Direct measurement of input loads for the wind turbine drivetrain under test on a nacelle test bench

Nacelle test benches with the capability of 6‑DOF (degree of freedom) load application for wind turbines have experienced a fast growth in the last decade. Numerous test campaigns have demonstrated their unique abilities in efficiently validating the wind turbine drivetrains under various design load cases. With new types of wind turbines becoming ever larger, the validation on a nacelle test bench tends to become a standard validation task in the development of wind turbine prototypes. Due to high load levels, the load measurements on the nacelle test bench are often not carried out directly in front of the DUT (device under test). This can cause losses in accuracy and dynamic behavior in the measurement. In this work we take the “DyNaLab” nacelle test bench at Fraunhofer IWES as an example (Fig. 1). The torque is measured on three link elements of the coupling, the axial force on each link element being measured by means of strain gauge technology. The torque is calculated using three forces and their lever arms with respect to the center of rotation. However, since the torque measured still needs to travel through the moment bearing of the load application unit (LAU) before reaching the DUT, additional uncertainty will be caused by the bearing friction torque, which is affected significantly by the non-torque loads applied and the temperature. The non-torque loads (bending moments, shear forces and thrust force) are calculated from the forces acting on the six hydraulic cylinders which are in turn determined by the hydraulic pressure measurement in each cylinder. The major disadvantages here are the friction forces in the cylinders as well as the dynamic effect of the LAU inertia, which are difficult to take into account in the calculation. The load measurements on the test bench are designed with the focus on the test bench operation. With the development of the test method and the growth of test program, more interest and requirements have arisen in better quality of the measurements, including aspects of dynamical behavior, linearity and accuracy. To address the demand of direct torque measurement with metrological traceability, different approaches have been proposed based on the force-lever principle [1, 2]. Another approach based on fiber-optic strain sensors is presented by Gutierrez al. [3] For load measurement in all six DOFs, a design study of multi-component has been carried out by Gnauert al. [4].

At DyNaLab, efforts have been made in a number of test campaigns to measure the input loads directly in front of the DUT, as shown in Fig. 2. The shaft adapter originally responsible only for connecting the DUT to the test bench is selected as the position where the input loads to the DUT are measured. Since the output flange of the test bench cannot fit the input shaft of each DUT, a shaft adapter is almost always necessary for a new test campaign. Usually, the shaft adapter has a structure of hollow cylinder between the connection flanges. This provides a very good condition for load measurement and accessibility for the instrumentation. In context of load measurement, the shaft adapter is also referred to as a measurement adapter.

Although the strain gauge full bridges are intended to measure the corresponding strains caused by specific loads, the raw signals of all the physical channels are electrical signals by nature. In engineering, the unit of the strain gauge raw signals is commonly \(mV/V\). To obtain the load, a relationship between the specific load \(L\) and the corresponding raw signal \(\delta\) needs to be determined. Usually, a linear relationship is assumed. For convenience of analysis, the linear relationship shown in Eq. 1 is adopted in this paper, where the offset parameter \(b\) compensates the signal offset at zero load and the sensitivity parameter \(a\) describes the slope of the linear relationship.

The most straightforward and common way in the sensor technology is to calibrate the measured signal with a reference load transducer, whose measurement is metrologically traceable to the national standard. This is shown in Fig. 6(a). However, due to the high levels of the loads that the adapter is supposed to measure, calibrations are usually either technically or economically not feasible. As an alternative, the factor \(a\) (the gain or sensitivity parameter) in the relationship can be obtained via analysis of the measurement chain. The parameters of the Wheatstone bridge and the strain gauges are needed to convert the electrical signal first into a strain signal in \(\mu m/m\). Their geometry and material properties are then used to obtain the signal of the load from this strain signal. This process is shown in Fig. 6(b) and is the one commonly used in the DyNaLab. The drawback of this process is the accumulation of uncertainty along the measurement chain. The uncertainty of each parameter in each conversion of the signal makes a contribution to the final measurement uncertainty of the load. Therefore, great care needs to be taken at each step and for each parameter in the process. Another drawback of the process is that the \(b\) factor in Eq. 1 (the zero point) cannot be determined through analysis of the measurement chain. This issue can only be solved by a nulling procedure via test.

As mentioned above, the measurement adapter shown in Fig. 5 has been instrumented for an industrial test campaign. An industrial multi-MW wind turbine has been tested for more than half a year, providing a number of opportunities to verify the load measurement on the adapter.

Since the adapter rotates together with the main shaft, the signals of loads in all the directions are measured in the rotating CSYS. With the help of additional measurement of the angular position, the loads in the rotating CSYS can be converted into loads in the stationary CSYS. In this case, a MEMS-based (Micro-Electro-Mechanical Systems) inclinometer is deployed on the drivetrain to measure the adapter’s angular position. An inclinometer measures the position with respect to the direction of gravity and is therefore very convenient to be instrumented. Unlike the rotary encoder, the inclinometer is deployed on the rotating part and therefore can be connected to the same DAQ system for the load measurement.

Apart from the torque and the thrust force, which can be adopted directly without change, bending moments and shear forces in orthogonal directions along \(20^{\circ}\) and \(110^{\circ}\) are used in the rotational CSYS to calculate the loads in the stationary CSYS, shown in Fig. 10. An example of the bending moments before and after the transformation is given in Fig. 11. For comparison, the loads measured by the test bench’s hydraulic system are also plotted in the figure. It can be seen that the bending moments measured by the adapter and by the test bench are in very good agreement. To protect the commercial interest of the turbine manufacture, the measured loads shown in the figures of this paper are often normalized. Since the loads in each figure or sub-figure are normalized using the same scale, the scientific characteristics of the figures are maintained.

Similarly, the comparison can also be carried out for loads in other directions. Fig. 12 shows another measurement where bending moments and shear forces change simultaneously or alternately during the course of the test. The loads are only compared in the stationary CSYS. The results also show a good match between the measurement on the adapter and the measurement from the test bench.

Directly after the industrial test campaign mentioned above, the same DUT and adapter have been used again for an EU research project named WindEFCY [8]. This project has provided a unique opportunity to also measure the torque with a calibrated highly accurate 5 MN m torque transducer [9, 10] (referred to as torque transducer below) from the German national metrology institute Physikalisch-Technische Bundesanstalt (also known as PTB). The behavior and accuracy of the torque measurement on the measurement adapter can therefore be studied with measurements of the transducer as the reference.

As shown in Figs. 13 and 14, the torque transducer is integrated into the test assembly between the measurement adapter and the DUT. Several additional adaptation structures are used to connect the transducer to the assembly. The load application unit of the test bench is shut down during the whole project to protect the torque transducer, which is not designed to withstand large bending moments and shear forces at the level found in a wind turbine.

To demonstrate why it is necessary to measure the torque at multiple positions on the adapter, the measurement using all 8 SHR channels is compared with the measurements with only 4 and 2 channels of the SHR full bridges installed on the adapter. Positions are always selected to be equally distributed on the circumference. For the 4‑position torque signal, SHR20, SHR110, SHR200 and SHR290 are chosen, while for the 2‑position torque signal, SHR20 and SHR200 are used. In Fig. 15, the differences in the torque measurements with 2, 4 or 8 positions of strain gauge channels are presented. Compared with the traditional 2‑position torque measurement, the use of 4 and 8 positions can greatly reduce the noise in the signal. From the plot of the power spectrum density (PSD) in the lower part of the figure, the torque based on 4 and 8 positions clearly has lower power levels on almost all the first 8 orders of the rotational speed. The 2‑position torque signal has the largest oscillation power on the second order, which can be successfully reduced with the 4‑position measurement. A slight increase in the 4th order can be seen on the signal of the 4‑position measurement, which is effectively reduced with the 8‑position measurement. Generally, it can be summarized that the measurement of more positions greatly reduces the noise in the low frequency range with the possibility of a slight increase in the high frequency range. A similar effect is also expected for the torque measurement on other similar applications, for example on the main shaft of a wind turbine. Considering the benefit and the extra effort needed for the instrumentation, torque measurement at 4 positions can be considered as a proper balance for industrial applications.

Comparing the 8‑position torque measurement on the adapter with the measurement of the torque transducer, the signal quality of the torque from the adapter can be checked and the torque measurement can be calibrated with the PTB transducer as the reference.

In Fig. 16, the torque measurements of the same profile as in Fig. 15 are shown. The difference in the signal levels in the upper part of the figure indicates a sensitivity error in the measurement of the adapter, which corresponds to the factor \(a\) in Eq. 1. Calibration based on a series of designed profiles similar to the one shown in the figure has been carried out. According to this calibration, the sensitivity parameter \(a\) should be increased by 4.3%. This magnitude of error in \(a\) is a result of all the uncertainty contributions in the analysis of the whole measurement chain shown in Fig. 6. The calibration also gives an offset correction of the measurement, and as a result the calibrated torque measurement on the adapter \({T}^{{}^{\prime}}\) can be expressed as a function of the original measurement \(T\):

The factor \(k\) addresses the sensitivity error of the measurement and has a value of \(k\approx 1.043\). With the calibration repeated on different days and under different conditions, it is found that the \(k\) factor is very stable, although a slight effect of the temperature can be observed. Throughout the calibrations studied, the \(k\) factor has been observed to drift by a maximum of 0.002 in line with the temperature changes. Other than the stable \(k\), the offset parameter \(d\) is more prone to drift with temperature change. A total drift range for \(d\) of about 200 kN m was observed in the calibrations studied. It is worth pointing out that the drift happens slowly over a longer period of time and can be effectively reduced by regularly carrying out the nulling process. However, to study the drift behavior of the measurement, no nulling process has been carried out during the WindEFCY project, since a drift-free torque signal was already available on the reference torque transducer of the PTB. For future work, there are motivations to compensate the signal drift in the measurement from the outset. The most important reason is to reduce the drift during the long-term tests, which can last for several hours. Another reason is to reduce the number of repetitions of the nulling process to give more test time. Concrete actions are planned to compensate the signal drift with the help of additional temperature sensors installed on the adapter.

To check the dynamic behavior of the adapter torque measurement, the signal is compared with the torque transducer measurement in a form of PSD analysis. This is shown in the lower part of Fig. 16. It is seen that the PSD of the torque measured by the adapter (based on 8 positions) has the same or similar levels as that of the torque transducer on all the first 20 harmonic orders of the rotation frequency. Therefore, the dynamic behavior of the torque signal from the measurement adapter can be considered to reach a level similar to that of the torque transducer in the examined frequency. In line with the analysis in Fig. 15, it has proven necessary to perform the torque measurement with strain gauges at 8 positions.

This paper discussed the challenge and necessity of direct load measurement on nacelle test benches. A detailed solution based on strain gauge instrumentation on the shaft adapter has been presented. A campaign has provided chances for the application and verification of the solution. It is shown in the paper that the direct measurement of torque and non-torque loads together on the adapter in front of the device under test (DUT) is feasible. Using strain gauges in different configurations and at multiple positions on the measurement adapter, signals of the transferred loads in almost all directions except for the thrust force can be successfully picked up and separated from each other. The relationships between loads and raw signals are determined through analysis of the whole measurement chain including mechanical property of the adapter and the strain gauge instrumentation. A signal nulling method based on specially designed test procedures is presented. Measurements of bending moments and shear forces show a good match with the corresponding measurements from the hydraulic system of the test bench. Moreover, the torque measurement is checked and calibrated up to 5 MN m with a state-of-the-art torque transducer. A sensitivity discrepancy of around 4.3% and an additional temperature-dependent offset drift have been found in the torque measurement on the adapter. Analysis of the dynamic behavior in the torque signals has shown that measurement using strain gauges at multiple positions can effectively reduce the noise in the signal. The torque measurement with 8 positions can achieve a similar low noise level to that of the torque transducer. Measurements with 4 positions can be a good compromise for similar industrial applications. Further research will focus on reducing the temperature-related signal drift in all the load measurements with the help of locally installed temperature sensors.