Module 2- The Feedback Control Loop and its Dynamics

Question 1

What are the two types of controllers

Answer 1

On-Off controller
Proportional Controller

Question 2

What is the other name for the on-off controller

Answer 2

Two Position Controller

Question 3

Explain how the on- off controller works

Answer 3

It has two extremes/ position of operation. It is either totally on or totally off ( fully opened or fully closed)

Question 4

Explain how the Proportional controller works

Answer 4

It opens or closes in proportion to the difference between the Setpoint and the PV

Question 5

Define Feedback Control

Answer 5

measuring the controlled variable after a change has occurred, then signaling an additional corrective change

Question 6

What is critical in determining the type of control strategy used

Answer 6

The type of process and its speed

Question 7

Control Loop Four Basic Functional Blocks

Answer 7

Process
Measurement
Automatic Controller
The Final Control Element

Question 8

What is the additional control loop functional block

Answer 8

Transmission Media

Question 9

Define Transmission Media

Answer 9

technology used for transmitting signals from 1 loop device to another

Question 10

What is the purpose of the process block

Answer 10

to make a product of a desired quality and or quantity

Question 11

For the purpose of control theory define the process

Answer 11

an action in which material and or energy is modified to a different form

Question 12

In majority of control loops what dictates the behavior of the loop and as a result it is important to….

Answer 12

the process functional block, this is why it is important to include the characteristics of the processes in process control theory

Question 13

Measurement block purpose

Answer 13

to measure the value of the
process output variable (in the case of the feedback loop), and to convey the value to the automatic controller.

Question 14

The measurement block is an interface between

Answer 14

the process and the controller
and
the process and human operator

Question 15

Measurement block usually comprises of… state its purpose as well

Answer 15

a transmitter
transmits an analog or a digital signal whose value is indicative
of the magnitude of the process’s output variable

Question 16

Some times measurement may require more than one device to… give an example

Answer 16

measure a single variable
Flow rate is measured with head type flow element such as an orifice plate, differential pressure transmitter and a square root extractor

Question 17

What does the square root extractor do

Answer 17

linearize the measurement wrt to flow

Question 18

The square root extractor can be in what two places

Answer 18

the transmitter itself ie the measurement block (smart transmitter) or the mounted in the control room (automatic controller) despite the location it is still considered part of the measurement block

Question 19

Define local controller

Answer 19

A controller located in the process area and does need the value of the pv to be transmitted to the control room

Question 20

When may transmitters not be necessary

Answer 20

when low level electronic signal generating devices are used such as Thermocouples

Question 21

Define Final Actuator

Answer 21

the device that regulates the supply of material and/or energy to affect the desired value of the controlled variable

Question 22

The primary function of the automatic controller is to

Answer 22

continually compare the measurement signal to the desired value or setpoint, of
the controlled variable

Question 23

What doe the controller do if there is a difference in the SP and measurement signal

Answer 23

he controller takes corrective action by changing its output,
which in turn adjusts the final actuator

Question 24

What does the final actuator change and why

Answer 24

The final actuator changes the supply of material and/or energy to
the process, in order to bring the controlled variable closer to the
setpoint

Question 25

The automatic controller can sometimes be made up of two different blocks they are

Answer 25

a summer and a control modes block

Question 26

Another name for summer

Answer 26

error detector deviation detector offset detector

Question 27

Function of error detector

Answer 27

compares the setpoint to the measured value

Question 28

The summer must have... for the loop to function properly since...

Answer 28

algebraic signs included since the algebraic sign of the error is important

Question 29

The error detector in a pneumatic controller is

Answer 29

a mechanical flapper nozzle mechanism

Question 30

The error detector in a electronic controller is an

Answer 30

electronic solid state amplifier

Question 31

In a microprocessor-based the error is developed by

Answer 31

the numerical difference between the sp and the measurement signal

Question 32

what does the controller mode block do

Answer 32

it acts on the error signal to determine the behavior of the controller's output signal

Question 33

What is executed in the controller mode block

Answer 33

the on-off or proportional control with its integral (reset) and derivate(rate) modes are executed

Question 34

What does the derivative / rate mode handles

Answer 34

how frequent the error occurs rate of change of the error

Question 35

integral in addition to proportional mode deals with

Answer 35

duration of error

Question 36

PID Control

Answer 36

Proportional Integral Derivative

Question 37

Proportional mode deals with

Answer 37

the magnitude of the error

Question 38

PI Control

Answer 38

Proportional control and integral control

Question 39

Why is it important to have calibrated measurement devices

Answer 39

to maintain the efficiency of the process

Question 40

Other not commonly used, 4 types of transmission media

Answer 40

Sonic, hydraulic, capillary and electromagnetic

Question 41

A pneumatic signal is transmitted over a distance through...., using a range....

Answer 41

cubing using a range of 20-100KPa (3-15 psig )

Question 42

In pneumatics the air/gas is

Answer 42

filtered and compressed to be at the required pressure and and cooled, therefore it is conditioned for this purpose

Question 43

In pneumatics sometimes the signal range is often changed to... in order

Answer 43

a multiple of the standard 20-100 kPa range eg 40-200kPa or 60-300kPa in order to increase the forces applied to the valve actuator

Question 44

How is pneumatic response time increased

Answer 44

by a valve positioner/air amplifier supplying a larger volume of air

Question 45

The electronic media often uses a...for which a signal range .... is used

Answer 45

current loop of 4-20mA

Question 46

The most common voltage range used is ... and how is it obtained

Answer 46

1-5V he 1 - 5 V range is developed by passing 4 - 20 mA through a 250 ohm resistor.

Question 47

What magnitude of electronic signals are referred to as high level signals

Answer 47

1-5 V 4-20mA 0-10V

Question 48

Another form of signal, which can be considered electronic is

Answer 48

the transmission of values in the form of digital codes

Question 49

define fibre optics

Answer 49

Light beams, modulated with digital codes, are used as the carrier frequency.

Question 50

Whatever the transmission medium, it has two design features which enhance the dynamics of the feedback control loop:

Answer 50

It must be instantaneous. or very nearly so. * It must be compatible with the controller, transmitter and valve type

Question 51

Ti

Answer 51

Temperature indicator

Question 52

TiT

Answer 52

Temperature indicating Transmitter

Question 53

TiC

Answer 53

Temperature indicating Controller

Question 54

TV

Answer 54

Temperature valve

Question 55

TY

Answer 55

current to pneumatic transducer

Question 56

Control loop signals designations b=

Answer 56

measurement signal

Question 57

Control loop signals designations c=

Answer 57

controlled variable

Question 58

Control loop signals designations e=

Answer 58

error, deviation or offset

Question 59

Control loop signals designations m =

Answer 59

manipulated variable

Question 60

Control loop signals designations m꜀=

Answer 60

controller output

Question 61

Control loop signals designations r=

Answer 61

setpoint or desired value

Question 62

Control loop signals designations q=

Answer 62

load variable

Question 63

the success of the controller to maintain the efficiency of the process depends on

Answer 63

how accurately it can reduce error to zero and maintain the controlled variable at the desired value aka sp

Question 64

What four factors determine the loop dynamics

Answer 64

* Response time of the process * Dead time of the process * Single capacity/single resistance process * Multiple capacity/multiple resistance process

Question 65

why does the control strategy of a control loop need to consider response time

Answer 65

The control strategy of any feedback loop has to consider the response time of the process to make changes to itself

Question 66

Define response time of a process

Answer 66

the total process time delay

Question 67

When it comes to response time what does the controller have to consider

Answer 67

the time delay by the manipulated variable

Question 68

If the response time is not consider what can happen

Answer 68

the loop will be poorly designed and can oscillate out of control and the controller will over correct making things worse

Question 69

How can lo0ps be designed to be stable in regards to the response time

Answer 69

by mathematically determining their response time, to aloe the valve and controller characteristics to minimize effects of response time. loops can be designed so they are stable and their response time mathematically determined to allow valve and controller characteristics to minimize the effects of response time.

Question 70

Define Dead time

Answer 70

the duration of time where nothing happens the amount of time after a change is made in the input to the process before there is any change in the process output measurement