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

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EE 5340, SMU Electrical Engineering Department, © 19992 Electromagnetic Flowmeters VoVo + _ electromagnet blood vessel indicator dilution methods assume flow rate is constant, only measure average flow. EM flowmeters enable measurement of instantaneous flow.

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EE 5340, SMU Electrical Engineering Department, © 19993 Faraday’s Law voltage induced across electrodes velocity of blood (m/s) magnetic flux density (Wb/m 2 ) vector in direction of electrodes length of -a moving conductor in a (possibly constant) magnetic field will have a voltage induced across it response is maximized when,, and are mutually orthogonal

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EE 5340, SMU Electrical Engineering Department, © 19994 Toroidal Cuff Probe

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EE 5340, SMU Electrical Engineering Department, © 19995 DC Flowmeter n use DC (constant) magnetic field n half-cell potential results across each sensing electrode, in series with the flow signal, even with non-polarizable potentials n pick up stray ECG n basically doesn’t work well, and DC flowmeters are not used. n flow frequency range: 0 - 30 Hz

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EE 5340, SMU Electrical Engineering Department, © 19996 AC Flowmeter n frequency of : about 400 Hz n V o becomes amplitude modulated sine wave: 400 Hz carrier 0 flow need a phase-sensitive demodulator

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EE 5340, SMU Electrical Engineering Department, © 19997 Transformer Voltage VtVt + _ blood vessel plane of electrode wires should be parallel to magnetic field. Otherwise, get transformer voltage, V t, proportional to:

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EE 5340, SMU Electrical Engineering Department, © 19998 Transformer Voltage (cont.) t t t magnet current, i m (t) transformer voltage, v t (t) flow voltage, v f (t) 90 o out of phase 0 or 180 o out of phase, depending on flow direction

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EE 5340, SMU Electrical Engineering Department, © 19999 Removal of Transformer Voltage n Phantom Electrode n Gating Flow Voltage n Quadrature Suppression

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EE 5340, SMU Electrical Engineering Department, © 199910 Phantom Electrode VtVt + _ blood vessel adjust until transformer voltage = 0

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EE 5340, SMU Electrical Engineering Department, © 199911 Gating Flow Voltage t t t flow voltage, v f (t) magnet current, i m (t) transformer voltage, v t (t) sample flow voltage when transformer voltage = 0

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EE 5340, SMU Electrical Engineering Department, © 199912 Quadrature Suppression Discussed in Chapter 8 of text. To understand it fully, we must go over several modulation/demodulation methods: n Amplitude Modulation/Demodulation n Double Sideband Modulation /Demodulation n Quadrature Multiplexing/Demultiplexing

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EE 5340, SMU Electrical Engineering Department, © 199913 Amplitude Modulation/Demodulation A Modulation: Demodulation (envelope detector): C R + _ + _ : information-bearing signal carrier frequency

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EE 5340, SMU Electrical Engineering Department, © 199914 Double Sideband (DSB) Modulation/Demodulation modulation: demodulation: LPF m(t) can be bipolar carrier frequency and phase must be known carrier frequency this demodulator is phase sensitive

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EE 5340, SMU Electrical Engineering Department, © 199915 DSB Modulation/Demodulation (cont.) trigonometric identity: LPF

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EE 5340, SMU Electrical Engineering Department, © 199916 DSB Modulation/Demodulation (cont.) Frequency Domain: from frequency shifting property of the Fourier Transform: USB LSB 0

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EE 5340, SMU Electrical Engineering Department, © 199917 DSB Modulation/Demodulation (cont.) 0 0 LPF =

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EE 5340, SMU Electrical Engineering Department, © 199918 Quadrature DSB (QDSB) Modulation -allows one to transmit two different information signals, m 1 (t) and m 2 (t) using the same carrier frequency, this enables more efficient bandwidth utilization.

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EE 5340, SMU Electrical Engineering Department, © 199919 QDSB Demodulation LPF

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EE 5340, SMU Electrical Engineering Department, © 199920 QDSB Demodulation (cont.) Trigonometric Identities:

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EE 5340, SMU Electrical Engineering Department, © 199921 QDSB Demodulation (cont.) LPF

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EE 5340, SMU Electrical Engineering Department, © 199922 Quadrature Suppression -used to suppress transformer voltage vessel amp LPF oscillator magnet current generator 90 o phase shift LPF

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EE 5340, SMU Electrical Engineering Department, © 199923 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.

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EE 5340, SMU Electrical Engineering Department, © 199924 Case Study (cont.) SAFETY Patient Isolation --- Isolated patient ground. 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.

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EE 5340, SMU Electrical Engineering Department, © 199925 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.

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EE 5340, SMU Electrical Engineering Department, © 199926 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.

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EE 5340, SMU Electrical Engineering Department, © 199927 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.

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EE 5340, SMU Electrical Engineering Department, © 199928 Case Study (cont.) courtesy of Carolina Medical examples of electromagnetic flowprobes

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EE 5340, SMU Electrical Engineering Department, © 199929 Case Study (cont.): example of EM flowmeter courtesy of Carolina Medical

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Example Problem You are measuring the EEG of a patient and accidently choose two different types of electrodes for EEG lead. One of them has a source impedance.

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