Archive for December 18th, 2011

Industrial Control Basics – Ladder Diagrams

Sunday, December 18th, 2011
     The other day I pressed the button to activate my electric garage door opener and nothing happened.  I pushed again and again, still nothing.  Finally, I convinced myself to get out of the car and take a closer look.  A wooden board I had propped up against the side of the garage wall had come loose, wedging itself in front of the electric eye, you know, the one that acts as a safety.  The board was an obstruction to the clear vision of the eye.  It couldn’t see the light emitter on the other side of the door opening and wouldn’t permit the door opener to function.

     The basic manual control system we looked at last week operates similarly to the eye on a garage door opener.  If you can’t “close the loop,” you won’t get the power.  Last week’s example was as basic as things get.  Now let’s look at something a bit more complex.

     Words aren’t always the best vehicle to facilitate understanding, which is why I often use visual aids in my work.  In the field of industrial control systems diagrams are often used to illustrate things.  Whether it’s by putting pencil to paper or the flow diagram of software logic, illustrations make things easier to interpret.  Diagrams such as the one in Figure l are often referred to as “ladder diagrams,” and in a minute we’ll see why.

Figure 1

     Figure 1(a) shows a basic manual control system.  It consists of wires that connect a power switch and electric motor to a 120 volt alternating current power source.  One wire is “hot,” the other “neutral.”  The hot side is ungrounded, meaning that it isn’t electrically connected to the Earth.  The neutral side is grounded, that’s right, it’s driven into the ground and its energy is dissipated right into the earth, then returned back to the power grid.  In Figure 1(a) we see that the power switch is open and an air gap exists.  When gaps exist, we don’t have a closed electrical loop, and electricity will not flow.    

     Figure 1(b), our ladder diagram, aka line diagram, shows an easier, more simplified representation of the manual control shown in Figure 1(a).  It’s easier to decipher because there’s less going on visually for the brain to interpret.  Everything has been reduced to simple lines and symbols.  For example, the electric motor is represented by a symbol consisting of a circle with an “M” in it.

     Now, let’s turn our attention to Figure 2 below to see what happens when the power switch is closed.

Figure 2

     The power switch in Figure 2(a) is closed, allowing electric current to flow between hot and neutral wires, then power switch, and finally to the motor.  The current flow makes the motor come to life and the motor shaft begins to turn.  The line diagram for this circuit is shown in Figure 2(b).

     You might have noticed that the line diagrams show in Figures 1(b) and 2(b) have a rather peculiar shape.  The vertically running lines at either side depict the hot and neutral legs of the system.  If you stretch your imagination a bit, they look like the legs of a ladder.  Between them run the wires, power switch, and motor, and this horizontal running line represents the rung of the ladder.  More complicated line diagrams can have hundreds, or even thousands of rungs, making up one humongous ladder, hence they are commonly referred to as ladder diagrams.

     Next week we’ll take a look at two key elements in automatic control systems, the push button and electric relay, elements which allow us to do away with the need for human intervention.