Module 3 Lecture Outline: Introduction to Control Methods

  1. Feedback Controls:
    1. Objective is to keep the controlled variable equal to the desired set point
    2. A measurement device monitors the controlled variable
    3. A measurement signal is sent to the error detector to represent the condition
    4. The error detector compares its signals to the Set Point and produces an error signal
      1. 3 types of errors
        1. Set point is changed
        2. Disturbance appears
        3. Load demand
    5. The error signal is fed to the controller
    6. The controlled determines the action to be taken
    7. Te output of the controller causes the actuator to make adjustments
  2. Dynamics of Closed Loop Systems:
    1. Response Time: The is the time lag from the time beginning of a change to the time it takes to produce an output
    2. Time Duration: Time it takes for the signal to pass from one instrument loop to another instrument loop.
    3. Static Inertia of the Controlled Variable: This is the opposition to change of a variable when energy is applied. The energy must overcome the resistance and cause the variable to reach its desired state.
    4. Pure Lag: Amount of lag due to the capacity of the material. Amount of lag due to the type of material.
    5. Dead Time: Elapsed time between the instant a deviation of the controlled variable occurs and the corrective action begins.
  3. Feed-Forward Control: To minimize errors
    1. 2 conditions:
      1. Occurrence of a large disturbance
      2. Delays in the dynamic response
    2. See page 14 heat Exchanger
    3. 4 Rules:
      1. Single loop performance
      2. Sensors are needed to measure performance
      3. Secondary variable loops have a faster response time than primary loops
      4. There is a relationship between the final control element and important         disturbances
    4. See diagram feed-forward control 1
      1. Liquid passes through the heat exchanger
      2. Temperature senses the temperature and causes steam to flow in and keep the liquid at 140 degrees.
    5. 4 rules to consider if cascade control or Feed-Forward Control
      1. Single loop not acceptable for possible disturbances
      2. The measured variable sensor available to measure important disturbance
      3. Direct or causal relationship between final control element or important disturbance
      4. Secondary variable in loop has a feaster response time than the primary loop.
    6. See diagram feed-forward control 2
      1. Primary loop compensates for any changes from the set point.
      2. Secondary loop compensates for the inlet flow. Corrects for changes on input temperature before process occur in the primary process and outlet temperature.
  4. Cascade Control
    1. Used where there are large time constraints
    2. Uses Primary and Secondary loops
    3. These loops are controlled by the final element
      1. 2 fluids are mixed and heated using steam and a heat exchanger
      2. These use large time constraints
      3. Primary Loop Transfer:   
        1. Temperature for outer steam loop
        2. Sets the steam flowing
    4. Secondary Loop Transfer: inner loop
    5. Changes in either causes a change in the inner loop: Acts more quickly to steam
    6. Faster, precise control
    7. Inner loop tuned first
    8. Outer Loop tuned last
    9. Time Constance between primary and secondary loop is 5 to 10 times faster
    10. Response of a Control System
      1. How well does the system work?
      2. What is the set point?
      3. What type of disturbance occurs?
  5. System Responses:
    1. Start with a process tank
    2. Heat and mix 2 fluids
    3. 3 factors make a better system:           
      1. Dead Time Delay
        1. Time between when set-points and when the process starts
        2. Depends on the type of sensor
        3. Hold to a minimum
      2. Transient Response
        1. The time for a process variable to go from 10% to 90% of its final value after a set-point change’
        2. .Size of tank
        3. Amount of heat
      3. Steady-State Response
        1. This is the system response to the physical system response to a new output level requested by the input level
        2. Steady-State Errors:
          1. Lack of precision in control system components
          2. Disturbances or changes in the set point
          3. System design fault