Introduction to control system 1

Introduction to Control System
 Definition, Representation, Classification
 Controllers and algorithms
 Transfer Functions and gain of a system
 Introduction to Digital Computer Control

Control System : Definition and Representation
Introduction
In order to modify the behavior of a system so it behaves in a specific
desirable way over time, control is introduced.
For example, we may want the speed of a car on the highway to
remain as close as possible to 60 miles per hour in spite of possible
hills or adverse wind.
Or, we may want an aircraft to follow a desired altitude, heading, and
velocity profile independent of wind gusts.
Or, we may want the temperature and pressure in a reactor vessel in
a chemical process plant to be maintained at desired levels.
All these are being accomplished today by control methods.

Definition
A control system is a system, which provides the desired response
by controlling the output.
A control system is a collection of devices/systems which function
together to drive things in a desired direction, either from external
input or sensed conditions.
Control systems are a central part of industry and of automation.
Figure : Simplified block diagram of a control system

Why do we need Control System ?
You need to be pro-active!
You do not want to find out about a fault after it had occurred!
You have to expect the unexpected!
Control system eliminates the redundant manual controls and
reduce human errors that can cost a significant amount of money.
Control system should be evaluated frequently to ensure that the
processes are where they need to be and functioning efficiently and
effectively. This way, adherence to industry standards prevails while
providing the best service to the customers.
Control system engineering ensures that there is a strategic
method to improving productivity and enhancing the best practices
of a company.

Basic Components
Mainly consists of five basic components:
(1)Input : Source of power to the system.
(2)Process being controlled : Function of the system.
(3)Output : Result of function of the system.
(4)Sensing elements : To sense the output quantities/errors.
(5)Controller & Actuating devices : System that controls & system
that is controlled.

Control System : Classification
Type of Feedback
1. Open Loop Control System
Doesn’t have any feedback loops connected to it
Doesn’t depend upon its output i.e. in open loop feedback
systems, output is not used as the control variable for the
system.
One way signal flow systems.
Also known as Non-feedback systems. As there is no feedback
loop, no data is transferred back to the open loop systems.

Open Loop Control System (Contd.)
Advantages:
Output independent.
Disadvantages:
The output of the open loop system can be determined only by
its present state input that means the input of open loop system
will always return to zero, before the output returns to zero.
There is no chance to correct the transition errors in open loop
systems so there is more chance to occur errors.
Application:
Automatic washing machine, Electric bulb, Electric hand drier,
Time based Bread toaster, Water faucet, TV remote control,
Clothes drier, Stepper motor or servo motor, Inkjet printers,
Door lock system etc.

Type of Feedback
2. Closed Loop Control System
Does not take input from human operators.
Other than adjustment by control systems, they operate
automatically and independently.
Action is entirely dependent on the process variable.
Figure : Block diagram for a closed loop control system.

Closed Loop Control System (Contd.)
Working principle :
The error detector produces an error signal, which is the difference
between the input and the feedback signal. This feedback signal is
obtained from the block (feedback elements) by considering the
output of the overall system as an input to this block. Instead of the
direct input, the error signal is applied as an input to a controller.
So, the controller produces an actuating signal which controls the
plant. In this combination, the output of the control system is
adjusted automatically till we get the desired response. Hence, the
closed loop control systems are also called the automatic control
systems.
In regards to a heating system, for example, a closed loop might
maintain a temperature as a set point, automatically switching on
when temperature is below the set point. Open control, in contrast,
would enable individuals to set timers and turn instant on heat.

Advantages:
Reduce errors by automatically adjusting the systems input.
Improves stability of an unstable system.
Increases or reduces the systems sensitivity.
Enhances robustness against external disturbances to the
process.
Produces a reliable and repeatable performance.
Disadvantages:
To provide the required amount of control, a closed-loop
system must be more complex by having one or more
feedback paths.
If the gain of the controller is too sensitive to changes in its
input commands or signals it can become unstable and start
to oscillate as the controller tries to over-correct itself, and

Comparative Study/Difference Between the Major Types:

Types of Closed Loop Control System :
(a)Feedback Control System :
The output signal is sampled and then fed back to the input to
form an error signal that drives the system.
Feedback is comprised of a sub-circuit that allows a fraction of the
output signal from a system to modify the effective input signal in
such a way as to produce a response that can differ substantially
from the response produced in the absence of such feedback.

Feedback Control System (Contd.)
Advantages:
Circuit characteristics such as the systems gain and response
can be precisely controlled.
Circuit characteristics can be made independent of operating
conditions such as supply voltages or temperature variations.
Signal distortion due to the non-linear nature of the
components used can be greatly reduced.
The Frequency Response, Gain and Bandwidth of a circuit or
system can be easily controlled to within tight limits.

Feedback Control System (Contd.)
Types:
I. Positive Feedback : The set point and output values are
added together by the controller as the feedback is “in-
phase” with the input. It “increases” the systems gain.
II. Negative Feedback : The set point and output values are
subtracted from each other by the controller as the
feedback is “out of phase” with the input. It “decreases” the
systems gain.

(b)Feedforward Control System :
Avoids the slowness of the feedback control.
Detects the disturbance directly and takes an appropriate control
action in order to eliminate its effect on the process output.
Actions to be taken is predicted by the model.

Feedforward Control System (Contd.)
Advantages:
By modeling the range of disturbances emanating from that source
feedforward control can prepare an appropriate response.
Once an upstream disturbance is identified feedforward
immediately seizes the initiative and prescribes an appropriate
change. It is proactive nature that either minimizes or eliminates
the negative effects of a known disturbance.
Feedforward’s use of a disturbance model allows it to assess the
full magnitude of the disturbance and to implement an appropriate
countermeasure.
If Feedforward control simply delivered a countermeasure as soon
as the disturbance was identified, then it might do more harm to
the process. But it is designed to deliver the countermeasure as the
disturbance hits the process, essentially nullifying the negative
effects of the disturbance.

(c)Adaptive Control System :
Adjust parameters based on current needs/able to adjust itself to
handle unknown model uncertainties.
Uses the information it gathers online to change itself and improve
its performance. It "adapts" to the feedback loop it is inside, and
develops into a better controller over time.

Adaptive Control System (Contd.)
Types:
I. Direct Method : The estimated parameters are directly
used in the adaptive controller.
II. Indirect Method: The estimated parameters are used to
build an estimation/estimated model. The estimated model
information is further/later used to adjust the controller.

Types : Considering I/P-O/P
1. SISO (single input single output)
2. MIMO (multiple input multiple output)
Types : Considering Signal Types
1. Continuous time control system
2. Discrete time control system

Introduction to control system 1

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