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This work presents an experimental platform for studying the locomotion of small-scale (<100 mg) legged microrobots. Robot chassis were fabricated with microscale 3D printing and embedded permanent magnets provide actuation. The design integrates a full rotational friction bearing in the hip joint, capable of actuation up to 150 Hz with no visible signs of wear after rotating at 100 Hz for over 1,000,000 cycles. The robot presented in this work weighs 1 mg and is observed running at speeds up to 37.3 mm/s (14.9 body lengths per second) providing initial insights on the dynamics of legged locomotion at ant-scales. INTRODUCTION Empirical biological scaling laws show an increase in relative velocity (body length per second) with decreasing body mass [1]. However, as robots are scaled down, their relative speeds pale in comparison to biological counterparts. An understanding of the dynamics of legged locomotion at insect scales is needed to inform design and control of microrobots. Large-scale robots, such as RHex (7 kg) [2], provide a platform for understanding both the morphology and control of legged locomotion. The large size of RHex enables versatility and flexibility within the platform, allowing design components, such as legs, to be changed easily. By incorporating microscale 3D printing with magnetic actuation, this work provides a versatile platform to study legged locomotion of robots with a mass over 1,000,000 times smaller than RHex. The final robot tested in this work is shown in Figure 1. Making ant-scale microrobotics, specifically robots less than 100 mg in body mass, is an impressive feat in itself. One of the more successful microrobots, an 80 mg thermally actuated microrobot, presented in [3] used thermal actuation to raise and lower legs. However, the stride length and stride frequency of this robot was limited, showing top speeds up to 6 mm/s (0.8 body lengths per second). The 10 mg solar powered microrobot presented in [4] incorporated actuation, control, and power to achieve a robot capable of autonomous movement, although not appreciable forward propulsion. More recently, the walking robot (18 mg) presented in [5] incorporated electrostatic actuators to generate foot paths in vertical and horizontal directions and demonstrated speeds up to 0.7mm/s (0.15 body lengths per second). These same microrobots can serve as physical models for understanding milligram-scale legged locomotion. Legged robots with lead zirconate titanate (PZT) were developed to investigate the dynamics of contact interactions in microrobotic legs [6], with more recent, similarly structured, legged microrobots weighing 4.4 g and 379 mg demonstrating walking at 0.06 body lengths per second and 0.1 body lengths per second, respectively [7]. The 25 mg robot fabricated in [8] incorporated compliant materials to create a microrobot with passively articulated legs. By taking advantage of the passive degrees of freedom and incorporating magnetic actuation this robot achieved top speeds of 5 body lengths per second. Similar to the experimental platform presented in [9], this work takes advantage of 3D printing to create a chassis for a legged robot. By incorporating permanent magnets as a form of wireless actuation the mechanical aspects of legged locomotion can be studied. Additionally, magnetic actuation provides insights into the torque requirements as well as the mechanical power required to achieve motion. Figure 1: Fabricated 1 mg, 2.5 mm x 1.6 mm x 0.7 mm quadrupedal microrobot pictured with a desiccated bullet ant head (Paraponera clavata). ROBOT DESIGN AND FABRICATION The robot design is similar to RHex [2] and the quadrupedal robots presented in [9]. Each leg of the robot is shaped like a “C”, providing a rolling contact with the ground. A friction bearing at each hip enables a full rotary motion of the leg from the torque on an embedded permanent magnet. The microrobots were fabricated using direct laser writing for 3D printing at the microscale. The process is described in Figure 2. A sacrificial layer of 20% dextran (Dextran 70 Sigma-Aldrich) was spin-coated on to an ITO-coated glass slide (Nanoscribe) at 1,500 rpm for 1 minute (Figure 2A). The microrobot structure is patterned using dip-in laser lithography (DiLL) with a Nanoscribe Photonic Professional GT using the 25X objective (Carl Zeiss Microscopy) and a negative tone photoresist (IP-S Nanoscribe). After printing, the microrobots were developed in propylene glycol monomethyl ether acetate (PGMEA) for 2 hours. The slide was removed from the PGMEA bath and rinsed with isopropyl alcohol (IPA) to wash away the PGMEA and excess photoresist. The microrobots were released from the sacrificial layer by dissolving the dextran with deionized (DI) water. The microrobots were then 978-1-940470-03-0/HH2018/$25©2018TRF 59 Solid-State Sensors, Actuators and Microsystems Workshop Hilton Head Island, South Carolina, June 3-7, 2018 DOI 10.31438/trf.hh2018.16 placed in a bath of IPA and sonicated overnight in a heated bath (3510 MTH Branson) to remove any excess photoresist in the rotary joints of the robot. Once fully cleaned, 250 μm cube magnets (C0005-10, SuperMagnetMan) were manually placed in the hip joints and secured with cyanoacrylate (Loctite 401). The final fabricated robot is shown in Figure 1. The orientation of the dipoles of the embedded magnets mechanically programs the gait for the microrobot. In this work, the dipoles were aligned in the same direction, creating a pronking (4-legged hopping) gait. Figure 2: Fabrication process for direct laser writing and embedded permanent magnets for actuation. EXPERIMENTAL SETUP Robots were actuated in a custom-made two-axis Helmholtz coil system, used to generate a two-dimensional rotating magnetic field, providing a torque on the embedded magnets and actuating the legs. A schematic of the interaction is shown in Figure 3. The coil brackets are 3D printed (UPrint Plus SE Stratasys) and wrapped with 22 gauge enameled magnet wire. The vertical coil pair has a nominal diameter of 120 mm and 60 turns of wrapping, while the horizontal pair has a nominal diameter of 160 mm and 50 turns of wrapping. To create a rotating magnetic field, each channel of a 2-channel arbitrary function generator (AFG3022C Tektronix) was connected to a current amplifier (TS200-0A Accel Instruments). The output of each channel of the function generator is set to balance the coils, creating a symmetric rotating field with a magnitude of 0.78 mT. The dipole of the 250 μm cube magnets was estimated from the volume and material of the magnets as 17.8 A-mm. Using the point-dipole model in [10], the maximum applied torque is calculated as

A 3D-PRINTED 1 MG LEGGED MICROROBOT RUNNING AT 15 BODY LENGTHS PER SECOND