Basics of PLC Programming
PLC Programming
Introduction
What is a PLC (Programmable Logic Controller)?
A Programmable Logic Controller, or PLC, is more or less a small
computer with a built-in operating system (OS). This OS is highly
specialized to handle incoming events in real time, i.e. at the time of
their occurrence.
The PLC has input lines where sensors are connected to notify upon
events (e.g. temperature above/below a certain level, liquid level
reached, etc.), and output lines to signal any reaction to the incoming
events (e.g. start an engine, open/close a valve, etc.).
The system is user programmable. It uses a language called "Relay
Ladder" or RLL (Relay Ladder Logic). The name of this language implies
that the control logic of the earlier days, which was built from relays,
is being simulated.
There are some other languages also used 1. Sequential Function chart 2.
Functional block diagram 3. structured Text 4. Instruction List
The PLCs' purpose in life[edit]
A PLC is primarily used to control machinery. Programs written for PLCs
consists in simple terms on instructions to turn on and off outputs
based on input conditions and the internal program. In this aspect, it
is similar to a computer. However, one designed to be programmed once,
and run repeatedly as needed. In fact, a crafty programmer could use a
PLC to control not only simple devices such as a garage door opener, but
their whole house, including switching lights on and off at certain
times, monitoring a custom built security system, etc.
Most commonly, a PLC is found inside of a machine in an industrial
environment. A PLC can run an automatic machine for years with little
human intervention. They are designed to withstand most harsh
environments.
History of PLCs
When the first electronic machine controls were designed, they used
relays to control the machine logic (i.e. press "Start" to start the
machine and press "Stop" to stop the machine). A basic machine might
need a wall covered in relays to control all of its functions. There are
a few limitations to this type of control.
- Relays fail.
- The delay when the relay turns on/off.
- There is an entire wall of relays to design/wire/troubleshoot.
A PLC overcomes these limitations, it is a machine controlled operation.
Recent developments
PLCs are becoming more and more intelligent. In recent years PLCs have
been integrated into electrical communications(Computer
network|networks)i.e., all the PLCs in an industrial environment have
been plugged into a network which is usually hierarchically organized.
The PLCs are then supervised by a control centre. There exist many
proprietary types of networks. One type which is widely known is SCADA
(Supervisory Control and Data Acquisition).
How the PLC operates
The PLC is a purpose-built machine control computer designed to read
digital and analog inputs from various sensors, execute a user defined
logic program, and write the resulting digital and analog output values
to various output elements like hydraulic and pneumatic actuators,
indication lamps, solenoid coils, etc.
Scan cycle
Exact details vary between manufacturers, but most PLCs follow a 'scan-cycle'format.
- Overhead
- Overhead includes testing I/O module integrity, verifying the user program logic hasn't changed, that the computer itself hasn't locked up (via a watchdog timer), and any necessary communications. Communications may include traffic over the PLC programmer port, remote I/O racks, and other external devices such as HMIs (Human Machine Interfaces).
- Input scan
- A 'snapshot' of the digital and analog values present at the input cards is saved to an input memory table.
- Logic execution
- The user program is scanned element by element, then rung by rung until the end of the program, and resulting values written to an output memory table.
- Output scan
- Values from the resulting output memory table are written to the output modules.
Once the output scan is complete the process repeats itself until the PLC is powered down.
The time it takes to complete a scan cycle is, appropriately enough, the
"scan cycle time", and ranges from hundreds of milliseconds (on older
PLCs, and/or PLCs with very complex programs) to only a few milliseconds
on newer PLCs, and/or PLCs executing short, simple code.
Basic instructions[edit]
Be aware that specific nomenclature and operational details vary widely
between PLC manufacturers, and often implementation details evolve from
generation to generation.
Often the hardest part, especially for an inexperienced PLC programmer,
is practicing the mental ju-jitsu necessary to keep the nomenclature
straight from manufacturer to manufacturer.
- Positive Logic (most PLCs follow this convention)
- True = logic 1 = input energized.
- False = logic 0 = input NOT energized.
- Negative Logic
- True = logic 0 = input NOT energized
- False = logic 1 = input energized.
- Normally Open
- (XIC) - eXamine If Closed.
- This instruction is true (logic 1) when the hardware input (or internal relay equivalent) is energized.
- Normally Closed
- (XIO) - eXamine If Open.
- This instruction is true (logic 1) when the hardware input (or internal relay equivalent) is NOT energized.
- Output Enable
- (OTE) - OuTput Enable.
- This instruction mimics the action of a conventional relay coil.
- On Timer
- (TON) - Timer ON.
- Generally, ON timers begin timing when the input (enable) line goes true, and reset if the enable line goes false before setpoint has been reached. If enabled until setpoint is reached then the timer output goes true, and stays true until the input (enable) line goes false.
- Off Timer
- (TOF) - Timer OFF.
- Generally, OFF timers begin timing on a true-to-false transition, and continue timing as long as the preceding logic remains false. When the accumulated time equals setpoint the TOF output goes on, and stays on until the rung goes true.
- Retentive Timer
- (RTO) - Retentive Timer On.
- This type of timer does NOT reset the accumulated time when the input condition goes false.
Rather, it keeps the last accumulated time in memory, and (if/when the
input goes true again) continues timing from that point. In the
Allen-Bradley construction, this instruction goes true once setpoint
(preset) time has been reached, and stays true until a RES (RESet)
instruction is made true to clear it.
- Latching Relays
- (OTL) - OuTput Latch.
- (OTU) - OuTput Unlatch.
Generally, the unlatch operator takes precedence. That is, if the
unlatch instruction is true then the relay output is false even though
the latch instruction may also be true. In Allen-Bradley ladder logic,
latch and unlatch relays are separate operators.
However, other ladder dialects opt for a single operator modeled after RS (Reset-Set) flip-flop IC chip logic.
- Jump to Subroutine
- (JSR) - Jump to SubRoutine
- For jumping from one rung to another the JSR (Jump to Subroutine) command is used.
About the Author
0 comments