Continuously variable transmissions
From FIRSTwiki
Contents |
Synopsis
This page does a good job of describing CVTs. The basic principle is that of two pulleys that are variable in size. Instead of shifting between gears as in a normal transmission, the sizes of these wheels change, effecitively producing an infinite number of gear ratios. This type of CVT is rarely seen, but it illustrates the point effectively.
Types of CVT
Intermeshing Cones
The basic principle here is that two cones are fashioned such that they can slide in and out of one another, effectively creating a v-shaped pulley that varies in size as the cones are moved in and out of one another. This type of CVT is often seen in motorcycles.
Toroidal
This type is illustrated below. Essentially two toroids are mounted across from one another, connected to shafts that serve as the input and output of the transmission. A secondary frictional wheel rotates within the two toroids transferring torque in different ratios depending on its position.
Super Simple
A wheel connected to the motor spins, and a smaller wheel mounted perpendicular to it slides along it's surface. Imagine a record player with a wheel instead of a read head, and imagine how the speed of rotation would change as the wheel got closer to the center of the record. This isn't very practicle either. See this ChiefDelphi thread for more information and images.
Application in FIRST
As with any drive train, drive trains in FIRST try to maximize the power output of the system. Though CVTs are very inefficient compared to more traditional gearing systems, they can be used to actually increase the effectiveness of the drive train because they can provide peak power over a at all RPMs and consequently at all times. On the other hand, a traditional system may provide even higher efficiency/ouput at one point (or several points in a system with multiple gears), it will be less effective at other RPMs.
Two teams, 64 and 190, have used a CVT in a FIRST robot, with mixed results (see this CD thread for more info). Several teams claim to have used CVTs, but have in fact used used a so-called "Planetary CVT". A planetary "CVT", such as the ThunderChickens' CCT, is not actually a CVT, and functions quite differently from a one.
