High precision current tracking pwm amplifier with optimal type 1 digital controller for generating magnetic field in mri systems

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HIGH PRECISION CURRENT TRACKING PWM AMPLIFIER WITH OPTIMAL TYPE 1 DIGITAL CONTROLLER FOR GENERATING MAGNETIC FIELD IN MRI SYSTEMS
S.Watanabe, P. Boyagoda and M.Nakaoka Yamaguchi University, Japan

This paper presents a two-paralleled PWM amplifier using switch-mode power current tracking technique in order to generate a gradient magnetic field in the Magnetic Resonance Imaging (MRI)system. This circuit has 8IGBTs at their inputs/outputs so as to realize further highpower density. A digital control scheme can minimize the current ripple and improve its control response in the Gradient Coils (GCs). It is proved that the proposed technique will highly enlarge the diagnostic target and improve the image quality of MRI.

unique digital control scheme for the circuit on the basis ofoptimal type-1 servo control. Furthermore, preview digital control is introduced into the amplifier with a corrective control scheme applied to improve the control of GC current and attain robustness. The effectiveness of the above scheme is evaluated through computer-aided analysis.

DIGITAL CONTROL SCHEME FOR PROPOSED CIRCUIT
The discrete system state equation of the power conversion circuitshown in Fig.1 can be written as

INTRODUCTION
MRl systems are utilized to diagnose moving parts in a human body where images should be taken in 50-200 milliseconds. The gradient power amplifiers in MRl systems have to generate very strong magnetic-fields to provide the necessary spatial resolution of the signals emitted by the hydrogen protons of a human body. To realize high-speed imaging,the gradient power amplifiers require higher output power as well as quick rise/fall response capability. Considering this backdrop, a new power conversion circuit for switch-mode gradient power amplifiers in MRI systems is proposed in this paper using a parallel connection of two conventional 4-switch full-bridge circuits with IGBTs at their inputs/outputs (Fig.l), and a

x(k + 1)= A x ( k ) +Bu(k - 1)
Y(k) =q k )

where X(k)=[ihl(k)~ iLaZ(k), iLbl(k), iLbZ(k), vCo(k), v C b ( k ) , iLgc(k)]TER7X', the system state vector, u(k-I)=[val is pl(k-I), V a 2 * p 2 ( k - 1 ) , V b l *p@-I), V b 2 * p 4 ( k - I ) ] T E & Q x ' iS the system input vector, and y(k)=[ihl(k), ihZ(k), il,&), i~&)]"R4"' is the system output vector. A, B, and Care real coefficient matrices with appropriatedimensions. Since digital control involves a time lag, it is necessary for the DSPs to consider the calculating time. Therefore, the input signal u(k-I) is calculated between the sampling time k-I and k.

Figure 1: A Construction of two paralleled bridge type power conversion circuit

Power Electronics and Variable Speed Drives, 21-23 September 1998, Conference Publication No. 456 0IEE 19919OPTIMAL TYPE-1 D I G l T a SERVO CONTROL
AII error signal vector e(k)&”’ is defined as e(k) = yr&) - y ( k ) where yr&)&” consists of the output reference signals. Even though the GC current is designed as the combination of two target currents, y,,f, the GC current will not achieve the desired value since, a current flows through the capacitor C, (C,). Hence, in order to overcome this error, acorrective expression is added as

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Figure 2: Concept of pulse generation procedure The power switch devices are turned on and off such that the input voltages to each of the LCR filters are given some pulses with magnitude E or 0 and width pi(k) (i = 1 tu 4 ) centered during sampling interval Ts. The pulse width vector p(k)=[pl(k),pz(k),p3(k),p4(k)lTEl?’’ will be derived in terms of the following equation. [c,(k)
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Although the LCR filters are connected with each of the bridge arms, these are not effective enough to eliminate the current ripple. The basic concept of pulse generation procedure to minimize the...