Automatic Process Control - 2

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Explain the purpose, operation, and give examples of on-off, proportional, proportional plus-reset, and proportional-plus-reset-plus-derivative control. Define proportional band and gain.


Questions and Answers
  • 1. 

    Which control mode would most likely cause a final control element to move from on to off, or from fully opened to fully closed?"

    • A.

      A. Two-position control

    • B.

      B. PID control

    • C.

      C. Rate control

    • D.

      D. Reset control

    Correct Answer
    A. A. Two-position control
    Explanation
    Two-position control is the control mode that would most likely cause a final control element to move from on to off, or from fully opened to fully closed. In two-position control, the final control element is either fully on or fully off, with no intermediate positions. This means that when the control signal reaches a certain setpoint, the final control element will quickly move to the fully closed or fully opened position, causing the process to turn off or on respectively.

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  • 2. 

    In a two-position control system, the amount of change that can occur in the process without causing a control system response is known as what?

    • A.

      A. Repeats per minute

    • B.

      B. Minutes per repeat

    • C.

      C. Dead zone

    • D.

      D. Gain

    Correct Answer
    C. C. Dead zone
    Explanation
    In a two-position control system, the dead zone refers to the amount of change that can occur in the process without causing a control system response. It represents a range of values where no action is taken by the control system, providing a buffer to prevent unnecessary adjustments. This allows for small variations or fluctuations in the process to be ignored, reducing unnecessary control system response and ensuring stability in the system.

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  • 3. 

    Which of these characteristics applies to two-position control?          

    • A.

      A. A proportional output signal is produced.

    • B.

      B. Reset and rate control features are combined

    • C.

      C. Output signals are based on integration.

    • D.

      D. A final control element is moved from one extreme to another.

    Correct Answer
    D. D. A final control element is moved from one extreme to another.
    Explanation
    Two-position control refers to a control system where the final control element, such as a valve or a switch, is moved from one extreme position to another. This means that the control element is either fully open or fully closed, with no intermediate positions. This type of control is commonly used in on-off control systems, where the control element is switched on or off based on the desired setpoint. It does not produce a proportional output signal, combine reset and rate control features, or base output signals on integration.

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  • 4. 

    A proportional controller can normally return a controlled variable to set point if what type of process disturbance occurs?

    • A.

      A. Continual

    • B.

      B. Temporary

    • C.

      C. Worsening

    • D.

      D. Permanent

    Correct Answer
    B. B. Temporary
    Explanation
    A proportional controller can normally return a controlled variable to set point if a temporary process disturbance occurs. This is because a proportional controller adjusts the control output in proportion to the error between the set point and the actual value of the controlled variable. When a temporary disturbance occurs, the controller detects the error and adjusts the control output accordingly, bringing the controlled variable back to the set point. However, if the disturbance is continual, worsening, or permanent, the proportional controller may not be able to fully compensate and return the controlled variable to the set point.

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  • 5. 

    In a proportional control system, what is the difference between the control point and the variable's set point called?"

    • A.

      A. Offset

    • B.

      B. Dead zone

    • C.

      C. Gain

    • D.

      D. Reset

    Correct Answer
    A. A. Offset
    Explanation
    In a proportional control system, the control point refers to the actual value of the variable being controlled, while the set point is the desired value for the variable. The difference between the control point and the set point is called the offset. This offset indicates how far off the control point is from the desired set point.

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  • 6. 

    The inverse of proportional band is known as what?"

    • A.

      A. Offset

    • B.

      B. Gain

    • C.

      C. Reset

    • D.

      D. MPR

    Correct Answer
    B. B. Gain
    Explanation
    The inverse of proportional band is known as gain. Gain refers to the ratio of the change in output to the change in input of a system. In the context of proportional control, gain represents the sensitivity of the controller to the error signal. A higher gain value means that the controller will respond more aggressively to changes in the error signal, resulting in a faster and larger correction. Conversely, a lower gain value will result in a slower and smaller correction. Therefore, gain is the correct answer as it represents the inverse relationship with the proportional band.

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  • 7. 

    Select the choice that most accurately completes this sentence. A narrow proportional band_________.

    • A.

      A. Is greater than a 100% PB

    • B.

      B. Minimizes cycling

    • C.

      C. Increases the offset in a process

    • D.

      D. Is less than a 100% PB

    Correct Answer
    D. D. Is less than a 100% PB
    Explanation
    A narrow proportional band refers to a smaller range within which the controller takes action to maintain the desired setpoint. It is less than a 100% proportional band, which means that the controller will respond more quickly to small deviations from the setpoint. This helps to minimize cycling and improve the overall stability and accuracy of the control system.

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  • 8. 

    The function of reset control is to adjust the output of a proportional controller to eliminate what?

    • A.

      A. Offset

    • B.

      B. Gain

    • C.

      C. Proportional band

    • D.

      D. Dead rate

    Correct Answer
    A. A. Offset
    Explanation
    The function of the reset control is to adjust the output of a proportional controller in order to eliminate offset. Offset refers to the difference between the desired setpoint and the actual output of the controller, and the reset control helps to reduce or eliminate this difference. By adjusting the output based on the error between the setpoint and the actual output, the reset control helps to bring the system back to the desired setpoint and minimize any offset.

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  • 9. 

     Which of these choices is another name for reset control?"

    • A.

      A. Derivative-plus-reset

    • B.

      B. Proportional-plus-integral

    • C.

      C. Proportional-plus-derivative

    • D.

      D. Integral-plus-derivative

    Correct Answer
    B. B. Proportional-plus-integral
    Explanation
    Proportional-plus-integral control is another name for reset control. This control strategy combines both proportional and integral control actions to regulate a system. Proportional control provides a response proportional to the error between the desired and actual values, while integral control integrates the error over time to eliminate steady-state errors. Together, they form the reset control strategy, which is commonly used in various control systems to improve stability and performance.

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  • 10. 

    Reset control action is most likely to be expressed in terms of what

    • A.

      A. A percentage

    • B.

      B. Gain

    • C.

      C. Offset

    • D.

      D. Minutes per repeat

    Correct Answer
    D. D. Minutes per repeat
    Explanation
    Reset control action is most likely to be expressed in terms of "Minutes per repeat" because reset control action refers to the time it takes for the controller to reset and start responding to a deviation from the set point. This parameter is typically measured in minutes per repeat, indicating the time it takes for the controller to make a complete adjustment cycle. Expressing reset control action in terms of minutes per repeat allows for better understanding and adjustment of the control system's response time.

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  • 11. 

    Which of these control modes increases, or repeats, the amount of proportional control until a controlled variable returns to set point?"

    • A.

      A. Rate control

    • B.

      B. Reset control

    • C.

      C. Gain control

    • D.

      D. Two-position control

    Correct Answer
    B. B. Reset control
    Explanation
    Reset control is a control mode that increases the amount of proportional control until a controlled variable returns to set point. It continuously adjusts the control output based on the error between the set point and the actual value, gradually increasing the control effort until the error is minimized. This helps to maintain stability and accuracy in the control system.

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  • 12. 

    Which of the following statements about rate control is true?"

    • A.

      A. Rate control normally exists by itself

    • B.

      B. Rate control responds to the speed at which a variable deviates from set point.

    • C.

      C. Rate control action is based on a mathematical function called integration.

    • D.

      D. Rate control action is often expressed in minutes per repeat. d. Rate control action is often expressed in minutes per repeat. d. Rate control action is often expressed in minutes per repeat.

    Correct Answer
    B. B. Rate control responds to the speed at which a variable deviates from set point.
    Explanation
    Rate control refers to the action taken to regulate the speed at which a variable deviates from its set point. It is not a standalone process but rather a component of a larger control system. The statement that rate control responds to the speed at which a variable deviates from set point is true and accurately describes the nature of rate control.

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  • 13. 

    Which of these control modes is sometimes referred to as proportional-plus-derivative control?"

    • A.

      A. Reset control

    • B.

      B. PB control

    • C.

      C. Rate control

    • D.

      D. Offset control

    Correct Answer
    C. C. Rate control
    Explanation
    Rate control is sometimes referred to as proportional-plus-derivative control because it combines the proportional control, which adjusts the control output based on the error between the setpoint and the process variable, with the derivative control, which adjusts the control output based on the rate of change of the error. This combination allows for more precise and responsive control of the process variable.

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  • 14. 

    In which of these control modes does a controller produce an instant boost in the proportional output signal to try to stop changes in the input as soon as they're detected?"

    • A.

      A. Reset control

    • B.

      B. PB control

    • C.

      C. Rate control

    • D.

      D. Offset control

    Correct Answer
    C. C. Rate control
    Explanation
    Rate control is the correct answer because it refers to a control mode where the controller instantly boosts the proportional output signal in order to quickly counteract any changes in the input that are detected. This mode is designed to respond rapidly to changes and prevent any further deviation from the desired setpoint. Reset control, PB control, and Offset control do not involve an instant boost in the output signal as soon as changes are detected, making them incorrect options.

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  • 15. 

    The 'D' in PID control stands for derivative, which is the same as what?"

    • A.

      A. Digital

    • B.

      B. Rate

    • C.

      C. Reset

    • D.

      D. Proportional

    Correct Answer
    B. B. Rate
    Explanation
    The 'D' in PID control stands for derivative, which is the same as the rate. The derivative term in PID control calculates the rate of change of the error signal and helps in predicting the future behavior of the system. It provides damping to the control system and helps in reducing overshoot and oscillations. Therefore, the correct answer is b. Rate.

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  • 16. 

    The I in PID control stands for integral, which is the same as what?"

    • A.

      A. Digital

    • B.

      B. Rate

    • C.

      C. Reset

    • D.

      D. Proportional

    Correct Answer
    C. C. Reset
    Explanation
    The I in PID control stands for integral, which is the same as "Reset". In PID control, the integral term calculates the accumulated error over time and adjusts the control signal accordingly. The integral term helps to eliminate steady-state errors and improve system response. Therefore, "Reset" is the correct term that represents the integral component in PID control.

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  • 17. 

    What mode of control combines the precise response action of reset control with the fast response action of rate control?"

    • A.

      A. Gain control

    • B.

      B. MPR control

    • C.

      C. Two-position control

    • D.

      D. PID control

    Correct Answer
    D. D. PID control
    Explanation
    PID control combines the precise response action of reset control with the fast response action of rate control. This control mode uses proportional, integral, and derivative actions to calculate the control output. The proportional action provides a response proportional to the error, the integral action eliminates any steady-state error, and the derivative action helps to anticipate and respond quickly to changes in the error. This combination of actions allows for both accurate and fast control of the system.

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  • 18. 

    Which of these control modes will most likely cause a final control element to move from one extreme to another?"

    • A.

      A. Two-position control

    • B.

      B. PID control

    • C.

      C. Rate control

    • D.

      D. Reset control

    Correct Answer
    A. A. Two-position control
    Explanation
    Two-position control is the control mode that will most likely cause a final control element to move from one extreme to another. This is because two-position control operates by turning the final control element fully on or fully off, depending on the setpoint. Therefore, when the setpoint changes, the final control element will move from one extreme position to the other in order to reach the new setpoint.

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  • 19. 

    A new steady-state value that's sometimes established for a controlled variable after a process disturbance is known as what?"

    • A.

      A. The gain

    • B.

      B. The PID

    • C.

      C. The control point

    • D.

      D. The reset point

    Correct Answer
    C. C. The control point
    Explanation
    The control point refers to the new steady-state value that is established for a controlled variable after a process disturbance. It represents the desired or target value that the system aims to maintain through control actions. The gain refers to the amplification factor of a control system, the PID (Proportional-Integral-Derivative) is a control algorithm used to regulate a system, and the reset point is not a term commonly associated with the establishment of a new steady-state value.

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  • 20. 

    The term 'gain' can be best described as the inverse of which of these choices?"

    • A.

      A. Offset

    • B.

      B. Reset

    • C.

      C. Dead zone

    • D.

      D. Proportional band

    Correct Answer
    D. D. Proportional band
    Explanation
    The term 'gain' refers to the amplification or increase in a signal. In the context of control systems, the proportional band is a measure of the gain or sensitivity of the system. A larger proportional band means a smaller gain, while a smaller proportional band means a larger gain. Therefore, the correct answer is d. Proportional band, as it is the inverse of gain.

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  • 21. 

    Which of these statements about reset control action is true?"

    • A.

      A. It is normally found in a two-position control system.

    • B.

      B. It is also called proportional-plus-derivative control.

    • C.

      C. It adjusts the output of a proportional controller to help eliminate offset.

    • D.

      D. It normally exists by itself.

    Correct Answer
    C. C. It adjusts the output of a proportional controller to help eliminate offset.
    Explanation
    Reset control action, also known as integral control, is used to eliminate offset in a control system. It adjusts the output of a proportional controller by integrating the error signal over time, which helps to drive the system towards the desired setpoint. This is typically achieved by adjusting the integral gain parameter in a PID controller. Reset control action is not normally found in a two-position control system and it is not the same as proportional-plus-derivative control. It is often used in combination with proportional and derivative control actions to achieve better control performance.

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  • 22. 

    Which of these control modes is designed to eliminate offset?"

    • A.

      A. Two-position

    • B.

      B. Proportional

    • C.

      C. Proportional-plus-rate

    • D.

      D. Proportional-plus-reset

    Correct Answer
    D. D. Proportional-plus-reset
    Explanation
    Proportional-plus-reset control mode is designed to eliminate offset. In this control mode, the controller adjusts the output based on the difference between the setpoint and the process variable, proportional to the error. Additionally, it also incorporates a reset action that continuously calculates and adjusts the output based on the integral of the error over time. This reset action helps to eliminate any residual offset in the system, ensuring that the process variable closely matches the setpoint.

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  • 23. 

    Integral control is also known as what?"

    • A.

      A. Rate control

    • B.

      B. Reset control

    • C.

      C. Gain control

    • D.

      D. Two-position control

    Correct Answer
    B. B. Reset control
    Explanation
    Integral control is also known as reset control because it refers to the control action that resets the controller output based on the accumulated error over time. This type of control continuously adjusts the controller output to eliminate any steady-state error. The reset action is proportional to the integral of the error and helps in maintaining the desired setpoint. It is commonly used in control systems to improve accuracy and stability.

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  • 24. 

    Which of these control modes responds to the speed at which a variable changes from set point?"

    • A.

      A. Two-position

    • B.

      B. Proportional

    • C.

      C. Proportional-plus-rate

    • D.

      D. Proportional-plus-reset

    Correct Answer
    C. C. Proportional-plus-rate
    Explanation
    Proportional-plus-rate control mode responds to the speed at which a variable changes from the set point. This means that it not only considers the difference between the actual value and the set point (proportional control), but also takes into account how quickly the variable is changing over time (rate control). This allows for a more dynamic and responsive control system, as it can adjust the output based on the rate of change of the variable.

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  • 25. 

    Since rate control responds only during changes in the input to a controller, it may allow what to occur      

    • A.

      A. Reset action

    • B.

      B. Integration

    • C.

      C. Offset

    • D.

      D. Gain

    Correct Answer
    C. C. Offset
    Explanation
    Rate control is a type of control system that only responds to changes in the input to a controller. It does not take into account the overall value or level of the input. As a result, rate control may allow for an offset to occur. An offset refers to a steady-state error or deviation from the desired setpoint that persists even after the controller has reached a stable state. Since rate control does not actively correct for this offset, it may allow it to occur.

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