Non-electrical parameters such as temperature, heart sound, blood pressure are measured from the human body with electronic equipment. Transducers are the devices that convert biological parameters to electrical signals. The process of conversion is Transduction. Generally, transducers convert one form of energy into another form of energy.
Transducers are of two types.
1. Active Transducers: It converts one form of energy into another form without using an external power source. Example: Photovoltaic cell. It converts light energy into electrical energy.
Types of Active Transducers
- Magnetic Induction Type
- Piezoelectric Type
- Photovoltaic Type
- Thermoelectric Type
2. Passive Transducers: It converts one form of energy into another form with the help of an external power source. It utilizes the principle of controlling DC voltage or AC carrier signal. Example: Strain Gauge, Load cell.
Types of Passive Transducers
- Resistive Type
- Inductive type
- Capacitive Type
Magnetic Induction Type Transducers
When an electrical conductor moves in a magnetic field, it changes the magnetic flux through the conductor. This produces a voltage, which is proportional to the rate of change of flux. Induced EMF is given as
Where B is the magnetic induction, l is the length of the conductor, and V is the velocity of the moving conductor.
The negative sign indicates that the direction of induced EMF and the direction of induced current are in the opposite direction.
The inverse magnetic effect is also true. When current passes through the electrical conductor placed in the magnetic field, mechanical force F acts on the conductor.
Applications Magnetic Induction Type Transducerscers
- Electromagnetic flow meter
- Heart sound Microphones
- Indicating instruments
- Pen motor in biomedical recorders
When compression or tension is applied to the crystal, charge separation occurs in the crystals. This produces electrical voltage resulting in Piezoelectric Effect. Piezoelectric transducers convert displacement or pressure into an electrical value. Barium Titanium, Rochelle salt, Lithium Niobate are few piezoelectric transducer materials.
Applications of Piezoelectric Transducers
- Piezoelectric Transducer acts as a pulse sensor to measure the pulse rate of a human.
When light or any other radiation of wavelength falls on the metal or semiconductor surface, it ejects electrons. This is the Photoelectric Effect. Photoemissive, Photoconductive and photovoltaic are the types of Photoelectric Transducers. Among these, Photovoltaic is an active transducer which generates an electrical voltage in proportion to the radiation incident on it.
Applications of Photovoltaic Transducers
- In Photoelectric Plethysmography silicon photovoltaic cells acts as pulse sensor.
- To measure sodium and potassium ion concentration in a sample using light absorption techniques.
These transducers work based on the Seebeck Effect. Seebeck effect states that, when two junctions of the thermocouple are at two different temperatures, it generates a potential voltage. The generated voltage is proportional to the difference in temperature between two junctions of the thermocouple.
Applications of Thermoelectric Transducers
- To measure physiological temperature in remote sensing circuits and biotelemetry circuits.
- In the doctor’s cold box to store plasma, antibiotics, etc.
The strain gauge, photoresistor, photodiode, phototransistor, and thermistor come under resistive type passive transducers. They have a common working principle, which states that the measured parameter results in a small change in the resistance of a transducer. Usually, a Wheatstone bridge measures the resistance change.
Applications of Resistive Transducers
- Finger-mounted strain gauge measures small changes in blood volume flowing via the finger.
- To measure intraarterial and intravenous pressure in the body.
- LDR or photoresistor measures the pulsatile blood volume changes.
A capacitor has two conducting surfaces. A dielectric medium acts separating gap between two surfaces. Capacitive transducers measure the change in displacement due to change in the area of conducting plates, the thickness of the dielectric medium and distance between the plates.
Applications of Capacitive Transducers
- Differential capacitive transducers measure blood pressure.
Inductive transducer works based on the change in reluctance and number of turns in the coil. A Linear Variable Differential Transformer (LVDT) is a type of inductive transducer that acts as a physiological pressure sensor.
Application of Inductive Transducers
- To measure tremor in patients suffering from Parkinson’s disease.