Exam 2 ME503

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1. What is Localization
Determining the position & orientation of the robot in a desired reference frame
2. Is an essential component in an autonomous system
Mobile robot localization
3. Localization Challenges
• Accuracy & Precision
• Pose not just position
• Sensor noise
• Error Propagation
4. Accuracy & Precision
How well does pose need to be known
5. Pose not just position
Heading, velocity, acceleration may be needed
6. Sensor noise
Price increases inversely with noise characteristics
7. Error Propagation
How is the error expected to develop
8. There are several (not mutually exclusive) approaches to localization (4)
• Odometry
• Active beacons
• Landmarks
9. What is odometer
use the kinematic model and wheel velocity measurements to estimate pose
use the wheel velocity and heading sensors (e.g. compasses, IMU) to estimate pose
11. Example of a active beacon
GPS
12. Example of landmarks
Stereo odometry
13. What is an encoder used for
An encoder is used to measure angular position or velocity
14. What is the principle of operation for an encoder
generate pulses as the shaft rotates
15. For an encoder the angular position is
proportional to the number of pulses
16. Incremental Encoders and what phase are the channels out
• Generate pulses and use two output channels (A and B) to sense position(Two channels or 4 states).
• Ch A and Ch B are 90 degrees out of phase
17. IN single ended or open collector encoders
only two channels are used (Ch A & Ch B)
18. In differential encoders
At least four channels are used (Ch A, Ch A’, Ch B, Ch B’) with Ch A’ 180± out of phase with Ch A and Ch B’ 180± out of phase with Ch A.

Channels are compared to reduce noise
19. Absolute encoders
Generate a unique word pattern for every position of the shaft. Number os signal lines determines the resolution.
20. Where would you put an encoder and why
Would put on the output shaft because of gear ratio
21. Why use gray code
Only one bit changes at a time. Good because only have to monitor one bit change at a time and it has fewer channels (more rigid)
22. With odometry what causes error and where do the biggest errors come from
errors are the result of wheel slip, propagate, leading to more uncertainty the longer the systems operates. Biggest error comes when turing so from theta
23. Why is error propagation in dead reckoning generally less than odometry
Due to in inclusion of heading sensors
24. What are two different sensor types used for heading estimation
• Magnetic compass
• inertial sensores (Accelerometers, gyroscopes, IMU)
25. Magnetometers
measure strength and or direction of magnetic field
26. compass sensor
magnetometer that determines heading angle of a vehicle using earth's magnetic field
27. Two types of magnetic compasses
• fluxgate compass (large mobile robotics)
• MEMs compass (small robotics)
28. The strength of earth's magnetic field is the _______ vector
resultant
29. Magnetic compass sensors experience distortion due to
metal objects or artificial magnetic fields
30. Compass may not always provide a
true absolute reference
31. Magnetic compass can still be used in the ____ reference and why
local reference, in all situations where the external influences are constant over time
32. Accelerometers are used to estimate and most use what type of sensors
mobile robot position and velocity and MEMS
33. Accelerometer readings and specification are reference to
gravitation acceleration
34. An accelerometer sensor detects acceleration in
one axis, is insensitive to accelerations in orthogonal axes
35. Accelerometers are used to measure
• inclination (if stationary or constant speed)
• vibration
36. Gyroscopes are the key components of
37. gyroscopes provide _____
they measure _____
inertial platforms for accelerometers and measure angular displacement and velocity
38. most well known gyroscope is the ____ and it consists of
fly wheel gyroscope and of a wheel whose angular momentum is such that the axis of rotation tends to remain fixed in space
39. Vibrating structure gyroscopes use
MEMs devices, most common today and operate on Coriolis force principle.
40. Coriolis force principle
to travel in a straight line, lateral velocity must be increased/decreased
41. Laser ring gyroscope
Highest accuracy and does not suffer from noise issues of MEMs gyroscope. Difference in wavelengths tells you the rate of rotation
42. IMUs refer to
3-,6-, or 9 axis units with each axis referred to as a DOF
43. These units incorporate a combination of 3-axis
• accelerometers
• gyroscopes
• magnetometers
44. Inertial measurements are either in terms of ______ and _____. And must be
acceleration or velocity, must be integrated in order to give positions
45. What is an Active beacon?
Signal must be ?
multiple bacons are used for ?
common types?
• emit a signal or energy that is read by an onboard sensor.
• signal is interpreted to determine distance
• triangulation
• IR and GPS
46. What is GPS
method of finding the position of an object with reference to a fixed global coordinate frame
Measures 6 DOF, IMU + GPS, uses sensor fusion through kaman filter (accelerometers,gyros,gps), and found in high cost applications
48. Difference in
GPS:
Inertial:
• Does not accrue error over time
• slow update rate
• gps position jumps

• high short term accuracy
• fast update rate
• smooth changes in position
49. Examples of Landmarks (4 examples)
• Passive buoys
• visual odometry
• LIDAR edge detectors
• Star trackers
50. Kalman filtering
used to estimate dynamic variables in the presence of noise
51. Kalman dynamic case components
• process model
• measurement model
• noise characteristics
52. Process model
• state space representation of dynamics
• linear discrete model
53. measurement model
• relatives measurements to process states
• linear discrete model
54. noise characteristics
55. Extended kalman filter
enables basic kaman filter equations to be applied by linearizing the system at each sample instant about the input and the state estimate
56. Pressure sensing can provide
altitude and depth
57. MEMs pressure sensor use either a
resistive or capacitive approach
58. resistive approach
a piezoresistive diaphragm displaces and the resulting resistance is measured with a wheatstone bridge
59. capacitive approach
the pressure difference changes the capacitance between electrodes
60. Resistive sensors are :
• small
• linear relationship
• temperature sensitive
61. Capacitive sensors are :
• bigger
• lower cost
• nonlinear
• good for high temp
62. What can still occur using inertial sensors and why
dead reckoning and do to major drift errors
63. Mapping in autonomous robots
assumes position and pose of the robot are known, sensors are used to locate and measure landmark positions, kaman filtering is often used to better estimate landmark positions
64. What is SLAM
robot is placed in an unknown location in an unknown environment. robot incrementally builds a map of the environment while simultaneously using this map to compute its location
65. What is active ranging. Is Active or Passive more accurate ?
Active ranging is the process of determining the distance to an object via active sensors

Active
66. what qualities of phase measurement and one application
• extreme active
• suffers from range ambiguity
• often used for relative motion

Application:Laser Vibrometers
67. what qualities of time-of-flight and an application
• limited resolution
• provides absolute range
• typically has a minimum range

Application: Laser rangefinders
68. Infrared sensors to measure distance also experience errors due to:
• hysteresis nonlinearity
• Low signal to noise
69. Time of flight sensors
• ultrasonic
• and laser
70. For use in mobile robotics, LIDAR data is typically treated in one of two ways:
• Occupancy grid
• Object segmentation
71. Trade off of Occupancy grid
Object segmentation
• occupancy grid - more memory
• object segmentation -more computational
72. Occupancy Grid
• Simple representation
• Well suited to graph search algorithms

• Inherently loses information
• Deals poorly with motion
73. Object Segmentation

• Keeps object information
• Enables object identification
• Provides ability to track objects
• More memory efficient than grid

• More complicated to implement
• More difficult to search
74. Line Segmentation Algorithms
• Split-and-Merge
• RANSAC
• Hough Transform
75. Split and merge is not robust to
outliers
76. Split and merge recursive algorithm
• A line is fit to the data
• The line is split if the variance is too high
• Adjacent lines are merged if close enough
• Iterate until all thresholds are achieved
77. RANSAC is highly
robust to outliers
78. RANSAC uses what method
non-deterministic method, relies on random sampling
79. Hough transform is a what scheme
voting
80. hough  transform is often used in ?
Good for what type of sensors ?
• vision processing
• vision base sensors
• end points are hard to compute
81. Algorithm Comparison:
Split and merge
Line-regression
RANSAC
Hough-Transform
• Speed     False Positives   Precision
• 1500              10               +++
• 400                10               +++
• 30                  30              ++++
• 10                  30              ++++
82. Sonar
Active
Passive

Listens for ambient acoustic energy
83. Sonar sensor uncertainties
exact position of detected object is unknown

sonar distortion
84. when do specular reflections occur
when the sonar beam hits a smooth surface at a shallow angle, and is therefore not reflected back to the robot, but outwards.
85. time-of-flight cameras
capture the whole 3D scene and is without moving parts
86. Example of a passive triangulation
stereo camera
87. Triangulation and active ranging
structured light sensor
88. Structured light sensors:
Sensor resolution is limited by ?
Sensor range is limited by ?
• Image resolution
• laser strength
89. Structured light is best used for
give objects profiles at high resolution
90. Doppler Effect is used with ____ to measure ______. One problem?
• active ranging and velocities
• may only detect moving objects
91. radar can be used as a ____ or _____? Radar can eight be _____ or ______
• ranging or speed sensor
• active or passive
92. ranging is accomplished using?
speed estimation occurs through ?
• time-of-flight
• doppler effect
 Author: coffeya ID: 309833 Card Set: Exam 2 ME503 Updated: 2015-11-02 00:54:14 Tags: Exam ME503 Folders: Description: Exam 2 ME503 Show Answers: