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Instrumentation and Drive Systems > Chpt 28 Basic Closed-Loop Control > Flashcards

Chpt 28 Basic Closed-Loop Control Flashcards

1
Q

how are control systems often described as

A

in terms of block diagrams

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2
Q

what is the system in the diagram known as

A

the plant

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3
Q

what is the control input

A

the parameter that we change to affect the plant

in the case of DC motor this will be the duty cycle drive of voltage to the motor

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4
Q

what is the output response

A

the magnitude of the control input

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5
Q

what is the output response

A

the parameter that we are interested in monitoring and controlling

for DC motor this will be the speed of the motor

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6
Q

what is the step response

A

the response of the system to an instantaneous change in the command or control input

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7
Q

what is an overdamped response

A

a response that rises slowly to the new value and always approaches from the direction of the new value of the system output prior to time t=0

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8
Q

what is an underdamped response

A

a response that exhibits a faster rise time but overshoots the target value and exhibits a decaying oscillation before settling

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9
Q

what is a rise time

A

the time taken for the value to reach the target value even if it overshoots

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10
Q

what is an overshoot

A

the amplitude the response goes above the target value before decreasing to go towards the target value again

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11
Q

what is the settling time

A

the time taken for the response to settle to a stable value

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12
Q

what is a critically damped response

A

a response that walks the fine line between over and under damped
it exhibits a quick rise time with no overshoot of the final target value
in practise it is difficult and often unnecessary to achieve true critical damping

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13
Q

what is open loop control

A

you map the response of a system to the control variable and then use that mapping to produce the desired output during operation

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14
Q

what is feedback

A

the operation of feedback is where the system gives information about the current state of the system

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15
Q

what is bang bang control

A

the control effort is either fully on or fully off and no attempt to made to modulate an intermediate value of control effort

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16
Q

what is important for bang bang control

A

the response of the system to the system to the control effort must be relatively slow compared to the rate at which the control effort can be switched on or off

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17
Q

where is bang bang control commonly used

A

in thermal control systems such as refrigerators baking ovens and heating/cooling systems in buildings

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18
Q

what is chatter in a bang bang system and how can it be overcome

A

chatter is the rapid turning of the actuator (eg a furnace) on and off when the measured parameter is close to the set point and moves slightly above and below it

to overcome the chattering effect two threshold values are set
the system is turned on till the measurement has reached the upper threshold where it will turn off the actuator and wont be switched on till it reaches the lower threshold

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19
Q

what is the gap between the two threshold setpoints known as

A

the deadband

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20
Q

what are the two conditions that must be met to consider Bang Bang control

A

first the system requirement is to allow for the variation in the output response are inherent in on-off control

second being that the combination of the dynamics of the system and the ‘strength’ of the actuator be such that the peak-to-peak variation in the controlled parameter can be limited enough to meet the requirements of the system

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21
Q

what is the simplified equation for control effort a linear closed loop control

A

Control effort = (Where we want to be - where we are) x scaling factor

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22
Q

what is error

A

the difference between ‘where we want to be’ and ‘where we are’

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23
Q

what is the scaling factor also known

A

gain

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24
Q

what is proportional gain

A

Kp

when the gain is applied directly to the error yielding an effort that is proportional to the error

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25
Q

how can the trapezoidal rule used in control systems

A

for the integral gain

Current Integral error = change in time x [(error at last step + current error)/2]

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26
Q

how to condense a history of error into a single number to be used for control

A

Control effort = (Error x Proportional Gain) + (Integral gain x sum of errors)

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27
Q

what is the scaling factor with the summation of errors known as

A

Ki

integral gain

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28
Q

what is the type of control with a proportional term and an integral term known as

A

PI control

29
Q

how does PI control work

A

proportional only control generated enough control effort to force the duty cycle to its maximum ( often referred to the Saturation of the actuator) by the time the proportional control effort starts to drop off ( after about 20 ms) the integral term has had time to accumulate an integrated error and starts to contribute significantly

the result is the slow rise of the motor speed to the desired set point

30
Q

what is the ability to overcome external disturbances

A

disturbance rejection

31
Q

what is integrator windup

A

a large accumulated error that caused a large command resulting in overshoot of the target speed

32
Q

what is the solution to integrator windup

A

integrator antiwindup
the easiest way to do this would be to monitor the commanded control effort, once it reaches 100% stop integrating the error as the system is already doing everything that it can do to reach the target
the same should be done for 0% duty cycle for the same logic

33
Q

what would adding a term that looked at the rate of change of the error and use it to reduce or increase the control effort

A

Control effort = Proportional Gain(Error + (Integral gain x sum of error)+ (derivative gain x dError/dT))

34
Q

what does adding the derivative control do to the system

A

adds stability to a system, the added stability compensates for the destabilising effect of the integral control

35
Q

what is the potential drawback of the derivative term

A

it may accentuate any noise present in the feedback signal

36
Q

what needs to be done to implement derivative control

A

need to keep track of what the error was the last time we updated the control effort so that we can develop a term that is the rate of change of error

37
Q

what is tuning

A

the process of selecting the proper gains to create a successful control system

38
Q

what does adding a proportional term do to the rise time

A

decreases

39
Q

what does adding a proportional term do to the overshoot

A

increases

40
Q

what does adding a proportional term do to the settling time

A

No change

41
Q

what does adding a proportional term do to the Steady state error

A

Decreases

42
Q

what does adding a integral term do to the Rise time

A

Decreases

43
Q

what does adding a integral term do to the Overshoot

A

Increases

44
Q

what does adding a integral term do to the Settling time

A

Increases

45
Q

what does adding a integral term do to the steady state error

A

Eliminates

46
Q

what does adding a derivative term do to the rise time

A

No Change

47
Q

what does adding a derivative term do to the overshoot

A

Decreases

48
Q

what does adding a derivative term do to the settling time

A

Decreases

49
Q

what does adding a derivative term do to the steady state error

A

No change

50
Q

what does increasing the proportional term do to the Rise time

A

Decreases

51
Q

what does increasing the proportional term do to the overshoot

A

Increases

52
Q

what does increasing the proportional term do to the Settling time

A

Increases

53
Q

what does increasing the proportional term do to the Steady state error

A

Decreases

54
Q

what does increasing the integral term do to the rise time

A

Decreases

55
Q

what does increasing the integral term do to the overshoot

A

Increases

56
Q

what does increasing the integral term do to the settling time

A

Decreases then Increases

57
Q

what does increasing the integral term do to the Steady state error

A

Eliminates

58
Q

what does increasing the derivative term do to the rise time

A

Increases

59
Q

what does increasing the derivative term do to the overshoot

A

Decreases

60
Q

what does increasing the derivative term do to the settling time

A

Decreases

61
Q

what does increasing the derivative term do to the Steady state error

A

No Change

62
Q

in Ziegler Nichols tuning what does ‘d’ represent

A

pseudo delay

63
Q

in Ziegler Nichols tuning what does ‘T’ represent

A

the process time constant and is arrived at by drawing a line tangent to the steepest part of the transition curve and extending the line upwards and downwards
T is the time difference between where the tangent intersects the original steady state plant output value and where the tangent intersects the final plant output value

64
Q

in Ziegler Nichols tuning what does ‘K’ represent

A

the value K is the measure of how much the plant output changed for a given command change

65
Q

what is the process gain given by Gp

A

Gp = K/change in control effort

66
Q

what is the Proportional gain given by in Ziegler Nichols open loop tuning

A

Kp = 1.2(T/d.Gp)

67
Q

what is the Integral gain given by in Ziegler Nichols open loop tuning

A

Ki = 0.5/d

68
Q

what is the Derivative gain given by in Ziegler Nichols open loop tuning

A

Kd = 0.5 . d

Instrumentation and Drive Systems (6 decks)

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