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Unformatted text preview: ME304/Spring 2005/EXPERIMENT 2 1 • Pre-laboratory reports are due 18.05.2005, Wednesday. (See procedure section (Item 7.1) of the laboratory sheets for details). Note that, you will not be able to attend the laboratories unless you submit this report. • Check ME304 web site for laboratory schedules and recent announcements. You need to see Asst. Gökhan BAYAR (C-204) for conflicts about the schedules no later than 18.05.2005, Wednesday. • Please feel free to ask any questions to your assistants about the laboratory work. MIDDLE EAST TECHNICAL UNIVERSITY MECHANICAL ENGINEERING DEPARTMENT ME304 – CONTROL SYSTEMS SPRING 2005 EXPERIMENT 2 CLOSED LOOP POSITION CONTROL OF A DC MOTOR 1. Introduction An automatic control system is a system in which we are controlling the state of a process, say the width and thickness of the strip being rolled in a steel mill. By means of transducers, it is possible to compare the actual width and thickness of the strip produced with that required. The system must be able, if there is a difference or error, to send modifying signals to an actuator, in this case the motor and gearing controlling the roller setting. Because the system is able to feed back information about the actual state for comparison with the desired state, it is called a closed loop system. In such system, the desired state is called an “input” or reference value and the actual state is called an “output”. The difference in comparison is called the error signal and the part of the system that carries out the comparison is the “error channel”. The input and the output quantities are compared to produce an “error” signal which actually operates the “forward path”, which is the whole of the system between the error signal and the final output. This description may be summarised in general terms as: 1. there is comparison of input and output to yield an error 2. the error operates the system 3. there is a power gain from error to output and these conditions are often taken as the definition of a “closed-loop control system”. The great advantage of a feedback or closed-loop system is that since it is “error operated” it contains the facility to compensate for any departure of the output from the required condition set by the input since this departure changes the error causing a correcting signal to be applied to the forward path. Whether the departure is caused by external loading, an internal disturbance in the system, or some change in the system parameters, an amplifier gain for instance, the system compensates for the change to an extent depending on the detailed design. The disadvantage of a feedback system is that due to the closed-loop path the system may tend to give an oscillatory response to any change of input, which may take some time to die out, or that the system may even become “unstable”....
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