Control Principles > Test 1 > Flashcards

Test 1 Flashcards

(55 cards)

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1
Q

What are factors that make control difficult?

A
  • Frequency and magnitude of disturbances
  • Process capacity
  • Lag time (including measurement lat time)
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2
Q

Lag definition

A
  • Delay in response time for process when input changes
  • Makes a process more difficult to control
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3
Q

Overshoot definition

A
  • Amount the PV exceeds setpoint following disturbance/change
  • None is ideal but not always practices
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4
Q

Response time definition

A

Time it takes an element to respond to change in measured value or produce 100% change in output from 100% change in input

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5
Q

Damping definition

A

Reduction of oscillation in a control system

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6
Q

Control action definition

A
  • Nature of controller (direct or reverse)
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7
Q

Control modes

A
  • Proportional (P) control
  • Integral (I) control
  • Derivative (D) control
  • Proportional-integral (PI) control (most common)
  • Proportional-integral-derivative control (PID)
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8
Q

Proportional control method:

A

Changing output of controller by an amount proportional to an error (AKA gain)

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9
Q

Integral control method:

A

Changing output of controller by an amount proportional to an error and its duration (AKA reset)

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10
Q

Derivative control method:

A

Changing output of controller in proportion to the rate of changes of the process variable (AKA rate)

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11
Q

Critically damped definition

A
  • When response time to an abrupt change is as fast as possible without overshoot
  • Under-damped when overshoot occurs
  • Over-damped when response time is slower than critical
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12
Q

Critical gain definition

A

A value of a system gain beyond which the loop becomes unstable

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13
Q

Loop gain definition

A

The product of all gains in a loop including:
- Sensor gain
- controller gain
- control valve gain
- Process gain

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14
Q

Cycling definition

A

An oscillation in the controlled variable

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15
Q

Hunting definition

A

Undesired motion of an automatic control system when the controlled variable oscillates around setpoint but doesn’t stabilize

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16
Q

Dead band/time definition

A

The value that a variable can change without causing a response

  • Undesirable in process control; usually expressed as a process of span
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17
Q

Deviation/offset definition

A

Difference between the actual measured value of the controlled variable and set point

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18
Q

Process definition

A

System/measurement being controlled (E.g. level, flow, temperature, pressure, etc.)

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19
Q

Process control definition

A

A system that combines measuring materials and controlling instruments in to an arrangement capable of automatic action

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20
Q

Process capacity definition

A

Maximum amount of energy/material that can be stored in a system/piece of equipment
- Smaller capacity systems have faster response times -> easier to control

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21
Q

Loop definition

A

Complete circuit with:
- Primary measuring element (input/transmitter)
- Final control element (output/valve)
- Controller

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22
Q

Loop (closed) definition

A

A control system that provides feedback to the controller thanks to changes made by final control element

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23
Q

Loop (open) definition

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A

Control system that sends a signal to final element but doesn’t verify results

24
Q

Manipulated variable definition

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A

The independent variable. I.e. the variable being controlled

25
Controller adjustments:
- Input meter - Output meter - Setpoint indicator - Setpoint adjuster - Output adjustment/manual adjustment - Auto-manual switch
26
What is bumpless/balanceless transfer?
When a controller is switched from manual to auto or auto to manual it automatically selects the last output value
27
When BBT is not an option...
a balance switch is provided. This allows the operator to adjust the manual output to correspond with the automatic output before switching
28
6 Basic Controller Types:
- Pneumatic indicating controller - Pneumatic recording controller - Electric analog indicating controller - Electric analog recording controller - Digital/Micro-processor based indicating controller - Digital/Micro-processor-based recording controller
29
4 Basic Controller Functions
- Receive a measurement value of variable being controlled - Comparing the value with a reference or desired value (setpoint) - Determining magnitude and direction of any deviation/error - Providing an output as a function of that deviation - Controllers look at a signal that represents the actual value of the process variable. compares this signal to the setpoint and acts on the process to eliminate any difference between these two values
30
Basic Components of a Controller:
- The process variable or measured variable is the signal from the transmitter - Set point generator produces a signal of the same type as the input - Transfer switch is the auto/manual switch - Manual control produces an output signal - An automatic controller contains a proportional band, reset, and rate adjustment as well as a direct/reverse switch
31
Controller Forms:
- On/off or tube position - Proportional only (P) - Proportional + Integral (PI) - Proportional + Integral + Derivative (PID)
32
Pneumatic Controller Components
- Set point adjustment produces a pneumatic output to the controller and also adjusts the set point indicator by turning the scale - Measured variable or process input from the transmitter goes to the controller and the process indicator - Controller compares the input to the set point and produces an output to eliminate any difference between these signals - Auto/manual switch determines whether automatic controller output or manual output goes through the relay to the valve - If the controller is in manual the rest of the control circuit has been eliminated and the manual regulator is adjusted to control the process - When starting up a process started in manual, bring the process manually up to set point before switching to automatic - This controller does not have BBT - The balance indicator or ball gauge must be in the center before switching from auto to manual or manual to auto
33
Proportional Principle of Operation:
- Set point signal is applied opposite to measured variable signal and both are 3-15 psi - As input signal increases -> flapper moves closer to nozzle -> creating back pressure -> causing output pressure to increase - As output pressure increases -> the pressure in the feedback bellows increases -> repositioning the force bar - Controller is balanced and process variable will be constant when the force in the feedback bellows balances with the input from the transmitter - The controller is now in a balanced position as the setpoint pressure equals the input pressure and the output is constant - If input pressure or setpoint pressure changes -> the output would change until the controller balances again
34
Proportional + Integral Principles of Operation:
- Same as proportional drawing with the addition of a 4th bellows and a restriction - The bellows and restriction produce a time constant that can be changed with the restrictor - If the flow of air into the bellow is reduced -> the bellows will take longer to fill -> the reaction time of the controller will be slowed down
35
Proportional Control Definition
A method of changing control by an amount proportional to the error
36
Proportional Band Definition
Percent of full scale through which the error signal must vary between
37
BBT Definition
- Bumpless balanceless transfer - To go from auto to manual while maintaining previous output (avoiding bumps/major hiccups
38
The adjustment on a proportional controller is commonly called...
Gain or PB
39
Proportional band responds to...
The magnitude and direction of the error signal (deviation of measured variable from set point -> MV from SP)
40
Proportional Control cannot consider...
The past of the error or possible future consequences of the error trend
41
Output change =
Input Change*Gain
42
As proportional band decreases...
Controller output increases
43
As gain decreases...
Controller output decreases
44
Wide bands correspond to...
Less sensitive controller settings -> narrow bands correspond to more sensitive controller settings
45
Offset Definition
Distance between measured variable and setpoint
46
The main limitation of plain proportional control is that...
It cannot keep the controlled variable at SP
47
The only time a proportional controller can maintain the process at setpoint...
Is at 50%
48
Reset Action
**A 2 mode controller responds to the magnitude and direction of error as well as offset - Reset action is also known as integral action - Reset action is a time adjustment - Continuous to change output as long as offset exists - Reset action stops when there is 0 offset *Reset is given in units: resets/minute or reset minutes
49
Reset Time Definition
The time necessary for reset action to duplicate the initial proportional response
50
Critical Gain Definition
A value of system gain beyond which the loop becomes unstable
51
Loop Gain Definition
Is the product of all gains in a loop including sensor gain, controller gain, control valve gain and process gain
52
Deadband Definition
Amount a variable can change without causing a response
53
Cycling Definition
An oscillation in the controlled variable
54
55
A 2 mode controller...
Responds to the magnitude and direction of error as well as offset

Control Principles (3 decks)

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