ee 5340, smu electrical engineering department, © 1999 1 carlos e. davila, electrical engineering...

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


2

Electromagnetic Flowmeters

B

u

L

Vo

+

_

V u B dLo

L

0

1

electromagnet

bloodvessel

indicator dilution methods assume flow rate is constant, only measure average flow. EM flowmeters enable measurement of instantaneous flow.


3

Faraday’s Law

V u B dLo

L

0

1

Vo: voltage induced across electrodesvelocity of blood (m/s)magnetic flux density (Wb/m2)vector in direction of electrodeslength of

u:B:

-a moving conductor in a (possibly constant) magnetic field will have a voltage induced across it

L:

LL1:

response is maximized when , , and are mutuallyorthogonal

u

B

L


4

Toroidal Cuff Probe

B


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


6

AC Flowmeter

frequency of : about 400 Hz Vo becomes amplitude modulated sine wave:

B

400 Hzcarrier

0 flow

need a phase-sensitive demodulator


7

Transformer VoltageB

u

L

Vt+ _

bloodvessel

plane of electrode wires should be parallel to magneticfield. Otherwise, get transformer voltage, Vt, proportionalto: dB

dt


8

Transformer Voltage (cont.)

t

t

t

magnet current, im(t)

transformervoltage, vt(t)

flowvoltage, vf(t)

90o outof phase

0 or 180o out of phase,depending onflow direction


9

Removal of Transformer Voltage

Phantom Electrode Gating Flow Voltage Quadrature Suppression


10

Phantom Electrode

u

Vt+ _

bloodvessel

adjust until transformervoltage = 0


11

Gating Flow Voltage

t

t

tflowvoltage, vf(t)

magnet current, im(t)

transformervoltage, vt(t)

sample flow voltage when transformer voltage = 0


12

Quadrature Suppression

Discussed in Chapter 8 of text. To understand it fully, wemust go over several modulation/demodulation methods:

Amplitude Modulation/Demodulation Double Sideband Modulation /Demodulation Quadrature Multiplexing/Demultiplexing


13

Amplitude Modulation/Demodulation

m t

A

A tc ccos

x tc

Modulation:

Demodulation (envelope detector):

C R x tc m t+

_

+

_

m t : information-bearing signalc: carrier frequency


14

Double Sideband (DSB) Modulation/Demodulation

m t

A tc ccos

x tc

modulation:

demodulation:

m t

2cos ct

x tc LPF x tb

m(t) can be bipolar

carrier frequency and phase must be known

c: carrier frequency

this demodulator isphase sensitive


15

DSB Modulation/Demodulation (cont.)

x t m t tb c2 2cos

cos cos2 12

1 2

x t m t t

m t m t tb c

c

1 2

2

cos

cos

trigonometric identity:

LPF m t


16

DSB Modulation/Demodulation (cont.)

Frequency Domain:

X jc

c

m t

from frequency shifting property of the Fourier Transform:

c

b b

M j

M j0

0 5 0. A M jc USB

LSB

0

b c


17

DSB Modulation/Demodulation (cont.) X jb

2c 2c

0 5 0. A M jc

0

A M jc 0

H j

0

LPF 1/ Ac=

M j


18

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.

m t1

cos ct

x tc

m t2

sin ct


19

QDSB Demodulation

m t1

2cos ct

x tc

m t2

2sin ct

LPF y t1

y t2LPF


20

QDSB Demodulation (cont.)

Trigonometric Identities:

cos cos2 12

1 2u u

sin cos2 12

1 2u u

cos sin sinu u u12

2

cos cos cos cosu v u v u v 12

sin cos sin sinu v u v u v 12


21

QDSB Demodulation (cont.)

x t m t t m t tc c c 1 2cos sin

y t t x t

m t t m t t t

m t t m t t

c c

c c c

c c

1

12

2

1 2

2

2 2

1 2 2

cos

cos cos sin

cos sin

LPF m t1

y t t x t

m t t m t t t

m t t m t t

c c

c c c

c c

2

22

1

2 1

2

2 2

1 2 2

sin

sin cos sin

cos sin

LPF m t2


22

Quadrature Suppression

-used to suppress transformer voltage

vessel amp LPF

oscillator

magnetcurrent

generator90o phase

shift

LPF

v tf

2cos ct

2sin ct

x tc

vt


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.


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.


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.


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.


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.


28

Case Study (cont.)

courtesy of Carolina Medical

examples of electromagnetic flowprobes


29

Case Study (cont.): example of EM flowmeter

courtesy of Carolina Medical

ee 5340, smu electrical engineering department, © 1999 1 carlos e. davila, electrical engineering...

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