Ok, last time I talked about some options for off-road vehicle mobility, and a few of my faves from JPL. This time it's all about designing an almost Sojourner-sized autonomous robot, about 640mm long. It's well inside the size allowance for the WCRS race of 1 meter max in width, height, and length, and I can add a manipulator arm and still be within 1 meter.
I've worked with aluminum before, but cutting and drilling with any accuracy takes time and care. There are modular systems, like OpenBeam or MicroRAX, but they are expensive and fairly small. I need something beefy to survive an off-road race.
Lucky for me ServoCity has released an amazing assortment of parts that use a common hole pattern, so they fit together easily, and they have pretty good specs on their website so designing is easy.
This is probably the zillionth design for a six wheeled rover that I've considered on paper, and I think it's the best balance of tough, simple, and lightweight. The top of the image is an overhead view of one of the rocker-bogie arms, and the bottom part is the side view.
These arms connect via a 12" wide body, so my only concern is lateral stability on a side slope... but I have to start somewhere! If you're curious, all the rectangular parts are based on ServoCity aluminum channel, and the wheels are 120mm off road R/C tires. Aluminum keeps it light; I think the chassis should weigh in around 2kg 'bare'; without motors, batteries, etc. As it gets assembled I'll post some pictures and stats.
The body of the rover, roughly equivalent to the WEB, or 'Warm Electronics Box' on Sojourner that holds the batteries and electronics, fits between the arms and extends down to about half way to the wheels. In my case I use the enclosure to force air across the motor drivers to keep them cool, instead of warm. If I can I'll make it even thinner and get some extra ground clearance. :)
One thing that isn't obvious about the rocker-bogie design is how the two sides connect, and why it works. On the smaller JPL rovers a geared system joins the two arms, and on the bigger Curiosity, a torsion bar crosses over the top of the body. In this way the body stays as level as it can (about half of the motion of rocker-bogie arms), and the wheels are roughly equally loaded, so they all tend to want to stay on the ground.
I'm going with the geared half-differential; each arm has a 1/4" rod to the center of the body, with a bevel gear on the end. The two gears from the arms are linked by a 3rd bevel gear at 90 degrees, so when one arm rotates one direction, the other arm is forced down in the opposite direction. Big thanks to Beatty Robotics and their Spirit II robot for such great work and pictures, and they use ServoCity parts, too!
Here is a view of the middle of differential gears (in red) which fits in the central bit of 12" aluminum channel:
I happen to have 3 spare metal bevel gears that fit 1/4" shafting, so all I need are some longer shafts and a bucket of bearings and clamps to make it all work.
Next time I'll describe some of the ways to link multiple microcontrollers together, and some of the ideas I have for sensors and automation.