Wind Turbine Blade Modal Testing Solution

Project: Wind Turbine Blade Modal Testing Solution

1. Sensor Selection & Mounting Method

Key Considerations:

      The first 6 natural frequencies (flapwise, edgewise, torsional modes) of wind turbine 

      blades are typically below 10 Hz, with the 1st flapwise mode as low as ~0.5 Hz.

•       Sensor Requirements:

  •       Low-frequency optimized (high sensitivity at <1 Hz)

  •       Large mass-type accelerometers (improved low-frequency response)

  •       Directional adjustability (for flapwise/edgewise measurements)

  •       Narrow operational bandwidth (reduces high-frequency noise interference)

Recommended Sensor:

      DT1A202E Low-Frequency Piezoelectric Accelerometer

•       Frequency Range: 0.1–200 Hz

•       Sensitivity: 10 V/g

•       Weight: ~200 g (requires mass effect compensation)

Mounting Methods:

•        Thick, soft double-sided adhesive tape (damped structural foam) for minimal stiffness impact.

•       Magnetic base (if blade surface permits).

2. Excitation Method

Two approaches are evaluated for a 48.8m blade:

(1) Force Measurement Method (Impact Testing)

     Tool: Impulse hammer with force sensor.

      Advantages:

• Controlled input energy.

• Clear frequency response function (FRF) calculation.

      Limitations:

• Difficult to excite large blades uniformly.

(2) Non-Force Method (Ambient/Free-Decay Excitation)

     Option A: Free-Decay Excitation

• Attach a mass block (size determined by FEM static analysis) to the blade tip.

• Cut the tether to trigger free vibration.

     Option B: Random Excitation

• Manually strike the blade at multiple points with a wooden rod.

     Advantages:

• No force sensor needed.

• Suitable for very low-frequency modes.

3. Sensor Mass Effect Compensation

    Challenge: Heavy sensors (~200 g) lower the blade’s natural frequencies.

    Solution:

• Use minimal sensor count (validated through iterative tests).

• Apply mass correction algorithms in post-processing.

Proposed Test System

Core Equipment:

1. SE-884U Dynamic Stress-Strain Testing & Analysis System

• 24-bit resolution, 0.1–10 kHz sampling rate.

• Supports simultaneous strain & acceleration measurement.

2. DT1A202E Low-Frequency Accelerometers

• Optimized for <1 Hz measurements.

3. Dynatronic Software Suite

• Real-time data acquisition.

• Modal analysis (OMA/EMA modes).

• Mass effect compensation tools.

4. Impact Hammer (for force method)

• Adjustable tip hardness for frequency bandwidth control.

Test Workflow

   1. Sensor Deployment

•    Install 6–12 accelerometers along the blade span (fewer points → reduced mass effect).

   2. Excitation

•    Force method: Strike at 3–5 key points (root, mid-span, tip).

•    Non-force method: Use free-decay or random excitation.

   3. Data Acquisition

•    Record time-domain responses (≥5 min for ambient vibration).

   4. Modal Analysis

•    Extract natural frequencies, damping ratios, mode shapes.

•    Apply mass correction if needed.

Key Advantages

✔ Low-Frequency Precision – Reliable detection of 0.5 Hz modes.
✔ Flexible Excitation – Supports both impact and ambient methods.
✔ Minimal Mass Impact – Optimized sensor count & post-processing correction.
✔ Integrated Software – Streamlines FRF calculation & modal parameter extraction.

Applications

• Blade health monitoring.

• Validation of FEM models.

• Aeroelastic stability studies.

This solution complies with IEC 61400-23 for wind turbine testing. Let me know if you need further customization!