The Armadillo Goes Mapping

Several weeks ago I built a new, small robot platform that could be used as a testbed for the ArbotiX-ROS bindings. That robot is the Armadillo. The Armadillo has a differential drive base with a pretty decent payload,
a FitPC2 brain, and a 4DOF arm. He's also sporting a Hokuyo URG-04LX-UG01 laser range finder.

The poor Armadillo was sitting around for quite a while until I upgraded his motor drivers to handle the extra weight of the platform. However, he's now fully operational. To test out whether his odometry would be good enough to work with the ROS navigation stack, he was driven around to build a series of maps.

The first map is of the first floor of my house. The Armadillo started in the living room, when down the hallway and into the kitchen, and then returned. The map came out pretty good, I plan to collect a map of the complete house later this week.

A second, much larger map was made of the CS department hallways. This one had some issues. In particular the scan matching was creating false positives, which "shortened" the hallways. I'm still hopeful this can be made to work though, with a bit more parameter tweaking. Below, the image on the left is the map from gmapping, and the image on the right is a raw odometry-based costmap in RViz:

I'm going to try and keep this blog a bit more up to date from now on.... we'll see if it actually happens.

-Fergs

ROS and Other Ramblings

I've been a bit ineffective at finishing things lately, and thus haven't had much to say in this blog. Issy got a new laser cut body, some teaser pictures are over at TRC, but I haven't gotten back to software development on him yet.

I've been a bit distracted playing around with navigation and motion control under ROS. During July I built a "Poor Man's LIDAR" (or PML for short), out of a long range IR sensor and an AX-12. The image to the left shows the PML mounted on ROSalyn, an iRobot Create based ROS-powered robot I've recently assembled at the University lab. I'm using a new ArbotiX-derivative board to control an AX-12 pan and tilt and the PML.

I actually bought the sensor about 18 months ago -- originally to put on REX (may he rest in pieces), but hadn't gotten around to actually hooking it up until recently (partially inspired by the successes that Bob Mottram had). All in all, it works fairly well -- way better results than I ever had with a sonar ring, but of course nowhere near a true LIDAR. The PML results are broadcast within ROS as if they were actually produced by a laser scanner. You can see the scan (black dots) and then a costmap_2d generation (red dots are lethal objects, blue dots are expanded version for motion planning) showing up in the RVIZ view (the robot is at the end of our hallway, the range of the costmap is 3m, less than the 5 or so meters that the laser can trace out, so there's no costmap generation of the walls inside the rooms in the distance, just the laser scan dots).

All of this ROS work is towards the goal of producing a very robust, and extensive ROS package for the ArbotiX. The core of the package allows the ArbotiX to control AX-12 servos, read/write digital IO, and read analog inputs -- all within ROS. There's also extensions to control differential drive robots, or NUKE-powered walkers, using the standard "cmd_vel" topic -- and publish odometry such that the bots could be tied into the ROS navigation stack. Version 0.1 is now in SVN, although the ROS API is quite unstable and will be changing drastically in 0.2 (to a much nicer, and more robust interface, which also sets up several features I want to implement further down the line).

-Fergs

Arduino Common Knowledge, Part 1 of N: LEDs Without Resistors

Some time ago I read a book called "C++ Common Knowledge". It consisted of a series of short 2-3 page articles, each detailing some important, but often misunderstood detail of C++. Recently, I've answered a number of Arduino/Electronics questions that seem to have this same feeling to them. This is the first in a series of short articles I'm going to call "Arduino Common Knowledge".

Today, we'll start with an idea that seems to go against all safety warnings: you can blink an LED directly off an Arduino/AVR pin. Yep, no resistors. We've probably all experienced the fun of plugging an LED in between our 5V and ground rails, and watching it glow brightly for a second before exploding and smelling quite awful -- so it seems natural we'd always want to install a current-limiting resistor to stop this problem.

But here's the interesting part: your Arduino/AVR is a current-limiting device. The I/O pins can typically only source about 15-20mA of current. Thus, if we connect our LED between the pin and ground, and toggle the pin high, our LED glows nicely without exploding.

There are a few caveats to this though: you can't do this on every pin on the AVR at the same time, or even a large number of pins. The ATMEGA168 data sheet specifies that the Absolute Maximum DC current between Vcc and GND is 200.0mA. Since the AVR core and periphrasis such as the UART/SPI/ADC draw some current of their own, we clearly can't control a huge number of LEDs this way. Also note that if we leave the LEDs on for a long time, you'll probably notice the AVR getting a bit warm sourcing that much current.

Next Up: Pull-Up Resistors (and Why You Need Them)

Another Day of Printing (and Issy Building)

I got another day of solid printing out of the Makerbot. Finished printing Issy's legs, printed his head camera mount, and the first segment of the tail. The head camera mount is a fairly massive print, at 3.5" x 1.75" and an inch tall. The first attempt warped massively after the first 3 layers and had to be tossed. I got a much better raft on the second one, and it came out fine (some warpage, but not enough to ruin the print). I'm hoping to crank out the snout and second tail section tomorrow.

Unfortunately, no pictures. My camera is on vacation in Florida. But, here's a rendering of the final head/snout with the MS Lifecam cameras:

Printing Grippers

The Makerbot has gotten a decent workout this past week, printing legs for Issy and (among other parts) a gripper for an upcoming bot. The third time really is the charm: with all the designs for this gripper and Issy's legs, the third design was the one I stuck with. Here's some pics of the aforementioned HS-55 based grippers I'm creating for a 2011 robowaiter entry:



















Each gripper has two printed parts: the HS-55 mount and the C-bracket. The servo mount is the same part for both the left and right sides but I had to slightly tailor the C-brackets to get decent range of motion. Each HS-55 mounts to an AX-12 C-bracket.

The gripper fingers are made of 1/16" thick 5052, about 3/4" wide, mounted with two 2mm screws. Instead of a bearing, I'm using a 3/16" Chicago bolt on the non-driven end of the printed C-bracket. Preliminary tests show the grippers have no problem holding the robowaiter plate.

-Fergs

Issy's New Legs

One of the key features desired for Issy3 is a set of feet with tactile feedback. I've played with FSR versions in the past, and they tend not to survive that long.

Two afternoons of prototyping and printing has yielded the first of Issy's new feet. I have one leg assembled, but I'll still have to get some different hardware tomorrow to tweak it completely.

The foot consists of three printed parts: the outer case, the foot pad, and a retainer insert inside the case. A screw connects throw the foot pad and the retainer, and a spring pushes the foot downward. There will be a little spring steel connector on top that will get touched by the screw -- acting as a simple switch. A vinyl footpad gives traction.

Each foot takes about an hour of printing on the Makerbot.

-Fergs

Robogames Wrap-Up

Well, it's almost a month since Robogames, but it's been hectic with the end of the semester. The semester is now over though, so it's robot time again.

Issy did terrible in Mech Warfare. He walked like a champ, but had numerous other issues. On Friday afternoon his gun literally exploded inside. Thankfully, I found a Sports Authority around the corner and purchased an extra gun(have I mentioned how much more convienient of a venue San Mateo is over the old place?). Apparently though the Trendnet camera decided that 6V was just too much - it stopped working on Saturday, and Issy was dunn for. He's already been torn down, and retired from Mech Warfare.

He'll be rebuilt over the course of the next month or so, this time with a Fit-PC2, stereo camera head, and a tail. He's going to be running ROS, which ought to be cool. Here's a teaser shot of what he will (hopefully) look like:

On other fronts, I'll be posting some other robot goodies over the coming days. I'm currently working on some finishing software touches for our first "social" robot at Albany, Nelson. I'm also working on a robowaiter entry for next April.

-Fergs