How can you implement a state machine with a PLC?

1 Define the states

The first step is to define the states of your system and what triggers the transitions between them. A state is a mode of operation or a phase of a process that has a distinct set of actions and outputs. A transition is a change from one state to another based on an input or a condition. You can use a state diagram to visualize the states and transitions of your system and how they relate to each other. A state diagram consists of circles representing the states and arrows representing the transitions.

2 Assign the variables

The next step is to assign the variables that you will use to represent the states and transitions in your PLC program. You can use different types of variables depending on your PLC model and programming language, but a common choice is to use boolean or integer variables. A boolean variable can have only two values: true or false. An integer variable can have multiple values: 0, 1, 2, etc. You can assign a boolean variable to each state and set it to true when the state is active and false when it is inactive. Alternatively, you can assign an integer variable to the state machine and set it to a different value for each state.

3 Write the code

The final step is to write the code that will execute the state machine logic in your PLC program. You can use different programming languages depending on your PLC model and preference, but a common choice is to use ladder logic or structured text. Ladder logic is a graphical language that uses symbols and contacts to represent the logic. Structured text is a textual language that uses statements and expressions to represent the logic. You can write the code in a separate function block or subroutine and call it from your main program. The code should consist of three parts: a part that checks the inputs and conditions and decides which state to activate, a part that performs the actions and outputs for each state, and a part that resets the variables for the next cycle.

4 Example: Traffic light controller

To illustrate how to implement a state machine with a PLC, let's look at an example of a traffic light controller. A traffic light controller is a system that controls the color and timing of the traffic lights at an intersection. The system has four states: green, yellow, red, and off. The transitions between the states are triggered by timers or switches. The system has three outputs: red light, yellow light, and green light. The system has two inputs: power switch and emergency switch.

The state diagram for the traffic light controller is shown below:

[Insert state diagram image]

The variables for the traffic light controller are shown below:

[Insert variable table]

The code for the traffic light controller in ladder logic is shown below:

[Insert ladder logic code]

The code for the traffic light controller in structured text is shown below:

[Insert structured text code]