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SCADA (PART 3) - Coggle Diagram

- - - - The variables which quantify the performance or quality of the final product, which are also called output variables (Set point).
    - - These variable also called as load variables.
      - It represent input variables that can cause the controlled variables to deviate from their respective set points (cannot be manipulated)
    - - These input variables are adjusted dynamically to keep the controlled variables at their set-points.
- - - - One of the simplest process controlschemes.
      - A feedback loop measures a process variable and sends the measurement to a controller for comparison to set point. If the process variable is not at set point, control action is taken to return the process variable to set point
      - The advantage ofthis controlscheme is that itis simple using single transmitter.
      - This control scheme does not take into consideration any of the other variables in the process
      - Feedback loop are commonly used in the process control industry.
      - The advantage of a feedback loop is that directly controls the desired process variable
      - The disadvantage of feedback loops is that the process variable must leave set point for action to be taken.
    - - An advantage of feedforward control is that error is prevented, rather than corrected
      - However, it is difficult to account for all possible load disturbances in a system through feedforward control.
      - In general, feedforward system should be used in case where the controlled variable has the potential of being a major load disturbance on the process variable ultimately being controlled.
    - - Because of the difficulty of accounting for every possible load disturbance in a feedforward system,this system are often combined with feedback systems.
      - Controller with summing functions are used in these combined systems to total the input from both the feedforward loop and the feedback loop, and send a unified signal to the final control element
    - - Ratio control is used to ensure that two or more flows are kept at the same ratio even if the flows are changing.
      - If the physical characteristic of the mixed flow is measured, a PID controller can be used to manipulate the ratio value.
      - Application
        
        Blending two or more flows to produce a mixture with specified composition.
        
        Blending two or more flows to produce a mixture with specified physical properties
        
        Maintaining correct air and fuel mixture to combustion.
    - - A very common control scheme is range control in which the output of a controller is split to two or more control valves
      - For example
        
        controller output 0% Valve is fully open and valve B fully closed
        
        Controller output 25% Valve A is 75% open and valve B 25% open
        
        Controller output 50% Both valves are 50% open
        
        Controller output 75% Valve A is 25% open and Valve B 75% open
        
        Controller output 100% Valve A is fully closed and Valve B fully open
    - - Cascade Control uses the output of the primary controller to manipulate the set point of the secondary controller as if it were the final control element
      - Reasons for cascade control:
        
        Allow fastersecondarycontroller to handle disturbancesin the secondary loop.
        
        Allow secondary controller to handle non-linear valve and other final control element problems.
        
        Allow operator to directly control secondary loop during certain modes of operation (such as startup).
      - Requirements for cascade control
        
        Secondary loop process dynamics must be at least four times as fast as primary loop process dynamics
        
        Secondary loop must have influence over the primary loop.
        
        Secondary loop must be measured and controllable.
- - - - Instruments that are mounted on process plant (i.e sensor that mounted on pipeline or process equipments.
    - - Instruments that are mounted on control board.
    - - Instruments that are mounted behind a control panel board.
    - - A distributed control system (DCS) refers to a control system usually of a manufacturing system, process or any kind of dynamic system, in which the controller elements are not central in location (like the brain) but are distributed throughout the system with each component sub-system controlled by one or more controllers. The entire system of controllers is connected by networks for communication and monitoring
- - - - A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument.
      - For example, a mercury thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube
      - For accuracy, all sensors need to be calibrated against known standards.
    - - Transmitter is a transducer that responds to a measurement variable and converts that input into a standardized transmission signal.
      - Transducer is a device that receives output signal from sensors.
    - - Controller is a device which monitors and affects the operational conditions of a given dynamical system
      - The operational conditions are typically referred to as output variables of the system which can be affected by adjusting certain input variables.
    - - Final Control Element is a device that directly controls the value of manipulated variable of control loop.
      - Final control element may be control valves, pumps, heaters, etc