A 3D-PRINTED 1 MG LEGGED MICROROBOT RUNNING AT 15 BODY LENGTHS PER SECOND
A 3D-PRINTED 1 MG LEGGED MICROROBOT RUNNING AT 15 BODY LENGTHS PER SECOND
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3D 打印的 1 MG 腿微型机器人，以每秒 15 个体长的速度运行
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发表时间：
2018
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通讯作者：
S. Bergbreiter
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作者：
R. S. Pierre;W. Gosrich;S. Bergbreiter
这项工作提供了一个用于研究小型（<100 mg）有腿微型机器人运动的实验平台。机器人底盘采用微型 3D 打印技术制造，嵌入式永磁体提供驱动。该设计在髋关节中集成了一个全旋转摩擦轴承，能够以高达 150 Hz 的频率驱动，以 100 Hz 的速度旋转超过 1,000,000 次循环后没有明显的磨损迹象。这项工作中展示的机器人重 1 毫克，观察到其运行速度高达 37.3 毫米/秒（每秒 14.9 个身体长度），为蚂蚁鳞片的腿式运动动力学提供了初步见解。引言 经验生物标度定律表明，相对速度（每秒身体长度）随着体重的减少而增加[1]。然而，随着机器人尺寸的缩小，它们的相对速度与生物机器人相比就相形见绌了。需要了解昆虫尺度的足式运动动力学，以便为微型机器人的设计和控制提供信息。大型机器人，例如 RHex（7 kg）[2]，为理解腿式运动的形态和控制提供了一个平台。 RHex 的大尺寸实现了平台的多功能性和灵活性，允许轻松更改设计组件（例如支腿）。通过将微型 3D 打印与磁驱动相结合，这项工作提供了一个多功能平台来研究质量比 RHex 小 1,000,000 倍以上的机器人的腿式运动。这项工作中测试的最终机器人如图 1 所示。制造蚂蚁大小的微型机器人，特别是体重小于 100 毫克的机器人，本身就是一项令人印象深刻的壮举。一种更成功的微型机器人是[3]中介绍的 80 毫克热驱动微型机器人，它使用热驱动来抬高和降低腿部。然而，该机器人的步幅和步频受到限制，最高速度可达 6 毫米/秒（每秒 0.8 个身体长度）。 [4] 中提出的 10 毫克太阳能微型机器人集成了驱动、控制和电源，以实现能够自主移动的机器人，尽管没有明显的向前推进力。最近，[5] 中提出的步行机器人 (18 mg) 结合了静电致动器，可在垂直和水平方向生成步行路径，并展示出高达 0.7 毫米/秒（每秒 0.15 倍身体长度）的速度。这些相同的微型机器人可以作为物理模型来理解毫克级的腿式运动。开发采用锆钛酸铅 (PZT) 的腿式机器人是为了研究微型机器人腿中接触相互作用的动力学 [6]，最近的、结构类似的腿式微型机器人重 4.4 克和 379 毫克，分别以每秒 0.06 个身体长度和每秒 0.1 个身体长度行走 [7]。 [8] 中制造的 25 mg 机器人采用了顺应性材料，以创建具有被动铰接腿的微型机器人。通过利用被动自由度并结合磁驱动，该机器人实现了每秒 5 个身体长度的最高速度。与[9]中提出的实验平台类似，这项工作利用 3D 打印来创建腿式机器人的底盘。通过将永磁体作为无线致动的一种形式，可以研究腿式运动的机械方面。此外，磁驱动可以深入了解扭矩要求以及实现运动所需的机械功率。图 1：制造的 1 毫克、2.5 毫米 x 1.6 毫米 x 0.7 毫米四足微型机器人，图中带有干燥的子弹蚁头 (Paraponera clavata)。机器人设计和制造 机器人设计类似于 RHex [2] 和 [9] 中提出的四足机器人。机器人的每条腿都呈“C”形，提供与地面的滚动接触。每个臀部的摩擦轴承使腿部能够通过嵌入式永磁体上的扭矩进行完整的旋转运动。这些微型机器人是使用直接激光写入进行微型 3D 打印制造的。该过程如图 2 所示。将 20% 葡聚糖 (Dextran 70 Sigma-Aldrich) 的牺牲层以 1,500 rpm 的速度旋涂到 ITO 涂覆的载玻片 (Nanoscribe) 上 1 分钟（图 2A）。使用浸入式激光光刻 (DiLL) 和 Nanoscribe Photonic Professional GT 使用 25X 物镜（卡尔蔡司显微镜）和负性光刻胶 (IP-S Nanoscribe) 对微型机器人结构进行图案化。打印后，微型机器人在丙二醇单甲醚醋酸酯 (PGMEA) 中开发 2 小时。将载玻片从 PGMEA 浴中取出并用异丙醇 (IPA) 冲洗以洗掉 PGMEA 和多余的光致抗蚀剂。通过用去离子 (DI) 水溶解葡聚糖，将微型机器人从牺牲层中释放出来。然后将微型机器人 978-1-940470-03-0/HH2018/$25©2018TRF 59 固态传感器、执行器和微系统研讨会南卡罗来纳州希尔顿黑德岛，2018 年 6 月 3-7 日 DOI 10.31438/trf.hh2018.16 放置在 IPA 浴中，并在加热浴中超声处理过夜（3510 MTH Branson）去除机器人旋转接头中多余的光刻胶。完全清洁后，将 250 μm 立方体磁铁（C0005-10，SuperMagnetMan）手动放置在髋关节中，并用氰基丙烯酸酯（Loctite 401）固定。最终制造的机器人如图 1 所示。嵌入式磁体偶极子的方向以机械方式对微型机器人的步态进行编程。在这项工作中，偶极子沿同一方向排列，形成了前蹲（4 条腿跳跃）步态。图 2：直接激光写入和用于驱动的嵌入式永磁体的制造工艺。实验设置机器人在定制的两轴亥姆霍兹线圈系统中驱动，用于生成二维旋转磁场，在嵌入式磁铁上提供扭矩并驱动腿。图 3 显示了交互示意图。线圈支架采用 3D 打印 (UPrint Plus SE Stratasys) 并用 22 号漆包漆包线包裹。垂直线圈对的标称直径为120毫米，缠绕60匝，水平线圈对的标称直径为160毫米，缠绕50匝。为了创建旋转磁场，将 2 通道任意函数发生器 (AFG3022C Tektronix) 的每个通道连接到电流放大器 (TS200-0A Accel Instruments)。函数发生器每个通道的输出被设置为平衡线圈，产生大小为 0.78 mT 的对称旋转场。根据磁体的体积和材料估计 250 μm 立方体磁体的偶极子为 17.8 A-mm。使用[10]中的点偶极子模型，最大施加扭矩计算如下
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