Control Engineering Application

Prepared by: Dr. Sam Sung Ting

Types of Control SystemOpen loop

No comparison of the controlled variable with the desired input Fixed output, no effect on control action Large variation of control variables, as a result of external disturbances

Advantages:

o Simple in Constructiono Easy to maintaino Economic in operationo No stability problem involved

Disadvantages:o Need careful calibrationo Large variation due to the effect of disturbances

Figure 19.1: open loop control systems

Close loop

Direct signal to control action Errors are considered for the difference between measured value and desired input The output signal fed back and form loop to the system The closure of loop permits the comparison of the putput signal with reference point

Advantages:

o Ability to provide fast and precise controlo Automatic operating- save time and simplify works

Disadvantages:o Inexpensiveo Need high precise of design and fabrication

Figure 19.2: block diagram of a typical closed loop system

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.

disturbance variables - these are also called "load" variables and represent input variables that can cause the controlled variables to deviate from their respective set points.

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set-point change - implementing a change in the operating conditions. The set-point signal is changed and the manipulated variable is adjusted appropriately to achieve the new operating conditions. Also called servomechanism (or "servo") control.

disturbance change - the process transient behavior when a disturbance enters, also called regulatory control or load change. A control system should be able to return each controlled variable back to its set-point.

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Control Terminology(2)

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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, despite variations in

x1(t). Flow rate w2 can be adjusted for this purpose.

Terminology:

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

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

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

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

Overall balance:

Component A balance:

1 20 (1-1)w w w

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

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

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

SPx x SPx x2 1x 2w

12 1 (1-3)

1SP

SP

x xw w

x

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• Equation 1-3 is the design equation for the blending system.

• If our assumptions are correct, then this value of will keep at . But what if conditions change?

xSPx

Control Question. Suppose that the inlet concentration x1 changes with time. How can we ensure that x remains at or near the set point ?

As a specific example, if and , then x > xSP.

SPx

1 1x x 2 2w w

Some Possible Control Strategies:

Method 1. Measure x and adjust w2.

• Intuitively, if x is too high, we should reduce w2;

2w

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• Manual control vs. automatic control

• Proportional feedback control law,

2 2 (1-4)c SPw t w K x x t

1. where Kc is called the controller gain.

2. w2(t) and x(t) denote variables that change with time t.

3. The change in the flow rate, is proportional to the deviation from the set point, xSP – x(t).

2 2 ,w t w

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Method 2. Measure x1 and adjust w2.

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

• One approach: Consider Eq. (1-3) and replace and with x1(t) and w2(t) to get a control law:

1x

2 2;w w

1x 2w

12 1 (1-5)

1SP

SP

x x tw t w

x

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• Because Eq. (1-3) applies only at steady state, it is not clear how effective the control law in (1-5) 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.

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Classification of Control Strategies

Method Measured Variable

Manipulated Variable

Category

1 x w2FB

2 x1 w2 FF

3 x1 and x w2 FF/FB

4 - - Design change

Table. 1.1 Control Strategies for the Blending System

Feedback Control:• Distinguishing feature: measure the controlled variable

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

Engineering Usage vs. Social Sciences

• 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…

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Feedforward Control: Distinguishing feature: measure a disturbance

variable

• Advantage:

Correct for disturbance before it upsets the process.

• Disadvantage:

Must be able to measure the disturbance.

No corrective action for unmeasured disturbances.Ch

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Closed-loop Artificial Pancreas

controller sensorpump patient

glucose setpoint

u

yr

measured glucose

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Block diagram for temperature feedback control system

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Figure 1.6 Block diagram for composition feedback control system on Fig. 1.4.

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electronic or pneumatic controller

Justification of Process Control

Specific Objectives of 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

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• General Requirements of control system design:

1. Safety. It is imperative that industrial plants operate safely so as to promote the well-being of people and equipment within the plant and in the nearby communities.

2. Environmental Regulations. Industrial plants must comply with environmental regulations concerning the discharge of gases, liquids, and solids beyond the plant boundaries.

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10.1 Introduction • General Requirements of control system

design:

3. Product Specifications and Production Rate. In order to be profitable, a plant must make products that meet specifications concerning product quality and production rate.

4. Economic Plant Operation. It is an economic reality that the plant operation over long periods of time must be profitable. Thus, the control objectives must be consistent with the economic objectives.

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10.1 Introduction • 10.1.1 Steps in Control System Design• After the control objectives have been formulated,

the control system can be designed. • The design procedure consists of three main steps:

1. Select controlled, manipulated, and measured variables.

2. Choose the control strategy (multiloop control vs. multivariable control) and the control structure (e.g., pairing of controlled and manipulated variables).

3. Specify controller settings.

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Introduction • 10.1.2 Control Strategies

• Multiloop Control:Each output variable is controlled using a

single input variable.• Multivariable Control:Each output variable is controlled using more

than one input variable.

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10.2 THE INFLUENCE OF PROCESS DESIGN ON PROCESS CONTROL

• Traditionally, process design and control system design have been separate engineering activities.

Thus in the traditional approach, control system design is not initiated until after the plant design is well underway and major pieces of equipment may even have been ordered.

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THE INFLUENCE OF PROCESS DESIGN ON PROCESS CONTROL

• This approach has serious limitations because the plant design determines the process dynamic characteristics, as well as the operability of the plant.

• In extreme situations, the plant may be uncontrollable even though the process design appears satisfactory from a steady-state point of view.

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THE INFLUENCE OF PROCESS DESIGN ON PROCESS CONTROL• A more desirable approach is to consider process

dynamics and control issues early in the plant design. This interaction between design and control has

become especially important for modern processing plants, which tend to have a large degree of material and energy integration and tight performance specifications.

As Hughart and Kominek (1977) have noted: "The control system engineer can make a major contribution to a project by advising the project team on how process design will influence the process dynamics and the control structure”.

General Feedback Control Loop

Y s(s)

C(s) U(s)Y sp (s)G c(s)

Y(s)

D(s)

G a(s) G p(s)

G s(s)

G d(s)

E(s)+ - ++

Closed Loop Transfer Functions

• From the general feedback control loop and using the properties of transfer functions, the following expressions can be derived:

1)()()()(

)()()(

)(

)(

sGsGsGsG

sGsGsG

sY

sY

scap

cap

sp

1)()()()(

)(

)(

)(

sGsGsGsG

sG

sD

sY

scap

d

Characteristic Equation

• Since setpoint tracking and disturbance rejection have the same denominator for their closed loop transfer functions, this indicates that both setpoint tracking and disturbance rejection have the same general dynamic behavior.

• The roots of the denominator determine the dynamic characteristics of the closed loop process.

• The characteristic equation is given by:

01)()()()( sGsGsGsG scap

Feedback Control Analysis

• The loop gain (KcKaKpKs) should be positive for stable feedback control.

• An open-loop unstable process can be made stable by applying the proper level of feedback control.

Characteristic Equation Example• Consider the dynamic behavior of a P-only

controller applied to a CST thermal mixer (Kp=1; tp=60 sec) where the temperature sensor has a ts=20 sec and ta is assumed small. Note that Gc(s)=Kc.

cc

c

KK

ssK

1

15.1

1

1200

form,standardtheintogrearranginAfter

01120

1

160

1

equationsticcharacteritheintongSubstituti

p