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