A b s t r a c tTo provide COSMOS, a dynamic model baaed manipulator control system, with an improved dynamic model, a PUMA 560 arm waa diaaaaembled; the inertial propertiea of the individual links were meaaured; and an ezplicit model incorporating all ofthe non-zero meaaured parametera waa deriued. The ezplicit model of the PUMA arm has been obtained with a derivation procedure comprised of aeveral heuristic rulea for simplification. A aimplijied model, abbreviated from the full ezplicit model with a 1% aignijicance criterion, can be evaluated with 305 calculationa, one fifth the number required by the recuraive Newton-Euler method. The procedure used to derive the model ia laid out; the meaaured inertial parametera are preaented, and the model ia included in an appendiz. I n t r o d u c t i o nThe Implementation of dynamic control systems for manipulators has been hampered because the models are difficult to derive and computationally expensive, and because the needed parameters of the manipulator are generally unavailable. Recursive methods for computing the dynamic forces have been available for several years [Luh, Walker and Paul 1980a; Hollerbach 19801. Several authors have proposed and simulated the use of RNE in control systems [Luh, Walker and Paul 1980b; Kim and Shin 19851; and [Valavanis, Leahy and Sardsi 19851 have used the RNE to control a PUMA -600 arm. The RNE algorithm has also found use in the computation of forward dynamics for simulation [Walker and Orin 1982; Koozekanani et al. 19831, and nominal trajectory control [VukobratoviE and Kirfanski 19841. The RNE meets the need for calculation of dynamic forces in these applications, but does not offer several advantages available provided by an explicit model. The explicit model allows of the calculation decomposition based on a significance criterion or other criteria, and provides a more direct solution for dynamic simulation.The tremendous size of an explicit dynamic model is the greatest barrier to its realization. Correspondingly, a considerable portion of the effort spent investigating dynamic models for control has been directed toward efficient formulation and automatic generation of the manipulator equations of motion. Programs for automatic generation of manipulator dynamics are reported in [Likgeois et al. 1976;Megahed and Renaud 1982;Cesareo size of the models generated by these programs varies widely; and there is little consensus on the question of whether the explicit models can be made sufficiently compact to be used for control. Aldon and Likgeois [1984] present an algorithm for obtaining efficient dynamic models; but none-the-less recomend the use of recursive algorithms for real time control, claiming that the complete results are too complicated for real-time control of robots.As we show, explicit dynamic models of manipulators that are more computationally efficient than the alternative recursive algorithms can be obtained. The computational cost of the RNE algorithm, the full explicit PUMA model, and the explicit PWMA mo...

For control applications involving small displacements and velocities, friction modeling and compensation can be very important. In particular, the modeling of presliding displacement (motion prior to fully developed slip) can play a pivotal role. In this note, it is shown that existing single-state friction models exhibit a nonphysical drift phenomenon which results from modeling presliding as a combination of elastic and plastic displacement. A new class of single state models is defined in which presliding is elastoplastic: under loading, frictional displacement is first purely elastic and then transitions to plastic. The new model class is demonstrated to substantially reduce drift while preserving the favorable properties of existing models (e.g., dissipativity) and to provide a comparable match to experimental data.

Magnetic resonance imaging (MRI) is a widely used method for non-invasive study of the structure and function of the human brain. Increasing magnetic field strengths enable higher resolution imaging; however, long scan times and high motion sensitivity mean that image quality is often limited by the involuntary motion of the subject. Prospective motion correction is a technique that addresses this problem by tracking head motion and continuously updating the imaging pulse sequence, locking the imaging volume position and orientation relative to the moving brain. The accuracy and precision of current MR-compatible tracking systems and navigator methods allows the quantification and correction of large-scale motion, but not the correction of very small involuntary movements in six degrees of freedom. In this work, we present an MR-compatible tracking system comprising a single camera and a single 15 mm marker that provides tracking precision in the order of 10 m and 0.01 degrees. We show preliminary results, which indicate that when used for prospective motion correction, the system enables improvement in image quality at both 3 T and 7 T, even in experienced and cooperative subjects trained to remain motionless during imaging. We also report direct observation and quantification of the mechanical ballistocardiogram (BCG) during simultaneous MR imaging. This is particularly apparent in the head-feet direction, with a peak-to-peak displacement of 140 m.

Magnetic resonance imaging (MRI) is a widely used method for non-invasive study of the structure and function of the human brain. Increasing magnetic field strengths enable higher resolution imaging; however, long scan times and high motion sensitivity mean that image quality is often limited by the involuntary motion of the subject. Prospective motion correction is a technique that addresses this problem by tracking head motion and continuously updating the imaging pulse sequence, locking the imaging volume position and orientation relative to the moving brain. The accuracy and precision of current MR-compatible tracking systems and navigator methods allows the quantification and correction of large-scale motion, but not the correction of very small involuntary movements in six degrees of freedom. In this work, we present an MR-compatible tracking system comprising a single camera and a single 15 mm marker that provides tracking precision in the order of 10 m and 0.01 degrees. We show preliminary results, which indicate that when used for prospective motion correction, the system enables improvement in image quality at both 3 T and 7 T, even in experienced and cooperative subjects trained to remain motionless during imaging. We also report direct observation and quantification of the mechanical ballistocardiogram (BCG) during simultaneous MR imaging. This is particularly apparent in the head-feet direction, with a peak-to-peak displacement of 140 m.

No abstract

When designing an identification experiment for a system described by nonlinear functions such as those of manipulator dynamics, it is necessary to consider whether the excitation is sufficient to provide an accurate estimate of the parameters in the presence of experimental noise. It is shown that the convergence rate and noise immunity of a parameter identifi cation experiment depend directly on the condition number of the input correlation matrix, a measure of excitation. The sensitivity of an identification experiment to unmodeled dynamics is also studied; a dimensionless measure of this sensitivity—bias susceptibility—is proposed and related to excitation. The issue of how exciting a trajectory may be is addressed, and a method is presented to maximize the exci tation. Two identification experiments reported in the litera ture are studied; analysis of these experiments shows that intuitively selected trajectories may provide poor excitation, and considerable improvement results from employing the optimization to maximize excitation. A typical improvement is the reduction of parameter convergence time from 15 min to 33 s. This work illustrates the difficulty of maintaining persistent excitation during the experimental identification of complex models.

The friction behavior of a brush type d.c. servo-motor driven mechanism with gearing is explored. The standard kinetic plus viscous friction model is found t o describe the dominant friction behavior. In addition, a dependence of friction upon position is identified and negative velocity dependence is observed at lolu velocities. A friction model is developed and used t o precompute motion iorques. The application of precomputed torques in an open loop fashion results in motions accurate to within a f e w percent.

Purpose: To introduce and evaluate a method of prospective motion correction for localized proton magnetic resonance spectroscopy (1H-MRS) using a single-camera optical tracking system. Materials and Methods:Five healthy participants were scanned at 3T using a point-resolved spectroscopic sequence (PRESS) with a motion-tracking module and phase navigator. Head motion in six degrees was tracked with a Retro-Grate Reflector (RGR) tracking system and target via a mirror mounted inside the bore. Participants performed a series of three predetermined motion patterns during scanning.Results: Left-right rotation (Rz) (average 12 ) resulted in an increase in the total choline to total creatine ratio (Cho/Cr) of þ14.6 6 1.5% (P ¼ 0.0009) for scans without correction, but no change for scans with correction (þ1.1 6 1.5%; P ¼ 0.76). Spectra with uncorrected Z-translations showed large lipid peaks (skull) with changes in Cho/Cr of À13.2 6 1.6% (P ¼ 0.02, no motion correction) and À2.2 6 2.4% (P ¼ 0.51) with correction enabled. There were no significant changes in the ratios of N-acetylaspartate, glutamateþglutamine, or myo-inositol to creatine compared to baseline scans for all experiments.Conclusion: Prospective motion correction for 1H-MRS, using single-camera RGR tracking, can reduce spectral artifacts and quantitation errors in Cho/Cr ratios due to head motion and promises improved spectral quality and reproducibility.