- Biomedical Transducers Definition: Biomedical transducers are devices that convert biological parameters into electrical signals, enabling the measurement of factors like temperature and blood pressure.
- Active Transducers: These transducers convert energy without an external power source, examples include photovoltaic cells and piezoelectric transducers.
- Passive Transducers: These transducers need an external power source to function, examples include strain gauges and photoresistors.
- Magnetic Induction Transducers: These active transducers generate voltage when an electrical conductor moves through a magnetic field, useful in medical devices like heart sound microphones.
- Thermoelectric Transducers: These transducers work based on the Seebeck effect, generating voltage from temperature differences, and are used in remote sensing and medical storage.
Non-electrical parameters such as temperature, heart sound and blood pressure are measured from the human body with electronic equipment. Biomedical Transducers are the devices that convert those biological parameters to electrical signals. The process of conversion is transduction. Generally, transducers convert one form of energy into another form of energy.
Two common transducer classes are active and passive.
Active Transducers: These convert one form of energy into another without needing an external power source. For example, a photovoltaic cell converts light energy into electrical energy.
Types of Active Transducers
- Magnetic Induction Type
- Piezoelectric Type
- Photovoltaic Type
- Thermoelectric Type
2. Passive Transducers: These convert one form of energy into another with the help of an external power source. They utilize 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
Active Transducers
Magnetic Induction Type Transducers
When an electrical conductor moves through a magnetic field, it changes the magnetic flux, producing a voltage proportional to the flux change rate.
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 shows that the induced EMF opposes the change in flux, so the induced current is set in that opposing sense.
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
Piezoelectric Transducers
When compression or tension is applied to the crystal, charge separation occurs in the crystals. This produces electrical voltage resulting in the piezoelectric effect. Piezoelectric transducers convert displacement or pressure into an electrical value. Barium titanate, Rochelle salt and lithium niobate are a few piezoelectric transducer materials.
Applications of Piezoelectric Transducers
- Piezoelectric Transducer acts as a pulse sensor to measure the pulse rate of a human.
Photovoltaic Transducers
When light or any other radiation of suitable wavelength falls on a metal or semiconductor surface, it can eject electrons. That emission is the photoelectric effect. Photoemissive, photoconductive and photovoltaic are 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 a silicon photovoltaic cell can act as one form of pulse sensor; photodiodes are also widely used.
- To measure sodium and potassium ion concentration in a sample using light absorption in some laboratory methods.
Thermoelectric Transducers
These transducers work based on the Seebeck Effect. The 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 a doctor’s cold box, thermocouples can monitor storage temperature of plasma, antibiotics and similar supplies; thermistors are also common in that role.
Passive Transducers
Resistive Transducers
Resistive transducers include strain gauge, photodiode, phototransistor and thermistor. For the resistive members of that group, the measured parameter causes a small change in the transducer resistance. A Wheatstone bridge typically measures this resistance change. A photodiode is an optical junction device, listed here with the other optical pickups.
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.
Capacitive Transducers
A capacitor has two conducting surfaces. A dielectric medium acts as the separating gap between two surfaces. Capacitive transducers measure a change in displacement from a change in the area of conducting plates, the thickness of the dielectric medium or the distance between the plates.
Applications of Capacitive Transducers
- Differential capacitive transducers measure blood pressure.
Inductive Transducers
An 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 can act as a physiological pressure sensor.
Application of Inductive Transducers
- To measure tremor in patients suffering from Parkinson’s disease.
