Vibration Isolation in Optical Test Systems
- Thursday, 01 September 2011
In the world of precision motion control there is a continuous pursuit of higher performance, whether it’s resolution, speed, stability, accuracy or step size. Even if a specified device appears on paper to be capable of achieving excellent results, the design of the mechanics and the platform stability play an integral role in achieving peak performance.
There is certainly an aspect of “art” in optical system design as these unique inventors look for new, innovative techniques to shape, bend, and manipulate light, either through the creation of new optical elements or by combining multiple elements. Equally important parts of the design are the mechanical elements that support, move, or control each component in the optical path. Each of these mechanical elements contributes to the stability (or instability) of the output beam and can be a hidden source of error if not properly designed or constructed.
A common source of instability within a light beam path is vibration. Vibration control systems that include, typically, vibration isolators and optical tables, are intended to minimize the impact of environmental vibration. The optical table serves as a common base for the whole opto-mechanical assembly. Opto-mechanical components such as posts, rods, and mounts, as well as positioning stages, are made to anchor optical elements in place so that the optical paths will be undisturbed by environmental impacts such as vibration. The final result depends on the whole “structural loop,” which encompasses support structures, motion control systems, and optomechanical elements.
Consider the innocuous optical post used by many to support optical mounts, sensors, and even motorized positioners. Typically these posts are machined from stainless steel or aluminum and serve as the foundation for many optical set-ups (Figure 1). When properly specified and installed they provide a rigid support to bring components to the proper beam height. However, when they are used incorrectly or installed improperly they can be a source of significant angst since the subtle vibration issues they may cause are typically blamed on the vibration control platform or motorized stage stability (i.e. proportional, integral, and differential (PID) problems or deadband hunting). To better understand this effect, testing was conducted on various diameter posts (0.5-in., 1.0-in., and 1.5-in.) of equal height (4-inches), to quantify the structural characteristics of each. Contact stiffness of the attachment was carefully controlled as it can be a significant factor affecting resonance vibration of the post.
The dynamic performance of the posts was characterized by the dynamic compliance, which is the ratio of the displacement measured in horizontal direction at the top of the post, to the excitation force applied to the post, as a function of frequency. The dynamic compliance shows the natural frequencies and the level of damping of the assembly.