In the Olden days industrial automation was done with complex hardwired Relay base control system.
These relay-based control systems:
1. Consumed a lot of power,
2. Had huge wear& tear and
3. Space needed to create even simple automation was problematic.
4. Thousands of relays could be necessary to automate a simple factory process.
5. The hardware nature made it difficult to alter the process. Changes would require rewiring and careful updating of the documentation.
6. If even ONE wire was out of place or one relay failed, the whole system would become faulty.
In 1968 the programmable logic controller or PLC was developed by General Motors to replace complicated relay circuits in the industrial plants. PLC provided several advantages over earlier automation systems.
A Programmable logic controller or PLC is an industrial computer that accepts real-time parameters from various field instruments and operate the output device based on the logic program.
Now let’s see an example in which is a switch is supposed to control the operation of a motor.
In a traditional control system, the switch would be directly wired to the motor.
In a PLC system the switch and the motor would be wired to the PLC. Now the PLC’s control program would complete the electrical circuit between two, allowing the switch to control the motor.
Now lets consider that we need the motor to run exactly after 5 minutes when the switch is on. In traditional Circuit we would need to connect a timer and do some rewiring.
Now Consider the same condition in a PLC system, in the PLC system by making a slight change in the PLC control program, we can make the motor run exactly after 5 minutes of switching on the switch.
This is one of the important features of the PLC. If you want a device in a PLC system to behave differently or to control a different process element all you have to do is change the control program.
In a traditional control system making this type of change would involve physically changing the wiring between the devices which is costly and time consuming.
Main components of a PLC.
PLC comes in many shapes and sizes, understanding the hardware is the first step of working with PLC.
Rack is the backbone of all PLC systems. The rack provides the necessary mounting to hold all the models that will reside in it. Depending on the need of control system it can be ordered in different standard sizes to hold more modules.
The Rack has a Backplane which allows the modules to be plugged in and
When a Power supply is added it will pass power to another modules. When a CPU module is added it supplies a data link allowing CPU to communicate with the modules inserted.
Power supply module is the power house that energizes the PLC to carry out its function. The power supply module provides power to the PLC by converting the available incoming AC power to DC power as required by the modules to operate properly
This DC power is transferred across the backplane of the rack. Usually in bigger PLC’s power supply module will be a separate module. While in small and medium PLC’s power supply module will be already attached to the rack.
Central processing unit or CPU:
CPU is a microprocessor-based system which receives the data from other models and makes decisions based on the logic programs stored in it by the user.
The CPU module can be divided into three sections. The processor section, the memory section and other integrated circuits to control monitoring and communications. The processor section is where the data from the other modules is processed and makes the decision needed by the PLC.
The memory section provides storage location to store programs without losing information when turning OFF machines. Communication section allows CPU to communicate with another module.
Every PLC’s must have some means of receiving information from sensors, switches and other instruments installed in the plant for this input models are used. The module which interacts with input signal is called Input Module. These modules are information providers to CPU in the PLC.
Usually, PLC’s have two types of inputs. They are Digital Inputs and Analogue Inputs.
Based on this PLC input modules are classified into :
Digital Input module and Analogue Input Module.
Sinking and Sourcing Concept in PLC:
Sinking and sourcing is a very important concept related to PLC modules. We have already made a CHAPTER on this.
Digital input module or DI module or Discrete input module:
Digital input module is the module which is used to connect digital input to the PLC.
Let’s consider a switch, the switch has only two states, that is either will be ON, or it will be OFF. This type of inputs which has only two states is known as Digital Inputs.
In Digital Circuits these two states are represented by logic 1 and logic 0. Generally a logic 1 represents a high voltage, while logic 0 represents a low voltage.
For an input module of 24v DC, 0V DC acts as OFF state, while 24VDC acts as ON state. In order for a PLC to be aware of a digital sensor state, PLC must receive a high or low voltage from the sensor through digital input module. This type of inputs can be from switches, push buttons, limit switches, or proximity sensors
Analogue Input module.
Now let’s considered that we need a continuous measurement of level of liquid in a tank of height 10 meter.
For this level transmitter is installed on the tank. As we know the output of the level transmitter will be a continuous signal representing the real-time level of liquid in the tank. At any point of time, level of liquid can be any value between 0 and 10m.
So, the output of the transmitter cannot be represented by two States like a digital signal. So, a continuous signal called analogue signal is used. The module which deals with analogue inputs from the plant is known as Analogue input module.
As discussed, Analogue input is a continuous input from the field to the PLC whose value continually varies over time. The commonly used analogue signals are 4-20 ma current signal and 0 to 10 VDC voltage signal.
As you know the PLC is an electronic device which deals with digital values, so the analogue input module must have an Analogue to Digital converter.
Now the CPU of our PLC has information from instruments in the plant through digital input module and analogue input module.
As the CPU has user designed logic program, CPU runs this program using the real-time information from the input modules and decides corrective actions which should be taken in the plant to achieve the target process.
For this PLC must have some means of connecting and sending the signals to the output devices like solenoid valves and motors, For this Output Module are used.
Like input modules, output modules also mainly of two types, Digital output module and Analogue output module.
Analogue Output Module
The Analogue output is a continuous output from the PLC to the field devices.
For example, to control the speed of a motor or brightness of a light, PLC use analogue output signal.
PLC analogue output module contains digital to analogue converter to convert PLC’s digital commands into analogue electrical quantities.
Digital Output Module
Digital Outputs at logic 0 and logic 1 output from the PLC. It is a process control output from the PLC to the field. It is usually used to ON or OFF field instruments.
Examples are motors, lights etc that needs just be switched ON or OFF.