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
EE 5340, SMU Electrical Engineering Department, © 1999
1
Electromagnetic Flowmeters
L1
Vo 

B
blood
vessel

  
u  B  dL
0

L
+
Vo
_

u
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
2
Faraday’s Law
-a moving conductor in a (possibly constant) magnetic
field will have a voltage induced across it
L1
Vo 

  
u  B  dL
0
Vo : voltage induced across electrodes

u:
 velocity of blood (m/s)
B: magnetic flux density (Wb/m2)

of electrodes
L: vector in direction

L1: length of L

 
response is maximized when u , B , and L are mutually
orthogonal
EE 5340, SMU Electrical Engineering Department, © 1999
3
Toroidal Cuff Probe

B
EE 5340, SMU Electrical Engineering Department, © 1999
4
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
5
AC Flowmeter



frequency of B : 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
6
Transformer Voltage
blood
vessel

B

L

u
+ Vt _
plane of electrode wires should be parallel to magnetic
field. Otherwise, get transformer voltage, Vt, proportional

to: dB
dt
EE 5340, SMU Electrical Engineering Department, © 1999
7
Transformer Voltage (cont.)
magnet
current, im(t)
t
90o out
of phase
transformer
voltage, vt(t)
t
flow
voltage, vf(t)
t
0 or 180o
out of phase,
depending on
flow direction
EE 5340, SMU Electrical Engineering Department, © 1999
8
Removal of Transformer Voltage



Phantom Electrode
Gating Flow Voltage
Quadrature Suppression
EE 5340, SMU Electrical Engineering Department, © 1999
9
Phantom Electrode
blood
vessel
adjust until transformer
voltage = 0

u
+ Vt _
EE 5340, SMU Electrical Engineering Department, © 1999
10
Gating Flow Voltage
magnet
current, im(t)
t
transformer
voltage, vt(t)
t
flow
voltage, vf(t)
t
sample flow voltage when transformer voltage = 0
EE 5340, SMU Electrical Engineering Department, © 1999
11
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
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12
Amplitude Modulation/Demodulation
mt  : information-bearing signal
c : carrier frequency
Modulation:
A
mt 
S
Ac cosct 
xc t 

Demodulation (envelope detector):
+
xc t 
_
C
+
R m t 
_
EE 5340, SMU Electrical Engineering Department, © 1999
13
Double Sideband (DSB) Modulation/Demodulation
modulation:
Ac cosct 
mt 
demodulation:
xc t 
c : carrier frequency
xc t 

2 cosct 

m(t) can be bipolar
this demodulator is
phase sensitive
xb t 
LPF
m t 
carrier frequency and phase must be known
EE 5340, SMU Electrical Engineering Department, © 1999
14
DSB Modulation/Demodulation (cont.)
xb t   2mt  cos2 ct 
trigonometric identity:
1
cos   1  cos 2 
2
2
 xb t   mt 1  cos2ct 
 mt   mt  cos2ct 
LPF
m t 
EE 5340, SMU Electrical Engineering Department, © 1999
15
DSB Modulation/Demodulation (cont.)
Frequency Domain:
M  j
b  c
M  j0
mt  
 b
b

from frequency shifting property of the Fourier Transform:
X c  j
LSB
0.5 Ac M  j 0
 c
0
USB
c
EE 5340, SMU Electrical Engineering Department, © 1999

16
DSB Modulation/Demodulation (cont.)
X b  j
Ac M  j 0
0.5 Ac M  j 0
 2c
0
2c
H  j

1/ Ac
LPF

=
0
  j
M

EE 5340, SMU Electrical Engineering Department, © 1999
17
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.
cosct 
m1t 
m2 t 

S
xc t 

sinct 
EE 5340, SMU Electrical Engineering Department, © 1999
18
QDSB Demodulation
2 cosct 

xc t 

y1t 
LPF
y2 t 
LPF
m1t 
m 2 t 
2 sinct 
EE 5340, SMU Electrical Engineering Department, © 1999
19
QDSB Demodulation (cont.)
Trigonometric Identities:
1
1  cos 2u
2
1
2
sin u  1  cos 2u
2
1
cos u sin u  sin 2u
2
1
cos u cos v  cosu  v   cosu  v 
2
cos2 u 
1
sin u cos v  sinu  v   sinu  v 
2
EE 5340, SMU Electrical Engineering Department, © 1999
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QDSB Demodulation (cont.)
xc t   m1t  cosct   m2 t  sinct 
y1 t   2 cosct  xc t 
 2m1 t  cos2 ct   2m2 t  cosct  sinct 
 m1 t 1  cos2ct   m2 t  sin2ct 
LPF
m 1t 
y2 t   2 sin c t  xc t 
 2m2 t  sin 2  c t   2m1t  cos c t  sin c t 
 m2 t 1  cos2 c t   m1t  sin2 c t 
LPF
m
 2 t 
EE 5340, SMU Electrical Engineering Department, © 1999
21
Quadrature Suppression
-used to suppress transformer voltage
amp
vessel
vt
magnet
current
generator
LPF
90o phase
shift
oscillator
xc t 

LPF
v f t 

2 sinct 
2 cosct 
EE 5340, SMU Electrical Engineering Department, © 1999
22
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
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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
24
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
25
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
26
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
27
Case Study (cont.)
examples of electromagnetic flowprobes
courtesy of Carolina Medical
EE 5340, SMU Electrical Engineering Department, © 1999
28
Case Study (cont.): example of EM flowmeter
courtesy of Carolina Medical
EE 5340, SMU Electrical Engineering Department, © 1999
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