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Instrumentation and Signal Processing • Signals usually need to be processed in various ways. For example, the output signal from a transducer (sensor) may well be contaminated with unwanted electrical “noise”. The electrodes attached to a patient’s chest when an ECG is taken measure the very weak electrical voltage changes due to the activity of the heart and other muscles. The signal is largely affected by “mains pickup” due to electrical interference from the power supply. • Processing the signal using a filter circuit may remove or reduce the unwanted part of the signal. The filtering of signals to improve signal quality or to extract important information is done by digital techniques rather than by analog techniques. Riadh W. Y. Habash, SITE, 2002 1 Instrumentation System Physical Signal Transducer Analog Output Analog Processing Analog Processing M U L T I P L E X E R ADC Computer Display DAC Riadh W. Y. Habash, SITE, 2002 2 Transducers • • • • • A transducer is an electronic device that converts energy from one form to another. Common examples include microphones, loudspeakers, thermometers, position and pressure sensors, and antenna. No transducer is 100-percent efficient; some power is always lost in the conversion process. Usually this loss is manifested in the form of heat. Transducers measure position and angle, pressure, flow rate, and temperature, etc. Transducers may also be divided into Analog or Digital, depending on whether they provide a continuously varying signal (Analog) or only two possible states (Digital) Wheatstone bridge circuit is an example that converts a resistance change to a voltage change. Other examples are: strain gages, potentiometers, differential transformers, pressure transducers, fluid-flow transducers, and temperature transducers. For details see the textbook: pp. 281-285. Riadh W. Y. Habash, SITE, 2002 3 Bridge Circuits [http://infoeng.ee.ic.ac.uk/~dario/courses/3e_instrumentation/1Sensors.pdf] • The sensitivity of a sensor can be enhanced by incorporating a bridge. Riadh W. Y. Habash, SITE, 2002 4 Types of Transducers • Passive – Voto-voltaic • Solar cells; portable exposure meters. – Piezo-electric • Gas igniters; migrophones; stress/strain guages. – Thermo-electric • Thermocouples. – Electromagnetic • Antennas! See the Figure! Riadh W. Y. Habash, SITE, 2002 5 • Active – Variable resistor • Thermistors; strain guages; photoconductors; diodes. – Hall effect • Current and field. – Optoelectronics • Avalanche diode; – Variable reactance • Variable inductance • Variable capacitance Riadh W. Y. Habash, SITE, 2002 6 Examples of Transducers • Microphone • Loudspeakers Riadh W. Y. Habash, SITE, 2002 7 Transducer: Sensor and Actuator [http://www.signal.uu.se/Courses/CourseDirs/Sensors/Sensor_Intro00.PDF] Riadh W. Y. Habash, SITE, 2002 8 Types of Energy [http://www.signal.uu.se/Courses/CourseDirs/Sensors/Sensor_Intro00.PDF] Riadh W. Y. Habash, SITE, 2002 9 Amplifiers! You know them! [http://www.ctjc.com/tutorial.htm] Riadh W. Y. Habash, SITE, 2002 10 Filters A filter is an electrical device which removes parts of the spectrum of a signal passed through it. [http://www.sfu.ca/sca/Manuals/ZAAPf/i/Index_Tech.html] • High-Pass Filter • Low-Pass Filter • Band-Pass Filter • Band-Pass Reject Filter Riadh W. Y. Habash, SITE, 2002 11