kinematics, dynamics, and sensors of autonomous robotswolff/aa/aa20080125_lect02_public.pdf · –...

Applied Mechanics

© Krister Wolff, PhD, Chalmers Univ. of Tech.Autonomous Agents 2008

Kinematics, dynamics, and sensors of

autonomous robots2008-01-25

Lecture 2

Applied Mechanics


Robotic hardware• Sensors

– measure some physical characteristic, e.g. light intensity, and transform it into electrical signals.

• Actuators (motors)– physical devices that transform energy into

mechanical motion.

• Microcontrollers– electronical devices for analyzing the sensory

information, making decisions, and transmitting commands.

Applied Mechanics


Sensors• Aid in localization• Obstacle avoidance• Monitor the internal state of the robot

– temperature– battery level– wheel speed– etc.

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Infrared (IR) proximity sensors• Used in order to detect nearby objects

• Consists of:– Light-emitting

diode (LED)– Light detector

(phototransistoror photodiode)

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IR LED and IR detector

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IR sensor working principle• Ic is controlled by

the light intensityon the detectorelement

=> Vin = R2Ic

0 < Vin < 5 [V]

• Light intensity is measured by the detector.

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IR range sensor• SHARP GP2D12

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IR ranging• Return a distance

measurement• Based on geometrical

relationships(triangulation)

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Optical (wheel) encoders• Slotted disc on the wheel axis, and a detector

• Determine robot's position and heading

• Incremental measurementsof the distance travelled by each wheel of the robot:Odometry, dead reckoning

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3-bit absolute optical encoder• 8 (23) logical states:

• Measure the absolute position of a rotary shaft

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Incremental optical encoders• Simple incremental encoder (A):

• Quadrature encoder (A+B): Direction of rotation can be determined

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Odometry• Prone to errors:

– wheel spin– wheel slip– robot sliding

• Errors accumulate quickly– especially when turning

• Frequent calibrationis necessary

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Ultrasound (sonar) sensor• SOund NAvigation and Ranging• Time-of-flight measurement:

– "ultrasound echolocation"

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SONAR Challenges• Foreshortening:

– The axis of the beam is at an angle to the wall.

• False reflections:– Make objects look further away than they are.

• Sensitive to texture

range returned

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Homing sensors• Can be used for locating e.g. a charging

station• IR beacon +

photodetectors

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Inertial navigation system • Accelerometer

– measures linear accelerations

• Gyroscope– measures angular motion

• Integration => Translation can be determined.

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Global Positioning System (GPS)

• Satellite-based sensing system– GPS uses timing signals from at least four satellites to

establish a position.

• Differential GPS:– Two GPS receivers; one on robot, the other stationary

=> error can be estimated

• Sensor Resolution:– GPS: 10-15 meters– DGPS: up to a few centimeters

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GPS Challenges• Satelite signals can be blocked:

– Amidst tall buildings (urban canyons)– Forested areas or mountain peaks– Does not work indoors in most buildings

• INS complementary system.• ... and compasses as well!

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Laser range finder• Hokuyo URG-04LX• Time-of-flight measurement.

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Laser range finder

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Actuators• Different motors:

– Electrical motors (AC, DC)– Piezoelectrical– Pneumatic motors– Hydraulic motors– Combustion engines

• We focus on electrical, direct-current (DC) motors!

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Lorentz force• Conducting wire• Current I• Magnetic field B

=> F = I x B

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DC motor principle• Current through closed loop of wire:

=> Forces acting on opposite sides of the loop => the loop turns!

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Direct-current (DC) motor• The parts of a standard DC motor are:

– the stator• providing the magnetic field

– the rotor• containing the coils (loops of wire)

– the commutator• reverses the current through the coils every half

turn

– the brushes• conduct the coils with the current (power supply)

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Direct-current (DC) motor

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Gears• Motor + gear =>

slow down (or up!) the rotational speed

• Increase/decreasegenerated torque

• Gear ratio: G=win/wout= torquein/torqueout

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Controlling a DC motor• Rotational velocity is proportional to the

applied voltage.

• The generated torque is proportional to the current.

• Refer to Sect. 1.1.2 in Handout 1 for a DC motor model.

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DC Motor Control (simplified)• The H-bridge circuit

– Four switches are controlled by a microprocessor

– The switch configuration determine the direction in which current is allowed to pass through the motor.

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DC Motor Control (simplified)• Opening and closing

switches at different rates

=> different average voltages across the motor

• Pulse-width modulation (PWM)

• 0%, 25%, 75%, and 100% Duty Cycle

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DC Servo Motors• Built-in potentiometer

and PID regulator for positioning=> PWM control

• Built-in gear box

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Microcontrollers

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The Canonical Computer• The Von Neumann Architecture

– Central Processing Unit, CPU– Ports, or I/O devices– Memory– System bus

• Implemented in silicon:microprocessor

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Board of Education• Single-chip

microprocessor + ''Motherboard''=> Microcontroller

• Board for Parallax' Basic Stamp modules.

• Ports:– 16 I/O– 4 pwm utg.– serial

• SDK, support, etc.• Refer to:

www.parallax.com

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BasicX Microcontroller• BasicX-24P• 83,000 BASIC-

instructions/sec.(BS2: 4000)

• EEPROM: 32K bytes• Max programlength:

8000+ lines• RAM 400 bytes• I/O pins: 21• 32 bit floats• www.basicx.com/

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Computational Hardware• The EyeBot microcontroller

– fully integrated single board microcontroller

Thomas Bräunl, 2002

EyeBot microcontroller,front view

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EyeBot microcontroller

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Microcontrollers, examples:• EyeBot

– Motorola MC68000, ”complete”, mobile robot MC, incl. vision.• Basic Stamp,

– Parallax, based on Microchip’s PIC™, Starter/development kits.• MAVRIC II

– ATmega128, Atmel, fusion of many sensors and digital I/O, servos and motor drivers.

• BrainStem™ GP 1.0 Module– 40 Mhz RISC processor, robotics development environment

including modules, hardware accessories, software• The Gumstix,

– small computer that runs Linux 2.6, based on Intel XScale®, 200/400MHz

Applied Mechanics


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