By Patrick Wong
Critical biomedical experiments can be seriously affected by vibrations. When tabletop vibrations are not controlled, sensitive instrumentation is affected and research results are often seriously degraded. And, in applications such as electrophysiology, microinjection and cell manipulation, tabletop vibrations can cause complete failure of the experiment. For example, in precision work, such as the study of cell signaling in living brain slices conducted by Stephen Smith at Stanford University, even micron-scale relative motion between tabletop components can mean experimental failure. These minute vibrations can shake electrodes out of cells or they can cause mechanical tip displacement, which may rupture cells and make it necessary to repeat the experiment. As a result, it has become important to design worktables for the biomedical laboratory that are capable of providing a level of vibration control that allows ultraprecision.
There are five primary sources of disturbances that affect mechanical alignment in precision worktables. These include: ground or floor vibrational inputs; airborne vibrations (acoustic); vibrations generated by equipment or apparatus within the system; load changes of quasi-static forces that act on the system; and thermal changes such as heat source or ambient temperature changes. Although no vibration control system can completely eliminate every source of instability, one fact is clear: the better the vibration control, the better the equipment performs.
What are the alternatives?
The key to minimizing the effect of vibrations on sensitive equipment is to create a worktable situation where all parts of the system are moving together, rather than shifting around relative to each other. To accomplish this stability, the mounting surface of the worktable must have a high degree of rigidity and it must be isolated from environmental vibrations. Although a variety of crude vibration isolation methods (that is, the use of tennis balls positioned under cement blocks) have been tried in many laboratories, three main vibration control technologies have become dominant: (1) granite blocks, (2) aluminum honeycomb core tables with broadband, visco-elastic dampers, and (3) steel honeycomb cores with tuned-spring mass hydraulic dampers.
As critical experiments such as video microscopy, electrophysiology and ultramicrotomy require interferometric accuracy from all instruments used on the tabletop, vibration control worktables must be designed to meet a demanding set of performance requirements. Granite slabs, the conventional solution for isolating vibrations, have been tried in some laboratories. However, they are bulky and have many drawbacks in crowded biomedical laboratories. For example, they can weigh a ton or more, making structural support and size major obstacles. They are also limited in their ability to provide the extremely high level of vibration control needed to perform delicate experiments, such as the study of living brain slices mentioned earlier, as they are easily excited by both acoustic and mechanically transmitted vibrations.
While it may seem difficult to induce vibrations in a block weighing thousands of pounds, granite cannot match the dynamic rigidity (the ability to damp, or resist the deformation that results from internal and external vibrational inputs) of a far lighter damped steel-honeycomb construction. Mass that does not contribute to stiffness only decreases the resonant frequencies of a structure, thus increasing displacements. Granite blocks subjected to vibrations have pronounced resonance peaks. Also, as the internal damping of granite is low, the material continues to vibrate, or 'ring', long after the stimulus ceases. These micron-scale motions reduce the accuracy, precision and repeatability of critical experiments.
The use of lightweight honeycomb structures for worktables offers a greater level of vibration control as well as a much higher level of flexibility for the biomedical laboratory where space and transportability are important considerations. The core structure of a honeycomb table provides a high strength-to-weight ratio and a stiffer, more stable platform. The low weight of honeycomb structures keeps the resonant frequency high--minimizing large-amplitude, low-frequency displacements. Additionally, internal damping dissipates the vibrational energy of the tabletop by converting small amplitude mechanical vibrations to heat. This lowers resonant motion peaks across the entire vibration frequency spectrum. Because every table has dominant bending and torsional modes (each of which has a characteristic frequency and table deformation shape), optimal damping of these resonant modes vastly improves the stability of the table. Two primary technologies are used for damping: broadband, visco-elastic dampers and tuned spring mass hydraulic dampers.
Tuning out resonances
The broadband, visco-elastic dampers indiscriminately absorb a moderate amount of energy over a wide range of frequencies. But, as they are not tuned to damp dominant table resonances, they are much less effective than tuned dampers. Additionally, the damping characteristic of the visco-elastic material changes over time, making future vibration control performance unpredictable.
The use of multiple tuned dampers, each individually tuned to minimize table motion at specific resonance modes, is a much more effective solution (see Fig. 1). Tuned dampers concentrate damping where it is needed most, at the frequencies of the dominant resonance modes. Because broadband dampers are designed to provide moderate damping over a wide range of frequencies, they are not as effective at damping the dominant modes of vibration.
FIG. 1 Tuned dampers (left) concentrate damping at the frequencies of dominant resonance modes. As broadband dampers (right) are designed to provide moderate damping over a wide range of frequencies, they are not as effective at damping the dominant modes of table vibration.
Although honeycomb cores may be constructed from either steel or aluminum, steel typically provides greater vibration control. The comparison is best illustrated by looking at compliance curves (Fig. 2). Basically, compliance curves show the displacement amplitude of a point on a body per unit of impulse force applied. The greater the compliance, the more easily the structure moves as a result of applied force. The compliance curve for the steel tabletop in figure 2 demonstrates that the table bending with the same force is about 1.8 x 10-1 mm N-1, or roughly a factor of 11 more rigid than the standard aluminum honeycomb tabletop of the same size.
FIG. 2 The compliance curve for the steel tabletop (top) shows that the table bending with the same force is a factor of 11 more rigid than the standard aluminum tabletop (bottom) of the same size.
Isolating floor vibrations
Most biomedical laboratories are plagued with floorborne vibrations resulting from, for example, elevators, compressors, air conditioners, and transformers, causing the table structure to resonate. As a result, the tabletop must be isolated from these 'resonance' vibrations as well as other tabletop vibrations. Of the main methods of isolating floorborne vibrations, pneumatic isolators offer the greatest level of vibration control. This is because they combine the fast 'rolloff' of the simple harmonic oscillator-at vibration frequencies above isolator resonance and the low amplification of the damped harmonic oscillator near resonance.
In a pneumatic isolation system, the isolated mass is supported by a piston that rests on a flexible rolling diaphragm. Under the diaphragm are two pressurized air chambers connected by a small orifice. Air moves from the upper to the lower chamber through the orifice, dissipating energy and reducing amplification at the isolator's natural frequency.
Vibration control systems that are capable of interferometric accuracy are not only an important consideration in the biomedical laboratory, they are essential. When conducting experiments, that require ultraprecision, the floor, air and other room vibrations found in a typical laboratory can cause research delays or even experimental failure. As a result, the lightweight steel-honeycomb core worktable with pneumatic isolators has emerged as a useful vibration control device for the biomedical laboratory.