Unit 3a Industrial Control SystemsSections:Basic Elements of an Automated SystemLevels of AutomationProcess Industries vs. Discrete Manufacturing IndustriesContinuous ControlDiscrete ControlComputer Process ControlSupervisory ControlEnterprise Control

Automation DefinedAutomation is the technology by which a process or procedure is accomplished without human assistance

Automation has cost-benefitsControlSystemPowerProgramInstructionsTransformation Process

Program of InstructionsSet of commands that specify the sequence of steps in the work cycle and the details of each stepCNC part program, Robot program, AS/RS program, etc.

Work Cycle ProgramNumber of steps in the work cycleManual participation in the work cycle (e.g., loading and unloading workparts)Process parameters - how many must be controlled?Operator interaction - does the operator enter processing data?Variations in part or product styles

Control System

Control ArchitectureLevel 0Level 1Level 2Level 3Level 4Industrial Automation(Shop Floor)Business Information(Business Office)

Logical SignalAutomatic Control - Level 0 and 1 ControllerActuators SensorsOutputVariablesInputParameters(Level 2)ProcessInputsErrorFeedback Signal

Sensors Level 0PhysicalMediumSensingElementConditioningTargetHandlingTemperatureResistanceVoltageInformationSignal

Actuators Level 0

Advanced Automation FunctionsSafety monitoringMaintenance and repair diagnosticsError detection and recovery

Safety MonitoringUse of sensors to track the system's operation and identify conditions that are unsafe or potentially unsafeReasons for safety monitoringTo protect workers and equipmentPossible responses to hazards:Complete stoppage of the systemSounding an alarmReducing operating speed of processTaking corrective action to recover from the safety violation

Maintenance and Repair DiagnosticsStatus monitoringMonitors and records status of key sensors and parameters during system operationFailure diagnosticsInvoked when a malfunction occursPurpose: analyze recorded values so the cause of the malfunction can be identifiedRecommendation of repair procedureProvides recommended procedure for the repair crew to effect repairs

Error Detection and RecoveryError detection functions:Use the systems available sensors to determine when a deviation or malfunction has occurredCorrectly interpret the sensor signalClassify the errorError recovery possible strategies:Make adjustments at end of work cycleMake adjustments during current work cycleStop the process to invoke corrective actionStop the process and call for help

Industrial Control SystemsThe automatic regulation of unit operations and their associated equipment as well as the integration and coordination of the unit operations into the larger production system

Process vs. Discrete IndustriesProcess industriesProduction operations are performed on amounts of materialsLiquids, gases, powders, etc.

Discrete manufacturing industriesProduction operations are performed on quantities of materialsParts, product units

Variables and ParametersVariables - outputs of the processParameters - inputs to the processContinuous variables and parameters - they are uninterrupted as time proceedsDiscrete variables and parameters - can take on only certain values within a given range

Types of ControlContinuous control - variables and parameters are continuous and analogDiscrete control - variables and parameters are discrete, mostly binary discrete

Maintain the value of an output variable at a desired levelParameters and variables are usually continuousSimilar to operation of a feedback control systemMost continuous industrial processes have multiple feedback loopsExamples: Chemical reaction (temperature, pressure, etc.); Position control of gripper at end of a robot arm

Types of Continuous Process ControlRegulatory controlFeed forward controlSteady-State optimizationAdaptive controlOn-line search strategiesOther specialized techniquesExpert systemsNeural networks

Regulatory ControlObjective - maintain process performance at a certain level or within a given tolerance band of that levelAppropriate when performance relates to a quality measurePerformance measure is sometimes computed based on several output variablesPerformance measure is called the Index of performance (IP)Problem with regulatory control is that an error must exist in order to initiate control action

Regulatory Control

Feedforward ControlObjective - anticipate the effect of disturbances that will upset the process by sensing and compensating for them before they affect the processMathematical model captures the effect of the disturbance on the processComplete compensation for the disturbance is difficult due to variations, imperfections in the mathematical model and imperfections in the control actionsUsually combined with regulatory controlRegulatory control and feedforward control are more closely associated with process industries

Feedforward Control Combined with Feedback Control

Steady-State OptimizationClass of optimization techniques in which the process exhibits the following characteristics:Well-defined index of performance (IP)Known relationship between process variables and IPSystem parameter values that optimize IP can be determined mathematicallyOpen-loop systemOptimization techniques include differential calculus, mathematical programming, etc.

Steady State (Open-Loop) Optimal Control

Adaptive ControlBecause steady-state optimization is open-loop, it cannot compensate for disturbancesAdaptive control is a self-correcting form of optimal control that includes feedback controlMeasures the relevant process variables during operation (feedback control)Uses a control algorithm that attempts to optimize some index of performance (optimal control)

Adaptive Control Operates in a Time-Varying EnvironmentThe environment changes over time and the changes have a potential effect on system performanceExample: Supersonic aircraft operates differently in subsonic flight than in supersonic flightIf the control algorithm is fixed, the system may perform quite differently in one environment than in anotherAn adaptive control system is designed to compensate for its changing environment by altering some aspect of its control algorithm to achieve optimal performance

Three Functions in Adaptive ControlIdentification function current value of IP is determined based on measurements of process variablesDecision function decide what changes should be made to improve system performanceChange one or more input parameters Alter some internal function of the controllerModification function implement the decision functionConcerned with physical changes (hardware rather than software)

Adaptive Control System

On-Line Search StrategiesSpecial class of adaptive control in which the decision function cannot be sufficiently definedRelationship between input parameters and IP is not known, or not known well enough to implement the previous form of adaptive controlInstead, experiments are performed on the processSmall systematic changes are made in input parameters to observe effectsBased on observed effects, larger changes are made to drive the system toward optimal performance