Short Control Logic Tutorial For Beginners

First, we have to start by saying that PLC, or programmable logic controller, is a compact computer that features specific operating system. This particular operating system is highly optimized and specialized to handle most operations in real time.

You probably know that PLC has input lines, and sensors can connect with them to notify you on different issues such as when the temperature reaches a certain level. It also features actuators that connect to output lines with the main idea to signal or affect reactions to specific and incoming events such as an open valve, start an engine, etc.

You can check this by entering on Allen Bradley PLC training, and we decided to present you a slight introduction:

PLC Program Languages

You probably know that system is completely user programmable and it features language called Relay Ladder Logic or Relay Ladder. The name will present you the idea from the early days when PLCs came from relays.

The most common languages used for PLC programming are:

  • Functional block diagram
  • Sequential function chart
  • Structured text
  • Instruction list
  • Continuous function chart

Brief History On PLCs:

Let us start from the beginning. PLC or programmable logic controller is a digital computer used in industrial processes to handle automation. You can use it to control machinery on assembly lines, light fixtures, and even amusement rides.

You can find it in many industries, which means that PLC comes with multiple arrangements of both analog and digital inputs and outputs as well as immunity to electrical noise, possibility to withstand harsh temperatures and resistance to impact and vibration due to the robust casing.

Software that controls machine operation features in non-volatile and battery-backed-up memory. It is a great way to produce real-time output results promptly.

Before PLC became popular, safety interlock logic, as well as sequencing and control in automotive industry, features cam timers, relays, dedicated closed-loop controllers and drum sequencers.

More information on PLC history, you can check by clicking here.

This was the problem for manufacturers, because that was expensive and time-consuming, especially if you want to update facilities with latest models each year. At the same time, the change was frustrating because electricians had to rewire everything to change its characteristics individually.

That is the main reason why industries started to implement general-purpose programmable devices in the form of digital computers that will conduct control of processes. Early computers were perfect for checking temperature, power quality, and cleanliness.

The industrial computer is different from general population devices because:

  • It can easily tolerate the harsh environment
  • It supports bit-form output and input which means that you can extend it further
  • You don’t need years of training to use it
  • It operates real time

The idea is to get a computer that will have a fast response time so that you can control it in real time. Of course, speed depends on the nature of the process, but if you have in mind that most industrial processes use timescale, you will be able to check the response measured in milliseconds.

The Largest Industry That Uses PLCs Is Still Automotive Industry

The main idea of first PLCs was to replace relay logic systems. They feature ladder logic language, which contains similar schematic diagram used in relay logic. The best thing about it is the possibility to understand everything without extensive training demands.

On the other hand, today you can program PLC in various ways from specially adapted forms of C or BASIC to relay-derived ladder logic. Early models didn’t have programming terminals that could provide you an entire graphical representation of logic.

The main problem with relays was limitations of this particular type of control such as:

  • Relays are not that durable
  • You will get delay due to on/off sequence
  • You will need too much space for relays
  • Maintenance is challenging and requires lots of time

PLC can overcome these problems, and due to the machine-controlled operation, you will not need to hassle much to work with it. The latest change in the world of PLC created more intuitive and intelligent machines that can easily connect to the network and communicate with other computing devices.

If you want to find more on the usage of PLCs in the automotive industry, you have to visit this particular website:

Basic PLC Components

As we all understand by now, the purpose of PLC is to control and read analog and digital inputs from a wide array of sensors. It uses the input information to execute the logic program and to write results by using analog and digital output values for different output elements such as pneumatic actuators, solenoid coils, indication lamps and many more.

The process of scanning depends on the manufacturer, but most of them use a similar concept from left to right.

  • Overhead – Overhead uses testing input/output module It will later verify whether program changed and if pc hasn’t locked up. It includes the possibility to communicate with other software and other external devices such as Human Machine Interfaces (HMIs).
  • Input Scan – You will get a snapshot of analog and digital values to the input memory
  • Logic Execution – After getting a snapshot, it will scan a program element by element, which will finally lead to results on output memory table.
  • Diagnosis and communication – This particular feature is common for different disciplines so that you can determine cause and effect. It is mostly used in computer science and engineering to determine causes of mitigations, symptoms and to provide a solution in case of a mistake.
  • Output Scan – It is final result that will show you memory table inside the output models.

As soon as scan completes, the process will repeat itself until you power it down. You should have in mind that the time needed to complete one scan cycle we know as scan cycle time, and it can range from hundreds of milliseconds to a few milliseconds on most advanced PLCs.