We derive thrust, roll, and pitch authority from a single propeller and single motor through an underactuated mechanism embedded in the rotor itself.
This allows new types of conventionally-capable micro air vehicles which only require two motors for practical control. This contrasts with the many servos and linkages of conventional helicopters or the many drive motors found in quadrotors.
Conventional UAV formats applied to smaller and smaller micro air vehicles lead to significant design challenges. A swashplate-controlled coaxial helicopter must find room in its mass, size, and cost budget for four actuators (two big rotor motors, two swashplate servo motors) and a complex linkage assembly. A quadrotor must similarly support four motors and face the practical problems of rapidly shrinking rotors. Our new rotor system provides significant system simplifications while retaining all the advantages of cyclic control.
The main motor directly drives the propeller hub, which is itself connected to the propeller blades by two inclined hinges. The hinge geometry couples blade lead-and-lag oscillations to a change in blade pitch. Instead of only driving the motor with a steady torque, we add a sinusoidal component in phase with the rotation of the rotor to induce a cyclic pitch variation. The amplitude and phase of this control signal determines the magnitude and direction of the vehicle response.
This paper presents a new concept a MAV propulsion system capable of using a minimum number of actuators in dual rolls. This simplifies and lowers the cost of MAVs. Removing complex swash plates and reducing the number of actuators reduces the number of parts, thus increases reliability (fewer parts to fail), reduces maintenance costs, reduces vehicle mass, and reduces manufacturing costs. Experimental results for the actuator response are presented along with a demonstration of a full flight vehicle using this system for both active stability and maneuvering.
We have shown that a hinged, underactuated rotor can mimic the behavior of traditional cyclic control systems in small MAVs without requiring either additional servomotor actuators or complex linkage systems. Both the magnitude and response time of the resulting control moments are sufficient for stabilizing and maneuvering a small, 358 g coaxial MAV.
In future work, we wish to develop a technique for determining optimal geometric design parameters for power-efficient operation given application constraints on required moments, thrust, and rotor size. This will allow us to evaluate the system-level power efficiency of this technology verses other control strategies while taking into account the associated actuator and structure material weights. The ultimate aim of this technology is to achieve reductions in system complexity and actuator count that may enable future small, simple, and low cost micro air vehicles.
J. Paulos and M. Yim, “An underactuated propeller for attitude control in micro air vehicles,” in Intelligent robots and systems (iros), 2013 ieee/rsj international conference on, Tokyo, Japan, 2013.