isladelobos
Sun 08 February 2009, 06:28
Repeatability
Repeatability is a statistical term associated with accuracy, it describes how a point is
repeated. If a robot joint is instructed to move by the same angle from a certain point a
number of times, all with equal environmental conditions, it will be found that the
resultant motions lead to differing displacements (Fig. 1.4). Although a target is always
missed by a large margin, if the same error is repeated, then we say that the repeatability
is high and the accuracy is poor. Repeatability does not describe the error with respect to
absolute coordinates. System repeatability is the positional deviation from the average of
displacements. For example, +-0,2 mm indicates that any point might be as much as 0,2
mm beyond or short of the center of the repeatability pattern.
Most robot manufacturers provide a numerical value for the repeatability rather than the
accuracy of their robots. The reason is that the accuracy depends upon the particular load
that the gripper carries. A heavier weight causes larger deflections of the robot links and
larger load on the joints, which degrade the accuracy, while the repeatability value,
however, is almost independent of the gripper load.
The repeatability of robots will usually be better than the accuracy, it is normally
measured in hundredths of an inch. Repeatability definition which is currently used is the
ISO 9283 definition (ISO / TC 184 / SC 2 / WG 2).
Figure 1.4: Example of representation of resolution, accuracy, and repeatability of a robot
arm [Kor85].
Figure 1.4: Example of representation of resolution, accuracy, and repeatability of a robot
arm [Kor85].
Repeatability is a statistical term associated with accuracy, it describes how a point is
repeated. If a robot joint is instructed to move by the same angle from a certain point a
number of times, all with equal environmental conditions, it will be found that the
resultant motions lead to differing displacements (Fig. 1.4). Although a target is always
missed by a large margin, if the same error is repeated, then we say that the repeatability
is high and the accuracy is poor. Repeatability does not describe the error with respect to
absolute coordinates. System repeatability is the positional deviation from the average of
displacements. For example, +-0,2 mm indicates that any point might be as much as 0,2
mm beyond or short of the center of the repeatability pattern.
Most robot manufacturers provide a numerical value for the repeatability rather than the
accuracy of their robots. The reason is that the accuracy depends upon the particular load
that the gripper carries. A heavier weight causes larger deflections of the robot links and
larger load on the joints, which degrade the accuracy, while the repeatability value,
however, is almost independent of the gripper load.
The repeatability of robots will usually be better than the accuracy, it is normally
measured in hundredths of an inch. Repeatability definition which is currently used is the
ISO 9283 definition (ISO / TC 184 / SC 2 / WG 2).
Figure 1.4: Example of representation of resolution, accuracy, and repeatability of a robot
arm [Kor85].
Figure 1.4: Example of representation of resolution, accuracy, and repeatability of a robot
arm [Kor85].