Fundamentals of Biomedical Engineering

PASSIVETRANSDUCERS

1.The passive transducer consists of a usually passive circuit element which changes its value as a function of the physical variable developed by physiological signal to be measured. There are only three passive circuit elements that can be used to change voltage at the output of the cercuit according to the physical variable : (1) resistors (2) capacitors and (3) inductors. The passive transducer is part of a cercuit normally an arrangement similar to a wheatstone bridge which is powered by an ac or dc excitation.

2.Principle of Wheatstone Bridge. The circuit is as shown in the figure. These are four

resistances R1, R2, R3 and R4 connected to a DC source (V0). The voltmeter (V) indicates the difference of potential between junction ‘A’ and ‘B’. The value of potential

Any variation in resistance in any of arm will vary the potential between junction ‘A’ and ‘B’ which can be read by the voltmeter. In unbonded strain gauge, the arrangement is made such that resistance in arms R1 and R4 is reduced by ∆R and resistance in the arms R2 and R3 are increased by ∆R which gives potential at junction ‘A' = V0

‘A’ and ‘B’ where ∆R depends upon the

variable input.

+ Vo

Resistive Passive Transducers

3.Potentiometer : An ordinary potentiometer can be used to convert displaement or rotary motion into a change of resistance. In linear displacement potentiometer, the reading of the voltmeter at point ‘C’ = v/l

× l1 which depends upon the position of the pointer ‘C’. Similarly rotational

displacement potentiometer, the pointer ‘C’ rotates as per the input variable.

Linear Displacement

C

Rotational Displacement

4.Strain gauge : The resistance of a resistive element is proportional to length and inversely proportional to area i.e.,R = r l/A (r = resistivity, l = length and A = area). If a tensile force is applied to extend its length and reduce area, then resistance of the resistive element will increase. Similarly if

compressive force is applied, its resistance will decreases. We define gauge factor as

∆R / R

G = ∆L/L . The strain gauge can be (1)

unbonded and (2) bonded in order to obtain sufficient resistance for each arm (four arms of wheatstone bridge). Several turns of thin wire are used between two posts (refer to figure). Four posts are mounted on stationary part and other four posts are connected to the transducer which can move to right or left with respect to the stationary part. If

moving part moves to right, resistive elements R2 and R3 are relaxed tension. As explained in the principle of wheatstone, the change of resistance is indicated by the voltmeter which depends upon the linear movement of the transducer. In the bonded strain gauge as shown is the figure, a thin resistance wire is shaped in a zigzag manner and it is cemented between two paper / foil covers.

The compact strain gauge can be easily cemented to the surface of a structure / body and any change in surface dimension is indicated by the change of the resistance of the strain gauge.

5.Solved Example (strain gauge). A strain

gauge (l = 0.1 meter) is bonded to a surface (Area = 4 cm2) having modulus of elasticity E = 200 G N/m2. The strain gauge unstrained resistance is 200 ohm and gauge factor (G) = 10. When load is applied, the resistance changes by 0.01 ohm. Find the stress and load applied.

Stress σ = ε × E

= 0.5 × 10–5 × 200 × 109 = 1 × 106 N/m2

Force = σ × A

= 1 × 106 × 4 × 10–4 = 400 N

INDUCTIVE PASSIVETRANSDUCERS

1.Variable induction : The property of inductance is varied in the cercuit to change

the output voltage in accordance with the input variable. The inductance L = n2 Gµ

(n = number of turns in coil, G = form factor of coil and µ = permeability of core material inside the coil). Though induction can be varied by any of these three parameters, but generally arrangement is made to change the permeability to achieve variation in induction of the cercuit as per the variable input. In the passive induction transducer, the core is made of a soft magnetic material which changes the induction of the coil when it is moved inside or outside, thereby the output of ac signal is

changed as per the displacement of the core in the coil.

Core

AC Signals

Induction Displacement Transducer

2.Variable reductance : In this, core remains stationary inside the coil but the air gap in the magnetic path of the core is varied to change the net permeability, thereby varying the output signal as per the input variable (displacement).

Displacem ent

Plate

iron

Core

Output

Variable Reductance Transducer

3.Linear Variable differential transformer (LVDT). The transducer consists of a transformer with one primary and two secondary windings. The secondary windings are connected as shown in the figure so as that their induced voltages oppose each other. If the core is positioned in central position as shown in the figure, the voltage induced in both secondary windings is equal and opposite, thereby the output voltage is zero. If the core is moved upwards, the voltage in secondary 1 is greater than voltage in secondary 2. Similarly the voltage in secondary 2 will be greater than voltage in secondary 1 if the core moves down. The output voltage will vary as per the movement of the core which is changing as per input variable.

LVDT

PASSIVE CAPACITANCETRANSDUCERS

1.Variable capacitance : The capacitance (C) of a capacitor having two parallel plates of area ‘A’ which are separated by a distance

A

‘d’ is : C = Εo Εr d (Ε0 = dielectric

constant of free space and Εr = relative dielectric constant). The capacitance can be changed by varying any of the parametes but it is parameter ‘d’ (separation between plates) which is usually changed in the transducer. In the linear displacement type capacitance transducer, one plate of capacitor is fixed while other plate is movable to change the capacitance as per the input variable. In the angular displacement capacitance transducer, one plate is fixed while other plate rotates to change the capacitance as per the input variable.

Capacitation

Displacem ent ‘d’

Variation of Capacitance with Displacement

Fixed

plate

θ

M ovable

Capacitance

Angular displacement (θ)

Variation of Capacitance with Angular Displacement

TEMPERATURE MEASUREMENT

1.The physiological state of an individual is indicated by his body temperature. It has been seen that a person in shock has reduced blood pressure in circulating system which results into low body temperature. Infection and illness are usually reflected by a high body temperature. Special heated incubators are used for maintaining the body temperature of infants. The temperature of the joint of an arthritic patient is closely linked with the amount of local inflammation. The temperature can be measured by (1) thermocouples (2) thermistor and (3) radiation and fiber optic detectors. The principle of thermocouple has already been explained in para 5 of this chapter.

2.Thermistors: It is a shortened word for thermo and resistor which means that they are semi conductors having a high negative temperature coefficient. The resistance of

thermistors decreases as temperature increase while resistance increases as temperature decreases. The resistance of thermistor can be given as

Thermistors can be formed into disks, beads, rods or any desired shapes. Thermistor probes are available with resistance from a few hundred ohms to several megohms. Most thermistor thermometers use the principle of wheatstone bridge to obtain a voltage output which varies as per input temperature.

3.Infrared thermometers: Our skin is perfect emitter of infrared radiation and the energy emitted is proportion to the body temperature. A device sensitive to infrared radiation can measure the emitted energy from a patient without clothing (room

temperature 21ºC) and directly indicates the body temperature. Such type of thermometers can detect areas of poor circulation, locate breast cancer or other unknown sources of heat in the body. The thermograph is an infrared thermometer incorporated into a scanner which can be used to scan entire surface of body or some part of body like a television camera. The infrared energy detected in scanning is used to modulate the intensity of a light beam so that to get the image on the photographic film in which the brightness depends on the detected infrared radiation. The image is called a thermogram.

4.Transduction principle & applications:

Biomedical transducer consists of two parts:

(1)sensing element and (2) transduction element. A detector or sensing element is that part of a transducer which responds to a physical phenomenon or its change. A transduction element transforms the output of a sensing element to an electrical output. Hence transduction element acts as a secondary transducer:

5.Transduction principle : Several basic physical variables and the transducers available for measurement are listed as under :

variable can be transformed into one of the physical variable (as listed in the table) which can be measured very conveniently. This is known as transduction principle.

PHYSIOLOGICAL SIGNALS AND TRANSDUCERS

6.Force transduction : A force summing member is used for the conversion of physical variables. The force can be transformed into (1) surface strain (figure ‘a’) (2) displacement (figure ‘b’) (3) output voltage in LVDT (figure ‘c’) (4) photo resistivity (figure ‘d’).

Strain gaugeForce

Figure (a)

Transduction w ith strain gauge

Force

Displacement

Figure (b)

Transduction in D isplacem ent

Force

Screen

Transduction in variable resistivity

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7.Transduction for displacement, velocity and acceleration : The parameters of displacement (D), velocity (V) and acceleration are interlinked as under :

we can also write above relations as under

D = ∫V ∂t = ∫∫ A∂2t V = ∫ A∂t

If we know one out or three variable, then we can find out other two variables by differentiation or integration. Though velocity and displacement transducers are readily available, but their applications in biomedical are difficult. Therefore displacement and velocity are measured by indirect methods like magnetic or optical methods.

8.Pressure transduction : Pressure is measured using diaphragm which gets deformed under pressure. The deformation is measured with the help unbonded strain gauge or LVDT. The output of these devices varies as per input pressure variable. The transducers using flat or corrugated diaphragms are designed to work on the principle of variable capacitance or reluctance. The diaphragms are usually for moderate pressure ranges and bourdon tubes are used for high pressure ranges. The diaphragm type transducers infect measure gauge pressure (the blood pressure at one side of diaphragm which gets deformed against atmospheric pressure). The absolute pressure can be measured if there is a vacuum at one side of the diaphragm.

9.Analog to Digital transduction: Digital data can be easily processed by computers which require that the output of transducers or instruments should be in digital form. If the output of a transducer is not in digital form, a device to convert the output from analog to digital form has to be used. Generally transducers contain encoded disks

or rulers with digital pattern which are photographically transformed on them. These patterns can be decoded with the help of a light source and photodiode or photo transistor. The encoded disk rotates and a digital signal indicating its position is obtained in digital form.

PHYSIOLOGICAL SIGNALS AND TRANSDUCERS

10.Biopotential electrodes:The biopotential electrodes can be grouped as under :

(a) Microelectrodes: They measure bioelectric potential near or within a single cell. Their tips are sufficiently small to penetrate a single cell to get the potential from the cell.

(b) Body surface electrodes: These electrodes do not penetrate the skin or cell but they are fixed on the surface of the body to measure the potential. ECG, EEG and EMG are obtained by using these electrodes. The floating electrodes are the latest

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version of these type of electrodes which eliminate direct contact of metal with skin by use of electrolyte paste or jelly, thereby permitting conductive paste between metal and skin.

(c) Needle electrodes : These electrodes are designed to penetrate the skin to record EEG potential of a region of the brain or EMG potential of a muscle. They are infact sharp and small subdermal needles to easily penetrate the scalp for EEG. They are required to penetrate up to surface at some depth of the skin which is parallel to brain or muscle.

Fill in the gaps

1.------- is a device which converts one form of energy into another. (a) Transducer (b) biomechanism

2.The ------- transducer directly converts input variable into electrical signal. (a) active (b) passive

inversely proportional (b) inversely proportional, proportional

6.The wheat stone bridge principle is used in

------- strain gauge to find out input variable. (a) mercury (b) unbonded

7.In LVDT, the induced voltages of two secondary windings ------- each other. (a) add (b) oppose

(b) amplifying

10.The ------- element acts as a secondary transducer. (a) sensing (b) transduction

INTRODUCTION

1.Signal processing (same as signal conditioning) has a rich history and its importance is evident in diverse fields like radar, data communication, nuclear sciences and biomedical engineering. In applications like EEG or systems for speech transmission or speech recognition, we like to extract some characteristic parameters. Alternatively we may like to remove interference such as noise from the signal or to modify the signal to present it in a form which is more easily interpreted by an expert. Also a signal transmitted from input to output stage or

over a communication channel is corrupted in a variety of ways like distortion, fading and insertion of background noise. In such cases, processing of the signals is essential.

2.To understand signal processing and its requirement, we take the example of wheat and the process involved in converting the wheat into flour. We have to process the wheat in the first stage and then analyse the flour we get. Depending upon the grade required for the flour, we filter the contents and reprocess the whole thing till the wheat is transformed into flour of the required grade. Signal processing may be seen in a similar manner.