1. What is control system
A control system is a system, which provides the desired response by controlling the
output. ... Traffic lights control system is an example of control system. Here, a sequence
of input signal is applied to this control system and the output is one of the three lights that
will be on for some duration of time.
What is a PLC?
PLC stands for “Programmable Logic Controller”. A PLC is a computer specially designed to
operate reliably under harsh industrial environments – such as extreme temperatures, wet,
dry, and/or dusty conditions. PLCs are used to automate industrial processes such as a
manufacturing plant’s assembly line, an ore processing plant, or a wastewater treatment
plant.
Types of PLCs
The two main types of PLC are fixed / compact PLC and modular PLC.
Compact PLC
Within a single case, there would be many modules. It has a fixed number of I/O modules and
external I/O cards. So, it does not have the capability to expand the modules. Every input and
output would be decided by the manufacturer.
Modular PLC
This type of PLC permits multiple expansion through “modules”, hence referred to as
Modular PLC. I/O components can be increased. It is easier to use because each component
is independent of each other.
A Modular Type PLC
PLC are divided into three types based on output namely Relay output, Transistor output, and
Triac Output PLC. The relay output type is best suited for both AC and DC output devices.
2. Transistor output type PLC uses switching operations and used inside microprocessors.
According to the physical size, a PLC is divided into Mini, Micro, and Nano PLC.
Controller
Definition: Controller is defined as, it is a basic
element of the system itself or external to the system
which control the plant or process.
Example : ON/OFF relays, Control valves, actuators etc.
(ili) Input
Definition: Input of a control system is defined as an
excitation or signal given to the control system from an
external energy source to achieve the specified task.
Example : Desired set temperature in A.C. system, set
values, set times etc.
(iv) Output
Definition: Output of a control system is defined as, it
is actual response given by the system as per requirement
after applying input to the system.
Example: Controlled Temperature in A.C. system,
Desired position, Actual parameters etc.
Basics of Control System
In studying control engineering, we need to define
important terms that are necessary to describe control
systems, such as plants, controller, inputs, outputs.
disturbances, feedback control.
Parts of Control System
(i) Plant or Process
(i) Controller
(in) Input
(iv) Output
(v) Disturbances
(a) Internal Disturbance
(b) External disturbance
(vi) Feedback control
(vii) Feedback control systems
b
Fig. 1.2.2 : Parts of control system
3. Open loop
system is defined as system in
which output depends on input but the input or controlling
input action is totally independent on the output.
Other examples of open loop systems
1. Electric fan.
2.
Lamp with switch.
3. Coffee maker.
4
Washing machine.
5.
Hand dryer.
6.
Water heater.
Advantages of open loop systems
1. These systems are simple in construction and due to
that cost is low.
2. These systems are more stable.
3. Maintenance of such system is easy.
4.
Installation is easy.
5. It consumes less power for operation.
6. Useful when output is difficult to measure.
D isadvantages of open loop systems
1. Less accurate.
2. Slow response.
3. These systems are not reliable.
4. Output affected by internal/external disturbances.
5. It cannot sense environmental changes.
6.
Calibration is required time to time to achieve more
accuracy.
7. Small bandwidth.
Close loop system is defined as a system in
which the input controlling action is somehow dependent
4. on the output that means regulating action changes as per
the current value of the system output.
Other examples of closed loop systems
Voltage stabilizer
Missile launching system.
3. Shig stabilization systern
DC motor speed control.
5. Closing and opening of gate.
b.
Satellite tracking system.
7.
Air traffic controller
Advantages of close loop systems
ACCuraCy is hign.
Useful in hazardous or more remote areas
3,
rast response.
These systems can sense environmental changes as well
as internal disturbanrac
SLarge dandwidin
More reliable for operation
Disadvantages of close loop systems
Complex construction hence difficult to desien due to
that cost is high.
Installation is difficult also maintenance is required
time to time
These systems are less stable.
I consumes more power
Lot of consideration is required during design.
Difference between plc and microcontroller
PLC has lots of built-in capabilities to make things happen behind the scenes, without
the user programming needed if a microcontroller is used. PLCs have software watchdogs to
keep an eye on the executing program, and hardware watchdogs that monitor I/O modules
and devices.
In general the Routh stability criterion states a polynomial has all roots in the open left half
plane if and only if all first-column elements of the Routh array have the same sign.
