Nowadays Industrial automation technology plays an important role in Manufacturing industries to meet Quality and Productivity. Industrial automation technology uses advanced & Robust embedded controllers like PLC or DCS to control the process in a predefined & desired manner.
Note: Both PLC & DCS are the types of Industrial automation controllers, which share almost the same working principles. However, the type of controller is selected based on the nature of the system. In short, PLCs for Discrete controls & for Small/Medium-sized applications and DCSs for Continues controls & Large sized applications. In this blog, we are going to see a PLC-based control system.
The following diagram compares Human body parts with PLC based control system for a better understanding of the PLC’s basic concept.
The eye sees (senses) the surrounding sends the signals to the brain, brain manipulates, and gives the commands to the Hands or Legs to make the necessary movements corresponding to the scene which sensed by the eye.
We can compare,
– Eye to Sensor, which gives the Input to the PLC. Inputs are necessary to know the status of the control system to make the correct/required action.
– Brain to the CPU, which make calculation according to the Logic made for the proper/required action to be carried out.
– Hands/Legs to the Actuators, which acts based on the command from the CPU.
PLC Architecture
Simple PLC Architecture consists of the followings,
CPU – Central Process Unit, makes all the necessary calculations to achieve the desired control system.
Input Module – Input devices reads/senses the status of control system and sends the signals to the CPU via Input modules
Output Module – CPU sends the Desired calculated outputs to the output devices via output module.
- Input Devices are such as Limit Switches, RTD, Thermocouple, Pressure transmitters, Proximity sensors, level Transmitters, etc.,
- Output Devices are such as Solenoid Valves, Control Valves, Motors, Pumps, Fans, etc.,
Look into the following example of the Tank Level Control System.
Above Control system consists Solenoid valve, High Level switch, Low Level Switch and Pump to maintain the tank level.
– Let us assume the sequence of operation as follows,
- If the Tank level is below the high level (sensed by LSH) then the Solenoid valve opens and water flows into the tank.
- If the Tank level is above the Low level (Sensed by LSL) then Pump starts and water drains from the tank.
In this example.
LSL and LSH are Input devices – These sensors are used to give the status of the Tank to the PLC via Input Modules.
Solenoid valves and Pumps are Output devices – These are getting commands from the PLC via Output modules I.e. makes the action as per the sequence programmed in the PLC.
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