2. Transducer
• Transducers are devices which convert one form of energy into another.
• Now a days it is the usual practice to convert all non-electric phenomenon associated
with the physiological events into electric quantities.
• Variation in electric circuit parameters like resistance, capacitance and inductance in
accordance with the events to be measured, is the simplest of such methods.
3. There are number of factors decide the choice of a particular transducer to be used for
the study of a specific phenomenon.
• The magnitude of quantity to be measured.
• The order of accuracy required.
• The static or dynamic character of the process to be studied.
• The site of application on the patient’s body, both for short-term and long-term
monitoring.
• Economic considerations.
4. Classification of Transducers
1. By the process used to convert the signal energy into an electrical signal.
Active Transducers—a transducer that converts one form of energy
directly into another. For example: photovoltaic cell in which light energy is
converted into electrical energy.
Passive Transducers—a transducer that requires energy to be put into it
in order to translate changes due to the measurand.
2. By the physical or chemical principles used. For example: variable
resistance devices, Hall effect devices and optical fibre transducers.
3. By application for measuring a specific physiological variable. For example:
flow transducers, pressure transducers, temperature transducers, etc.
5. Performance characteristics of a transducer
• Static characteristics
• Accuracy- It shows that how close the output reading of the instrument to
the correct value.
• Precision- How closely individual computed measured value agrees with
each other.
• Sensitivity- It describes transfer ratio of output to input.
• Drift-It defines the amount by which an instrument sensitivity of
measurement varies as the ambient condition changes.
• Threshold- If the input of an instrument is gradually increasing from zero,
the input will have to reach a certain minimum level before the change in
the instrument output readings is of large enough magnitude to be
detectable that minimum level is called threshold.
• Resolution- Smallest change in the measured variable at which the
instrument respond.
6. • Dynamic characteristics- It is the study of behaviour of an instrument between
the time measured quantity(input) changes value and time when the instrument
output attain a steady state.
• For any dynamic system, the order of the differential equation that describes
the system is called the order of the system. Most medical instrumentation
systems can be classified into zero-, first-, second-, and higher-order systems.
• Zero Order- It has an ideal dynamic performance, because the output is
proportional to the input for all frequencies and there is no amplitude or
phase distortion. A linear potentiometer used as a displacement trasduser
is a good example of a zero-order transducer.
• First Order- The first-order transducer or instrument is characterized by a
linear differential equation. The temperature transducers are typical
examples of first order measuring devices.
• Second Order- The output varies up to second order differentiation then
we call it as second order instrumentation system. A typical example a
second-order system is the spring—mass system of the measurement of
force.
• Other characteristics- There are other characteristics also which determine the
performance and choice of a transducer like Input and output impedance, overlead
range, Recovery time after overload, Excitation voltage, Size and weight
8. Variable Resistance Transducers
These are of two types – Resistive track transducers and Strain Gauges.
• Resistive Track Transducers- Wiping contact may either move round a circular
resistive track or linearly along a straight track
For example- Caliper Myograph, Spirometer
• Strain Gauge- Resistive transducer that can convert a mechanical response to
electrical response through a fractional change in the resistance
9. Variable Capacitance Transducers
Works on following relationships
Where
C= Capacitance
A= Area of plates
k =Dielectric constant (relative permittivity of medium)
εo=8.854 X 10-12 F/m
d= Distance b/w plates
• C changes as distance between plates or area of plates change.
• ΔC is the converted into an electrical signal that is rectified, amplified and recorded
10. Variable Inductance Transducers
Changes in the inductance can be used to measure displacement. The inductance can
be changed either by varying its physical dimensions or by changing the permeability
of its magnetic core. The core having a permeability higher than air can be made
to move through the coil in relation to the displacement. The changes in the
inductance can be measured using an ac signal which would then correspond to the
displacement.
LVDT (Linear Variable Differential Transformer)
LVDT is an absolute position or displacement transducers that converts a position or
linear displacement of this core from a mechanical reference(position) into a
proportional electrical signal that contains a phase and a amplitude.
11. • Consists of 3 coils (1 primary and 2 secondary).
• Primary (central coil) is excited ay an AC which produce an electric field that
induces equal voltages across secondary coils.
• Secondary coils are connected in series opposition so that resultant secondary
voltage is zero.
• The insertion of a core imbalances the system and voltage generated by
secondaries are no longer equal
12. Piezoelectric Transducers
Certain crystalline materials when subjected to mechanical stress, develop surface
electric charges. These are categorized as anisotropic materials (Electrical and
mechanical properties differ along different directions)
On application of pressure, the charge Q developed along a particular axis is given by
Q = dF coulomb
where d is the piezo-electric constant (expressed in Coulombs/Newton, i.e. C/N) and F
is the applied force. The change in voltage can be found by assuming that the system
acts like a parallel— plate capacitor where the voltage Eo across the capacitor is charge
Q divided by capacitance C. Therefore
Eo = Q/C = dF/C
The capacitance between two parallel plates of area ‘a’ separated by distance ‘x’ is
given by C =ε.a/x
where ε is the dielectric constant of the insulator between the capacitor plates. Hence,
Eo = (d/ε) .(F/a).x = g.P.x
where d/ ε = g is defined as the voltage sensitivity in volts, P is the pressure acting on
the crystal
per unit area and x is the thickness of the crystal.
13. Applications
• Crystal microphones for phonocardiography.
• Catheter tip sensors for intracardiac phonocardiography.
• Medical Ultrasound and Ultrasonic blood flow meters.
• Occlusive cuff method for blood pressure measurement.
14. Pressure Transducer
Pressure is a very valuable parameter in the medical field and therefore many devices
have been developed to effect its transduction to measurable electrical signals. The
basic principle behind all these pressure transducers is that the pressure to be measured
is applied to a flexible diaphragm which gets deformed by the action of the pressure
exerted on it. This motion of the diaphragm is then measured in terms of an electrical
signal.
The most commonly employed pressure transducers which make use of the diaphragm
are of the following types:
• Capacitance manometer—in which the diaphragm forms one plate of a capacitor.
• Differential transformer—where the diaphragm is attached to the core of a
differential transformer.
• Strain gauge—where the strain gauge bridge is attached to the diaphragm.
15. Strain Gauge Pressure Transducer
Nearly all commercially available pressure monitoring systems use the strain gauge
type pressure transducers for intra-arterial and intravenous pressure measurements.
The transducer is based upon the changes in resistance of a wire produced due to small
mechanical displacements. A linear relation exists between the deformation and
electric resistance of a suitably selected gauge (wire, foil) over a specified range.
• Unbonded Strain Gauges- Most of the pressure transducers for the direct
measurement of blood pressure are of the unbonded wire strain gauge type. The
arrangement consists in mounting strain wires of two frames which may move with
respect to each other.
16. The outer frame is fixed and the inner frame which is
connected to the diaphragm upon which the pressure
acts, is movable. A pressure Papplied in the direction
shown (Fig. 3.5) stretches wires B and C and relaxes
wires A and D. These wires form a four arm active bridge.
The moving frame is mounted on springs which brings it
to the central reference position when no pressure is
applied to the diaphragm.
The unbonded strain gauge transducers are preferred
when low pressure measurements are to be made since
hysteresis errors are much less than would be the case if
wire gauges were bound to the diaphragm. Unbonded
strain gauge transducers can be made sufficiently small,
which are even suitable for mounting at the tip of a
cardiac catheter.
17. Bonded Strain Gauges: The bonded strain gauge consists of strain-sensitive gauges which
are firmly bonded with an adhesive to the membrane or diaphragm whose movement is to
be recorded.
By using a pair of strain gauges and mounting them one above the other, the changes in the
resistances of the two gauges arising from the changes in the ambient temperature can be
cancelled. Also, one strain gauge would increase while the other would decrease in
resistance when the pressure is applied to them.
Silicon Bonded Strain Gauges: In recent years, there has
been an increasing tendency to use bonded gauges made
from a silicon semiconductor instead of from bonded wire or
foil strain gauges. This is because of its higher gauge factor
resulting in a greater sensitivity and potential for
miniaturization.
18. Transducers for body temperature measurement
The most popular method of measuring temperature is by using a mercury-in-glass
thermometer. However, they are slow, difficult to read and susceptible to contamination.
Also, reliable accuracy cannot be attained by these thermometers. The continuous
reading facility of electronic thermometers obviously lends itself to such applications.
Electronic thermometers are convenient, reliable and generally more accurate in practice
than mercury-in-glass thermometers for medical applications. They mostly use probes
incorporating a thermistor or thermocouple sensor which have rapid response
characteristics. The probes are generally reusable and their covers are disposable.
Thermocouples- Thermocouples measure the temperature difference between two
points, NOT absolute temperature.
When two wires of different materials are joined together at either end, forming two
junctions which are maintained at different temperatures, a thermo-electromotive force
(emf) is generated causing a current to flow around the circuit. This arrangement is called
a thermocouple.
19. Thermistor- A thermistor is a type of resistor with resistance varying according to
its temperature.
Principle:
ΔR = f (ΔT)----working p/p
ΔT = f (ΔR)----recording p/p
Assuming, as a first-order approximation, that the relationship between resistance
and temperature is linear, then:
ΔR = kΔT
Where,
ΔR = change in resistance, ΔT = change in temperature
k = first-order temperature coefficient of resistance
Thermistors can be classified into two types depending on the sign of k.
• PTC: If k is positive: device is called a positive temperature coefficient (PTC)
thermistor, or posistor.
• NTC: If k is negative: device is called a negative temperature coefficient (NTC)
thermistor.
20. Radiation Thermometry- Any material placed above absolute zero temperature
emits electromagnetic radiation from its surface. Both the amplitude and frequency of
the emitted radiation depends on the temperature of the object.
Infrared thermometers have significant advantages over both glass and thermistor
thermometers used orally, rectally or axillarily. They eliminate reliance on conduction
and instead measure the body’s natural radiation. They use an ideal measurement site—
the tympanic membrane of the ear which is a function of the core body temperature. It
is a dry, non-mucous membrane site that minimizes risks of cross-contamination.The
disadvantage of infrared thermometers is their high cost as compared to other types of
thermometers.
