1 CMPUT 412 Localization Csaba Szepesvári University of Alberta TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AA A A A A AA

2 Contents  To localize or not to localize  Noise and aliasing; odometric position estimation  Map representation  Belief representation  Probabilistic map-based localization  Other examples of localization system  Autonomous map building "Position" Global Map PerceptionMotion Control Cognition Real World Environment Localization Path Environment Model Local Map

3 Localization, Where am I? Odometry, Dead Reckoning Localization base on external sensors, beacons or landmarks Probabilistic Map Based Localization Perception

4 Challenges of Localization  Knowing the absolute position (e.g. GPS) is not sufficient Indoor environments Relation to other (possibly moving) objects  Planning needs more than location  Factors influencing the quality of location estimates Sensor noise Sensor aliasing Effector noise Odometric position estimation

5 To Localize or Not?

6  Constraints on navigation: navigation without hitting obstacles detection of goal location  Possible solution: follow always the left wall detect that the goal is reached

7 Behavior Based Navigation

8 Model Based Navigation

9 Noise and aliasing

10 Sensor Noise  Where does it come from? Environment: e.g. surface, illumination … Measurement principle: e.g. interference between ultrasonic sensors  What does it cause? Poor information  What to do? Integrate  Over sensors  Over time

11 Sensor Aliasing  What is it? Same reading, multiple possible locations (states)  How generic? Very..  What is the result? Insufficient information  What to do? Integrate  Over multiple sensors  Over time

12 Effector Noise, Odometry, Dead Reckoning  Wheel encoders are precise  Use it for keeping track of the robot’s location  Does this work??  What is “odometry and dead reckoning”? Position update is based on proprioceptive sensors  Odometry: wheel sensors only  Dead reckoning: also heading sensors  How do we do this? The movement of the robot, sensed with wheel encoders and/or heading sensors is integrated to the position.  Pros: Straight forward, easy  Cons: Errors are integrated -> unbound Improvement: additional heading sensors (e.g. gyroscope)

13 Odometry

14 Errors in Odometry  Error types Deterministic (systematic) Non-deterministic (non-systematic)  What to do with them? Calibration can eliminate deterministic errors Non-deterministic errors: leave with them!  Major Error Sources: Limited resolution during integration (time increments, measurement resolution …) Misalignment of the wheels (deterministic) Unequal wheel diameter (deterministic) Variation in the contact point of the wheel Unequal floor contact (slipping, not planar …) …

15 Classification of Integration Errors  Range error: integrated path length (distance) of the robots movement sum of the wheel movements  Turn error: similar to range error, but for turns difference of the wheel motions  Drift error: difference in the error of the wheels leads to an error in the robots angular orientation Over long periods of time, turn and drift errors far outweigh range errors! Consider moving forward on a straight line along the x axis. The error in the y-position introduced by a move of d meters will have a component of dsin, which can be quite large as the angular error  grows.

16 The Differential Drive Robot

17 Growth of Pose Uncertainty for Straight Line Movement  Note: Errors perpendicular to the direction of movement are growing much faster!

18 Growth of Pose Uncertainty for Movement on a Circle  Note: Error ellipse does not remain perpendicular to the direction of movement!

19 Map Representations

20 Map Representation  Issues 1.Map precision vs. application 2.Features precision vs. map precision 3.Precision vs. computational complexity  Continuous Representation  Decomposition (Discretization)

21 Representation of the Environment  Environment Representation Continuous Metric  x,y, Discrete Metricmetric grid Discrete Topologicaltopological grid  Environment Modeling Raw sensor data, e.g. laser range data, grayscale images  large volume of data, low distinctiveness on the level of individual values  makes use of all acquired information Low level features, e.g. line other geometric features  medium volume of data, average distinctiveness  filters out the useful information, still ambiguities High level features, e.g. doors, a car, the Eiffel tower  low volume of data, high distinctiveness  filters out the useful information, few/no ambiguities, not enough information

22 Continuous Line-Based Representation a)Architecture map b)Representation with set of infinite lines

23 Exact Cell Decomposition

24 Fixed Cell Decomposition

25 Adaptive Cell Decomposition

26 Topological Decomposition

27 Topological Decomposition node Connectivity (arc)

28 Challenges  Real world is dynamic  Perception is still a major challenge Error prone Extraction of useful information difficult  Traversal of open space  How to build up topology (boundaries of nodes)  Sensor fusion  …

29 Belief Representations

30 Belief Representation  Continuous map with single hypothesis  Continuous map with multiple hypotheses  Discretized map with discrete pdf  Discretized topological map with discrete pdf

31 Belief Representation: Characteristics  Continuous Precision bound by sensor data Typically single hypothesis pose estimate Lost when diverging (for single hypothesis) Compact representation and typically reasonable in processing power  Discrete Precision bound by resolution of discretisation Typically multiple hypothesis pose estimate Never lost (when diverges converges to another cell) Important memory and processing power needed. (not the case for topological maps)

32 A Taxonomy of Probabilistic Models More general More specific discrete HMMs continuous HMMs Markov Loc semi-cont. HMMs Bayesian Filters Bayesian Programs Bayesian Networks DBNs Kalman Filters MCMLPOMDPs MDPs Particle Filters Markov Chains S t S t-1 S t S t-1 O t S t S t-1 A t S t S t-1 O t A t Courtesy of Julien Diard S: State O: Observation A: Action

33 Single-hypothesis Belief – Continuous Line-Map

34 Single-hypothesis Belief – Grid and Topological Map

35 Grid-base Representation – Multi-Hypothesis  Cell size around 20 cm 2. Courtesy of W. Burgard

36 Probabilistic, Map-based Localization

37 The Problem  Problem: Odometry alone: uncertainty growth uncontrolled!  Why we do not get lost? “Look around”  Incorporate other sensory information!  Key problem: How to fuse observation?

38 Belief Update  Action update action model ACT o t : Encoder Measurement, s t-1 : prior belief state increases uncertainty  Perception update perception model SEE i t : exteroceptive sensor inputs s’ 1 : updated belief state decreases uncertainty

39 Why Does This Work??

40 The Five Steps for Map-Based Localization 1. Prediction based on previous estimate and odometry 2. Observation with on-board sensors 3. Measurement prediction based on prediction and map 4. Matching of observation and map 5. Estimation -> position update (posteriori position)

41 Methods  Aim is to calculate P( X t | Y 1,A 1,…, A t-1,Y t ) “posterior of state given the past observations and actions” == belief state “Filtering”  Methods – How is the belief state represented? Discretization  Markov Localization Continuous  Kalman Filter and its variants Particle cloud  Particle Filters

42 Markov  Kalman Filter  Markov localization Robust  localization starting from any unknown position  recovers from ambiguous situations Costly  Kalman filter localization Moderately robust Inexpensive

43 Markov Localization  Explicit, discrete representation for the probability of all positions in the state space.  Representation grid topological graph  Update: Sweeps through all possible states

44 Probability Theory Basics  P(A): Probability that A is true. e.g. p(r t = l): probability that the robot r is at position l at time t  We wish to compute the probability of each individual robot position given actions and sensor measures.  P(A|B): Conditional probability of A given that we know B. e.g. p(r t = l| i t ): probability that the robot is at position l given the sensors input i t.  Product rule:  Bayes rule:

45 Fusing in Sensory Input  Bayes rule: Map from a belief state and a sensor input to a refined belief state (SEE): p(l): belief state before perceptual update process p(i |l): probability to get measurement i when being at position l  consult robots map, identify the probability of a certain sensor reading for each possible position in the map p(i): normalization factor so that sum over all l for L equals 1.

46 Action Update  Bayes rule: Map from a belief state and a action to new belief state (ACT): Summing over all possible ways in which the robot may have reached l.  Markov assumption: Update only depends on previous state and its most recent actions and perception.

47 Case Study: Topological Map  The Dervish Robot  Topological Localization with Sonar  1994 AAAI

48 Dervish  Topological map of office-type environment

49 Dervish – Belief Update  Update of belief state for position n given the percept-pair i p(n¦i): new likelihood for being in position n p(n): current believe state p(i¦n): probability of seeing i in n (see table)

50 Dervish Move Update

51 Grid Map  Fine fixed decomposition grid (x, y, ), cell size: 15 cm x 15 cm x 1° Action and perception update  Action update:  Perception update: Courtesy of W. Burgard

52 Grid Map -- Challenge  Challenge: calculation of p(i | l) Combinatorial explosion! Use model (sensor, map) Assumptions  Measurement error: mean zero Gaussian  Non-zero chance for any measurement  Failure mode Courtesy of W. Burgard

53 Grid Map 1.Start  No knowledge at start, thus we have an uniform probability distribution. 2.Robot perceives first pillar  Seeing only one pillar, the probability being at pillar 1, 2 or 3 is equal. 3.Robot moves  Action model enables to estimate the new probability distribution based on the previous one and the motion. 4.Robot perceives second pillar  Base on all prior knowledge the probability being at pillar 2 becomes dominant

54 Grid Map  Example 1: Office Building Position 3 Position 4 Position 5 Courtesy of W. Burgard

55 Grid Map Challenges  Fine fixed decomposition grids result in a huge table Huge processing power needed Large memory  Reducing complexity: Randomized Sampling / Particle Filter Approximated belief state by representing only a ‘representative’ subset of all states (possible locations) E.g update only 10% of all possible locations The sampling process is typically weighted, e.g. put more samples around the local peaks in the probability density function However, you have to ensure some less likely locations are still tracked, otherwise the robot might get lost

56 Summary  To localize or not?  Noise, noise, noise  Odometry  Map representations  Belief representations  Filtering – Markov localization