Glossary

The following is an explanation of commonly used terms in the motion control field.

Accuracy - is the difference in position between the actual position of the payload and the programmed position of the payload. It is measured in (inches or [microns]). Link: Understanding Accuracy and Repeatability

Cogging - is defined as a preferred position of the table with respect to the stationary base when using iron core brushless motors. It is caused by a reluctance change between the poles of the permanent magnets and the edge of laminated core of the coil assembly. This occurs when the motor is not powered up.

Flatness - is the variation of the vertical position of the stage as it travels down its length. Better flatness can be achieved using the AB and XR stages. It is measured in (in/ft or [microns/m])

How Big Is Micron - Use the example and illustration bellow to better understand exactly how big a micron is. The scale of the diagram is 1000 to 1 ratio.

The thickness of a standard piece of paper measures 0.004 inch (0.1016 mm).

In comparison, a circle of 0.001 inch (0.0254 mm) diameter is shown. Note how much smaller it is than the piece of paper. 1 micron is 1/100 of the paper thickness.

A circle with a diameter of 1 micron is shown. A micron measures 0.000039 inch (0.001 mm) diameter is shown. Note the extremely small size of this circle compared to the piece of paper.

Repeatability - is defined as the range of the final positions of the moving table of the stage after a series of repetitive moves to the same point. It is measured in (± inches or ± microns)

Straightness - is the variation of the horizontal position of the stage as it travels down its length. Better flatness can be achieved using the AB and XR stages. It is measured in (in/ft or [microns/m])

Understanding Linear Encoder Resolution

Resolution: Resolution is a term defining the available number of divisions per inch (millimeter) for linear encoders. Resolution is typically specified in lines per inch (millimeter).

Typical linear encoder resolutions, in order from lowest to highest.

5-micron encoder - A 5-micron encoder resolution means that over 1 meter of travel there are 200,000 different points that the encoder can read. Each point on the encoder scale/tape is exactly 5 micron (0.005 mm) away from the next.

1-micron encoder – A 1-micron encoder resolution means that over 1 meter of travel there are 1 million different points that the encoder can read. Each point on the encoder scale/tape is exactly 1 micron (0.001 mm) away from the next.

0.1-micron encoder - A 0.1-micron encoder resolution means that over 1 meter of travel there are 10 million different points that the encoder can read. Each point on the encoder scale/tape is exactly 0.1 micron (0.0001 mm) away from the next.

Note: The final positioning resolution of a properly tuned closed loop servo system with an encoder is typically ±2 encoder counts. So with a system that has a 1 micron resolution encoder on it, you can expect to achieve a final position that is within ±2 microns of the desired position.