If you have read some of the earlier posts about my large robot ERN-E you might recall I mentioned that it was rather large and heavy (20kg), and that I was thinking of a redesign to produce a smaller and lighter robot, more compatible with being in a house and around people.
Problems with ERN-E 1.0
I had been thinking along the lines of getting a more compact drive train. ERN-E has some relatively large 300mm diameter wheels, originally intended to allow travelling over small bumps and steps. It turns out that any significant step creates a large lean as the robot is quite tall.
The drive motors are from cordless drills, and while these work well the proverbial challenge for custom robot builders is attaching the motor to the wheel or drive-train. I found the easiest way was to leave the drill chuck on and tighten it onto the shaft of the pinion sprocket. The downside of that was the length of the motors limited the minimum width of the chassis and the placement of the motors.
Connecting the motor and wheel is a chain, sourced as a spare part for a mini dirt motorbike. To avoid the complications associated with shortening the chain the motors were mounted some distance back from the wheels.
The drive system worked reasonably well. The power of the motors was well matched to the size of the wheels and the weight of the robot. To drive the robot for any reasonable time I had a 12Ah SLA battery for the motors. To provide a clean power supply for the electronics I had a second battery - 7.2Ah. Together these batteries weigh 5.5kg, a significant proportion of the overall weight and a serious challenge to producing a lighter more compact robot.
What Does the Robot Do?
I guess every robot builder has heard this question, and as some bright roboteer once said - It makes me happy. I enjoy robotics as a hobby, so allowing me to tinker with a robot and solving challenges is enough for my robot to do. My wife would like it to do all of the housework.
That said, I would like to experiment with autonomous way-finding. That involves obstacle detection and avoidance, and SLAM (simultaneous location and mapping). I'd like to add voice-command (speech recognition), and speech output. Vision processing to detect faces, recognise navigation features, and follow objects is also something I'd like to do. On top of that the robot could be fun and entertaining, so should have an assortment of those sorts of features.
Those advanced features require a fairly powerful processor, and ERN-E 1 uses an on-board PC running ROS. While the PC could be moved off the robot with a wireless communications link I'd prefer to keep the robot self-contained. That starts dictating the minimum size and weight.
Candidate Chassis
There are a lot of pre-built or kitset chassis available these days, most of which would be a reasonable start.
The Turtle-Bot was one contender, but the Roomba base left me wondering about the ability to negotiate small bumps and a range of floor surfaces. I also looked at the Eddie chassis and other similar offerings.
After looking at these I was intending to re-build a custom chassis using a different set of motors and wheels. For one thing most of the robot kits are relatively expensive once purchased and shipped to New Zealand way down under. Secondly, I already had a good supply of aluminium stock and other hardware in ERN-E, and don't mind chassis building.
While researching motors and wheels I was being asked by my family what they could get me for my recent birthday (a reasonably significant ag). Looking through some websites we came across the demonstration video for the Wild Thumper 6WD chassis. That looks like FUN - just as a radio-control toy. It should also serve nicely as a chassis for an autonomous outdoor all-terrain robot. After a lot of hesitation on my part, and egged on by my wonderful family the order was placed!
I am now fully into planning my new robot. The very grippy tyres supplied might limit turning-on-the spot on carpet, so we will have to trial indoor turning performance and possible get some alternate tyres from a radio-control car shop. The maximum recommended payload is 5kg so a lot of the planning is based around minimising weight in all of the components - after my experience with ERN-E 1, that is not a bad thing. With the weight of the chassis the robot should weigh in at under 10kg.
Adding the ability to properly travel around outdoors, including on grassy slopes opens up new challenges (infrared distance sensors don't work well in bright sunlight) and new opportunities - like GPS location.
Stay tuned for more updates.
