ee 5340 7340 introduction to biomedical engineering electromagnetic flowprobes
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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

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

electromagnetic flowmeters
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

faraday s law
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

toroidal cuff probe
Toroidal Cuff Probe

EE 5340, SMU Electrical Engineering Department, © 1999

dc flowmeter
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

ac flowmeter
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

transformer voltage
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

transformer voltage cont
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

removal of transformer voltage
Removal of Transformer Voltage
  • Phantom Electrode
  • Gating Flow Voltage
  • Quadrature Suppression

EE 5340, SMU Electrical Engineering Department, © 1999

phantom electrode
Phantom Electrode

blood

vessel

adjust until transformer

voltage = 0

_

Vt

+

EE 5340, SMU Electrical Engineering Department, © 1999

gating flow voltage
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

quadrature suppression
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

amplitude modulation demodulation
SAmplitude Modulation/Demodulation

: information-bearing signal

Modulation:

carrier frequency

A

Demodulation (envelope detector):

+

+

C

R

_

_

EE 5340, SMU Electrical Engineering Department, © 1999

double sideband dsb modulation demodulation
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

dsb modulation demodulation cont
DSB Modulation/Demodulation (cont.)

trigonometric identity:

LPF

EE 5340, SMU Electrical Engineering Department, © 1999

dsb modulation demodulation cont16
DSB Modulation/Demodulation (cont.)

Frequency Domain:

from frequency shifting property of the Fourier Transform:

LSB

USB

0

EE 5340, SMU Electrical Engineering Department, © 1999

dsb modulation demodulation cont17
DSB Modulation/Demodulation (cont.)

0

LPF

0

=

EE 5340, SMU Electrical Engineering Department, © 1999

quadrature dsb qdsb modulation
SQuadrature 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

qdsb demodulation
LPF

LPF

QDSB Demodulation

EE 5340, SMU Electrical Engineering Department, © 1999

qdsb demodulation cont
QDSB Demodulation (cont.)

Trigonometric Identities:

EE 5340, SMU Electrical Engineering Department, © 1999

qdsb demodulation cont21
QDSB Demodulation (cont.)

LPF

LPF

EE 5340, SMU Electrical Engineering Department, © 1999

quadrature suppression22
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

electromagnetic flowprobe case study cliniflow ii carolina medical
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

case study cont
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

case study cont25
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

case study cont26
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

case study cont27
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

case study cont28
Case Study (cont.)

examples of electromagnetic flowprobes

courtesy of Carolina Medical

EE 5340, SMU Electrical Engineering Department, © 1999

case study cont example of em flowmeter
Case Study (cont.): example of EM flowmeter

courtesy of Carolina Medical

EE 5340, SMU Electrical Engineering Department, © 1999

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