Introduction to Process Control

Chapter One

sensor

processcontrol element

controller

Control Terminology

Controlled Variables - These are the variables which quantify the performance or quality of the final product, which are also called output variables.

Manipulated Variables - These input variables are adjusted dynamically to keep the controlled variables at their set-points.

Disturbances - These are also called the “load” variables and represent inputs that can cause the controlled variables to deviate from their respective set points.

Servo control - The set-point signal is changed and the manipulated variable is adjusted appropriately to achieve the new operating conditions.

Regulatory control – The set-point is fixed at a constant value. When any disturbance enters the system, the manipulated variable is adjusted to drive the controlled variable back to its fixed set-point.

Control Terminology

Ch

apte

r 1

Why do we need automatic process control?

• Industrial processes are not static but rather very dynamic; they are continuously changing as a result of many types of disturbances.

• It is principally because of this dynamic nature that control systems are needed to continuously and automatically watch over the variables that must be controlled.

Illustrative Example: Blending System

Notation:

• w1, w2 and w are mass flow rates

• x1, x2 and x are mass fractions of component A

Assumptions:

1. w1 is constant

2. x2 = constant = 1 (stream 2 is pure A)

3. Perfect mixing in the tank

Control Objective:

Keep x at a desired value (or “set point”) xsp, by adjusting flow

rate w2, despite variations in x1..

Terminology:

• Controlled variable (or “output variable”): x

• Manipulated variable (or “input variable”): w2

• Disturbance variable (or “load variable”): x1

Design Question: What value of is required to have 2w?SPx x

Overall balance:

Component balance:

1 20 (1)w w w

1 1 2 2 0 (2)w x w x wx

(The overbars denote nominal steady-state design values.)

• At the design conditions, . Substitute Eq. (1), and , then solve Eq. (2) for :

SPx x SPx x2 1x 2w

12 1 (3)

1SP

SP

x xw w

x

Control Question

Suppose that the inlet concentration changes with time. How can we ensure that x remains at or near the set point ?

As a specific example, if and , then

.

SPx

2 2w w1 1x x

SPx x

1x

Possible Control Strategies

Method 1. Measure x and adjust w2.Intuitively, if x is too high, we should reduce w2

and vice versa.

1. Manual control

2. Automatic control with proportional feedback control law.

2 2 (4)c SPw t w K x x t

Method 2. Measure x1 and adjust w2.

Thus, if x1 is greater than , we would decrease w2 so that

One approach: Consider Eq. (3) and replace and with x1(t) and w2(t) to get a control law:

1x

2 2;w w

1x 2w

12 1 (5)

1SP

SP

x x tw t w

x

Because Eq. (3) applies only at steady state, it is not clear how effective the control law in (5) will be for transient conditions.

Method 3. Measure x1 and x, adjust w2.

• This approach is a combination of Methods 1 and 2.

Method 4. Use a larger tank.

• If a larger tank is used, fluctuations in x1 will tend to be damped

out due to the larger capacitance of the tank contents.

• However, a larger tank means an increased capital cost.

Classification of Control Strategies

Method Measured Variable

Manipulated Variable

Category

1 x w2FBa

2 x1 w2 FF

3 x1 and x w2 FF/FB

4 - - Design change

•

Feedback Control

• Distinguishing feature: – measure the controlled variable.

– It is important to make a distinction between negative feedback and positive feedback.

• Advantages:– Corrective action is taken regardless of the source of the

disturbance.– Reduces sensitivity of the controlled variable to disturbances

and changes in the process (shown later).• Disadvantages:

– No corrective action occurs until after the disturbance has upset the process, that is, until after x differs from xsp.

– Very oscillatory responses, or even instability

Feed Forward Control

• Distinguishing Feature: Measure a disturbance.

• Advantage: Correct for disturbance before it upsets the process.

• Disadvantage:– Must be able to measure the disturbance– No corrective action for unmeasured

disturbances.

Justification of Process Control• Increased product throughput• Increased yield of higher valued products• Decreased energy consumption• Decreased pollution• Decreased off-spec product• Increased Safety• Extended life of equipment• Improved Operability• Decreased production labor

Economic Incentives - Advanced Control 

1. Measurement and Actuation

2. Safety, Environment and Equipment Protection

3a. Regulatory Control

4. Real-Time Optimization

5. Planning and Scheduling

Process

3b. Multivariable and Constraint Control

(days-months)

(< 1 second)

(< 1 second)

(seconds-minutes)

(minutes-hours)

(hours-days)

Figure 1.7 Hierarchy of process control activities.

Figure 1.9 Major steps in control system development