THE INTRODUCTION OF AUTOMATIC PROCESS CONTROL Xining Ye

THE INTRODUCTION OF AUTOMATIC

PROCESS CONTROL

Xining Ye

GOAL:

Maintaining process variables (temperatures, pressures, flows, compositions, levels) at a desired operating value.

Processes are dynamic in nature, and changes are always occurring. The important variables those related to safety, product quality, and production rates will not achieve design conditions.

POINTS:

1.1 PROCESS CONTROL SYSTEM

1.2 IMPORTANT TERMS

1.3 TYPES OF CONTROL SYSTEMS

1.4 CONTROL STRATEGIES

1.5 SUMMARY


• Manual process control

• Automatic process control


Condensate return

T(t)

T

Steam

Ti(t)

Process fluid

Figure 1-1 Heat exchanger

The purpose of this unit: To heat the process fluid from some inlet temperature, Ti(t), up to a desired outlet temperature, T(t).

Condensate return

T(t)

T

Steam

Ti(t)

Process fluid

Figure 1-1 Heat exchanger

In this process many variables can change, causing the outlet temperature to deviate from its desired value. If this happens, some action must be taken to correct for this deviation.

• Manual process control

(1) Measure the temperature T(t);

(2) Compare it to its desired value

(3) Based on this comparison, decide what to do to correct for any deviation. The steam valve can be manipulated to correct for the deviation.

Condensate return

T(t)

T

Steam

Ti(t)

Process fluid

How it works?

If the outlet temperature T(t) is above its desired value, the steam valve can be throttled back to cut the steam flow(energy) to the heat exchanger; If the outlet temperature T(t) is below its desired value, the steam valve could be opened more to increase the steam flow to the heat exchanger.

Condensate return

T(t)

T

Steam

Ti(t)

Process fluid

Disadvantages of manual process control

(1) The operator should look at the temperature frequently to take corrective action whenever it deviates its desired value.

(2) Different operators would make different decisions as to how to move the steam valve, resulting in inconsistent operation.

(3) This corrective procedure would require a large number of operators.

So, We would like to accomplish this control automatically. Without requiring intervention from the operator.

• Automatic process control:

Fig. 1-2 Heat exchanger control loop

T

Steam

Ti(t)

Process fluid

T(t)

TT

TC

SP

Condensate return

Sensor

Final control element

Transmitter

Controller

(1) Measure the outlet temperature of the process stream by a sensor(thermocouple, resistance temperature device, thermisters, etc.)

(2) Transmitter transmits the signal to the controller

(3) Controller compare the signal to the desired value, and decides what to do to maintain the temperature at its desired value.

(4) The controller sends a signal to the final control element to manipulate the steam flow.

Three components of all control systems:


(1)Sensor/transmitter( 检测 / 变送 ). The primary and secondary elements.(2) Controller( 控制器 ). The brain of the control system.

(3) Final control element( 执行器 ). A control valve, but not always. (variable-speed pumps, conveyors and electric motors)

T

Steam

Ti(t)

Process fluid

T(t)

TT

TC

SP

Condensate return

Sensor


Transmitter

Controller

Three basic operations:


(1) Measurement(M). Measuring the variable to be controlled

(2) Decision(D). Based on the measurement, the controller decides what to do to maintain the variable at its desired value.

(3) Action(A). As a controller’s decision, the system must take an action. This is usually accomplished by the final control element.

T

Steam

Ti(t)

Process fluid

T(t)

TT

TC

SP

Condensate return

Sensor


Transmitter

Controller

1.2 IMPORTANT TERMS • Controlled variable( 被控变量 )(process variable, measurement). The variable that must be controlled at some desired value.

• Set point( 设定值 ). The desired value of the controlled variable.


T

Steam

Ti(t)

Process fluid

T(t)

TT

TC

SP

Condensate return

Sensor


Transmitter

Controller

•Controlled object or Process( 被控对象或过程 ). The object that need to be controlled.

• Disturbance ( 干扰 )(upset). Any variable that causes the controlled variable to deviate away from the set point.

• In the heat exchanger, possible disturbances. Inlet process temperature Ti(t), the process flow f(t), The energy content of the steam, ambient condition, process fluid composition and fouling.

T

Steam

Ti(t)

Process fluid

T(t)

TT

TC

SP

Condensate return

Sensor


Transmitter

Controller

1.2 IMPORTANT TERMS


• Manipulated variable( 操纵变量 ). The variable used to maintain the controlled variable at its desired value.

• NOTE. Disturbances are always occurring in processes, transient conditions are very common. It is because of these disturbances that automatic process control is needed. If there were no disturbances, design operating conditions would prevail, and there would be no necessity of continuously “monitoring” the process.

With these preceding terms defined, we can say:

1.2 IMPORTANT TERMS

The objective of an automatic process control system is to adjust the manipulated variable to maintain the controlled variable at its set point in spite of disturbances.

• Why control is important?

(1) Safety: Prevent injury to plant personnel, protect the environment by preventing emission and minimizing waste and prevent damage to the process equipment.

(2) Maintain product quality (composition, purity, color, etc.) on a continuous basis and with minimum cost.

(3) Maintain plant production rate at minimum cost.

So, we can say that the reasons for automation of process plants are to provide safety and at same time maintain desired product quality, high plant throughput, and reduce demand on human labor.

Two types of control system:

(1) Regulatory control( 定值控制 ): In some processes the controlled variable deviated from the set point because of disturbances, regulatory control refers to systems designed to compensate for these disturbances.

1.3 TYPES OF CONTROL SYSTEM

(2) Servo control( 随动控制 ): In some processes, the most important disturbance is the set point itself. That is, the set point may be changed as a function of time. Servo control refers to control systems designed for this purpose.

In the process industries, regulatory control is far more common that servo control.

Points ：

(1) Feedback control (closed-loop control)

(2) Feedforward control (open-loop control)


(3) Choose a proper control system

反馈控制 ( 闭环控制 )

前馈控制 ( 开环控制 )

• The block diagrams of feedback control

• The characteristics of feedback control

Points:


• How it works?



•How it works?

If the inlet process temperature decreases, thus creating a disturbance, its effect must propagate through the heat exchanger before the outlet temperature decreases. Once the outlet temperature changes, the signal from the transmitter to the controller also changes.

T

Steam

Ti(t)

Process fluid

T(t)

TT

TC

SP

Condensate return

Sensor


Transmitter

Controller




•How it works?

It is then that the controller becomes aware that a deviation from set point has occurred and it must compensate for the disturbance by manipulating the steam valve. The controller then signals the valve to increase its opening and thus increase the steam flow.

T

Steam

Ti(t)

Process fluid

T(t)

TT

TC

SP

Condensate return

Sensor


Transmitter

Controller


The response of feedback control (closed-loop control)


At first the outlet temperature decreases because of the decrease in inlet temperature, but then it increases, even above the set point and continuous to oscillate until it finally stabilizes.

This oscillatory response is typical of feedback control and shows that it is essentially a trial and error operation. That is, when the controller notices that the outlet temperature has decreased below the SET POINT, it signals the valve to open. But the opening is more

TIME

TIME

TIME

Fig.1-3 Response of feedback control

INLE

T

TE

MP

ER

AT

UR

EO

UT

LET

T

EM

PE

RA

TU

RE

CO

NT

RO

LLE

R

OU

TP

UT


Than required. Therefore, the outlet temperature increases above the SET POINT. Noticing this, the controller signals the valve to close again somewhat to bring the temperature back down. This trial and error continued until the temperature reached and stayed at SET POINT.

TIME

TIME

TIME

Fig.1-3 Response of feedback control

INLE

T

TE

MP

ER

AT

UR

EO

UT

LET

T

EM

PE

RA

TU

RE

CO

NT

RO

LLE

R

OU

TP

UT

• The block diagrams of feedback control system

Fig 1-4 Block diagrams of closed-loop control systems

Controller Final control

element

Process

or plant

SP Output

Or

Controlled

variable

Sensor/

Transmitter

- Summing

junction

Disturbance 2 Disturbance 1

The advantage of feedback control


Compensate for all disturbances

The result of any disturbance entering the process is to make the controlled variable deviate from the SET POINT. Once the controlled variable deviates from the set point, the controller changes its output to return the controlled variable to SET POINT(its desired value). The feedback control loop does not know, nor does it care, which disturbance enters the process. It only tries to maintain the controlled variable at set point, and in this way compensates for all disturbances. The feedback controller works with minimum knowledge of the process. Actually, the only information it needs is in which direction to move, and how much to move is usually adjusted by trial and error.


The disadvantage of feedback control


Can compensate for a disturbance only AFTER the controlled variable has deviated from the set point because of the disturbance.

Can not give the controlled variable a timely control (laggard 滞后的 )


T

Steam

Ti(t)

Process fluid

T(t)

TT

TC

SP

Condensate return

Sensor


Transmitter

Controller

• The block diagram of feedforward control

• The characteristics of feedforward control

Points:


• How it works?

The feedforward control is a very common control strategy in the process industries. It is the simplicity that accounts for its popularity.

The objective of feedforward control is to measure the disturbances and compensate for them before the controlled variable deviates from the set point. If applied correctly, the controlled variable deviation would be minimum.

• How it works?

Suppose that “major” disturbance is the inlet temperature Ti(t). To implement feedforward control, this disturbance must first be measured and then a decision made as to manipulate the steam valve to compensate for them.


TT

T

Steam

Ti(t)

Feedforward controller

SP

T(t)

Condensation retun

Fig 1-5 Feedforward control

Fig 1-5 shows this control strategy.



measure the inlet temperature

Feedforward controller makes the decision about how to manipulate the steam valve to maintain the controlled variable at set point.

TT

T

Steam

Ti(t)


SP

T(t)

Condensation retun

• The Block diagrams of Feedforward control

Fig 1-6 Block diagrams of feedforward control

Controller Final control

element

Process

or plant

SP

Process flow

Output

Or

Controlled

variable

Sensor/

Tranmitter

Disturbances


Feedforward control cannot compensate for all disturbances that enter the process

The disadvantage of feedforward control


T

Steam

Ti(t) f(t)


SP

TT

T(t)

Condensation retun


In this example, The feedforward control system can compensate only one of disturbances. If any of the other disturbances enter the process, this strategy will not compensate for it, and the result will be a permanent deviation from set point of the controlled variable.

The disadvantage of feedforward control


It has the characteristic of forward control

So, if we use this strategy correctly, the controlled variable will not deviate set point.

The advantage of feedforward control

TT

T

Steam

Ti(t)


SP

T(t)

Condensation retun


Some examples:

Washing machine

Oven

Microwave oven

Air conditioner

Feedforward control system

Feedback control system

How to choose a proper control system?

? :

• Feedback control system

Review:

• Feedforward control system

(3) Choose a proper control system

• Proper Control System

• Can get the output that a process need

• Low cost

Feedforward Control System

Feedback Control System

Can not compensate all disturbances

Can compensate all disturbances

Simple structure Low cost

Complex structure

High cost

• Contrast

Trade-off:• Simplicity and low cost

• Complexity and higher cost

• ChoiceAn open-loop

system

A closed-loop

system

Premise: Ensure the requirement of industrial production

• Feedforward control with feedback control

T

Steam

Ti(t) f(t)


SP

TT

T(t)

Condensation retun

TC

TT

+

+

Fig 1-6 Feedforward control with feedback control

In this example, Feedforward control now compensate for the “major” disturbance; feedback control compensate for all other disturbances.

• feedforward control with feedback control

T

Steam

Ti(t) f(t)


SP

TT

T(t)

Condensation retun

TC

TT

+

+

Notice: the three basic operations, M,D,A are still present in this more “advanced” control strategy.

The sensors and transmitters perform the measurement.

Both feedforward and feedback controller make the decision.

The steam valve takes action.

Fig 1-6 Feedforward control with feedback control

• The need for automatic process control

• The principles of a control system, we can use three letters to describe, M, D and A

1.5 SUMMARY

• Present the basic components of a process control system: sensor/transmitter, controller, and final control element

1.5 SUMMARY

• Present two types of control strategies: Feedforward control or feedback control, we also discussed their advantages and disadvantages,

• Give the principles of choosing the proper control system

Documents

THE INTRODUCTION OF AUTOMATIC PROCESS CONTROL Xining Ye