Evolutionary intelligent design of gradient amplifiers
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Evolutionary/ Intelligent Design of Gradient Amplifiers. Greig Scott. Prepolarized Magnetic Resonance Imaging Lab, Department of Electrical Engineering, Stanford University. Goals. Gradient Amplifier Problem Statement The venerable Techron 8607 Feedback Control and Compensation

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Evolutionary intelligent design of gradient amplifiers

Evolutionary/ Intelligent Design of Gradient Amplifiers

Greig Scott

Prepolarized Magnetic Resonance Imaging Lab,

Department of Electrical Engineering, Stanford University


Goals

Goals

  • Gradient Amplifier Problem Statement

  • The venerable Techron 8607

  • Feedback Control and Compensation

  • PWM design evolution & digital control.

  • Advanced topologies for ripple reduction.

  • Gradient coil inductance ramifications

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Gradient driver problem

coil

Gradient Driver Problem

h=8500A/T/m: 3G/cm is 250A

200ms rise time to 3 G/cm is SR 150 (150T/m/s)

400 kVAR Amp

Ldi/dt: 1275 Volts

I*R: 25 Volts

Voltage rail: 1500V

250A

L~1mH

R~0.1W

1300 Volts

25 Volts

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Techron 8607

Techron 8607

x20 (single) x40 (master/slave)

Techron

+

8607

-

R1

magnet

+

-

R-C damp

x1/20

i

R2

current

transducer

Master/Slave: ~200V, 100 A linear gradient amplifier

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Opamp dc gain

OPAMP DC GAIN

OP27 1.8 Million

NE5532 0.2 Million

LT1007 20 Million

OPA227 100 Million

GBW~8 MHz, SR 2.3-8 V/us

Higher DC gain minimizes gradient 1/f noise and drift

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Basic bridge power stage

Basic Bridge Power Stage

a

c

Linear or PWM H arm.

Isolated transformer

Can boost supply

Can place in series.

Techron placed 2 in series.

+

-

b

d

a

c

+

-

d

b

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Power stage freq response

Power Stage Freq. Response

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Power stage noise

Power Stage Noise

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Fluxgate current transducers

Fluxgate Current Transducers

N

750

10

W

Ideal transformer to DC

Danfysik Ultrastab 866

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Danfysik 866 freq response

Danfysik 866 Freq. Response

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Evolutionary intelligent design of gradient amplifiers

18 bit, 500ksps (eg AD767x ADC) ENOB~ 17 bits

LEM Hall device

18 bit ADC floor

For 4V reference, 18uV rms noise

For 500 ksps, ~35nV/rtHz floor or ~4uA/rtHz.

Primary current noise uA/rt(Hz)

Ultrastab fluxgate

High speed high resolution ADC noise floor can now digitize current sensor.


Feedback loop noise

Feedback Loop Noise

+

e

x

u

S

A

B

S

S

S

-

y

n

n

n

3

1

2

C

S

S

n

n

4

5

U/X = AB/(1+ABC) transconductance

U/n3 = 1/(1+ABC) -> 0 power noise

U/n4 = ABC/(1+ABC) ~ 1 sensor noise

High loop gain ABC minimizes noise to sensor level

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Loop gain

ghRa

(s+1/RaCa)

LG

1+LG

-ghR2

LG~

I/V =

R2

s

L(s+R/L)

R1

Loop Gain

+

Loop gain

g

x

-

R

R1

Co

Transfer function:

L

Ca

Ra

i

R2

h

y

v=hi

Gradient coil adds up to –90 degrees. Opamp integrator at –90

Degrees. Ra and Ca cancel coil phase shift at high frequency.

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Compensation network

Compensation Network

Set RaCa = L/R @ crossover frequency

Bandwidth

Co

Co kills high freq. gain

Ra

Ca

-

Higher bandwidth allows more low freq loop gain & more noise reduction.

R2

+

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Output impedance

Output Impedance

Ideal Current source with scaled RC-C network

+

g

x

-

R

R1

Co

Z

Cp

Cs

Ca

Ra

i

R2

h

Cp =Co*R2/gh

Cs = Ca*R2/gh

R = Ra*gh/R2

y

v=hi

Zout

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Proportional integral control

Ca

Ra

-

R2

+

+

Proportional Integral Control

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Evolutionary intelligent design of gradient amplifiers

Feedforward & Feedback System

Feedforward

+

+

+

G

+

-

R

L

Feedback

x1/20

H

Set:

then

Integrator gives infinite gain & 0 loop error at DC. Feedforward does not change feedback dynamics


Pwm basics

PWM Basics

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Series bridges

Series Bridges

Feedforward voltage boost PWM

Vm

PWM

+

-

-

+

+

Linear

-

Vsb

Rsense

+

magnet

agnd

-

+

-

PWM

Linear feedbackcontrol

-

+

PWM

Vsa

Isolated Linear or PWM bridges can be placed in series

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Mosfet vs igbt

MOSFET

Majority carrier device

On voltage drop 10-100V at high (~600A) current.

Higher switch frequency

Easy to parallel

IGBT

Minority carrier device

Superior conduction. Vce sat 2-3 volts at 600A.

Higher breakdown V

Double current density

New devices +ve Tc so parallel connections possible.

MOSFET vs IGBT

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Insulated gate bipolar transistor

Insulated Gate Bipolar Transistor

Powerex CM600HA-24H: 1200V, 600A.

Vce-sat: 2.1-2.4V for 600A

30kHz hard, 60-70kHz soft

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Motor torque control

Motor Torque Control

Apex Microtechnology SA-03 hybrid PWM: 22.5kHz, 30A

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Gradient topologies

Gradient Topologies

  • Stack PWM and linear amp in series

    • High voltage for high inductance coil.

  • Parallel PWM amplifiers.

    • High current for low inductance coil.

  • 20kHz to 60kHz switch frequencies

  • Digital PI control of feedback.

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Quasi linear

Quasi-linear

Va

  • Va, Vb add discrete voltage steps of +/-300, +/-900V

  • Linear: +/- 150V

  • Total V: +/-1350V

  • Current feedback control of linear amplifier only.

Isolated supplies

Linear amplifier

coil

Mueller, Park IEEE APEC 1994?

Vb

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Paralleled bridge configuration

Paralleled Bridge Configuration

Inductor current imbalance

coil

IGBTs: 1200V/300A, 20 kHz, driven in 90 degree phase steps

Ripple current: [email protected]

Takano et al. IEEE IECON’99 p785.

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Bi modal pwm supply

Bi-modal PWM Supply

31 kHz

62.5 KHz

600V IGBT

1200V IGBT

PWM mode for <400V

  • V>400: variable supplies switch.

  • Phase shifted so 2x62.5=125kHz switch rate.

  • V<400: PWM switches

  • Amplifier is dual gain depending on PWM stage.

400V

400V

400-800V variable supply

Steigerwald, IEEE PESC 2000 p643

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Digital control of 4 parallel bridge pwm

Digital Control of 4 Parallel Bridge PWM

4-parallel bridge

filter

+

20kHz, 17bit PWM

+

+

_

_

Voltage control

10 bit A/D

coil

Current loop control

18 bit A/D

Current transducer

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Evolutionary intelligent design of gradient amplifiers

700V, 31.25kHz

Ldi/dt

IR

200V, 62.5kHz

+

+

+

-

720MHz DSP & FPGA

700V, 31.25kHz

18bit

Sabate IEEE PESC 2004,p261


Advanced methods

Advanced Methods

  • Quasi-resonant low loss switching

  • Balanced PWM current amplifier

  • Notch Ripple filters

  • All target low loss, higher effective switching frequency and lower ripple.

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Transformer assisted quasi resonant commutated pole

Transformer Assisted Quasi-Resonant Commutated Pole

load

Implements Zero-Voltage Switching (ZVS) using TQRCP. Switching losses reduced.

Fukuda et al, IEEE Conf. Industrial Automation & Control 1995

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Opposed current interleaved amplifier ocia

Opposed Current Interleaved Amplifier (OCIA)

a

a

b

b

Crown Balanced Current Amplifier 1998

Ripple frequency double that of standard bridge

Load excitation

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Opposed current interleaved amplifier ocia1

Opposed Current Interleaved Amplifier (OCIA)

load

Ripple frequency double that of standard H bridge

Crown Balanced Current Amplifier 1998

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Ripple cancellation filters

Ripple Cancellation Filters

Notch filter action introduces passband zero at ripple frequency

Transformers act to inject equal and opposite ripple currents but not signal currents.

Sabate, IEEE APEC 2004, p792

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Gradient coil inductance impact on amplifier design

Gradient Coil Inductance: Impact on Amplifier Design

N turn gradient, inductance L, resistance R,

-> V, I.

N/2 turn gradient, inductance L/4, resistance R/4,

-> V/2, 2I.

Split N turn gradient, inductance ~L/2, resistance R/2 each,

-> V/2 per coil, same I.

Gradient L can allow substantial change in device voltage

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Summary

Summary

  • PWM designs now standard.

  • Full digital control.

  • Design conflict: How to structure IGBT stages with finite voltage and current limits, and switch speed.

  • Gradient coil inductance choice can impact amplifier topology.

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Summary1

Summary

  • New precision opamps (eg LT1007) improve 1/f noise by ~100 times.

  • Current transducer low 1/f drift.

  • Gradient amplifier is ideal current source with RC-C shunt network.

  • Voltage boost designs still have same basic stability analysis.

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Danfysik 866 noise

Danfysik 866 Noise

6 Turns

10 ohm R

Noise floor:

20 nV/rt Hz

1.6 pA/rt Hz

0.2uA/rt Hz

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Feedforward ldi dt control

Feedforward Ldi/dt Control

Ldi/dt control

g

x

g

x

R1

Ya

Z

R1

Ya

Yb

Z

i

R2

Yb

h

R2

i

y

h

v=hi

y

v=hi

Voltage boost control is feedforward, so dynamics is same.

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