Understanding Modal Analysis

Definition: What is Modal Analysis?

Modal analysis is the process of studying and characterizing the inherent dynamic properties of a structure or mechanical system. These properties—specifically its natural frequencies, damping ratios, and mode shapes—are known as the system’s “modal parameters.” Modal analysis determines the unique ways in which a structure will naturally tend to vibrate if excited. This information is fundamental to designing structures that can withstand dynamic forces, as well as for troubleshooting and solving complex vibration problems.

The Goal: Identifying Modal Parameters

Every structure has a unique set of modal parameters that are determined by its physical properties (mass, stiffness, and damping). The goal of modal analysis is to identify these:

• Natural Frequencies (or Resonant Frequencies): These are the specific frequencies at which the structure will vibrate with the largest amplitude when excited. There are multiple natural frequencies for any given structure.
• Damping Ratios: This parameter quantifies how quickly the structure’s vibrations will decay. It is a measure of the energy dissipation within the structure.
• Mode Shapes: A mode shape is the specific pattern of deformation or deflection that the structure undergoes when vibrating at one of its natural frequencies. Each natural frequency has a unique corresponding mode shape.

By identifying these parameters, engineers can fully understand and predict how a structure will respond to any dynamic load it might encounter during operation.

Types of Modal Analysis

There are two primary approaches to determining a structure’s modal parameters:

1. Experimental Modal Analysis (EMA)

EMA, also known as a “bump test,” involves measuring the structure’s response to a known, controlled input force. It is the most common method for testing real-world hardware. The process involves:

1. Exciting the structure with a measured force, typically from an instrumented impact hammer (which has a force sensor in its tip) or an electrodynamic shaker.
2. Measuring the vibration response at one or more locations on the structure using accelerometers.
3. Calculating the Frequency Response Function (FRF) for each measurement point, which is the ratio of the output vibration to the input force.
4. Using specialized software to analyze the set of FRFs to extract the natural frequencies, damping, and mode shapes. The software can then animate the mode shapes to visualize how the structure deforms at each natural frequency.

2. Operational Modal Analysis (OMA)

OMA is used when it is impractical or impossible to apply a controlled input force, or when it’s important to understand the structure’s behavior under actual operating conditions. In OMA, only the output response of the structure is measured (using accelerometers) while it is being excited by its normal operational or ambient forces (e.g., wind on a bridge, road inputs on a car, or operational forces in a running machine). Advanced algorithms are then used to extract the modal parameters from the response-only data. This is a more complex but sometimes necessary approach.

3. Analytical Modal Analysis (FEA)

This is a purely theoretical approach using computer models, most commonly Finite Element Analysis (FEA). Engineers create a virtual model of the structure and the software calculates the predicted modal parameters. EMA is often performed to validate and refine the accuracy of these FEA models.

Applications of Modal Analysis

• Troubleshooting Resonance Problems: Its most common application. If a machine has high vibration, modal analysis can determine if a structural natural frequency is being excited by an operating force.
• Design Validation: Engineers use it to confirm that a new product’s natural frequencies are not close to any known excitation frequencies (e.g., engine RPM, blade pass frequency).
• Structural Modification: If a resonance problem is found, the modal model can be used to perform “what-if” analyses to determine the most effective way to fix it (e.g., “Where should I add a stiffener to move this natural frequency higher?”).
• Structural Health Monitoring: Changes in a structure’s modal parameters over time can indicate the presence of damage, such as a crack.

← Back to Main Index