Introduction to control system 1

Introduction to control system 1

1. Introduction to Control System Definition, Representation, Classification Controllers and algorithms Transfer Functions and gain of a system Introduction to Digital Computer Control

2. Control System : Definition and Representation Introduction In order to modify the behavior of a system so it behaves in a specific desirable way over time, control is introduced. For example, we may want the speed of a car on the highway to remain as close as possible to 60 miles per hour in spite of possible hills or adverse wind. Or, we may want an aircraft to follow a desired altitude, heading, and velocity profile independent of wind gusts. Or, we may want the temperature and pressure in a reactor vessel in a chemical process plant to be maintained at desired levels. All these are being accomplished today by control methods.

3. Control System : Definition and Representation Definition A control system is a system, which provides the desired response by controlling the output. A control system is a collection of devices/systems which function together to drive things in a desired direction, either from external input or sensed conditions. Control systems are a central part of industry and of automation. Figure : Simplified block diagram of a control system

4. Control System : Definition and Representation Why do we need Control System ? You need to be pro-active! You do not want to find out about a fault after it had occurred! You have to expect the unexpected! Control system eliminates the redundant manual controls and reduce human errors that can cost a significant amount of money. Control system should be evaluated frequently to ensure that the processes are where they need to be and functioning efficiently and effectively. This way, adherence to industry standards prevails while providing the best service to the customers. Control system engineering ensures that there is a strategic method to improving productivity and enhancing the best practices of a company.

5. Control System : Definition and Representation Basic Components Mainly consists of five basic components: Input : Source of power to the system. Process being controlled : Function of the system. Output : Result of function of the system. S ensing elements : To sense the output quantities/errors. Controller & Actuating devices : System that controls & system that is controlled.

6. Control System : Classification Type of Feedback Open Loop Control System Doesn’t have any feedback loops connected to it Doesn’t depend upon its output i.e. in open loop feedback systems, output is not used as the control variable for the system. One way signal flow systems. A lso known as Non-feedback systems. As there is no feedback loop, no data is transferred back to the open loop systems.

7. Control System : Classification Open Loop Control System (Contd.) Advantages: Output independent. Disadvantages: The output of the open loop system can be determined only by its present state input that means the input of open loop system will always return to zero, before the output returns to zero. There is no chance to correct the transition errors in open loop systems so there is more chance to occur errors . Application: Automatic washing machine, Electric bulb, Electric hand drier, Time based Bread toaster, Water faucet, TV remote control, Clothes drier, Stepper motor or servo motor, Inkjet printers, Door lock system etc.

8. Control System : Classification Type of Feedback 2. Closed Loop Control System Does not take input from human operators. Other than adjustment by control systems, they operate automatically and independently. Action is entirely dependent on the process variable. Figure : Block diagram for a closed loop control system.

9. Control System : Classification Closed Loop Control System (Contd.) Working principle : The error detector produces an error signal, which is the difference between the input and the feedback signal. This feedback signal is obtained from the block (feedback elements) by considering the output of the overall system as an input to this block. Instead of the direct input, the error signal is applied as an input to a controller . So, the controller produces an actuating signal which controls the plant. In this combination, the output of the control system is adjusted automatically till we get the desired response. Hence, the closed loop control systems are also called the automatic control systems. In regards to a heating system, for example, a closed loop might maintain a temperature as a set point, automatically switching on when temperature is below the set point. Open control, in contrast, would enable individuals to set timers and turn instant on heat.

10. Control System : Classification Closed Loop Control System (Contd.) Advantages: R educe errors by automatically adjusting the systems input. Improves stability of an unstable system. Increases or reduces the systems sensitivity. Enhances robustness against external disturbances to the process. Produces a reliable and repeatable performance. Disadvantages: T o provide the required amount of control, a closed-loop system must be more complex by having one or more feedback paths. If the gain of the controller is too sensitive to changes in its input commands or signals it can become unstable and start to oscillate as the controller tries to over-correct itself, and eventually something would break.

11. Control System : Definition and Representation Comparative Study/Difference Between the M ajor Types:

12. Control System : Classification Closed Loop Control System (Contd.) Types of Closed Loop Control System : (a)Feedback Control System : T he output signal is sampled and then fed back to the input to form an error signal that drives the system. Feedback is comprised of a sub-circuit that allows a fraction of the output signal from a system to modify the effective input signal in such a way as to produce a response that can differ substantially from the response produced in the absence of such feedback.

13. Control System : Classification Closed Loop Control System (Contd.) Feedback Control System (Contd.) Advantages: Circuit characteristics such as the systems gain and response can be precisely controlled. Circuit characteristics can be made independent of operating conditions such as supply voltages or temperature variations. Signal distortion due to the non-linear nature of the components used can be greatly reduced. The Frequency Response, Gain and Bandwidth of a circuit or system can be easily controlled to within tight limits.

14. Control System : Classification Closed Loop Control System (Contd.) Feedback Control System (Contd.) Types: Positive Feedback : T he set point and output values are added together by the controller as the feedback is “in-phase” with the input. It “increases” the systems gain. Negative Feedback : The set point and output values are subtracted from each other by the controller as the feedback is “out of phase” with the input. It “decreases” the systems gain.

15. Control System : Classification Closed Loop Control System (Contd.) Types of Closed Loop Control System : (b)Feedforward Control System : A voids the slowness of the feedback control. D etects the disturbance directly and takes an appropriate control action in order to eliminate its effect on the process output. Actions to be taken is predicted by the model.

16. Control System : Classification Closed Loop Control System (Contd.) Feedforward Control System (Contd.) Advantages: By modeling the range of disturbances emanating from that source feedforward control can prepare an appropriate response. Once an upstream disturbance is identified feedforward immediately seizes the initiative and prescribes an appropriate change. It is proactive nature that either minimizes or eliminates the negative effects of a known disturbance. Feedforward’s use of a disturbance model allows it to assess the full magnitude of the disturbance and to implement an appropriate countermeasure. If Feedforward control simply delivered a countermeasure as soon as the disturbance was identified, then it might do more harm to the process. But it is designed to deliver the countermeasure as the disturbance hits the process, essentially nullifying the negative effects of the disturbance.

17. Control System : Classification Closed Loop Control System (Contd.) Types of Closed Loop Control System : (c)Adaptive Control System : A djust parameters based on current needs/able to adjust itself to handle unknown model uncertainties. Uses the information it gathers online to change itself and improve its performance. I t "adapts" to the feedback loop it is inside, and develops into a better controller over time.

18. Control System : Classification Closed Loop Control System (Contd.) Adaptive Control System (Contd.) Types: Direct Method : T he estimated parameters are directly used in the adaptive controller. Indirect Method: The estimated parameters are used to build an estimation/estimated model. The estimated model information is further/later used to adjust the controller.

19. Control System : Classification Types : Considering I/P-O/P SISO (single input single output) MIMO (multiple input multiple output) Types : Considering S ignal Types Continuous time control system Discrete time control system