Pi Robot

Pi Robot Rebuilds for 2012

Santa's elves were busy over the holidays giving Pi Robot a tune-up in preparation for the New Year. It is time to finally put Pi to work around the house but first a few improvements to Pi's overall design were necessary.

First, Pi's old rectangular base was too big and boxy to get in and out of tight spaces so something smaller and rounder would be required.
Second, Pi's arms (or arm) would need some way to reach objects as low as the ground and as high as a table top or counter. While people tend to use their flexible knees or torso to adjust height, Pi has neither.
Third, the design should be as modular as possible so that, for example, the base could be disconnected from the rest of the robot and run on its own for simple navigation experiments.
Finally, the whole system would have to run off a single battery with as much run time between recharges as possible.

Having experimented with the Willow Garage TurtleBot lately, the use of stackable circular plates seemed like a good design for the base. Unfortunately, the iRobot Create rides too close to the ground to function as a true service robot--for one thing, it cannot get over carpet thresholds or throw rugs. So I would have to use a new drive train consisting of better motors, wheels and controller.

Fortunately, the folks at the George Tech Healthcare Robotics Lab already solved the arm problem: mount it on a vertical track so that the whole arm can move up and down as shown below:

Fellow robot club member and Willow Garage software engineer Michael Ferguson recognized the genius of this design and built a less expensive version he calls Maxwell, shown below:

Mike brought Maxwell to our latest club meeting and I had a chance to pick his brain regarding design details. Mike uses a piece of T-slot extruded aluminum as the vertical track and a 20" linear motor to move the arm up and down. He also uses some top-of-the-line Dynamixel servos for the arm and an Asus Xtion Pro Live 3D camera instead of the Kinect. (The Asus has the nice feature of running solely off a USB port without the need for an additional 12V power supply like the Kinect.) Inspired by Mike's work, and with some very helpful feedback from fellow club members on the Home Brew Robotics forum, I fired up Google Sketchup, a 3D drawing program, and put together a similar design for Pi Robot shown here:

As you can see, the result is a hybrid of Maxwell and the TurtleBot. After collecting all the needed parts, I grabbed a hack saw and power drill and five days later, Pi was reborn in the form shown below:

The new Pi Robot does not yet have the linear motor to move the arm up and down and the weight distribution will have to be modified for the final version but it's a good start. The arm is the standard TurtleBot arm using low end AX12 servos. Even so, you can see that it can handle the weight of a fairly hefty wine glass. Here is a shot inside the base:

The circular device at the top (front of the base) is the Hokuyo laser scanner and it is nicely tucked away out of harm's way. The big black and white rectangular object is a 12V 10000 mAh LiFePO4 battery which weighs about half as much as a lead acid battery of the same capacity. Below that is the ArbotiX microcontroller and Pololu 30A motor driver. The motors (Pololu 100:1 220 oz-in 100 RPM) are mounted underneath the base. The wheels (approx. 5" diameter) are from BaneBots with a fairly soft rubber tire which works well on carpet and hard surfaces (thanks again to Mike for the recommendation). As you can see, it's a little cramped in there so the final design will use slightly larger plates. Total weight of the robot is 17.4 lbs (7.9 kg).

The brain of the robot is a Zotac dual core mini-ITX that sits on the second plate and runs off the 12V battery. One can also use a laptop.

That's it for now. Stay tuned for the many adventures of the new Pi Robot in 2012!