ISR research accomplishments

Anti-backlash drive systems for multi-degree freedom devices
US Patent 5,245,263 Sept. 14, 1993

Research team

Lung-Wen Tsai (ME/ISR)
Sun-Lai Chang

Accomplishment

The need for the systems. Most industrial robots use gear trains for power transmission to allow actuators to be located in desirable positions. Gear trains are also used for torque amplification. Backlash is a persistent problem in such machines because of the tooth clearances. These clearances are necessary to prevent gear teeth jamming due to manufacturing errors or thermal expansion.

Backlash introduces discontinuity, uncertainty and impact in mechanical systems which, in turn, makes accurate control of a manipulator difficult. End-effector positioning accuracy is also compromised due to backlash.

Precision gears, spring-loaded split gear assemblies, and precise mechanical adjustment are often used to overcome these difficulties. However, these techniques do not completely eliminate the backlash and can increase the cost of manufacturing and assembling. Therefore, a way to reduce or eliminate the backlash problem is urgently needed.

Prior anti-backlash techniques using an active actuator technique using unidirectional drives have included U.S. Pat. No. 3,512,425 where two motors drive a load through separate trains of gears. The motors are unidirectionally operated, but drive torque of each motor is always of an opposite sense to eliminate backlash. Such a system is limited to a single degree of freedom.

What do the systems accomplish? This patent is an innovative concept for eliminating gear backlash in a multiple-DOF manipulator. This reduces noise and vibration associated with gear trains and, at the same time, improves the accuracy and stability of a multi-degree-of-freedom system. The concept:

Uses redundant unidirectional drives to assure positive coupling of gear meshes at all times.
Includes a systematic methodology for enumerating a class of unidirectional-drive gear mechanisms. To demonstrate, some typical two- and three-DOF robot manipulators have been sketched.
Derives actuator torques as functions of either joint torques or end-effector dynamic performance requirements.
Provides a redundantly driven gear mechanism with a fail-safe advantage. Except for the loss of backlash control, it can continue to function when one of its actuators fails to work.
Provides flexibility where, if high accuracy is not important, then it is possible to control the actuators in such a way that no antagonism; i.e., anti-torque application, exists among the actuators so as to achieve maximum dynamic performance.

For more information

View this patent at Delphion.
Related ISR technical reports: 90-38 | 92-85 | Related ISR PhD thesis: 91-7