Alexander Robotics

After sketching the robot and selecting a motor, my next step was planning out how to connect the motors to the wheels for propulsion. In other words, figuring out the robot’s drive train. The motor selection step and drive train design can be switched or worked in parallel.

Initial design — and failure

My original plans for the robot called for mounting a motor directly to each of the two back wheels and placing two caster wheels in the front. I crafted a rudimentary prototype by screwing a pneumatic model airplane wheel to a mounting hub, coupled to the motor shaft. I mounted the two motors to a frame of thin slabs of wood using Polulu’s motor bracket.

Around the time of building this prototype, the rather self-explanatorily titled book ”Building Robot Drive Trains” arrived at my doorstep. Early in the book, the flaws in coupling a wheel to a motor directly became obvious, too often invoking the “Duh, what was I thinking?” response. The biggest flaw in direct coupling is the robot’s requirement to carry a payload around 14 kg. The radial load — the perpendicular force on the motor’s shaft — would be too much if the shafts had to support the weight of the robot. A high radial load stresses the motor’s bearing and wears down its life expectancy.

Decoupling

I needed to decouple the motor from the wheel. Back to sketching.

My three options from a high-level view were: connect the motor and wheels via gears, using belt and pulley, or via an extended shaft. Hehe. I ruled out gears because I did not want to deal with the precision necessary in placing the gears and lining up the teeth. It was a toss-up between belt and pulley versus putting another shaft between the motor and wheel. I ended up drawing plans for both.

The belt and pulley design involved coupling the motor to a pulley and another pulley to a shaft, which spun the wheel, and then connecting the two pulleys using a belt.

The big choice on a belt and pulley setup is v-belt or timing belt. A v-belt is smooth and shaped like a stunted letter V. The tension turns the pulleys. A timing belt is notched and lines up with the timing pulley’s teeth. The timing belt has the benefits of transferring torque more efficiently and allowing less slippage. I consider the v-belt’s property of allowing slippage a feature. For example, if the robot is being stopped by an outside force while the motor is still going, the slippage lets the motor still spin rather than burn up from the resistance. I decided to try both a v-belt and timing belt setup and test empirically.

To extend the motor’s shaft, I connected a shaft between the wheel and motor’s shaft. I used pillow blocks to support the shaft and handle the radial load from the wheels. A pillow block is a mount, bored through with a ball bearing. The perpendicular stress on the shaft is distributed to each bearing. To connect the motor shaft to the extended shaft, I utilized shaft couplings.

Where to buy

My biggest complaint about building robots, a complaint that does not apply to the world of software, is where to find parts. I hope when the Personal Robot market matures, more standardization will take place with the hardware aspects. Perhaps when the term PR becomes as prevalent as PC.

McMaster.com. McMaster-Carr is an industrial parts supplier, complete with a wide selection of pulleys, belts, bearings, wheels, and thousands of other components. McMaster-Carr is to hardware what DigiKey and Mouser are to electronics. To boot, the website is impressively easy to navigate (please take some notes, DigiKey) and includes helpful explanations detailing what to look for in each component. The downside: a substantial markup for each part.