Introduction to Process Control

Control system

a system that provides the desired response by controlling the output

the output is controlled by varying input

In control systems, the ______ is controlled by varying _______

Diagram of a control system

• Traffic light system

• Toaster

• Thermostat

• Coffee maker

• Washing Machine

Examples of a control system

1. To maintain a process at a desired, constant operating condition (temperature, pressure, composition, etc.) in the face of disturbances

2. To force it to follow a desired trajectory with time

What are the objectives of a control system?

ELSCC

1. Encounter a process

2. Learn how it behaves

3. Specify how you wish to control it

4. Choose an appropriate equipment

5. Check if applying control has improved things

Process of a Control system

Error = [Set point value] – [Measurement signal of controlled variable]

Generalized Process Control system equation

MCF

1. measurement (often several)

2. control strategy (embedded in a controller)

3. final element for implementing the control action

Elements of a Control system

a valve, heater or other variable input

The final element is usually __________________

Measurements

A wide range of on-line measuring devices (sensors) exists in the process industries, the most common being those for flow rate, pressure, liquid level, temperature, pH, and other selective measures of chemical composition.

Classical PID (proportional-integral-derivative)

______________________ controllers still dominate feedback strategy in industrial applications.

p - signal to the final control element

ps - nominal value

e - error signal

What do these symbols represent:

p

ps

e

Final Element

The control action signal calculated by the controller is sent to the final control element, a device which implements the change of a suitable input to the system.

Flow rate

The variable input as the final element is usually this

controlled variable

the process variable that we want to maintain at a particular value

set point

the desired value of the controlled variable

disturbances

any process variables that can cause the controlled variable to change. variables that we have no control over

manipulated variable

Process variable that is adjusted to bring the controlled variable back to the set point

• disturbance rejection

• set point tracking

Common Attributes of Control Systems

disturbance rejection

The ability to maintain the process variable at its desired value in spite of disturbances that might be experienced

set point tracking

The ability to move the process variable from one setting to a new desired setting

• Continuous time vs. Discrete-time Control Systems

• SISO vs. MIMO Control Systems

• Open Loop vs. Closed Loop Control Systems

• Feedback vs. Feedforward Control Systems

Classifications of Control Systems

type of the signal

Control systems can be classified as continuous time control systems and discrete time control systems based on the ________________ used

continuous

In continuous time control systems, all the signals are ______________ in time

continuous in time

In discrete time control systems, there exists one or more ________ time signals

Number of inputs and outputs

Control systems can be classified as SISO control systems and MIMO control systems based on the ________________ used

SISO (Single Input Single Output) Control systems

They have one input and one output

MIMO (Multiple Input Multiple Output) Control systems

They have more than one input and more than one output

Open loop

—the measured value of the controlled variable is not fed back to the controller (input), i.e. the control action is independent of the desired output

Closed loop

the measured value of the controlled variable is fed back to the controller, i.e. the control action is dependent on the desired output

Sprinkler system

Example of an open-loop system

Feedback control

using information about the deviation of the system from its desired state (error signal) to control the system information (to adjust the process inputs)

Feedforward control

measure disturbances and compensate for them before the controlled variable deviates from the set point, i.e. information about expected conditions are used to adjust the process inputs

Oven

Example of feedback control

egg toss

Example of feedforward control

EMSAM

● Enhanced process safety

● Meeting ever-stricter product quality specifications

● Satisfying environmental constraints

● Address operational constraints

● More efficient use of raw materials and energy and increase profitability

Advantages of Process Control

MTSC

• Measurements

  • inadequate/inaccurate measurements

• Time

  • long time delays in either the process or the measurements

• System

  • varying or nonlinear nature of the system

• Control loops

  • interaction of several quasi-independent control loops

Problems in Process Control

• Suppress influence of external disturbances

• Ensure stability of a chemical process

• Optimize performance of a chemical process

Issues to be Addressed in Process Control

digitally calculated control signals

Replacement of analogue controllers by ________________________, even for conventional PID controllers

Conventional controller

the controller parameters are often adjusted ("tuned") on-line, although prior knowledge of the system to be controlled will provide some guidance in their choice

Model-based controller

makes use of a model of the process (either fundamentally-based or empirical) to decide on the appropriate control strategy to be implemented to achieve the desired objective. In principle, these can lead to improved performance, but, in practice, their performance may be strongly dependent on the accuracy of the model used.