As technology advanced throughout the years, there came a need for a device that would help facilitate the industrial process; something reliable that would help control the manufacturing of goods on assembly lines, something that would be reliable and offer process fault diagnosis. Out of this need was born the programmable logic controller. A programmable logic controller (or PLC) is a specialized computer used to control machines and process. It has a programmable memory that works to store instructions and specific functions. Read on to find out more about PLC’s, their history and how they come in handy in today’s world.
A PLC is an industrial computer control system that monitors the state of input devices and makes decisions based on a custom program which controls the state of the output devices. It can be used in a production line or in the functioning of a machine.
The History of Programmable Logic Controllers
Technology is constantly advancing. We are always finding issues that need improving and working on coming up with ways to make things run more smoothly. Before the invention of PLCs, those using machinery noticed that hardwired panels were very difficult to work with, making it time consuming to wire, debug and change them out. When it got to a point that the necessity to replace them was apparent, General Motors noted some requirements for the computer controllers that would replace the hard wired panels. These included that they be solid state and not mechanical, would be easy to modify output and input devices, be easily programmed and maintained and be able to function well in an industrial environment.
IN 1969, the first PLC model, the 084, was invented by Dick Morley. It offered the same functionality as the existing relay logic systems but was programmable, reusable and reliable. It could withstand a harsh industrial environment, used a battery backup, had no hard drive, could start in seconds and used Ladder Logic for programming.
Advantages of PLCs
Programmable Logic Programmers have several advantages over hard drives. These are as follows:
- They eliminate much of the hard wiring that was necessary with conventional relay control circuits. The program PLCs use takes the place of much of the wiring that was previously needed.
- They are more reliable than the equipment that was previously used. This is due to their ability to be programmed. Once a program has been written and tested, it can be downloaded to other PLCs. And, since all the logic is contained in the unit’s memory, there is no chance of making a logic wiring error.
- PLCs are also more flexible. Updates can be made easily by having the original equipment manufacturers send out a new program. This is simpler than wiring or rewiring a circuit, a previously used method for updates. End users can modify the program in the field as necessary.
- PLCs also cost less than the relay control logic they replaced. In fact, savings have been so considerable that relay control is becoming obsolete, except for in power applications. Generally, if an application requires more than 6 control relays, a PLC will be a more affordable option.
- Programmable Logic Controllers are also offers better communication, having the ability to communicate with other controllers or computer equipment. This ability allows them to be networked to perform functions including supervisory control, data gathering, monitoring devices and process parameters and the downloading and uploading of programs.
- PLCs work in real time meaning they are an overall faster system. It also means that an event that takes place in the field will result in an operation or output taking place within the PLC. This is convenient for machines that process thousands of items per second and objects that spend only a fraction of a second in front of a sensor.
- Programmable Logic Controllers are also easier to troubleshoot making maintenance and repair simpler. This is due to its resident diagnostic and override functions which allow users to trace and correct software and hardware programs easily. PLCs have a control program that can be watched in real time as it works to find and fix problems.
Parts of a Programmable Logic Controller and How It Operates
To summarize how a PLC operates, it contains a Central Processing Unit (CPU) which tells the PLC how to execute Control Instructions that are in the User’s Program. The program is stored in ‘nonvolatile’ memory ensuring that the program will not be lost if power is removed. The CPU also helps the PLC communicate with other devices including I/O Devices, Programming Devices, Networks and other PLCs. The CPU also helps it perform general activities like communications and internal diagnostics.
There are four basic steps that occur in the PLC to keep it operating. These are as follows:
Input scan: Detects the state of all input devices connected to the PLC
Program Scan: Executes the user created program logic
Output Scan: Energizes and de-energizes all output devices connected to the PLC.
Housekeeping: Includes communications with programming terminals, internal diagnostics, etc.
Of course, a PLC has a variety of parts that help it operate. Read on to find out what these parts are and how they help the device to function.
Power Supply: Just like any other device that relies on electricity, Programmable Logic Controllers have a power supply. These supply DC power to other modules that plug into the rack. In large PLC systems, the power supply usually does not supply power to the field devices. In smaller systems, the power supply will be used for this purpose.
Processor: The processor is considered the “brain” of the PLC. It consists of a microprocessor which implements the logic and controls the communication between the modules. It is designed so that circuits can be entered in relay ladder logic form. It accepts input data from various sensing devices, executes the stored user program and sends output commands to control devices.
I/O Section: Programmable Logic Controller contains an I/O section, consisting of input and output modules.
- Input Modules: These form the interface where the input field devices are connected to the controller. “Field” and “real world” are terms that refer to the external devices that must be physically wired to the system.
- Output Modules: These form the interface where the output field devices ae connected to the controller. PLCs use an optical isolator that works by using light to electrically isolate the internal components from the input and output terminals.
Programming Device: A personal computer is the most common type of programming device used with a programmable logic controller. The software in the computer is responsible for all for the creating, editing, documenting, storing and troubleshooting of the program. It communicates with the PLC using a serial of parallel data communications link.
Hand held programming devices can also be used to program small PLCs. They are compact, inexpensive and easy to use but do not have the capability to display as much logic on the screen as a computer monitor does. They are often used on factory floors to troubleshoot, modify programs and transfer programs to multiple machines.
PLC Mixer Process Control Problem: The PLC contains a mixer motor which automatically stirs the liquid in the devices process vat when temperature and pressure reach preset values. There is also an alternate manual pushbutton control of the motor. The temperature and pressure sensor switches close their respective contacts when conditions reach their preset values. The motor starter coil is energized when the pressure and temperature switches are closed or when the manual pushbutton is pressed.
Things to Consider If You Are In the Market for a Programmable Logic Controller
If you are in the market to purchase a programmable logic controller, there are some things you should consider before making a decision. These are as follows:
- Whether the system will be powered by AC or DC voltage.
- Whether the PLC has enough memory to run the user program you are using
- Whether the system runs fast enough to meet your application’s requirements
- What type of software you require
- Whether the PLC will be able to manage the number of inputs and outputs required
- If the PLC can handle analog inputs and outputs as needed
- How you are going to communicate with the PLC
- Whether you need network connectivity and, if so, whether this can be added to the PLC
- Whether the system will be located in one place of whether it will be spread out over a large area
Once you have determined these things, you may want to familiarize yourself with basic acronyms that are commonly used in reference to your PLC as well the programming language that will be used. Though Ladder Logic is still the most common PLC programming language, it is not the only one. Other programming languages include Function Block Diagram (FBDI), Structured Text (ST), Instruction List (IL) and Sequential Function Chart (SFC).
Programmable logic controller has made great strides in the way businesses operate and in technology in general. They have become a necessary component in the ease of use of machinery. If you are in business in a factory setting and do not have a PLC, you may want to look into how you can use one of these devices to increase your productivity.