Monte Carlo Localization

Daniel Lowd

CS154: Robotics


I implemented the Monte-Carlo localization mostly as described, but with a few modifications.  First, in my normalization code, I generated uniformly distributed random particles if for some reason not every particle was assigned.  (In my first implementation, I would sometimes end up with a few particles at (0, 0), because they hadn't been assigned new locations.)  Second, I decided to determine and display the average absolute and overall errors on the particles after normalization.  This way, I could get an objective sense of the precision and accuracy of the code.  I coupled this with a hack that would place the robot automatically in four consecutive locations, rather than relying on keyboard movement or mouse placement, to generate statistics on the effect of different actual and modeled noise levels.

Since this was somewhat beyond the scope of the project, I did not follow through to completion.  I observed, for example, that the accuracy and precision would vary from run to run, but I did not attempt to describe that distribution, only gain some intuition into the effects of the different factors.  I did these experiments with the simple square map using 100 particles and had pretty good success.

Qualitative Analysis

With actual and modeled noise of 10.0 inches (standard deviation), the localization performs pretty well.  The following screenshots show some of the progression when running the robot in part of the loop, but done by mouse.  The darker colored circles are the particles selected by the algorithm.  The lighter colored circles are too unlikely and thus being cut.

Quantitative Analysis

Through automated experiments, it was possible to gain a more precise understanding of the relationship between precision and noise.  When there was no noise introduced to the sonars, less noise in the model yielded more precision, when measured in terms of average distance from the true location to each particle after a certain amount of localization.

With noise of standard deviation 10.0 inches, the graph looks somewhat different:

Here we can see that underrepresenting the noise has detrimental effects, even in a simple box-like room.  However, overrepresenting the noise does lead to less precision, as before.  It should be noted that this is precision, not accuracy.  Though not shown here, accuracy tended to be very good for the case with no noise across all variation of model noise.  For the case with noise, accuracy was good with noise modeled at the actual level of the noise or somewhat greater, even up through 120 inches of noise.  240 inches was too much to effectively localize, which makes sense, since the maximum sonar value is 255.

An example of this failure can be seen below.  While the average location of the particles is somewhat near the robot's actual position, it's still pretty vague.  More discerning localization is more effective than this.

Maze Challenges

The maze map, unfortunately, proved to be much more challenging.  Given the complexity of this map, I did not attempt to do testing as automatically or methodically as with the simpler one.  Simply to be able to consistently localize would be considered a victory.  It was only with lower levels of noise that I was able to manage this.  (Alternately, more particles might help, but I did not experiment with that.)  The following image shows an example of successful localization, but just barely.  10 inches of model noise but no actual noise were used, with 100 particles.  Even with these parameters, it nearly failed based simply on initial particle placement.  Of the nearby particles, only one ended up being identified as useful, and most of the likely clumps of particles were quite far away.  It should also be noted that knowing where the robot really is and letting that information influence the next placement of the robot really is cheating, and probably makes this method work better than it otherwise would.  A true test would be to implement entropy-seeking behavior or to place the robot in a random location and show a human only the particles.

Source Code -- Complete source code to my implementation of the Monte-Carlo Localization code.