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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. Gradient Amplifier Problem Statement The venerable Techron 8607 Feedback Control and Compensation

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Evolutionary/ Intelligent Design of Gradient Amplifiers

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  1. Evolutionary/ Intelligent Design of Gradient Amplifiers Greig Scott Prepolarized Magnetic Resonance Imaging Lab, Department of Electrical Engineering, Stanford University

  2. 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 PMRIL Stanford Electrical Engineering

  3. 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 PMRIL Stanford Electrical Engineering

  4. 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 PMRIL Stanford Electrical Engineering

  5. 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 PMRIL Stanford Electrical Engineering

  6. 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 PMRIL Stanford Electrical Engineering

  7. Power Stage Freq. Response PMRIL Stanford Electrical Engineering

  8. Power Stage Noise PMRIL Stanford Electrical Engineering

  9. Fluxgate Current Transducers N 750 10 W Ideal transformer to DC Danfysik Ultrastab 866 PMRIL Stanford Electrical Engineering

  10. Danfysik 866 Freq. Response PMRIL Stanford Electrical Engineering

  11. 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.

  12. 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 PMRIL Stanford Electrical Engineering

  13. 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. PMRIL Stanford Electrical Engineering

  14. 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 + PMRIL Stanford Electrical Engineering

  15. 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 PMRIL Stanford Electrical Engineering

  16. Ca Ra - R2 + + Proportional Integral Control PMRIL Stanford Electrical Engineering

  17. 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

  18. PWM Basics PMRIL Stanford Electrical Engineering

  19. 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 PMRIL Stanford Electrical Engineering

  20. 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 PMRIL Stanford Electrical Engineering

  21. Insulated Gate Bipolar Transistor Powerex CM600HA-24H: 1200V, 600A. Vce-sat: 2.1-2.4V for 600A 30kHz hard, 60-70kHz soft PMRIL Stanford Electrical Engineering

  22. Motor Torque Control Apex Microtechnology SA-03 hybrid PWM: 22.5kHz, 30A PMRIL Stanford Electrical Engineering

  23. 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. PMRIL Stanford Electrical Engineering

  24. 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 PMRIL Stanford Electrical Engineering

  25. Paralleled Bridge Configuration Inductor current imbalance coil IGBTs: 1200V/300A, 20 kHz, driven in 90 degree phase steps Ripple current: 250mA@80kHz Takano et al. IEEE IECON’99 p785. PMRIL Stanford Electrical Engineering

  26. 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 PMRIL Stanford Electrical Engineering

  27. 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 PMRIL Stanford Electrical Engineering

  28. 700V, 31.25kHz Ldi/dt IR 200V, 62.5kHz + + + - 720MHz DSP & FPGA 700V, 31.25kHz 18bit Sabate IEEE PESC 2004,p261

  29. 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. PMRIL Stanford Electrical Engineering

  30. 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 PMRIL Stanford Electrical Engineering

  31. Opposed Current Interleaved Amplifier (OCIA) a a b b Crown Balanced Current Amplifier 1998 Ripple frequency double that of standard bridge Load excitation PMRIL Stanford Electrical Engineering

  32. Opposed Current Interleaved Amplifier (OCIA) load Ripple frequency double that of standard H bridge Crown Balanced Current Amplifier 1998 PMRIL Stanford Electrical Engineering

  33. 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 PMRIL Stanford Electrical Engineering

  34. 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 PMRIL Stanford Electrical Engineering

  35. 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. PMRIL Stanford Electrical Engineering

  36. 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. PMRIL Stanford Electrical Engineering

  37. Danfysik 866 Noise 6 Turns 10 ohm R Noise floor: 20 nV/rt Hz 1.6 pA/rt Hz 0.2uA/rt Hz PMRIL Stanford Electrical Engineering

  38. 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. PMRIL Stanford Electrical Engineering

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