Introduction to Industrial Automation

Example 4.4:  A Three-Speed Machine Automation

One complex industrial machine is operating by utilizing a motor with three speeds. The operation 

of the machine is achieved through four pushbuttons. The button b

0

 is for STOP, while the but-

tons b

1

, b

2

, b

3 

are for the slow, medium, and high speeds, correspondingly. Moreover, we would like 

to have the medium speed to be commanded only if the motor is operating in slow speed before, 

and the high speed to be commanded only if the motor is operating in the medium speed before. 

The corresponding automation diagram for this problem (states S

0

, S

1

, S

2

, and S

3

) is indicated in 

Figure 4.11. In cases of complex applications or equivalently of state diagrams with a lot of states, 

the states should be coded through a binary code for the proper functionality of the presented 

methodology, which will allow for the proper extraction of the logical expressions needed for the 

design of the automation circuit. As an indicative guide to how many binary variables are needed 

for state diagrams with n states, the code should contain m binary variables so that the relation 

n ≤ 2

m

 is valid. Issues such as the best coding selection, the multitude of possible binary assign-

ments, the absence of a systematic method for determining the required binary code, and others 

can be looked up in the related literature. However, in this book, we will suggest a simple design 

rule according to which “there should not be a synchronous value change of two Boolean variables 

as we are moving from one state to another one” and thus the most appropriate coding for this 

case is Gray coding.

b

0

AB

S

0

Low speed

S

1

S

2

S

3

b

1

b

0

b

2

b

3

b

0

00

01

11

10

Medium

speed

High

speed