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Electronics Paper 2, Nov/Dec. 2008  
Questions: 1 2 3 4 5 6 7   Main
General Comments

Question 7

(a)Explain the following terms:
    1. open loop system;
    2. closed loop system.

(b)In a feedback motor speed control system, state the functions of the following:

    1. error detector;
    2. power amplifier;
    3. tachogenerator.



The expected answers were:
(a)(i)     An open-loop control system is one in which control action does not utilize
            information concerning the output state.

    (ii)   A closed-loop control system utilizes feedback information on the output state for
            the initiation and sustenance of control action.

(b)(i)    Error detector – compares feedback information with set reference level to
            determine nature of desired control action.

     (ii)   Power amplifier – amplifies the ‘error signal’ and its output feeds the motor.

    (iii)   Tacho-generator located in the feedback path functions to provide information
            about the status of the output (speed converted to signal) as feedback information.

This question was on amplifiers.

Candidates performed poorly in part (a).

Candidates were able to identify the functions of error detector and power amplifier but majority failed in the case of tachogenrator.

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