Robotics Flashcards
Classifications of robots
- Manual handling devices - device with multi degrees of freedom actuated by an operator
- Fixed sequence - device that performs successive stages of a task according to a predetermined and fixed program
- Variable sequence - performs the successive stages of a task according to a predetermined but programmable method
- playback robot - human operator performs the task manually by leading the robot, motions recorded for later playback , robots repeats the motions according to the recorded information
- Numerical control - operator supplies robot with motion programme rather than teaching it the task manually
- Intelligent robot- ability to understand its environment and successfully complete a task despite changes in the surrounding condition
Robot mechanical subsystems
- mechanical subsystem composed of rigid bodies
- a sensing subsystem
- an actuation subsystem
- a controller
- an information processing subsystem
Wrist DoF
Roll - rotation about the X-axis of global coordinate system
Pitch - rotation about the Y-axis of global coordinate system
Yaw - rotation about the Z-axis of global coordinate system
Proximity sensors
Inductive sensor - sense existence of a metallic object due to a change in inductance
Capacitative - sense existence of gas, liquid or metals that cause a change in capacitance
Hal effective - work based on interaction between voltage of semiconductor material and magnetic fields
Sonic, ultrasonic & optical - work based on the reflection or modification in an emitted signal by objects
End effector considerations
- shape & size of grasped object
- uniformity or tolerance of handled objects
- are surfaces smooth, what is the coefficient of friction
- what is the spacing of objects
- how will they be oriented and presented
- what placement and spacing dimensions can be modified
- what are placement tolerances
- what are grasping forces or pressure limits
- are complementary tools needed
- is compliance necessary for assembling parts
- special environmental considerations
Cleanroom design guidelines
- minimise number of moving parts
- place all moving parts below the substrate
- enclose and seal the robot interior and evacuate generated particles
- use internal robot cabling only
- apply coating or treatment to external robot surfaces
- use cleanroom approved lubricants
- use stainless steel screws and washers
- minimise the contact surface of end-effectors
- brushless motors
- direct-drive trains to eliminate belts
Transformation of kinetic geometry
- determination of the trajectory in cartesian coordinate space
- transformation of the cartesian trajectory into equivalent joint coordinate space
- generation of motor torque commands to realise trajectory
Euler angles
precession = inverse Cos(r33)
nutation = inverse Tan(r31/r32)
spin angle = inverse Tan (r13/r23)
CCD camera principle
- photons of light striking the individual semiconductor elements in an array creates a charge on each element
- individual charges are coupled sequentially to the output, where they are amplified, converted to a digital value and stored in a frame buffer
Contour tracing algorithm
- random search algorithm used to find a boundary
- if pixels ahead are different, go ahead
- if both pixels 0, turn right
- if both pixels 1, turn left
General functions required of mobile robot systems
- environmental perception
- environmental modelling
- navigation
- task planning
- interaction
- safety monotoring
Environmental perception methods
- bumper switch
- ultrasonic sensor
- laser sensor
- CCD stereo camera
SCARA configuration
- two horizontal revolute joints
- widely used for assembly
- prismatic joint provides compliant vertical motion
Closed kinematic chain
- parallel linkages
- active and passive joints
- passive joints coordinates are functions of active joints and robot geometry
Electric actuated robots
- cleaner, quieter and more precise than hydraulic and pneumatic
- effective at high speeds so high gear ratio is needed
Typical robot joint actuation
- first axis direct motor drive
- second axis band drive
- third axis belt drive
- fourth axis linear ball-screw drive
Hydraulic advantages & disadvantages
ADV
- superior at high load carrying
- high power-to-weight ratios
- actuator control through solenoid or servo valve
DIS
- power supply is bulky
- fast & accurate servos are constly
- leak & maintenance limit their use
AC construction
- single phase or three phase
- coils on outside, magnets in the middle
- no. of windings and frequency of power fed to coils make the speed of the motor
- windings on outside can be cooled better
- best in constant speed applications
Permanent magnet DC
- coils on rotor, magnets on outside
- low inductance, low friction
- lots of electrical noise due to brush sparks, brushes can wear out and get clogged with dirt
- torque and speed can be controlled
- high gyroscopic effect due to coils on rotor
Stepper motors
- capable of precise incremental shaft motion
- low rotational mass so they can stop accurately, similar construction to AC & DC
- cant handle sudden changes in loads
Transmission types
Direct drive: high torque, low speed, eliminate free play & smooth torque transmission
Band drives: up to 10:1 drive ratios. actuator mass moved away from joint
Belt drives: up to 100:1 with multiple stages, tension controlled by idlers
Gear drives: spur or helical, sealed reliable and low paintenance
Harmonic: elastic geared ring, no backlash, lightweight, high gear ratios
Types of end-effector
PASSIVE & PREHENSILE
- parallel axis, typically pneumatic actuated, must be synchronised, close around jaw centre
- parallel/rotary jaws, not parallel, jaw centre changes
- four-bar linkage, rotary motion converted into parallel jaw motion, changing jaw centre
- chuck style jaws for cylindrical parts, very strong grip
Active end-effectors
- aim to mimic human hand, allow 3 dimensional grasping, can grasp irregular objects
- control is difficult as there are variations in contact area, fingers continually making/breaking contact
Remote Compliance Centre device (RCC)
- allows force/deflection properties of end effector to be tailored to suit a task
- allows an assembly robot to account for positioning errors
- can reduce damage, lower contact forces vie lower horizontal and rotational stiffness’
Payload considerations
- payload is the weight or mass of the end effector
- acceleration forces and moments are considered part of the load
- force calculations for payloads must be determined at all critical locations
6 human grasping modes
- spherical
- cylindrical
- hook
- lateral
- palmar
- tip
Link offset D
displacement along the Zi-1 axis to go from the link i-1 to the link i perpendicular
Joint angle Theta
rotation about Zi-1 required to align Xi-1 with Xi
positive rotation is clockwise looking in direction of Zi-1
Link length Ai
length of the perpendicular i.e. displacement required in the Xi direction to bring the origin frame {i-1} coincident with that of frame {i}
Link twist alpha(i)
rotation required about Xi to make Zi-1 coincident with Zi ( positive rotation is clockwise looking in the direction of Xi)
Common controller algorithm
- determine start location joint angles
- determine end location joint angles
- determine duration for complete movement
- for each joint determine a trajectory which would implement the move with correct duration
- move the 6 joints simultaneously according to individual trajectories
Determining straight line trajectory
- trajectory generation needs to be carried out in cartesian space
- trajectory is applied to linear coordinates and angles representing end-effector location
- trajectories are sampled to produce a set of transforms spanning the whole movement
- perform inverse kinematics on each of these gives a set of joint angle vectors for the manipulator to follow
- computer then solves the inverse kinematics repetitively
Image interpretation process
- image capture
- image processing
- pattern recognition
- feature extraction
- object identification - scene analysis
