Re-inventing the robot wheel
January 1, 2013
Anyone wanting to play with articulated robotic systems like arms, legs, pan tilt cameras etc will invariably end up using the ubiquitous hobby servo, a handy little gear motor that comes with its own angle sensor and closed loop position control. You just plug them in, send a position pulse and away you go. You can even hack them to produce continuous rotation for wheeled platforms, albeit without any closed loop control.
When it comes to bots on wheels there is no direct equivalent to the hobby servo, no integrated solutions, just lots of component parts (gear motors, motor drivers, sensor boards, wheels, etc) that need to be cobbled together before you can set your wheels in motion. The results can be less than ideal, with motors sticking out of the wheel at right angles and taking up room in your chassis along with the tangle of wires connecting motors to driver boards and encoders.
Given the lack of a ‘servo wheel’ I thought it was time for a small revolution in robot wheel technology (pun intended!) so a few years ago I set up a company called Creative Robotics and started working on a solution – the HUB-ee wheel - a wheel hub with a gear motor and open source electronics with a motor driver and quadrature encoder, all squashed into a 60mm diameter by 20mm package. We also made sure that when the tyre is removed you can use it with tank tracks (from Solarbotics)
The idea of a powered wheel hub isn’t entirely new, wheel hub motors for vehicles date back to the 1900’s, Ferdinand Porsche used electric hub motors for a car way back in 1897 and today they can be found in golf carts, bicycles, electric luggage and even sports cars. Ours is the first (that we know of) that is designed specifically for small robots.
The first version of our HUB-ee wheels still lacks the handy closed loop control of the hobby servo. The open source PCB has a Toshiba TB6593FNG motor driver which needs a PWM signal and a couple of binary direction signals in order to set the speed, direction and brake mode, and it has an incremental encoder that reads a 32 stripe reflective sensor (the same as the excellent Wheel Watcher kit) and gives you a two channel quadrature signal. Plug all this into an Arduino and you can craft a proper PID speed control loop for your wheel (or use the library we are working on!).
A proper closed loop controller PCB is in development so in the next few months we will introduce a version that provides speed and direction control from a single servo pulse thanks to an embedded microcontroller, all of which will be open source, including the firmware.
What is it good for?
Just like the hobby servo, there will never be a ‘one size fits all’ solution (which is why we are working on quite a few different models for the future), and so our first version will not work for everyone, but there are some robot designs that just become so much easier when all the essential drive components are wrapped up inside the wheel. For starters, you can literally bolt the wheel onto anything you want (a cardboard box for example) and get an instant robot.
If you are building a simple differential steering bot you can end up with a much neater design – no motors sticking into the centre of the robot, right where the batteries should be. This is handy for all terrain bots as well because there is no motor to snag on obstacles. Below are a few photos and videos of some of the platforms we have built, you can also see some of them in action via our Youtube Chanel.
HUB-ee wheels are just starting to come onto the market - Expect to see them appear with distributors over the next month or two!
Unobot - so called because of the Arduino Uno that provides the brains. The lack of sticky-out-motor gives plenty of space for batteries ...
... and you can get much better ground clearance. Great for all terrain vehicles and tanks.
You can daisy chain wheels together so they all run in sync from the same control signals – Tanks and other multi wheel behemoths are easy.
And you can do some fancy stuff with servo steering – bolt a servo to the wheels …
... and make your own Mars rover? ...
... complete with rocker suspension!
With a few cleverly designed laser cut parts (using Delrin) you can make this neat self leveling steerable wheel.
A version with LEGO® NXT compatibility is in the works ...
... or you can hook it up to an Arduino.
This trike bot just wouldn’t work with conventional motors and wheels. The rear wheel doesn’t have any sticky-out bits and no external drive components, just a cable. It even has suspension made possible with laser cut polycarbonate springs.