Direct-drive mechanism
A direct-drive mechanism is a mechanism design where the force or torque from a prime mover is transmitted directly to the effector device without involving any intermediate couplings such as a gear train or a belt.
History
In the late 19th century and early 20th century, some of the earliest locomotives and cars used direct drive transmissions at higher speeds. Direct-drive mechanisms for industrial arms began to be possible in the 1980s, with the use of rare-earth magnetic materials. The first direct-drive arm was built in 1981 at Carnegie Mellon University.Today the most commonly used magnets are neodymium magnets.
Design
Direct-drive systems are characterized by smooth torque transmission, and nearly-zero backlash.The main benefits of a direct-drive system are increased efficiency and being a simpler design with fewer moving parts. Major benefits also include the ability to deliver high torque over a wide range of speeds, fast response, precise positioning, and low inertia.
The main drawback is that a special type of electric motor is often needed to provide high torque outputs at low rpm. Compared with a multi-speed transmission, the motor is usually operating in its optimal power band for a smaller range of output speeds for the system.
Direct-drive mechanisms also need a more precise control mechanism. High-speed motors with speed reduction have relatively high inertia, which helps smooth the output motion. Most motors exhibit positional torque ripple known as cogging torque. In high-speed motors, this effect is usually negligible, as the frequency at which it occurs is too high to significantly affect system performance; direct-drive units will suffer more from this phenomenon unless additional inertia is added or the system uses feedback to actively counter the effect.
Applications
Direct-drive mechanisms are used in applications ranging from low speed operation to high speedsSome electric railway locomotives have used direct-drive mechanisms, such as the 1919 Milwaukee Road class EP-2 and the 2007 East Japan Railway Company E331. Several cars from the late 19th century used direct-drive wheel hub motors, as did some concept cars in the early 2000s; however, most modern electric cars use inboard motor, where drive is transferred to the wheels, via the axles.
Some automobile manufacturers have managed to create their own unique direct-drive transmissions, such as the one Christian von Koenigsegg invented for the Koenigsegg Regera.