Repeatability describes the accuracy of repeatedly moving to a position with the same speed and acceleration. A value of ±5 to 10 arcseconds is currently considered standard in robotics. Other applications, such as positioning, require precision down to a hundredth of a millimetre. Excellent repeatability can be achieved, for example, by calibration of the robot or of other applications.
Repeatability, positioning accuracy and transmission accuracy
Besides repeatability, positioning accuracy is another key parameter. Positioning accuracy defines the speed at which a desired position can be reached. Another important aspect here is the transmission speed, which reflects the position error of the initial motion that is transmitted at the gear output shaft. Transmission accuracy results primarily from the production tolerances and the pins installed in the case, and is specific to each individual gearbox. Positioning accuracy is defined primarily by transmission error and hysteresis loss.
The precision of a gearbox depends on numerous gear-specific characteristics. Precision-related gear data includes, for example, the hysteresis loss, which results from a combination of friction and backlash, the rigidity of the gear, and positioning and transmission accuracy.
High precision throughout the entire life of the gearbox
The special design of Nabtesco cycloidal gears, which use cams and rollers, allows reduction ratios from 30:1 to more than 300:1 with only two or three gear stages – with extreme precision throughout their entire life (hysteresis loss of 0.2 to 1 arc.min). In addition, the very low transmission ratio fault ensures very smooth-running precision, so that the gear output speed remains very constant, with minimal fluctuation. Nabtesco gears feature repeatability of ±5 to 10 arcseconds.
Robots with the precision of a machine tool
The integration of gear-specific data and characteristics in the robot controller allows exact compensation for deviations from the ideal behaviour, improving robot precision by a factor of 9. In the future it will be possible to implement robots with pin-point and path accuracy within hundredths of a millimetre. High-precision handling and processing tasks can then be carried out by a robot with the precision of a machine tool. That enables maximum flexibility with no compromise in quality.
Precision-related gear effects:
- Backlash / hysteresis
- Transmission error
- Elasticity / rigidity
- Reverse backlash
- Gear friction