Transcript Robot Basics

Fundamentals of Robot
Technology
Integral Parts of a Robot
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Robot Anatomy
Drive System
Control System
Sensors
Actuators / End Effectors
Degrees of Freedom (DoF)
• Joint : relative motion between two parts of
the robot body.
• Joint provides the robot with degree-offreedom of motion.
• In most cases, 1 DoF is associated with a
joint.
• Robots are often classified according to
total number of DoF they posses.
Links are rigid components of the robot
manipulator
Robot Anatomy: Joints & Links
Linear joint, L
Orthogonal Joint, O
Rotational Joint, R
Twisting Joint, T
Revolving Joint, V
basic joints
Revolute Joint
1 DOF ( Variable - )
Prismatic Joint
1 DOF (linear) (Variables - X)
Spherical Joint
3 DOF ( Variables - X, , Z)
Example
2
3
4
1
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
3 DoF wrist assembly
Degrees of Freedom
6 Basic Robot Configurations
Polar
Jointed-Arm
Cylindrical
Mobile
Cartesian
SCARA
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. it’s (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
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.
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)
Reverse Transformation: Going from world space to joint space
Two possible configurations to achieve the position
cos(A+B) = cosA cosB – sinA sinB
sin(A+B) = 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
Using
Square both sides and add the two
cos2 =
Also
(x2 + y2 - L12- L22 ) / 2 L1 L2
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]
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
Seesaw Physics
T = Torque
F = Force
r = radius
T = rF sin
T = rF
Meshing Gears
• LEGO Gears
40T
8T
16T
24T
1T Worm
24T
Crown
Bevel
Worm Gears
• Pull one tooth per revolution
3
1
2
• Result is a 24:1 gearbox
4
Motors
• 9V Gear Motor
• ~ 150 mA
• 300 RPM (no load)
Motors
• 9V Micro Motor
• 20-30 RPM
Mounting
Motors
Lego Motors
Motor
Normal
speed
(RPM)
Torque
(Lego units)
3240
880 bricks X
bump
Torque
(metric
units)
at a speed of
1.760 Kg X
cm
40 RPM
370
1920 bricks
X bump
3.840 Kg X
cm
15 RPM
36
64 bricks X
bump
0.128 Kg X
cm
36 RPM
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.
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
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.
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.
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 S148 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.
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.
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.
Sensors:
Anything that detects the state of
the environment.
•Light sensing
•Heat sensing
•Touch sensing
•Rotational
•Sonar
•Radar
•Infra-red
There are four main factors to consider in choosing a sensor.
1) Cost: sensors can be expensive, especially in bulk.
2) Environment: there are many sensors that work well and
predictably inside, but that choke and die outdoors.
3) 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.
4) 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.
Gas Sensor
Gyro
Accelerometer
Pendulum Resistive
Tilt Sensors
Metal Detector
Piezo Bend Sensor
Gieger-Muller
Radiation Sensor
Pyroelectric Detector
UV Detector
Resistive Bend Sensors
Digital Infrared Ranging
CDS Cell
Resistive Light Sensor
Pressure Switch
Miniature Polaroid Sensor
Limit Switch
Touch Switch
Mechanical Tilt Sensors
IR Pin
Diode
IR Sensor w/lens
Thyristor
Magnetic Sensor
IR Reflection
Sensor
Magnetic Reed Switch
IR Amplifier Sensor
Hall Effect
Magnetic Field
Sensors
Polaroid Sensor Board
IRDA Transceiver
Lite-On IR
Remote Receiver
Radio Shack
Remote Receiver
IR Modulator
Receiver
Solar Cell
Compass
Compass
Piezo Ultrasonic Transducers
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
Resistive Bend Sensor
Potentiometer
Light Sensor
• Good for detecting direction/presence of light
• Non-linear resistance
• Slow response
Cadmium Sulfide Cell
Applications
Sensor
• Measure bend of a joint
Sensors
• Wall Following/Collision
Detection
Sensor
• Weight Sensor
Lego tips:
Structure
• Common pitfall when trying to increase
mechanical robustness:
Structure
• The right way:
Structure
• The right way:
Connector pegs
• Black pegs are tight-fitting for locking
bricks together.
• Grey pegs turn smoothly in bricks for
making a pivot
Car Turn
Differential Gear
Differential Drive
Where D represents the arc length of the center of the robot
from start to finish of the movement.