Introduction to Industrial Automation

3.5.1   Starting a Machine with Canceling Ability

Every machine, when in operation, is performing specific actions and delivers a specific result. 

However, there are cases of machines where their operation, after their start, are dependent spe-

cifically on a particular result. In cases of malfunction, where no specific results are produced, the 

machine should stop, while in the normal operation the machine should continue to operate. The 

involved time from the starting of the machine until the creation of the final result varies with 

respect to the specific application. Subsequently, the operation of such a machine with canceling 

ability is presented through the following example.

Let’s assume a pump that transfers a liquid from Tank A into Tank B, as indicated in Figure 

3.22. Moreover, we assume that the pump is self-lubricated by the same liquid that is passing 

through the tank (e.g., in the case of water, the pump is hydro-lubricated). In case that the pump is 

in operation without being lubricated, which is equivalent to the situation that no liquid is passing 

through the pump, it is more than likely that the pump will be destroyed (burnt). For this reason, 

it is desirable to design an automation circuit for putting the pump in operation; if there is a flow 

of the liquid in the pipe, the pump will continue to operate, while if there is no flow, the overall 

operation will be canceled, which means that the operation of the pump should be stopped auto-

matically. Since the liquid needs some necessary time (T) sec to reach from Tank A to the pump, 

the automation circuit should cancel the operation of the pump only after this specific time of T 

plus some small-time tolerance (specifically T = distance from tank A to flow switch/flow velocity). 

The existence of a liquid flow in the pipe is detected through the utilization of a flow switch, which 

is placed closed to the pump. The case of no flow might be caused by various factors, including the 

Pump

Flow switch

A

B