Posts Tagged ‘industrial control’

Industrial Control Basics – Introduction to Electric Relays

Tuesday, January 3rd, 2012
     I’ve always considered science to be cool.  Back in the 5th grade I remember fondly leafing through my science textbook, eagerly anticipating our class performing the experiments, but we never did.  For some reason my teacher never took the time to demonstrate any.  Undeterred, I proceeded on my own.

     I remember one experiment particularly well where I took a big steel nail and coiled wire around it.  When I hooked a battery up to the wires, as shown in Figure 1 below, electric current flowed from the battery through the wire coil.  This set up a magnetic field in the steel nail, thereby creating an electromagnet.  My electromagnet was strong enough to pick up paper clips, and I took great pleasure in repeatedly picking them up, then watching them unattach and fall quickly away when the wires were disconnected from the battery.

Figure 1

 

     Little did I know then that the electromagnet I had created was similar to an important part found within electrical relays used in many industrial control systems.  An example of one of these relays is shown in Figure 2.

Figure 2

 

     So, what’s in the little plastic cube?  Well, a relay is basically an electric switch, similar to the ones we’ve discussed in the past few weeks, the major difference being that it is not operated directly by human hands.  Rather, it’s operated by an electromagnet.  Let’s see how this works by examining a basic electrical relay, as shown in Figure 3.

 Figure 3

 

     The diagram in Figure 3 shows a basic electric relay constructed of a steel core with a wire coil wrapped around it, similar to the electromagnet I constructed in my 5th grade experiment.  If the coil’s wires are not hooked up to a power source, a battery for example, no electric current will flow through it.  When there is no current the coil and steel core are not magnetic.  For purposes of our illustration and in accordance with industrial control parlance, this is said to be this relay’s “normal state.”

     Next to the steel core there is a movable steel armature, a kind of lever, which is attached to a spring.  On one end of the armature is a pivot point, on the other end is a set of electrical switch contacts.  When the relay is in its normal state, the spring’s tension holds the armature against the “normally closed,” or N.C., contact.  If electric current is applied to the wire leading to the pivot point on the armature while in this state, it will be caused to flow on a continuous path through the armature and the N.C. contact, then out through the wire leading from the N.C. contact.  In our illustration, since the armature does not touch the N.O. contact, an air gap is created that prevents electric current from traveling through the contact from the armature.

     Next week we’ll see how these parts come into play within a relay when electric current flows through the coil, turning it into an electromagnet.

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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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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.

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Industrial Control Basics

Sunday, December 4th, 2011
     When I was a child in school I loved field trips.  They didn’t happen too often, but when they did they were a welcomed break from the routine of the classroom. Once we went on a tour of a large factory that made telephones.  During the tour we walked amongst gargantuan machines, conveyor belts, furnaces, boilers, pumps, and compressors, all energized and working together to transform raw materials into telephones.  Sequences of manufacturing and assembly operations, from the simple to the most complex, were carefully orchestrated with no apparent human intervention.

     The equipment in the telephone factory was certainly impressive to watch, and our tour guides did a fine job of explaining what was happening, except for one important detail.  I realized after we left that no one had explained who or what was actually controlling the machinery.  I realized even then that machines can’t think for themselves.  They can only do what humans tell them to do.

     I didn’t know it at the time, but the telephone factory setup included some interesting examples of industrial control systems.  Industrial control systems can be broken down into two basic categories, manual controls and automatic controls.  Manual controls work as their name implies, that is, someone must manually press a button or throw a switch to initiate factory operations.  This involves continual monitoring of processes, coupled with hands-on activities to keep everything working.

     Automatic controls still require human intervention to some extent, such as initiating operations, but once that’s done they move into self-regulation mode until the operations are shut down at the end of production.  Employees are thus freed up to spend time doing things which are not automated.  Automatic controls are excellent at handling mundane, repetitive tasks that humans tend to get quickly bored with.  Boredom leads to a lack of attention, and this may lead to accidents, so utilizing automatic controls often makes for a safer work environment.

     Next time we’ll begin our examination of how manual and automatic controls work within the context of an industrial setting.  To begin, we’re going to take a virtual field trip back to the telephone factory and look at some basic industrial control examples.

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