Post on 16-Oct-2021
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MECH 498: Introduction to Robotics
Actuation, Sensing, and Design
M. O’Malley
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Manipulator Mechanical Design• Particular structure of a manipulator influences kinematic and
dynamic analysis• The tasks that a manipulator can perform will also vary greatly with a
particular design (load capacity, workspace, speed, repeatability)
• The elements of a robotic system fall roughly into four categories– The manipulator mechanism & proprioceptive sensors– The end-effector or end of the arm tooling– External sensors (e.g. vision system) or effectors (e.g. part feeders)– The Controller
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Manipulator Mechanical Design –Task Requirements
• Robots usually don’t fit the ideal of universally programmable devices
• Task Specific Design Criteria– Number of degrees of freedom– Workspace– Load capacity– Speed– Repeatability accuracy
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Task Requirements - Number of DOF
• The number of DOF in a manipulator should match the number of DOF required by the task.
– Minimizes cost (hardware, computing power, and power consumption)
– Minimizes size/weight
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Task Requirements
• Not all the tasks required 6 DOF for example:– End effector with an axis of symmetry - Orientation
around the axis of symmetry is a free variable,– Placing of components on a circuit board - 4 DOF
• Dividing the total number of DOF between a robot and an active positioning platform
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Task Requirements• Workspace (Work volume, Work envelope)
– Placing in the work space of the manipulator– Singularities– Collisions
• Load Capacity– Size of the structural members– power transmission system– Actuators
• Speed– Robotic solution must compete on economic basis– Process limitations - Painting, Welding– Maximum end effector speed versus cycle time
• Repeatability & Accuracy– Matching robot accuracy to the task (painting - spray spot 8 +/-2 “)– Accuracy function of design and manufacturing (Tolerances)
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Kinematic Configuration
• Joints & DOF -– For a serial kinematic linkages, the number of joints
equal the required number of DOF• Overall Structure
– Positioning structure (link twist 0 or +/- 90 Deg, 0 off sets)
– Orientation structure• Wrist
– The last n-3 joints orient the end effector– The rotation axes intersect at one point.
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Kinematic Configuration - Cartesian
• Joints– Joint 1 - Prismatic– Joint 2 - Prismatic– Joint 3 - Prismatic
• Inverse Kinematics - Trivial• Structure -
– Stiff Structure -> Big Robot– Decoupled Joints - No singularities
• Disadvantage– All feeder and fixtures must lie “inside” the robot
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Kinematic Configuration - Cartesian
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Kinematic Configuration -Articulated
• Joints– Joint 1 - Revolute -Shoulder– Joint 2 - Revolute - Shoulder– Joint 3 - Revolute - Elbow
• Workspace– Minimal intrusion– Reaching into confined spaces– Cost effective for small workspace
• Examples– PUMA– MOTOMAN
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Kinematic Configuration -Articulated
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Kinematic Configuration - SCARA
• Joints– Joint 1 - Revolute– Joint 2 - Revolute– Joint 3 - Revolute– Joint 4 - Prismatic– Joints 1,2,3 - In plane
• Structure– Joint 1,2,3, do not support weight (manipulator or weight)– Link 0 (base) can house the actuators of joint 1 and 2
• Speed– High speed (10 m/s), 10 times faster then the most articulated
industrial robots• Example
– SCARA (Selective Compliant Assembly Robot Arm )
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Kinematic Configuration - SCARA
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Kinematic Configuration - Spherical
• Joints– Joint 1 - Revolute (Intersect with 2)– Joint 2 - Revolute (Intersect with 1)– Joint 3 - Prismatic
• Structure– The elbow joint is replaced with prismatic joint– Telescope
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Kinematic Configuration - Spherical
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Kinematic Configuration -Cylindrical
• Joints– Joint 1 - Revolute– Joint 2 - Prismatic– Joint 3 - Prismatic
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Kinematic Configuration -Cylindrical
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Kinematic Configuration - Wrist
• Joints– Three (or two) joints with orthogonal axes
• Workspace– Theoretically - Any orientation could be
achieved (Assuming no joint limits)– Practically - Severe joint angle limitations
• Kinematics– Closed form kinematic equations
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Kinematic Configuration - Wrist
• Three intersecting orthogonal Axes– Bevel Gears Wrist
• Limited Rotations
• Three Roll Wrist (Cincinatti Milacron)• Three intersecting non-orthogonal
axes• Continuous joint rotations (no limits)• Sets of orientations which are
impossible to reach
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Kinematic Configuration - Wrist
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Kinematic Configuration - Wrist
• Non intersecting axes wrist
• A closed form inverse kinematic solution may not exist
• Special Cases (Existing Solutions)– Articulated configuration
• Joint axes 2,3,4 are parallel– Cartesian configuration
• Joint axes 4,5,6 do not intersect 22
Actuation Schemes
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Reduction & Transmission Schemes
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Kinematic ConfigurationsDesign
• Decide degrees of freedom first• Then choose kinematic configuration to
obtain the best– Workspace– Dynamic properties– Use of actuators and sensors– Accuracy
• A general, 6 dof manipulator is usually classified by the first 3 dof plus a wrist
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Workspace Attributes
• Design efficiency• How much material is needed to build different
designs with the same workspace?• Length sum
• Structural length index
• (W = workspace volume, di = distance between joint limits)
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Condition of Workspace
• When the manipulator is near a singular point, actions of the manipulator are said to be poorly conditioned.
• Singular conditions are given by• Thus, use the Jacobian as a measure of
manipulator dexterity
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Manipulability Measure (vel)
• Yoshikawa defines manipulability as
• For a nonredundant manipulator
• A good manipulator has a high w over large areas of its workspace
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Manipulability (acc/force)• Asada examines the eigenvalues λI and
eigenvectors of the Cartesian mass matrix
• Graphically, this can be represented as an inertia ellipsoid
• This is the equation of an n-dimensional ellipsoid, where n is the dimension of X– Axis directions are eigenvectors and lengths are .– See Craig, figure 8.12.
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Redundant structures
• Can be useful for avoiding collisions while operating in cluttered work environments
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Closed Loop structures
• So far, we have only considered serial chain manipulators
• Closed loop, or parallel, structures can be stiffer and more precise
• But, they typically decrease joint ranges and therefore workspace size
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6dof Parallel manipulator
• Stewart Platform– (inverse kinematics easy, forward kinematics hard!)
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DOF for closed loop system
• DOF not readily obvious• Grubler’s formula for closed chain manipulators
states
– Where F is the total dof in the mechanism– l is the number of links (including the base)– n is the total number of joints– fi is the dof associated with the ith joint
• Stewart F = 6(14-18-1)+36 = 6
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Actuators and Sensors
• First, choose general kinematic structure• Next, choose actuation
– Actuator– Reduction– Transmission
• Finally, select sensors• (And then control)
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Actuator Location• Direct Drive
– Placed at the joint– Simple and high controllability– No transmission or reduction elements
• However, speed reduction is often required because many actuators are suited to high speeds and low torques
• Also, weight/inertia of actuators affect the dynamics, so the actuators are placed at or near the robot base. Thus, a transmission system must be used.
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Reduction & Transmission
• Gears produce large reductions in a compact configuration
• Disadvantages:– backlash and friction
• Gear ratio: relationship between input and output speeds & torques
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Types of gears• Spur Gears
– (parallel shafts)• Bevel gears
– (orthogonal shafts)• Worm gears/cross helical
gears– (skew shafts)
• Rack & Pinion• Consider load, wear and
frictionSRL, Georgia Tech
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• Flexible Transmission– Bands, Cables, Belts
• Capstan drive used in haptic devices• Need large preloads to ensure the cable stays
engaged
Jake Abbott, JHU
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Actuator Types
• Electric motors– DC (direct current)– Brushed– PM (permanent
magnet)• Pneumatic Actuators
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PM DC brushed motors
• How do they work?– Rotating armature with coil
windings is caused to rotate relative to a permanent magnet
– current is transmitted through brushes to armature, and is constantly switched so that the armature magnetic field remains fixed.
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DC Motor Components
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DC motor terms
• Cogging– Tendency for torque output to ripple as the
brushes transfer power• Friction/damping
– Caused by bearings and eddy currents• Stall torque
– Max torque delivered by motor when operated continuously without cooling
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Motor Equations
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Pneumatic Actuators
• How do they work?– Compressed air pressure is used to transfer
energy from the power source to robotic device
• Many different types• Concerns are
– friction – bandwidth
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Robot Sensors
• Manipulators– Proprioception, Force
• Mobile Robots– Dead reckoning, Tactile and proximity,
Ranging, etc.• Recommended reading:
– Mobile Robots by Joseph J. Jones and Anita M. Flynn
– Sensors for Mobile Robots by H.R. Everett
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Manipulator Sensors
• Primary concern is proprioception• Kinesthesia/Proprioception/Force:
– A sense mediated by end organs located in muscles, tendons, and joints.
– Stimulated by bodily movements.
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Potentiometers
• Produce a voltage proportional to shaft position
• Voltage divider
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Potentiometers
• Problems:– Friction (for backdriveable systems like haptic
devices)– Noise– Resolution– Linearity
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Optical Encoders• How do they work?• A focused beam of light aimed at
a matched photodetector is interrupted periodically by a coded pattern on a disk
• Produces a number of pulses per revolution (Lots of pulses = high cost)
• Quantization problems at low speeds
• Absolute vs. referential
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Optical Encoders
• Phase-quadrature encoder– 2 channels, 90° out of phase– allows sensing of direction of rotation– 4-fold increase in resolution
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Hall-Effect Sensors
• How do they work?– A small transverse voltage is generated across a
current-carrying conductor in the presence of a magnetic field
– (Discovery made in 1879, but not useful until the advent of semiconductor technology.)
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Hall-Effect Sensors
• Amount of voltage output related to the strength of magnetic field passing through.
• Linear over small range of motion• Need to be calibrated• Affected by temperature, other magnetic objects in the
environments
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Measuring Velocity
• Differentiate position– advantage: use same sensor as position
sensor– disadvantage: get noise signal
• Alternative– for encoders, measure time between ticks
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Digital differentiation
• Many different methods• Simple Example:
– Average 20 readings = P1– Average next 20 readings = P2where t is the
the period of the servo loop• Differentiation increases noise!
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Time-between-ticks
• Encoders fare poorly at slow velocities– There may be very few ticks during a single servo loop
• Instead, use a specialized chip (PLC) that measures time between ticks– Fares worse at high velocities
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External sensors• Computer Vision
– Use vision to determine linkage position
• Magnetic– e.g., Ascension flock of
birds• Force
– Commercial load cells/force sensors
– Direct application of strain gages
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Mobile Robot Sensing
• Transducing vs. understanding• Levels of abstraction:
– Is it light or dark?– Is there a wall to the left?– Who just walked in the room?
• Algorithms are required to determine the desired information from basic sensor data
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Example
• Robart II– sonar, infrared,
bump, microwave motion, burglar alarm, surveillance camera, earthquake, & flood
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Example
• Hannibal– Force, touch, color, potentiometer, force-sensing
whisker, gyroscope, pitch-and-roll, small camera, near-infrared rangefinder
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Sensitivity
• Sensor output: r• Measured physical quantity: x• Sensitivity is S• Typical sensors output a voltage
corresponding to the value of x, so amplifiers can improve sensitivity
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Range• Signals should be detected and amplified so that
the output falls in the correct range• Consider a signal where 0 = nothing and 255 =
maximum
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More Sensor Metrics• Resolution:
– What is the smallest change you can measure?• Signal to Noise Ratio
– The signal is useful information and noise is anything else. A low ratio is bad!
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Light Sensors
• Photoresistors (photocell)– Variable resistance, like a
potentiometer• Phototransistors
– Greater sensitivity• Photodiodes
– Highest sensitivity, but low output requires amplifier
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Proximity Detectors• Near-infrared detectors
(IRs)– Signifies whether
something is present within a cone of detection
– Emitter-detector pair• Pyroelectric detector
– Output changes with small changes in temperature over time
– Detects radiation in the range of (8-10 µm)
– Useful for sensing of humans
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Mobile Robot force sensors
• Microswitches for bumpers• Bend Sensors (conductive ink)
– 3-5x resistance change• Force-sensing resistors
– Several orders of magnitude resistance change
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Cameras
• Video camera technology is rapidly changing
• CCD cameras can pick up near-infrared light
• On-board processing• Cell phone revolution
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Sound sensors
• Microphones• Piezoelectric Film
– Really senses vibrations• Sonar
– Measure time-of-flight with emitter-detector pair (ping, then echo)
– Very commonly used in advanced robotics research
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Use of SONAR data
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Position and Orientation
• Encoders on Wheels– Dead reckoning– Does not account for slip
• Rate gyroscope– Determines speed of rotation
• Tilt sensors (e.g., mercury switch)• Compass
– ~45 degrees error due to metal components/indoors• GPS
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Robot State
• Battery level– Time until recharging is required
• Stall current– Detects when wheels are not turning– Needs to respond slowly
• Temperature– Of motors, microprocessor
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Commercial Mobile Robot
• Roomba vacuum cleaner from iRobot