Browsing by Subject "Digital control"

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    ItemOpen Access
    Control of uncertain sampled-data systems: An Adaptive posicast control approach
    (Institute of Electrical and Electronics Engineers Inc., 2017) Abidi K.; Yildiz, Y.; Annaswamy A.
    This technical note proposes a discrete-time adaptive controller for the control of sampled-data systems. The design is inspired from the Adaptive Posicast Controller (APC) which was designed for time-delay systems in continuous time. Due to the performance degradation caused by digital approximation of continuous laws, together with the problem of assuming time-delays as integer multiples of sampling intervals, the benefits of APC could not be fully realized. In this technical note, these approximations/assumptions are eliminated. In addition, a disturbance observer is incorporated into the controller design which minimizes the effect of disturbances on the system. Extension to the case of uncertain input time-delay is also presented. The proposed approach is verified in simulation studies. © 1963-2012 IEEE.
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    ItemOpen Access
    A digitally controlled class-e amplifier for MRI
    (2016-08) Poni, Redi
    Radio-frequency (RF), or B1field is used for slice selection purposes in Magnetic Resonance Imaging (MRI). In an MRI scanner thisfield is handled by the RF chain which consists on a pulse generator unit, a high power amplifier and a transmit coil. Relatively low effciency linear power amplifiers are used. These amplifiers are placed in the system room, far from the transmit coil. In this work we propose to design the amplifier and the coil as a single unit, aiming to decrease cost and complexity while improving performance. Splitting the coil into multiple elements makes possible to drive these elements with individual amplifiers in Transmit Array (TxArray) mode. Also these elements are integrated as the load network of the amplifier, in this case a high effciency Class-E amplifier. The Class-E amplifier was modified for the intended application and it was digitally controlled. For the pulse generation two dierent methods were applied. One was by controlling separately the phase and amplitude of the pulse. The other method generates simultaneously phase and amplitude by controlling the switching pattern of the amplifier. An amplifier of output power 100W with effciency up to 88% was developed. As a step toward 36 channel complete system, 2 element prototype's operation was tested in a 3T Tim Trio Siemens scanner. In overall, Class-E amplifier is shown to be a promising candidate for on-coil RF excitation in TxArray in terms of size, cost, effciency and complexity.
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    ItemOpen Access
    Highly efficient 300 W modified class-E RF amplifiers for 64 MHz transmit array system
    (2017-12) Tu Zahra, Fatima
    The conventional MRI system uses high power linear RF amplifiers which are placed away from scanner room, have low efficiency, high cost and need a cooling system. We aim to use parallel transmit system with on-coil amplifiers that has shown to be advantageous in improving B1 field homogeneity, slice selectivity and SAR reduction. In this work the on-coil class-E RF switching amplifier is suggested for MRI to decrease the cost and complexity of the current system while improving performance. The amplifier is digitally controlled and for pulse generation purpose supply modulation is used. The transmit coil acts as the load network of the amplifier so the need for matching circuit is eliminated. The amplifier has an output power of 300 W with maximum efficiency of 92%. The efficiency does not drop below 75% in 1 MHz bandwidth. The performance of amplifier at high temperature is also evaluated and it is established that at low duty cycles due to high efficiency no cooling system is required but for high duty cycle applications a cooling system might be needed for the uninterrupted operation. The ultimate goal of this research is to design a 32-channel transmit array using on-coil amplifiers, as a step forward towards this goal a prototype for two channels is designed. Our results depict that the behavior of the class-E amplifiers under coupled operations is acceptable. The dual-channel prototype was tested on Scimedix 1.5 T and no artifacts were observed in the images due to the presence of amplifiers near transmit coils inside the bore. To sum up, the class-E amplifier is proved to be a favorable candidate for on-coil applications in RF excitation due to its small size, reduced complexity and high efficiency.