Using Axis Control Valves in Wind Turbine Applications

Wind Power is a rapidly developing part of the Power Generation Industry where Moog has specialized for many years due to its unique expertise in motion control. As this article describes, Moog is helping to solve some real-world operational and performance challenges for wind turbine manufacturers and users. Two key challenges our customers are facing today involve trends in the wind turbine market: the size of the machines is increasing rapidly and the need for enhancements in energy efficiency, especially for wind park locations with low or unsteady wind speed expectations. Moog offers a variety of solutions to meet theses needs including both hydraulic and electric pitch control. This article focuses on Moog’s hydraulic solution involving a decentralized motion control architecture and the use of one of the most sophisticated valves in existence to provide hydraulic pitch control.


The trend towards increased turbine size is creating even greater challenges for users trying to maintain and operate systems on a 24/7 basis. Today’s installed machines have a nominal power output of 2 or 3 MW and the next generation being developed will have a nominal power of 5MW and more. To add to the complexity of this environment, these turbines are often installed in off-shore wind parks. The trend towards enhancements in energy efficiency is driven by the need to expand wind park locations to improve the return of investment and allow installations in places previously considered less suitable. These challenges are often addressed through blade design, where optimized profiles allow higher output at the same wind speed. This requires a sophisticated pitch control system, as the blades become more sensitive to squalls and storms.

The Moog Solution

In wind turbine application, the requirements are some of the highest in the world regarding the need for reliability of the machines, systems and components. In particular, the design of a pitch control system is key, as a breakdown could lead to critical machine conditions or even – in a worst case scenario – to a destruction of the whole turbine. A hydraulic pitch control system is typically used in this application due to the following advantages:

  • Easy and reliable storage of energy for failsafe movements in hydraulic accumulators
  • Low risk of actuator break down, due to simple and robust design of hydraulic cylinders
  • In a power failure situation, the blade will in all circumstances enter the safe position
  • The high power density of the hydraulic axis requires less room than the electric version

Figure 1 shows an example of a hydraulic pitch control system.

Pitch control systems are located in the hub of the machine, which means that power and signals have to be connected from the (rotating) hub to the nacelle. The connection of hydraulic power can be done easily via a Fluid Rotary union (FRU). To reduce the number of electric signals which have to be connected via slip ring, the preferred solution is a FIELD-bus.

Figure 2 shows the block diagram of an implemented solution. The Moog Servo-Proportional Valve with Integrated Digital Axis Control execute different tasks including:

  • Position demand interpolation with built-in trajectory generator
  • PID position control
  • Axis synchronization (parallel control of three axes)
  • Self monitoring of the inner valve control loops and the axis control loops
  • Communication with the host (PLC)

While the position signals from the transducers are supplied in an extra CAN-bus system in order to reduce bus load on the external CAN-bus system, the redundant CAN-bus topology allows a controlled machine shutdown in emergency situations. This is achieved by using a decentralized control architecture. Each valve includes the whole logic to control its own position loop and communicate independently with the PLC system, while it monitors the signals of its adjacent axis. The following are some key safety monitoring functions:

  • Loss of position signal (caused by transducer failure or cable break)
  • Loss of command signal (caused by bus interruption or PLC breakdown)
  • Loss of communication to adjacent axis (caused by bus interruption, cable break or valve malfunction)
  • Recognition of emergency situation identified from an adjacent axis

Due to the digital approach and the use of standardized communication protocols and device profiles, the system can easily be supervised remotely, offering huge advantages in this application where access is so problematic. There are several possibilities to achieve a remote control system:

  • The CAN network messages are translated into the turbine’s network bus protocol with use of a gateway
  • Use of wireless technology (Bluetooth or WLAN)

Rationale for Using CAN

Due to the required slip ring for transmission of bus data from the stationary nacelle to the rotating hub, the use of CAN is highly recommended. While Moog’s family of Axis Control Valves are available with several FIELD-bus interfaces (e.g. Profibus, EtherCAT), the CAN-bus has proven to be very robust and offer the highest reliability in the field.


While this article focuses on a specific hydraulic solution, Moog works with the world’s leading Wind Power Companies to realize a variety of high performance motion control solutions including electric pitch control, slip rings for data transmission and testing machinery. Moog has worked closely with OEM’s in this industry to address their needs for technology that ensures safety of the turbine, better energy efficiency in previously unsuitable locations, more effective monitoring, and the highest reliability. For the machine builder and user, these benefits translate into lower operating costs, greater safety, opportunities to use this technology in more locations and easier maintenance and troubleshooting. Best of all, our solutions such as the Axis Control Valve have now has been proven with thousands of wind turbine systems in the field.

About the Author

Walter Lenz is a Development Engineer who is responsible for digital valves electronics. He has been with Moog since 1996 working in electronics and software development engineering and is teamleader of the EFB valves electronics development since 2004. He graduated from University of Applied Science Furtwangen in 1995 as degreed engineer in fine mechanics Engineering.