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Chapter 1
Introduction to Industrial
Control Systems
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Introduction
The Industrial Revolution represented the
change from manufacturing processesbased upon one-at-a-time production tomass production because of the advent ofthe steam engine. Eventually, the electricmotor and hydraulic systems replacedsteam and Industrial Controls weredeveloped to automate the manufacturingprocess.
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Industrial Control Classifications
Motion and Process Control Motion control: position or velocity
Process control: temperature, pressure,
flow, level, pH, humidity, or chemicalcomposition
Batch processing
Continuous processing
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Industrial Control Systems automatically monitor manufacturing
processes being executed and take appropriate corrective action if
the operation is not performing properly.
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Motion Control Automatic control system that controls the
physical motion or position of an object.Motion control is also referred as servos or
servo mechanisms. Example: robot arm
performing welding, CNC machine toolequipment
Three characteristics:
1- control position, speed, acceleration2- measurement of motion or position
3- instant response to commands (faster than
process control 4
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Process Control
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One or more variables are regulated during the manufacturing of a
product. This can include temperature, pressure, flow rate, liquid
or solid levels.
Response time of PC is typically slowvaries from seconds to
minutes and is the type of industrial control used in manufacturing
Two Categories: batch and continuous
1- Batch Processing aka sequential process
is a sequence oftimed operations. Ex. Machine producing cookies. See P5 in book
2Continuous Process aka instrumentation: one or more
operations are being performed as the product is being passed
through a process. The raw materials enters and leaves severalstations in each process step. It can hours, days or weeks. Ex.
Nuclear pwr distribution, waste water treatment, oil refining,
natural gas distribution
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Typical Industries
Process industries Petroleum
Chemical
Power
Pulp and paper
Food and beverage
Textile
Pharmaceutical
Waste Treatment
Water Treatment
Discrete parts Automotive
Electrical/electronic manufacturing
Instrument/parts manufacturing
Toys
Sporting Goods
Appliances
Tools
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Open- and Closed-Loop Systems
How systems are controlled can also definethe control process
Open-loop systems depend upon a (manual)
command signal to inform the output devicewhat to do. Has no comparator.
Closed-loop systems are self-correcting andallow for continuous operation without
interruption.
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Open- and Closed-Loop Level Control
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Open-loop level control
requires manual operation to
maintain proper level with noautomatic feedback. It is the
simplest way to control a
system. Has to reach a steady
state or balance condition.Closed-loop level control allows
for unattended continuous levelcontrol with feedback
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Closed-loop level control allows for unattended level control.
Feeback control systems were developed between WWI and
WWII.
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Elements of Open- and Closed-Loop Systems
PP 8 9 in book
Controlled Variable Measurement Device Feedback Signal Set Point Error Detector Error Signal Controller Actuator Manufacturing Process Disturbance
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More feedback control references
An error can develop in 3 ways in feedback control
1. The set point is changed
2. A disturbance appears
3. The load demand varies
See p. 12 for good example
There is Positive and Negative feedback.
Ex. A home thermostat uses negative feedback tokeep its temperature correct, a uses positive
feedback to get/find the stronger signal
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Process Lag Time
Describes the time it takes for a correctiveaction to be reflected in the process. It takestime to re-establish a balanced condition.
This is referred to a pure lag and is proportional
to the amount of its mass.
Factors influencing lag time are:
Inertia - mass
Capacity
physical quantity
Resistance type of material
Dead-time ex. liquid temperature in a pipe12
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Feed-Forward Controltakes corrective action after an error develops
Two factors can make feedback controlineffective:
Large magnitude disturbances
Large process lag times
Feed-forward control prevents errors fromoccurring; however, feed-forward control isseldom used alone, typically in conjunction
with feedback systems. Not perfect due to other disturbances and used only
for critical apps.
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Examples of Feed-Forward Controls
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Feed-forward control of level Feed-forward control loop