Backlash describes direction-dependent lost motion under a stated load procedure.
RCI 005 · Parameter guide
Three errors that should not share one column.
A practical separation of lost motion, angle mapping error and repeated positioning behavior in geared robot joints.
Transmission error describes the output-angle deviation over a rotation, even without a direction reversal.
Repeatability describes a system returning to a target under a defined procedure; it does not prove absolute accuracy.
Backlash is a reversal phenomenon
Backlash is commonly observed when the input reverses direction and the output does not immediately follow. Gear tooth clearance, spline interfaces, bearings, preload and elastic deformation can all affect the measured dead band. The value depends on the applied torque, direction, measurement point, temperature, wear state and whether compliance is removed from the result.
A datasheet value such as 15 arcmin or ±0.25° is therefore not a complete test method. It is still a useful reported boundary, but RCI stores the unit, sign convention and missing load condition rather than silently treating every published backlash number as equivalent.
Transmission error can exist without a reversal
Transmission error is the difference between actual output angle and the ideal output angle implied by the nominal ratio. It can vary periodically with gear position and may reflect tooth geometry, eccentricity, assembly and elastic effects. A joint can have low backlash yet still show meaningful transmission error across a revolution.
For control and calibration, the engineer needs the error trace or harmonic content, not just a maximum number. Whether the error is measured unloaded, under torque or after compensation changes its system impact.
Repeatability belongs to the complete positioning system
Repeatability asks how closely a system returns to the same pose under a specified sequence. It includes sensing, controller behavior, load, approach direction, temperature and structural stiffness. Absolute accuracy asks how close the achieved pose is to the commanded physical reference; it is a different metric.
An integrated actuator encoder resolution is not joint accuracy, and joint accuracy is not end-effector accuracy. Resolution, accuracy, repeatability, backlash and transmission error occupy different levels of the evidence model.
The minimum publishable record
For any lost-motion or positioning claim, RCI seeks the metric name, angular location, load, direction or approach procedure, temperature, number of cycles, measurement instrument, product revision and whether compensation was enabled.
- Keep arcmin, degrees and radians convertible, but retain the original unit.
- Do not infer transmission error from encoder resolution.
- Do not infer end-effector repeatability from a single joint specification.
- When a vendor uses “precision” without a definition, store the wording and mark the definition unknown.
RCI operating rule
A missing definition is publishable evidence about data quality. It is not a reason to invent a cleaner metric.
Selected primary sources
These links support the examples and product claims used in this guide. They do not imply that every general engineering statement is a quotation from one source.