Posts Tagged ‘pushbutton’

Industrial Control Basics – Electric Relay Ladder Diagram

Sunday, January 22nd, 2012
     My daughter will be studying for her driver’s license exam soon, and I can already hear the questions starting.  “What does that sign mean?  Why does this sign mean construction is ahead?”  Symbols are an important part of our everyday lives, and in order to pass her test she’s going to become familiar with dozens of them that line our highways.

     Just as a triangle on the highway is a symbol for “caution,” industrial control systems employ a variety of symbols in their diagrams.  The pictures are shorthand for words.  They simplify the message, just as hieroglyphics did for our early ancestors who had not yet mastered the ability to write.  Ladder diagrams and the abstract symbols used in them are unique to industrial control systems, and they result in faster, clearer interpretations of how the system operates.  

     Last week we analyzed an electric circuit to see what happens when we put a relay to use within a basic industrial control system, as found in Figure 1.

Figure 1

     Now let’s see how it looks in an even simpler form, the three-rung ladder diagram shown in Figure 2.

 

Figure 2

     In industrial control terminology the electric relays shown in ladder diagrams are often called “control relays,” denoted as CR.  Since a ladder diagram can typically include many different control relays, they are numbered to avoid confusion.  The relay shown in Figure 2 has been named “CR1.” 

     Our ladder diagram contains a number of symbols.  The symbol on the top rung which looks like two parallel vertical lines with a diagonal line bridging the gap between them represents the N.C. contact.  This symbol’s vertical lines represent an air gap in the N.C. contact, the diagonal line is the relay armature which performs the function of bridging/closing the air gap.  This rung of the ladder diagram represents the contact when the relay is in its normal state.  

     In the middle ladder rung the N.O. contact symbol looks like two parallel vertical lines separated by a gap.  There is no diagonal line running through it since the relay armature doesn’t touch the N.O. contact when this particular relay is in its normal state.  The wire coil and steel core of this relay are represented by a circle on the bottom ladder rung.  The contact and coil symbols on all three rungs are labeled “CR1” to make it clear that they are part of the same control relay.

     Other symbols within Figure 2 represent the red and green bulbs we have become familiar with from our initial illustration.  They are depicted as circles, R for red and G for green, with symbolic light rays around them.  The pushbutton, PB1, is represented as we have discussed in previous articles on ladder diagrams.

     Just as road sign symbols are faster than sentences for drivers speeding down a highway to interpret, ladder diagrams are faster than customary illustrations for busy workers to interpret. 

     Next time we’ll expand on our electric relay by introducing latching components into the control system that will allow for a greater degree of automation. 

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Industrial Control Basics – Electric Relay Example

Saturday, January 14th, 2012
     When a starving monkey is faced with two buttons, one representing access to a banana, the other cocaine, which will he push?  The cocaine, every time.  The presence of buttons usually indicates a choice must be made, and electric relays illustrate this dynamic.

     Last week we looked at a basic electric relay and saw how it was used to facilitate a choice in electricity flow between two paths in a circuit.  Now let’s see what happens when we put a relay to use within a basic industrial control system making use of lit bulbs.

Figure 1

 

     Figure 1 shows an electric relay that’s connected to both hot and neutral wires.  At the left side is our pushbutton and the hot wire, on the right two bulbs, one lit, one not, and the neutral wire.  No one is depressing the pushbutton, so an air gap exists, preventing current from flowing through the wire coil between the hot and neutral sides.  With these conditions in place the relay is said to be in its “normal state.”

     The relaxed spring positioned on the relay armature keeps it touching the N.C. contact.  This allows current to flow between hot and neutral through the armature and the N.C. contact.  When these conditions exist the red bulb is lit, and this is accomplished without the need for anyone to throw a switch or press a button.  In this condition the other lamp will remain disengaged and unlit.

     Now let’s refer to Figure 2 to see what happens when someone presses the button.

Figure 2

 

     When the button is depressed the air gap is eliminated and the coil and wire become magnetized.  They will attract the steel armature closer to them, the spring to expand, and the armature to engage with the N.O. contact.  Under these conditions current will no longer flow along a path to light the red bulb because an air gap has been created between the armature and N.C. contact.  The current instead flows through the N.O. contact, lighting the green bulb.  It will stay lit so long as someone holds the button down.

     If our monkey were faced with the scenarios presented in Figures l and 2 and a banana was placed in the position of the red bulb, the cocaine in the position of the green, he might find that the regular delivery of bananas that takes place when the relay is in the N.C. contact position is enough to keep him happy.  In this state he might be so full of bananas he won’t want to expend the energy to engage the button into the N.O. contact position for the delivery of cocaine. 

     Next time we’ll revisit the subject of ladder diagrams and see how they are used to denote the paths of electric relays. 

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Industrial Control Basics – Electric Relay Operation

Monday, January 9th, 2012
     It’s a dark and stormy night and you’ve come to the proverbial fork in the road.  The plot is about to take a twist as you’re forced to make a decision in this either/or scenario.  As we’ll see in this article, an electric relay operates in much the same manner, although choices will be made in a forced mechanical environment, not a cerebral one.

      When we discussed basic electric relays last week we talked about their resting in a so-called “normal state,” so designated by industrial control parlance.  It’s the state in which no electric current is flowing through its wire coil, the coil being one of the major devices within a relay assembly.  Using Figure 3 of my previous article as a general reference, in this normal state a relaxed spring keeps the armature touching the N.C. switch contact.  While in this state, a continuous conductive path is created for electricity through to the N.C. point.  It originates from the wire on the left side, which leads to the armature pivot point, travels through the armature and N.C. contact points, and finally dispenses through the wire at the right leading from the N.C. contact.

     Now let’s look at an alternate scenario, when current is made to flow through the coil.  See Figure l, below.

Figure 1

     Figure 1 shows the path of electric current as it flows through the wire coil, causing the coil and the steel core to which it’s attached to become magnetized.  This magnetization is strong, attracting the steel armature and pulling it towards the steel core, thus overcoming the spring’s tension and its natural tendency to rest in a tension-free state.

     The magnetic attraction causes the armature to rotate about the pivot point until it comes to rest against the N.O. contact, thus creating an electrical path en route to the N.O. wire, on its way to whatever device it’s meant to energize.  As long as current flows through the wire coil, its electromagnetic nature will attract the armature to it and contact will be maintained with the N.O. juncture.

     When current is made to flow through the wire coil, an air gap is created between the armature and the N.C. contact, and this prevents the flow of electric current through the N.C. contact area.  Current is forced to follow the path to the N.O. contact only, effectively cutting off any other choice or fork in the road as to electrical path that may be followed.  We can see that the main task of an electric relay is to switch current between two possible paths within a circuit, thereby directing its flow to one or the other.

     Next time we’ll examine a simple industrial control system and see how an electric relay can be engaged with the help of a pushbutton.

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Industrial Control Basics – Pushbuttons

Monday, December 26th, 2011
     I always enjoy watching impatient people waiting for an elevator.  They press the button, and if it doesn’t come within a few seconds they press it over and over again, as if this will hurry things up.  In the end they must resign themselves to the fact that the elevator will operate in its own good time.

     Pushbuttons, although simple in appearance like the big, red “Easy” button that’s featured in a certain business supply chain’s commercials, are actually complex behind the scenes.  They perform important functions within the industrial control systems of a huge diversity of mechanized equipment.

     Last week we introduced ladder diagrams, used to design and document industrial control systems, and we’ll now see how they depict the action of pushbuttons within two commonly used industrial settings, the “normally open” and the “normally closed.” 

Figure 1

 

     Figure 1(a) shows a pushbutton hooked up to an electric motor.  When no one is pressing it a spring in the pushbutton forces the button to rest in the up position, allowing an air gap to exist in the electrical circuit between hot and neutral and preventing current from flowing.  This type of switch is characterized as a “normally open” switch in industrial control terminology.

     In Figure 1(b) someone depresses the button, compressing its spring and closing the air gap, which allows current to flow and the motor to operate

     Figure 1(c) shows the ladder diagram version of 1(a). 

     Now let’s take a look at Figure 2 to see a different type of pushbutton, one that’s  characterized as “normally closed.”

Figure 2 

 

     “Normally closed” refers to the fact that when no one is depressing the button, the normal operating position is for the air gap to be absent, allowing electrical current to flow and the motor to operate, as shown in Figure 2(a). 

     Figure 2(b) shows that an air gap is created when the button is depressed and the spring holding the mechanism into the normally closed position is forced down.  This action interrupts electrical current and causes the motor to stop.   

     Figure 2(c) shows the simplified line drawing version of 2(a).

     You can imagine how strained your finger would be if it had to press down on that button with any frequency or duration.  Next time we’ll see how electrical relays work alongside pushbuttons to give index fingers a break.

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