Simultaneous Localization and Mapping

1. Simultaneous Localization and Mapping Presented by Lihan He Apr. 21, 2006

2. Outline • Introduction • SLAM using Kalman filter • SLAM using particle filter • Particle filter • SLAM by particle filter • My work : searching problem

3. Introduction: SLAM SLAM: Simultaneous Localization and Mapping A robot is exploring an unknown, static environment. • Given: • The robot’s controls • Observations of nearby features The controls and observations are both noisy. • Estimate: • Location of the robot -- localization where I am ? • Detail map of the environment – mapping What does the world look like?

4. a2 at a1 at-1 … x1 x2 xt xt-1 … x0 o2 o1 ot ot-1 … m Introduction: SLAM Markov assumption State transition: Observation function:

5. p(xt) p(mt) or mt p(x1) p(m1) or m1 p(xt-1) p(mt-1) or mt-1 … Prior distribution on xt after taking action at Introduction: SLAM Method: Sequentially estimate the probability distribution p(xt) and update the map. Prior: p(x0)

6. Introduction: SLAM Map representations 1. Landmark-based map representation Track the positions of a fixed number of predetermined sparse landmarks. Observation: estimated distance from each landmark. 2. Grid-based map representation Map is represented by a fine spatial grid, each grid square is either occupied or empty. Observation: estimated distance from an obstacle using a laser range finder.

7. Represent the distribution of robot location xt(and map mt) by a Normal distribution Introduction: SLAM Methods: The robot’s trajectory estimateis a tracking problem 1. Parametric method – Kalman filter Sequentially update μt and Σt 2. Sample-based method – particle filter Represent the distribution of robot location xt(and map mt)by a large amount of simulated samples. Resample xt (and mt) at each time step

8. Location error Map error Introduction: SLAM Why is SLAM a hard problem? Robot location and map are both unknown. • The small error will quickly accumulated over time steps. • The errors come from inaccurate measurement of actual robot motion (noisy action) and the distance from obstacle/landmark (noisy observation). When the robot closes a physical loop in the environment, serious misalignment errors could happen.

9. SLAM: Kalman filter Update equation: Assume: Prior p(x0) is a normal distribution Observation function p(o|x) is a normal distribution Then: Posterior p(x1), …, p(xt) are all normally distributed. Mean μtand covariance matrix Σt can be derived analytically. Sequentially update μt and Σt for each time step t

10. Assume: State transition Observation function Kalman filter: Propagation (motion model): Update (sensor model): SLAM: Kalman filter

11. localization mapping SLAM: Kalman filter The hidden state for landmark-based SLAM: Map with N landmarks: (3+2N)-dimentional Gaussian State vector xt can be grown as new landmarks are discovered.

12. Idea: • Normal distribution assumption in Kalman filter is not necessary • A set of samples approximates the posterior distribution and will be used at next iteration. • Each sample maintains its own map; or all samples maintain a single map. • The map(s) is updated upon observation, assuming that the robot location is given correctly. SLAM: particle filter Update equation:

13. Particle filter: Assume it is difficult to sample directly from But get samples from another distribution is easy. We sample from , with normalized weight for each xit as The set of (particles) is an approximation of Resamplextfrom ,with replacement, to get a sample set with uniform weights SLAM: particle filter

14. Particle filter (cont’d): 0.4 0.3 0.2 0.1 SLAM: particle filter

15. Choose appropriate Transition probability Choose Then Observation function SLAM: particle filter

16. Algorithm: Let state xt represent the robot’s location, 1. Propagate each state through the state transition probability . This samples a new state given the previous state. 2. Weight each new state according to the observation function 3. Normalize the weights, get . 4. Resampling: sample Ns new states from are the updated robot location from SLAM: particle filter

17. are the expected robot moving distance (angle) by taking action at. Measured distance (observation) for sensor k Map distance from location xtto the obstacle SLAM: particle filter State transition probability: Observation probability:

18. SLAM: particle filter • Lots of work on SLAM using particle filter are focused on: • Reducing the cumulative error • Fast SLAM (online) • Way to organize the data structure (saving robot path and map). Maintain single map: cumulative error Multiple maps: memory and computation time • In Parr’s paper: • Use ancestry tree to record particle history • Each particle has its own map (multiple maps) • Use observation tree for each grid square (cell) to record the map corresponding to each particle. • Update ancestry tree and observation tree at each iteration. • Cell occupancy is represented by a probabilistic approach.

19. SLAM: particle filter

20. Searching problem Assumption: • The agent doesn’t have map, doesn’t know the underlying model, doesn’t know where the target is. • Agent has 2 sensors: • Camera: tell agent “occupied” or “empty” cells in 4 orientations, noisy sensor. • Acoustic sensor: find the orientation of the target, effective only within certain distance. • Noisy observation, noisy action.

21. Searching problem • Similar to SLAM • To find the target, agent need build map and estimate its location. • Differences from SLAM • Rough map is enough; an accurate map is not necessary. • Objective is to find the target. Robot need to actively select actions to find the target as soon as possible.

22. Searching problem • Model: • Environment is represented by a rough grid; • Each grid square (state) is either occupied or empty. • The agent moves between the empty grid squares. • Actions: walk to any one of the 4 directions, or “stay”. Could fail in walking with certain probability. • Observations: observe 4 orientations of its neighbor grid squares: “occupied” or “empty”. Could make a wrong observation with certain probability. • State, action and observation are all discrete.

23. Searching problem In each step, the agent updates its location and map: • Belief state: the agent believes which state it is currently in. It is a distribution over all the states in the current map. • The map: The agent thinks what the environment is . • For each state (grid square), a 2-dimentional Dirichlet distribution is used to represent the probability of “empty” and “occupied”. • The hyperparameters of Dirichlet distribution are updated based on current observation and belief state.

24. Belief state update: the set of neighbor states of s where Probability of successful moving from sjto s when taking action a From map representation and with Neighbor of s in orientation j Searching problem

25. Assumeat step t-1, the hyperparameter for state S is At step t, the hyperparameter for state S is updated as and are the posterior after observing o given that the agent is in the neighbor of state s. If the probability of wrong observation for any orientation is p0,then p0 if o is “occupied” 1-p0 if o is “empty” prior can be computed similarly. Searching problem Map update (Dirichlet distribution):

26. Searching problem Belief state update: a=“up” a=“right” Map representation update: a=“right” a=“up”

27. Searching problem Choose actions: Each state is assigned a reward R(s) according to following rules: Less explored grids have higher reward. Try to walk to the “empty” grid square. Consider neighbor of s with priority. x x