Compact platform rides on dual-role isolators

Irvine, CA--Vibration isolation platforms support analytical balances, microscopes, and other instruments. Conventional isolation systems are relatively expensive, typically costing more than $2,000. Such systems usually employ pneumatic isolators to isolate platforms from vertical floor vibrations. A piston resting on a flexible rolling diaphragm supports the isolated mass, and the diaphragm also supports the piston on a large spring chamber.

In damped systems, air flows between the spring chamber and a second chamber called the damping chamber via a small orifice. As air flows through the orifice, energy is dissipated, reducing the amplification of the isolator at resonance.

Newport Corp. saw a need for a small, lightweight, low-cost isolation platform capable of offering excellent isolation. Designed by Senior Design Engineer Nick Eddy and Senior Designer George Ecker, this isolation system enables Newport's BenchTopisolation platforms to meet these requirements. Their design combines both horizontal and vertical isolation into a single package, and greatly improves the linkage between the isolator's two chambers. In fact: "The result of this design is that our spring chamber is basically the entire volume of the isolator," asserts Bowie Houghton, engineering manager, vibration control.

Houghton explains that horizontal isolation typically relies upon a pendulum. Given that the total planned depth of the new platform was about two inches, the pendulum used in the new system would be short, achieving less-than-optimal isolation. Newport's team decided to make the pendulum integral with the diaphragm used to isolate.

In this arrangement, the (approximately) 1.50-inch-long pendulum flexes the rubber diaphragm as it moves. In effect, the design produces a pendulum and spring in series, pulling the horizontal natural frequency of the isolation platform lower than it could be with a simple pendulum. Thus, a single assembly can perform both horizontal and vertical isolation. (Three or four of these assemblies, depending upon the model specified, support Newport's isolation platform.)

When engineers design a classic pneumatic isolator, the small orifice that connects the spring chamber and damping chamber causes air to flow in a nonlinear manner. Sizing the orifice for micro-motion means that when large floor motion occurs, not enough air flows through the opening to damp the motion. Conversely, if you size the orifice to handle large floor motions, you underdamp for micro-motion.

In the system designed by Houghton and his team, linkage between the damping chamber and the compliance (spring) chamber is provided by a laminar-flow damper--a sintered-bronze filter--instead of a simple orifice. Air flows through thousands of tiny orifices simultaneously, creating laminar flow.

Basically, says Houghton, the entire volume of the isolator becomes the compliance (spring) chamber. Considered from the viewpoint of the physics involved, the damping chamber and the compliance chamber run together into a hybrid chamber, because of the excellent transmission characteristics of the sintered-bronze component. This approach enables the engineers to produce an isolator assembly 1/3 smaller than classical units. Laminar-flow damping reduces settling time by 50% compared to units that employ conventional orifice damping.

Achieving the project's goals proved challenging. Initially, the isolator was to consist of a metal, bell-shaped part bonded to a rolling rubber diaphragm. Cost constraints caused Houghton and his associates to give up on the metal part, and they turned to DiaCom Corp. of Amherst, NH, to build a diaphragm capable of housing the isolation module's pendulum.

Charles Schwab, DiaCom's chief engineer, says that the company's initial prototype was made from a single layer of polyester impregnated with nitrile elastomer. Its shape was that of a truncated cone. To give the diaphragm some structural strength, engineers molded a 6061-T6 aluminum ring into the diaphragm's base, just above its convolution. This ring increased the effective diameter of the rolling diaphragm, and thus its active surface area. Unfortunately, test parts' walls could not support the forces generated by 80 to 100 psi air, and collapsed. The pendulum ripped through the single fabric layer, causing leaks. Finally, the ring proved difficult to bond into the diaphragm.

After several iterations, Schwab and his colleagues determined where stresses tended to build up in the component. They then decided to make the upper part of the diaphragm, the part that fits over the isolation assembly's pendulum, from two layers of square-woven polyester. This construction prevented any further ripping around the pendulum. The flexible convolution at the cone-shaped structure's base required only one layer of material. To capture the ring, DiaCom shaped it to conform to the surface at which the part's conical upper half meets the diaphragm. After coating the ring with adhesive, they support the ring within a mold at four points, and inject rubber around it.

The open space between the diaphragm's wall and the wall of the chamber that surrounds it forms the isolator's compliance chamber. In use, this isolator assembly actually supports the isolation platform and isolates both horizontal and vertical motion. At maximum load capacity (225 lbs per set of four isolators), the BenchTop platform exhibits vertical and horizontal natural frequencies of less than 3.5 Hz.

Newport molds most of the platform's components from polycarbonate. The company offers seven different platform sizes ranging from 15-0 inches to 30- 36 inches; the largest units weight approximately 50 lbs.

Additional details, custom diaphragm...Contact Bud Comstock, DiaCom Corp., 5 Howe Dr., Amherst, NH 03031, (603) 880-1900.

Additional details, isolation platform...Contact Bowie Houghton, Newport Corp., 1791 Deere Ave., Irvine, CA 92714, (714) 863-3144.