Process control overview

Kiến thức cơ bản về kiểm soát công nghệ, tổng thể process control, phù hợp cho các ngành nghề tự động hóa, logic control, năng lượng và dầu khí

Concept of Process Control

Moataz Sherif

Senior Instrumentation and Control Engineer

Concept of Process Control

● Introduction

● Process Control Definition

● Basic Elements of Control Loop

● Open Loop and Closed Loop Control

● Closed Loop Control Modes

● Sensors and Transducers

● Standard Instrument Signals

● Smart Transmitters

4

Introduction

Industrial Instrumentation

● Instrumentation is the science of automated

measurement and control.

● The first step, naturally, is measurement If we can’t

measure something, it is really pointless to try to control it.

6

Industrial Instrumentation

● Once we measure the quantity we are interested in, we

usually transmit a signal representing this quantity to an

indicating or computing device where either human or

automated action then takes place.

Industrial Instrumentation

● If the controlling action is automated, the computer sends

a signal to a final controlling device which then influences

the quantity being measured.

8

Control System

● A system which responds to input

signals from the process and/or

from an operator and generates

output signals causing the process

to operate in the desired manner

● The control system include

Control system can used as a

● Basic Process Control System

● Safety Instrumented System

● Combined BPCS-SIS

Basic Process Control System

● Basic Process Control System (BPCS) is a system which responds to input signals from the process, its associated equipment, other programmable systems and/or an

operator and generates output signals causing the process and its associated equipment to operate in the desired

manner but which does not perform any safety instrumented functions with a claimed SIL ≥ 1 12

Pneumatic Control System

Programmable Logic Control (PLC)

14

Turbo machinery Control System

Boiler Control System

16

Safety Instrumented System

SIS takes some other names

● Trip and Alarm system

● Emergency Shutdown System (ESD)

● Safety Shutdown System

● Safety Interlock System

● Safety Related Control System

BPCS vs SIS

18

● SIS is a protection layer located to prevent the Hazards

from occurring

Safety Instrumented System

The system consists of

● Sensors

● Logic Solver

● Final control element

Process Control Definition

20

Process Control Definition

● A process is broadly defined as an operation that uses

resources to transform inputs into outputs.

● It is the resource that provides the energy into the process for the transformation to occur.

Process Control Definition

● Each process exhibits a particular dynamic (time varying)

behavior that governs the transformation.

● That is, how do changes in the resource or inputs over time affect the transformation.

● This dynamic behavior is determined by the physical

properties of the inputs, the resource, and the process itself.

22

Process Control Definition

● The manipulated variable (MV) is a measure of resource being fed into the process, for instance how much thermal energy.

● A final control element (FCE) is the device that changes the

value of the manipulated variable.

● The controller output (CO) is the signal from the controller to the final control element.

24

● The process variable (PV) is a measure of the process output

that changes in response to changes in the manipulated variable.

● The set point (SP) is the value at which we wish to maintain the process variable at.

Process Control Definition

● Process control is the act of controlling a final control element to change the manipulated variable to maintain the process variable

at a desired set point.

● A corollary to our definition of process control is a controllable

process must behave in a predictable manner

● For a given change in the manipulated variable, the process

variable must respond in a predictable and consistent manner.

26

Basic Elements of Control Loop

Basic Elements of Process Control

● Controlling a process requires knowledge of four basic

elements:

○ the process itself

○ the sensor that measures the process value

○ the final control element that changes the manipulated

variable

○ the controller. 28

Basic Elements of Process Control

● Input devices used to see what’s going on in the process

● Control Systems make decisions based on process inputs,

operator inputs, and control software

● Output devices control the process

Basic Elements of Process Control

30

Basic Elements of Process Control

Open Loop and Closed Loop

Control

32

Open Loop Control

● The open-loop control is where output variable does not have any influence on the input variable.

● In open loop control the controller output is not a function

of the process variable.

Open Loop Control

34

Open Loop Control

● the controller output is fixed at a value until it is changed by an

Example for Open Loop Control

36

Example for Open Loop Control

● A system consists of the "valve" with the output variable

"volumetric flow" and the input variable "control valve setting".

● This system can be controlled by adjusting the control valve This allows the desired volumetric flow to be set

● if the applied pressure fluctuates, the volumetric flow will also

fluctuate

● In this open system, adjustment must be made manually

Example for Open Loop Control

38

Closed Loop Control

● process where the controlled variable is continuously

monitored and compared with the reference variable

● Depending on the result of this comparison, the input

variable for the system is influenced to adjust the output

variable to the desired value despite any disturbing

influences

Closed Loop Control

● Closed loop control is also called feedback or regulatory control.

● The output of a closed loop controller is a function of the error.

● Error is the deviation of the process variable from the set point and is defined as

E = SP - PV

40

Example for Closed Loop Control

Closed Loop Control

● The controller now passes a signal to the manipulating element

dependent on the deviation

● If there is a large negative deviation, that is the measured value of the volumetric flow is greater than the desired value the valve is

closed further

● If there is a large positive deviation, that is the measured value is

smaller than the desired value, the valve is opened further.

42

Example for Closed Loop Control

Closed Loop Control

● Setting of the output variable is normally not ideal:

○ If the intervention is too fast and too great, influence at the

input end of the system is too large This results in great fluctuations at the output.

○ If influence is slow and small, the output variable will only

approximate to the desired value.

44

Closed Loop Control Modes

Closed Loop Control Modes

● Closed loop control can be, depending on the algorithm that determines the controller output:

Manual Control Mode

● In manual control an operator directly manipulates the

controller output to the final control element to maintain a

desired setpoint.

● Used in abnormal conditions when maintenance is required

for measuring instruments.

Manual Control Mode

48

On-Off Control Mode

● provides a controller output of on or off in response to error.

On-Off Control Mode

● Upon changing the direction of the controller output, deadband is the value that must be traversed before the controller output will

change its direction again.

50

On-Off Control Mode

PID Control Mode

● provides output that changes from 0 to 100% in response to error.

52

PID Algorithm

● A proportional-integral-derivative controller (PID controller) is a

common feedback loop component in industrial control systems

PID Algorithm

● The PID can adjust process outputs based on the history and rate

of change of the error signal, which gives more accurate and stable control.

● PID controllers can be easily adjusted (or "tuned") to the desired

application.

54

● Kd: Derivative Gain - Larger Kd decreases overshoot, but slows

down transient response.

PID Algorithm

1- Proportional:

● To handle the immediate error, the error is multiplied by a

constant Kp (for proportional), and added to the controlled

quantity

● Kp is only valid in the band over which a controller's output is

proportional to the error of the system.

56

PID Algorithm

2- Integral:

● To learn from the past, the error is integrated (added up) over a

period of time, and then multiplied by a constant KI (making an

average), and added to the controlled quantity

PID Algorithm

3- Derivative:

● To handle the future, the first derivative (the slope of the error)

over time is calculated, and multiplied by another constant KD, and also added to the controlled quantity.

58

PID Interacting Algorithm

PID Interacting Algorithm

● The series or "interacting" form, where the output of each part of

the controller is used as the input for another part, so that separate

P, D and I controllers are connected together in series.

● This is effectively how older pneumatic and analog electronic

controllers worked It is the more restricted form of the two.

60

PID Non-interacting Algorithm

PID Non-interacting Algorithm

● The parallel or "non-interacting" form, where the P, I and D parts

of the controller are all given the same error input in parallel and their output is added together

● This allows independent adjustment of the proportional, integral and derivative constants.

62

PID response graph

PID response graph – single-step change

64

Cascade Control Mode

● Cascade control uses the output of a primary (master or outer)

controller to manipulate the set point of a secondary (slave or

inner) controller as if the slave controller were the final control

element.

Cascade Control Mode

66

Cascade Control Mode

● The purpose of cascade control is to achieve greater stability of the primary process variable by regulating a secondary process variable

in accordance with the needs of the first

● An essential requirement of cascaded control is that the secondary process variable be faster-responding than the primary process

variable.

Cascade Control Mode - Example 1

68

Cascade Control Mode - Example 1

● heated air is used to evaporate water from a granular solid

● The primary process variable is the outlet air exiting the dryer,

which should be maintained at a high enough temperature

● This outlet temperature is fairly slow to react, as the solid material mass creates a large lag time.

Cascade Control Mode - Example 1

● There are several parameters influencing the temperature of the

outlet air.

● These include air flow, ambient air temperature, and variations in steam temperature

● If any of these parameters were to suddenly change, the effect

would be slow to register at the outlet temperature

● Correspondingly, the control system would be slow to correct for

Cascade Control Mode - Example 1

Cascade Control Mode - Example 1

● Installing a second temperature transmitter at the inlet duct of the dryer, with its own controller to adjust steam flow at the command of the

primary controller will be a great solution

● Now, if any of the loads related to incoming air flow or temperature vary, the secondary controller (TC-1b) will immediately sense the change in

dryer inlet temperature and compensate by adjusting steam flow through the heat exchanger Thus, the “slave” control loop (1b) helps stabilize the

“master” control loop (1a) by reacting to load changes long before any

effect might manifest at the dryer outlet

72

Cascade Control Mode - Example 2

Cascade Control Mode - Example 2

● The “secondary” or “slave” flow controller works to maintain

feedwater flow to the boiler at whatever flow rate is desired by the level controller If feedwater pressure happens to increase or

decrease, any resulting changes in flow will be quickly countered

by the flow controller without the level controller having to react

to a consequent upset in steam drum water level.

74

Cascade Control Mode - Example 2

● Thus, cascade control works to guard against steam drum level

instability resulting from changes in the feedwater flow caused by factors outside the boiler.

● As stated previously, the slave (flow) controller effectively shields the master (level) controller from loads in the feedwater supply

system, so that master controller doesn’t have to deal with those

loads.

Feedforward control

● It is based on that if all significant loads on a process variable are monitored, and their effects on that process variable are well-understood.

● A control system programmed to take appropriate action

based on load changes will shield the process variable from

any ill effect

76

Feedforward control

● The feedforward control system uses data from load sensors

to predict when an upset is about to occur, then feeds that

information forward to the final control element to

counteract the load change before it has an opportunity to

affect the process variable.

Feedforward control

78

Feedforward control

● Feedback control systems are reactive, taking action after to changes in the process variable occur

● Feedforward control systems are proactive, taking action

before changes to the process variable can occur.

Feedforward control

80

Feedforward control

● We have a liquid level control system on an open tank, where three

different fluid ingredients are mixed to produce a final product

● A level transmitter (LT) measures liquid level, while a level controller

(LC) compares this level to a Setpoint value, and outputs a signal calling for a certain amount of discharge flow

● A cascaded (slave) flow controller (FC) senses outgoing flow via a flow

transmitter (FT) and works to maintain whatever rate of flow is “asked”

Feedforward control

● The level control system acts to keep liquid level constant in the vessel,

ensuring adequate mixing of the three ingredients

● Being a feedback level control system, it adjusts the discharge flow rate in response to measured changes in liquid level Like all feedback control

systems, this one is reactive in nature: it can only take corrective action

after a deviation between process variable (level) and Setpoint is detected

82

Feedforward control

Feedforward control

● Let us now change the control system strategy from feedback to

feedforward It is clear what the loads are in this process: the three

ingredient flows entering the vessel If we measure and sum these three

flow rates, then use the total incoming flow signal as a setpoint for the

discharge flow controller, the outlet flow should (ideally) match the inlet flow, resulting in a constant liquid level

● Being a purely feedforward control system, there is no level transmitter

(LT) any more, just flow transmitters measuring the three loads

84

Feedforward control

● If all flow transmitter calibrations are perfect, the summing of flow ratesflawless, and the flow controller’s tuning robust, this level control systemshould control liquid level in the vessel by proactive effort (“thinking

ahead”) rather than reactive effort (“after the fact”)

● Any change in the flow rate of ingredients A, B, and/or C is quickly

matched by an equal adjustment to the discharge flow rate So long as

total volumetric flow out of the vessel is held equal to total volumetric