Monte-Carlo Localization: from Tragedy to Triumph!

Daniel Lowd

CS 154: Robotics

Spring 2003


We live in uncertain times, times of war, times of economic hardship, times of cultural change.  In such a world, we must hold our beliefs carefully, having the courage to apply them yet the grace to evolve them.  Belief without action yields nothing, but brittle beliefs are easily broken.  Robotics is much the same: machines that interact with the world face uncertainties at every level.  I hope to address one of the most basic questions: where am I?  For the first few weeks, I will focus on resolving a robot's position in a known and static environment.  Through Monte-Carlo Localization [1], a robot may combine the information of many readings in many positions, narrowing its position down to the few most probable.  For the final weeks, I will attempt to build the map at the same time, a process known as Simultaneous Localization And Mapping, or SLAM.  The methods of Monte-Carlo Localization may be combined with expectation maximization (EM) to yield the most likely map and position, given a set of locations and sensor readings.

None of these methods is new: all have been published and implemented on multiple systems.  I hope to explore some of the limitations of these methods when implemented on the Nomad 200 in a simulator.  Through simulation, I can easily add random and systematic errors in noise or positioning, then observe the effects on the algorithm.  My goals are first accuracy, then robustness, then flexibility.  A robot algorithm must first be effective before other factors are worth considering.  I consider reliability in the face of uncertainty and errors to be an important secondary goal, given the unpredictability of the real world.  Flexibility in this case represents making fewer assumptions so the algorithm is effective in more environments with fewer modifications required.


Part 1: Map reading and particle filtering

I first attempted to implement MCL by working on the MCL assignment, starting from Zach Dodds' code.  Methods were included for reading in a map file, computing the distance to the nearest wall in each direction, updating particle locations, and moving the robot using the keyboard.  All of this proved to be quite helpful.  After some hacking, I managed to get a mostly working implementation.  I played around with parameters a bit and tried out a few maps to verify that it seemed to work, but did not do extensive experimentation.  This system represented a step towards a viable implementation, rather than the solution itself.  It should be noted that I never had much success localizing in the maze I developed.  Part of this is likely issues with my implementation that I need to address.  Part of it is also probably parameter tuning.  I didn't spend too much time on this, since I was eager to spend my time tuning the final algorithm, with all features implemented.

This critical weakness of this first implementation was that it required absolute knowledge of the robot's angle.  If the robot were equipped with a compass, this assumption could perhaps be made.  However, I wished to relax it, so my robot could localize itself starting from any position or angle, with no special knowledge other than its sonars and an accurate map.  I also decided to support maps with angled walls, not just vertical and horizontal ones.  This functionality seemed important in the real world, where not everything is straight.

The major challenge of these two features turned out to be not robotics so much as geometry.  The problem to be solved could be summarized as determining the point where a ray intersects a line segment, if the two intersect at all.  Given a location and angle for the robot, I had to determine the distance to the nearest wall, each wall being represented as a line segment, so I could compare this theoretical distance with actual sonar readings.  When the robot's sonars only point in the cardinal directions, this is trivial.  When the sonars may be at arbitrary angles, this requires substantial angle normalization, use of arctangent, and finally, the law of sines.  Debugging it turned out to be very difficult, so I wrote a separate program to generate random test cases of a form that would be easy to solve and compare my algorithm to the known answer.  This helped me uncover the final bug in the algorithm, as well as a number of bugs in my testing code.  All told, it took many hours to restore the original functionality of my program with this new method.

Even when this was done, it was hard to use it with sonar because the sonar was less accurate and less consistent.  Even with no noise added, the sonar disagreed with my calculations on the distance to a wall.  Part of this showed up as a systematic error -- the sonar underestimated the distance by about 9-10 inches, though it varied.  I suspect that this is because my calculations work from the center of the robot, while sonars are positioned on the outside, so the result should be off by the radius of the robot.  I tried accounting for this in my computations, but there were so many other errors that it was hard to tell how much effect it was really having on the results.  Another source of error could be how sonar works -- my calculations assume straight lines, while sonar goes out in a cone.  Still, this should not have affected the experiments I was running, which used a perpendicular wall.

Another sonar-related issue was corners.  Specifically, a sonar may bounce off of a corner, resulting in the maximum reading rather than the appropriate distance.  This can cause all of the most accurate position guesses to be discarded in favor of less accurate ones.  I attempted to remedy this by skipping past any sonars that read the maximum reading, and only focusing on ones that provided more specific distance information.  Now, however, it seems to be having more difficulty closing in on the right location.

I also found it necessary to mess with the number of particles and the standard deviation used in the model a fair bit.  The reason for this seems to be that the standard deviation in the model must account for not only the error in the sonar readings, but also the error in particle placement.  If the sonar reading is off by a distance of 10.0 units and the nearest particle is 10.0 units away from the correct location, then the actual error may be as great as 20.0.  If the standard deviation of the error is assumed to be 10.0, then the result is two standard deviations from the mean rather than one.  This gets still more complicated with a three-dimensional state space, as it's harder to quantify the effect of being off by 10 degrees, and only 1/18th of the particles will randomly be within 10 degrees of the true angle anyway.

In fact, that very reason may be part of why I had so little success once I added robot angle as a third variable.  The system tended to converge to inaccurate areas or not converge at all during my limited experiments.  Experimentation was slower than I anticipated, since the graphics routines are so inefficient.  Generating a bitmap and writing it to a file each time might be faster.  100 points is manageable, but 5,000 or even 500 is noticeably slow.  (Even though drawing points rather than circles did provide a sufficient speed-up.)

Overall, I feel I've made a great deal of progress, but localization is clearly not complete.  Conceptually, it's done -- the algorithm is fully implemented in the code.  Practically, it doesn't perform very well at all.  Experimentation, debugging, tuning, and analysis will hopefully suffice to improve it.

Part 2: Dual-MCL and a Hybrid Algorithm

Simultaneous localization and mapping seemed far too ambitious, when localization itself was broken.   The figure below shows an extreme example.  It was possible to confuse the algorithm so that almost all possible points were outside the room!

I decided to try out my code on the 2D localization problem, since I had already solved that for a short homework assignment.  What I uncovered was the same systematic error mentioned before.  By taking into account the radius of the robot into account, I was able to mostly fix this problem.

Once this modification (and perhaps a few other fixes) had been made, I was able to successfully do 2D localization in a maze.  Sometimes.  When the particles were placed right to begin with.  What I was really trying to accomplish was consistently successful 3D localization.

To tackle this, I implemented a more complicated version of Monte-Carlo localization: by adding particles sampled from the configuration space, conditioned on the last sensor reading, one can theoretically achieve a much more robust solution [2].  Unfortunately, the article I read did not specify all the details.  For example, how is one supposed to sample particles from the configuration space?  Furthermore, how does one determine the probability that each came from one of the particles from the last time step?  Though several methods were listed, none was clear enough for me to be sure of how it applied.  They made numerous references to kd-trees, so I took some time to learn about kd-trees and how they worked.

My final conclusion was that I could sample particles based on sensor readings by first uniformly generating particles, then computing their theoretical sensor readings, and finally building a kd-tree keyed on the 16 different readings.  The sampling process would then consist of taking a set of sensor readings, adding noise, and finding the point with the nearest readings, as cached in the kd-tree.  By adding different random noise each time, I could sample many of these at random, and was not restricted to selecting only the k nearest neighbors of the actual sensor reading.  This seemed probablistically justified.

The weighting process I used was to generate a guess of the newly sampled particle's possible previous location and scale its likelihood based on the density of the prior particles in that area.  Since density can be measured as points/unit volume, I was build a kd-tree of the prior particles, keyed on location in three dimensions, and used the size of the hyper-rectangles at the leaves as indications of the relative density at various locations.

I successfully implemented all of this in several steps.  First, I developed my own kd-tree code.  Though code is publicly available, I needed something efficient, flexible, and with as little encapsulation as possible.  I wanted to be able to traverse the tree myself rather than relying on some object method to give me the information.  I wrote a test program to help me debug it, and only continued when I was satisfied that it worked.

Next, I wrote the code for building the uniformly-sampled sensor readings kd-tree.  This took additional debugging, because I suddenly had to deal with duplicate values in my kd-trees.  For a while, there were issues with infinite recursion, since the kd-tree building code couldn't split up the many points for which all sensor readings are 255 inches.  Eventually, I modified the tree structure so that a leaf could contain many data points and finished debugging it.

The sampling code and weighting code were more straightforward, though they had some bugs at first, especially the weighting code.  When finished, I was able to place the robot in various locations and see what particles it generated.  Some locations seemed to work better than others.

Here is an example of successful sampling: since the space is rectangular, close to square, sensor readings that indicate a corner could imply any of the four corners.  The algorithm accurately selects points from all corners.

It seems to work fairly well in the maze as well.  Since the maze is complicated and has many similar-looking locations, the particles are spread out a fair bit, but some of them are quite near the true location of the robot.  This is a good sign.

Another location in the maze furhter confirms this.  Not only are some particles near the robot's true location, there are particles in many other dead-ends that look similar, as one would expect.

Unfortunately, this method has at least one serious weakness.  This figure shows the particles generated when all sensors had their maximum readings, 255.0.  One would expect more particles in the middle near the actual location of the robot.  Unfortunately, it is very unlikely that a central point will ever be chosen.  There are 16 sensor readings and each receives noise representing the sensor error and additional noise for each sampled particle.  Since the error added is Gaussian, there's a 50% chance that any given sensor reading is changed so that it is less than 255.0.  The probability that all of them remain at 255.0 is thus 1/2^16 = 1/64,000.  While the added sampling noise could change some of those values back to 255.0, it's likely to change just as many in the opposite direction.  Since the noisy sensor readings have values under 255.0, the sampled particles are likely to be less than 255.0 inches from some wall, which is exactly what we see.

This shows the biggest weakness of my overall approach: maxed out sensor values are not modeled accurately.  With a real robot, if the actual distance is 1,000, it would be highly unlikely for the sensor to read 240.0 or 236.0.  But supposing one assumes that actual sensors are noisy the exact same way as simulated in my code.  Then in order to accurately model the probability density function of the sensors' true values, one would need to include priors for each distance.  That is, since a true distance over 255.0 is much more common than a true distance of 236.0, if the sensor value is 240.0 (after noise), then 255.0 may be its more likely original value than 236.0, even though 236.0 is closer.  I thought a bit about ways to correct this -- for example, use the uniformly sampled sensor readings to construct a probability distribution to fix this -- but decided that would be outside the scope of my project.

The second-biggest weakness of my approach also has to do with sensor modeling.  In the robot's displayed location, all of its sensor readings are 255.0 except for one.  Because it's so close to the wall, one one sonar bounces back to the robot; the rest bounce away and never return.  Since only one of the 16 sensors is less than 255.0, we have almost the same distribution as shown before, when the robot was in the middle.  My clearance computations would have predicted much lower readings on diagonal sonars than actually occurs here; therefore, more accurate points will rarely be chosen in such a situation.  Better sensor modeling would fix this problem.  I declared this problem outside of the scope as well, and hoped the situation would be tolerable overall.

When I combined the two methods, I did so as suggested in the paper: select some fraction of the particles by sampling given the sensor readings; select the remainder by moving all previous particles are reweighting, as in regular MCL.  These methods together did seem to yield a better solution than either alone.  I tried several values for the fraction before settling on 0.1, which seemed to be enough to fix the problems with my original MCL solution.

This shows a successful localization on a simple map, using the more robust method.  Unfortunately, it took 32 steps to achieve this solution.  Due to the random placement of particles to begin with or the noise added to the sensor readings, the algorithm quickly selected one of the two likely positions shown and placed all of the particles there.  Due to the symmetry of this map, both solutions should be equally likely, but due to chance, only one was selected.  The particles added by sampling did help give the other solution a chance, but it remained the minority over many, many moves.  The reason for this is that once one solution had more particles, the competing solution had to have more accurate particles in order to change the distribution.  However, the solution with more particles was more likely to have at least some particles that were very accurate.  Over 32 steps, it did stabilize, and I believe it will in general, but it can be slow.

Trying this on the maze map worked, too, but also took a while.  This shows the early hypothesis of the algorithm: note that very few points are near the true location of the robot.  However, over time this was eventually fixed, as shown in the figure belows.  Overall, this modified algorithm is clearly more robust than MCL, even with poorly modeled sensors and improper probabilities.


[1] F. Dellaert, D. Fox, W. Burgard, and S. Thrun. Monte Carlo localization for mobile robots. In Proceedings of the IEEE International Conference on Robotics and Automation (ICRA), 1999.

[2] S. Thrun, D. Fox, W. Burgard, and F. Dellaert. Robust Monte Carlo Localization for Mobile Robots.  Artificial Intelligence, vol 128, 1-2, pp 99-141, 2001.


Source code for Monte-Carlo localization, and various testing code: