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The programme examines the experimental determination of frequency response using a resistive capacitive network. An analytical technique for finding the closed loop response of a system, having e...xperimentally determined the open loop response, is developed.
Metadata describing this Open University video programme
Module code and title: T391, Control engineering
Item code: T391; 03
First transmission date: 03-05-1978
Published: 1978
Rights Statement:
Restrictions on use:
Duration: 00:24:30
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Producer: Edward Smith
Contributors: Dick Fendrich; Roger Loxton
Publisher: BBC Open University
Keyword(s): Control engineering; Frequency response
Footage description: Dick Fendrich introduces the programme. To review student ideas about frequency testing, he displays a frequency response for a simple R.C. network on an oscilloscope screen. Fendrich freezes the input and output traces for this network on the oscilloscope and takes measurements to work out the amplitude ratio and phase shift. He goes on, briefly to explain the effect of interaction on the network and to eliminate this effect. Roger Loxton uses a schematic diagram to describe a set-up which will measure the amplitude ratio and phase shift of a pump. Dick Fendrich, with a pump and measuring apparatus, applies a sinusoidal demand voltage to the pump. Both input and output voltages are displayed on the oscilloscope and then on a transfer function analyser. Fendrich explains why the transfer analyser allows more accurate measurements to be taken than the oscilloscope. Roger Loxton plots the readings from the above experiment on to a Bode graph and a Nichols graph. He goes on to explain why more measurements are necessary. Dick Fendrich performs the pump experiment again this time with a different input frequency. He notes the amplitude ratio and phase lag. Roger Loxton plots these measurements on his Bode Nichols graphs. Several more measurements are plotted and a curve is drawn through them. Over shots of a diagram of a flow control system, Roger Loxton explains how such a system can be designed as a unity feedback system which can then use Nichols graphs to predict closed loop behaviour from open loop characteristics. He goes on to explain why the amplitude ratio has to be modified to account for the addition of a controller to the system. The modified curves are plotted. Loxton takes curve plotted above and superimposes it on a Nichols graph which has plotted on it axes of constant closed loop gain and constant closed loop phase. This allows him to pick out the corresponding closed loop gain and phase for his experiment. He plots these on a Bode graph. Loxton goes on to explain how the resulting Bode curve can be used to project the transient behaviour of the closed loop system. The curve is compared to a second order model and found to look quite similar. By using this second order analogy, Loxton predicts a 15% overshoot for the pump experiment. In conclusion, Dick Fendrich runs the experiment and reports a 15% overshoot.
Master spool number: 6HT/72698
Production number: 00525_5319
Videofinder number: 1097
Available to public: no