Source: Robin Ritz, Institute for Dynamic Systems and Control, ETH Zurich.

The IDSC Tailsitter has been designed at the Institute for Dynamic Systems and Control, ETH Zurich, as a testbed for novel control algorithms for tailsitter vehicles. The goal of the project is to develop controllers that enable agile and robust flight for all flight regimes, such that the full potential of these vehicles can be exploited.

Airframe based on a Clark Y profile. Source: Robin Ritz, Institute for Dynamic Systems and Control, ETH Zurich.

The airframe of the vehicle is based on a Clark Y profile and designed such that for regular flight the pitching moment vanishes for zero flap angle, and such that the aerodynamic neutral point coincides with the vehicle’s center of gravity. The objective of this design is to allow agile maneuvers for all flight regimes and to avoid flap angle saturation problems caused by large trim angles.

Internal structure. Source: Robin Ritz, Institute for Dynamic Systems and Control, ETH Zurich.

The internal structure of the vehicle is built upon a core of a carbon sandwich composite material. The motors and servos are attached to the core using 3D-printed mountings, where the servos are placed such that they can be directly connected to the flaps. Further, a set of wing profile ribs and two carbon fiber tubes on the side for protection are mounted to the core structure. The electronics are placed at the center of the vehicle, and the battery at the front such that the center of gravity is at the desired position. All parts of the vehicle are screwed together (no glue is used), in order to simplify the replacement of broken components. The tailsitter’s wingspan is 48cm and the total weight is 150g.

The vehicle is controlled by a global controller enabling recovery to hover from any initial condition. A cascaded control architecture is used: Based on position and velocity errors an outer control loop computes a desired attitude keeping the vehicle in coordinated flight, while an inner control loop tracks the desired attitude using a lookup table with precomputed optimal attitude trajectories. The controller runs with an update rate of 500Hz on a Cortex M4F microprocessor. The vehicle is able to communicate with a ground station using an nRF51822 chip from the company Nordic Semiconductor. Communication can be established for recording telemetry data or for sending high-level commands to the vehicle, such as “take-off” or “land”. However, the vehicle is able to fly autonomously, the trajectories that are being tracked are stored on-board.

The on-board state estimation algorithms and technology are provided by the ETH spin-off company Verity Studios.

The attitude control algorithm is presented in the research paper “A Global Strategy for Tailsitter Hover Control”, International Symposium on Robotics Research (ISRR), 2015.

Researchers

Robin Ritz and Raffaello D’Andrea
Institute for Dynamic Systems and Control (IDSC), ETH Zurich, Switzerland
Flying Machine Arena

Technical details

Airframe: Custom design based on a Clark Y profile
Electronics: Custom design using a Cortex M4F microprocessor
Battery: Thunder Power RC G8 Pro Lite 25C LiPo 480mAh 2S
Motors: Hacker A05-13S
Motor controllers: Dys SN20A Mini ESCs with SimonK firmware
Propellers: 5×3 GWS EP-5030
Flap servos: MKS DS65K
Infrastructure: Flying Machine Arena

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• Video: Quadrocopter learns from its mistakes, perfects slalom air racing

See all the latest robotics news on Robohub, or sign up for our weekly newsletter.

Tailsitter in the Flying Machine Arena. Source: Robin Ritz, Institute for Dynamic Systems and Control, ETH Zurich.

In August 2014 Google [x] announced that they had been secretly developing a drone delivery program to rival Amazon’s Prime Air. Called Project Wing, the system was based on a mechanically simple “tailsitter” design – a hybrid vehicle that takes off vertically like a multirotor but is also able to fly horizontally like a fixed-wing airplane. Following field tests, however, Google scrapped the design, saying that it was difficult to control and didn’t perform well in high winds. Since then, researchers at ETH Zurich have developed a new algorithm for robustly controlling a tailsitter flying machine in hover position. Using the algorithm, the tailsitter is able to recover from any orientation, including upside down. ETH has not yet demonstrated its algorithm in real-world conditions (e.g., high winds, GPS localization, different payload distributions). However, given the robustness of the solution shown in the video, perhaps Google should not dismiss the tailsitter design too quickly for its next design iteration.  

– Robohub Editors

A tailsitter provides hover capability by pointing the nose and thrust direction upwards. For fast forward flight, the vehicle tilts to a near-horizontal orientation, resulting in efficient lift production due to the conventional wing design. Compared to other fixed-wing, hover-capable aircraft such as tiltrotors and tiltwings, the major advantage of a tailsitter is its mechanical simplicity: no mechanism for changing the direction of the propulsion system has to be added, saving weight and increasing reliability.

The algorithm demonstrated in the video is based on Optimal Control. Optimal trajectories for a set of initial orientations are precomputed and the corresponding initial desired body rates are stored in a small lookup table (500 entries). An outer control loop running at 50 Hz reads the desired body rates for the current orientation from the lookup table, and feeds them to an inner control loop. The inner control loop runs at 1000 Hz and tracks the desired body rates with the two propellers and the two flaps using feedback from a rate gyroscope. The resulting real-time computational complexity is extremely low.

Tailsitter’s propellers and motors. Source: Robin Ritz, Institute for Dynamic Systems and Control, ETH Zurich.

The propellers are driven by brushless Hacker A05-13S motors that are attached to the wings by 3D-printed mountings. While the vehicle is hovering, it is crucial that a large part of the flaps are within the propeller-induced airstream, in order to keep the flaps effective and the system highly maneuverable. The motors are controlled by ZTW Spider 12A ESCs, which run custom SimonK firmware that enables a fast response to thrust set point changes. Since the custom SimonK firmware is tuned for the fastest possible response, the motor controllers get quite hot and are therefore placed in the propeller airstream for cooling.

Tailsitter’s underbelly. Source: Robin Ritz, Institute for Dynamic Systems and Control, ETH Zurich.

Two radios are attached on the bottom side of the vehicle; a WiFly module for high-bandwidth telemetry, and a low-latency, frequency-hopping 2.4 GHz radio link that receives the control commands from the ground station. The total system latency including the motion capture system and the radio link is around 35 milliseconds.

Tailsitter’s flaps. Source: Robin Ritz, Institute for Dynamic Systems and Control, ETH Zurich.

The flaps are actuated by MKS DS65K servos providing a fast response on flap angle changes. The flaps of the original frame are designed for fixed-wing forward flight and do not provide enough control authority for the hover flight regime. Therefore their area is enlarged by carbon fibre sheets. This increases the effectiveness of the flaps by a factor of four. The maximum flap deviation is 30 degrees.

Tailsitter’s flight management unit (FMU). Source: Robin Ritz, Institute for Dynamic Systems and Control, ETH Zurich.

A Pixhawk PX4 Flight Management Unit (FMU) runs the onboard control loop that tracks the desired body rates using a rate gyroscope which is also provided by the FMU. Safety routines that switch off the actuators if the vehicle does not receive control commands anymore or if the battery voltage is too low are also implemented on the FMU.

The algorithm is presented in research paper “A Global Strategy for Tailsitter Hover Control”, submitted to International Symposium on Robotics Research (ISRR), 2015.

Researchers
Robin Ritz and Raffaello D’Andrea
Institute for Dynamic Systems and Control, ETH Zurich
Flying Machine Arena

Technical details
Frame: Robbe Mini Wing RC styrofoam airframe
Electronics: Pixhawk PX4 Flight Management Unit
Battery: Thunder Power RC G6 Pro Lite 25C LiPo 350mAh 2S
Motors: Hacker A05-13S
Motor controllers: ZTW Spider 12A ESCs with SimonK firmware
Propellers: 5×3 GWS EP-5030
Flap servos: MKS DS65K
Infrastructure: Flying Machine Arena

• Multiple tethered quadrotors whip around a pole together at high speeds 
• Quadrocopter failsafe algorithm: Recovery after propeller loss
• Three new quadrotor videos demonstrate agile control and the power of machine learning
• Video: Quadrocopter learns from its mistakes, perfects slalom air racing

See all the latest robotics news on Robohub, or sign up for our weekly newsletter.