Introduction to Industrial Automation

4.3   State  Diagrams

4.3.1   Classical State Diagrams

The design methodology of a state diagram and its logical processing is based on a specific pro-

cedure, and is a graphical approach in designing automation circuits, with its main aim to sys-

tematically simplify the design procedure, especially in big, complicated industrial automations. 

In general, the state diagram accurately represents the states of a complex industrial automation 

system, e.g., a complex machine, as well as the electrical signals that force the automated system 

to change from one operating state to another. After the proper design of the state diagram, 

the extraction of Boolean logical expressions that describe each one of these states can be easily 

derived, and thus the final automation circuit can be further extracted, as will be presented using 

a more simplified approach. In this methodology, the most important thing is the identification of 

the exact states and the transition signals, and not the design of the automation itself, as presented 

in the previous chapters with ad-hoc methodology. Specifically, the methodology in design auto-

mation with the state diagram approach involves the following steps:

  1. A detailed, extended description of the desired operation of the complex machine or the 

industrial automation and definition of the operating states

  2. Construction of the state diagram based on the design rules that will be described 

subsequently

  3. Extraction of logical Boolean expressions

  4. Design of the industrial automation based on the derived logical expressions

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  Introduction to Industrial Automation

The state diagram itself is a direct method of modeling industrial systems that contain a set of 

logical variables, where their number defines the order of the system. In general, the state of an 

industrial system or a complex machine can be considered as a set of logical values that contains 

these logical variables. For example, the state of a complex machine can be the following set of 

logical values: “The first motor is in operation, the valve is energized, the second motor is not in 

operation”; where in most cases and for simplification purposes it can be equivalently character-

ized by verbal terms like “the machine is getting ready”. The state diagram is constructed accord-

ing to the following rules:

  1. We denote with circles the different states of the complex machine or industrial system. In 

every circle, we define the corresponding state.

  2. Every possible transition among different states is denoted by an arrow connecting the cor-

responding states. The arrow’s direction denotes the transition’s direction.

  3. On every directed arrow, we denote the Boolean variable or the logical expression of the 

Boolean variables that cause the change of state. We denote “Turn OFF” as the logical 

expression of the variables that cause the change of the dual variable value, characterizing 

the changed state, from 1 to 0, and “Turn ON” as the logical expression of variables that 

cause the corresponding change of the same dual variable value from 0 to 1.

  4. For all the dual variables of the state diagram we calculate the following expression:

 

X Turn OFF Turn ON Present State of X variable

=

+

⋅

(

))

 (1)

    The “Present State of X variable” is used as a memory type element that updates the cur-

rent value of the variable. As will be presented subsequently, this term has the same role as 

the “self-latching contact” principle, which was presented in Section 3.1.1, describing the 

empirical methodology of circuit design.

  5. Based on the extracted and simplified logical expressions from Step 4, we design the indus-

trial automation circuit.

The described procedure for the logical design of the automation circuits, based on the state dia-

gram and logical expression (1), are explained and discussed further through the following appli-

cation examples.