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The electrical outputs of many different transducers have often to be analysed simultaneously in time, for example, the pressure and temperature variations during the firing cycle of a combustion e...ngine. Obviously, if measurements of a system taken from different transducers relate to different times (i.e. phase errors) or if one transducer is not sensitive enough or overloaded (i.e. amplitude errors), then calculations based on these measurements would be liable to error. This programme shows how particular transducers can be chosen to adequately respond to a particular measurand. The specification of a transducer is usually given in terms of its phase and amplitude response to sinusoidal signals covering a band of frequencies, In general a typical measurand has a complex waveform, but the process called Fourier Analysis allows any waveform to be analysed in terms of sine waves of different frequencies and amplitudes. Conversely Fourier Synthesis is the synthesis of a complete waveform from a set of sine waves. The output signals (i.e. response) from an accelerometer transducer are displayed using a swept frequency sine wave generator driving a mechanical shaker on which the accelerometer is fixed. A large display oscilloscope plots vertically the amplitude phase responses against the frequency of the shaker plotted on the horizontal axis. The effect of 'damping' the transducer shows how the peak in the responses at the resonant frequency of the transducer is removed at the expense of sensitivity. Mechanical models demonstrate the Fourier synthesis of a "square" wave from four "sine waves". Each sine wave is started at the same time (exactly in phase with each other) but with different amplitudes and frequencies so as to generate a non-sinusoidal summation. The effect of an 80° phase change of just one sine wave demonstrates the importance of phase information in the analysis of the response of a transducer to a complex waveform. A similar "square" wave is synthesised electronically and then analysed back into its fundamental frequency components (i.e. spectrum). It is shown that whilst phase shifts cause a dramatic change in the waveform, there is no change apparent on the spectrum analyser, since it is only sensitive to frequency and amplitude.
Metadata describing this Open University video programme
Module code and title: T291, Instrumentation
Item code: T291; 06; 1975
First transmission date: 08-06-1976
Published: 1975
Rights Statement:
Restrictions on use:
Duration: 00:24:00
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Producer: Tony Jolly
Contributors: Gary Alexander; Gaby Smol
Publisher: BBC Open University
Keyword(s): Amplitude; Combustion engine; Firing cycle; Fourier analysis; Oscilloscope; Phase; Sine waves; Synthesis
Master spool number: 6HT/71820
Production number: 00525_5268
Videofinder number: 1067
Available to public: no