Introduction to Feedback and Control Systems PDF

Introduction to Feedback and Control Systems Dr. Aaron Don M. Africa 1 Numerous applications are all around us like Electric Fan Aircon Washing Machine Human Beings  Control System Definition A Control System consist of feedforward (or control) el...

Introduction to Feedback and Control Systems Dr. Aaron Don M. Africa 1 Numerous applications are all around us like Electric Fan Aircon Washing Machine Human Beings  Control System Definition A Control System consist of feedforward (or control) elements and processes (or plants) assembled for the purpose of controlling the output of the processes.  Example is a furnace Feedforward/Control Elements are called Fuel Valves. Plant/Process : Fuel-valve actuators Other subsystems such as thermostats acts as sensors to measure the temperature of the room Input; stimulus Output; response Control System Desired response Actual Response Input – Is the stimulus or command applied to a control system Output – The actual response obtained from a control system Advantages of Control Systems With control systems we can move large equipment with precision that would otherwise be impossible  Early Elevators ere controlled by hand ropes or an elevator operator. There are safety breaks at that time an innovation in early elevators Elevator input and output  We build control systems for four primary reasons 1) Power Amplification 2) Remote Control 3) Convenience of input form 4) Compensation for disturbance.  A radar Antenna positioned by the lower power rotation of a knob at the input, requires a large amount of power for its output rotation. A control system can produce the needed power amplification or Power Gain  Robots designed by control system principles can compensate for human disabilities. Control systems are also useful in remote or dangerous locations. For example, a remote controlled robot arm can be used to pick up materials in contaminated environments.  Rover was built to work in contaminated areas at Three Mile Island in Middleton, PA, where a nuclear accident occurred in 1979. The remote controlled robot’s long arm can be seen at the front of the vehicle. History of Control Systems Feedback control systems are older than humanity. Numerous biological control systems were built into the earliest inhabitants of our planet Brief history of Human Designed Control Systems The Greeks began engineering feedback systems around 300 BC. A water clock invented by Ktesibios operated by having water trickle into a measuring container at a constant rate. Water clock of Ktesibios Steam Pressure and temperature control Regulation of steam pressure began on 1681 with Dennis Papin’s invention of the safety valve The concept was further elaborated on by weighting the valve top if the upward pressure exceeds its weight.  In the 17th century, Cornelis Drebbel in Holland invented a purely mechanical control system for hatching eggs.  In 1745 speed controlled was applied to a windmill by Edmund Lee. This is done by applying a windmill fantail  the 18th Century James Watt In invented the flyball speed governor to control the speed steam engines In this device two spinning flyballs rise as rotational speed increases A steam valve connected to the flyball mechanism closes with the ascending flyballs and opens with descending flyballs, thus regulating the speed. Stability, Stabilization, and Steering Control Systems as we know today began to crystallize in the latter half of the 19th Century In 1868 James Clerk Maxwell published the stability criterion for a third-order system based on the coefficients of the differential equations  Edward John Routh In 1874 Edward John Routh using a suggestion from William Kingdon Routh Clifford that was ignored earlier by Maxwell, was able to extend the stability criterion to 5th order system He also pioneered the Routh–Hurwitz stability criterion a method to know if a control system is stable  Henry Bessener During the 2nd half of the 1800s the development of control systems focused on steering and stabilizing ships. In 1874 Henry Bessener, using a gyro to sense a ship’s motion and applying power generated by the Ship’s hydrolic system, move the ship’s saloon to make it stable  th 20 Century Developments It was not until the early 1900s that automatic steering ships were achieved. In 1922 the Sperry Gryscope Company installed an automatic steering system that uses the elements of compensation and adaptive control  In 1948 Walter R. Evans, working in the aircraft industry, developed a graphical technique to graph the roots of a characteristic equation of a feedback system whose parameters changed over a particular range of values This technique is now known as the root locus  Control Systems are not limited to Science and Industry Example is an Aircon Control System consisting of a Thermostat that detects and adjust temperature Open-loop control system – A control system in which the control action is independent of the output Closed-loop control system – Is one in which the control action is somehow dependent on the output  Two outstanding features of open loop systems 1) Their ability to perform accurately is determined by their calibration problems of instability 2) They are not usually troubled by problems of instability Calibrate – to establish or reestablish the input-output relation to obtain the desired system accuracy.  Examples Open Loop: Most automatic toasters are open loop systems because they are controlled by a timer. Closed Loop: An autopilot mechanism of an airplane is a closed-loop (feedback) control system  Transient Response – This type of response is important. In the case of the Elevator, a slow transient response makes passengers impatient. Whereas excess in rapid response makes them uncomfortable In a computer transient response contributes to the time required to required to read from or write to computer disk storage.  Steady-State Response: This response resembles the input and is usually what remains after the transients have decayed to zero. – Example an Elevator that stopped in the fourth floor or a disk drive that stopped on the correct track – We are concerned with the accuracy of the Steady State response.  Feedback Feedback is a characteristic of closed loop control systems which distinguish them from open loop system It is the property of a closed loop system which permits the output to be compared into the input of the system so that the appropriate control may be formed as some function of the output and input.  Summing point z + + x+y+z x + y  Definitions Plant (or process, or control system) g2 – the system controlled by the feedback control system. Controlled output c – the output variable of the plant, under the control of the feedback control system Forward Path – is the transmission path from the summing point to the controlled output c Feed forward (control) elements g1 – Typically includes controllers, compensators, amplifiers. Control Signal – is the output signal of the feed forward elements g1 applies as input to plant g2 Feedback path – is the transmission path from the controlled output c back to the summing point Feedback elements h – establish the functional elements between the controlled output c and the primary signal b Reference input r – an external signal applied to the feedback of the control system usually at the first summing point. Primary feedback signal b - is the function of the controlled output c, algebraically summed with the reference input r Actuating (or error) signal –reference input signal r plus or minus the primary signal b Automobile power steering apparatus  Activity 1 Figure 1 Give the functional block diagram of Figure 1. Assume positive feedback  Activity 2 Give examples of OPEN and CLOSED Control Systems ,draw its block diagram and identify the parts. (4 Each)