Friday, January 30, 2009
This illustration shows how a motor can be mounted inside of a PRT “Pod-Car” wheel.
I was first inspired by a design from an electric “concept car” where the wheels were actually motors. The idea is simple. If you plug in a motor but hold the shaft, it will spin, (winding up the cord until it unplugs itself). So why not feed the cord through the shaft somehow? That way the motor itself would be a wheel, instead of mounting a wheel on the shaft? (Or more likely, a sprocket, gear or pulley) It turns out this idea is actually widely used. A Google search revealed that the concept is commonly employed to move conveyor belts, (drum motors) and to electrify bicycles in China (hub motors) and in fans, especially computer and ceiling fans.
Despite the obvious simplicity (and efficiency) of the idea, apparently the motors don’t perform well in terms of torque, as compared with a much higher speed motor fitted with a reduction gear. Also they are not exactly cheap or easy to find, so I bit the bullet and researched some more conventional motors.
My switching design calls for hard wheels, so I put in a bunch of them to have the traction to climb steep smooth slopes. I don’t really know how many is really optimum, at this point, but I checked out some “NEMA” standard sized brushless motors, and found that the NEMA 42 standard comes with sufficient torque that if each or the wheels had a motor within they could pull (together) with between 300 and 800 lbs. of forward power, while having a top speed of 60 mph. (Sorry, younger and euro readers, I am not natively metric and feeling lazy) Obviously, reducing the top speed increases torque proportionately, but it also worthy to note just how much of a trade-off is required between the weight, the steepness of the climbs and the top speed. There is no free lunch. For example a combined vehicle/payload weight of 1600 lbs. will probably not make a 45-degree climb.
It is also worth noting that the separate motors solve the fact that my system has no differential gears. For those who do not know what a differential gear is, here is a brief explanation – On car, when turning a corner, the outside wheels have further to travel than the inside wheels. Therefore they have to revolve faster, making more revolutions than the inside wheels. The differential gear accommodates this, preventing one or both wheels from slipping on the pavement, which would create wear and partial loss of control. Even though the right and left wheels in my PRT design are fairly close together, the same phenomena would apply to a lesser extent. With separate right and left motors, however, the RPMs of the wheels can be precisely controlled to actually create the appropriate steering forces.