Biomedical Engineering Reference
In-Depth Information
FIGURE 10.12
Linear translational (a) and angular (b) displacement transducers.
EXAMPLE PROBLEM 10.5
Calculate the change in output voltage of a linear potentiometer transducer that undergoes a
20 percent change in displacement.
Solution
Assuming that the current flowing through the transducer is constant, from Ohm's law,
D
V
¼
I
D
R
Hence, since the resistance between the sliding contact and one terminal of the resistor is
linearly proportional to the displacement, a 20 percent change in displacement will produce a
20 percent change in the output voltage of the transducer.
Elastic Resistive Transducers
In certain clinical situations, it is desirable to measure changes in the peripheral volume
of a leg when the venous outflow of blood from the leg is temporarily occluded by a blood
pressure cuff. This volume-measuring method is called plethysmography and can indicate
the presence of large venous clots in the legs. The measurement can be performed by
wrapping an elastic resistive transducer around the leg and measuring the rate of change
in resistance of the transducer as a function of time. This change corresponds to relative
changes in the blood volume of the leg. If a clot is present, it will take more time for the
blood stored in the leg to flow out through the veins after the temporary occlusion is
removed. A similar transducer can be used to follow a patient's breathing pattern by
wrapping the elastic band around the chest.
An elastic resistive transducer consists of a thin elastic tube filled with an electrically
conductive material, as illustrated in Figure 10.13. The resistance of the conductor inside
the flexible tubing is given by
r
I
A
R
¼
ð
10
:
5
Þ
where r is the resistivity of the electrically conductive material (in
O
m),
l
is the length,
and
A
is the cross-sectional area of the conductor.
