Reduction of magnetic field fluctuations in powered magnets for NMR using inductive measurements and sampled-data feedback control

Mingzhou Li, Jeffrey L. Schiano, Jenna E. Samra, Kiran K. Shetty, William W. Brey

    Research output: Contribution to journalArticlepeer-review

    12 Scopus citations

    Abstract

    Resistive and hybrid (resistive/superconducting) magnets provide substantially higher magnetic fields than those available in low-temperature superconducting magnets, but their relatively low spatial homogeneity and temporal field fluctuations are unacceptable for high resolution NMR. While several techniques for reducing temporal fluctuations have demonstrated varying degrees of success, this paper restricts attention to methods that utilize inductive measurements and feedback control to actively cancel the temporal fluctuations. In comparison to earlier studies using analog proportional control, this paper shows that shaping the controller frequency response results in significantly higher reductions in temporal fluctuations. Measurements of temporal fluctuation spectra and the frequency response of the instrumentation that cancels the temporal fluctuations guide the controller design. In particular, we describe a sampled-data phase-lead-lag controller that utilizes the internal model principle to selectively attenuate magnetic field fluctuations caused by the power supply ripple. We present a quantitative comparison of the attenuation in temporal fluctuations afforded by the new design and a proportional control design. Metrics for comparison include measurements of the temporal fluctuations using Faraday induction and observations of the effect that the fluctuations have on nuclear resonance measurements.

    Original languageEnglish (US)
    Pages (from-to)254-264
    Number of pages11
    JournalJournal of Magnetic Resonance
    Volume212
    Issue number2
    DOIs
    StatePublished - Oct 1 2011

    All Science Journal Classification (ASJC) codes

    • Biophysics
    • Biochemistry
    • Nuclear and High Energy Physics
    • Condensed Matter Physics

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