Programmable Logic Controller Internal Operations.
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Transcript of Programmable Logic Controller Internal Operations.
Programmable Logic Controller Internal Operations
Chapter PLC3 Slide 2
Learning Objectives
covers the internal operations of a PLCillustrates the concept of scan timeexplains I/O response timediscusses practical issues of input signal interface with PLCdiscusses practical issues of output signal interface with PLC
This topic
Chapter PLC3 Slide 3
Applications of PLCs:
Sequence control
Motion Control
Process Control
Field bus
Communication
Data Management
Chapter PLC3 Slide 4
Internal Operation of PLCs: Input / Output Addressing
Chapter PLC3 Slide 5
Internal Operation of PLCs: Input / Output Addressing
INPU
TO
UTPU
T000.00 010.01
PROGRAM
Chapter PLC3 Slide 6
Internal Operation of PLCs: Input / Output Addressing
INPU
TO
UTPU
T000.00 010.01
PROGRAM
Reads Input Executes Program Sets Output
Chapter PLC3 Slide 7
Input / Output Addressing
These outputs are latched and they retain their status until they are updated by the next I/O copying routine.
Input / Output Processing
Reads Input Executes Program On Output
Most common method is mass input/output copying
Reads All Input Status
ExecutesProgram
Turns on
OutputDecides which Outputs to turn on
Updates Status at
Input RAM
read one at a time
Stores Output
Status at Output RAM
Transfer to Output Module
Chapter PLC3 Slide 8
I/O Refresh
The process of writing from RAM to the output relays and reading input relay status to RAM is known as I/O Refresh.
Copy all inputsinto RAM
Fetch, decode and execute all instructions in sequence
Copy all outputs from O/PRAM to output unit and input all inputs to I/P RAM
Time depends on length of total programe.g. 1K program = 5 ms
Fixed length delay e.g.5 ms
START I/O Copy Program Execution End I/O Copy Program
I/O Refresh.
Chapter PLC3 Slide 9
Scan Time
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.
On power up, CPU goes through a series of internal processes
ReadsInput
SetsOutput
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.
Executes Program
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.
Chapter PLC3 Slide 10
Scan Time
ReadsInput
SetsOutputExecutes Program
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.
ReadsInput
SetsOutputExecutes Program
One Scan One Scan
Chapter PLC3 Slide 11
ReadsInput
SetsOutputExecutes Program
ReadsInput
SetsOutputExecutes Program
Scan Time
One Scan One Scan
INPUT SIGNAL comes in ….. And DETECTED !!
Case 1 :
Chapter PLC3 Slide 12
ReadsInput
SetsOutputExecutes Program
ReadsInput
SetsOutputExecutes Program
Scan Time
One Scan One Scan
No INPUT SIGNAL comes in …..
And DETECT nothing !!
Case 2 :
Some INPUT SIGNAL comes in …..
And DETECTED !!
Chapter PLC3 Slide 13
ReadsInput
SetsOutputExecutes Program
ReadsInput
SetsOutputExecutes Program
Scan Time
One Scan One Scan
INPUT SIGNAL comes in ….. And NOT DETECTED !!
Case 3 :
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 14
Interfacing On-Off Sensors with PLCs
Mechanical Switches
or ‘Dry Contacts’
DC proximity sensors
AC/DC 2-wire
Proximity sensors
relay output solid state output
NPN PNP1
2
3
4
5
COM
NC
NO
24V Load
sink
+24V DC
(-)
+
-
Load
source
+24V
(-)
-
+
Load 24V DC
Sensors
Chapter PLC3 Slide 15
Interfacing On-Off Sensors with PLCs
solid state output
NPN PNP
Load
sink
+24V DC
(-)
+
-
Load
source
+24V
(-)
-
+
NPN (current sinking) sensor must be matched with a source type input card.
Input Cards can be broadly classified into two categories:
a) source type (or common positive), &
b) sink type (or common negative).
PNP (current sourcing) sensor must be matched with a Sinking type input card.
Chapter PLC3 Slide 16
When Sensor is OFF…
Interfacing On-Off Sensors with PLCs: Input Units and Interface Circuits
NPN (current sinking) sensor must be matched with a source type input card.
When Sensor is ON…
No Signal Signal is detected
SENSOR Sinks
PLC Sources
Chapter PLC3 Slide 17
Interfacing On-Off Sensors with PLCs: Input Units and Interface Circuits
When Sensor is ON…SENSOR Souces
PLC Sinks
PNP (current sourcing) sensor must be matched with a Sinking type input card.
Chapter PLC3 Slide 18
Interfacing On-Off Loads with PLCs
Output Module of PLCs are used to drive loads, such as
• solenoids,
• lights,
• buzzers,
• motor contactors, etc
These loads are powered by an External Power Source not within the PLC.
Power
Supply
Output ModuleOutput Module
Chapter PLC3 Slide 19
Types of Output Modules
npn pnp
Transisterised
Triac Relay
Chapter PLC3 Slide 20
Interfacing On-Off Loads with PLCs
Load type Load current Response Time
RELAY
TRIAC
Transistorised (npn/ pnp)
Characteristics
Type of Output
AC/ DC
AC
DC
2A Typical
1A Typical
1A Typical
10ms Typical
3ms Typical
1ms Typical
Factors affecting selection of Output Module:
Chapter PLC3 Slide 21
Interfacing On-Off Loads with PLCs
LOAD
Transisterised NPN Type
Chapter PLC3 Slide 22
Interfacing On-Off Loads with PLCs
Transisterised PNP Type
LOAD
Chapter PLC3 Slide 23
Interfacing On-Off Loads with PLCs
Triac 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
Interfacing On-Off Loads with PLCs
Relay Type
LOAD
Chapter PLC3 Slide 27
This chapter :
Summary
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