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Process Control > FPC Test 1 > Flashcards

FPC Test 1 Flashcards

1
Q

What does PV stand for?

A

Process Variable

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2
Q

What does SP stand for and what is it?

A

Set Point. This is the desired value of the process variable.

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3
Q

What does E stand for? What is it and how is it calculated?

A

Error. The difference between set point and process variable.
E = PV - SP

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4
Q

What is M?

A

Manipulated Variable. This is used to control the PV.

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5
Q

What are the two parts of a cascade control system and what does each part do?

A

Master/primary loop - Reads process temperature and tries to keep it at SP. Generates error signal and hence generates SP of slave loop.
Slave/secondary loop - Provides a specific amount of M depending on signal from master loop.

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6
Q

What are two advantages of cascade?

A

Minimises disturbance in PV if there is a change by quickly accounting for it by changing M.
It accounts for any control valve issues.

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7
Q

Under what condition can cascade be used and why?

A

Slave loop must be faster than master loop because the secondary loop must correct the change before the primary loop has a chance to see it.

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8
Q

Describe what on-off control is and where is it often used?

A

It is a form of control which only has an on or off setting without any control valve to give an in between. It is often used in domestic control systems.

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9
Q

What is a deadband?

A

The sensitivity that the sensor has in the control system.

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10
Q

What may deadtime do to the PV in terms of the deadnband.

A

It may cause the PV to oscillate slightly outside the deadband.

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11
Q

What is the equation to calculate M?

A

M = KcE + C
Kc - Controller Gain
E - Error
C - Constant when E = 0

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12
Q

What is the velocity form of the M equation?

A

dM = Kc * dE

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13
Q

How do you calculate Kc when given proportional band?

A

Prop. Band = 100/Kc

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14
Q

What is proportional band?

A

The percentage change in error which is required to change the controller output by 100%.

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15
Q

What is the equation for change in error?

A

dE = dPV - dSP

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16
Q

What are the expressions for dE and dM in a steady operation?

A 
dE = -dSP
dM = -Kc * dSP
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17
Q

What is offset?

A

The idea that proportional control can never fully eliminate error and the PV will never reach the SP.

18
Q

What happens in proportional control when Kc is increased?

A

PV gets closer to SP and offset decreases but oscillations increase and system becomes less stable.

19
Q

What control is used to eliminate offset?

A

Integral Control

20
Q

What is the integrated equation for M in integral control?

A

M = Kc / Ti * INT(E.dt)

Ti - Integral time constant

21
Q

What is the differential equation for M in integral control?

A

dM/dt = Kc / Ti * E

22
Q

How does integral action affect oscillation? What is done about this?

A

It increases it. Kc is reduced to counteract this.

23
Q

How is integral action increased?

A

By decreasing Ti.

24
Q

What is derivative action and why is it used?

A

It is trying to predict the error if it is changing quickly and this decreases the oscillations after integral action meaning you can increase Kc.

25
Q

What is the equation for M in derivative action?

A

M = Kc * Td * dE/dt

Td - derivative time constant

26
Q

What does increasing Td do?

A

It makes the oscillations more stable.

27
Q

Why can you not use PID for flow systems?

A

Flow is noisy (has random spikes) which are amplified by derivative action, meaning that derivative action will make the system unsteady.

28
Q

What problem arises when using derivative action?

A

Derivative spikes which cause error.

29
Q

If SP is unchanged, how can you express the error caused by a derivative spike?

A

PV(n) - 2PV(n-1) + PV(n-2)

30
Q

What is a load change?

A

An external disturbance which affects PV.

31
Q

What is the difference between control tuned for SP change and Load change?

A

Control tuned for load change reaches the set point quicker after an external disturbance.
Control tuned for SP change reaches a new set point faster than control tuned for load change because the latter overcompensates.

32
Q

In theory, which controllers should be used for SP that doesn’t change?

A

Proportional to PV.

33
Q

In theory, which controllers should be used for primary and which for secondary in a cascade system?

A

Primary proportional to PV, secondary proportional to error.

34
Q

In practice, what are all controllers proportional to and why?

A

PV, because values such as Kc, Ti and Td can be used to alter how the controller reacts to change.

35
Q

What are the advantages and disadvantages of tuning through trial and error?

A

+Simple
+Involves all real components in closed loop

  • Time consuming
  • Different people will have different opinions on if a controller is well tuned
  • Usually results in non-optimum tuning
36
Q

How do you tune by trial and error?

A

Set Ti and Td to 0.
Keep increasing Kc in steps of 20% until system starts oscillating
Reduce Ti so that 1/Ti starts increasing, do this in steps of 50% and reduce Kc to maintain stability
Continue adjusting these constants until the loop is well tuned
Add the derivative, starting with Ti/4, and decrease Kc to maintain stability
Adjust all 3 constants until well tuned

37
Q

What is quarter decay?

A

When the second kick is four times smaller than the first error kick.

38
Q

What is the penalty function and what is the equation for it which minimises error?

A

It is when you change the parameters of the controller to minimise the area between PV and SV after a change.
ITAE = INT(Et.dt)

39
Q

What is the settling time?

A

The time taken for the error to reduce to 5% and stay within that range.

40
Q 
What if the time constant is:
Equal to process lag?
Greater than process lag?
Less than process lag?
Which is preferred?
A

Change in SP will have same time constant as the open loop response to a change in MV
Tuning will be more robust and more tolerant to changes in process dynamics.
Response will be fast giving more MV overshoot

Time constant should be greater than lag

Process Control (2 decks)

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