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Unformatted text preview: EECS 216 Fall 2010 Lab 4: Feedback Control Part I: Intro & Pre-lab Assignment 1 Introduction In this laboratory project we will develop the idea of feedback control, the process of adjusting the input to a system as a function of a measured variable so as to obtain a desired response. Stated like this, the concept seems very vague, almost as vague as the notion of a system seemed to you in the first few weeks of this course. We will try to make the notion of feedback control more precise by looking at a particular example, the idle speed control loop in your car. After that, well discuss different kinds of control. When you press on the accelerator pedal of your car, the engine produces more power. When you take your foot off the pedal, the engine does not stop 1 , rather, it runs at a fixed idle speed, such as 750 RPM (revolutions per minute). Moreover, the engine maintains a constant idle speed even when the power load on the engine changes, such as when the air conditioner cycles on or off, or, assuming you have your foot on the brake, when you shift the transmission from park to neutral to drive. How is it possible for the engine to idle at a fixed speed under a variety of loads? The answer is feedback control. A sensor measures the speed of the engine and a mechanism, called a feedback controller, automatically adjusts the power requested 2 from the engine so as to maintain a constant idle speed. The engine system illustrates all the basic components of what we call a feedback control system . The key elements are: (i) A measured Quantity that is to be regulated to a desired value (e.g., engine idle speed); this is typically defined to be the system output ; (ii) An input that can be varied so as to change the value of the output (e.g., throttle position); (iii) An element that determines how to adjust the input so as to drive the output to a desired value; this element is called the controller . The overall system is shown in Fig. 1.0.1, where the comparator computes the error between the desired output value and the current output value, and the controller seeks to drive this error to zero. Such a feedback control system is also known as a closed-loop control system due to the closed signal path that connects all four components (e.g., Comparator to Controller to Plant to Measurement to Comparator). Desired Output Response- Comparator- Controller- Plant- Output Measurement 6 Figure 1.0.1: Closed-Loop Control System. The plant is the generic term for the system to be controlled. 1 It may if you have a hybrid electric vehicle, or HEV. 2 Power is varied by opening and closing the throttle, which regulates air flow and hence engine power, and/or by changing the spark timing, which regulates power as well....
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- Spring '08