Introduction to Industrial Automation

3.2.6   Machine Operation with Automatic or Manual Control

Let’s consider a machine, as part of a production line, which operates through a command from 

a sensor (e.g., a pump being controlled by a level sensor, or a conveyor belt being controlled by a 

limit switch). For the time that the corresponding sensor operates properly, there is no problem 

with the existing automation. If we assume the sensor has an unwanted fault, then the machine is 

not able to operate, which causes a problem in the production line (most likely the whole produc-

tion line will be stopped). A solution to this problem would be to replace the faulty sensor as soon 

as possible assuming such additional sensor exists; however, a delay in the production line may still 

occur, which on occasion might generate a significant profit loss.

To avoid such problematic time delays, and until the restoration of the fault, it is acceptable that 

the machine will operate manually via an operator action overriding the faulty sensor. The ability 

to control the operation of a machine in an automatic manner is denoted as “A” (based on a sensor) 

or through the classical START-STOP commands is denoted as “H”, indicated in the automation 

circuit shown in Figure 3.10a. Moreover, there are plenty of cases where the overriding of the sensor 

command is desired, for example in Figure 3.10b, where in the corresponding circuit a time relay 

with a 24-hour period is being utilized instead of a sensor. With the switch RS

1-0-2

 in the A position, 

the machine will operate automatically in the time periods that we have initially programmed.

N

T

1

C

50 Hz 230 V

R

e

RS

1-0-2

0

1

2

T

2

T

1

T

2

T

1

Circuit features:

•Periodic operation of machine C

•Reoperation 

T

1

OFF

ON

t

t

C

RS

1-0-2

0

2

T

2