Capacitive and Inductive Transducers

Capacitive Transducers:

Capacitive transducer is a measurement device in which variations in pressure upon a capacitive element proportionally change the element’s capacitive rating and thus the strength of the measured electric signal from the device. As we know here the capacitance of the transducer varies with respect to the external stimulas. Normally a capacitive transducer uses a stationary plate and a movable plate. The stationary and the movable plates are separated by aair or vacuum dielectric. The movable plate changes its position under the influence of an external stimulus. As we know the capacitance depends on the area of plates, the movement of plate causes a change in capacitance. This is the basic working principle of a capacitive transducer.

Advantages of Capacitive transducers:

Compared with optical, piezo-resistive and inductive transducers, capacitive transducers have many advantages.
1. Low cost,
2. Less power usage,
3. Good stability,
4. High resolution,
5. Better speed,
6. Good frequency response
7. They require very little force to operate..

Disadvantages of Capacitive transducers:

1. The performance is severely affected by dirt and contaminants as they change the dielectric constant.
2. They are sensitive to temperature variations
3. Metallic parts must be insulated from each other

Biomedical Application:

1. It is mainly used for blood pressure measurements.
2. Also used for displacement or position measurement.
3. They can detect motion, acceleration, flow and many other variables.

Inductive Transducers:

The inductance of a coil can be changed either by changing its physical dimension or by changing the effective permeability of its magnetic core. In transducers using inductive elements we normally use the second principle. The effective permeability can be changed by moving a core, which is having a permeability higher than, the air through the coil. But the disadvantage with basic type of inductive transducer is that the inductance of coil is not related linearly to the displacement of coil if large displacements occur accidently. It is to avoid this disadvantage that we use LVDTs. The existing inductive transducers have a relatively small output power and often require amplification of their outputs.

Transducers and Sensors

A transducer is a device that performs the conversion of one form of variable into another. Normally in biomedical applications, the transducer input is in non-electrical form and the output will be in electrical form. Actually transducers and electrodes are a part of a group of devices called sensors. Major difference between transducers and electrodes is that transducers make use of some transducible element for the measurement. But the electrodes directly measure the signals. A sensor means a device used to measure a particular parameter either by changing the desired signal to electrical signal or by changing ionic flow into electron flow.

Two types of principles are involved in the process of converting non-electrical variable into electrical signal. Depending on these principles the transducers are mainly classified into two types. - Active transducers and Passive transducers


An active transducer is one which gives its output without the use of excitation voltage or modulation of a carrier signal. One property of active transducer is that it converts non-electrical energy into electrical energy and vice-versa.
The various types of active transducers are
a.Magnetic induction type transducers
b. Piezoelectric type transducers
c. Photovoltaic type transducers
d. Thermoelectric type transducers

A. Magnetic induction type Transducers:

We know that when an electrical conductor is moved in a magnetic field in such a way that flux through the conductor is changed, a voltage is induced. The induced voltage will be proportional to the rate of change of magnetic flux which in turn is proportional to the conductor movement.. The induced emf is given by, e = Blv

Biomedical Applications:
1. These type of transducers are used in electromagnetic flow meters to measure blood flow.
2. Also used in heart sound microphones.

B. Piezoelectric type transducers:

Piezoelectricity is the ability of some materials to generate an electric field or electric potential in response to applied mechanical stress. The effect is related to a change of polarization density within the materials volume and is also reversible-means the production of stress or strain when an electric field is applied. A piezoelectric crystal (such as Quartz) can produce a voltage under deformation by compression or tension. This is called piezoelectric effect. This effect is the basic working principle of piezoelectric type transducers. So they convert displacement or pressure into an electrical variable.

Biomedical Application:
Mainly used in pulse sensing measurements.

C. Photo voltaic type of transducers:

Photoelectric effect is the ejection of electron from a metal or semi conductor surface when it is illuminated by light or any other radiation of suitable wavelength. So a photoelectric transducer generates electrical voltage in proportion to the radiant energy incident on it.

Biomedical Application:
Used in pulse sensors.

D. Thermoelectric type transducers:

These types of transducers are based on the seeback effect. It states that when two junctions of a thermocouple are maintained at different temperatures, an emf is generated which wil be proportional to the temperature difference between the junctions. Thermocouples are widely used type of temperature sensor for measurement and control. They are inexpensive, interchangeable and are supplied with standard connectors and can measure a wide range of temperatures. Also we can use thermistors as active transducers. Here its resistance value changes with change in temperature. The material used in a thermistor is usually a ceramic or polymer.

Advantages: Small size, Low cost, Fast response, Wide temperature range, High accuracy

Disadvantages: Thermistors are unsuitable for wide temperature ranges, Less stable at high temperatures, Non-linear temperature-resistance curve.

Biomedical Application:
1. Used in the measurement of physiological temperature
2. In biotelemetry systems to measure temperature.


Passive transducers convert the physiological parameter (such as blood pressure, temperature etc) into an electrical output using a DC or AC excitation voltage. One important property of the passive transducers is that they are not reversible. Passive components such as resistors, capacitors, inductors are used to make the passive transducers. They require an external power to operate and the output is a measure of some variation in the passive components.

Wheatstone Bridge
Many transducers make use of the principle of Wheatstone bridge. In many biomedical transducers using Wheatstone bridge, all four resistances are equal under balanced condition. The balanced condition is varied when any of the resistances varies. Normally during the measurement, it is designed in such a way that the resistance value of a particular resistance varies and hence balance is lost. By analyzing this change in resistance, the parameter can be measured indirectly.

Electrodes in Biomedical Instrumentation

Electrodes are devices that convert  ionic potentials into electronic potentials.  The type of electrode used for the measurements depends on the anatomical location of the bioelectric event to be measured. In order to process the signal in electronic circuits, it will be better to convert ionic conduction into electronic conduction. So simply bio-electrodes are a class of sensors that transdues ionic conduction into electronic conduction. The purpose of bio-electrodes is to acquire bioelectrical signals such as ECG, EMG, EEG etc.

Electrodes are mainly classified into two. They are perfectly polarized electrodes and perfectly non-polarized electrodes. There are a wide variety of electrodes which can be used to measure bioelectric events. The three main classes of electrodes are Microelectrodes, Body Surface electrodes and Needle electrodes.

A. Microelectrodes

Microelectrodes are electrodes with tips having tips sufficiently small enough to penetrate a single cell in order to obtain readings from within the cell. The tips must be small enough to permit penetration without damaging the minute cell. The main functions of microelectrodes are potential recording and current injection. Microelectrodes are having high impedances in mega ohn range because of their smaller size. Microelectrodes are generally of two types. With the use of a microelectrode or an array of microelectrodes, researchers can gather all sort of information regarding living organism.
a. Metal type b. Micropipette type

a. Metal microelectrode: Metal microelectrodes are formed by electrolytically etching the tip of fine tungsten to the desired size and dimension. Then the wire is coated almost to the tip with any type of insulating material. The metal-ion interface takes place where the metal tip contacts the electrolyte. The main features of metal microelectrodes are
1. Very good S/N ratio
2. Strong enough to penetrate
3. High biocompatibility

b. Micropipette: The micropipette type of microelectrode is a glass micropipette with its tip drawn out to the desired size. The micropipette is filled with an electrolyte which should be compatible with the cellular fluids. A micropipette is a small and extremely fine pointed pipette used in making microinjections. A commercial type of micropipette is shown in figure below.

B. Body Surface Electrodes:

Surface electrodes are those which are placed in contact with the skin of the subject in order to obtain bioelectric potentials from the surface. Body surface electrodes are of many sizes and types. In spite of the type, any surface electrode can be used to sense ECG, EEG, EMG etc. The various types of body surface electrodes are discussed below. Major body surface electrodes are

1. Immersion electrodes: They are one of the first type of bioelectric measuring electrodes. Immersion electrodes were simply buckets of saline solution in which the subject placed his hands and feet. So it was not a comfortable type of measurement and hence it was replaced with plate electrodes.

2. Plate electrodes: These electrodes were separated from subject’s skin by cotton pads socked in a strong saline solution. The plate electrodes have generally smaller contact area and they do not totally seal on the patient. The electrode slippage and displacement of plates were the major difficulties faced by these type of electrodes because they have a tendency to lose their adhesive ability as a result of contact with fluids on or near the patient. Since these types of electrodes were very sensitive, it led to measurement errors.

3. Floating electrodes: These types of electrodes can eliminate the movement errors (called artifacts) which is a main problem with plate electrodes. This is done by avoiding any direct contact of the metal with the skin. So the main advantage of floating electrodes is mechanical reliability. Here the conductive path between the metal and the skin is the electrolyte paste or jelly.

4. Disposable electrodes: Normally plate electrodes, floating electrodes etc can be used more than one time. This requires the cleaning and cares after each use. We can use disposable electrodes which can be used only once and be dsposed after the use. These types of electrodes are now widely used.

5. Suction electrodes: These type of electrodes are well suited for the attachment to flat surfaces of body and to regions where the underlying tissue is soft, due to the presence of contact surface. An advantage of these type of electrodes is that it has a small surface area. These types of electrodes are mainly used for the measurement of ECG. Suction electrodes used a plastic syringe barrel to house suction tubing and input cables to an AC amplifier.

6. Ear clip & Scalp electrodes: These type of electrodes are widely used in the measurement of EEG exclusively. Scalp electrodes can provide EEG easily by placing it over bare head. A typical ear clip electrode is shown in figure below. The most common method for EEG measurement is 10 – 20 electrode placement system and here we use scalp electrode usually. They can avoid measurement errors and movement errors. During labour internal monitoring may be needed and is usually in the form of an electrode placed under the baby’s scalp. It is called fetal scalp electrode which is used to monitor baby’s heartbeat while still in uterus.

C. Needle Electrodes:

To reduce the interface and noise (artifact) caused due to electrode movement, during the measurement of EEG, EMG etc we can use small sub-dermal needle electrodes which penetrate the scalp. Actually the needle electrodes are not inserted into the brain. They nearly penetrate the skin. Generally they are simply inserted through a small section of the skin just beneath the skin parallel to it.
The needle electrodes for EMG measurement consist of fine insulated wires placed in such a way that their tips are in contact with the muscle, nerve or other tissues from which the measurement is made. The needle creates the hole necessary for insertion and the wires forming the electrodes are carried inside it. A typical EEG needle electrode is shown in figure.

One of the main advantage of needle electrodes is that they are less susceptible to movement errors than surface electrodes. Also the needle electrodes have lower impedances when compared to surface electrodes as it makes direct contact with the sub-dermal tissues or intracellular fluid.

CRO and its Applications

An oscilloscope is previously called as an oscillograph. It can be informally known as a scope, CRO (for cathode-ray oscilloscope), or DSO (for the more modern digital storage oscilloscope). It is a type of electronic test instrument that allows observation of constantly varying signal voltages. The output of a CRO will be usually as a two-dimensional graph of one or more electrical potential differences using the vertical or y-axis, plotted as a function of time (horizontal or x-axis). Many signals (e.g. sine, cosine etc) can be converted to voltages and can be displayed this way. By changing the mode of CRO into transfer characteristics, we can see the transfer characteristics of signals. The transfer characteristics of a signal give the variation of the output wave with respect to the variation to the input wave. The Signals may be either periodic or repeat constantly, so that the multiple samples of a signal which is actually varying with respect to time can be displayed as a steady picture in the CRO. Many oscilloscopes (storage oscilloscopes) are able to capture non-repeating waveforms for a specified period of time, and are able to produce a steady display of the captured segment.

Oscilloscopes are mainly used to observe the correct wave shape of an electrical signal. Oscilloscopes are usually calibrated so that the two axes voltage and time can be easily read as well as possible by the eye. This will yields to the measurement of the peak-to-peak voltage of a waveform. It also allows checking the frequency of periodic signals, the time between pulses, the time taken for a signal to rise to full amplitude, which is usually called as the rise time, and relative timing of several related signals.

The applications of Oscilloscopes are in the fields of sciences, engineering, medicine, and telecommunications industry. For the maintenance of electronic equipment and laboratory work, general-purpose instruments are used. Special-purpose oscilloscopes are used for analyzing an automotive ignition system or to display the waveform of the heartbeat as an electrocardiogram. If we consider some practical example, some computer sound software allows the sound being listened to be displayed on the screen as by an oscilloscope. Thus CRO can be used in many applications in the emerging world technologies. Most of all, the CRO can be considered as the eye of an electronic engineer. In other words an electronic engineer cannot see his outputs without the help of a CRO.

Some special storage CRTs are used to maintain a steady display of a single brief signal in case of advanced storage oscilloscopes. By digital storage oscilloscopes (DSOs) with thin panel displays, fast analog-to-digital converters and digital signal processors, CROs were later largely outdated. DSOs without integrated displays are known as digitizers. Digitizers are available at lower cost, and it uses a general-purpose digital computer to process and display the required waveforms.