SOLUTION: Further Control Systems | My Assignment Tutor

Unit 54 Further ControlSystemsBasics of control systemsDr Hind ChebboControl•To cause a machine or process to function in apredetermined manner.•To energize or de-energize an output, or to set a datatable bit or bits to on or off, by means of a userprogram.What is controller•A circuit that accepts inputs:• One input is the action (command signal)• The other is the measurement signal (feedback/feedforward) compares these inputs and determines theoutput reaction.Controller•A unit that controls a machine or process. It can be,but not limited to a:• PLC – Programmable Logic Controller• PAC – Programmable Automation Controller• Robot controller with digital I/O• Relay panel• Computer data acquisition (DAQ)What is a Control system• An interconnection of components forming a system configurationwhich will provide a desired system rWhat is a process•Any operation to be controlled.(A natural, progressively continuing operation ofdevelopment, marked by a series of gradual changesthat succeed one another in a relatively fixed way andlead to a particular result or end).Types of control•Open loop control system• Closed loop control systemOpen loop control systemsA control system that has no means ofcomparing the output with the input forcontrol purposes.Open loop control systemsDid the Conveyor start, and if it did, isit running at the right speed?Closed Loop Control SystemA closed loop control system utilizes an additional signal thatmeasures the actual output. It then compares the actualoutput with the desired setpoint, which in turn adjusted thecontroller to produce the desired output response.Feedback•Feedback• The signal or signals returned from a controlled machineor process to denote its response to the command signal.• Feedback Element (feedback device)• An element that converts motion, position, pressure, flowor temperature to and electrical signal for comparison tothe command signal.Feedback•Feedback Loop• A closed signal path in which feedback is compared withthe commanded value to obtain a corrective error signal.•Feedback Signal• The measurement signal indicating the value of a directlycontrolled variable, which is compared to the commandedvalue to obtain the corrective error signalClosed Loop Control SystemFeedbackFeedforward bias• Control action in which information concerning upstream conditions isconverted into corrective commands to minimize the deviations of thecontrolled variable.• Feedforward is predominantly used to enhance PID control to improvesystem response.• A feedforward control system sends information about a disturbance to thecontrol before it affects the process. This type of control system can also bereferred to as predictive control.• Systems where the setpoints come from, or are modified by, sensedconditions upstream of the process being controlled. Sometimes calledcascade controllers. Cascade suggests that a change in an upstreamvariable will affect, or cascade down to, later control systems.FeedForward BiasFeedforward bias-ExampleRatio control• Ratio control is another common type of feedback control system.This control scheme is often confused with cascade and/orfeedforward control but they are quite different.• Ratio control is most often used with processes that have two ormore streams of material that need to be continually mixed togetherto form a composition of materials in the final mixture.• An example could be that if the blending of two ingredient is 10 to 1(10:1), then for every liter per minute of one material that is flowing,10 liters per minute of the second material needs to flow.Ratio controlWhat is a System•A combination of controller(s), actuators andsensors that act together to perform a desiredobjectiveControl Hierarchy•Local Control• Control is located at the device to be controlled.• Distributed Control• A control system organization in which factory or machinecontrol is divided into several subsystems, each managedby a separate controller, yet all interconnected to form asingle entityControl Classification•Nonlinear• The control is either ON or OFF. Most physical controlsystems are nonlinear.•Linear• The control is variable. It is not completely ON orcompletely OFF but can be.Control Classification• Servo System• A feedback control system in which the output is some mechanicalposition, velocity or acceleration. The terms servo system andposition, velocity or acceleration controls are used synonymously.Basic servo control systemAdaptive control system•An automatic control scheme in which the controlleris programmed to evaluate its own effectiveness andmodify its own control parameters to respond todynamic conditions occurring in or to the processwhich affect the controlled variables.Affects of control systemsDisturbanceAn undesirable signal that tends to adversely affect the value of theoutput of a system.• Types of disturbances:• Internal• ExternalExamples of disturbance• Power Line• Externally Conducted Noise• Transmitted Noise• Ground Loops• Inductive Driven Devices• SCR’s Firing• Motor Drives• WeldersControl Response Characteristics• Steady State Performance• The ability of a system output to maintain SP following a disturbance.• Transient Performance• The response of a system to maintain SP with abrupt changes to inputs or following anabrupt disturbance.• Response Time (Settling Time• The time required for a system to achieve steady state after an input change orfollowing a disturbance. This time should be as short as possible.• Overshoot• The condition when the output temporarily exceeds the desired SP.• Oscillatory Behavior• A system that is not at steady state but is continuously varying above and below theSP.References• Morriss, S. B. (1995). Automated manufacturing systems: actuators,controllers, sensors, and robotics. New York, NY: Glencoe/McGrawHill.• Hunter, R. P. (1978). Automated process control systems concepts andhardware. Englewood Cliffs, NJ: Prentice-Hall, Inc. Rexford, K.B.(1997).• Electrical control for machines. Albany, NY: Delmar Publishing.