Non-contact velocity measurement: Optical sensors for wind turbines
Non-contact velocity measurement: Optical sensors for wind turbines
Wind turbines are operated by means of determined rpm control concepts to optimize the efficiency and prevent damages or the destruction of the turbine. The power generator works with the highest efficiency at a fixed revolution speed. In the standard operating mode the pitch of the rotor blades is constantly adjusted to maintain this constant revolution speed.

However, high wind velocities and the corresponding excessive speeds can cause mechanical stress and consequently lead to damages, and in the worst case destroy the turbine. Damage prevention strategies include orienting the blades away from the wind, securing the turbine with brakes, or turning the blades in sail position and initiating idle operation of the turbine. Alternatively, modern turbines feature a so-called storm control function which enables safe operation in almost any wind speed. This technology requires highly precise, reliable rpm measurement.

The demands on the sensor system are highly challenging: safety requirements necessitate redundancy, and the system must withstand adverse environmental conditions and high mechanical stress on the turbine’s rotor shaft. For maximum efficiency, velocity control requires precise rpm data. Solutions based on rotary encoders or proximity switches, which have been used in the past, have proven susceptible to errors and to require high maintenance – moreover, their mechanical integration is very complex and the relatively low resolution is a limiting factor for an accurate rpm measurement.

Optical alternative
The OPTIPACT sensor from INTACTON is an alternative for velocity measurement in wind turbines. The sensors, which survey moving surfaces and feed the control system with the resultant data, autonomously measure planar movements. Their operating principle is based on the detection and tracing of characteristic structures within the visible surface which are optically recorded and correlated by a 2D-camera system.

In wind turbines, OPTIPACT sensors are installed with the integrated 2D camera focusing the revolving hub or any moving surface. From the measured data, they generate a signal similar to that of an incremental encoder. In addition by means of a reference mark on the hub, OPTIPACT can monitor not only the revolutions per minute but also the rotary position of the hub – this additional information can be used for monitoring the revolutions and to position the rotor. Since the easy-to-install, cost-efficient systems drastically decrease installation and maintenance efforts, even the redundant installation of two units is economically sound. For more than one year, OPTIPACT sensors have been tried and tested under extreme conditions in wind turbine applications. The results proved that the signals supplied by OPTIPACT are very well suited for rpm control. The units can drastically improve efficiency in comparison to conventional systems.

In addition to INTACTON’s know-how about optical sensors, sister companies from the FRABA group also contributed to the application’s success. POSITAL, for instance, has extensive experience with various fieldbus connection options, rotary encoders for wind turbines, and further safety-oriented applications.

Small, flexible, low-cost units
OPTIPACT sensors consist of an LED illumination unit, a 2D camera, miniaturized transmitting and receiving optics, and integrated analysis and interface electronics. Measuring 98 x 42 x 47 mm and weighing merely 250 grams, the units can be easily integrated into almost any application. X-axis and y-axis position data enable surveillance of movement in any direction – unlike in other systems, adjustment and calibration is unnecessary. The sensors are housed in an IP65-protected aluminum enclosure. They require a supply voltage of optionally 5V or 10V…30V . An incremental interface and an RS232 interface are available for communication with a higher-level PLC. INTACTON implements fieldbus systems or SSI on request. The sensors’ completely modular design enables easy adaptation to specific requirements. The energy-efficient LED illumination has the added advantage of requiring no light bulbs that need exchanging. Unlike conventional optical systems, the sensor is completely maintenance-free.

Robust, maintenance-free solution
The camera’s field of view on the measured surface is 8 x 8 mm. Since it needs to observe only a small part of this array, dirt particles hardly impair its reliable operation. If the sensor is mounted with a vertically downward orientation, there will be hardly any dirt accumulation at all on the lens. Due to relatively large camera pixels and sufficiently luminous LEDs, very fine dust will not directly impair the measurement results. A control loop integrated in the sensor adapts the exposure time to the current conditions.

INTACTON GmbH specializes in the development, production and distribution of miniaturized optical sensors for industrial applications. The company belongs to the FRABA group which dates back to 1918, when its predecessor Franz Baumgartner elektrische Apparate GmbH, a producer of relays, was established. Today, the group consists of six independent companies that develop and produce industrial sensor technology and safety equipment.

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