Programmable Logic Controller

Programmable Logic Controller (PLC) is Industrial grade computer that programmed to use in control function. With PLC, many wires in conventional relay are eliminated.

In 1960, electromechanic, timer, counter, and sequencer used as standard of control function in industrial. Many control panels are containing hundred of relay equipment and the cables are very long. Low reliability and high maintenance cost. Control panel upgrading are also high cost. In 1968, hidramatic division in general motor uses instrumentation that now are called PLC.

Advantages of PLC

Parts of PLC

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• Processor
• CPU
  CPU is the brain of PLC. Usually processor contains of microprocessor that is used to implement logic and communicate with module. CPU is controlling PLC activities and designed so that user can input his desired program logic to ladder programming.
  PLC programs are executed as looping process starting from read an inputs, executing programs, diagnostics and communication, and finally updating output status. This process are looping as long as PLC in run mode. There are switch that has three mode, RUN, PROG, and REM. 
• Memory
  Knowing memory capacity is an important requirement to decide the type of processor. The higher its capacity, the higher it could contain coil and contact. The memory part that save I/O status called I/O table. Each bit can have 1 or 0 value depends on the input.  
• I/O
  I/O is one part that connecting all field devices and provide interface between CPU and field devices. There are two configurations of I/O, Fixed I/O and Modular I/O. Fixed I/O is cheap but not flexible. Also in a certain model, when something has broken, all parts must be replaced. Modular I/O is more expensive rather than Fixed I/O because its flexibility. When some parts has broken, it can be replace only the broken parts. Each I/O will has an address so the controller identify, monitor, and control them. 
• Discrete I/O Module
  This kind of module has only two state ON or OFF, so the input and output will be limited to some devices that have two state like switch, pushbutton, limitswitch, lamp, relay, solenoid, motor starter, and others.
  Specifications:
  -   Nominal input voltage, declared in magnitude 5v/24v/230v and type AC/DC. In module input usually planned has +/-10% from input voltage rating. In DC input module, input voltage can work 24-60v in operational condition.
  -   Threshold input voltage, there are two condition 1 as ON state minimum voltage or 0 as OFF state maximum voltage.
  -   Nominal input current, this value is the minimum value that can be received from discrete input device. This value relate to input voltage as threshold to secure from noise or as a signal leakage detector.
  -   Temperature rating, this value show the maximum temperature where the I/O will have the best operational condition.
  -   ON/OFF delay input, known as response time.
  -   Output voltage
  -   Output current is value that show the maximum current in the burdened module.
  -   Electric isolation
  -   Point in each module, generally, discrete module has 8 to 32 circuit, low-end controller has 2 to 4 circuit, and high-end has 32 to 64.
  Discrete input module do four task in control system:
  -   Receive signal from field devices
  -   Convert input signal to the right voltage level corresponding to PLC rating
  -   Isolate PLC from fluctuating voltage or current input signal
  -   Send signal to processor from sensor
• Analog I/O Module
  In analog I/O, the state not only two, but can be infinity. Nowadays, PLC can implement both discrete and analog, usually the value of I/O varies between 0 to 20mA, 4 to 20mA, and 0 to 10v. Input voltage has two types, unipolar and bipolar.
  Specifications:
  -   Channel in each module, there are two connection, single-ended or differential. If the normal module has 16 single-ended connection, it will has 8 connection in differential connection. Single-ended has more vulnerable with noise.
  -   Voltage/current I/O range must be match with the used device.
  -   Resolution shows the accuracy of analog to digital value.
  -   Impedance (MΩ) and capacitance (pF) input value that must be match with external device that connected with module.
• Special I/O Module
• Fast speed counter module used as interface that needs a fast counter in PLC. This part is separated from the processor. The counter rate is about 0 to 100kHz. 
• Encoder counter module
• Stepper motor module
• Power Supply
• Programming Equipments

To understand how PLC works, observe illustration below,

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Motor is used to stir liquids when it achieve some temperature and pressure that have been set up. To operate motor manually, there is push button in control panel. Process is equipped by thermal and pressure sensor that when liquid reach certain value it will trigger the motor. That control problem can be solved by using relay circuit in ladder programming like in image below,

motor will start to rotate when pressure and temperature achieve some value that have been set up or when manual push button is pushed.

To input program, an instruction is written one by one to CPU memory from programming device. Each input and output devices are given an address so PLC knows theirs connections. The I/O address will be different depends on PLC type and brand. When PLC start at run mode, controller will monitor input device and updating output based on the program. One cycle program in PLC is called scan, PLC scan time show PLC speed response. Output will be updated every one cycle scan time has been completed.

PLC is categorized based on its function, number of I/O, price, and physical size. Generally, nano type is smallest size that has less than 15 I/O points, next is micro type 15--128 I/O points, medium type 128--512 I/O points, and big type has more than 512 I/O points.

There are three types of PLC based on its application,

• Singleended
  Singleended is standalone PLC that processing tasks one by one and can't be used in communicate with computer or other controllers.
• Multitask
  Multitask is PLC that can processing few tasks at one time. 
• Control Management
  PLC Control Management is used in large-scale control process. This type must have an ability to communicate with other PLCs or computers.