As part of a sustainable energy solution, wind power has demonstrated great potential to satisfy energy demands with a clean, safe, and highly efficient energy source. Consequently, wind energy is on the rise, reaching an output of nearly 57,000 Megawatts in 2007 and continuing to grow. However, continued focus is needed on ways to build, test, deliver, and maintain wind turbines more efficiently. One way to achieve this is through OEMs choosing components that contribute to overall system efficiency. One such component is the magnetostrictive sensor, which delivers linear-position sensor technology that improves the turbine's energy efficiency while also providing greater safety and reducing maintenance costs.

How it works

Magnetostrictive position sensors are essentially sonic wave sensing devices. By using a high resolution clock, accurate absolute position between a fixed reference point and moving magnet can be determined by the time it takes for a sonic wave to travel that distance. Another benefit of magnetostrictive position sensing is that the position magnet does not touch the waveguide, so there are no parts to wear out, resulting in long sensor life.

In a linear-position sensor or liquid-level sensor, a sonic strain pulse is induced in a specially designed magnetostrictive waveguide by the momentary interaction of two magnetic fields. One field comes from a movable permanent magnet that passes along the outside of the sensor tube, while the other comes from a current pulse or interrogation pulse applied along the waveguide. Interaction of the two magnetic fields produces a strain pulse, which travels at sonic speed along the waveguide until the pulse is detected at the sensor head.

Magnet position is determined with high precision by measuring the elapsed time between application of the interrogation pulse and arrival of the resulting strain pulse. The benefit: Accurate non-contact position or liquid-level sensing is achieved with absolutely no wear on sensing components. For example, R-Series sensors from MTS Systems Corp., Cary, N.C., are rugged components that provide absolute linear-position measurement. They can survive impact forces of 100 g and vibration forces to 30 g, significantly increasing their longevity, especially in the harsh environments found in wind turbine applications.

Wind turbine efficiency, pitch control

Regarding wind turbine efficiency, MTS has developed rod-style sensors featuring Temposonics magnetostrictive technology made especially for direct stroke measurement in hydraulic cylinders, such as the inside of rotor blade positioning mechanisms. The pressure-resistant sensor pipe installs into a bore in the cylinder piston head and rod assembly. The pipe protects the sensing element containing the magnetostrictive waveguide that carries the measuring signal. A ring magnet, secured on the piston head, travels over the sensor pipe without contact and provides the position signal through the pipe's wall.

An important aspect of wind turbine efficiency is pitch control: The turbine's input power is controlled by dynamically adjusting rotor blade angle. A control system using independent hydraulic actuators turns each blade at an angle between 0° and 90°. Under low wind conditions, the angle of the turbine blades is adjusted to achieve maximum efficiency and maintain constant electric power output. Because variations as small as a single degree can change the aerodynamics, accurate position feedback provided by magnetostrictive sensors is critical. When wind velocity exceeds the maximum permissible generator output, blades are adjusted away from the optimum position to reduce aerodynamic efficiency and maintain suitable rotational velocity.

Actively controlling input power enables peak efficiency, while reducing stress on the rotor, tower, and foundation for increased safety and longevity. Accurate position and velocity measurements also help support early diagnosis of variation in turbine operation. Sensors also can be used in brake control applications to ensure greater safety during harsh weather conditions or during turbine maintenance.

Information provided by Matt Hankinson, technical marketing manager, MTS Systems Corp., Sensors Division. For more information, visit www.mtssensors.com.