Chapter 7    The Feedback Loop

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Check Your Reading             Study Questions             Thought Questions

The questions will be categorized according to the four key concepts in the chapter.

Loop Elements Performance Measures
Selecting CV and MV Control Approaches

 

 

 

 

 

 

 

 

 

 

 

Check Your Reading

Solidify the Concepts
7.1
Which of these control loop elements are typically very fast and very accurate?

Loop Elements

Engineers must select the appropriate equipment for each element.

 

Electrical Transmission

 

Signal Conversion

 

 Pneumatic Transmission

 		 

 Process
 

 Sensor

 		 

 Controller Calculation
 
 Final Element, Valve  
 

 

 

 

 

 

 

 

 

 

 

 

 

7.2
Which of the following statements are true?

Loop Elements

 

The control valve always remains within its limits when used for control; it is never fully closed or open.

 

 

 Most control signals are transmitted 50 meters or less.

 
 

 The selection of which valve to manipulate influences the behavior of the controlled variable.
 

 

 

 

 

 

 

 

 

 

 

7.3
A control valve % opening is usually changed by adjusting an electric motor that moves the stem position.

Loop Elements

TRUE     FALSE

 

 

 

 

 

 

 

 

 

7.4
It is possible to measure the temperature, pressure and flow of highly corrosive materials, like acid.

Loop Elements

 

 

 

 

 

 

TRUE     FALSE

 

 

 

 

 

 

 

 

 

 

 

7.5
When performing a process reaction curve experiment which elements of the loop are included in the resulting dynamic model?

Loop Elements

 

Electrical Transmission

 

Signal Conversion

 

 Pneumatic Transmission

 		 

 Process
 

 Sensor

 		 

 Controller Calculation
 
 Final Element, Valve
 		 
 

 

 

 

 

 

 

 

 

 

 

7.6
The behavior of the controlled variable, CV, is important because...

Selecting CV and MV

 

 

 

it was selected to achieve one or more of the 7 control objectives.

 
 the sensor measures this value.
 
 large fluctuations create messy trend plots.

 

 

 

 

 

 

 

 

7.7
The behavior of the manipulated variable, MV, is important because...

Selecting CV and MV

 

 

 

large fluctuations might damage equipment.

 

 

 high frequency fluctuations might wear out the valve.

 
 

 large fluctuations might cause disturbances elsewhere in the plant.

 
 
 the signal transmission could not keep up with fast fluctuations.

 

 

 

 

 

 

 

 

 

 

 

7.8
Dynamic data for the distillate product quality has been collected from the tower under normal operation WITHOUT feedback control.  What would be some reasons for the variation shown in the data?

Performance Measures

 

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7.9
Which of the following criteria are required  for a manipulated variable?
(Distinguish criteria that are (i) required, (ii) desirable, and (ii) not relevant.)

Selecting CV and MV

 

The ability to compensate for large disturbances.

 

 

 

Automated valve to influence the selected controlled varible.

 

 

  

Fast speed of response.

 

 

  

Causal relationship between the valve and the controlled variable.

 


 

The ability to adjust the manipulated variable rapidly and with little upset to the remainder of the plant.

 

 

 

 

 

 

 

 

 

 

7.10
Which of the five approaches to control would you apply to each of the following variables?

Control Approaches

Engineers chose the most appropriate, not always the highest technology.

1. Temperature in a garage.

2. Temperature in a oil storage tank that could freeze in winter.

3. Temperature of soup being heated on a stove.

4. Temperature of a CSTR making pharmaceuticals.

5. Temperature in a closed vessel with an exothermic chemical reaction.