Wednesday, March 13, 2013
BIOINTRUMENTATION ( INPUT TRANSDUCER )
Riddick "Online Input transducer Input transducers convert physical, chemical or biological phenomena to proportional electrical signals. There are two kind of input transducers : active tranducers which require electrical energy for their operation ( e.g. strain gauges). And passive transducer which do not require, but instead produce electrical energy. ( e.g. piezoeleectric transducers ). 3.1. Rotary motion and muscle displacement transducers Figure 7 shows an active transducer wich converts angular movement of a shaft to an electrical signal. A precision ball bearing allows the shaft to rotate with little fiction, and a small metal vane moves with the shaft within coils of wire. The reaction of the magnetic field around these coils to the metal vane creates variations of an electrical current in the coils, which may then be used by the remainder of the instrument system. The operation of this transducer is somewhat similiar to that of a variable reluctance high- fidelity record playing catrigde. The DC output voltages produced by additional circuitry located within the transducer case in directly proportional to the shaft rotation. The polarity of the output voltage is positive for clockwise shaft rotation, negative for counterclockwise. Another transducer element similiar in appearance and function to that shown in figure 7 converts angular movement of a shaft to an electrical signal by changing the capacitance between concentric cylinders . The inner cylinder , to which the shaft is attached, has two axial voids machined it in and rotates on precision ball bearing to assure low friction and small breakaway torque. The outer cylinder is made of insulating material with two conductive inserts which serve as capacitance plates. During rotation of the innner cylinder the defferential capacitance increases or decreases depending upon the direction of rotation. An electronic circuit located within the transducer housing convert the capacitance variation into output voltage changes proportional to shaft rotation. The transducer has greater sensitivity, lower inertia and less breakaway torque than the inductive transducer, therefore it can be used for the recording of muscle contractions of less force and amplitude than is possible with most other types of motion transducers. Fig. 8. Transducer element of fig. 7 fitted with lever and mounting bar for recording contractions of cardiac or smooth muscle. Lever arms may be attached to these transducers, by means of which they may be used to record muscle movements as in experiments on the responses of cardiac muscle. ( see fig.8 ). 3.2 Pulse transducers A transducer designed to monitor pressure changes accompanying pulsatile blood flow in extremities in shwon in fig. 9. This pressure transducer uses a piezoeleetric crystal which generates minute electrical signals when compressed. The crystal similiar to that employed is some microphones and phonograph cartridges. When this transducer is attached to a finger as in chapter 12, pulsatile pressure variations caused by arterial expansion will produce a signal output. Figure 9. Piozoeleetric transducer for recording peripheral pulse. 3.3 Temperature Transducers Temparature may be detected and Transduced using thermocouples, thermistors, bimetallic strips, etc. A device quite common bioinstrumentation is the thermistor : its resistance varies with temperature. Usually a Wheatstone –brigde circuit is used with thermistor probes to convert the resistance change into voltage variations. Thermistors are available in a wide variety of resistance rangers, and may be connected to the instrumentation system from some distance by means of a 2 wire cable. 3.4 Prerssure transducers It is often desirable to record gauge absolute or differetial presurres, such a arterial blood pressure or air pressure within a closed system. The electronic pressured transducer shown in figure 10 is used to measure and / or record direct fluid pressure in vessels or cavicities of organisms. It can also be used to measure air pressure. This instrument converts the pressure against its stainless steel diagphragm moves between two fixed capacitor plates in response to the applied pressure. The deferential capacitance changes is converted to an electrical signal by the electronic circuitry located within the transducer housing, The use of this pressure transducer is detailed in the experimental chapter dealing with blood pressure measurement. Fig. 10. Pressure transducer for use with fluid systems. Such a device is suitable for the monitoring of blood pressure via a canulla, but can be used with most all non-coorosive fluids, including air. Other transducer which may be encountered iclude those for light ( e.g. photocells), radioactivity ( e.g. Geiger-counter ), liquid drops, etc. For the subsequent experiments however, only electrodes and transducer for mechanical movement and pressure will be required. ( FROM GROUP (1).AMINUDDIN, BUYUNG, SINTHA, RIBAS, ( IKOR FK UNAIR )
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