6.3 Dynamic vibration absorbers
4 min read•july 30, 2024
Dynamic vibration absorbers are clever devices that reduce unwanted shaking in machines and structures. They work by adding a secondary mass-spring system tuned to match the problematic vibration frequency, creating opposing forces that cancel out the shaking.
These absorbers are most effective when designed for specific frequencies, but damping can broaden their range. Key factors include the mass ratio between absorber and main system, and careful tuning of parameters like stiffness and damping for optimal performance.
Dynamic Vibration Absorbers
Principles and Functionality
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- Passive mechanical devices reduce unwanted vibrations in primary systems by adding secondary mass-spring systems
- Utilize anti- concept tuning absorber's to match primary system's excitation frequency
- Create reactive forces opposing primary system motion, canceling vibrations at target frequencies
- Effectiveness depends on mass ratio between absorber and primary system, and damping characteristics of both
- Optimal for systems with known, constant operating frequencies
- Introduce new degree of freedom, resulting in coupled system with two natural frequencies
- Expanded working range achieved by introducing damping, trading peak response reduction for effectiveness bandwidth
System Dynamics and Performance Factors
- Most effective when designed for specific excitation frequencies
- Mass ratio between absorber and primary system impacts vibration reduction level
- Damping in absorber broadens effective frequency range
- Adaptive or semi-active absorbers adjust properties in real-time for varying excitation frequencies
- Performance evaluated using frequency response functions (FRFs)
- Key metrics include peak amplitude reduction, frequency band width of vibration reduction, and new resonance peaks
- Time domain analysis provides insights into transient response and steady-state behavior
Tuning Dynamic Vibration Absorbers
Parameter Selection and Optimization
- Primary tuning parameters mass, stiffness, and carefully selected for optimal performance
- Natural frequency of absorber typically set equal to problematic frequency of primary system
- Optimal mass ratio determined based on desired vibration reduction and practical constraints (weight, space)
- Absorber spring stiffness calculated using relationship: (ω: natural frequency, k: spring constant, m: mass)
- Optimal damping ratio determined through methods like H∞ optimization technique
- Numerical optimization techniques (genetic algorithms, particle swarm optimization) employed for complex systems
Advanced Tuning Strategies
- Adaptive absorbers adjust properties for effectiveness across wider frequency ranges
- Semi-active absorbers utilize controllable elements (magnetorheological fluids, piezoelectric actuators) for real-time tuning
- Multi-modal absorbers designed to address multiple problematic frequencies simultaneously
- Nonlinear absorbers exploit beneficial nonlinear effects for enhanced performance
- Robustness considerations account for uncertainties in system parameters and operating conditions
- Sensitivity analysis performed to understand impact of parameter variations on absorber performance
Performance of Dynamic Vibration Absorbers
Analysis Techniques and Metrics
- Vibration reduction ratio quantifies absorber effectiveness by comparing system response with and without absorber
- Modal analysis techniques reveal effects on mode shapes and natural frequencies of combined system
- concept crucial for analyzing performance in isolating sensitive equipment from base excitations
- Statistical energy analysis (SEA) predicts high-frequency performance in complex systems
- Finite element analysis (FEA) simulates absorber behavior in detailed structural models
- Experimental modal analysis verifies theoretical predictions and fine-tunes absorber designs
Performance Evaluation and Optimization
- Frequency response analysis reveals amplitude reduction and new resonance characteristics
- Time domain simulations assess transient behavior and settling time
- Parametric studies identify optimal absorber configurations for given system requirements
- Trade-off analysis balances vibration reduction, frequency bandwidth, and practical constraints
- Robustness evaluation ensures consistent performance under varying operating conditions
- Life cycle analysis considers long-term effectiveness and maintenance requirements
- Cost-benefit analysis weighs vibration reduction benefits against implementation costs
Applications of Dynamic Vibration Absorbers
Structural Applications
- Tall buildings employ tuned mass dampers at top to counteract wind-induced oscillations (Taipei 101)
- Bridge structures mitigate pedestrian-induced and wind-induced vibrations (London Millennium Bridge)
- Offshore platforms use absorbers to reduce wave-induced motions
- Power transmission towers incorporate absorbers to minimize galloping and wind-induced vibrations
- Stadium roofs and floors utilize absorbers to control crowd-induced oscillations
Mechanical and Industrial Applications
- Rotating machinery (turbines, engines) employ absorbers to reduce torsional vibrations and improve stability
- Automotive applications reduce engine vibrations, improve ride comfort, and minimize noise transmission (crankshaft pulley dampers)
- Aerospace industry uses absorbers to reduce structural vibrations from engines and aerodynamic forces
- Precision manufacturing and measurement equipment incorporate miniature absorbers to isolate sensitive components
- Industrial piping systems use absorbers to control flow-induced vibrations
- Wind turbines employ absorbers to reduce blade and tower vibrations, improving efficiency and longevity
Key Terms to Review (16)
Automotive suspension systems: Automotive suspension systems are mechanical systems that connect a vehicle's body to its wheels, allowing for relative motion between them while providing comfort and stability during movement. These systems play a critical role in managing vehicle vibrations and impacts from the road, ensuring that passengers experience a smooth ride while maintaining control and handling of the vehicle. By effectively isolating vibrations from road irregularities, these systems also contribute to enhanced safety and performance.
Broadband absorbers: Broadband absorbers are devices designed to dissipate vibrational energy across a wide range of frequencies, rather than being limited to a narrow band. These absorbers are particularly useful in applications where vibrations can occur at multiple frequencies, ensuring more effective vibration control and minimizing resonance issues.
Building sway control: Building sway control refers to the techniques and systems used to minimize lateral movements and vibrations in structures caused by environmental forces such as wind and earthquakes. Effective sway control enhances a building's stability, safety, and comfort for occupants by reducing the amplitude of oscillations during such events, allowing for safer design practices in urban environments.
Damping Ratio: The damping ratio is a dimensionless measure that describes how oscillations in a mechanical system decay after a disturbance. It indicates the level of damping present in the system and is crucial for understanding the system's response to vibrations and oscillatory motion.
Equation of Motion: An equation of motion describes the relationship between the forces acting on a system and its resultant motion, typically in the context of oscillatory systems. It provides a mathematical framework to analyze the dynamics of systems under various conditions, such as free and forced vibrations, damping, and external excitations. This concept is fundamental to understanding how mechanical systems respond to disturbances and is essential for designing effective vibration control strategies.
Frequency matching: Frequency matching refers to the condition where the natural frequency of a dynamic vibration absorber aligns with the frequency of the system it is intended to control. This alignment is crucial as it allows the absorber to effectively counteract and reduce vibrations by maximizing energy transfer, thereby minimizing unwanted oscillations in the primary system. The effectiveness of a dynamic vibration absorber is highly dependent on this synchronization.
Frequency Response Function: The frequency response function (FRF) describes the relationship between the output and input of a system in the frequency domain, allowing engineers to analyze how a system responds to various frequencies of excitation. This function is crucial for understanding dynamic behavior and stability, as it provides insights into resonance, damping, and the overall performance of mechanical systems under different loading conditions.
Multi-degree-of-freedom absorbers: Multi-degree-of-freedom absorbers are systems designed to reduce vibrations by utilizing multiple connected mass-spring-damper components, allowing them to target several frequencies simultaneously. These absorbers are more complex than single-degree-of-freedom systems and can effectively control vibrations in structures or mechanical systems subjected to multiple modes of excitation. By addressing various vibration modes, they improve overall performance and stability in applications ranging from buildings to automotive engineering.
Natural Frequency: Natural frequency is the frequency at which a system tends to oscillate in the absence of any external forces. It is a fundamental characteristic of a mechanical system that describes how it responds to disturbances, and it plays a crucial role in the behavior of vibrating systems under various conditions.
Passive Absorber: A passive absorber is a device designed to reduce vibrations by dissipating energy through damping mechanisms without requiring any external power source. These absorbers work by tuning their natural frequency to match that of the vibrating system, allowing them to effectively minimize oscillations and enhance system stability. Their main goal is to absorb unwanted vibrations and prevent them from affecting the performance of mechanical systems.
Resonance: Resonance is a phenomenon that occurs when a system is driven at its natural frequency, leading to a significant increase in amplitude of oscillation. This effect can cause systems to behave in unpredictable and potentially damaging ways, and it's important in understanding how various vibrations interact with materials and structures.
Response Amplitude: Response amplitude refers to the maximum displacement or movement experienced by a vibrating system when subjected to an external force or input. It quantifies how much the system reacts to vibrations, and it is influenced by factors such as damping, frequency, and the system's inherent properties. Understanding response amplitude is essential for analyzing how systems behave under dynamic conditions, particularly in the presence of damping mechanisms or when utilizing devices designed to mitigate vibrations.
Rubber isolators: Rubber isolators are flexible components used to reduce the transmission of vibrations and shocks from one part of a mechanical system to another. They provide a cushion that absorbs energy, preventing excessive movement and noise, which is essential in various applications such as machinery, automotive systems, and structural supports.
Transmissibility: Transmissibility is a measure of how much vibration is transmitted from one part of a mechanical system to another, often evaluated in terms of force or displacement. It plays a critical role in assessing the effectiveness of vibration isolation systems, as it determines how well these systems can reduce or control the transmission of vibrations to sensitive components or structures.
Tuned mass damper: A tuned mass damper is a device used to reduce vibrations in structures by utilizing a secondary mass that oscillates out of phase with the primary structure's motion. This system is particularly effective in controlling resonant vibrations, helping to stabilize buildings, bridges, and other structures subject to dynamic loads like wind or earthquakes. The design involves careful tuning of the mass and spring properties to ensure maximum effectiveness at specific frequencies.
Viscoelastic Materials: Viscoelastic materials are substances that exhibit both viscous and elastic characteristics when undergoing deformation. This means that they can store energy like an elastic material but also dissipate energy like a viscous fluid, making them unique in their response to stress and strain. The interplay between these two behaviors allows viscoelastic materials to effectively absorb vibrations, which is critical in applications related to damping, isolation, and vibration absorption.