Chapter 1 Introduction to Control System

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Chapter 1

Introduction to Control System

Adapted from:

Kilian, C. T. (2001), Modern Control Technology: Components and Systems

Delmar

Objectives

Distinguish between open
-
loop and
closed
-
loop control systems.

Understand control system block
diagrams.

Explain transfer functions.

Differentiate between analog and digital
control systems.

Know how process control systems
work.

Know how servomechanisms work.

Control Systems

Control System
: A collection of
components working together under the
direction of some intelligence.


Modern control systems use electronic
intelligence.



Classifications of control systems:


Regulatory System



Maintains a parameter at or near a setpoint



Follow
-
up System



Causes the output to follow a predefined path.



Event Control System



Controls a sequential series of events.

Open
-
Loop Control

Controller independently calculates the
exact voltage or current required for the
set point without sensing the process.


Controller never knows the effect of the
actuator on the process.


Appropriate where actuator actions are
very repeatable and reliable.

Control System Block Diagram

The motor rotates at a reliable speed, such
as 5 degrees/second. Based on that, reliable
positioning can be performed.

Closed
-
Loop Control

The output process is monitored by a
sensor
providing

feedback
, allowing the
controller to make adjustments to the
actuator.

The comparator compares the set point
to the actual and produces an error.

Error = set point


Actual


The controller works

to minimize the error

signal.


Zero error indicates

actual = set point.

The difference in the actual and setpoint
produce an error, which the controller
responds too.

Control strategies can be simple or
complex as how to minimize the error.


Closed
-
loop is preferable to open
-
loop
because the control system can
produce reliable, repeatable
performance even when the
components cannot.


Transfer Functions

A control system is collection of
components and circuits to perform a
useful function.


Each component converts energy from
one form to another.


A Transfer Function is a mathematical
relationship between the input and the
output of a control system component.


TF =
OUTPUT


INPUT


More accurately,

TF =

O
UTPUT



INPUT


For a steady
-
state system,

Gain = TF
Steady
-
State


A pressure sensor has an output of 0
-
10V through a pressure range of 0 to
2000 PSI. Calculate the transfer
function.

A pressure sensor has an output of
4mA
-
20mA through a pressure range of
500 to 1000 PSI. Calculate the transfer
function.


For an open
-
loop system, the product of each
components Transfer Function is the
system’s transfer function.



Analog Control Systems

In an analog control system, the
controller consists of traditional analog
devices and circuits


linear amplifiers.


Analog control provides instantaneous
changes.



Digital Control System

In digital control systems, the controller
is often a microprocessor or
microcontroller.


The controller repeats a program over
and over. Each repetition is an
iteration
or

scan
.


The required scan time is dependent on
the process being controlled.

Classifications of Control
Systems

Process control refers to a control
system that oversees some industrial
processes so that a uniform, correct
output is maintained.


Control Parameters, such as
temperature, are monitored and
adjusted to ensure the output product is
uniform.

Classic Heat Control

Paint Mixing Control

Manual Control

Automatic Flow Control

Automatic Color Control

Continuous Process Control


Continuous flow of material or product,
such as paint mixing.


Batch Process


A process with a beginning and end, may
be repeated over and over.



Individual Local Control

For a process “flow”, if the line needs to
adjusted, each independent controller
needs to be adjusted.

Direct Computer Control

Advantage: Local processes can be
monitored and adjusted.

Disadvantage: If controller goes down,
line is down.

Distributed Control

Best of both worlds: Each process is
independent, but supervised from a
central location.

Sequentially Controlled Systems

A series of defined tasks to be
performed.

Time
-
Driven


Each operation in the sequence is
performed for a certain amount of time.
May be open
-
loop control.

Event
-
Drive
:


Each operation is performed until some
event goal is reached. Must be closed
-
loop control.

Motion Control

Broad term to describe processes where
things are moving.


Servomechanism:

Traditional term to a
closed
-
loop system that directs the
precise movement of an object, such as
an antennae.



Numerical Control

(NC): type of digital
control on machine tools (lathes, mills)
which use a numeric coordinate system
to define the movement of components,
typically in X,Y, Z coordinates.


Today's systems can accept data
directly from CAD drawing information
for the control of the operation.
Computer
-
aided Manufacturing

(CAM).



Computer
-
Integrated Manufacturing
(CIM) uses computers integrated in the
customer order, raw supply orders,
machining and assembly and shipping.

Robotics
: Industrial control robotics are
classic examples of position control
systems for manufacturing.

Pick
-
and
-
place robots
, the simplest
type, pick up parts and place them
nearby, such as onto a printed circuit
board.




Objectives
-

Review

Distinguish between open
-
loop and
closed
-
loop control systems.

Understand control system block
diagrams.

Explain transfer functions.

Differentiate between analog and digital
control systems.

Know how process control systems
work.

Know what servomechanisms do.