Rotary Throttle Actuator – Case Study

For Unmanned Autonomous Drone (UAV)

BAE Systems are at the forefront of UAV technology and development. They were in the process of adapting their High Endurance Rapid Technology Insertion (Herti) UAV drone for use in commercial, non-military applications. Drones offer significant advantages in many situations but, as with many things, the advantages often come with their own share of disadvantages. By definition, an autonomous drone lacks the capability for human intervention in a failure situation. Consequently, regulations demand that, in a non-military situation, controls must incorporate redundancy for increased safety in case of failure. On board software also needs to be DO178 compliant.

For the Herti, the requirement was to find an actuator that could position the throttle with the necessary levels of accuracy and repeatability and that also incorporated redundancy in a failure situation to meet stringent regulations.

The first port of call for BAE was to source a commercial off the shelf (COTS) system. But most of those that were appropriate for this type of installation did not possess the requisite redundancy capability. Equally so, modification of the units to meet that capability was not possible. In addition, the COTS solutions were heavier than the ideal weight and their shape and size would have necessitated adaptation to the physiology of the aircraft itself in order to fit in the required position. It seemed like an impossible situation to resolve.

Enter OTM. Following BAE’s dismissal of the COTS as not viable, OTM were actually their fourth in line choice to be asked to solve the problem. But, our in-house engineering design expertise has been making the impossible possible for over 50 years. Our development team rose to the challenge to succeed where others had failed. They designed and built a bespoke rotary throttle actuator incorporating a dual wound motor that allowed 50% of the motor to fail without preventing it from continuing to work. In addition, a communication sensor was built in to confirm throttle position and communicate back to the flight control computer.

As an extra fail-safe, the actuator was mirrored on the opposite side of the throttle so, should the entire unit fail, the back-up would immediately kick in. The sensors were programmed to communicate with each other and operate in step so, in the event of failure, the other sensor would immediately assume control without any loss of throttle position information. Likewise, should either sensor fail, the other would assume control to ensure seamless operational transition.

In controlled, automated systems, the level of repeatability – the measure of how consistently the system can return to the same commanded position – is often a trade-off against accuracy or, conversely, a system can demonstrate high accuracy but poor repeatability. OTM’s solution combines both high repeatability and accuracy without the associated compromise. The positional feedback accuracy of the system is 0.2° offering immediate response and an associated high degree of fuel control.

The technical brief had been well and truly met. Just one of OTM’s many specialities over the years has always been our ability to match technical know-how and power efficiency with the physical needs of our clients. The OTM solution delivered the necessary 10Nm of operating torque, was tailored to fit the exact space on the aircraft and came in 30% lighter than any of the COTS solutions.

Features of the OTM Rotary Actuator for UAVs

  • Hazardous environment tested to -50 and +120 °C
  • Positional feedback accuracy – 0.2°
  • Operating Torque – 10Nm
  • Response rate – 45° per second
  • Actuator weight – 3.5kg
  • DO178 compliant
  • Dual redundancy
  • Quadruple redundancy at half power

The actuator has been rigorously tested to operate successfully in the hostile conditions as required by the application. With the possibility existing that, at any time, one of these could be flying overhead, it all combines to provide an ultra-safe operational environment.