Pitch Actuator

Articles on this page:

• Friction in single actuator pitch systems
• How do I ignore pitch actuator dynamics?
• Pitch acceleration limits not obeyed

Problem

For single actuator pitch systems, is the friction defined for the whole system or for the individual blades?

Solution

When you select “Single Actuator Pitch System”, the pitch system is considered as one rotational degree of freedom.

The constant part of the friction terms (highlighted in red below) which act directly on this single degree of freedom, represent friction acting on the whole system pitch.

For the load dependent friction (highlighted in green below), the additional friction is calculated based on the loads on each blade, then summed up over the three blades before adding the necessary friction value to the pitch actuator DoF.

So, the green parts are “per blade”, but the red parts are “for the whole system”.

Keywords

Single actuator; Pitch system; Friction

Problem

I want to ignore the pitch dynamics, but Bladed won't let me use a zero time constant or instantaneous passive response.

Solution

In Bladed, it is not possible to completely ignore pitch dynamics. This is because the rigorous structural dynamics approach requires that the pitch acceleration must be defined. This is necessary for getting the correct inertial loading in the structure. If the controller were to have the power to instantly change the pitch position or rate from one communication interval to the next, then the pitch acceleration would be undefined. So for this reason, it is not possible to ignore the pitch dynamics.

However, in version 4.6 and later, there is the possibility to "Track External Hardware" in the pitch actuator UI. When using this, you can use the SetHardwarePitchAngle and SetHardwarePitchRate in the external controller API and the pitch actuator will track the hardware position and rate with a fully defined pitch motion (i.e. including acceleration)

Keywords

Pitch actuator

Problem

I have defined pitch acceleration and/or rate limits in the pitch actuator Setpoint Trajectory Planning but the pitch rate and acceleration do not obey these limits. Why is this?

Solution

The acceleration limits are set in the Setpoint Trajectory Planning (STP) part of the pitch actuator definition. STP determines the relationship between the “Controller demanded pitch angle” and the “Limited demanded pitch angle”. STP ensures that the “Limited demanded pitch angle” obeys the acceleration and rate limits that have been set. So, this determines the relationship between the black & red lines in the image below.

The relationship between the “limited demanded pitch angle” and the pitch angle is governed by the relationship below (image from pitch actuator screen). The pitch acceleration is calculated at each time step by rearranging this equation calculate d^2 x/dt^2. An example calculation corresponding to the time history above is given in this spreadsheet.

As seen above, the pitch acceleration depends on the pitch angle, pitch rate and “limited demanded pitch angle”, so this is not directly subject to the acceleration limits defined in STP.

If you want to limit the pitch acceleration, you could consider adding torque limits on the actuator, which will effectively limit the acceleration that the actuator can achieve. Alternatively, you could choose a slower pitch response (e.g. frequency 1Hz) which will result in a less aggressive response to the difference between the pitch angle and the “limited demanded pitch angle”.

Interaction with safety system: The behaviour of pitch systems when performing safety actions varies depending on the type of pitch actuator. Some pitch actuators use an entirely different control path for safety action – eg a hydraulic system might connect an accumulator directly to the pitch cylinder, bypassing any active control and therefore any acceleration limits – whereas other pitch actuators use the normal control mechanism but with demands from different sources, and so could have acceleration limits, but not necessarily the same ones as in normal operation.

The Bladed pitch actuator model is a simplified approximation and cannot represent all the details of every real world pitch system. If you wish to represent your pitch system more accurately it is recommended to implement the pitch safety action in the External Controller DLL rather than in the Bladed safety system. Then Bladed will see it as normal operation rather than safety action, and apply the Setpoint Trajectory Planning limits.

Alternatively, users could create a pitch DLL to replace the Bladed pitch actuator model (but this does involve a greater amount of work than the above option).

Keywords

Pitch actuator; Rate limits; Acceleration limits