Emphasis on MultiPoint Response Control and Force Limited Vibration Testing

1. Introduction

Single axis electrodynamic shaker testing is widely used to qualify aerospace, automotive, and electronic hardware. While this approach is well established, traditional acceleration-controlled testing does not always represent real operational conditions. The interface between the shaker and the test article is typically much stiffer than real-world mounting conditions. This can result in unrealistic force levels and, in some cases, over-testing, particularly at resonant frequencies.

To improve test realism and reduce unnecessary risk, modern testing approaches incorporate:

• Multipoint response monitoring and limiting, even in single shaker, single axis tests
• Force Limited Vibration Testing (FLVT), a methodology developed and widely adopted by agencies such as NASA

These methods help ensure that test conditions better reflect real-world environments while maintaining control over the applied loads.

2. MultiPoint Response Control in Single Axis Testing

Even in a single axis test, a structure rarely responds in a simple way. Complex bending, local resonances, and structural modes can lead to significant variations in response across the test article. Relying on a single control accelerometer can result in an incomplete or misleading picture of the system response. Areas away from the control point may experience significantly higher or lower levels of vibration.

Modern vibration controllers address this by allowing:

• Multiple monitoring channels placed at key structural locations
• Response limits defined for each channel, including acceleration, velocity, or strain
• Automatic drive reduction when any monitored channel exceeds its defined limits

This approach is often referred to as multipoint limiting. Although excitation is still single axis, the system is effectively controlled across multiple points to improve test accuracy and safety.

3. Notching as a Control Strategy

Notching is the process of reducing the input excitation at specific frequencies where the test article exhibits excessive response. In vibration testing, resonances can significantly amplify motion beyond the applied base input. Without proper control, this can lead to damage or unrealistic stress levels. By applying notching, the controller reduces the drive level in targeted frequency regions while maintaining overall test objectives. This ensures that response limits are not exceeded at critical points in the structure. Proper notching is widely recognised as an essential practice in both sine and random vibration testing, particularly when testing sensitive or high-value hardware.

4. Force Limited Vibration Testing

4.1 Motivation

Acceleration based control alone does not guarantee that the applied loads are realistic. The combination of a stiff shaker interface and a resonant structure can artificially increase interface forces, leading to test conditions that do not reflect real operational environments. Force Limited Vibration Testing, often referred to as FLVT, addresses this limitation by introducing force as a control parameter.

4.2 Core Principles

FLVT works by measuring the interface force between the shaker and the test article and applying limits to that force during testing.

Force limits can be determined using methods such as:

• Impedance based analysis
• Semi empirical approaches
• Apparent or effective mass calculations

If measured forces exceed the defined thresholds, the controller automatically reduces the input drive at those frequencies. This results in a physically meaningful form of notching based on load rather than just acceleration.

4.3 Sensor Technologies

FLVT relies on accurate measurement of dynamic interface forces. This is typically achieved using piezoelectric multi axis force sensors capable of measuring forces in multiple directions. These sensors provide the necessary feedback for closed loop control, allowing the system to regulate force in real time. Proper installation, calibration, and mounting are critical to ensure accurate measurements and reliable test results.

4.4 Industry Adoption

Force limiting methods are well established in aerospace and space qualification programs. They are documented in guidance such as NASA standards and have been applied in major programmes by organisations such as NASA and ESA. This demonstrates that FLVT is a validated and widely accepted approach for improving test realism and protecting test hardware.

5. Practical Application in Single Axis Testing

5.1 Pretest Characterisation

A low-level sine sweep is often used before full level testing to identify resonant frequencies and estimate structural response. This information can be used to define appropriate force limits and notching strategies.

5.2 Typical Test Configuration

A robust test setup may include:

• A primary control accelerometer located at the base
• Multiple monitoring accelerometers placed at key structural locations
• A force sensor at the mounting interface for FLVT applications

Modern vibration controllers support features such as:

• Drive notching and limiting
• Spectral and RMS limits
• Safety interlocks and automatic shutdown

5.3 Real Time Control

During the test, the system continuously monitors response and force levels. When predefined limits are exceeded, the controller applies notching in the affected frequency regions. This ensures that the test remains within safe and realistic bounds without compromising overall test objectives.

5.4 Documentation and Validation

Best practice guidelines recommend documenting all notching events and force limits applied during testing.

This information is important for:

• Correlating test results with predicted flight or operational loads
• Supporting qualification and certification requirements
• Ensuring traceability and repeatability of test procedures

6. Summary

Single axis vibration testing can be significantly improved using multipoint response control and force limiting techniques. Multipoint monitoring ensures that multiple areas of the structure are properly controlled and protected, while notching helps prevent excessive response at resonant frequencies. Force Limited Vibration Testing provides a more realistic representation of operational loading by directly controlling interface forces, rather than relying solely on acceleration. Together, these advanced strategies improve test accuracy, reduce the risk of over testing, and help ensure that vibration testing more closely reflects real world conditions.