MEMS AIRFOIL WITH INTEGRATED INCHWORM MOTOR AND FORCE SENSOR

We demonstrate a MEMS-actuated aerodynamic control surface integrated with a piezoresistive force-sensing platform and fabricated in a simple 3-mask silicon-on-insulator (SOI) process. This 47-mg actuator/sensor system generates 0.25 mN of aerodynamic lift force in 23 m/s airflow while operating at 40V, with a rotational displacement of 11 degrees, and a slew rate of 100 degrees/s. This is the first use of an electrostatic inchworm motor to actuate an aerodynamic control surface and generate lift. INTRODUCTION Small unmanned aerial systems (sUAS) and micro-air-vehicles (MAVs) have demonstrated useful applications such as search and rescue, aerial photography, crop inspection and biopsy, and industrial chimney inspection [1]–[3]. Scaling these systems down to the pico-air-vehicle (PAV) realm, where dimensions and masses less are than 5 cm and 500 mg, will improve energy consumption, decrease cost, increase ensemble density, and increase data granularity of unmanned aerial systems [3]. Recent advances in mesh networking, MEMS technology, and novel propulsion methods are increasing the feasibility of PAVs [3]. Some of these PAVs include flapping wings, hovering ionocrafts, and potentially ion jet planes [3]–[5]. These vehicles will require millimeter-scale control surfaces in order to control their flight. Some MEMS control surfaces used arrays of MEMS actuators to manipulate airflow over a centimeter-scale to meter-scale delta wing [6]–[8]. These MEMS arrays aren’t suitable for PAVs in their current form because their array size is too large. In [9], Wood et al. developed miniature piezoelectric actuators that were eventually used to actuate the ailerons on a 2 gram microglider [10]. Millimeter-scale control surfaces that are suitable for PAVs will resemble control surfaces like these. In addition to piezoelectric actuators, electrostatic inchworm motors are also suitable for millimeter-scale control surfaces. Inchworm motors are easy to integrate with transmissions and mechanisms by using simple silicon-on-insulator (SOI) fabrication processes [11]. In MARSS 2017, we reported the design and fabrication of a millimeter-scale MEMS control surface using electrostatic inchworm motors, but did not demonstrate its ability to generate aerodynamic forces [12]. This control surface could be improved by an integrated force measurement system. Integrated force sensing eliminates the need for complicated and cumbersome external force measurement systems, and it enables force feedback applications. Integrated force measurements can be used to improve microbotic control systems. For example, B. Yang et al. used simulated motor force outputs along with machine learning and dynamic simulations of a microrobotic hexapod to design an optimized hexapod gait [13]. Integrated motor force sensors will be necessary in order to evaluate and tune control schemes like these when they are implemented on a physical microrobot. Several piezoresistive and capacitive force sensors for microrobotic applications have been developed [14]–[16]. The systems in [15] and [16] both were fabricated in an SOI process compatible with an SOI inchworm motor system. Xu et al. integrated a capacitive force sensor with an electrostaticallyactuated microgripper, which demonstrated the integration of force sensors with actuator systems in SOI processes [17]. This paper describes the design of a MEMS control surface actuation system integrated into a piezoresistive force sensor using a simple SOI process. The SOI process included a 550 m singlecrystal silicon (SCS) substrate, a 2 m oxide layer, a 40 m SCS device layer, and a metallization layer. The process steps were a substrate deep reactive ion etch (DRIE), a device-layer DRIE, a metal deposition, and a sacrificial oxide etch.