Fundamentals of Robot Technology

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Fundamentals of Robot Technology

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Integral Parts of a Robot

• Robot Anatomy
• Drive System
• Control System
• Sensors
• Actuators / End Effectors

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Degrees of Freedom (DoF)

• Joint relative motion between two parts of the
  robot body.
• Joint provides the robot with degree-of-freedom
  of motion.
• In most cases, 1 DoF is associated with a joint.
• Robots are often classified according to total
  number of DoF they posses.

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• Links are rigid components of the robot
  manipulator

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Robot Anatomy Joints Links
Linear joint, L
Orthogonal Joint, O
Rotational Joint, R
Twisting Joint, T
Revolving Joint, V
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basic joints
Revolute Joint 1 DOF ( Variable - ?)

Prismatic Joint 1 DOF (linear) (Variables - X)
Spherical Joint 3 DOF ( Variables - X, ?, Z)
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Example
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There are two more joints on the end effector
(the gripper)
This robot arm has SIX revolute joints A
revolute joint has ONE degree of freedom ( 1 DOF)
that is defined by its angle
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3 DoF wrist assembly
Degrees of Freedom
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6 Basic Robot Configurations
Polar
Cylindrical
Cartesian
Mobile
SCARA
Jointed-Arm
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Kinematics the motion of bodies We are
interested in two kinematics topics Forward
Kinematics (angles to position) What you are
given The length of each link The
angle of each joint What you can find The
position of any point (i.e. its (x, y,
z) coordinates) Inverse Kinematics (position to
angles) What you are given The length of each
link The position of some point on the
robot What you can find The angles of each
joint needed to obtain that position
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Point Representation RR Robot
Position of the end of the arm Pj (?1, ?2)
joint space Pw (x, y) world space World
space is useful when the robot must communicate
with other devices.
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Forward Transformation Going from joint space to
world space We can determine the position of the
end of the arm in world space By defining a
vector for Link 1 and another for Link 2. r1
L1 cos?1, L1 sin?1 r2 L2 cos(?1
?2), L2 sin(?1 ?2) Adding these two
vectors yields the coordinates x and y of the
point Pw x L1 cos?1 L2 cos(?1 ?2) y
L1 sin?1 L2 sin(?1 ?2)
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Reverse Transformation Going from world space to
joint space Two possible configurations to
achieve the position Using cos(AB)
cosA cosB sinA sinB sin(AB) sinA cosB
sinB cosA Rewrite the coordinates x L1 cos?1
L2 cos?1 cos?2 L2 sin?1 sin?2 y L1 sin?1
L2 sin?1 cos?2 L2 cos?1 sin?2
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Square both sides and add the two cos?2
(x2 y2 - L12- L22 ) / 2 L1 L2 Also tan?
L2 sin?2 / ( L2 cos?2 L1) tan? y / x Using
tan(A B) (tanA tanB) / ( 1 tanA
tanB) tan?1 y(L1 L2 cos?2) -x L2 sin?2 /
x(L1 L2 cos?2) - yL2 sin?2
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Drive Systems/Actuators

• Hydraulic
• Larger Robots
• Greater speed strength
• Larger floor space required
• Rotary vane actuators for rotary motion
• Hydraulic pistons for linear motion
• Electric
• Accuracy repeatability is better
• Smaller floor space
• Stepper motors or servo motors
• Drive train/gear systems for rotational
• Pulleys or similar systems for linear motion.
• Pneumatic
• Smaller robots with fewer DoF
• Pick-and-place with fast cycles
• Pneumatic pistons

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Seesaw Physics
T Torque F Force r radius T rF sin? T
rF
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Meshing Gears
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• LEGO Gears

40T
8T
16T
Bevel
1T Worm
24T
24T Crown
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Worm Gears

• Pull one tooth per revolution

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1
2
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Result is a 241 gearbox
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Motors

• 9V Gear Motor
• 150 mA
• 300 RPM (no load)

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Motors

• 9V Micro Motor
• 20-30 RPM

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Mounting Motors
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Lego Motors
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Stepper motors A stepper motor's shaft has
permanent magnets attached to it, together called
the rotor. Around the body of the motor is a
series of coils that create a magnetic field that
interacts with the permanent magnets. When these
coils are turned on and off the magnetic field
causes the rotor to move. As the coils are turned
on and off in a certain sequence the motor will
rotate forward or reverse. This is called the
phase pattern and there are several types that
will cause the motor to turn. Common types are
full-double phase, full-single phase, and half
step. To make a stepper motor rotate, you must
constantly turn on and off the coils. If you
simply energize one coil the motor will just jump
to that position and stay there resisting change.
This energized coil pulls full current even
though the motor is not turning. This is the main
way steppers generate heat, when at standstill.
This ability to stay put at one position rigidly
is often an advantage of stepper motors. The
torque at standstill is called the holding
torque.
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Because steppers can be controlled by turning on
and off coils, they are easy to control using
digital computers. The computer simply energizes
the coils in a certain pattern and the motor will
move accordingly. At any given time the computer
will know the position of the motor since the
number of steps given can be stored. This is true
only if some outside force of greater strength
than the motor has not interfered with the
motion. An optical encoder could be attached to
the motor to verify its position but this is not
necessary. A stepper motor can be run in
"open-loop" mode (without feedback of an encoder
or other device). Most stepper motor control
systems will have a home switch associated with
each motor that will allow the software to
determine the starting or reference "home"
position.
http//www.cs.uiowa.edu/jones/step/types.html
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Servo motors Take a normal DC motor that that
has one coil (2 wires). If you attach a battery
to those wires the motor will spin continuously
Reversing the polarity will reverse the
direction. Attach that motor to the wheel of a
robot and watch the robot move, note the speed.
Now add a heavier payload to the robot, what
happens? The robot will slow down due to the
increased load. The computer inside of the robot
would not know this happened unless there was an
encoder on the motor keeping track of its
position. So, in a DC servo, the speed and
current drawn are affected by the load. For
applications that the exact position of the motor
must be known, a feedback device like an encoder
MUST be used. The control circuitry to perform
good servo of a DC motor is MUCH more complex
than the circuitry that controls a stepper motor.
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A Servo is a small device that has an output
shaft. This shaft can be positioned to specific
angular positions by sending the servo a coded
signal. As long as the coded signal exists on the
input line, the servo will maintain the angular
position of the shaft. As the coded signal
changes, the angular position of the shaft
changes. In practice, servos are widely used in
radio controlled devices and robots.
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Servos are extremely useful in robotics. The
motors are small, as you can see by the picture
below, have built in control circuitry, and are
extremely powerful for their size. A standard
servo such as the Futaba S-148 has 42 oz/inches
of torque, which is pretty strong for its size.
It also draws power proportional to the
mechanical load. A lightly loaded servo,
therefore, doesn't consume much energy. The guts
of a servo motor are shown in the picture below.
You can see the control circuitry, the motor, a
set of gears, and the case. You can also see the
3 wires that connect to the outside world. One is
for power (5volts), ground, and the white wire
is the control wire.
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So, how does a servo work? The servo motor has
some control circuits and a potentiometer (a
variable resistor, aka pot) that is connected to
the output shaft. This pot allows the control
circuitry to monitor the current angle of the
servo motor. If the shaft is at the correct
angle, then the motor shuts off. If the circuit
finds that the angle is not correct, it will turn
the motor to the correct direction until the
angle is correct. The output shaft of the servo
is capable of traveling somewhere around 180
degrees. A normal servo is used to control an
angular motion of between 0 and 180 degrees. A
normal servo is mechanically not capable of
turning any farther due to a mechanical stop
built on to the main output gear.
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The amount of power applied to the motor is
proportional to the distance it needs to travel.
So, if the shaft needs to turn a large distance,
the motor will run at full speed. If it needs to
turn only a small amount, the motor will run at a
slower speed (proportional control) How do you
communicate the angle at which the servo should
turn? The control wire is used to communicate
the angle. The angle is determined by the
duration of a pulse that is applied to the
control wire. This is called Pulse Coded
Modulation. The servo expects to see a pulse
every 20 milliseconds (.02 seconds). The length
of the pulse will determine how far the motor
turns. A 1.5 millisecond pulse, for example, will
make the motor turn to the 90 degree position
(often called the neutral position). If the pulse
is shorter than 1.5 ms, then the motor will turn
the shaft to closer to 0 degrees. If the pulse is
longer than 1.5ms, the shaft turns closer to 180
degrees.
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Sensors Anything that detects the state of the
environment.

• Light sensing
• Heat sensing
• Touch sensing
• Rotational
• Sonar
• Radar
• Infra-red

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• There are four main factors to consider in
  choosing a sensor.
• Cost sensors can be expensive, especially in
  bulk.
• Environment there are many sensors that work
  well and predictably inside, but that choke and
  die outdoors.
• Range Most sensors work best over a certain
  range of distances. If something comes too close,
  they bottom out, and if something is too far,
  they cannot detect it. Choose a sensor that will
  detect obstacles in the range you need.
• Field of View depending upon what you are doing,
  you may want sensors that have a wider cone of
  detection. A wider field of view will cause
  more objects to be detected per sensor, but it
  also will give less information about where
  exactly an object is when one is detected.

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Gas Sensor
Gyro
Accelerometer
Metal Detector
Pendulum Resistive Tilt Sensors
Piezo Bend Sensor
Gieger-Muller Radiation Sensor
Pyroelectric Detector
UV Detector
Resistive Bend Sensors
CDS Cell Resistive Light Sensor
Digital Infrared Ranging
Pressure Switch
Miniature Polaroid Sensor
Limit Switch
Touch Switch
Mechanical Tilt Sensors
IR Sensor w/lens
IR Pin Diode
Thyristor
Magnetic Sensor
Polaroid Sensor Board
Hall Effect Magnetic Field Sensors
Magnetic Reed Switch
IR Reflection Sensor
IR Amplifier Sensor
IRDA Transceiver
IR Modulator Receiver
Radio Shack Remote Receiver
Solar Cell
Lite-On IR Remote Receiver
Compass
Compass
Piezo Ultrasonic Transducers
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Resistive Sensors

• Bend Sensors
• Resistance 10k to 35k
• Force to produce 90deg 5 grams
• www.jameco.com 10
• Potentiometers
• Fixed Rotation Sensors
• Easy to find, easy to mount
• Light Sensor
• Good for detecting direction/presence of light
• Non-linear resistance
• Slow response

Resistive Bend Sensor
Potentiometer
Cadmium Sulfide Cell
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Applications
Sensor

• Measure bend of a joint
• Wall Following/Collision Detection
• Weight Sensor

Sensors
Sensor
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Lego tips Structure

• Common pitfall when trying to increase mechanical
  robustness

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Structure

• The right way

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Structure

• The right way

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Connector pegs

• Black pegs are tight-fitting for locking bricks
  together.
• Grey pegs turn smoothly in bricks for making a
  pivot

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Car Turn
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Differential Gear
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Differential Drive
Where D represents the arc length of the center
of the robot from start to finish of the movement.