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Control Engineering Application Prepared by: Dr. Sam Sung Ting.
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Transcript of Control Engineering Application Prepared by: Dr. Sam Sung Ting.
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Control Engineering Application
Prepared by: Dr. Sam Sung Ting
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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
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Figure 19.1: open loop control systems
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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
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Figure 19.2: block diagram of a typical closed loop system
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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.
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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
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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”.
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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)+ - ++
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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
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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
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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.
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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