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The basis for analysis of a system is the foundation provided by linear system theory which assumes a cause-effect relationship for the components of a system. Therefore, a component or process to be controlled can be represented by the block diagram shown in the slide. The input-output relationship represents the cause-effect relationship of the process, which in turn represents a processing of the input signal to provide an output signal variable, often with a power amplification.
Examples: Refrigerator Motor control Home thermostat
Advantages: Relatively insensitive to external disturbances and internal variations in system parameters. Can control action of the output device or function.
Example: Level monitoring. Level sensor sends back information while and after the level changes. Refer to the next slide.
In this feedback control system the input is the material entering the pipe. The output is the material exiting the tank. The equipment being controlled is the valve and the feedback (measured parameter) is the tank level by means of the float. The feedback from the float adjusts the valve to maintain a preset level in the tank.
The system shown in this slide is used to control the mixing of soda pop with artificial coloring. The first controller is a feedforward controller that receives a “final mix color” setpoint, subtracts sensory information about the concentration of the colorant on its way to the mixing tank and outputs an “amount of colorant” setpoint to the next controller. The second controller, a standard closed loop feedback controller, controls flow of colorant to the mixing tank. The feedforward control system is predictive because it corrects out-of-tolerance conditions before they actually occur. It could be called cascade, because it uses an open loop, feedforward, “final mix color” control system to change the “amount of colorant” setpoint that is provided to the closed loop, colorant flow control loop.
If a feedforward signal comes from a sensor measuring environmental conditions, it is called a disturbance signal. If the signal is provided manually, it is called an offset signal or bias.
This slide depicts a ratio controller. A ratio controller is a variation of the feedforward control but in many ways is quite different. It could be called a “feedsideways” controller. Its purpose, as shown in this slide, is to use a sensor in one stream to control the processes in one or more parallel streams.
The ratio controller’s sensor measures the concentration of colorant, but not to control the flow of the colorant. Instead, the sensor provides a setpoint to two parallel fluid streams (soda water and flavor), so that the output from the mixing tank is the correct ratio to color, flavor and soda water.
Another definition for Distributed control would be: There is an independent control function for each individual control action with some coordination amongst all.
Internal disturbances are disturbances that are generated within the system.
External disturbances are disturbances that are introduced to the system from an outside source.
Control Systems Basics
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Control System Basics
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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.
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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
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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
Computer data acquisition (DAQ)
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What is a Control System?
An interconnection of components forming a system
configuration which will provide a desired system
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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).
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Main Categories of Control
Open loop control system
Closed loop control system
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Open Loop Control System
A control system that has no means of
comparing the output with the input for
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Open Loop Control System
Did the Conveyor start, and if it did,
is it running at the right speed?
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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 Output
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Closed Loop Control System
Feedback speed an position
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Is a pop-up toaster and example of an Open
Loop or Closed Loop control system?
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The signal or signals returned from a controlled
machine or process to denote its response to the
Feedback Element (feedback device)
An element that converts motion, position, pressure,
flow or temperature to and electrical signal for
comparison to the command signal.
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A closed signal path in which feedback is compared
with the commanded value to obtain a corrective error
The measurement signal indicating the value of a
directly controlled variable, which is compared to the
commanded value to obtain the corrective error signal
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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
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.
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Process Variable (PV)
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soda pop out
soda pop in
Final mix color
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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
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.
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Color to flavor
Color to soda water
Soda water in
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What is a System?
A combination of controller(s),
actuators and sensors that act together
to perform a desired objective.
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Control is located at the device to be controlled.
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.
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The control is either ON or OFF. Most physical control
systems are nonlinear.
The control is variable. It is not completely ON or
completely OFF but can be.
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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.
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Basic Servo Control System
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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
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Affects on Control Systems
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An undesirable signal that tends to adversely affect
the value of the output of a system.
Types of disturbances:
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Examples of Disturbance
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Control Response Characteristics
Steady State Performance
The ability of a system output to maintain SP following a disturbance.
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
The condition when the output temporarily exceeds the desired SP.
A system that is not at steady state but is continuously varying above
and below the SP.
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Morriss, S. B. (1995). Automated manufacturing systems:
actuators, controllers, sensors, and robotics. New York, NY:
Hunter, R. P. (1978). Automated process control systems
concepts and hardware. Englewood Cliffs, NJ: Prentice-Hall,
Rexford, K.B. (1997). Electrical control for machines. Albany,
NY: Delmar Publishing.
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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
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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
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This Power Point is a simple overview of Industrial Control Systems Basics