Positional accuracy and repeatability are two distinct concepts often used to describe the performance of mechanical systems in the industrial sector.
Repeatability is generally greater than positional accuracy.
Positional Accuracy
Positional accuracy is a specific value, not a range, that indicates the degree of deviation between the target position that a mechanical system intends to achieve in a single movement and its actual position.
It reflects the system's accuracy in reaching the target position in a single movement.
In simple terms, it refers to the deviation between the actual position at which a piece of equipment stops and the position it should reach.
Positional accuracy is a defined value (e.g. 0.01 mm) rather than a range (e.g. ±0.02 mm).
The error in positional accuracy depends on the manufacturing inaccuracies of the transmission components.
For example, if an axis is supposed to move 100 mm but actually moves 100.01 mm, the 0.01 mm difference represents positional accuracy. If a lead screw is configured to move 100 mm and the actual movement varies from 99.99 mm to 100.01 mm, its positional accuracy will be 0.02 mm (100.01 – 99.99 = 0.02) .
Repeatability
Repeatability refers to the accuracy with which a mechanical system can repeatedly reach the same target position over multiple movements. It demonstrates the system's consistency and repeatability to achieve the same target position across multiple movements, which is crucial for applications requiring repetitive movements.
In layman's terms, it is the error produced when positioning the same location multiple times. Repeatability is expressed as a range (e.g., ±0.02).
The error in repeatability is related to play in the transmission components. Furthermore, repeatability is influenced not only by the accuracy and resolution of the grid scale (displacement sensor), but also by errors in the mechanical system such as guidance system, transmission system, and structural rigidity.
These factors can reduce positional accuracy and, to some extent, affect repeatability.
For example, if an axis is required to move 100 mm, it may actually move 100.01 mm on the first attempt and 99.99 mm on a repeated attempt. The error between these two measurements, 100.01 – 99.99 = 0.02 mm, represents repeatability. Repeatability is expressed as ±x, with the above error represented as ±0.01 mm or ±(0.02/2).
For example, if a lead screw is set to move 100mm and more than 100 moves, the actual distance varies from 99.988mm to 99.992mm, its repeatability is ±0.002mm.
