6 - Localisation & Mapping

Question 1

Name 3 methods of localisation

Answer 1

Based on external sensors, beacons and landmarks

Odometry (proprioceptive sensors)

Map-based (no external sensors and landmark, only onboard sensors)

Question 2

Discuss GPS positioning

Answer 2

GPS provides metre-level outdoor positioning using information from multiple satellites

Question 3

What are problems with GPS?

Answer 3

Atmospheric conditions limit precision unless corrected using a reference

Can be power hungry

Limited accuracy

Poor in urban environments

Need infrastructure

Question 4

Discuss beacon based localisation

Answer 4

Generally limited accuracy

External references can be used for triangulation

City line of sight may be blocked

Signals may be jammed

Question 5

What is a Home vector?

Answer 5

Estimated location = vector relative to home

Question 6

What is Odometry?

Answer 6

An approximation of the location of a robot can be obtained by repeatedly computing the distance moved and the change direction from the velocity of the wheels in a short period of time

Question 7

What is used to try and reduce the accumulated errors in Odometry?

Answer 7

Heading sensors, such as a gyroscope

Question 8

What is Visual Odometry?

Answer 8

Odometry is not limited to wheel encoders, in visual odometry consecutive camera images are used to estimate velocity

Question 9

What is the solution to the problem of self-localisation?

Answer 9

Use a map. Combine odometry with sightings of known landmarks/environmental features

Question 10

Name 5 sources of Odometry errors

Answer 10

Misalignment of wheels

Unequal wheel diameter

Variation in the contact point

Unequal floor contact of the wheel

Limited resolution during integration

Question 11

What are the 3 types of Odometry error?

Answer 11

Range, turn and drift

Question 12

What is a Range rrror?

Answer 12

Sum of the wheel motion leads to an error in the integrated distance of the robot’s movement

Question 13

What is a Turn error?

Answer 13

Difference of the wheel motions leads to an error in the robot’s final orientation

Question 14

What is a Drift error?

Answer 14

Difference in the error of the wheels leads to an error in the robot’s angular orientation

Question 15

What does SLAM provide?

Answer 15

The best of both worlds by using Odometry for rapid position updates which are realigned periodically using the map

Question 16

What is a map?

Answer 16

A collection of elements or features at some scale of interest and a representation of the spatial and semantic relationships among them

Question 17

What can maps be used for?

Answer 17

Localisation, planning, mobile manipulation and human-robot interaction

Question 18

Why is mapping highly labour intensive?

Answer 18

Exploration (global coverage), Measurement, Validity, Currency

Question 19

Name 4 types of maps

Answer 19

Metric, topological, semantic, hybrid

Question 20

What is a Metric map?

Answer 20

Records the location of objects in an absolute coordinate system

Question 21

What is a Topological map?

Answer 21

Records the connections (edges) between a set of places (nodes)

Question 22

What is a Semantic map?

Answer 22

Records semantic information (metadata), includes segmentation, place/object naming, function

Question 23

What is a Hybrid map?

Answer 23

Combines two or more map types

Question 24

What are 2 types of Metric map?

Answer 24

Continuous/vector format and discrete/raster format

Question 25

What is a continuous/vector metric map?

Answer 25

Points, linear or curved segments, surface patchess

Question 26

What is a Discrete/raster metric map?

Answer 26

Occupancy grids

Question 27

What is an Occupancy Grid an example of?

Answer 27

Metric Map

Question 28

What is an Occupancy Grid?

Answer 28

Where each cell is associated with a probability that the cell is occupied. It’s updated using the current robot pose and depth measurements from range-finding sensors

Question 29

What are the 4 regions in an Occupancy Grid?

Answer 29

Probably occupied

Probably empty

In the shadow of the detected object so status unknown

Outside the beam so status unknown

Question 30

What is Probabilistic robotics?

Answer 30

Explicit representation of uncertainty using the calculus of probability theory

Question 31

What can the edges in a Topological map also represent?

Answer 31

Actions needed to get from one node to the next, or direction and distance

Question 32

Discuss path planning in Topological maps

Answer 32

Can determine a route by standard graph search methods such as A* or Dijkstra, if the distances or costs are added to the edges

Question 33

What are 5 advantages of Metric Maps over Topological Maps?

Answer 33

Detailed, quantitative, sub-symbolic representation

Good for representing and avoiding known, static obstacles

High computational cost of storage and processing

Requires very accurate position tracking (reliance on accurate odometry and range findder sensors)

How to determine an appropriate resolution?

Question 34

What are 5 advantages of Topological maps over Metric Maps?

Answer 34

Abstract, qualitative, symbolic representations

May be more persistent/robust to environment dynamics

Low computational cost (efficient path planning, scale better to large environments)

Require accurate place recognition (problem of perceptual aliasing, what if 2 or more places look alike)

How to determine what makes a place?