of bias and noise for the methods, and a torso phantom with cardiac insert was used to evaluate the performance of the methods for experimentally acquired data. The cross-talk compensation methods substantially improved lesion contrast and significantly reduced quantitative errors for simultaneously acquired data. Thallium image normalized mean square error (NMSE) was reduced from 0.522 without cross-talk compensation to as low as 0.052 with model-based cross-talk compensation. This is compared with a NMSE of 0.091 for the subtraction-based compensation method. The application of a preliminary model for cross-talk arising from lead fluorescence x-rays and collimator scatter gave promising results, and the future development of a more accurate model for collimator interactions would probably benefit simultaneous Tc/Tl imaging. Model-based compensation methods provide feasible cross-talk compensation in clinically acceptable times, and they may ultimately make simultaneous dual-isotope protocols an effective alternative for many imaging procedures.

A survey of robot interaction control schemes with experimental comparison

A great many control schemes for a robot manipulator interacting with the environment have been developed in the literature. This paper is aimed at presenting a survey of robot interaction control schemes for a manipulator, the end effector of which comes in contact with a compliant surface. A salient feature of the work is the implementation of the schemes on an industrial robot with open control architecture equipped with a wrist force sensor. Two classes of control strategies are considered, namely, those based on static model-based compensation and those based on dynamic model-based compensation. The former provide a good steady-state behavior, while the latter enhance the behavior during the transient. The performance of the various schemes is compared in the light of disturbance rejection, and a thorough analysis is developed by means of a number of case studies.

Simultaneous technetium-99m/thallium-201 SPECT imaging with model-based compensation for cross-contaminating effects

Simultaneous technetium-99m/thallium-201 SPECT imaging with model-based compensation for cross-contaminating effects

Predictive model-based compensation schemes for robust speech recognition

For practical applications speech recognition systems need to be insensitive to differences between training and test acoustic conditions. Differences in the acoustic environment may result from various sources, such as ambient background noise, channel variations and speaker stress. These differences can dramatically degrade the performance of a speech recognition system. A wide range of techniques have been proposed for achieving noise robustness. This paper considers one particular approach to model-based compensation, predictive model-based compensation, which has been shown to achieve good noise robustness in a wide range of acoustic environments. The characteristic of these schemes is that they combine a speech model with an additive noise model, a channel model and, in the general case, a speaker stress model, to generate a corrupted-speech model. The general theory of these predictive techniques is discussed. Various approximations for rapidly performing the model combination stage have been proposed and are reviewed in this paper. The advantages and the limitations of such a predictive approach to noise robustness are also discussed. In addition, methods for combining predictive schemes with schemes which make use of speech data in the new environment, adaptive schemes, are detailed. This combined approach overcomes some of the limitations of the predictive schemes.

Application of reconstruction-based scatter compensation to single- and dual-isotope SPECT imaging

The presence of scatter in single-photon emission computed tomography(SPECT) data can lead to degraded contrast and inaccurate quantitation in reconstructed images. A promising approach to compensate for these effects is to model the process of scatter during the image reconstruction process. Such reconstruction-based scatter compensation (RBSC) has a number of advantages over subtraction-based approaches, but difficulty in modeling the complex process of scatter has heretofore limited both the usefulness and evaluation of this approach. In this work, the utility of performing RBSC in single- and dual-isotope SPECTimaging was investigated. Singular value decomposition techniques were used to study some fundamental issues regarding the reconstructibility and noise properties of scatter in SPECT. The results were used to determine energy window schemes and RBSC approaches that led to the best performance for Tc-99m and Tl-201 imaging. Model-based compensation methods were also developed for use with simultaneously acquired Tc-99m/Tl-201 data. Finally, fast and practical implementations of RBSC were developed that can reconstructimages with fully 3D scatter compensation in clinically acceptable times. The conclusions of this work indicate that RBSC can be fast and practical, and may be the scatter compensation method of choice for both single- and dual-isotope SPECTimaging.

Compensation method based on formularizing hysteresis of piezoelectric tube scanners

Based on a circular arc scanning model and the formularizing of hysteresis curves of piezoelectric tubes, a three-dimensional scanning model of piezoelectric tubes is presented and used to compensate distortions of measured images scanned by a piezoelectric tube. This model-based compensation method can correct three-dimensional distortions simultaneously with two key parameters which are obtained just by measuring vertical deformation-voltage characteristics of the piezoelectric tube. Experiments showing the effectiveness of this method are also discussed in this article.