Introduction - Process Control - Lecture Slides, Slides of Process Control

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Introduction to Process Control

Chapter One

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

• Chapter

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.

Assumptions:

1. w 1 is constant
2. x 2 = constant = 1 (stream 2 is pure A)
3. Perfect mixing in the tank

Control Objective:

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

rate w 2, despite variations in x 1 ..

Terminology:

• Controlled variable (or “output variable”): x
• Manipulated variable (or “input variable”): w 2
• Disturbance variable (or “load variable”): x 1

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

Overall balance:

Component balance:

0 = w 1 (^) + w 2 − w (1)

w x 1 1 (^) + w x 2 2 (^) − wx = 0 (2)

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

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

x = x SP x = xSP x 2 (^) = 1 w 2

1 2 1 1 (3)

SP SP

x x w w x

Possible Control Strategies

Method 1. Measure x and adjust w2.

Intuitively, if x is too high, we should reduce w 2

and vice versa.

1. Manual control

2. Automatic control with proportional feedback

control law.

w 2 (^) ( ) t (^) = w 2 (^) + K (^) c  xSP − x t ( ) (4)

Method 3. Measure x 1 and x, adjust w 2.

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

Method 4. Use a larger tank.

• If a larger tank is used, fluctuations in x 1 will tend to be damped

out due to the larger capacitance of the tank contents.

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

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.