NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Blade Testing Equipment 

Development and 

Commercialization

Cooperative Research and Development 

Final Report

CRADA Number: CRD-09-346

NREL Technical Contacts: David Snowberg and 

Scott Hughes

CRADA Report

NREL/TP-7A10-55342

April 2013
NOTICE

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Cover Photos: (left to right) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

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1 

Cooperative Research and Development Final Report

In accordance with Requirements set forth in Article XI.A(3) of the CRADA document, this document is 

the final CRADA report, including a list of Subject Inventions, to be forwarded to the Office of Science 

and Technical Information as part of the commitment to the public to demonstrate results of federally 

funded research.

CRADA Number: CRD-09-346

CRADA Title: Blade Testing Equipment Development and Commercialization

Parties to the Agreement: MTS Systems Corp

Joint Work Statement Funding Table showing DOE commitment: 

Estimated Costs NREL Shared Resources

Year 1 $ 163,000.00

Year 2 $ 00.00

Year 3 $ 00.00

TOTALS $ 163,000.00

Abstract of CRADA work: 

Blade testing is required to meet wind turbine design standards, reduce machine cost, and reduce the 

technical and financial risk of deploying mass-produced wind turbine models. NREL’s National Wind 

Technology Center (NWTC) in Colorado is the only blade test facility in the U.S. capable of performing 

full-scale static and fatigue testing of multi-megawatt-scale wind turbine blades. Rapid growth in wind 

turbine size over the past two decades has outstripped the size capacity of the NWTC blade test facility 

leaving the U.S. wind industry without a suitable means of testing blades for large land-based and 

offshore turbines.

This CRADA will develop and commercialize testing technologies and test equipment, including scaling 

up, value engineering, and testing of equipment to be used at blade testing facilities in the U.S. and 

around the world.

Summary of Research Results: 

This CRADA resulted in the successful commercialization of a wind turbine blade fatigue test system. The 

hardware and control software for this fatigue test system were developed by MTS in collaboration with 

NREL. The system used for testing is called the Universal Resonance EXcitation (UREX) test system (see 

subsequent UREX photographs). MTS has sold UREX systems to multiple blade test labs as a result of this 

CRADA. The UREX is currently the only known wind turbine blade fatigue test system available in the 

market as a commercial product. 
2 

The UREX utilizes a blade test technology developed at NREL prior to this CRADA. This technology is a 

method of conducting accelerated fatigue tests of full-scale wind turbine blades. Testing is conducted at 

the fundamental resonant frequency of the blade test system. This technology provides an efficient 

means to input fatigue loads into a wind turbine blade test article. The loads are introduced to the test 

article through the oscillation of mass at the fundamental frequency. NREL licenses this technology to 

MTS for the UREX systems manufactured and sold by MTS.

The following activities were completed through this CRADA: 

• MTS and NREL collaborated to develop a hardware specification for a resonant fatigue blade 

test system

• MTS developed a design for a resonant blade fatigue test system (UREX)

• NREL participated in a UREX design review at the MTS facility

• MTS developed a prototype UREX based on the NREL-developed resonant blade fatigue test 

technology

• MTS developed a resonant tracking algorithm

• NREL tested the prototype UREX and resonant tracking algorithm on a full-scale 50-meter wind 

turbine blade (see subsequent completed test matrix)

o UREX was controllable and tunable

o UREX capable of inputting loads for 50m blade fatigue test

o Resonant tracking algorithm capable of automatically tracking blade resonance during 

test

• MTS commercialized prototype UREX based on NREL developed resonant test technology

Below are several photographs of the commercial UREX system.

Commercial MTS UREX resonant blade fatigue test system module developed under CRADA (PIX 

17637)
3 

Dual-station UREX configuration for flapwise fatigue test to 50m blade (PIX 20448)

UREX configured for edge-wise fatigue test to 50m blade (PIX 20445)
4 

As series of tests demonstrating the performance of the MTS UREX system were conducted at NREL, at 

the National Wind Technology Center near Boulder, Colorado. These trials were developed to test the 

UREX system under conditions and events that would normally be observed during blade testing of 

mega-watt scale blades. Testing was conducted on a 50-m blade used for a 2+ MW machine. Raw data 

from the testing is proprietary to the blade manufacturer.

Initial testing focused on flapwise testing, where the UREX actuators were aligned in the blade’s flapwise 

orientation, providing load excitation (actuator displacement) at the system’s flapwise natural 

frequency. During the initial testing, the system was tuned for safe and stable operation. Tuning 

included programming the MTS servo-control system parameters and feedback systems to obtain a 

stable control which matched the desired command. The primary sensor used for feedback in the 

testing was an accelerometer. System hydraulics were tuned through adjustments to accumulator 

preload settings. The interface between the MTS hydraulic controller and the data acquisition system 

was checked and verified to have proper communication. The initial test runs used a light oscillating 

mass attached to the UREX actuators, about 170 kg for each actuator, ramping the displacement of the 

UREX actuators. For each increment of oscillating weight the system was demonstrated to be stable. 

Load cases were run to establish the sensitivity of the response to the control frequency.

Once operation at target test loads was achieved, the test matrix was expanded to include operational 

cases to include off-resonant operation, edgewise, torsional, and combined loading cases. The system 

was operated varying the frequencies and displacements of the UREX actuators to achieve pure 

edgewise loads with flap excitation. Dual axis cases were tested and evaluated by running one UREX 

actuator at the flap natural frequency and one at the edge natural frequency. Similar testing was 

conducted at the flap, edge, and torsional natural frequencies. Cases were run whereby the phase lag 

between actuators was modified. The MTS resonant tracking software was demonstrated for several 

load cases. The following table provides a top-level view of the load cases performed during the 

demonstration test.

Table 1 – Completed Test Matrix (in chronological order)

Date & 

Time

UREX mass x2 

total (kg)

Actuator 

Stroke 

(±mm)

UREX frequency (Hz) Test Notes

110517 1039 170 30 (approx flap resonance) 1st flap

110517 1120 170 30 (approx flap resonance) Same as 110517_1039, but 

stop hydraulics.

110517 1536 170 30 (approx flap resonance) 1st flap

110517 1606 170 60 (approx flap resonance) 1st flap

110517 1623 170 90 (approx flap resonance) 1st flap

110517 1641 170 120 (approx flap resonance) 1st flap

110517 1708 170 150 (approx flap resonance) 1st flap

110517 1715 170 140 (approx flap resonance) 1st flap
5 

Date & 

Time

UREX mass x2 

total (kg)

Actuator 

Stroke 

(±mm)

UREX frequency (Hz) Test Notes

110517 1721 170 140 ramp up (approx flap resonance) Ramp up actuator stroke 

1105181002 170 140 (approx flap resonance) 1st flap

1105181012 170 140 (+1% flap resonance) 1st flap

1105181021 170 140 (+2% flap resonance) 1st flap

1105181031 170 140 (+3% flap resonance) 1st flap

1105181048 170 140 (-1% flap resonance) 1st flap

1105181058 170 140 (-2% flap resonance) 1st flap

1105181108 170 140 (-3% flap resonance) 1st flap

110519 0930 420 56 (approx edge resonance) Edge excite

110519 1005 420 30 (approx flap resonance) 1st flap

110519 1019 420 60 (approx flap resonance) 1st flap

110519 1033 420 90 (approx flap resonance) 1st flap

110519 1050 420 120 (approx flap resonance) 1st flap

110519 1106 420 140 (approx flap resonance) 1st flap

110519 1755 670 30 (approx flap resonance) 1st flap

110520 0854 670 45 (approx flap resonance) 1st flap

110520 0906 670 30 (approx flap resonance) 1st flap

110520 0918 670 60 (approx flap resonance) 1st flap

110520 0929 670 75 (approx flap resonance) 1st flap

110520 0940 670 90 (approx flap resonance) 1st flap

110520 0950 670 105 (approx flap resonance) 1st flap

110520 1001 670 120 (approx flap resonance) 1st flap

110520 1122 670 60 (approx flap resonance) LE actuator lags TE actuator by 

45°. 

110520 1134 670 60 (approx flap resonance) LE actuator lags TE actuator by 

90°. 

110520 1145 670 60 (approx flap resonance) LE actuator lags TE actuator by 

135°. 

110520 1156 670 60 (approx flap resonance) LE actuator lags TE actuator by 

180°. 

110520 1411 670 45 (approx edge resonance) Edge excite

110520 1432 670 60 (LE actuator)

(TE actuator)

Dual axis

110520 1447 670 60 (LE actuator)

(TE actuator)

Dual axis

110520 1506 670 60 0 (LE actuator)

0 (TE actuator, 72° phase lag)

Actuator phase lag

110523 1648 1020 50.8 Resonant search
6 

Date & 

Time

UREX mass x2 

total (kg)

Actuator 

Stroke 

(±mm)

UREX frequency (Hz) Test Notes

110524 1030 1020 50.8 Resonant tracking

110524 1042 1020 50.8 Resonant search

110524 1139 1020 50.8 Resonant tracking

110524 1157 1020 50.8 Resonant tracking

110524 1211 1020 30 & 15 0.1 to 1.44 Frequency sweep Frequency sweep

110524 1355 1020 15 (approx flap resonance) 1st flap

110524 1403 1020 30 (approx flap resonance) 1st flap

110524 1409 1020 40 (approx flap resonance) 1st flap

110524 1415 1020 50 (approx flap resonance) 1st flap

110524 1420 1020 60 (approx flap resonance) 1st flap

110524 1425 1020 70 approx flap resonance) 1st flap

110524 1437 1020 50.8 Resonant tracking

110524 1534 1020 Varied 0.601 to 0.92 (sweep) Sweep demonstration

110524 1648 

to 1911

1020 50.8 Resonant search and tracking

110525 1512 1020 50.8 Resonant tracking

110526 0931 1020 2 (approx 2nd flap resonance) 2nd flap excite

110526 0947 1020 5 (approx 2nd flap resonance) 2nd flap excite

110526 1010 1020 5 1.42 to 3.7 (sweep) Frequency sweep

110526 1458 1020 1 (LE)

5 (TE)

(LE)

(TE)

1st and 2nd flap simultaneous

110526 1506 1020 1 (LE)

10 (TE)

(LE)

(TE)

1st and 2nd flap simultaneous

110526 1514 1020 2 (LE)

10 (TE)

(LE)

(TE)

1st and 2nd flap simultaneous

110526 1521 1020 3 (LE)

10 (TE)

(LE)

(TE)

1st and 2nd flap simultaneous

110526 1527 1020 3 (LE)

15 (TE)

(LE)

(TE)

1st and 2nd flap simultaneous

110527 0922 1020 10 (LE)

10 (TE)

(LE)

(TE)

Dual axis

110527 0930 1020 10 (LE)

30 (TE)

(LE)

(TE)

Dual axis

110527 0942 1020 20 (LE)

30 (TE)

(LE)

(TE)

Dual axis

110527 0955 1020 30 (LE)

30 (TE)

(LE)

(TE)

Dual axis

110527 1013 1020 30 (LE)

40 (TE)

(LE)

(TE)

Dual axis

110527 1022 1020 40 (LE)

40 (TE)

(LE) 

(TE)

Dual axis

110527 1030 1020 0 (LE)

40 (TE)

(LE)

(TE)

Dual axis
7 

Date & 

Time

UREX mass x2 

total (kg)

Actuator 

Stroke 

(±mm)

UREX frequency (Hz) Test Notes

110527 1037 1020 1 (LE)

40 (TE)

(LE)

(TE)

1st flap and torsion 

simultaneously

110527 1311 1020 1 3.0 (180° phase shift between 

each actuator)

Torsion with 180° phase shift 

between actuators

110527 1020 ? ? 1st and 2nd flap (PV algorithm 

check)

Subject Inventions Listing: 

none

Report Date: 

November 30, 2012

Responsible Technical Contacts at Alliance/NREL: 

David Snowberg and Scott Hughes

This document contains NO confidential, protectable, or proprietary information.