Acceptable vs. Unacceptable Oscillations

When tuning a servo, you will be forced to make judgment calls on the servo's performance. To determine whether or not a servo has satisfactory performance, you will need to decipher when a servo has acceptable or unacceptable oscillations.

Here are some common criteria guidelines that should be considered when evaluating a servo's performance:

Measured Overshoot
ex: 25% overshoot on a step input.
Visible overshoot
ex: The visible motion appears untuned.
Audible Noise
ex: No audible noise is heard when the stage is inside the production enclosure.
Payload slippage due to servo chatter
ex: The wafer cannot slip on the stage > 0.5 um when a move is made.

It is important to set your requirements and guidelines BEFORE you start to tune your machine in order to avoid the risk of making compromising judgments later in the tuning process. If clear guidelines are set, it will be easier to determine what is considered a passing or failing performance.

Example

Here is an example of a servo performance specification.

50 % maximum overshoot on a 100 count step. (measure of stability)
No audible noise is heard after the stage has settled.
No stage dithering continues after the motion is done.
Minimum Settling Time (when no tuning has been performed)

How do I choose my own spec?

See Choosing Specifications for a Machine for more information.