Industrial Control Basics – Manual Control

 You’ve probably heard the saying, “asleep at the switch.”  It’s usually associated with some sort of disaster, found later to have been caused by human error.  Someone wasn’t paying attention, and something very bad happened.  The meltdown of the Soviet nuclear power plant Chernobyl in 1986 comes to mind.  You may be surprised to learn that the saying has its origins in the world of industrial controls, or more specifically, manual controls, as we’ll see in this article.

     Last week when we opened our discussion on manual controls, we talked about how they work just as their name implies, that is, someone must manually press a button or throw a switch in order to initiate a factory operation.  In other words, a manual control requires human intervention to initiate an action, such as pushing the start button.  The machine will then continue to run until a person hits the stop button. 

     Let’s go now on a virtual field trip into a telephone factory to see how a basic manual control system works.  It has a conveyor belt operated by an electric motor, and this motor is connected by wires and a power switch to a 120 volt power source of alternating current. Figure 1 illustrates what we mean.  It shows that when the power switch is in the open position, a physical air gap exists within the electrical circuit.  This prevents electricity from flowing through the wire because electricity can’t jump over gaps.

Figure 1 – Open Power Switch

     Enter a human into the scenario, someone who grabs the power switch handle and manually closes it, eliminating the air gap.  See Figure 2.

Figure 2 – Closed Power Switch

     When the power switch is closed, a metal conductor bridges the gap, causing electricity to flow through the metal conductor to the electric motor in the circuit.  This brings life to the conveyor belt.  As long as the power switch remains closed, the conveyor belt will continue to operate. 

     That’s it, that’s a basic manual control system.  It’s simple to operate, but it does have one major flaw.  It requires constant monitoring by a human.  Aside from opening and closing a power switch, humans are required to monitor operations, in case something goes wrong.  The operator watching over an industrial machine performs the same function as the pilot on a plane, that is, to start-stop operations, and to intervene in case of an emergency.  Computers fly modern jets.  Pilots serve as trouble shooters when the unanticipated disaster situation occurs, because computers can’t yet creatively problem solve.