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EE 5340/7340 Introduction to Biomedical Engineering Electromagnetic Flowprobes

EE 5340/7340 Introduction to Biomedical Engineering Electromagnetic Flowprobes. Carlos E. Davila, Electrical Engineering Dept. Southern Methodist University slides can be viewed at: http:// www.seas.smu.edu/~cd/ee5340.html. Electromagnetic Flowmeters. blood vessel. +. V o. _.

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EE 5340/7340 Introduction to Biomedical Engineering Electromagnetic Flowprobes

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  1. EE 5340/7340 Introduction to Biomedical EngineeringElectromagnetic Flowprobes Carlos E. Davila, Electrical Engineering Dept. Southern Methodist University slides can be viewed at: http:// www.seas.smu.edu/~cd/ee5340.html EE 5340, SMU Electrical Engineering Department, © 1999

  2. Electromagnetic Flowmeters blood vessel + Vo _ electromagnet indicator dilution methods assume flow rate is constant, only measure average flow. EM flowmeters enable measurement of instantaneous flow. EE 5340, SMU Electrical Engineering Department, © 1999

  3. Faraday’s Law -a moving conductor in a (possibly constant) magnetic field will have a voltage induced across it voltage induced across electrodes velocity of blood (m/s) magnetic flux density (Wb/m2) vector in direction of electrodes length of response is maximized when , , and are mutually orthogonal EE 5340, SMU Electrical Engineering Department, © 1999

  4. Toroidal Cuff Probe EE 5340, SMU Electrical Engineering Department, © 1999

  5. DC Flowmeter • use DC (constant) magnetic field • half-cell potential results across each sensing electrode, in series with the flow signal, even with non-polarizable potentials • pick up stray ECG • basically doesn’t work well, and DC flowmeters are not used. • flow frequency range: 0 - 30 Hz EE 5340, SMU Electrical Engineering Department, © 1999

  6. AC Flowmeter • frequency of : about 400 Hz • Vo becomes amplitude modulated sine wave: 400 Hz carrier 0 flow need a phase-sensitive demodulator EE 5340, SMU Electrical Engineering Department, © 1999

  7. Transformer Voltage blood vessel _ Vt + plane of electrode wires should be parallel to magnetic field. Otherwise, get transformer voltage, Vt, proportional to: EE 5340, SMU Electrical Engineering Department, © 1999

  8. t t t Transformer Voltage (cont.) magnet current, im(t) 90o out of phase transformer voltage, vt(t) flow voltage, vf(t) 0 or 180o out of phase, depending on flow direction EE 5340, SMU Electrical Engineering Department, © 1999

  9. Removal of Transformer Voltage • Phantom Electrode • Gating Flow Voltage • Quadrature Suppression EE 5340, SMU Electrical Engineering Department, © 1999

  10. Phantom Electrode blood vessel adjust until transformer voltage = 0 _ Vt + EE 5340, SMU Electrical Engineering Department, © 1999

  11. t t t Gating Flow Voltage magnet current, im(t) transformer voltage, vt(t) flow voltage, vf(t) sample flow voltage when transformer voltage = 0 EE 5340, SMU Electrical Engineering Department, © 1999

  12. Quadrature Suppression Discussed in Chapter 8 of text. To understand it fully, we must go over several modulation/demodulation methods: • Amplitude Modulation/Demodulation • Double Sideband Modulation /Demodulation • Quadrature Multiplexing/Demultiplexing EE 5340, SMU Electrical Engineering Department, © 1999

  13. S Amplitude Modulation/Demodulation : information-bearing signal Modulation: carrier frequency A Demodulation (envelope detector): + + C R _ _ EE 5340, SMU Electrical Engineering Department, © 1999

  14. Double Sideband (DSB) Modulation/Demodulation modulation: m(t) can be bipolar carrier frequency demodulation: this demodulator is phase sensitive LPF carrier frequency and phase must be known EE 5340, SMU Electrical Engineering Department, © 1999

  15. DSB Modulation/Demodulation (cont.) trigonometric identity: LPF EE 5340, SMU Electrical Engineering Department, © 1999

  16. DSB Modulation/Demodulation (cont.) Frequency Domain: from frequency shifting property of the Fourier Transform: LSB USB 0 EE 5340, SMU Electrical Engineering Department, © 1999

  17. DSB Modulation/Demodulation (cont.) 0 LPF 0 = EE 5340, SMU Electrical Engineering Department, © 1999

  18. S Quadrature DSB (QDSB) Modulation -allows one to transmit two different information signals, m1(t) and m2(t) using the same carrier frequency, this enables more efficient bandwidth utilization. EE 5340, SMU Electrical Engineering Department, © 1999

  19. LPF LPF QDSB Demodulation EE 5340, SMU Electrical Engineering Department, © 1999

  20. QDSB Demodulation (cont.) Trigonometric Identities: EE 5340, SMU Electrical Engineering Department, © 1999

  21. QDSB Demodulation (cont.) LPF LPF EE 5340, SMU Electrical Engineering Department, © 1999

  22. LPF LPF magnet current generator 90o phase shift oscillator Quadrature Suppression -used to suppress transformer voltage amp vessel EE 5340, SMU Electrical Engineering Department, © 1999

  23. Electromagnetic Flowprobe: Case Study- Cliniflow II, Carolina Medical SPECIFICATIONS ACCURACY Electrical Zero --- Automatic zero for occlusive or non-occlusive zero reference. Calibrate Signal --- -1V to +1V in 0.1V steps @ 0.2 sec/step. Flowmeter Calibration Accuracy --- +/-3% of full scale after a 5 second warm-up. (Includes the effect of gain and excitation variation.) DC Drift --- +/-5mV after a 5 second warm-up. Linearity --- +/-1% maximum full scale. EE 5340, SMU Electrical Engineering Department, © 1999

  24. Case Study (cont.) SAFETY Patient Isolation --- Isolated patient ground. <10uA RMS leakage @ 120V RMS. Breakdown >2500V RMS. Equipment Isolation --- External connections to recorders, etc, are optically isolated to preserve patient protection even when connected to external equipment. Electrical Isolation --- Designed to comply with UL544 specifications. No exposed, non-isolated metal surfaces available to the operator or patient. EE 5340, SMU Electrical Engineering Department, © 1999

  25. Case Study (cont.) INPUT CHARACTERISTICS Autoranging --- Overall gain, full scale recorder output amplitude, flow rate range indicator and decimal point location are automatically programmed by the selected probe. Probe Excitation --- 450 or 475Hz square-wave, 0.5 Ampere +/-l%. Amplifier Input --- Differential >30 megohm plus 50pF. CMRR >/- or =80dB @ 60Hz. Defibrillator protected. EE 5340, SMU Electrical Engineering Department, © 1999

  26. Case Study (cont.) OUTPUT CHARACTERISTICS Flow Range --- 5 milliliters/min to 19.99 liters/min depending on probe selected. Gain --- Automatically preset by the probe used. Flow Indicator --- 3.5 digit red L.E.D. display, automatic calibration, automatic flow direction indicator. Outputs PULSATILE: Single ended, +/-lOV (20Vp-p) full scale. MEAN: single ended, +/-1.999V (4Vp-p) full scale. BOTH: capable of driving 1 kohm minimum load. Short circuit protected. Isolated from power or chassis ground. EE 5340, SMU Electrical Engineering Department, © 1999

  27. Case Study (cont.) Frequency Response --- Front panel selectable, 3dB down @ 12Hz, 25Hz, 50Hz or 100Hz. Output Noise PULSATILE: 11OmV typical @ 100Hz response, 30mV typical @ 12Hz response. (Varies with the probe used and the frequency response setting.) MEAN: 5mV maximum. EE 5340, SMU Electrical Engineering Department, © 1999

  28. Case Study (cont.) examples of electromagnetic flowprobes courtesy of Carolina Medical EE 5340, SMU Electrical Engineering Department, © 1999

  29. Case Study (cont.): example of EM flowmeter courtesy of Carolina Medical EE 5340, SMU Electrical Engineering Department, © 1999

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