Transcript STEPPER MOTORS

L
INTRODUCTION TO
MOTORS, SERVOS
AND STEPPERS
Deepali Mittal
Harshad Sawhney
Mechanical Aspects of Robotics
DC MOTORS
• Designed to work on DC power.
• Mainly of three types :
– Permanent Magnet Type
– Brushed DC Motors
– Brushless DC Motors
DC MOTORS
• These are simple motors used for
simple purpose.
• Apply +12V (generally) and they
start rotating. Apply -12V and they
rotate in the opposite direction!
• Speed can be reduced by
reducing the voltage supplied,
however that’s not an efficient
way of doing things.
• Torque and speed are interrelated. A 10rpm motor has
relatively high torque. Generally a
100rpm – 300rpm
THEORY OF SERVOMECHANISMS
• Closed loop control whose output is some mechanical
position or velocity set by a reference input(required
output).
• A servomotor consists of an electric motor ,feedback
device, electronic controller.
• The feedback device for servomotors is typically an
encoder or resolver built into motor frame
SERVO MOTORS
• Servo motors have three wires: power, ground,
and signal.
• The power wire is typically red, and should be
connected to the 5V pin on the Arduino board.
The ground wire is typically black or brown and
should be connected to a ground pin on the
Arduino board.
The signal pin is typically yellow, orange or white
and should be connected to a digital pin on the
Arduino board.
• Note that servos draw considerable power, so if
you need to drive more than one or two, you'll
probably need to power them from a separate
supply (i.e. not the +5V pin on your Arduino). Be
sure to connect the grounds of the Arduino and
external power supply together.
SERVO MOTORS
• They are used for specific purposes (for
instance, in grippers, etc.)
• They have a restricted rotation of 180 or 360
degree. The least rotation angle possible is 1
degree.
• They are driven through microcontrollers.
Power comes separately from a battery (12V,
generally).
• Arduino has one line command for driving a
servo . Using an Atmega PWM needs to be
generated to drive a servo.
• Speed can be reduced by sending the
commands at a delay. This doesn’t reduce
the efficiency of the motor.
SERVO MOTORS
• The basic behind Servo Control is Pulse Width
Modulation.
• Pulse Parameters :
• Minimum Pulse
• Maximum Pulse
• Repetition Rate
• The general concept is to simply send an ordinary logic
square wave to your servo at a specific wave length,
and your servo goes to a particular angle (or velocity if
your servo is modified). The wavelength directly maps to
servo angle
DRIVING THE MOTOR
• The microcontrollers like Atmega16 and Arduino have a
current rating of 5-10 mA.
• The normal DC motor’s current ratings start from 150 mA
and above.
• So the motor cannot be directly attached to the
Atmega, hence a motor-driver is used.
• Generally, L298, L293 are used to drive simple DC
motors. The
servo motors already have a power cable and a
different control cable which can directly be connected
to the microcontrollers.
MOTOR DRIVER
STEPPER MOTORS
• Electromechanical device which
converts electrical pulses into discrete
mechanical movements.
• The speed of the motor shafts rotation
is directly related to the frequency of
the input pulses
• Length of rotation is directly related to
the number of input pulses applied.
• Brushless motors.
• Full torque at stand still.
• Open loop control.
ADVANTAGES AND WHEN TO USE
• Precise positioning.
• Use whenever controlled movement is
required.
• Where you need to control rotation
angle, speed, position and synchronism.
DISADVANTAGES
• Not easy to operate a high speeds.
• Open Loop Control so it could have errors.
• Higher torque stepper motors are very
costly.
UNIPOLAR VS BIPOLAR MOTORS
UNIPOLAR VS BIPOLAR MOTORS
• The bipolar motor needs current to be driven in
both directions through the windings.
• The centre tap on the unipolar motor allows a
simpler driving circuit, limiting the current flow to
one direction.
• Drawback :In unipolar motor there is limited
capability to energize all windings at any time,
resulting in a lower torque compared to the bipolar
motor.
PHASES
• Generally two phases.
• But three or five phases also
exist
Two Phase Stepper
STEPPER DIAGRAMS
STEP ANGLE CALCULATION
• Right Hand Rule used to determine magetics flux
direction
• the torque is proportional to the number of winding
turns and the current
• Step angle=360 /(NPh * Ph)=360/N
• NPh = Number of equivalent poles per phase =
number of rotor poles
• Ph = Number of phases
• N = Total number of poles for all phases together
STEPPING MODES
• Wave Drive (1 phase on): A -> B -> A’ -> B’
• Disadvantage:Unipolar use only 25% and bipolar only 50% of
the total motor winding. Hence not maximum torque used.
• Full Step Drive (2 phases on): AB -> A’B -> A’B’ ->AB’
• Unipolar motor uses only 50% while the bipolar motor uses
the entire winding.
• Half Step Drive (1 & 2 phases on): AB -> B -> A’B ->
A’ -> A’B’ -> B’ -> AB’ -> A
• Microstepping : currents in the windings are
continuously varying to be able to break up one full
step into many smaller discrete steps.
POWER DISSIPATION
• A stepper motor rated at 6V and 1A per phase and
has two phases.
• Then P= V X I
• P=6 X 2 = 12 W
• Select a motor driver suitably.
TORQUE VS SPEED
Maximum Holding Torque
Torque
Speed
ARDUINO CODE
MOTOR DRIVERS
•
•
•
•
H-bridge
L293/L293D
Timer
Pulse Width Modulation
H-BRIDGE
• An electronic circuit that enables a voltage to be
applied across a load in either direction.
HOW TO RUN MOTORS THROUGH AN
MCU ?
• Never connect the MCU pins directly to the motors.
• The Atmega16 has a current rating of 5-10 mA.
• The normal DC motor’s current ratings start from 150
mA and above which arises a need for a Motor and
MCU interface.
• So the motor cannot be directly attached to the
ATmega, hence a motor-driver is used… (eg:
L293/L293D, L298, etc…)
L293/L293D MOTOR DRIVER
It has two H-bridges, hence two motors can be driven.
Note: A motor driver doesn’t amplify current, it only acts as
switches. An L293 is nothing but four switches.!
L293
• Drivers are enabled in pairs, with drivers 1 and 2 being
enabled by the EN Pin. For EN=1 or +5V, the outputs
are active and in phase with their inputs.
• When EN Pin is low, the output is low(disconnected),
irrespective of the input.
• Asserting both logics to either high or low will stop the
motor.
L298 VS L293
• L293 is a quadruple half-H driver while L298 is a dual
full-H driver
• Output current per channel = 1A for L293 and 2A for
L298.
• Protective Diodes against back EMF are provided
internally in L293D but must be provided externally in
L298.
SPEED CONTROL
• To control motor speed, we can use pulse width
modulation or PWM, applied to Enable Pins of L293.
• PWM is the scheme in which the duty cycle of a
square wave output from the microcontroller is varied
to provide a varying average DC output.
• What actually happens by applying a PWM pulse is
that the motor is switched ON and OFF at a given
frequency. In this way, the motor reacts to the time
average of the power supply.