# Robotics by jennyyingdi

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```									             Robotics

Actuators. Motors. Sensors.
NXT-G logic.
Lab: Measurements
Homework: Task moodle assignment!
Mechanics readings. Prepare summary.
Postings.
Next class: Closer look at programming and
build & program robot with bumper (use of
touch sensor)
Recap
• Build: attention to detail.
• Review construction: parts used for attaching,
stability.
• Refine & experiment
– Challenge to do the exact job
– My process: took 4-5 iterations. Added sound block.
Changed from rotations to degrees.
• 360 degrees in a rotation (of the motor), not the wheel.
– NOTE: more power means faster, not more distance if
you are specifying rotations or degrees
Inserted to
be sure
when
turning
move over

9 rotations
too many
Gentle
curve
Steering
• Steering can accept an input value: -100
to 100.
• When we get to variables, can set a
variable with a number and use data wires
to set exact steering
Joints
• Robots have joints (think of joints of the
body) that connect links
• A robot joint [generally] provides 1 degree
of freedom (d.o.f.)
– [Note: a Carnegie Mellon robot travels around
on a ball: work of Ralph Hollis. More later or
report in posting]
• Rotational versus linear motions
IBM Box frame robot:
3 linear motors
3 rotational
1 (jaw) linear gripper
Preview

• Kinematics
– Each joint is set to specific position, where is
endpoint?
• move my shoulder, move my elbow, move my wrist, raise my
finger, where is tip of finger?

• Inverse Kinematics
– What to set each joint in order to have the endpoint at
a given point (orientation)
• how much do I move each joint in order to have the tip of my
finger touch my nose?
Actuators
• One type of actuator is a motor: motors make
the robot [a joint of the robot] move
• Many types of motors
– Stepper motor allows for precise positioning
– DC or AC Servo motor allows for feedback … for
precise positioning
• “is it there yet?”
– Hydraulic or pneumatic pistons
• May or may not involve gears/gearing
of specific type of motor, other types of actuator.
Sensors
• Machine (electro-mechanical) device that
detects something about external world
– Generates a signal back to the program
• Refer to
– Programmable, sensory robots as robots that
• can be programmed
• Programming can involve/use sensory input
Sensors
• Contact, touch, switch
• Proximity
• Light
– Light level
– Vision systems: BIG topic
• Sound
– Sound signal (range)
– Speech systems: BIG topic
Sensors (& joints)
• Locality of sensing
• Where is ‘the robot’ when the ultrasonic
sensor (proximity) says <20 cm??
• In positioning and movement, need to
provide clearance for ALL the robot + any
Calibration
• Process to set (define) relationship
between strength (size) of thing being
measured and the measurement scale
• REQUIRED for most sensors
• May be required EACH TIME sensor is
used (in specific environment)
• This will be the case for the light sensor
Strain gauge
• Category of touch, but…
• Detects some amount of force (strain, push
back)
– Physical force deforms material that produces change
in resistance (electrical quantity). This electrical
quantity is what is measured.
• Common in industrial robotics
• The Lego touch sensor is NOT a strain gauge. It
is off or on
– States are off, pressed, [just] released
– Bump = pressed and released
• Good tutorial on www.societyofrobots.com
Logic
• Event driven programming in high[er] level language
such as Flash ActionScript: specify event and handler
and the system does everything else.
– var mytimer:Timer: new Timer(1000)
• In other places, need to specify testing explicitly. In
NXT-G, need to put in wait blocks and other blocks,
perhaps use switch blocks and loop blocks.
• NOTE: Flash & other languages generate the code that
does the checking and re-checking…
NXT-G wait until
• Next class will do move until bump sensor.
Here Wait until certain NXT button
pressed
• unlimited motors move
wait until button pressed
brake motors block
Preview: NXT-G logic
• Logic block: takes 1 or 2 inputs and
performs
– AND, OR, XOR, NOT

• What do you do with result?
– Input to looping block
– Input to switch block
– Input to …
NXT-G
•   Arithmetic blocks
•   Comparison blocks
•   Variables
•   Input from sensors AND motors and
calculations and (Bluetooth) messages
NXT-G Looping
• Loop block: block can contain other blocks
• Looping forever, sensor, count, logic, time
– System inserts the appropriate block
Preview: NXT-G switch
• Switch block can have sensor or value to
control
• Think of this as IF/ELSE
Preview: NXT-G
•   Set up two (or more) sequences of blocks.
•   NXT-G generates coding.
•   This may produce essentially the same as
periodic checks along one thread.
General comment
• Program may set up to check for
sequence/set of events over and over
– Duty cycle
– Event loop
• Can give the program too much to do.
• Think of sentry doing guard duty, adding
checkpoints.
Mechanics 101
Very, very loose definitions
Torque used for defining the power of the motor.
Torque = force * distance. Motor can apply that force at that
distance.
Velocity is speed in a direction—a vector as opposed to a scalar
quantity. Acceleration refers to changes in velocity. Robots need
acceleration to start and to overcome gravity and friction.
For wheels, velocity = circumference * rpm.
Revolutions Per Minute depend on torque from motor AND friction
from wheels against specific surface
Gears/gearing is to put the torque where you want it.

Work load of robot is the robot itself plus the payload (what it needs to
carry)
Considerations for robots
• Want motor that is powerful enough to do
the job
– Do the job most of the time
• Not too powerful, because powerful
generally means expensive and heavy
• Robot may need to carry itself + tooling +
parts
– Payload refers that to that which pays for the
job.
Lab: challenges
• Measurements: measure & record distance
traveled and time taken
– 180 degrees, 1 rotation, 5 rotations at power levels
25, 50, 75.
– 1 second, 5 seconds [time taken should be close to
inputs]
Prepare neat tables to show.

• Navigate field with obstacles (fixed = static)
– Go around obstacle (chair or stool). You decide route.
Time it. Prepare report to show.
Suggestion

• You can use
the wait until
block with the
NXT buttons.
This gives you
time to make
the
measurements
Homework
Read statics, dynamics and gear sections of
http://www.societyofrobots.com/mechanics_dynam
ics.shtml
Or
Read about gears, torque, motors in Perdue book
(in Library)
Or
Find 'spec' data (torque, rpm, etc.) on Lego
Mindstorms motor
Write summary, including definition of terms,
to turn in next class!

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