  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.
