Introduction to Industrial Automation

3.1.2   The Principle of “Command”

In the wiring diagram of Figure 3.1a, the operation of the machine “C” is controlled by the 

human-based actions on the switch RS, while this operation is presented more clearly in Figure 

3.2a. However, it is possible that the operation of the machine is controlled in an autonomous 

manner, not from a human-based operation, but through the utilization of a switching contact, 

e.g., a contact of a light sensor, as presented in Figure 3.2b. In this example, when the photocell 

is not detecting an object, it has its internal contact open and thus the machine cannot operate. 

When the photocell detects an object, the previously open contact closes and thus the operation 

of the machine is allowed. In this case and in all other similar ones, the operation of the machine 

is based on the “command” from the utilized sensor. In both cases depicted in Figure 3.2, the 

machine is being operated through a contact, either by the switch RS or the sensor switch. In 

the second case, the definition of “command” is describing the absence of a human operator. An 

additional difference in the described cases is the meaning of the distance. In the first case, in 

Figure 3.2a, the operating switch is usually very close to the rest of the automation circuit, either 

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  Introduction to Industrial Automation

in the same electrical cabinet containing the automation circuit or in a nearby control panel. In 

the second case, in Figure 3.2b, the sensor cannot be placed inside or close to the cabinet with the 

automation circuit, since this device is placed in the environment needing the measurement of a 

specific quantity and thus proper wiring should be performed from the sensor to the automation 

cabinet. Hence, the definition of “command” also includes the meaning of the distance or the 

meaning of a control command from a distance.

From an automation circuit design perspective, it should be mentioned that the points in a cir-

cuit, where the “command” from a sensor is being utilized, are denoted by “

∅

”. In most cases, we 

are not designing the sensor device in the automation circuit, but we are simply notating the kind 

of the command and switching contact as it will be analyzed subsequently. Finally, the machine 

operation control through a sensor with an SPST output switching contact is commonly referred 

as a “two-wire command”.