
Figure 2  Older power chairs use a belt drive, which is easy to modify
There are many varieties of power chairs and shopping scooters, and often the older ones use a belt and pulley drive, which is good for hacking because you can alter the gear rations by changing the diameter of the pulleys. A larger pulley on the motor means more speed and less torque, whereas a smaller pulley on the motor means slower speeds and more torque. Most power chairs travel at a speed between a fast walk and a jog and are capable of carrying several hundred pounds so the gear rations used are probably just perfect for a powerful robotics base.
Many newer power chairs have direct drive, having the wheels attached directly to the output shaft on the motors. These types of transmission systems are very easy to use in robotics projects due to their simplicity, and as long as you are happy with the fixed speed, these are actually a better choice for robotics projects. Wheel diameter is also important if you plan to take your robot into rough off road terrain, with larger diameter wheels being better. Many older power chairs using belt drive have large diameter wheels in the range of 26" or 24" whereas the newer direct drive chairs usually use 12" or 10" diameter wheels. Some shopping scooters even have small 6 inch wheels, and although these smaller wheels are not all that good for off road terrain, they will certainly be fine for most urban terrain.
To calculate how fast a robot will travel, you first need to know how far your robot will travel when its drive wheels make one full rotation. To calculate this value (which is the circumference of a circle), multiply the total diameter of the drive wheel by PI (3.1415). So, for Oberon's total wheel diameter of 16 inches, I would multiply 16 x 3.1415 to get 50.264. This means that my robot would travel approximately 50 inches if the drive wheel were to make one full revolution. Calculating top speed once you know how far your robot will travel for each drive wheel revolution is easy now; just multiply that number by the final rpm of the drive wheel. So if Oberon's drive wheels were turning at 80 rpm, I would multiply 50.264 x 80 to get a top speed of 4021.12 inches per minute. 4021 inches per minute may seem like recordbreaking speed, but once we divide that by 63360 (the number of inches in a mile) we end up with .0634 miles per minute. Multiplying that answer by 60 will tell us how many miles per hour our robot will travel (.0634 x 60 = 3.8). So, after all that math, I calculated that Oberon would travel at 3.8 miles per hour if its 16 inch diameter drive wheels were turning at 80 rpm, not bad for an outdoor robots top speed. So, the formula to calculate how fast in miles per hour your robot will travel if you know both the drive wheels total diameter and RPM would be: speed = ((wheel diameter x 3.1415 x RPM) / 63360) x 60.
Figure 3  The belt drive and pulley system using a belt tensioner
The power chair that I am salvaging for parts has a belt drive system that is kept tight by a pair of idler pulleys that force the belt tight around the motor drive pulley. I plan to reduce the complexity of the transmission by simply using the correct size belt to keep the tension, replacing it after I am done installing the motors and drive wheels. Belts are available in every possible length, so it just seemed more logical to leave out the tensioner pulleys and use the proper length belt.


