The Programmable Logic Controller (PLC) has been one of the most evolving components of the automation industry, quick to take on changes that would enhance its capabilities and functionalities. PLCs have become completely assimilated into all major industries that require some sort of automation to complete their processes.

Relay Circuitry

The automotive industry in particular was the first one to adopt to PLCs and reap their advantages. Before these controllers, processes were usually carried out through relay circuitry, something that was a headache for operators. Even the slightest of change in the car model design forced engineers to modify the existing circuitry, which made the entire system too rigid. Contractor and relay controls were widely used for all types of control tasks, and were referred to as hard-wired controls. This meant that electricians had to go through great lengths to get the system working, everything a change was made. Starting with the design, electrical components had to be selected, installed and tested. Due to the complexity of the circuitry, any error took considerable amount of time to identify and then rectify. All this added to costs as well as lack of productivity.

What are PLCs?

But since the advent of PLCs, automotive industries have become flexible and are ready to take on all kinds of changes with extreme ease. PLCs are solid-state control devices that have the ability to monitor, control and automate electrical systems within industrial environments.

For simplicity, PLCs may be divided into three major portions:

• Input modules: these are capable of accepting digital or analog signals from devices such as sensors. The raw signals are converted into logical ones and sent to the CPU.
• Central Processing Unit: just like a computer, the PLC’s CPU makes all the decisions based on the inputs collected and executes any program stored in its memory.
• Output modules: these modules convert control instructions sent by the CPU into a digital/analog signal. The signal is sent to a connected device such as an actuator or stepper motor.

In addition to these major blocks, PLCs are also capable of accepting peripheral devices or communication cards through expansion slots.

PLCs may be seen as a programmable form of relay circuitry, eliminating most hard-wiring from the control system. Hard wiring is only existent at the output-end where field devices are placed. A PLC can be programmed using a vendor-specific language, usually ladder-logic and can be flashed at any instant.

Talking to Electrical Systems

The title of this very article states that PLCs can allow us to talk to electrical systems. How? PLCs use their own language for communication. It depends on the vendor, but usually all PLCs understand ladder logic. PLCs can be programmed to “sense” through their input modules, process the data into useful information, and then based on this information execute a specific software or hardware application. A better insight of this concept can be gained, once you’re familiar with the various components that make up the inputs and outputs of PLCs.

• Sensors: these devices are used to convert a physical parameter into an electrical one. The electrical signal may be digital or analog. Sensors connect to the PLC’s input. Examples of sensors include temperature sensor, pressure sensor, light sensor, etc.
• Actuators: these are output devices and connect to the output modules, acting on signals sent by the PLC. Actuators convert an electrical signal into a physical one, for instance starting up a motor or varying its speed, depending on the PLC’s output signal.
• Discrete Input: discrete input signals are those that are sent by digital sensory devices. These signals can take on either of the two forms, i.e. ON/OFF or High/Low. Devices capable of producing such signals include push buttons, limit switches, pressure activated sensors, etc. PLCs convert these signals into a stream of zero or one depending on the on/off condition.
• Discrete Output: just as discrete input signals have limited representation; discrete output signals can also be either ON or OFF. Devices that can understand discrete outputs include contractors, switches, relays, etc.
• Analog Input: analog input signals are constantly varying, taking on multiple, unexpected values. Examples of devices that generate analog signals include temperature sensor, intensity sensor, microphone, etc. Analog inputs usually consist of varying currents or voltages.
• Analog Output: PLCs that generate analog output are capable of driving analog devices such as transducers, motors, etc. The outputs usually vary in terms of DC voltage.

Controlled by the CPU, the inputs are processed according to programmed requirements, and output in the form of analog/digital signals, controlling the electrical system which may consist of relays, contactors, HVAC, renewable energy resources, and so on. PLCs are much more user friendly than the technology that was previously utilized. They are equipped with integrated diagnostics and override functions, that allow immediate documentation of all processes and changes going in within the PLC. At the end of the day, they provide unmatched scalability, safety and cost-effectiveness to any electrical system. 

Premier Automation offers industry specific technology solutions to help our customers increase their industrial productivity, efficiency and overall systems performance.

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