control systems basics
TRANSCRIPT
1Northampton Community College
Control System Basics
2Northampton Community College
Control
To cause a machine or process to function
in a predetermined manner.
To energize or de-energize an output, or to
set a data table bit or bits to on or off, by
means of a user program.
3Northampton Community College
What is a Controller?
A circuit that excepts inputs:
One input is the action (command signal)
The other is the measurement signal
(feedback/feed forward) compares these
inputs and determines the output
reaction.
4Northampton Community College
Controller
A unit that controls a machine or process. It
can be, but not limited to a:
PLC – Programmable Logic Controller
PAC – Programmable Automation Controller
Robot controller with digital I/O
Relay panel
Computer data acquisition (DAQ)
5Northampton Community College
What is a Control System?
An interconnection of components forming a system
configuration which will provide a desired system
response.
ProcessInput Output
6Northampton Community College
What is Process?
Any operation to be controlled.
(A natural, progressively continuing operation of
development, marked by a series of gradual
changes that succeed one another in a relatively
fixed way and lead to a particular result or end).
7Northampton Community College
Main Categories of Control
Open loop control system
Closed loop control system
8Northampton Community College
Open Loop Control System
A control system that has no means of
comparing the output with the input for
control purposes.
Controller Process
Input Output
9Northampton Community College
Open Loop Control System
Motor
ControllerMotor
Conveyor
Did the Conveyor start, and if it did,
is it running at the right speed?
10Northampton Community College
Closed Loop Control System
A closed loop control system utilizes an additional signal
that measures the actual output. It then compares the actual
output with the desired setpoint, which in turn adjusted the
controller to produce the desired output response.
Controller Process OutputCommand
Signal,
Setpoint
Measurement
+-
Error
11Northampton Community College
Closed Loop Control System
Motor
ControllerMotor
Conveyor
Rotary Encoder
Feedback speed an position
12Northampton Community College
Classroom Discussion
Is a pop-up toaster and example of an Open
Loop or Closed Loop control system?
13Northampton Community College
Feedback
Feedback
The signal or signals returned from a controlled
machine or process to denote its response to the
command signal.
Feedback Element (feedback device)
An element that converts motion, position, pressure,
flow or temperature to and electrical signal for
comparison to the command signal.
14Northampton Community College
Feedback
Feedback Loop
A closed signal path in which feedback is compared
with the commanded value to obtain a corrective error
signal.
Feedback Signal
The measurement signal indicating the value of a
directly controlled variable, which is compared to the
commanded value to obtain the corrective error signal
15Northampton Community College
Feedback
Input
Output
Float feedback
H
16Northampton Community College
Feedforward (bias)Control action in which information concerning upstream
conditions is converted into corrective commands to
minimize the deviations of the controlled variable.
Feedforward is predominantly used to enhance PID control
to improve system response.
A feedforward control system sends information about a
disturbance to the control before it affects the process. This
type of control system can also be referred to as predictive
control.
Systems where the setpoints come from, or are modified
by, sensed conditions upstream of the process being
controlled. Sometimes called cascade controllers.
Cascade suggests that a change in an upstream variable
will affect, or cascade down to, later control systems.
17Northampton Community College
Feedforward (bias)
ControllerControl
OutputSetpoint +
-
Error++
Kf
Process Variable (PV)
Feedforward Path
18Northampton Community College
Feedforward Example
Motor+-
+-
Colored
soda pop out
Flow
Sensor
ValveAmount
of colorant
required
Setpoint:
Color
sensorColorant
In
Uncolored
soda pop in
Setpoint:
Final mix color
19Northampton Community College
Ratio Control
Ratio control is another common type of feedback control
system. This control scheme is often confused with cascade
and/or feedforward control but they are quit different.
Ratio control is most often used with processes that have
two or more streams of material that need to be continually
mixed together to form a composition of materials in the final
mixture.
An example could be that if the blending of two ingredient is
10 to 1 (10:1), then for every liter per minute of one material
that is flowing, 10 liters per minute of the second material
needs to flow.
20Northampton Community College
Ratio Control
Motor
Motor+-
x
+-
x
Setpoint:
amount of
soda water
required
Setpoint:
amount of
flavor
required
Color to flavor
ratio setting
Color to soda water
ratio setting
Colorant
in
Fully
mixed
soda pop
out
Flow sensor
Flow sensor
Flavoring in
Soda water in
Color sensor
Valve
Valve
21Northampton Community College
What is a System?
A combination of controller(s),
actuators and sensors that act together
to perform a desired objective.
22Northampton Community College
Control Hierarchy
Local Control
Control is located at the device to be controlled.
Distributed Control
A control system organization in which factory or
machine control is divided into several subsystems,
each managed by a separate controller, yet all
interconnected to form a single entity.
23Northampton Community College
Control Classifications
Nonlinear
The control is either ON or OFF. Most physical control
systems are nonlinear.
Linear
The control is variable. It is not completely ON or
completely OFF but can be.
24Northampton Community College
Control Classifications
Servo System
A feedback control system in which the output is some
mechanical position, velocity or acceleration. The terms
servo system and position, velocity or acceleration
controls are used synonymously.
25Northampton Community College
Basic Servo Control System
Encoder Digital
Signal
Command Signal
to motor
Controller
D/A Converter
Encoder
AnalogServo
Amp
PWM
Motor
26Northampton Community College
Adaptive Control System
An automatic control scheme in which the
controller is programmed to evaluate its own
effectiveness and modify its own control
parameters to respond to dynamic conditions
occurring in or to the process which affect the
controlled variables.
27Northampton Community College
Affects on Control Systems
28Northampton Community College
Disturbance
An undesirable signal that tends to adversely affect
the value of the output of a system.
Types of disturbances:
Internal
External
29Northampton Community College
Examples of Disturbance
Power Line
Externally Conducted
Noise
Transmitted Noise
Ground Loops
Inductive Driven
Devices
SCR’s Firing
Motor Drives
Welders
30Northampton Community College
Control Response Characteristics
Steady State Performance
The ability of a system output to maintain SP following a disturbance.
Transient Performance
The response of a system to maintain SP with abrupt changes to
inputs or following an abrupt disturbance.
Response Time (Settling Time)
The time required for a system to achieve steady state after an input
change or following a disturbance. This time should be as short as
possible.
Overshoot
The condition when the output temporarily exceeds the desired SP.
Oscillatory Behavior
A system that is not at steady state but is continuously varying above
and below the SP.
31Northampton Community College
Bibliography
Morriss, S. B. (1995). Automated manufacturing systems:
actuators, controllers, sensors, and robotics. New York, NY:
Glencoe/McGraw-Hill.
Hunter, R. P. (1978). Automated process control systems
concepts and hardware. Englewood Cliffs, NJ: Prentice-Hall,
Inc.
Rexford, K.B. (1997). Electrical control for machines. Albany,
NY: Delmar Publishing.
32Northampton Community College
Bibliography
Industrial automation glossary: a helpful guide to allen-bradley
technical terms. (1993). Milwaukee, WI: Allen Bradley.
Patrick, D, & Stephen, F. (1997). Industrial process control
systems. Albany, NY: Delmar Publishers.
Maloney, T. (2001). Modern industrial electronics. Upper
Saddle River, NJ: Prentice-Hall.
Hughes, T. (2007). Measurement and control basics.
Research Triangle Park, NC: International Society of
Automation.
33Northampton Community College
Bibliography
Patrick, Dale & Fardo, Stephen. Industrial Process Control
Systems. Delmar Publishers 1997. ISBN 0-8273-6386-9
Maloney, Timothy, J. Modern Industrial Electronics 4th
Edition. Prentice Hall 2001 ISBN 0-13-015676-0