This page is dedicated to explaining the basic concept of vibration and technology used to suppress its affects on high precision microscopes and other tooling that vibration creates detrimental problems in quality and performance. For a more detailed overview, Contact Us and request our Vibration Technology handout.
Vibration is a mechanical movement around an equilibrium point. In other words, it is a mechanical phenomenon where a dynamic external force is applied onto structures or floors, causing the oscillation motion that repeats itself after an interval of time. Among the sources of vibration, the low frequency vibration is not perceptible during daily activities, and can adversely affect high-performance metrology tools in the fields of semiconductors, display manufacturing, electron microscopy, photonics and life sciences.
Classification of Vibration
Periodic noise is defined as the known magnitude of the excitation acting on a vibratory system at any given time. It is generally caused by rotating machinery. Random noise is caused by unpredictable excitation such as wind velocity, road roughness, foot and vehicular traffic, and ground motion during various activities.
Sources of Vibrations in Laboratories
- Ground vibrations indicate all the factors that cause vibrations on a floor. The factors include foot and vehicular traffic. external noises. wind blowing against building, seismic activity, HVAC systems and many other types of mechanical equipment in the nearby vicinity depending on how the building either dampens those vibrations or transmits them doe to the building construction.
- Acoustic noises are the factors that directly apply a force to a payload, such as loud noise, wind blowing from fans or the opening and closing of doors.
- A direct force is a force that directly applied to a payload on a platform, including cables connecting all the equipment and motorized linear stages.
Natural Frequency and Resonance
The natural frequency is the frequency at which the system resonates or oscillates when the system is not disturbed by an external force. A higher stiffness and a lower mass indicates the high natural frequency and a lower stiffness and a higher mass indicates the low natural frequency. As will be explained later, isolation solutions also have resonant frequency.
Resonance occurs if a forced frequency coincides with a natural frequency of the system resulting in large oscillations. Frequency at which the response amplitude is a relative maximum is known as resonant frequency. Materials like elastomers and springs often used for vibration isolation can actually make the vibration of the payload worse if their natural or resonant frequency is near the same frequency as the vibration to be attenuated.
Vibration isolation is defined as the process to isolate an object from sources of vibration. The theory of vibration isolation is to make the natural frequency of the system lower than the forced frequency and to suppress the resonance at the natural frequency of the system. As technology advances. a vibration isolation technique is essentially required to isolate vibrations from high-performance metrology tools.
Transmissibility (T) indicates the ratio of the amplitude of the vibration transmitted to an isolated payload to that of the exciting vibration. The efficiency of the vibration isolation improves with the lower natural frequency, meaning the lower transmissibility is the better vibration isolation performance.
The frequency ratio is a function of the forced frequency and the natural frequency of the system and is used as an evaluation criterion to determine vibration isolation performance.
As the frequency ratio reaches 1.414FN and T becomes less than 1 for all values greater than this, the isolation effect takes place. On the other hand. when the transmissibility ratio is smaller than 1.414. then vibration is amplified. Therefore. the transmissibility frequency (1.414FN) is reasonable value to determine the limit frequency of each natural frequency because the transmissibility frequency (1.414FN) is unaffected by the damping value. If the frequency ratio is equal to 1. then vibration amplitude is maximized (i.e. resonance occurs when the forced frequency and the natural frequency coincides).
Damping is reduction or restraining of mechanical oscillations by dissipating the energy stored in an oscillatory system. An un-damped spring leads the peaks of vibration amplitude at the resonant frequency. On the other hand. the damped spring decreases the vibration amplitude at resonance. However, there is a trade off between vibration isolation performance and damping that the vibration isolation performance degrades as damping increases.
Damping ratio is a system parameter denoted by ζ (zeta) that can vary from un-damped (ζ=0) to under-damped.
Principles of a PASSIVE Vibration Isolation System
A passive vibration isolation system consists of three components: an isolated mass (payload), a spring (K) and a damper (C) and they work as a harmonic oscillator. The payload and spring stiffness define a natural frequency of the isolation system . While the spring (isolator) reduces floor vibrations from being transmitted to the isolated payload. the damper eliminates the oscillation that is amplified within the isolation system. In most cases. the passive isolation systems employ a pneumatic spring due to its low resonant frequency characteristic that provides outstanding vibration isolation and damping.
While the simple composition of isolation system can achieve the maximum vibration isolation efficiency, there are also limitations. such as a resonance phenomenon in the low frequency range. a longer settling time. and lack of control-ability.
Principles of a ACTIVE Vibration Isolation System
An active vibration isolation system consists of feedback and feed-forward control systems with integrated sensors and actuators to isolate the most sensitive equipment from low frequency vibration which passive isolation systems commonly amplify those vibrations due to resonant frequencies. The extremely sensitive sensors detect incoming vibrations in all six degrees of freedom and a digital controller processes the measured vibration data received from the sensors into the digital signals. Then, the controller sends the signals to the actuators and the actuators cancel the vibrations by generating equal and opposite forces.
Comparison of PASSIVE versus ACTIVE Vibration Isolation Systems
Generic Vibration Criteria
Vibration Criterion (VC) curves are widely used and accepted throughout the world as a basis for designing vibration sensitive technical facilities and evaluating the performance of instruments and tools. Equipment operational problems that are caused by vibration can be prevented if vibration conditions on the floor of the building comply with the VC curve appropriate to the vibration requirements. In other words, for an installation site to comply with vibration requirements, the measured one-third octave band velocity spectrum must lie below the appropriate VC curve.