Tuesday, 20 June 2017

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

ACTIVE 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:

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.

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