PLC

List of free online PLC simulators and How to use an Online PLC simulator?

Start scenario of a motor in PLC ladder logic

What is an Online PLC Simulator?

PLC Simulation is a way of developing and testing algorithms.PLC simulation software is designed to develop control logic as well as analyse the system with a systematic model.

Learning a PLC simulator makes PLCs debugging easier. PLC software programs may be hard to get a hang of and sometimes requires a little training to apprehend the correct functions and the full range of programmable logic controllers. This is one reason why the usage of a simulator may be a great teaching technique for people who want to learn PLC programming.

PLC simulation control layout allows at the beginning of the development system earlier than the electric and mechanical components are activated. Engineers can create, test, and validate software program application simulations of controllers without the threat of damaging a device or system.

List of Online PLC Simulators

Here are some of the free online PLC simulators to check and verify algorithms. They are

  1. PLC Simulator
  2. PLC Fiddle
  3. PLC Online
  4. Vlabs

Comparison of Free Online PLC Simulators

S. NoNameLanguage SupportedFunction blocks supportedURLFree/ Licence
     1PLC SimulatorLadder logicBool, Number, Timer, Counter, Math blocksClick Here     Free
     2PLC FiddleLadder logicBool, Number, Timer, Counter, Math blocksClick Here     Free
     3PLC OnlineLadder logicBool, Number, Timer, Counter, Math blocksClick HereNeed to login
     4VlabsLadder logicBool, Number, Timer, Counter, Math blocksClick Here     Free
List of free online PLC simulators

What is ladder logic?

Ladder diagrams are specialized schematics for describing industrial control logic systems.They are called “ladder” diagrams because it looks like a ladder, it has two vertical rails and as many horizontal lines called “rungs”.

Why Ladder Logic is Popular?

Ladder logic is the most popular technique of PLC programming as it is a graphical programming language that looks like an electrical schematic drawing. Engineers, electricians, and college students already recognized with electric schematic drawings find the transition from an electrical circuit to the ladder logic pretty easy, as compared to different text-based programming languages.

How to draw a ladder logic?

Ladder logic diagrams are generated in the same way as to relay logic circuits. Ladder logic diagrams use symbolic notation to express logic functions rather than circuit elements.

There are 7 basic parts of a ladder diagram which are detailed below to understand how to draw ladder logic diagram.

  • Rails
  • Rungs
  • Inputs
  • Outputs
  • Logic Expressions
  • Address Notation & Tag Names
  • Comments

Click here to know more about ladder logic

What is Rail?

A ladder line has rails, which might be drawn in vertical strains on the pinnacle of the page. If they’re in a relay logic circuit, they’ll represent the active and 0 voltage connections of the electricity supply, in which the current flows from left to right.

What is a Rung?

The rungs are drawn as horizontal lines that connect the rails to logical expressions. If they had been in a relay logic circuit, they replicate the wires connecting the power of the switching and relay elements.

Ladder logic representation shows Ladder Rail and Rung
Ladder Logic representation

What is a Branch?

A branch is formed when two or more instructions are connected in parallel. Ladder logic circuits are always branching ranges, with multiple branch levels.

Below shown is an example of a branch

Representation of a branch in PLC ladder logic
Branch representation

What is Coil (Output)?

The coil is usually connected to a PLC output, but it can also be used as a memory. The coil used as memory does not physically open or close anything, but internally stores the state of the current path to which it is connected.

If there is a true command path before the coil, then the coil is energized and is therefore true. If the coil command is true, it means it is physically on /open.

If there is a false command path before the coil, then the coil is de-energized and is therefore false. If the coil command is false, it means it is physically off/close.

Coil representation in a PLC ladder logic
Coil representation

What is NO/NC Contact (Input)?

NO/NC contact is a commonly used digital input element in a ladder logic program to store the value from a digital input device from field like limit switch.

Normally, open (NO): The circuit will be in open state when the contacts are not working (no current flows).  When actuation happens the relay contact moves to closed state and hence the circuit closes (current flows).

Normally closed (NC): The circuit will be in closed state when the contacts are not working (current flows).  When actuation happens the relay contact moves to open state and hence the circuit opens (no current flows).

NO and NC contact representation in a PLC ladder logic
NO & NC Contacts

What is Timer?

A timer is an instruction in ladder logic program to measure the time elapsed after a particular event in the logic to achieve the control action.

The timer comes in two basic configurations on-delay timer and off-delay timer.

An “on-delay” timer activates an output only when the input to the timer has been active for a time period defined in the timer element.

An “off-delay” timer de-activates an output only when the input to the timer has been inactive for a time period defined in the timer element.

What is a Counter?

A counter is an instruction that either counts up (increment) or counts down (decrements)  an integer as the bit goes false to true (0 to 1).

Counter instructions come in three basic types:

  • Up counters,
  • Down counters, and
  • Up/Down counters.

What is a Function block?

A function block is created with a set of instructions which needs to be used frequently in the control logic. The function block usually will have inputs and outputs attached to it, the number of inputs and outputs depends on the instruction configured inside the function block. Ex: Math Function block – Add block

Add function block in a PLC ladder logic
Add Function block

How to use an Online PLC Simulator?

Now we are going to look at how to do a PLC program at PLC simulator, PLC simulator is an online PLC simulator right in your browser. It is an ideal way to understand PLC concepts for industrial automation needs.

Below we have explained how to use one of the free online PLC simulator.

Step 1: Open the browser and load the PLC simulator.

 Click here to load the PLC simulator

Step 2: Add variables also known as tags

Type Variable name à then choose Variable data type à click to submit

Adding variables using online PLC simulator
Adding variables

Step 3: After adding all the variables, let’s add elements to the diagram.

To add an element, select Rung and click desired element icon in the toolbox as shown below.

Note: The elements added can be moved to desired position using arrows.

Adding elements using online PLC simulator
Adding elements
Adding elements using online PLC simulator
Adding elements

Step 4: Now we need to assign tag elements. For assigning tag elements click on the element and select the variable which you want to assign as shown below

Adding element properties using online PLC simulator
Adding element properties
Adding element properties using online PLC simulator
Adding element properties

 Step 5: After configuring all the elements, click the Green arrow ( ) icon to start the simulation.

Note: To change the state of variables click on its value on the action bar.

Logic Description:

Start Scenario

When the operator operates the start button, the value of the start variable is set to true for a moment and falls back to false.  But the motor coil is still energized by a signal passing through the branch with the motor normally open contact.

Start scenario
Start Scenario

Stop Scenario

When the operator operates the stop button to turn off the motor, the value of the stop variable is set to true and opens the contact. The motor coil gets de-energized and stops the motor.

Stop scenario
Stop Scenario

Level Switch Scenario

Level switch has been included in to the logic as a permissive to start the motor, if level switch value is true operator will be able to start the motor.

Click here for Overhead Water tank automatic level control PLC ladder logic program. 

In this similar fashion other simulators can also be explored for learning purpose.

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