[Paper Review] Bee$^+$: A 95-mg Four-Winged Insect-Scale Flying Robot Driven by Twinned Unimorph Actuators

Xiufeng Yang, Ying Chen+3 more|arXiv (Cornell University)|May 6, 2019

Biomimetic flight and propulsion mechanisms24 references4 citations

TL;DR

Bee+ is a 95-mg four-winged microrobot driven by two pairs of twinned unimorph actuators, enabling independent wing flapping and improved controllability over two-winged counterparts. The design achieves stable hovering and attitude control through a quadrotor-inspired controller, demonstrating enhanced yaw stability and reduced actuation complexity compared to prior insect-scale flyers.

ABSTRACT

We introduce Bee$^+$, a 95-mg four-winged microrobot with improved controllability and open-loop-response characteristics with respect to those exhibited by state-of-the-art two-winged microrobots with the same size and similar weight (i.e., the 75-mg Harvard RoboBee). The key innovation that made possible the development of Bee$^+$ is the introduction of an extremely light (28-mg) pair of twinned unimorph actuators, which enabled the design of a new microrobotic mechanism that flaps four wings independently. A first main advantage of the proposed design, compared to those of two-winged flyers, is that by increasing the number of actuators from two to four, the number of direct control inputs increases from three to four when simple sinusoidal excitations are employed. A second advantage of Bee$^+$ is that its four-wing configuration and flapping mode naturally damp the rotational disturbances that commonly affect the yaw degree of freedom of two-winged microrobots. In addition, the proposed design greatly reduces the complexity of the associated fabrication process compared to those of other microrobots, as the unimorph actuators are fairly easy to build. Lastly, we hypothesize that given the relatively low wing-loading affecting their flapping mechanisms, the life expectancy of Bee$^+$s must be considerably higher than those of the two-winged counterparts. The functionality and basic capabilities of the robot are demonstrated through a set of simple control experiments.

Motivation & Objective

• To develop a more controllable and stable insect-scale flying microrobot than existing two-winged designs.
• To address the yaw instability common in two-winged microrobots by increasing actuator count and wing count.
• To reduce fabrication complexity while maintaining high performance through monolithic twinned unimorph actuators.
• To demonstrate open-loop response and closed-loop control of attitude and position in a 95-mg flying robot.
• To validate the feasibility of four-winged flapping-wing design for enhanced control authority and reduced wing-loading.

Proposed method

• The robot uses two pairs of twinned unimorph actuators fabricated monolithically, enabling independent control of four wings.
• Each actuator pair drives two wings in a synchronized flapping mode, increasing control authority from three to four inputs.
• A control strategy based on quadrotor control principles was adapted, using a combination of attitude and position controllers.
• The thrust force direction is controlled via differential flapping amplitudes and phases to generate desired net forces and torques.
• Yaw control is achieved using the ISP (inter-sweep phase) method, which was found most effective for the prototype.
• Dynamic analysis and simulations were used to estimate thrust, damping, and steering torque characteristics.

Experimental results

Research questions

• RQ1 Can a four-winged microrobot with independent actuation achieve better controllability than two-winged counterparts?
• RQ2 How does increasing the number of actuators from two to four affect yaw stability and control authority?
• RQ3 Can twinned unimorph actuators enable a compact, low-mass, and easily fabricated flapping mechanism at the insect scale?
• RQ4 What control strategy enables stable hovering and attitude regulation in a 95-mg flying microrobot?
• RQ5 How does wing-loading and flapping mode influence the robot's open-loop response and flight stability?

Key findings

• Bee+ achieved stable hovering with roll and pitch oscillations confined within ±10°, indicating effective attitude control.
• The robot successfully took off and maintained upright orientation for a significant duration, demonstrating sufficient lift generation.
• Position control experiments showed approximate tracking of reference signals for the first second, with increasing error due to actuator saturation and limited integral action.
• The ISP method was identified as the most effective yaw control strategy among the three evaluated methods.
• The use of twinned unimorph actuators reduced fabrication complexity and enabled a lightweight, compact design with 28 mg actuator mass.
• Simulations and analyses confirmed that the four-wing configuration naturally dampens rotational disturbances, improving yaw stability over two-winged designs.