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A transducer is a device that converts one form of energy or variable into another form of energy or variable. A transducer is required to convert physiological variables into electrical signals which are easier to be further processed.
We have a number of factors which are considered in choice of a particular transducer to be used for the study of a specific phenomenon. These factors include:
Any transducer is used in association with suitable electronic circuitry to give a final electrical output of convenient amplitude; for data storage and processing. The relationship between the input variable and output variable can be:
Numerous methods have been developed for the conversion of non-electric phenomenon that is associated with the physiological events into electrical quantities. Many physical, chemical & optical properties and principles can be employed in the construction of transducers that are used in the Biomedical Instrumentation Systems. These transducers can be classified in four ways:
An active transducer directly converts input variable into electrical signals. For example photovoltaic cell in which light energy is converted into electrical energy. Active transducers do not require any external source of power for their operation. Depending upon the principle of transduction, they are further classified as:
Passive transducer modifies either the excitation voltage or modulates the carrier signals. Passive transducers require an external source of power to make them operative. They are further classified according to the principle of transduction as:
A good example of a passive transducer application is the variable resistance placed in a Wheatstone bridge in which the voltage at the output of the circuit reflects the physical variable. In this application, the actual transducer is a passive circuit element that needs to be powered by an ac or dc excitation signal.
Find out more about: Fingertip Pulse Oximeter Blood Oxygen Saturation Monitor
2. The Physical or chemical principles used for example:
3. By application for measuring a specific physiological variable. Examples include:
4. Digital, in digital transducers, a direct digital output of the amplitude of the measurand is obtained. There are a number of digital transducers like: Encoders which give digitally coded output pulses, and event counting type which provide a frequency output.
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Table: Type of active transducers, working principle and typical applications
| Type of Transducer | Principle of Operation | Typical Application | |
| 1. | Moving Coil Generator | Motion of a coil in a magnetic field induces an output voltage. | Used in measurement of velocity and vibration. |
| 2. | Photovoltaic | A voltage is generated in a semiconductor junction in a solar when simulated by a radiant light energy. | Physiological signals modulate light intensity and variation is measured as current output of solar cell |
| 3. | Piezoelectric effect | An emf is generated when an external force is applied to a crystalline material like Quartz. | This type of transducer is used for measurement of sound, vibration, acceleration, and pressure variation. |
| 4. | Thermocouple | When one of the junctions of two dissimilar metals is heated and the other is cooled then an emf is generated across the two junctions. | This transducer is used to measure temperature, heat flow and radiation. |
A biomedical transducer has two elements:
A sensing element is that part of the transducer which corresponds to any physical phenomenon or its change. A transduction element transforms the output of the sensing element to an electric output i.e. the transduction element works as a secondary transducer.
In medical applications, the physiological variables can be transformed into some suitable variable which can be measured conveniently. This is known as transduction principle.
Table: Physical Variables and Transducers used
| Physical Variables | Transducers | |
| 1 | Displacement | Variable resistance Variable Inductance Variable reluctance Variable Capacitance Strain gauge |
| 2 | Velocity | Magnetic inductance |
| 3 | Surface strain | Strain gauge |
| 4 | Force/pressure | Strain gauge Piezoelectric |
| 5 | Temperature | Thermocouple Thermistor |
| 6 | Light/infrared | Photovoltaic Photoresistor |
| 7 | Magnetic field | Hall effect |
A force sensing element is used for the conversion of force into physical variables. The force can be transformed into:
The figure below show workings of these transducers with force transduction:
Pressure is a very important parameter used in many medical devices as a principle of transduction. The basic principle behind these pressure transducers is that 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 deformation is measured by a strain gauge or LVDT. In essence we have 3 main pressure transducers which make use of the diaphragm:
Displacement transducers can be used in both direct and indirect systems of measurement e.g. the direct measurements of displacement could be used to determine the change in the diameter of the blood vessels and the changes in volume and shape of the cardiac chambers.
The indirect measurements of displacement are used to quantify the movement of liquids through the heart valves. Displacement transducers can be conveniently converted into pressure transducers by attaching a diaphragm to the moving member of a transducer such that the pressure is applied to the diaphragm. A good application for this is the detection of the movement of the heart indirectly through the movement of a microphone diaphragm.
Displacement measurements form the basis of many transducers for measuring pressure, force, acceleration, temperature, etc. for example we can show displacement (D), velocity (V) and acceleration (a) are interlinked:
If we know one out of the above three variables, then we can find the other two variables either by differentiation or by integration. Though velocity and displacement transducers are readily variable, their application in medical field are difficult, hence displacement and velocity are measured by indirect methods like magnetic or optical methods.
We look at several types of transducers used in medical instrumentation systems.
This type of transducer consists of a transformer with one primary winding and two secondary windings. The secondary windings are connected in such a way that their induced voltages oppose each other as shown below:
If the core is positioned in the central position, with respect to both the secondary windings, the voltage induced in each of the secondary windings is equal and opposite in direction; thereby the net output voltage from them is zero. If the core is moved upwards the voltage induced in the secondary winding 1 is greater than that induced in the secondary winding 2. Similarly the voltage in the secondary winding 2 will be greater than that induced in the secondary winding 1, if the core is moved down. Therefore, the output voltage will vary as per the movement of the core which is moving as per the input variable. The output voltage can be further processed to be displayed e.g. calibrated in terms of mm of displacement.
For the range of displacements from about 0.1 mm to 10 mm, a simple photoelectric transducer is often suitable. This consists of a constant light source, a variable width slit, and a photocell. The light must be maintained at a constant voltage from a regulated supply. The collimating lens produces a parallel beam of light which is intercepted by a slit whose width is varied by displacement. The light is then focused on a suitable photocell whose output is a measure of the displacement. For large displacements, a single slit is used; for small displacements, greater sensitivity can be obtained if a number of slits are used.
A potentiometer can be used to convert linear or rotary displacement into a proportional change of resistance. The resistance between two terminals on this device is related to the linear or angular displacement of a sliding tap a long resistance.
In the linear displacement potentiometer shown below, the reading of the voltmeter at point C is given as:
Fig –linear displacement potentiometer
For the rotational displacement potentiometer shown below, pointer C rotates as per the input variable, the reading of the voltmeter at point C is given as:
The capacitance of a parallel plate capacitor depends upon the area of the two parallel plates and the distance between the parallel plates i.e. the capacitance C is proportional to the Area A of a plate, inversely proportional to the plate spacing d (dielectric thickness) and depends on the nature of the dielectric.
The capacitance can be changed by varying any of the 3 parameters excluding εo. It is the parameter d which is usually changed in such type of transducers.
In linear displacement type capacitance, one plate is made to move in order to change the capacitance as per the input variable. In the angular displacement capacitance transducer, one plate remains fixed while the other plate rotates to vary the capacitance as per the input variable.
The induction of a coil can be changed by varying its physical dimension or by changing the permeability of its magnetic core. Changes in the inductance can be used to measure displacement by varying any of the 3 coil parameters given in the following equation:
L = n2Gμ
Where,
L = Inductance of the coil
N = number of turns of a coil
μ =permeability of the medium
G = Geometric form factor
The core having 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.
In this type of transducer, the core remains stationery inside the coil but the air gap in the magnetic path of the core is varied in order to change the permeability of the coil.
The output signals from the coil vary as per the input variables. In this case, the displacement is of the iron plate from the core.
The electric characteristics of, p-n semi conductive junctions diodes and transistors are affected by light. The incident light generates electron-hole pairs. Photodiodes and Phototransistors are constructed to enhance current using this property. Photodiodes and Phototransistors are generally made from Silicon but if response in the infrared region is required, Germanium devices are used. The diode or transistor is fitted with lens or a window to admit light on to the junction. The diode is reverse-biased and the reverse current increases with increase in radiation. This current can then be amplified using OPAMPs. The spectral response and photocell characteristics are shown below:
Some circuits using a photocell for amplifying the current are shown in the figure below; Arrays of photodiodes coupled with an amplifier in a single encapsulation.
Photoconductive cells
These are devices whose resistances changes with incident light. They are thin films of materials like Selenium, Germanium, Silicon or metallic halides and sulphides like Cadmium Sulphate deposited on a ceramic base, encapsulated in a plastic, glass or metal case. They are widely used in densitometers, electronic camera shutters, automatic garage door openers etc.
Photomultiplier Tubes
It is a vacuum tube with a window for light to pass onto a photoemissive cathode which emits electrons in accordance with the intensity of light radiation. This is followed by an electron multiplier section which consists of a number of anodes called dynodes which emit a large number of secondary electrons for every electron impinging on it, thus producing a large current. The potential of each dynode is 90 V above the previous dynode.
The piezoelectric effect is a property of natural crystalline substances to develop electric potential along a crystallographic axis in response to the movement of charge as a result of mechanical deformation.
Piezoelectric transducers depend on the fact that many crystals show the property of piezo-electricity. If stressed along a suitable axis, an electric charge appears between opposite faces of the crystal. Conversely, if a voltage is applied between the faces, the crystal deforms along the original axis. Piezoelectric materials normally used in electronic equipment are either Quartz, or Synthetics such as Barium Titanate.
If the stress along the axis is maintained, the charge produced leaks away, both around the crystal and through the associated measuring circuit. This limits the use of piezoelectric strain gauges to the measurement of sudden changes in applied force or of alternating forces, rather than to the measurement of slowly varying or steady forces.
Piezoelectric materials have a high resistance and therefore, when a static deflection is applied, the charge leaks through the leakage resistor. It is therefore important that the input impedance of the voltage measuring device must be higher than that of the piezoelectric sensor.
Piezoelectric transducers are used in many medical instrumentation applications for example; they are used in detection of korotkoff sounds in non-invasive blood pressure measurements. They are used in ultrasonic scanners for imaging and blood flow measurements and they are also used in external and internal phonocardiography.
If a wire is subjected to tension, it increases in length and decreases in diameter; provided that, tension is within the elastic limits of the wire, it will return to its original shape when the tension is removed. Both the increase, in length and the decrease in diameter contribute to an increase in electrical resistance; which is quite accurately proportional to the tension applied.
The strain in the wire is defined as extension per unit length, δl/l. The increase in the resistance per unit resistance, δR/R is related to the strain by a gauge factor, which depends on the wire material and is usually about 2.
Whenever axial tensile force is applied to any metallic element, it increases the length and decreases the area of the element. The resistance of the element is proportional to its length and inversely proportional to its area R α L/A hence the resistance of the element will increase depending upon the magnitude of the axial force applied. The resistance will similarly decrease when a compressive axial force is applied on the element. The gauge factor is defined as:
By connecting the wire as one arm of a Wheatstone bridge, the tension applied can be measured. However, the temperature coefficient of resistance of the wire will be about 1% for every 2.5 °C, so this system is unusable in practice. To compensate for ambient temperature changes, it is usual to make all 4 arms of the bridge; tension-detecting elements, this also gives four times the output for a given tension.
Two types of strain gauge are in common use. The Unbonded and Bonded strain gauges.
Unbonded Strain Gauge
In order to obtain sufficient resistance for each arm (four arms of Wheatstone bridge), several turns of a thin wire are put in between two ends/posts. Four posts are mounted on the stationery part of the transducer and other four posts are mounted on the moving part of the transducer. The moving part can move to right or left with respect to the stationery part. The diagrams below show more details.
In case the moving part moves towards the left, then the resistances R1 and R2 increases due stretching while it decreases in R3 and R4 due to relaxing of resistive wires. The change in the resistance as per the linear movement of the moving part of the transducer is indicated by the voltmeter reading.
Many commercial forms of unbonded strain gauge are available; they are often used in parallel with a much heavier spring, to increase the force required to give maximum output.
The Bonded Strain Gauge
This consists of a very fine etched metal film attached to a thin elastic backing; in use four strain gauges are rigidly cemented by an epoxy resin to the surface whose extension is to be measured, and connected as a Wheatstone bridge so that two are lengthened and two either unchanged or shortened.
This type of transducer gives the output directly in the digital format. They are encoded disks or rulers with digital patterns photographically etched on glass plates. These patterns are decoded using a light source and an array of photodetectors (photo-diodes or photo-transistors). A digital signal that indicates the position of the encoding disk is obtained which represents the displacement being measured. An example of the patterns of digital encoders is shown below:
This encoder consists of a cylindrical disk with the coding patterns arranged in concentric rings, having a defined number of segments on each ring. The number of segments on the concentric rings in a binary count (32-16-8-4-2) from a total of 32 (16 conductive and 16 non-conductive) on the outside ring to two on the inside ring, each angular position of the disk would have a different combination of segments which will indicate the position of the shaft on which the disk is mounted.
In most cases the body temperature is measured using a mercury-in-glass thermometer, however these type of thermometers, are slow, difficult to read and are easily susceptible to contamination. They are also, not with a reliable accuracy.
Continuous or frequent sampling of temperature is important in critical medical applications. Electronic thermometers are reliable, convenient, and generally more accurate than the mercury-in-glass thermometers. In most medical applications, they use probes that incorporate a thermistor or thermocouple sensor which have rapid response characteristics.
We discuss more on Resistance Thermometry, Thermistors, and Thermocouples towards the end of this article. In the meantime, we can look at the Radiation Thermometry.
Any material placed above absolute zero temperature emits electromagnetic radiation from its surface. The amplitude and frequency of the emitted radiation depends on the temperature of the object. The cooler the object, the lower frequency of the emitted waves; and less the power is emitted. The temperature of the object can be determined from the power emitted.
Infrared thermometers, measure the magnitude of infrared power (flux), in abroad spectral range, typically from 4 to 14 micrometres. They make no contact with the object measured.
The detectors used for measuring the emitted infrared radiation are Thermopiles, Pyroelectric sensors, Golay cells, Photoconductive cells, and Photovoltaic cells.
Hand-held infrared scanners are now available for monitoring the pattern of skin temperature changes. This measurement is based on a new type of sensor called, ‘’Pyroelectric sensor”.
A Pyroelectric Sensor develops an electric charge that is a function of its temperature gradient. The sensor contains a crystalline flake which is pre-processed to orient its polarized crystals. Temperature variation from infrared light striking the crystal changes the crystalline orientation, resulting in development of an electric charge. The charge creates a current which can be accurately measured and related to the temperature of the tympanic membrane.
The infrared Thermometer incorporated into a scanner can be used to scan the entire surface of the body or some part of the body just like a Television Camera and thereby provide a thermograph.
They have a high cost compared to other types of thermometers.
The Wheatstone bridge is used for measurement of unknown resistances by comparing with a standard resistance. The basic circuit is shown below:
Omitting the meter G for the moment, the circuit may be regarded as two voltage dividers ABD and ACD connected in parallel across the source voltage E.
Let us find the condition for these two potentials to be equal, so that inserting the meter G between B and C will produce no current from B to C through the meter. This will occur when,
In this condition, where the meter reads zero, the bridge is said to be balanced.
If R3 and R4 are equal resistors, balance occurs when R1 = R2; If R1 is an unknown variable resistor, R1 may be measured by setting R2 for balance. Whatever the ratio of R3 to R4, the same ratio of R1 to R2 will give the balance e.g. If R3 is 10 times R4 and the bridge is balanced, then R1 is 10 times R2. In this way, resistors larger or smaller than the standard R2 can be measured.
Although the condition of balance is independent of E, the deflection of the meter G for a small imbalance is directly proportional to E, and is greatest when the four arms are of equal resistance.
Wheatstone bridge is a highly accurate instrument as a laboratory instrument and although quite suitable for biomedical electronics workshop, it is rather slow to use.
The Wheatstone bridge is widely used for resistance thermometry. All metallic conductors increase in resistance as the temperature is raised; the most suitable for resistance thermometry are nickel and platinum. Both of these metals increase in resistance by about 0.4% for each degree Celcius rise.
Besides metals, a large range of semiconductor materials shows considerable variations with temperature, usually giving a fall in resistance as the temperature rises. This property is used in the construction of Thermistors. A typical Thermistor consists of a bead of semiconductor material between two fine platinum wires, and mounted in a small glass envelope. The resistance of a thermistor is usually quoted at 20°C; typically it will halve for every 25°C rise in temperature. The resistance is exponential as a function of temperature.
The Wheatstone bridge circuit is often used as a pseudo-bridge. In this form, three arms are fixed in resistance, and the fourth; which may be a resistor, is allowed to vary. The meter current is used as a measure of resistance of the fourth arm. If the fourth arm varies only over a small range near balance, the meter current is reasonably close to being directly proportional to the resistance of the fourth arm.
Thermistors are the oxides of certain metals like cobalt, manganese and nickel which have large negative temperature coefficient that is, the resistance of the thermistor shows a fall with increase in temperature. The resistance temperature relation for a thermistor is given by:
Where R and Ro are resistance values at temperatures, T and To respectively. β is a characteristic constant of the material. The reference To is generally taken as 25°C or 298K and β = 4000
Thermistors can be formed into disks, beads, rods or any other desired shapes. Thermistor probes are available with resistance varying from a few hundred ohms to several mega ohms.
Most Thermistor thermometers use the principle of Wheatstone bridge to determine voltage output which varies as per the input temperature.
A potentiometer is a device for comparing an unknown voltage with a standard voltage. The principle of working of a potentiometer is shown below:
The figure above is essentially a calibrated voltage divider. A long uniform wire is set up beside a scale calibrated in volts (In our case 0 to 1.5) and its ends connected to a 2 V battery A in series with a variable calibrating resistor R. Any voltage V from 0 to 1.5 V can be picked off the slide wire between the origin and the moving point. This voltage is connected in series with a sensitive meter and source of emf, E less than 1.5 V which is to be measured. Since E is connected in opposition to V, the meter will read zero when they are exactly equal.
The potentiometer is first calibrated by putting accurately known standard source of voltage at E (standard cells), setting the moving point to the value of the standard cell voltage on the scale, and adjusting R until the meter reads zero. The scale is now reading correctly for its whole length. An unknown emf, source may now be substituted at E, and the moving point adjusted until the meter again reads zero. The unknown emf is then read off the scale.
The potentiometer (in the form of a recording instrument) is widely used for the thermocouple thermometry. If two dissimilar metals are connected as shown in the figure below and the junctions 1 and 2 are held at different temperatures, a small emf will be generated, which will be proportional to the difference in temperature, and of a magnitude depending on the two metals. For temperature difference of 100°C the emf is typically about 5 mV. For accurate measurements, one junction is held at constant temperature (often at 0°C in melting ice) and the other junction is used as a temperature measuring probe. If it is impossible to maintain a melting ice reference, the reference junction can be maintained at room temperature, and the potentiometer fitted with a special compensating unit.
The amount of voltage change per degree temperature change of the junction varies with kinds of metals making up the junction. The voltage sensitivities of thermocouples made of various metals is given in the table below:
Table: Thermal emf for various types of thermocouple
| Type | Thermocouple | Useful range °C | Sensitivity at 20 °C (mV/°C) |
| T | Copper-constantan | -150 to +350 | 45 |
| J | Iron-constantan | -150 to +1000 | 52 |
| K | Chromel-alumel | -200 to +1200 | 40 |
| S | Platinum-rhodium (platinum (90%, rhodium (10%) | 0 to +1500 | 6.4 |
For medical applications, copper-constantan combination is usually preferred. With the reference junction at 0°C and the other at 37.5 °C the output from the Thermocouple is 1.5 mV.
As indicated earlier, potentiometer is used with a thermocouple to measure potential difference. In addition to this, moving coil movements are used as millivoltmeters to measure the thermocouple emf. They are directly calibrated in temperature units. Normally clinical thermocouple instruments reflecting galvanometers or light spot galvanometers are preferred to measure and display temperature. They can also be used to read on modern digital voltmeters.
This transducer consists of several thermocouples of the same material connected in series and arranged so that the hot junctions are at one focal point as shown in the figure below:
The output voltage is thus equal to nV, where n is the number of thermocouples in the assembly and V is the output of one thermocouple. The Thermopile is used in the optical radiation pyrometer where optical radiation from a heated source is focused on the hot junction. This avoids physical contact with the heated surface.
Table: Physiological Parameters and the Transducers used
| Physiological Parameter | Transducer required |
| Cardiac output (blood flow) | Dye dilution method Thermal dilution method Electromagnetic flowmeter and integrator |
| Heart rate | Electrocardiograph (ECG) Foetal phonocardiogram-ultrasonic method Scalp electrodes for foetal ECG |
| Phonocardiogram (heart sounds) | Crystal or moving coil microphone |
| Respiration rate | CO2 detectors Strain gauge detectors Thermistor Doppler shift transducer Impedance pneumography electrodes |
| Pneumotachogram (respiration flow rate) | BMR spirometers Fleisch pneumotachograph |
| Impedance Pneumogram | Surface or needle electrodes |
| Tidal volume (volume/breath) | Direct from spirometer Integrated from pneumotachogram |
| Impedance cardiogram | Surface or needle electrodes |
| Blood pressure (arterial, direct) | Unbonded wire strain gauge Bonded semiconductor strain gauge Capacitance pressure transducer Differential transformer (LVDT) |
| Blood pressure (arterial, indirect) | Low frequency microphone for picking up korotkoff sounds |
| Blood pressure (venous, direct) | Strain gauge pressure transducer with higher sensitivity |
| Blood flow (aortic or venous) | Electromagnetic flowmeter Ultrasonic Doppler flowmeter Tracer methods |
| Gas in expired air | Infrared sensors Mass spectrometer |
| Oximetry | Photoelectric pulse pick-up Photoelectric flow-through cuvette |
You can also read: Biomedical signal acquisition instruments
Each time when the heart contracts, blood flows with pulse pressure moving through the cardiovascular system. Higher pressure in the circulatory system is measurable. Even the pulse or higher pressure can be felt, if we place a finger on the radial artery in wrist. The pulse can be measured by:
There are two photoelectric methods of measuring pulse; they are:
In transmitted method, light is transmitted through the patient’s fingers as shown below:
The transmitted light, reaching the other side of the patient’s finger where Photoresistor is placed, depends upon the blood flow through the finger. The transmitted light reduces when the blood flow is more.
In reflection method, the Photoresistor is kept adjacent to the lamp and the reflected light from the blood flow in the patient’s finger reaches the Photoresistor. The change of current flow due to change in resistance of Photoresistor is recorded.
The blood vessel swells when blood pulse with higher pressure moves through it. A feeler pin with a leaf spring is used to move with the enlargement of blood vessel. The strain gauge is fixed on the leaf spring and its output depends on the movement of feeler pin which is moving with the displacement of the vessel wall.
In this method, a piezoelectric crystal is subjected to the displacement stresses in the wall of the blood vessel. The soft rubber diaphragm is used for the transmission of stresses. The pulse waveform is recorded using ECG recorder.
During breathing, muscular action causes the lungs to increase or decrease their volume for inspiration and expiration of air. The measurement of respiration rate can be measured by either strain gauge based chest transducer or by thermistor based transducer.
The strain gauge type transducer is held around the chest with the help of an elastic band. The respiratory movements to produce thoracic volume change result into the resistance variance of strain gauge, forming one arm of the Wheatstone bridge circuit. Hence, the output voltage, of the Wheatstone bridge is found to vary with the chest movements, hence, the output signals correspond to the respiratory activity.
There exist a difference in temperature of inspired and expired air during respiration. The change of temperature of air in respiration can be sensed using a Thermistor based transducer. The change of resistance of thermistor with respiration rate can be used in a Wheatstone bridge to generate an output signal.
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