There’s no mistaking the Blaest headquarters on the shores of the Limfjord in Norway, where rows of white wind turbine blades dominate the horizon. In 2005 the load bearing capacity of the blades was still measured with sandbags. Fast forward to today and the methods have long since been fully digitised, with electromechanical loading systems connecting the blade to the pulling stations fixed to the hall floor. During the three-month test phase, each rotor blade is swung in different directions with great force approximately four million times.
“Our job is to perform fatigue tests on blades so that wind turbine manufacturers can get their prototypes approved,” explains Blaest test engineer, Nicolai Vangsgaard. “The blades should last between 25 and 30 years, and we have to prove that the blade can withstand the theoretical load for which it is designed, as accurately as possible.” To this end, Blaest now uses PC-based control and measurement technology from Beckhoff.
Image copyright: Nicolai Franzen.
Longer blades require faster data acquisition
When the company decided to expand in 2018 with a new, larger test hall, the next logical step was to update the previous control system. This manages communication with the several hundred data acquisition boxes that run along the massive rotor blades and captures the measurement signals from the sensors. Previously a separate cable was required for each individual measurement point, which not only generated significant costs, but also created a tangle of cables that had to be attached to the blade and connected to the control system.
“Our primary goal was to have a more flexible system with a channel count we could expand at any time as the wind turbine blades became longer and stronger,” explains Vangsgaard. This is because the next generation of wind turbine blades will be 100 metres long, as opposed to the 70 to 80 metres found today. This means additional sensors, more measurement channels, and more data to be recorded and processed, with maximum synchronicity and accuracy. The data should also be collected as close as possible to each individual measurement point to avoid the kilometres of cables that previously had to be maintained and replaced. Since Blaest was using its own in-house software tailored to suit each customer, it was essential to come up with an automation system that was both open and easily adaptable.
Customised development for specific requirements
To minimise the complexity of the cabling, Beckhoff developed a decentralised, high-channel EtherCAT P Box for evaluating measurement bridges that capture the signals from the strain gauge sensors on the rotor blades. The I/O box module supports the evaluation of full, half and quarter bridges with 24 bits and sampling rates of up to 10 ksps. Parameters can all be set via EtherCAT using the CoE directory.
The EtherCAT P Box converts the analog sensor signals into digital measured values, in close proximity to the measurement point, which reduces the risk of cable interferences and distorted analog signals. As any test engineer will confirm, the shorter the cable, the more accurate the measurement. Moreover, EtherCAT P reduces the number of lines. “This concept was only possible because EtherCAT is able to handle large networks, while also offering an extremely low real-time update time,” adds Vangsgaard.
During a wind turbine test, around 950 billion measured values are recorded, which corresponds to 7 terabytes of data. The measured values of the 500 load cells are streamed into a database every 4 ms. The old measurement system required more than 10 km of cabling for each test setup. The current system based on EtherCAT P has a cable length of only 1 km and can easily be extended by additional load cells. Vangsgaard puts the savings from the reduced installation effort alone at $4000 per test structure.
“Blaest operates the largest EtherCAT Hot Connect system in the world,” he adds. “The fact that everything is preconfigured makes it easy to make changes to the configuration.” The control cabinets now simply consist of an IPC, a connection for EtherCAT P and a safety module. “Overall, it makes us faster and more adaptable, while also lowering our costs and allowing us to take more accurate measurements. Another huge bonus is that the openness of the system means our test centre is open to all wind turbine manufacturers, so we can adapt to pretty much anything,” he concludes.
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