Avionics is among the increasing number of fields relying on power electronics in its applications. One longstanding project in this area has been developing a system for individually controlling helicopter rotor blades.

To date, rotor blades are primarily controlled by a swashplate, hydraulic actuators located in the fuselage and mechanical linkages between the rotating swash plate and each blade arm. But this architecture severely limits the flexibility of blade pitch motion. A more complex pitch control scheme, in which each blade arm is controlled separately, can reduce the consequent noise and vibration. Rescue operations, in particular, benefit from this, as patients do not have to endure as much jarring. Currently, individual blade control (IBC) systems, which are added on to the swashplate, meet this challenge. However, extra hardware is then required to move the blades. An electro-mechanical actuator (EMA) placed at the root of the blade arm negates the need for the extra hardware and combines the primary and individual rotor blade control into one unit.

Fraunhofer IZM developed the power electronics for controlling the EMA for ZF Luftfahrttechnik GmbH, as part of the development of a technology demonstrator within the »Aviation Research Program IV«. The system requires careful construction and high reliability standards must be met. The EMA as a whole operates redundantly, as must the power electronics. Three power control units (PCU) are assigned for one EMA and the switching topology with three full bridges allows for independent operation of every motor phase. In the event of failure, only one phase has to be put out of operation.

We designed the complete system, beginning with the selection of the power switches and calculation of the power losses and ending with the design of the thermal path, at which point the materials were chosen, as the material determines the lifetime of the packaging.

The available build space is strictly limited, so a compact construction was developed: gate drive and control, load terminals and DC link are spread over several PCBs and stacked into levels. Additionally, the DCB is embedded into the PCB, and wires are bonded directly from DCB to PCB. This reduces the assembly by one level.

For encapsulation, a housing was developed that features a latticed lid. The silicon gel can then expand without detaching under the influence of centrifugal forces. This project has been qualified and the new EMA is now in operation.