Present electrically powered manipulators are still far inferior to human antis and unsatisfactory for many applications in terms of speed, accuracy and versatility. One of the reasons for this poor performance comes from the transmission mechanisms, such as gear trains, lead screws, steel belts, chains and linkages which are used to transmit power from the motors to the load and to increase the driving torque. These problems result from having complicated transmission mechanisms Dynamic response is poor because of the heavy weight or high compliance of the transmission. Fine movements and pure torque control are difficult because of the relatively large friction and backlash at the transmission.

Direct Drive motor applications that can accept trade off’s between high efficiency and high performance versus high cost and integrated packages are still few in number.

Direct drive motor technology eliminates backlash, reduces the number of mechanical components, and provides a stiff mechanical system for highly dynamic applications. Pre-tapped mounting holes and a hollow-through shaft permit a variety of machine designs.

Types of Direct Drive Motors

Direct Drive Motors are two types, Direct Drive Linear (DDL) Motors, and Direct Drive Rotary (DDR) Motors. Direct drive technology eliminates backlash, reduces the number of mechanical components, and provides a stiff mechanical system for highly dynamic applications. Pre-tapped mounting holes and a hollow-through shaft permit a variety of machine designs. Accuracy is a key reason direct-drive motors are chosen. In both DDL and DDR motors, the work piece is rigidly coupled to the motor. There are no transmission errors such as backlash, lead screw error, belt stretch, and gear tooth error.

Feedback devices for DDL and DDR motors are very accurate. The multi-speed resolvers used in DDR motors are 10 or 20 times more accurate than traditional resolvers. The scales used with DDL motors are often resolved to 100 nanometers or less. The problem of stick-slip, a phenomenon where the load is hard to move small distances with accuracy, is reduced or eliminated with direct drive. This is because the relatively high friction, high-compliance transmission components that commonly cause stick-slip in traditional mechanical transmissions are eliminated in direct-drive systems.

Advantages of Direct Drive Motors

  • Increased efficiency: The power is not wasted in friction i.e; from the belt,chain etc.
  • Reduced noise: Being a simpler device, a direct-drive mechanism has fewer parts which could vibrate, and the overall noise emission of the system is usually lower.
  • Faster and precise positioning. High torque and low inertia allows faster positioning times on permanent magnet synchronous servo drives. Feedback sensor directly on rotary part allows precise angular position sensing.
  • Drive stiffness. Mechanical backlash, hysteresis and elasticity is removed avoiding use of gearbox or ball screw mechanisms.
  • High torque at low rpm.
  • Longer lifetime: Having fewer moving parts also means having fewer parts prone to failure.

Limitations of Direct Drive Motors

  • The main disadvantage of the system is that it needs a special motor. Usually motors are built to achieve maximum torque at high rotational speeds, usually 1500 or 3000rpm.
  • direct-drive mechanisms need a more precise control mechanism. Low voltage variations on a high-speed motor, which is reduced to low rpms can go unnoticed, but in a direct-drive, those variations are directly reflected on the rotational speed.
  • Direct-drive motors do not fit every application. In most cases, they are more expensive than traditional motors and transmissions. That’s especially true when the transmission is used to gain large mechanical advantage such as with large gear-ratio gearboxes and fine-pitch lead screws. The feedback devices for direct drive are usually more expensive as well.
  • Direct drive systems do not have much friction. While this is usually an advantage, it can be a disadvantage when power is lost because some machines rely on friction to bring the machine to rest.
  • Finally, for engineers who are used to the design of traditional machines, it takes time to learn how to best apply direct-drive technology.