On fig. 1 shows a sectional view of a Bellking that uses two air chambers instead of one. These chambers are connected by a small hole. As the piston moves up and down, air is forced through the orifice, creating a damping force on the payload.
The damping is very strong for large piston movements and weak for smaller pistons. This allows the payload to be quickly stabilized without compromising low amplitude vibration isolation performance. This type of damping usually gives Q ≈ 3 for movements on the order of a few millimeters.
The damping provided by the bore is limited by a number of factors. The Bellking BK-PA isolator uses a different approach: multi-axis viscous fluid damping. These isolators extend the damping to a nearly critical level for applications that need it.
For example, semiconductor testing equipment often uses fast moving wafer transfer stages. BK-PA Vibration Isolators allow the payload to quickly stabilize after stage movement while still providing significant vibration isolation.
The insulator uses a high viscosity synthetic oil with very low outgassing, which is hermetically sealed in a separate air chamber of the insulator. The special geometry ensures that the isolator dampens both vertical and horizontal movement (in the X and Y directions) with equal efficiency.
The BK-PA® Isolators feature a strong but simple pendulum insulator that provides horizontal isolation. Like an air spring, the pendulum also generates ω0, which is independent of the payload and equals √g/l, where l is the length of the pendulum.
In BK-PA, the pendulum is actually a piston. The load is supported by a load plate which transfers its load to the bottom of the piston chamber through the load pins. The load pin contacts the bottom of the piston through a swivel thrust bearing.
As the payload moves sideways, the piston chamber rotates in the plane of the diaphragm like a driveshaft. Thus, a pendulum is formed with a length equal to the vertical distance from the roller in the diaphragm to the bottom of the loading pin.
Bellking’s Compact Sub-Hertz Pendulum system uses a different type of pendulum concept to reduce the horizontal resonant frequency to 0.3 Hz. This insulator uses a geometric arm to “fold” a 0.3 Hz pendulum into a housing less than 16 inches (400 mm) high.
An equivalent simple pendulum would be 110 inches (nearly 3 meters) high. For more information, readers can go to TMC Pneumatic Isolators for OEMs.
In most insulators, horizontal damping occurs by coupling the horizontal tilt. As the payload moves laterally, it acts on the insulator in the vertical direction (by tilting) to provide damping. Some systems, such as Bellking’s BK-PA isolators, use fluid damping to dampen horizontal movement.
At low amplitudes, the small friction in the rotating diaphragm and the low hydraulic resistance caused by the orifice affect the performance of the insulator. This is why it is important to use lower excitation levels when measuring their rate of transmission.
Three or more isolators are required to support the payload, usually four. Since there can only be three valves in a system (see the Equalizing Valves section), the two branches in a 4-station system must be connected as a master/slave combination.
Although the master/slave combination forms an effective reference point, the resulting damping effect is very different from that of a single (larger) insulator. Bellking recommendations include the use of at least four isolators (in addition to “circular” payloads such as NMR spectrometers). Placing these isolators under the payload can significantly impact system performance.
For small rigid payloads such as granite structures in semiconductor fabrication equipment, it is recommended to place the insulator close to the corner of the payload. This greatly improves the tilt stability of the system, reduces payload movement caused by on-board interference, and reduces system leveling and stabilization time.
Leveling time is defined as the time required for the valve system to bring the payload to the correct height and angle. The settling time is defined as the time required for the payload to stop after an impulsive disturbance.
For extended surfaces, including large optical tables, insulators should be placed below the surface nodal lines to minimize the effects of forces transmitted by the isolators to the table. For both types of payload, it is best to have the center of mass of the payload in the same plane as the effective fulcrum of the insulator. This improves system stability (see Gravitational Instability section) and eliminates horizontal and tilting payload movement.
There are various methods for removing uneven floors. Most Bellking insulators have a travel range of ±0.5 in. providing enough flexibility for almost any application. Some systems have leveling feet. If the floor is very uneven, it may be necessary to install supports for the insulators.
Some freestanding insulators or supports, such as rigid tripods, must be grouted into the floor if the floor surface is of poor quality. Fast-setting “ready-mixed concrete” or epoxy resins are suitable for this situation.
Pneumatic isolators can easily lift thousands of pounds of payloads. The insulators can be tied together with ‘tie-downs’, greatly reducing the risk of heavy objects tipping over, for example in accidents or earthquakes. Bellking ties are reinforced shaped channels that use limited layer damping to prevent resonance.
Such damping may not be required due to the high isolation efficiency of the insulator at these frequencies. In addition, the system can be equipped with anti-vibration mounts to prevent the payload from breaking off the insulators in extreme conditions.
The travel limit built into all Bellking isolators is an important safety feature. On fig. 1 internal “key” (yellow) prevents the system from overstretching even at pressures up to 120 psi. inch (830 kPa) under “no load” conditions.
With a possible force of thousands of pounds acting on the piston of the insulator, the insulator, without limiting its stroke, can turn into a cannon if it is suddenly unloaded. Safety features, including circuit pressure relief valves, do not inherently provide the high level of safety provided by mechanical stroke limits.
All rigid payloads use only three height control valves, even with ten insulators. Because three points define a plane, using more valves mechanically overloads the system and results in poor positional stability (like a four legged dining table) and constant air consumption. Proper placement and piping of the three valves is critical to optimal system performance.
Figures 2a and 2b show the general wiring for 4- and 6-column systems. The system consists of three valves, a pressure regulator/filter (optional), several quick-disconnect tees, and a hole “braid” on each insulator, which is a short piece of tubing with a hole in it.
The section is marked with a red ring that has a fitting at one end for the air supply line to the height control valve. The orifice limits the “gain” of the servo (mechanical valve system) to prevent oscillation. Some systems with a high center of gravity may require smaller holes to prevent instability. Bellking’s use fixed ports rather than adjustable needle valves due to their ease of use and long term stability.
For systems with four or more isolators, two or more isolators must be connected together. Usually the valve is installed next (for convenience) to the insulator, which is referred to as the “primary”. A remote isolator (S) using a master valve is called a “slave valve”.
The choice of legs as “leader” and “slave” affects the stability of the system (see the section on gravitational instability) and affects the dynamic behavior of the system. Dynamic performance is very important for semiconductor testing machines with fast moving stages.
There are a number of “rules of thumb” that can be used to make the right choice, although they may conflict with each other in some systems. Some experimentation may be required to determine the best choice.
In addition to the valve position, there are several different valve types to choose from. Bellking offers standard mechanical valves and precision mechanical valves. Standard valves are less expensive and have a positioning accuracy (dead zone) of about 0.1 in. (2.5 mm).
It has the property that with less movement, the valve is an airtight seal. This makes it suitable for systems requiring pressurized gas supplies. Precision valves provide positioning accuracy of 0.01 in. (0.3 mm) or better, but let a small amount of air through (all metal seats are used internally).
This limits their use in cylinder operations. Finally, Bellking offers electronic valve systems such as the Precision Electronic Positioning System with a position stability of approximately 0.0001 inches (approximately 2 microns). For more information, readers can refer to the Bellking PEPS and PEPS-VX pages.
For cleanroom applications, Bellking offers stainless steel mechanical valves and/or exhaust pipes.
Like a pen balancing on its tip, a payload supported below its COM is inherently unstable. As the payload tilts, its COM moves horizontally, further increasing the tilt. This is compensated by the rigidity of the pneumatic isolator, which tries to return the payload to a horizontal position.
The balance of the two forces determines the gravitational stability of the system. On fig. 3 shows the payload supported by two idealized pneumatic isolators. W is the width between the centers of the insulators, H is the height of the center of gravity of the payload above the point of effective support of the insulator, X is the horizontal position of the center of gravity from the center line of the insulator. . The stable region is expressed as:
This relationship is shown in Figure 3 as an inverted parabola defining stable and unstable regions of COM position. The second equation shows that stability increases with the square of the distance between insulators.
This is very important because it shows that the H/W aspect ratio does not determine the stability of the system (as some sources claim) and that the stable area is not a “pyramid” or “triangle”. However, real systems are not as simple as shown in Figure 3.
The A/V ratio in equations 1 and 2 represents the stiffness of the insulator. However, what is the correct V in a two-chamber insulator? Unlike the insulator shown in Figure 3, which has a fixed spring rate, real insulators have a frequency dependent spring rate.
At high frequencies, the opening between the two chambers successfully blocks the airflow, and only V can be used as the top air volume. At system resonance, the “effective” volume of air is between the upper volume and the total volume (upper plus lower). At low frequencies, the height adjustment valve gives the insulator a very high rigidity (corresponding to a very small V).
In addition, the height control valve also attempts to return the payload to a horizontal position. Here are some of the reasons why Equation 1 cannot be applied to double chamber insulators.
Instead, three zones can be assigned: stable, unstable, and marginal; the first two are based on “total” and “upper only” airflow, respectively. In addition, the stable area is different for axes parallel and perpendicular to the axis of the master/slave insulator.
Figure 4 defines two different axes for a quadrupedal system. The pitch axis is less stable because the left main/slave outriggers provide no pitch resistance at low frequencies (although they do resist pitch above about 1 Hz).
To compensate for this, the master/slave combination is chosen so that Wp is greater than Wr (rule 3 from the Balance Valve section). The stable region is the volume covered by the inverted parabola on both axes.
The A/V ratio is not universal and must be confirmed for each insulator and capacitance model, but (A/V)Top is about 0.1″-1 and (A/V)Tot is about 0.05″-1. On fig. 5 shows the region of edge stabilization of a two-chamber insulator.
Unfortunately, the COM of many systems ends up in this undefined region. These rules do not take into account the operation of the height control valve, which will always increase the stability of the system. These rules may change if the payload has a mass that can move (a pendulum or a liquid bath).
Equations 5 and 6 provide “rules of thumb” for calculating system stability. Like all such rules, system stability is only an approximation based on an “average” isolated system. It’s always better to use low COM.
The Bellking has an effective fulcrum approximately 7 inches below the top of the insulator. For lightly loaded insulators, these conditions underestimate the stability of the system. If the system violates these equations or is in a critical situation, stability can be improved by using special bulk insulators, counterweights, various isolation valves, etc.
Post time: Feb-23-2023