Transcript Introduction to Motors - University of Nebraska Omaha

Introduction to Motors
Understanding the CEENBot’s muscles
Presented by:
Alisa N. Gilmore, P.E.
Senior Lecturer, UNL Computer and Electronics Engineering Dept.
NSF ITEST SPIRIT Workshop Summer 2008
The Peter Kiewit Institute Omaha, NE
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A robot is “an autonomous system which
exists in the physical world, can sense its
environment, and can act on it to achieve
some goals.” – The Robotics Primer by Maja Mataric
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Autonomous – programmable
Exists in physical world – has a body
Can sense its world– has sensors
Can act on it – possesses effectors &
actuators, i.e. legs, arms (wheels) &
muscles (motors)
Robot Defined
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Motors in context of robotics, different types
of robots have different types of motors
Overview of motor types / characteristics
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Principle of operation of a DC motor
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Principle of operation of stepper motors
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CEENBot stepper motor operation/control
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◦ All motors convert electric energy to mechanical
motion
◦ Motor characteristics: AC or DC power source,
torque, speed performance
◦ Industrial robotics: AC servo motor
◦ Mobile robotics & Hobby robots: dc motor, dc servo
motor, and stepper motors
◦ Inside a DC motor
◦ Performance advantages of stepper motor over DC
motor and DC servo motor
Overview
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Either an AC or DC electrical energy
source serves as the input to the motor.
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The result is mechanical motion of the
output shaft, most often a rotation
about the shaft, provided the load
carried by the shaft does not exceed the
maximum load the motor is designed to
carry.
Motor Basics
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There are numerous ways to design a motor,
thus there are many different types of
motors.
The type of motor chosen for an application
depends on the characteristics needed in
that application.
These include:
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How fast you want the object to move,
The weight, size of the object to be moved,
The cost and size of the motor,
The accuracy of position or speed control needed.
Choosing a Motor
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The level of performance a motor can
provide is described by its parameters.
These include:
Rated Speed
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Torque
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HorsePower = Speed x Torque / 5252.11...
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Torque-Speed performance of a motor
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◦ Speed measured in shaft revolutions per minute
(RPM)
◦ Rotational force produced around a given point, due
to a force applied at a radius from that point,
measured in lb-ft (or, oz-in).
◦ A measure of work expended: 1 HP = 33,000 footpounds per minute.
Motor Parameters
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The different types of motors possess
different operating characteristics.
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A brief overview of some operation
characteristics of:
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AC motors
DC motors
DC servo motors
Stepper motors
Types of Motors
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When power is applied, AC motors turn in
one direction at a fixed speed.
Both reversable and non-reversable models
available
Usually high voltage (110V AC and up)
Inexpensive and commonly available
Optimized to run at a fixed, usually high
speed.
If the applied load is greater than the
capacity of the motor, the motor will stall and
possibly burn out.
AC Motor Characteristics
When power is applied, DC motors turn in one
direction at a fixed speed.
 They are optimized to run at a fixed, usually high
speed.
 Most common found in toys, hobby planes,
inexpensive robots, blender, toothbrush,
screwdriver, etc.
 Speed can be varied if a (pulse width
modulation) PWM controller is added.
 Almost all can be reversed.
 Inexpensive and commonly available.
 Not suitable for positioning unless some kind of
position feedback is added.
 If the applied load is greater than the capacity of
the motor, the motor will stall and possibly burn
out.
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DC Motor Characteristics
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Applications that require Servo motors
involve control of acceleration, velocity,
and/or position to very close tolerances.
These motors allow for fast starts, stops and
reversals, and very accurate control.
DC servo motors consist of a DC motor
combined with feedback for either position
or speed.
A servo system is closed loop with a motor,
feedback signal, desired input signal, and a
controller which constantly adjusts the
position or speed in reaction to the feedback.
Servo motor controllers are complex.
DC Servo Motors
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A stepper motor will not automatically turn when
power is applied.
It requires a separate controller circuit to cause
the motor to move.
Controllers for stepper motors are easier to
implement than closed loop servo systems.
Precise positioning is possible by keeping
count of steps, no feedback is required. It is
open loop.
They are inexpensive and commonly available,
especially in salvaged computer equipment.
Note: If the applied load is greater than the
capacity of the motor, the motor may not step,
thereby making precise positioning no longer
possible.
Stepper Motors
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DC Electric Motors use Direct Current (DC)
sources of electricity:
◦ Batteries
◦ DC Power supply
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Principle of How Motors Work:
Electrical current flowing in a loop of wire will
produce a magnetic field across the loop.
When this loop is surrounded by the field of
another magnet, the loop will turn, producing
a force (called torque) that results in
mechanical motion.
DC Electric Motors
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Motors are powered by electricity, but rely
on principles of magnetism to produce
mechanical motion.
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Inside a motor we find:
◦ Permanent magnets,
◦ Electro-magnets,
◦ Or a combination of the two.
Motor Basics
Magnets
◦ A magnet is an object that possesses a
magnetic field, characterized by a North and
South pole pair.
◦ A permanent magnet (such as this bar
magnet) stays magnetized for a long time.
◦ An electromagnet is a magnet that is created
when electricity flows through a coil of wire. It
requires a power source (such as a battery) to
set up a magnetic field.
Current Flowing through a coil or wire
LEFT: Current Enters A
North Pole on Top
RIGHT: Current Enters B (Reversed) North Pole on Bottom
Current in a coil creates a
Magnet
A Simple Electromagnet
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A Nail with a Coil of Wire
Q – How do we set up a magnet?
 A – The battery feeds current through the coil of
wire. Current in the coil of wire produces a
magnetic field (as long as the battery is
connected).
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A Simple Electromagnet
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A Nail with a Coil of Wire
S
N
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Q - How do we reverse the poles of this
electromagnet?
 A – By reversing the polarity of the
battery!
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The Electromagnet in a Stationary
Magnetic Field
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If we surround the electromagnet with a stationary magnetic
field, the poles of the electromagnet will attempt to line up
with the poles of the stationary magnet.
OPPOSITE
POLES
ATTRACT!
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The rotating motion is transmitted to the shaft, providing
useful mechanical work. This is how DC motors work!
DC Motor Operation Principles
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Once the poles align, the nail (and shaft) stops rotating.
How do we make the rotation continue?
By switching the poles of the electromagnet. When they line up
again, switch the poles the other way, and so on.
This way, the shaft will rotate in one direction continuously!
Brushed DC Motor Components
How the Commutator Works
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As the rotor turns, the commutator terminals
also turn and continuously reverse polarity of
the current it gets from the stationary
brushes attached to the battery.
Inside a Toy Motor
(Similar to TekBot Motor)
Inside the Motor, cont.
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The DC motors on the TekBot offer limited
speed control and low torque.
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The CEENBot uses a stepper motor for each
wheel.
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The stepper motors on the CEENBot enables
accurate wheel positioning with high holding
torque and allows for open-loop speed
control (wheel position feedback is option).
Advantages of Stepper Motor
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A stepper motor consists of:
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The stepper motor moves as the permanent rotor
magnet attempts to line up with the poles of the
electromagnets on the stator.
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The electromagnets are digitally switched to change
their pole orientation, which when done in a sequence
produces continuous rotation of the rotor.
◦ A permanent magnet rotating shaft (or rotor)
◦ Electromagnets on the stator – the stationary portion that
surrounds the motor
http://www.interq.or.jp/japan/se-inoue/e_step1.htm
Stepper Motor Operation
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The smallest step of angular rotation a
stepper motor can make is called its
resolution.
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Unlike the example, which had 90 degrees
per step resolution, real motors employ a
series of mini-poles on the stator and
rotor to increase resolution.
The same sequence of 4 stepping phases is
used to control this scenario. There is no
increase in control complexity.
http://www.interq.or.jp/japan/seinoue/e_step1.htm
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CEENBot stepper motors have a resolution of
1.8 degrees per step.
◦ Q: How many steps are needed to make 1 complete
wheel revolution?
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Because the rotor is fixed by magnetism in
the stationary condition, the stationary
torque is large. It allows one to make a
precise stop at some angle and hold it there.
◦ The CEENBot can better hold its position on a ramp.
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Speed control is achieved by digitally
cycling through the phases at a desired
speed of rotation.
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A microcontroller is used to reverse the
current after each step, which changes the
poles of the corresponding electromagnets.
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The stepper motor example is similar to the CEENBot
motor, except that it is unipolar.
It has 6 wires to connect, verses the 4 wires of the
bipolar stepper motors you will install on the CEENBot.
The difference is the bipolar provides greater torque since
an entire coil is energized instead of a half coil for each
state of the electromagnet.
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The unipolar is simplier to control since the two coils that
make up the stepper are centertapped, a wire is connected
midway on each coil and is tied to power. To reverse
power, simply alternate the grounding of one of the two
terminals connected to a coil. This reverses current flow,
and thus reverses the poles of the electromagnet.
However, only one half of each coil is energized at a time.
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Bipolar motors require a slightly more involved controller
that must reverse the current flow through the coils by
alternating the polarity of the terminals.
This is done simply with the aid of a microcontroller.
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Unipolar & Bipolar Steppers
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“The Difference Between Stepper Motors, Servos, and RC
Servos” by Roger Arrick
http://www.arrickrobotics.com/motors.html
Making Things – “General Information on Motors”
http://www.makingthings.com/teleo/products/documentati
on/app_notes/motors_general.htm
“How Stepper Motors Work” by Images Scientific
Instruments
http://www.imagesco.com/articles/picstepper/02.html
CEENBot Stepper Motor & PM DC Motor Testing Unit
Operations Manual by Ben Barenz, CEEN Student
Hansen Corp. “Servo motors” http://www.hansenmotor.com/servo-motors.htm
Animated operation of a Unipolar stepper motor:
http://www.interq.or.jp/japan/se-inoue/e_step1.htm
Basic Motor Theory by Reliance Electric:
http://www.reliance.com/mtr/mtrthrmn.htm
References