BIOMEDICAL SENSORS AND MEASUREMENT SYSTEMS.ppt
- 1. TRANSDUCERS
- 2. Basic Sensors and Principles 2
- 4. INTRODUCTION OF TRANSDUCERS • A transducer is a device that convert one form of energy to other form. It converts the measurand to a usable electrical signal. • In other word it is a device that is capable of converting the physical quantity into a proportional electrical quantity such as voltage or current. Pressure Voltage
- 5. BLOCK DIAGRAM OF TRANSDUCERS • Transducer contains two parts that are closely related to each other i.e. the sensing element and transduction element. • The sensing element is called as the sensor. It is device producing measurable response to change in physical conditions. • The transduction element convert the sensor output to suitable electrical form.
- 7. CHARACTERISTICS OF TRANSDUCERS 1. Ruggedness 2. Linearity 3. Repeatability 4. Accuracy 5. High stability and reliability 6. Speed of response 7. Sensitivity 8. Small size
- 8. TRANSDUCERS SELECTION FACTORS 1. Operating Principle: The transducer are many times selected on the basis of operating principle used by them. The operating principle used may be resistive, inductive, capacitive , optoelectronic, piezo electric etc. 2. Sensitivity: The transducer must be sensitive enough to produce detectable output. 3. Operating Range: The transducer should maintain the range requirement and have a good resolution over the entire range. 4. Accuracy: High accuracy is assured. 5. Cross sensitivity: It has to be taken into account when measuring mechanical quantities. There are situation where the actual quantity is being measured is in one plane and the transducer is subjected to variation in another plan. 6. Errors: The transducer should maintain the expected input- output relationship as described by the transfer function so as to avoid errors.
- 9. Contd. 7. Transient and frequency response : The transducer should meet the desired time domain specification like peak overshoot, rise time, setting time and small dynamic error. 8. Loading Effects: The transducer should have a high input impedance and low output impedance to avoid loading effects. 9. Environmental Compatibility: It should be assured that the transducer selected to work under specified environmental conditions maintains its input- output relationship and does not break down. 10. Insensitivity to unwanted signals: The transducer should be minimally sensitive to unwanted signals and highly sensitive to desired signals.
- 10. Classification of Transducers Transducers On The Basis of principle Used Active/Passive Primary/Secondary Analogue/Digital Capacitive Inductive Resistive Transducers/ Inverse Transducers Transducers may be classified according to their application, method of energy conversion, nature of the output signal, and so on. 10
- 11. • Active transducers : • These transducers do not need any external source of power for their operation. Therefore they are also called as self generating type transducers. I. The active transducer are self generating devices which operate under the energy conversion principle. II. As the output of active transducers we get an equivalent electrical output signal e.g. temperature or strain to electric potential, without any external source of energy being used. ACTIVE AND PASSIVE TRANSDUCERS
- 12. Example of active transducer: Piezoelectric Transducer
- 13. CLASSIFICATION OF ACTIVE TRANSDUCERS
- 14. • Passive Transducers : I. These transducers need external source of power for their operation. So they are not self generating type transducers. II. A DC power supply or an audio frequency generator is used as an external power source. III. These transducers produce the output signal in the form of variation in resistance, capacitance, inductance or some other electrical parameter in response to the quantity to be measured. PASSIVE TRANSDUCERS
- 16. PRIMARY AND SECONDARY 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, because they deal with the physical quantity to be measured. •The electrical device then convert this mechanical signal into a corresponding electrical signal. Such electrical device are known as secondary transducers.
- 18. CLASSIFICATION OF TRANSDUCERS According to Transduction Principle
- 19. CAPACITIVE TRANSDUCER: •In capacitive transduction transducers the measurand is converted to a change in the capacitance. • A typical capacitor is comprised of two parallel plates of conducting material separated by an electrical insulating material called a dielectric. The plates and the dielectric may be either flattened or rolled. • The purpose of the dielectric is to help the two parallel plates maintain their stored electrical charges. • The relationship between the capacitance and the size of capacitor plate, amount of plate separation, and the dielectric is given by C = ε0 εr A / d d is the separation distance of plates (m) C is the capacitance (F, Farad) ε0 : absolute permittivity εr : relative permittivity A is the effective (overlapping) area of capacitor plates (m2) CLASSIFICATION OF TRANSDUCERS According to Transduction Principle d Area=A Either A, d or ε can be varied.
- 20. The variable resistance transducers are one of the most commonly used types of transducers. The variable resistance transducers are also called as resistive transducers or resistive sensors. They can be used for measuring various physical quantities like temperature, pressure, displacement, force, vibrations etc. These transducers are usually used as the secondary transducers, where the output from the primary mechanical transducer acts as the input for the variable resistance transducer. The output obtained from it is calibrated against the input quantity and it directly gives the value of the input. CLASSIFICATION OF TRANSDUCERS According to Transduction Principle RESISTIVE TRANSDUCER:
- 21. Principle of Working of Variable Resistance Transducer The variable resistance transducer elements work on the principle that the resistance of the conductor is directly proportional to the length of the conductor and inversely proportional to the area of the conductor. Thus if L is the length of the conductor (in m) and A is its area (in m square), its resistance (in ohms) is given by: R = ρL/A Where ρ is called as resistivity of the material and it is constant for the materials and is measured in ohm CLASSIFICATION OF TRANSDUCERS According to Transduction Principle
- 22. RESISTIVE TRANSDUCER There are 4 type of resistive transducers. 1. Potentiometers (POT) 2. Strain gauge 3. Thermistors 4. Resistance thermometer
- 23. POTENTIOMETER • The potentiometer are used for voltage division. They consist of a resistive element provided with a sliding contact. The sliding contact is called as wiper. • The contact motion may be linear or rotational or combination of the two. The combinational potentiometer have their resistive element in helix form and are called helipots. • Fig shows a linear pot and a rotary pot.
- 24. INDUCTIVE TRANSDUCERS The variable inductance transducers work generally upon the following three principles. Change of self inductance Change of mutual inductance Production of eddy current CLASSIFICATION OF TRANSDUCERS According to Transduction Principle
- 25. LINEAR VARIABLE DIFFERENTIAL TRANSFORMER The transformer consists of a single primary winding and two secondary windings which are placed on either side of the primary. The secondaries have an equal number of turns but they are connected in series opposition so that the emfs induced in the coils OPPOSE each other. The position of the movable core determines the flux linkage between the ac-excited primary winding and each of the two secondary winding. With the core in the center, the induced emfs in the secondaries are equal, and since they oppose each other, the output voltage will be 0 V. When an externally applied force moves the core to the left-hand position, more magnetic flux links the left-hand coil than the right-hand coil and the Differential Output V0 = VS1 – VS2 Is in-phase with Vi as VS1 > VS2 .
- 26. The induced emf of the left hand coil is therefore larger than the induced emf of the right-hand coil. The magnitude of the output voltage is then equal to the difference between the two secondary voltages, and it is in phase with the voltage of the left-hand coil. Similarly, when the core is forced to move to the right, more flux links the right- hand coil than the left-hand coil and the resultant output voltage is now in phase with the emf of the right-hand coil, while its magnitude again equals the difference between the two induced emfs. Ideally the output voltage at the null position should be equal to zero. LINEAR VARIABLE DIFFERENTIAL TRANSFORMER
- 28. Transducer and Inverse Transducer Transducer, as already defined, is a device that converts a non-electrical quantity into an electrical quantity. Normally a transducer and associated circuit have a non-electrical input and an electrical output, for example a thermo- couple, photoconductive cell, pressure gauge, strain gauge etc. An inverse transducer is a device that converts an electrical quantity into a non-electrical quantity. It is a precision actuator having an electrical input and a low-power non-electrical output. For examples a piezoelectric crystal and transnational and angular moving-coil elements can be employed as inverse transducers. Many data- indicating and recording devices are basically inverse transducers. An ammeter or voltmeter converts electric current into mechanical movement and the characteristics of such an instrument placed at the output of a measuring system are important. A most useful application of inverse transducers is in feedback measuring systems.
- 29. THERMISTOR •Thermistor is a contraction of a term “thermal resistor”. •Thermistor are temperature dependent resistors. They are made of semiconductor material which have negative temperature coefficient of resistivity i.e. their resistance decreases with increase of temperature. •Thermistor are widely used in application which involve measurement in the range of 0-60º Thermistor are composed of sintered mixture of metallic oxides such as magnese, nickle, cobalt, copper, iron and uranium
- 30. Contd. •The thermistor may be in the form of beads, rods and discs. •The thermistor provide a large change in resistance for small change in temperature. In some cases the resistance of themistor at room temperature may decreases as much as 6% for each 1ºC rise in temperature.
- 31. Thermocouples-Thermoelectric Transducer Seebeck Effect In 1821 Thomas Seebeck discovered that when two dissimilar metals were in contact, a voltage was generated where the voltage was a function of temperature. The device, consisting of two dissimilar metals joined together, is called a Thermocouple and the voltage is called the Seebeck voltage.
- 32. As an example, joining copper and Constantan produces a voltage on the order of a few tens of milli-volts with the positive potential at the copper side. An increase in temperature causes an increase in voltage. When two dissimilar metals such a iron and copper are gained to form a closed circuit, current flow when one junction is at higher temperature and the other one is at lower temperature as shown in the figure. Thermocouples The emf driving the current is called a thermoelectric emf and the phenomenon is known as thermoelectric effect or Seeback effect. Usually a thermoelectric emf is very small. A pair of dissimilar metals welded together at their junction forms what is called a thermocouple. When several thermocouples are arranged in series, the emf is added together to give an appreciable output, this arrangement is called thermopile as shown in the figure.
- 34. Materials for thermocouple: 1. Melting point of thermocouple materials must be higher than the measuring temperature. 2. The dissimilar materials on joining should be able to produce large emf for accuracy of measurements. 3. Temperature is determined indirectly i.e. through calibrations of emf with temperature. As for as possible, the linear variation of emf with temperature is desired. 4. Thermocouple materials should be resistant to atmospheres in furnaces.
- 36. Cold junction compensation Application of see back effect to thermocouple requires that one end of the junction (cold) must be at constant temperature. The standard calibration data for all thermocouples are based on 0°C cold junction temperature. In Practice it may not be possible to keep cold junction at zero degree temperature. Hence standard data need to be corrected. One way is to add the environmental temperature to the value of temperature determined by thermocouple measurement. In another method, thermistor may be put in the thermo‐couple circuit. The voltage drop across thermistor depends on environmental temperature which then compensates for the error.
- 37. There will be at least two thermocouple junctions in the system. To contend with this, it is necessary that the temperature of one of the junctions be known and constant. Therefore, there is a fixed offset voltage in the measuring system. It was customary a long time ago to place this junction in a mixture of ice and water, thus stabilizing the temperature to 0 degree C as shown in figure. More modern techniques use electronic reference junctions that are not necessarily at 0oC. This junction is called the reference or cold junction due to the fact that this junction was in the ice bath.