What are Programmable Logic Controllers / Programmable Automation Controllers?
In any industrial automation application there are a variety of points being measured and controlled with each point playing an important role in maintaining
safety, quality, and efficiency throughout the entire application. Programmable logic controllers (PLCs) and programmable automation
controllers (PACs) are industrial control systems that continuously monitor these input measurements and use that information to control the state of
output devices based upon a custom logic program. Put simply, PLCs and PACs are specialized industrial computers used to control machines and processes.
The ability of PLCs and PACs to collect vital information and to change the process based on this information makes them ideal for almost any production line,
machine function, or process that requires high reliability control, ease of programming, and process fault diagnosis.
PLCs and PACs can be sized to fit just about any application, from small “bricks” which contain the central processing unit along with tens of inputs/outputs,
to large rack-mounted modular devices with thousands of inputs/output, and which are often networked to other PLC and SCADA systems. The ease with which PLC/PAC-based
control systems can be scaled and the ability to mix and match the type of input devices best suited for a particular application are major advantages to installing
such a system.
What are the differences between PLCs and PACs?
PLCs and PACs are very similar to one another in both form and function. They are similar enough, in fact, that PACs are often simply called PLCs, a more familiar term.
Despite the similarity, there are some notable differences between the two.
Generally speaking, PACs are better equipped to integrate with the outside world. This integration allows for remote access to your application and provides analytical
insights that are not available through a standard PLCs. PACs are also equipped with a more robust selection of programming paradigms.
Despite the brawn of a PAC, it is not always the best choice for an application. There is no need to invest in a high-powered PAC to run machinery that requires simple
programming. It all comes down to the complexities of the operation. For complex, large-scale automation applications, a PAC is going to be the best option but for smaller,
simpler operations, a PLC is often a better, and more cost-effective solution.
How Do PLCs and PACs work?
Components
Both PACs and PLCs need a few essential components to create a working system. For small-scale applications, brick-type PLCs and PACs generally include all these components
in a single package. Larger, modular units are required for large-scale applications.
The brains of the system is the CPU which controls the application by communicating with all connected devices and executing the logic program.
Input Modules interface with external sensors to provide the raw data used by the PLC/PAC prior to executing the logic program. PLCs and PACs can make
decisions based on proximity, temperature, humidity, pressure, flow, time, level, gas concentration, and operator input. Depending on the modules, accepted inputs include
but are not limited to discrete switches, voltage, current, thermocouple, RTD, motion, DC, AC, high speed counters, resistance, Modbus, Ethernet, serial, HART, and others.
Output Modules interface with the output devices that perform the operations indicated by the logic program. Output devices include valves, motor starters,
solenoids, actuators, horns/alarms, lights, relays, counters/totalizers, pumps, fans, heaters, coolers, and other devices.
Networking modules are typically added to expand to distributed I/O and other devices such as HMIs and SCADA systems.
The system must also include a power supply.
Order of Operations
Some basic steps in the operation of all PLCs and PACs include: Input Scan, Program Scan, and Output Scan. These steps continually take place in a repeating loop.
The Input Scan detects the state of all input devices that are connected to the PLC or PAC. Once there are values for all input devices, the Program
Scan executes the user-created program logic. Based upon the program logic, the Output Scan energizes or de-energizes all output devices connected to the system.
One cycle through these steps is called the Scan Time. The Scan Time is very fast, typically taking just a few milliseconds.
Programming Language
The key to PLCs and PACs is in the programming. In the hands of a skilled programmer, the full power of a PLC or PAC can be unleashed.
There are a number of programming languages that can be used for PACs and PLCs including Ladder Logic, Function Block Diagram, Structured Text, Instruction List, and C-Block.
The choice of which language to use is largely one of preference as they all, generally, are capable of providing a similar level of control over the process. The variety
of programming languages simply allows multiple ways of executing the necessary instructions with none “more correct” than another. Familiarity with the language is usually
the overriding factor though particularly complex programming many benefit from using a specific language. That being said, Ladder Logic is the most common programming
language used for basic PLCs.
PLCs and PACs are programmed using application software on personal computers. The computer is connected to the PLC or PAC through USB, Ethernet, RS-232, RS-485, or RS-422 cabling.
Things to Consider When Selecting a PLC or PAC:
- What process is being automated?
- Is this a new installation, the upgrade of an existing system, or a bit of both?
- How much I/O is required?
- What types of input is provided by the sensors?
- What type of signals are needed on the output to enable control?
- Do I need network connectivity and can it be added to my PLC?
- Is redundancy required?
- Are there any size, space, or environmental factors to consider?
- Who will install the system?
- Who will program the PLC or PAC?
If you have any questions regarding PACs or PLCs, please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
