Many types of equipment exhibit a very high center of gravity. Common examples include large probes such as Scanning Electron and Transmission Electron Microscopes. Other systems use precision granite bridges to support measurement instrumentation. In these cases the center of gravity can be higher than that recommended for flat optical platforms. Below is the common rule of thumb for a common optical table system.
When the center of gravity is outside the recommended range, the angular moment of the system can overcome the restoring force of the isolator’s pneumatic leveling system. This can be thought of as the problem facing top-heavy sailboats when the overturning moment against the sails overcomes the righting moment of the keel. Another analogy is the inherent instability of a bottom wing fighter aircraft when compared to a top wing Cessna. In all these cases, the instability can set up oscillatory motion as the overturning moment and the restoring force compete. In an optical pneumatic isolation system, the payload can pitch violently as these air valves attempt to compensate for the overturning moment. In fact, higher performance systems tuned to optimize response time and leveling accuracy may react worse than lower performance “soft” systems.
The solution to this problem is to use a cradle type platform. A cradle platform lowers the center of gravity and dramatically increases the stability of many systems. Over the years, Newport has built a large variety of cradle systems. In general, they consist of three or four outriggers connecting to the isolator at the top and supporting a platform at the bottom. Each design is built to customer specifications in our custom products group.
Shown are a variety of cradle systems. Cradle materials include welded steel, aluminum and honeycomb structures. As with any of our custom products, the platform can include holes, cutouts, multiples levels and even vertical structures. Any of our isolation systems may be used depending on load and size requirements. Docking systems and other options may be included. |