Neel Doshi

Self-assembly of a swarm of autonomous boats into floating structures

This paper addresses the self-assembly of a large team of autonomous boats into floating platforms. We describe the design of individual boats, the systems concept, the algorithms, the software architecture and experimental results with prototypes that are 1:12 scale realizations of modified ISO shipping containers, with the goal of demonstrating self-assembly into large maritime structures such as air strips, bridges, harbors or sea bases. Each container is a robotic module capable of holonomic motion that can dock in a brick pattern to form arbitrary shapes. Over 60 modules were built of varying capability. The docking mechanism is designed to be robust to large disturbances that can be expected in the high seas. The docking mechanism also incorporates adjustable stiffness so that the conglomerate can comply to waves representative of sea state three, and have the ability to dynamically stiffen as required. The component modules for autonomous assembly, docking and simultaneous collision-free planning as well as the software architecture are presented along with the description of experimental verification.

Model driven design for flexure-based Microrobots

This paper presents a non-linear, dynamic model of the flexure-based transmission in the Harvard Ambulatory Microrobot (HAMR). The model is derived from first principles and has led to a more comprehensive understanding of the components in this transmission. In particular, an empirical model of the dynamic properties of the compliant Kapton flexures is developed and verified against theoretical results from beam and vibration theory. Furthermore, the fabrication of the piezoelectric bending actuators that drive the transmission is improved to match theoretical performance predictions. The transmission model is validated against experimental data taken on HAMR for the quasi-static (1-10 Hz) operating mode, and is used to redesign the transmission for improved performance in this regime. The model based redesign results in a 266% increase in the work done by the foot when compared to a previous version of HAMR. This leads to a payload capacity of 2.9g, which is ~ 2× the robots mass and a 114% increase. Finally, the model is validated in the dynamic regime (40-150 Hz) and the merits of a second order linear approximation are discussed.

Bio-inspired mechanisms for inclined locomotion in a legged insect-scale robot

Legged locomotion is an open problem in robotics, particularly for non-level surfaces. With decreasing robot size, different issues for climbing mechanisms and their attachment and detachment appear due to the physics of scaling. This paper describes micro-scale phenomena for different adhesion methods that can be employed in microrobots. These adhesion methods are applied to a sub-2 gram legged robot, the Harvard Ambulatory MicroRobot (HAMR), by leveraging recent advances in milli- and micrometer-scale manufacturing. The presented designs utilize different passively oriented adhesives on the legs of the robot to improve inclined locomotion performance. A 3DoF ankle joint is designed and implemented and the effects of a passive tail are studied. As a result, HAMRs climbing capability is increased from 3° inclines to 22° inclines and 45° declines. Finally, an analytical model of leg and foot force generation is presented and compared with experimental force data from the attachment mechanism on a single-leg experimental setup.

Feedback control of a legged microrobot with on-board sensing

Full autonomy remains a challenge for miniature robotic platforms due to mass and size requirements of on-board power and control electronics. This paper presents a solution to these challenges with a 2.3g autonomous legged robot. An off-the-shelf optical mouse sensor is adapted for use on the Harvard Ambulatory Microrobot (HAMR) by reducing the sensor weight by 36% and achieving a position error below 11% when suspended 3mm above a cardstock surface. The position data is combined with data from a gyroscope for feedback control of both position and orientation. A microcontroller processes the sensor data and commands a controlled gait to HAMR that is powered by a battery, a boost converter and high voltage drive electronics. Solar cells are used as an alternative source providing enough power for autonomous operation of the robot. The resulting deviation for a controlled straight-line walk using both sensors to minimize lateral deviation and angular error is only 4.6%, compared to an error of 31% in an uncontrolled, straight-line walk.

The milliDelta: A high-bandwidth, high-precision, millimeter-scale Delta robot

A high-bandwidth, millimeter-scale Delta robot offers precise control for microscale applications. Delta robots have been widely used in industrial contexts for pick-and-place applications because of their high precision and speed. These qualities are also desirable at the millimeter scale for applications such as vibration cancellation in microsurgery and microassembly or micromanipulation. Developing a millimeter-scale Delta robot that maintains the characteristic input-output behavior and operates with high speed and precision requires overcoming manufacturing and actuation challenges. We present the design, fabrication, and characterization of an adapted Delta robot at the millimeter scale (the “milliDelta”) that leverages printed circuit microelectromechanical system manufacturing techniques and is driven by three independently controlled piezoelectric bending actuators. We validated the design of the milliDelta, where two nonintersecting perpendicular revolute joints were used to replace an ideal universal joint. In addition, a transmission linkage system for actuation was introduced to the laminate structure of the milliDelta. This 15 millimeter–by–15 millimeter–by–20 millimeter robot has a total mass of 430 milligrams and a payload capacity of 1.31 grams and operates with precision down to ~5 micrometers in a 7.01-cubic-millimeter workspace. In addition, the milliDelta can follow periodic trajectories at frequencies up to 75 hertz, experiencing velocities of ~0.45 meters per second and accelerations of ~215 meters per squared second. We demonstrate its potential utility for high-bandwidth, high-precision applications that require a compact design.

Phase control for a legged microrobot operating at resonance

We present an off-board phase estimator and controller for leg position near the resonance of the Harvard Ambulatory MicroRobots (HAMR) two degree-of-freedom transmission. This control system is a first step towards leveraging the significant increase in stride length at transmission resonance for faster and more efficient locomotion. We experimentally characterize HAMRs transmission and determine that actuator phase is a sufficient proxy for leg phase across the range of useful operating frequencies (1–120Hz). An estimator is developed to determine actuator phase using off-board position sensors and it converges within a cycle on average. We also fit a nonlinear dynamic model of the transmission to the experimental data, and utilize the model to determine a suitable open-loop resonant leg trajectory and define feed forward control inputs. This resonant (100Hz) trajectory is theoretically 50% more efficient than pre-resonant high speed running trajectories. The controller converges to this trajectory in 0.05 ± 0.02 seconds (5.3 ± 2.4 cycles) in air, and in 0.05 ± 0.01 seconds (4.7 ± 0.6 cycles) under perturbations that approximate ground contact.

High speed trajectory control using an experimental maneuverability model for an insect-scale legged robot

This paper presents an off-board trajectory controller for a range of stride frequencies (2–45 Hz) that enables zero-radius turns and holonomic control on one of the smallest and fastest legged robots, the Harvard Ambulatory MicroRobot (HAMR). An experimental model is used as the basis for control to capture the highly nonlinear response of the robot to input signals. Closed-loop trajectories are performed with an RMS position error at or below 0.3 body lengths (BL) using gaits at speeds up to 6.5 BL/s (29.4 cm/s) for straight-line and sinusoidal trajectories.

Controllable water surface to underwater transition through electrowetting in a hybrid terrestrial-aquatic microrobot

Several animal species demonstrate remarkable locomotive capabilities on land, on water, and under water. A hybrid terrestrial-aquatic robot with similar capabilities requires multimodal locomotive strategies that reconcile the constraints imposed by the different environments. Here we report the development of a 1.6 g quadrupedal microrobot that can walk on land, swim on water, and transition between the two. This robot utilizes a combination of surface tension and buoyancy to support its weight and generates differential drag using passive flaps to swim forward and turn. Electrowetting is used to break the water surface and transition into water by reducing the contact angle, and subsequently inducing spontaneous wetting. Finally, several design modifications help the robot overcome surface tension and climb a modest incline to transition back onto land. Our results show that microrobots can demonstrate unique locomotive capabilities by leveraging their small size, mesoscale fabrication methods, and surface effects.Some animals have multimodal locomotive capabilities to survive in different environments. Inspired by nature, Chen et al. build a centimeter-scaled robot that is capable of walking on water, underwater, on land, and transiting among all three, whose ‘feet’ break water by modifying surface tension.