transducers for bio medical

TRANSDUCERS FOR BIOMEDICAL APPLICATIONS

Presented By: Jaspreet Singh Mehrok SLIET, EIE Department India

Transducers

Transducer a device that converts primary form of energy into

other different energy form only for measurement purposes. Primary Energy Forms: mechanical, thermal,

electromagnetic, optical, chemical, etc.

Sensor It is a wide term which covers almost everything from

human eye to trigger of a pistol. Senses the change in parameter(specific).

CLASSIFICATION OF CLASSIFICATION OF TRANSDUCERSTRANSDUCERS

Active & Passive Transducers Absolute & Relative Transducers Direct & Complex Transducers Analog & Digital Transducers Primary & secondary Transducers On the basis of principle used

Active vs Passive Transducers: Active Transducers: Add energy to the measurement environment as

part of the measurement process. Requires external power supply.

Strain gauge, potentiometer & etc.

Passive Transducers : Do not add energy as part of the measurement process

but may remove energy in their operation. Does not require external power supply

Thermocouple, photo-voltaic cell & etc.

ANALOG & DIGITAL ANALOG & DIGITAL TRANSDUCERSTRANSDUCERS

ANALOG TRANSDUCER - The transducers which convert the input quantity into an analog output which is a continuous function of time.

DIGITAL TRANSDUCERS - The transducers which convert the input quantity into digital form means in the form of pulses.

PRIMARY vs SECONDARY PRIMARY vs SECONDARY TRANSDUCERSTRANSDUCERS PRIMARY TRANSDUCERS - Some transducers

contain the mechanical as well as electrical device. The mechanical device converts the physical quantity to be measured into a mechanical signal. Such mechanical device are called as the primary transducers.

SECONDARY TRANSDUCERS - The electrical device then convert this mechanical signal into a corresponding electrical signal. Such electrical device are known as secondary transducers

CLASSIFICATION ON THE BASIS OF CLASSIFICATION ON THE BASIS OF PRINCIPLE USEDPRINCIPLE USED

Capacitive Inductive Resistive Electromagnetic Piezoelectric Photoconductive Photovoltaic

Selecting a Transducer

What is the physical quantity to be measured? Which transducer principle can best be used to measure

this quantity? What accuracy is required for this measurement?

Fundamental transducer parameters Physical conditions Environmental conditions Compatibility of the associated equipment

Reducing the total measurement error : Using in-place system calibration with corrections

performed in the data reduction Artificially controlling the environment to minimize

possible errors

Transducers for Physiological Variable Measurements

• A variable is any quantity whose value changes with time. A variable associated with the physiological processes of the body is known as a physiological variable.

• Physiological variables occur in many forms: as ionic potential, mechanical movements, hydraulic pressure ,flows and body temperature etc.

• Different transducers are used for different physiological variables.

Electrical Activity Measurement Electrodes:

Electrodes convert ionic potential into electrical signals. Used for EEG, ECG, EMG, ERG and EOG etc. Different types of Electrodes are:

1) Surface Electrodes(no. Of muscles)

These electrodes are used to obtain bioelectric potentials from the surface of the body.

2) Needle electrodes(specific to a muscle) These electrodes are inserted into body to obtain localized

measurement of potentials from a specific muscle.3) Microelectrodes(cellular level record) Electrodes have tips sufficiently small to penetrate a single cell in

order to obtain readings from within cell.

Electrical Activity Measurement(cont.)

Working of Electrodes: When metal electrodes come in contact with electrolyte then ion-

electron exchange takes place as a result of electro-chemical reaction.

One cation M+ out of the electrolyte becomes one neutral atom M taking off one free electron from the metal

One atom M out of the metal is oxidized to form

one cation M+ and giving off one free electron e- to the metal.

Half-cell potentialOxidation and reduction processes take place when metal comes in contact with Electrolyte .Net current flow is zero but there exists a potential difference depends upon the position of equilibrium and concentration of ions. That p.d. is known as half-cell potential.

Over-potentialIf there is a current between the electrode and electrolyte then half-cell potential altered due to polarization is known as over-potential.



Types of Electrodes: Perfectly Polarizable Electrodes

- only displacementdisplacement current, electrode behave like a capacitorcapacitor

example: noble metals like platinum Pt

Perfectly Non-Polarizable electrode - current passes freely across interface,

- no overpotentialoverpotential

examples: - silver/silver chloride (Ag/AgCl), - mercury/mercurous chloride

Blood Pressure

Blood pressure is an important signal in determining the functional integrity of the cardiovascular system. Scientists and physicians have been interested in blood pressure measurement for a long time.

Blood Pressure Measurement

Blood pressure measurement techniques are generally put into two broad classes:

1) DIRECT TECHNIQUES

Direct techniques of blood pressure measurement, which are also known as invasive techniques, involve a catheter to be inserted into the vascular system.

2) INDIRECT TECHNIQUES

The indirect techniques are non-invasive, with improved patient comfort and safety, but at the expense of accuracy.

Transducers for Blood Pressure Measurement

Strain GaugesResistance is related to length and area of cross-section of the resistor and resistivity of the material as By taking logarithms and differentiating both sides, the equation becomes

Dimensional

piezoresistance

Strain gage component can be related by poisson’s ratio as

Transducers for Blood Pressure Measurement(cont.)

Gage Factor of a strain gage

G is a measure of sensitivity

Think of this as a Transfer Function!

Input is strain

Output is dR

Put mercury strain gauge around an arm or chest to measure force of muscle contraction or respiration, respectively

Used in prosthesis or neonatal apnea detection, respectively

Strain Gauges


Strain Gauges


An inductor is basically a coil of wire over a “core” (usually ferrous)

It responds to electric or magnetic fields

A transformer is made of at least two coils wound over the core: one is primary and another is secondary

Primary Secondary Displacement Sensor

Inductors and tranformers work only for ac signals

Inductive Pressure Sensors ( LVDT)


Capacitive Pressure Sensors

When there is difference in P1 & P2then diaphragm moves toward low pressure side and accordingly capacitance varies. So, capacitance becomes function of pressure and that pressure can be measured by using bridge ckt.

It can be used for blood pressure measurent.


Capacitive Pressure Sensors

Pressure

An example of a capacitive sensor is a pressure sensor.In parts a, the thin sensor diaphragm remains parallel to the fixed electrode and in part b, the diaphragm deflects under applied pressure resulting in capacitance change


The other pressure sensing approach, characterized by a diaphragm in front of the fibre optic link, is based on the light intensity modulation of the reflected light caused by the pressure-induced position of the diaphragm.

Fibre-optic pressure sensor

Blood Flow

A measure of the velocity of blood in a major vessel. In a vessel of known diameter , this can be calibrated as flow and is most successful accomplished in arterial vessels. Used to estimate heart output and circulation. Requires exposure of the vessel. Flow transducer surrounds vessel. Methods of measurement include

Electro-magnetic Ultrasonic principles Fibre-optic laser Doppler flowmetry

Blood Flow Measurement

Based on Faraday’s law of induction that a conductor that moves through a uniform magnetic field, or a stationary conductor placed in a varying magnetic field generates emf on the conductor:

When blood flows in the vessel with velocity u and passes through the magnetic field B, the induced emf e measured at the electrodes is.

L

de0

LBu

For uniform B and uniform velocity profile u, the induced emf is e=BLu. Flow can be obtained by multiplying the blood velocity u with the vessel cross section A.

Electromagnetic Flow meters

Blood Flow Measurement(cont.)

Electromagnetic Flow meter Probes

• Comes in 1 mm increments for 1 ~ 24 mm diameter blood vessels

• Individual probes cost $500 each

•Only used with arteries, not veins, as collapsed veins during diastole lose contact with the electrodes

• Needless to say, this is an INVASIVE measurement!!!

• A major advantage is that it can measure instantaneous blood flow, not just average flow.


Ultrasonic Flow meters

Based on the principle of measuring the time it takes for an acoustic wave launched from a transducer to bounce off red blood cells and reflect back to the receiver.

All UT transducers, whether used for flowmeter or other applications, invariably consists of a piezoelectric material, which generates an acoustic (mechanical) wave when excited by an electrical force (the converse is also true)

UT transducers are typically used with a gel that fills the air gaps between the transducer and the object examined


Ultrasonic Flow metersThe Doppler blood-flow measurement

Doppler blood flow detectors operate by means of continuous sinusoidal excitation. The frequency difference calibrated for flow velocity can be displayed or transformed by a loudspeaker into an audio output.


Fibre-optic laser Doppler flow metry

The basic scheme of fibre-optic laser Doppler flow metry is illustrated in figure . The light of a He–Ne laser is guided by an optical fibre probe to the tissue or vascular network being studied. The light is diffusely scattered and partially absorbed within the illuminated volume. Light hitting moving blood cells undergoes a slight Doppler shift. The blood flow rate is derived by the spectrum-analysis of the back-scattered signal, which presents aflow-dependent Doppler-shifted frequency.

Temperature

Systematic Temperature: A measure of the basic temperature of the complete organism. Measured by

thermometer, oral thermistor probe.

Skin Temperature: Measurement of the skin temperature at a specific part of body surface. Measured by

thermistors placed at surface of the skin , infrared thermometer or thermograph.

Temperature Measurement

Thermistors are made from semiconductor material.

Generally, they have a negative temperature coefficient (NTC), that is NTC thermistors are most commonly used.

Ro is the resistance at a reference point (in the limit, absolute 0).

Thermistor

Temperature Measurement(cont.)

Seebeck Effect

When a pair of dissimilar metals are joined at one end, and there is a temperature difference between the joined ends and the open ends, thermal emf is generated, which can be measured in the open ends.

This forms the basis of thermocouples.

In a bimetallic strip, each metal has a different thermal coefficient…this results in electromagnetic force/emf or bending of the metals.

Thermocouples


Fiber Optics

Most of the light is trapped in the core, but if the cladding is temperature sensitive (e.g. due to expansion), it might allow some light to leak through.

-> hence the amount of light transmitted would be proportional to temperature

-> since you are measuring small changes in light level, this sensor is exquisitely sensitive


Liquid-in-glass Thermometer

A common form of mercury-in-glass is a solid-stem glass thermometer shown in figure. When bulb comes in contact with temperature then mercury expands and gives direct value on main scale.

Respiration sensorsThe primary function of the respiratory system are to supply oxygen to the tissues and remove carbon-dioxide from tissues. Several types of transducers have been developed for measurement of respiration rate.

1)Strain Gauge type chest transducer The transducer is held by an elastic band which goes around the chest. The respiratory movements result in resistance change of the strain gauge element connected in Wheatstone bridge. The bridge output varies with chest expansion and yields signals corresponding to respiratory activity.

2) Thermistor Air is warmed during its passage through the lungs and there is a detectable temperature difference between inspired and expired air. Temperature change can be measured by thermistor and it gives rate of change of resistance and hence calibrated in terms of respiration rate.

Pulse sensors

Heart rate measurement is one of the very important parameters of the human cardiovascular system. The heart rate of a healthy adult at rest is around 72 beats per minute (bpm). Basically, the device consists of an infrared transmitter LED and an infrared sensor photo-transistor. The transmitter-sensor pair is clipped on one of the fingers of the subject. The LED emits infrared light to the finger of the subject. The photo-transistor detects this light beam and measures the change of blood volume through the finger artery. This signal, which is in the form of pulses is then amplified and filtered suitably and is fed to a low-cost microcontroller for analysis and display

Pulse Sensor(cont.)

The microcontroller counts the number of pulses over a fixed time interval and thus obtains the heart rate of the subject. Several such readings are obtained over a known period of time and the results are averaged to give a more accurate reading of the heart rate. The calculated heart rate is displayed on an LCD in beats-per-minute in the following format: Rate = nnn bpm

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