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Dynamite With a Laser Beam!
Wednesday, 29 August 2007

Ok, so there's no dynamite, but there is a laser beam. I've managed to get hold of a SICK PLS101-312 laser proximity scanner. This is very similar to the blue SICK LMS200 lasers that you may have seen on various robots, the only difference being that the PLS range of scanners are designed more for industrial automation as they have programmable zones that can be used to signal to machinery when an object is present, for safety cut-off etc. There's a good comparison table a little way down this page as well as a tutorial in interfacing with the device.

A SICK LIDAR wouldn't have been my first choice of laser scanner to buy, mainly due to it's size and the fact that it runs from 24V. However, this one was very cheap (eBay - no one else bid), so I didn't really have too much choice. I would much rather have had a HOKUYO URG laser as they are about the same size as an IP webcam, run from 5V and have USB on them. However, both types of laser are directly supported by Player/Stage and other various software, so this will give me opportunity to try out some things that I wouldn't otherwise be able to afford to do.

All of these laser scanners work in pretty much the same way - basically a laser is reflected from a spinning mirror outwards in a plane spanning 180 degrees. The device times how long it takes for the light to reflect back from objects that it hits, and works out how far away they are. This provides data which is almost as good as a map of the environment - and it does this ten times a second or more.

Unfortunately the black front visor/screen was missing on the unit I bought, so the one on there now came from a faulty unit, but it's scratched which makes the PLS see things that aren't there, and it also constantly complains that it needs cleaning. The unit works ok without it, but dust contamination is a big issue for these types of device, so I have a new screen arriving shortly. Here are some pictures of the PLS - you can see the mirror inside that rotates when it's turned on as well as the very scratched screen:

Here you can see the PLS dwarfing my IP web cam, which is similar in size to the HOKUYO laser I mentioned above - and also the largest thing I was planning to stick under my 914's lifting head. There are also two pictures that show the scan produced by the PLS, the large blue area on the screen is a distance of 4 metres / 12ft. You can also see that right in the middle of the scan it sees an object very close which isn't really there due to the scratched screen.

My next steps with the PLS will be to (somehow) mount it on the 914 and power it while the robot is mobile. Then I can use the localization, navigation and planning drivers that come with Player/Stage, along with PlayerNav. This will allow for a map of the environment to be supplied and for the Bot to locate itself within the environment, plan a path, then navigate to another location. All this is provided out of the box and seems to be as easy as dragging the Bot in a GUI from point A to point B - then watching it sort everything out and get there itself. Of course it's the laser that makes this possible because it makes it easy to match features of the physical environment to the map, and easy to see obstacles it has to avoid. The Wavefront, AMCL and VFH drivers/algorithms are provided with Player which take care of all the tricky stuff. Of course I can also write my own program that gives the Bot a new x/y grid reference on a map of my house for instance, but not have to worry about programming the actual navigation myself.

Until the next installment I'll leave you with these two YouTube videos. The SICK lasers were used on most of the DARPA Grand Challenge robot vehicles including the winner, Stanley, which has five similar SICK laser scanners on it's roof. The Stanford Racing Team are of course also entering the DARPA Urban Challenge - the first video is a nice run down of their urban robot vehicle which appears to have at least two SICK LMS291's on it. The second video is another laser scanner guided car at the VW test facility in Germany. This video is worth watching until the end where the car drives at top speed around the course by itself.

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