Repeatability

[isladelobos]

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].

[isladelobos]

One repeatability test

http://www.youtube.com/watch?v=ky8Pip6Qa_A

a good repeatability test system

http://www.youtube.com/watch?v=6OaX8ThRFKs

Mechmate testers is possible see this with a caliper?

Or a ballpen line or circle repeating this.

[Gerald D]

Sometimes I use the term predictability instead of repeatability. It gives a feeling of how predictable a machine will be when it responds to your command.

[Gerald D]

I think you asked how good the repeatability of the MM is . . . ?

In the X&Y directions always better than 0.1mm. And it does not matter from which direction you approach the position. Cannot measure any better with a standard caliper. The last time I played with measuring these numbers, we still had direct drive motors. With geared motors it should be even better.

In the Z direction we saw bigger differences in the repeatability, probably around 0.5mm, depending on whether the sun was shining on the table or not. There were big windows next to the machine, but now we have blocked out the sunlight permanently to get better repeatability.

In general, the stepper motor/pinion/rack with spring loading gives excellent repeatability. Flex of the table reduces the repeatability.

[Richards]

Since these machines are going to be used to cut wood, it's good to remember that wood is not stable. The thickness changes with temperature. The width changes with humidity. All kinds of things happen when the cutter relieves (or creates) internal stress. Try an experiment. Cut a bunch of parts (which should be identical in size right after they are cut). Put a few parts in a warm room, a few in a cold room, a few where the humidity is higher or lower than normal, a few in direct sunlight and a few in a dark, cool room. After a day or two, gather all the pieces together and compare sizes.

Your tolerance or repeatability, if tighter than the difference in sizes on the 'seasoned' wood parts, will make little or no difference in the final part size. Even here in Utah, where the desert air keeps things pretty dry, we try to assemble parts within a few hours of cutting, especially when tolerances are tight. I made several hundred doll cradles for a Christmas charity. At the end of the project, I searched around and gathered all the 'forgotten' pieces that had slipped behind other material. Some of the parts had to be pounded pretty hard with a dead-blow mallet to make them fit, and yet when they were first cut, they were near perfect size and fit together without tools.