09-11-2012, 04:26 PM
PWM Based Automatic Closed Loop Speed Control of DC Motor
Automatic Closed.pdf (Size: 207.21 KB / Downloads: 130)
Abstract-
The electric drive systems used in many industrial
applications require higher performance, reliability, variable
speed due to its ease of controllability. The speed control of DC
motor is very crucial in applications where precision and
protection are of essence. Purpose of a motor speed controller is
to take a signal representing the required speed and to drive a
motor at that speed. Microcontrollers can provide easy control of
DC motor. Microcontroller based speed control system consist of
electronic component, microcontroller and the LCD. In this
paper, implementation of the ATmega8L microcontroller for
speed control of DC motor fed by a DC chopper has been
investigated. The chopper is driven by a high frequency PWM
signal. Controlling the PWM duty cycle is equivalent to
controlling the motor terminal voltage, which in turn adjusts
directly the motor speed. This work is a practical one and high
feasibility according to economic point of view and accuracy. In
this work, development of hardware and software of the close
loop dc motor speed control system have been explained and
illustrated. The desired objective is to achieve a system with the
constant speed at any load condition. That means motor will run
at a fixed speed instead of varying with amount of load.
INTRODUCTION
Speed control of dc motor could be achieved using
mechanical or electrical techniques. In the past, speed controls
of dc drives are mostly mechanical and requiring large size
hardware to implement. The development has launched these
drives back to a position of formidable relevance, which were
hitherto predicted to give way to ac drives. Some important
applications are: rolling mills, paper mills mine winders,
hoists, machine tools, traction, printing presses, textile mills,
excavators and cranes. Fractional horsepower dc drives are
widely employed -as servo means for positioning and tracking
[1]. Controlled rectifiers provide a variable dc voltage from a
fixed dc voltage. Due to their ability to supply a continuously
variable dc voltage, controlled rectifier and dc choppers made
a revolution in modern industrial equipment and variablespeed
drives [2]. Adjustable speed drives may be operated
over a wide range by controlling armature or field excitation.
RELATED WORKS
A reasonable number of works have found in the literature,
regarding the employment of solid-state devices for the
control of dc drives. The paper of Kurnera, Dayananda and
Jayawikrama, elucidated the use of chopper in collaboration to
PC for the control of dc motor speed. Software was
developed, fed into a PC and consequently, commands were
given to the chopper via the computer for control of motor
speed [3].The use of standalone micro controller for the speed
control of DC motor is past gaining ground. Nicolai and
Castgnct have shown in their paper how a microcontroller can
be used for speed control. The operation of the system can be
summarized as: the drive form rectified voltage; it consists of
chopper driven by a PWM signal generated from a
microcontroller unit (MCU). The motor voltage control is
achieved by measuring the rectified mains voltage with the
analog to-digital converter present other micro controller and
adjusting the PWM signal duty cycle accordingly [5]. Another
system that uses a microprocessor is reported in the work of
khoel and Hadidi a brief description of the system is as
follows: The microprocessors computes the actual speed of
the motor by sensing the terminal voltage and the current, it
then compares the actual speed of the motor with the reference
speed and generates a suitable signal control signal which is
fed into the triggering unit. This unit drives an Hbridge Power
MOSFET amplifier, which in turn supplies a PWM voltage to
the DC motor [6].
System Overview
The motor to be controlled is fed by a DC source through a
chopper. The tachogenerator senses the speed, which gives
voltage as output. And this voltage is fed to the
microcontroller to drive the speed of the motor. The output
voltage of tachogenerator is provided to the microcontroller
and microcontroller determines the output voltage of the
chopper fed to the DC motor for desired speed.
The circuit is based on PWM technique. ATmega-8’s timer
portion has this special feature. By adjusting register values
(ocr, tccr etc) duty cycles can be controlled. When motor run
at 70% of duty cycle, the tachogenerator gives a Voltage
corresponds to that speed. Now if any load occurs, desired
speed will be decreased. Hence, the voltage drop will be less.
This voltage is fed into the ADC of microcontroller. By
comparing the previous value microcontroller can sense the
decrease in the speed. After sensing the load condition, it will
start increasing its duty cycle until it reaches the desired
speed. During overload condition, Microcontroller will try to
reach the desired speed by increasing duty cycle. But if at the
maximum duty cycle it fails to run it will show a message to
the user through A LCD panel. The message indicates
OVERLOAD. Now user can run the machine again at desired
speed by decreasing the load.
CONCLUSION
The microcontroller based closed-loop automatic speed
control of dc motor has been introduced. Controlling a
permanent magnet DC motor with speed feedback through a
tachogenerator will be implemented using an ATmega8L
microcontroller. The system will be made user friendly so that
Anybody can operate the system without any trouble. LCD
display will used to show the condition of the system.
Knowing the condition the user can change the amount of load
if necessary.