Block Diagram of PLC
PLC comprises of all above components as shown in figure 3-1 in block diagram of PLC, like Input card, output card, CPU (Central Processing unit), Power supply card (SMPS) and programming card.
There are some PLC’s which possess all components and some PLC’s are of such type which do not possess all components, we have to assemble it in a Rack.
Classification of PLC
On the basis of assembly of all components PLC can be classified as-
1. Rack Type.
2. Brick/Shoe box Type.
Description is given below-
1. Rack Type
Most medium to large PLC systems are assembled within a rack which contains
individual components like CPU, Input/Output card, programming card and Power Supply
card(SMPS).
Fig.3-2 and Fig.3-3 of Rack Type are given below-
2. Bricks/Shoe Box Type
In smaller PLC systems, all of these components may be contained in a single housing or "brick" which are sometimes referred to as "bricks type" or "shoebox type” PLCs
PLC can also be classified on the basis of Size which is described below-
1. Compact/Micro PLC- It covers up to 128 I/O’s and memories up to 2 Kbytes.
2. Modular- It covers up to 1024 I/O’s and memories up to 32 Kbytes.
3. Extreme- It covers up to 8000 I/O’s and memories up to 750 Kbytes.
Compact or Micro Series comes under the Bricks type PLC. Modular and Extreme Series
comes under the Rack type PLC. Some manufacturer like Mitsubishi classify the PLC in two
types-i.e. Micro and Modular, where Micro PLC covers upto 128 I/O and Modular PLC
covers upto 8000 I/O.
Description of PLC Components
POWER SUPPLY
Provides the voltage needed to run the PLC components, It is explained in detail at the
last of this chapter.
I/O MODULES
Provides signal conversion and isolation between the internal logic level signals inside the
PLC and the field’s high level signal.
- The I/O interface section of a PLC connects it to external field devices.
- The main purpose of the I/O interface is to condition the various signals received
from or sent to the external input and output devices.
- Input modules converts signals from discrete or analog input devices to logic levels
acceptable to PLC’s processor.
- Output modules converts signal from the processor to levels capable of driving the
connected discrete or analog output devices.
Here first We are describing the discrete or digital module/card, analog module has described
in analog programming section of this book.
DC INPUT MODULE
signal. Hence we can say it provides isolation between external field devices to CPU that’s
why it is also called optoisolator. Following figure is showing the IC which is performing the optoisolation,
PROGRAMMING DEVICE
The PLC is programmed using a specialty programmer or software on a computer that
can load and change the logic inside. Most modern PLCs are programmed using software on a PC or laptop computer. Older systems used a custom programming device which has their memory.
Memory of a PLC can be of two types namely: -
1. VOLATILE
2. NON-VOLATILE
Volatile memory loses its contents when power supply is turned OFF, whereas non-volatile
does not.
PLC will use non-volatile memory from a majority of user’s memory because the
program must be retained during the power off. It is imported that all non-volatile memory in a PLC uses some of the error checking to ensure that the memory has not changed. The other memories used in PLCs are
Description of various input devices
On the machine side the different input components are used. These input components are connected to input cards (modules) of PLC for feeding input signals to the CPU of PLC.
There are two types of input devices one is digital input and other is analog input devices.
As per ISA standard digital input signal is 24 V DC across the input terminal, typically it’s
having two states on or off.
Whereas analog signals provide a varying voltage, current or resistance value. As per ISA
standard analog input signal is 0 to 10V or 4-20mA. Some of the analog input components
are:
-Thermocouple (TC)
-Resistance Temperature Detector (RTD)
-Strain gauges
-LVDT
-Pressure Sensor
-Level Sensor etc.
In this chapter we will understand digital sensor, analog sensor will be covered in analog programming sections.
Some of the Digital input components are –
Push Buttons
Toggle switches
Emergency Switches
Limit Switches
Level Switches
Pressure Switches
Selector Switches
Temperature Switches
Sensors
Flow Switches
Proximity Switches
Read Switches
Float Switches
Input components are described as follows-
are of two types-
1. NO Push Button- When we press this push button from upper head signal (+24V)
goes to PLC & when we release the button , due to spring based mechanism , signal
goes to PLC is Zero.
2. NC Push Button-At normal condition PLC is getting +24V , as we press this push
button signal goes to PLC is zero and after releasing the switch PLC is again getting
+24V.
Emergency Switches/ Mushroom type push button
These are also called mushroom type push buttons, these are always NC type. These
are hand operated switches, in emergency when we press this switch it disconnects the whole supply from PLC and when we release our hand from button supply remains disconnected.
For re-establishing the connection of PLC we have to rotate clock-wise and release it.
Limit Switches
The switch body includes electrical contacts to energize and de-energize a circuit.
The operating head incorporates some types of lever arms or plunger which is also
referred as Actuator.
The standard limit switch is a mechanical device that uses physical contact to detect
the presence of an object (Target). When the target comes in contact with actuator, the
actuator is rotated from its normal position to operating position. This mechanical operation
activates the contacts within the switch body
Pressure Switches
Pressure Switches
These are input components used as a field device installed on the machine and are
connected with input module of PLC. Pressure switches are actually used to control the
pressure and have NO & NC switch over type of contacts, which changes the state when a
preset pressure threshold value is reached.
Temperature Switches
Thermostats are the devices which have a contact that is NO & NC. When the
temperature reaches the threshold value the contact changes the state from NO to NC. When the temperature value falls below the threshold value the contact resume its initial position.
Thermostats are widely used in refrigerators and in temperature controlling machines and
heating installations.
Proximity Switches/Sensors
Sensors are the components just like limit switches whose internal contact will
become ‘NO’ to ‘NC’ when it sense (detect) some object. The major difference between
mechanical switches and proximity sensors are that sensors actuated without any physical
contact with the object.
Digital Sensors Classification
The schematic symbols for the discrete sensor are drawn as limit switches in a diamond shaped box. In figure 3-16 symbols (a) and (b) are respectively N/O and N/C sensors.
