- National Electrical Manufacturers Association (NEMA)
Components
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Input and output module
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Processor
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Memory
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Power supply
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Detachable programming device
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History
1969 specifications (General Motors)
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Must be programmable (and reprogrammable)
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Designed to operate in an industrial environment
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Must accept 120 V ac signals from pushbuttons, switches
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Outputs must switch and continuously operate loads such as motors
and relays of two amps rating
Before the 1980s, programmable controllers were called PCs.
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PLC Functions
A PLC performs the functions of conventional relays, timers, counters
Advantages (over conventional devices)
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More flexibility
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Less space, more compact
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Higher reliability (less mechanical parts)
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Modular, can be easily connected to computer systems and to hardware 2
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Conventional devices
Relays
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Conventional devices
Relays (continued)
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Conventional devices
Relays (continued)
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Normally open
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Normally closed 3
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Conventional devices (continued)
Timers
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Operation
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Inputs are sampled and stored in memory
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Control program is executed
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Calculated values are used to update the outputs
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Scan time is generally of the order of milliseconds
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Application 4
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PLCs versus Computers
Conceptually very similar, though there are practical differences.
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PLCs are based on solid state technology and generally use
microprocessors as their processing unit.
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PLCs are used to simulate electromechanical devices that were typical
of
transfer lines and mass production.
PLC
Computer
executes simple program in milliseconds, scan
time is much smaller than 1/60 seconds
possibly complicated program, often timing is
not critical
objective is to make yes/no, 0/1 decisions
tasks may include floating point arithmetic,
manipulation of data
special purpose device, same program is
repeated continuously
general purpose, possibly different programs
no peripherals
need peripherals
programmed using ladder logic
programmed using high-level languages
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Why PLCs?
Advantages
Disadvantages
+ many inputs and outputs, excellent
for controlling and monitoring
many processes
+ designed for industrial
environments, robust and reliable
+ reprogrammable
+ modular
+ ideally suited to supervisory
control
+ easy to set up, good for FMS
environment
+ inexpensive
- do not use the full capabilities of
the microprocessor
- ladder logic programming style is
cumbersome and prone to mistakes
- only good for yes/no decisions
- do not handle continuous inputs
and outputs, not good for direct
control
- inflexible (compared to
microprocessors)
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Logic Control Systems
Consider a system with two inputs (A, B) and two outputs (X, Y)
where Y is on (true) if and only if A and B are both off (false), and
X is on if and only if A and B are both on (true) or both off (false).
1. Relays 5
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Logic Control Systems (continued)
2. Logic Gates
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X is on if and only if A and B are both on (true) or both off (false).
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Y is on (true) if and only if A and B are both off (false).
3. PLCs
B
A
X
Y
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Logic Control Systems - Representation
Computer program
if ((A=0) and (B=0)) or ((A=1) and (B=1)),
X:=1
else
X:=0;
if ((A=0) and (B=0))
Y:= 1
else
Y:=0;
Truth table
A
B
X
Y
0
0
1
1
1
0
0
0
0
1
0
0
1
1
1
0
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Logic Control Systems - Representation
Boolean Logic
X:= A.B
Y:= ~(A+B)
Ladder Logic
X
A
B
Y
A
B
A
B 6
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Logic Control Systems - Representation
PLC Program
STR
SHF 00 ENT
AND
SHF 01 ENT
OR SHF 11 ENT
OUT
SHF 10 ENT
STR NOT
SHF 00 ENT
AND NOT SHF 01 ENT
OUT
SHF 11 ENT
10
00
01
11
00
01
00
01
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Ladder Logic Diagrams
Input contacts (switches, relays,
photoelectric sensors, limit
switches)
Output loads (actuators, alarms,
lights)
Logical inverse (NOT)
Logical AND operation
Logical OR operation
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Ladder Logic Diagrams (continued)
Example 7
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Ladder Logic Diagrams
Push-button switch
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Ladder Logic Diagrams
Automatic weigh station on a conveyor
Ladder logic diagram
Response from sensors
Sensor X
Sensor Y
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20
Rotate base CCW
21
Rotate base CW
22
Lift arm
23
Lower arm
24
Extend arm
25
Retract arm
26
Close gripper
27
Open gripper
Application: Direct Robot Control 8
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Application (continued)
Sequence of operations
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Application (continued)
PLC Ladder Logic Diagram
(not all rungs shown)
Output
Limit switch
Necessary PLC inputs
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Application (continued)
PLC Code