Biomedical Engineering Reference
In-Depth Information
EXAMPLE PROBLEM 10.4
Calculate the voltage induced in a magnetic flow probe if the probe is applied across a blood
vessel with a diameter of 5
10
3
m and the velocity of blood is 5
10
2
m/s. Assume that
10
5
Wb/m
2
.
the magnitude of the magnetic field, B, is equal to 1.5
Solution
From Eq. (10.4),
10
5
Wb
m
2
10
3
m
10
2
m
10
10
V
V
¼
B
l
u
¼ð
1
:
5
=
Þð
5
Þð
5
=
s
Þ¼
37
:
5
(
: [Wb]
¼
[V
S])
Note
Practically, this device consists of a clip-on probe that fits snugly around the blood ves-
sel, as illustrated in Figure 10.11. The probe contains electrical coils to produce an electro-
magnetic field that is transverse to the direction of blood flow. The coil is usually excited
by an AC current. A pair of very small biopotential electrodes are attached to the housing
and rest against the wall of the blood vessel to pick up the induced potential. The flow-
induced voltage is an AC voltage at the same frequency as the excitation voltage. Using
an AC method instead of DC excitation helps to remove any offset potential error due to
the contact between the vessel wall and the biopotential electrodes.
Potentiometer Transducers
A potentiometer is a resistive-type transducer that converts either linear or angular
displacement into an output voltage by moving a sliding contact along the surface of a
resistive element. Figure 10.12 illustrates linear and angular-type potentiometric trans-
ducers. A voltage,
, between
the sliding contact and one terminal of the resistor is linearly proportional to the displace-
ment. Typically, a constant current source is passed through the variable resistor, and
the small change in output voltage is measured by a sensitive voltmeter using Ohm's law
(i.e.,
V
i
, is applied across the resistor,
R
. The output voltage,
Vo
I
¼
V/R
).
FIGURE 10.11
Electromagnetic blood flow transducer.
