PLC SYSTEM ( Principle Of Operation PLCs)

Inside The PLC

The PLC mainly consists of a CPU, memory areas, and appropriate circuits to receive input/output data. We can actually consider the PLC to be a box full of hundreds or thousands of separate relays, counters, timers and data storage locations. Do these counters, timers, etc. really exist? No, they don't "physically" exist but rather they are simulated and can be considered software counters, timers, etc. These internal relays are simulated through bit locations in registers.

What does each part do?

INPUT RELAYS-(contacts)These are connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors.

INTERNAL UTILITY RELAYS-(contacts) These do not receive signals from the outside world nor do they physically exist. They are simulated relays and are what enables a PLC to eliminate external relays. There are also some special relays that are dedicated to performing only one task. Some are always on while some are always off. Some are on only once during power-on and are typically used for initializing data that was stored.

COUNTERS-These again do not physically exist. They are simulated counters and they can be programmed to count pulses. Typically these counters can count up, down or both up and down. Since they are simulated they are limited in their counting speed. Some manufacturers also include high-speed counters that are hardware based. We can think of these as physically existing. Most times these counters can count up, down or up and down.

TIMERS-These also do not physically exist. They come in many varieties and increments. The most common type is an on-delay type. Others include off-delay and both retentive and non-retentive types. Increments vary from 1ms through 1s.

OUTPUT RELAYS-(coils)These are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or triacs depending upon the model chosen.

DATA STORAGE-Typically there are registers assigned to simply store data. They are usually used as temporary storage for math or data manipulation. They can also typically be used to store data when power is removed from the PLC. Upon power-up they will still have the same contents as before power was removed. Very convenient and necessary!!

PLC Operation

A PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 important steps. There are typically more than 3 but we can focus on the important parts and not worry about the others. Typically the others are checking the system and updating the current internal counter and timer values.

Step 1-CHECK INPUT STATUS-First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on? How about the second input? How about the third... It records this data into its memory to be used during the next step.

Step 2-EXECUTE PROGRAM-Next the PLC executes your program one instruction at a time. Maybe your program said that if the first input was on then it should turn on the first output. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. It will store the execution results for use later during the next step.

Step 3-UPDATE OUTPUT STATUS-Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true. After the third step the PLC goes back to step one and repeats the steps continuously. One scan time is defined as the time it takes to execute the 3 steps listed above.

PLC SYSTEM ( History of The PLC )

THE HISTORY OF THE PROGRAMMABLE LOGIC CONTROLLER


The history of the programmable logic controller is not extensive. In fact, it goes back only as far as 1968.

The massed manufacturer of automobile (or, for that matter, automobile components) involves many machines, all of which must be controlled. Consider, for example , a boring machine. If the boring machine is used to bore a hole in the center a place of metal, then there must be some control mechanism that prevent the boring machine from operating until the piece of metal is aligned properly. Also, this control mechanism must actuated the boring machine when the metal is aligned and reverse the direction of the bit when is completed. Prior to 1968,this control function was performed by control relays.

Control relays were effective ,but they suffered from several disadvantages. To begin with, relays are only capable of on/off control, so in order to design complicated control systems, many relays are needed. This makes the relay control scheme quite expensive .Control relays can be bulky, so a control system requiring many relays takes up much floor and cabinet space.
Control relays are power-hungry, and this high power consumption result in heat generation. When a relay fails ,either through an opening of the coil or pitting of the contacts. It is difficult to troubleshoot and locate the failed relay. But worst of all, relays are hardwired. Any change in the control program requires the rewiring of relays. This is external costly ,both in terms of labor and plant downtime, and (in case of General Motors ) is of critical importance ,since relay control systems must be changed each year due to model year differences.

In the late 1960's PLC’s were first introduced. The primary reason for designing such a device was eliminating the large cost involved in replacing the complicated relay based machine control systems. Bedford Associates (Bedford, MA) proposed something called a Modular Digital Controller (MODICON) to a major US car manufacturer. Other companies at the time proposed computer based schemes, one of which was based upon the PDP-8. The MODICON 084 brought the world's first PLC into commercial production.

When production requirements changed so did the control system. This becomes very expensive when the change is frequent. Since relays are mechanical devices they also have a limited lifetime which required strict adhesion to maintenance schedules. Troubleshooting was also quite tedious when so many relays are involved.

Now picture a machine control panel that included many, possibly hundreds or thousands, of individual relays. The size could be mind boggling. How about the complicated initial wiring of so many individual devices! These relays would be individually wired together in a manner that would yield the desired outcome.

These "new controllers" also had to be easily programmed by maintenance and plant engineers. The lifetime had to be long and programming changes easily performed. They also had to survive the harsh industrial environment.

That's a lot to ask! The answers were to use a programming technique most people were already familiar with and replace mechanical parts with solid-state ones diagrams, instruction lists, C and structured text all at the same time! PC's are also being used to replace PLC’s in some applications. The original company who commissioned the MODICON 084 has actually switched to a PC based control system.

In the mid70's the dominant PLC technologies were sequencer state-machines and the bit-slice based CPU. The AMD 2901 and 2903 were quite popular in Modicon and A-B PLC’s. Conventional microprocessors lacked the power to quickly solve PLC logic in all but the smallest PLC’s.

As conventional microprocessors evolved, larger and larger PLC’s were being based upon them. However, even today some are still based upon the 2903.(ref A-B's PLC-3) Modicon has yet to build a faster PLC than their 984A/B/X which was based upon the 2901.
Communications abilities began to appear in approximately 1973. The first such system was Modicon's Modbus. The PLC could now talk to other PLC’s and they could be far away from the actual machine they were controlling.

They could also now be used to send and receive varying voltages to allow them to enter the analog world. Unfortunately, the lack of standardization coupled with continually changing technology has made PLC communications a nightmare of incompatible protocols and physical networks. Still, it was a great decade for the PLC!

The 80's saw an attempt to standardize communications with General Motor's manufacturing automation protocol(MAP). It was also a time for reducing the size of the PLC and making them software programmable through symbolic programming on personal computers instead of dedicated programming terminals or handheld programmers. Today the world's smallest PLC is about the size of a single control relay! The 90's have seen a gradual reduction in the introduction of new protocols, and the modernization of the physical layers of some of the more popular protocols that survived the 1980's. The latest standard (IEC 1131-3) has tried to merge plc programming languages under one international standard. We now have PLC’s that are programmable in function block.

A PLC (i.e. Programmable Logic Controller) is a device that was invented to replace the necessary sequential relay circuits for machine control. The PLC works by looking at its inputs and depending upon their state, turning on/off its outputs. The user enters a program, usually via software, that gives the desired results.

PLC’s are used in many "real world" applications. If there is industry present, chances are good that there is a plc present. If you are involved in machining, packaging, material handling, automated assembly or countless other industries you are probably already using them. If you are not, you are wasting money and time. Almost any application that needs some type of electrical control has a need for a plc.

For example, let's assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on for. We can do this with a simple external timer. But what if the process included 10 switches and solenoids? We would need 10 external timers. What if the process also needed to count how many times the switches individually turned on? We need a lot of external counters.

As you can see the bigger the process the more of a need we have for a PLC. We can simply program the PLC to count its inputs and turn the solenoids on for the specified time. This site gives you enough information to be able to write programs far more complicated than the simple one above. We will take a look at what is considered to be the "top 20" plc instructions. It can be safely estimated that with a firm understanding of these instructions one can solve more than 80% of the applications in existence. That's right, more than 80%! Of course we'll learn more than just these instructions to help you solve almost ALL your potential plc applications.

The need for low-cost, flexible and easily commissioned control system has resulted in the development of PLC-standard units based on a hardware CPU and programmable memory. Originally designed as a replacement for the hard-wired relay and timer logic control system found in control panels, PLC’s provide ease and flexibility of control using software and executing simple logic instructions (usually in ladder diagram form). PLC’s have internal functions such as timers, counters and relays, making sophisticated control possible even with the lowest range of PLC.

A PLC operates by reading the input signals from a process and carrying out logic instructions (which have been programmed and stored in its memory) on these input signals, producing output signals to drive output actuators. Standard interfaces built in to the PLC’s allow them to be directly connected to process sensors and actuators (e.g. proximity switches and solenoids) without the need for intermediate circuitry or relays.

Through the use of PLC’s it became possible to modify a control system without having to disconnect or redirect a single wire; it is only necessary to change the control program (software) using a programming console or visual display unit terminal. PLC’s also require require much shorter installation and commissioning times than hardwired control systems.

Although PLC’s are similar to conventional computers n terms of hardware architecture, they have specific features suited to industrial control :

1) More rugged and has noise immune capability
2) Modular approach in construction, allowing easy replacement/addition of units (e.g

input/output (I/O)
3) Standard I/O connections and signal levels.
4) Easy to understand programming language (e.g. ladder diagram)
5) Easy to program and reprogram

The above features make PLC’s highly desirable in a wide variety of industrial-plant and process control applications.