close

Вход

Забыли?

вход по аккаунту

код для вставкиСкачать
Programmable Logic Controller
Internal Operations
Learning Objectives
This topic
covers the internal operations of a PLC
illustrates the concept of scan time
explains I/O response time
discusses practical issues of input signal interface with
PLC
discusses practical issues of output signal interface
with PLC
Chapter PLC3
Slide 2
Applications of PLCs:






Sequence control
Motion Control
Process Control
Field bus
Communication
Data Management
Chapter PLC3
Slide 3
Internal Operation of PLCs: Input / Output Addressing
Chapter PLC3
Slide 4
000.00
OUTPUT
INPUT
Internal Operation of PLCs: Input / Output Addressing
010.01
PROGRAM
Chapter PLC3
Slide 5
000.00
OUTPUT
INPUT
Internal Operation of PLCs: Input / Output Addressing
010.01
PROGRAM
Reads Input
Chapter PLC3
Executes Program
Sets Output
Slide 6
Input / Output Addressing
Reads Input
Executes Program
On Output
Input / Output Processing
Most common method is mass input/output copying
Reads
All Input
Status

Updates
Status at
Input
RAM
read one
at a time
Executes
Program
Decides which
Outputs to
turn on
Stores
Output
Status at
Output
RAM
Transfer to
Output
Module
Turns on
Output
These outputs are latched and they retain their status until they are updated by the
next I/O copying routine.
Chapter PLC3
Slide 7
I/O Refresh

The process of writing from RAM to the output relays and reading input
I/ORefresh.
Refresh.
relay status to RAM is known as I/O
START I/O Copy
Copy all
inputs
into RAM
Chapter PLC3
Program Execution
End I/O Copy
Fetch, decode and execute
all instructions in sequence
Copy all outputs from O/P
RAM to output unit and input all
inputs to I/P RAM
Time depends on length of
total program
e.g. 1K program = 5 ms
Fixed length delay
e.g.5 ms
Program
Slide 8
Scan Time
Reads
Input
Executes Program
Sets
Output
On power up, CPU goes through a series of internal processes
Data input and output when status of the input relays will be written into the
input portion of the I/O RAM and status of the output portion will be reflected in
the output relays.
Instruction execution which includes an executive routine in which the base
intelligence of the system is used to interpret the user program containing the
instructions to be executed.
Common processes, such as watchdog timer resetting, checking the I/O bus,
checking scan time, diagnostic operations and housekeeping on program
memory – processes. Communication windows which include the link service
of a host computer and communications with peripheral devices.
Chapter PLC3
Slide 9
Scan Time
Reads
Input
Executes Program
Sets
Output
One Scan
Reads
Input
Executes Program
Sets
Output
One Scan
 Scan time is the total time required for PLC to perform activities in each
scan cycle.
 Typical scan time for 500 instruction words varies between 3 ms and 10
ms with ladder logic programming.
Chapter PLC3
Slide 10
Scan Time
Case 1 :
One Scan
One Scan
Reads
Input
Executes Program
Sets
Output
Reads
Input
Executes Program
Sets
Output
INPUT SIGNAL comes in ….. And DETECTED !!
Chapter PLC3
Slide 11
Scan Time
Case 2 :
One Scan
One Scan
Reads
Input
Executes Program
Sets
Output
No INPUT SIGNAL comes in …..
And DETECT nothing !!
Chapter PLC3
Reads
Input
Executes Program
Sets
Output
Some INPUT SIGNAL comes in …..
And DETECTED !!
Slide 12
Scan Time
Case 3 :
One Scan
One Scan
Reads
Input
Executes Program
Sets
Output
Reads
Input
Executes Program
Sets
Output
INPUT SIGNAL comes in ….. And NOT DETECTED !!
Because of the cyclic nature of I/O copying routine, input signals of high frequency
which change state after the READ routine will not be recognised until the next copy
occurs.
Only certain PLCs incorporate a subroutine to interrupt the scan to pick up such
signals.
Chapter PLC3
Slide 13
Interfacing On-Off Sensors with PLCs
Sensors
Mechanical
Switches
or
‘Dry Contacts’
DC proximity
sensors
AC/DC
2-wire
Proximity sensors
24V DC
Load
solid state output
relay output
1
COM
2
NC
3
NO
+24V DC
+24V
+
+
4
5
PNP
NPN
source
Load
24V
Load
sink
-
Chapter PLC3
(-)
(-)
Slide 14
Interfacing On-Off Sensors with PLCs
Input Cards can be broadly classified
into two categories:
a) source type (or common positive), &
b) sink type (or common negative).
NPN (current sinking) sensor
must be matched with a
source type input card.
solid state output
PNP
NPN
+24V DC
+24V
+
PNP (current sourcing) sensor
must be matched with a
Sinking type input card.
+
source
Load
Load
sink
-
Chapter PLC3
(-)
(-)
Slide 15
Interfacing On-Off Sensors with PLCs: Input Units and
Interface Circuits
NPN (current sinking) sensor
must be matched with a
source type input card.
PLC Sources
When SENSOR
Sensor isSinks
OFF…
ON…
Signal
is detected
No Signal
Chapter PLC3
Slide 16
Interfacing On-Off Sensors with PLCs: Input Units and
Interface Circuits
PNP (current sourcing) sensor
must be matched with a
Sinking type input card.
PLC Sinks
When Sensor
is Souces
ON…
SENSOR
Chapter PLC3
Slide 17
Interfacing On-Off Loads with PLCs
Output Module of PLCs are used to drive
loads, such as
•
•
•
•
solenoids,
lights,
Output Module
Output Module
buzzers,
motor contactors, etc
Power
Supply
These loads are powered by an External Power
Source not within the PLC.
Chapter PLC3
Slide 18
Types of Output Modules
Transisterised
npn
Triac
Chapter PLC3
pnp
Relay
Slide 19
Interfacing On-Off Loads with PLCs
Factors affecting selection of Output Module:
Characteristics
Type of Output
RELAY
TRIAC
Transistorised
(npn/ pnp)
Chapter PLC3
Load type
Load current
Response Time
AC/ DC
2A Typical
10ms Typical
AC
1A Typical
3ms Typical
DC
1A Typical
1ms Typical
Slide 20
Interfacing On-Off Loads with PLCs
Transisterised NPN Type
LOAD
Chapter PLC3
Slide 21
Interfacing On-Off Loads with PLCs
Transisterised PNP Type
LOAD
Chapter PLC3
Slide 22
Interfacing On-Off Loads with PLCs
Triac Type
LOAD
Chapter PLC3
Slide 23
Interfacing On-Off Loads with PLCs
Relay Type
LOAD
Chapter PLC3
Slide 24
Interfacing On-Off Loads with PLCs
Relay Type
LOAD
Chapter PLC3
Slide 25
Interfacing On-Off Loads with PLCs
Relay Type
LOAD
Chapter PLC3
Slide 26
Summary
This chapter :





covers the internal operations of a Programmable
Controller
illustrates the concept of scan time
explains I/O response time
discusses practical issues of input signal interface with
Programmable Controller
discusses practical issues of output signal interface with
Programmable Controller
Chapter PLC3
Slide 27
1/--страниц
Пожаловаться на содержимое документа