Control Systems Basics
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This Power Point is a simple overview of Industrial Control Systems Basics
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Control Systems Basics
- 1. 1Northampton Community College Control System Basics
- 2. 2Northampton Community College Control To cause a machine or process to function in a predetermined manner. To energize or de-energize an output, or to set a data table bit or bits to on or off, by means of a user program.
- 3. 3Northampton Community College What is a Controller? A circuit that excepts inputs: One input is the action (command signal) The other is the measurement signal (feedback/feed forward) compares these inputs and determines the output reaction.
- 4. 4Northampton Community College 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)
- 5. 5Northampton Community College What is a Control System? An interconnection of components forming a system configuration which will provide a desired system response. ProcessInput Output
- 6. 6Northampton Community College What is Process? Any operation to be controlled. (A natural, progressively continuing operation of development, marked by a series of gradual changes that succeed one another in a relatively fixed way and lead to a particular result or end).
- 7. 7Northampton Community College Main Categories of Control Open loop control system Closed loop control system
- 8. 8Northampton Community College Open Loop Control System A control system that has no means of comparing the output with the input for control purposes. Controller Process Input Output
- 9. 9Northampton Community College Open Loop Control System Motor Controller Motor Conveyor Did the Conveyor start, and if it did, is it running at the right speed?
- 10. 10Northampton Community College Closed Loop Control System A closed loop control system utilizes an additional signal that measures the actual output. It then compares the actual output with the desired setpoint, which in turn adjusted the controller to produce the desired output response. Controller Process Output Command Signal, Setpoint Measurement + - Error
- 11. 11Northampton Community College Closed Loop Control System Motor Controller Motor Conveyor Rotary Encoder Feedback speed an position
- 12. 12Northampton Community College Classroom Discussion Is a pop-up toaster and example of an Open Loop or Closed Loop control system?
- 13. 13Northampton Community College Feedback Feedback The signal or signals returned from a controlled machine or process to denote its response to the command signal. Feedback Element (feedback device) An element that converts motion, position, pressure, flow or temperature to and electrical signal for comparison to the command signal.
- 14. 14Northampton Community College Feedback Feedback Loop A closed signal path in which feedback is compared with the commanded value to obtain a corrective error signal. Feedback Signal The measurement signal indicating the value of a directly controlled variable, which is compared to the commanded value to obtain the corrective error signal
- 15. 15Northampton Community College Feedback Input Output Float feedback H
- 16. 16Northampton Community College Feedforward (bias) Control action in which information concerning upstream conditions is converted into corrective commands to minimize the deviations of the controlled variable. Feedforward is predominantly used to enhance PID control to improve system response. A feedforward control system sends information about a disturbance to the control before it affects the process. This type of control system can also be referred to as predictive control. Systems where the setpoints come from, or are modified by, sensed conditions upstream of the process being controlled. Sometimes called cascade controllers. Cascade suggests that a change in an upstream variable will affect, or cascade down to, later control systems.
- 17. 17Northampton Community College Feedforward (bias) Controller Control Output Setpoint + - Error + + Kf Process Variable (PV) Feedforward Path
- 18. 18Northampton Community College Feedforward Example Motor+ - + - Colored soda pop out Flow Sensor Valve Amount of colorant required Setpoint: Color sensorColorant In Uncolored soda pop in Setpoint: Final mix color
- 19. 19Northampton Community College Ratio Control Ratio control is another common type of feedback control system. This control scheme is often confused with cascade and/or feedforward control but they are quit different. Ratio control is most often used with processes that have two or more streams of material that need to be continually mixed together to 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.
- 20. 20Northampton Community College Ratio Control Motor Motor+ - x + - x Setpoint: amount of soda water required Setpoint: amount of flavor required Color to flavor ratio setting Color to soda water ratio setting Colorant in Fully mixed soda pop out Flow sensor Flow sensor Flavoring in Soda water in Color sensor Valve Valve
- 21. 21Northampton Community College What is a System? A combination of controller(s), actuators and sensors that act together to perform a desired objective.
- 22. 22Northampton Community College Control Hierarchy Local Control Control is located at the device to be controlled. Distributed Control A control system organization in which factory or machine control is divided into several subsystems, each managed by a separate controller, yet all interconnected to form a single entity.
- 23. 23Northampton Community College Control Classifications Nonlinear The control is either ON or OFF. Most physical control systems are nonlinear. Linear The control is variable. It is not completely ON or completely OFF but can be.
- 24. 24Northampton Community College Control Classifications Servo System A feedback control system in which the output is some mechanical position, velocity or acceleration. The terms servo system and position, velocity or acceleration controls are used synonymously.
- 25. 25Northampton Community College Basic Servo Control System Encoder Digital Signal Command Signal to motor Controller D/A Converter Encoder Analog Servo Amp PWM Motor
- 26. 26Northampton Community College Adaptive Control System An automatic control scheme in which the controller is programmed to evaluate its own effectiveness and modify its own control parameters to respond to dynamic conditions occurring in or to the process which affect the controlled variables.
- 27. 27Northampton Community College Affects on Control Systems
- 28. 28Northampton Community College Disturbance An undesirable signal that tends to adversely affect the value of the output of a system. Types of disturbances: Internal External
- 29. 29Northampton Community College Examples of Disturbance Power Line Externally Conducted Noise Transmitted Noise Ground Loops Inductive Driven Devices SCR’s Firing Motor Drives Welders
- 30. 30Northampton Community College Control 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 an abrupt disturbance. Response Time (Settling Time) The time required for a system to achieve steady state after an input change or following 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 the SP.
- 31. 31Northampton Community College Bibliography Morriss, S. B. (1995). Automated manufacturing systems: actuators, controllers, sensors, and robotics. New York, NY: Glencoe/McGraw-Hill. Hunter, R. P. (1978). Automated process control systems concepts and hardware. Englewood Cliffs, NJ: Prentice-Hall, Inc. Rexford, K.B. (1997). Electrical control for machines. Albany, NY: Delmar Publishing.
- 32. 32Northampton Community College Bibliography Industrial automation glossary: a helpful guide to allen-bradley technical terms. (1993). Milwaukee, WI: Allen Bradley. Patrick, D, & Stephen, F. (1997). Industrial process control systems. Albany, NY: Delmar Publishers. Maloney, T. (2001). Modern industrial electronics. Upper Saddle River, NJ: Prentice-Hall. Hughes, T. (2007). Measurement and control basics. Research Triangle Park, NC: International Society of Automation.
- 33. 33Northampton Community College Bibliography Patrick, Dale & Fardo, Stephen. Industrial Process Control Systems. Delmar Publishers 1997. ISBN 0-8273-6386-9 Maloney, Timothy, J. Modern Industrial Electronics 4th Edition. Prentice Hall 2001 ISBN 0-13-015676-0
Editor's Notes
- The basis for analysis of a system is the foundation provided by linear system theory which assumes a cause-effect relationship for the components of a system. Therefore, a component or process to be controlled can be represented by the block diagram shown in the slide. The input-output relationship represents the cause-effect relationship of the process, which in turn represents a processing of the input signal to provide an output signal variable, often with a power amplification.
- Example: Window fan Room light Advantages: Simple Inherently stable Relatively inexpensive
- Examples: Refrigerator Motor control Home thermostat Advantages: Relatively insensitive to external disturbances and internal variations in system parameters. Can control action of the output device or function.
- Example: Level monitoring. Level sensor sends back information while and after the level changes. Refer to the next slide.
- In this feedback control system the input is the material entering the pipe. The output is the material exiting the tank. The equipment being controlled is the valve and the feedback (measured parameter) is the tank level by means of the float. The feedback from the float adjusts the valve to maintain a preset level in the tank.
- Example:
- The system shown in this slide is used to control the mixing of soda pop with artificial coloring. The first controller is a feedforward controller that receives a “final mix color” setpoint, subtracts sensory information about the concentration of the colorant on its way to the mixing tank and outputs an “amount of colorant” setpoint to the next controller. The second controller, a standard closed loop feedback controller, controls flow of colorant to the mixing tank. The feedforward control system is predictive because it corrects out-of-tolerance conditions before they actually occur. It could be called cascade, because it uses an open loop, feedforward, “final mix color” control system to change the “amount of colorant” setpoint that is provided to the closed loop, colorant flow control loop. If a feedforward signal comes from a sensor measuring environmental conditions, it is called a disturbance signal. If the signal is provided manually, it is called an offset signal or bias.
- This slide depicts a ratio controller. A ratio controller is a variation of the feedforward control but in many ways is quite different. It could be called a “feedsideways” controller. Its purpose, as shown in this slide, is to use a sensor in one stream to control the processes in one or more parallel streams. The ratio controller’s sensor measures the concentration of colorant, but not to control the flow of the colorant. Instead, the sensor provides a setpoint to two parallel fluid streams (soda water and flavor), so that the output from the mixing tank is the correct ratio to color, flavor and soda water.
- Another definition for Distributed control would be: There is an independent control function for each individual control action with some coordination amongst all.
- Internal disturbances are disturbances that are generated within the system. External disturbances are disturbances that are introduced to the system from an outside source.