Ranjana Sahai

Towards a 3g crawling robot through the integration of microrobot technologies

This paper discusses the biomimetic design and assembly of a 3g self-contained crawling robot fabricated through the integrated use of various microrobot technologies. The hexapod structure is designed to move in an alternating tripod gait driven by two piezoelectric actuators connected by sliding plates to two sets of three legs. We present results of both the kinematic and static analyses of the driving mechanism that essentially consists of three slider cranks in series. This analysis confirmed the force differential needed to propel the device. We then review various other microrobot technologies that have been developed including actuator design and fabrication, power and control electronics design, programming via a finite state machine, and the development of bioinspired fiber arrays. These technologies were then successfully integrated into the device. The robot is now functioning and we have already fabricated three iterations of the proposed device. We hope with further design iterations to produce a fully operational model in the near future

Soft curvature sensors for joint angle proprioception

We introduce a curvature sensor composed of a thin, transparent elastomer film (polydimethylsiloxane, PDMS) embedded with a microchannel of conductive liquid (eutectic Gallium Indium, eGaIn) and a sensing element. Bending the sensor exerts pressure on the embedded microchannel via the sensing element. Deformation of the cross-section of the microchannel leads to a change in electrical resistance. We demonstrate the functionality of the sensor through testing on a finger joint. The film is wrapped around a finger with the sensing element positioned on top of the knuckle. Finger bending both stretches the elastomer and exerts pressure on the sensing element, leading to an enhanced change in the electrical resistance. Because the sensor is soft (elastic modulus E ∼ 1 MPa) and stretchable (>350%), it conforms to the host bending without interfering with the natural mechanics of motion. This sensor represents the first use of liquid-embedded elastomer electronics to monitor human or robotic motion.

Development of piezoelectric bending actuators with embedded piezoelectric sensors for micromechanical flapping mechanisms

This paper presents the fabrication and the testing of piezoelectric unimorph actuators with embedded piezoelectric sensors which are meant to be used for the actuation of the Micromechanical Flying Insect (MFI). First the fabrication process of a piezoelectric bending actuator comprising a standard unimorph and a rigid extension is described together with the advantages of adding such an extension. Then the convenience of obtaining an embedded piezoelectric sensor by a simple and inexpensive variation of the fabrication process is pointed out. A model for the sensor embedded into a unimorph actuator with rigid extension is derived together with its flat response band limits. Calibration steps are also outlined which allow, despite residual parasitic actuator-sensor coupling, the use of the actuator with the embedded sensor for measuring position and inertial forces when external mechanical structures are driven. An experiment is carried out which validates the model for the actuator/sensor device under desired operating conditions. Preliminary application of the fabricated device to the MFI is also presented where the mechanical power fed into the wing is estimated.

Elastic Element Integration for Improved Flapping-Wing Micro Air Vehicle Performance

This paper studies flapping-wing micro air vehicles (FWMAV) whose transmission mechanisms use flexures as energy storage elements to reduce needed input power. A distinguishing feature of the proposed four-bar mechanism is the use of rubber-based flexures in two of its joints. These lightweight and compact flexures have been used for the first time in the design of an FWMAV whose projected total weight is approximately 3 g. This paper discusses in detail how the flexures were designed and how the challenges associated with their fabrication were met. Flexure stiffnesses were chosen based upon a simple, computationally efficient model of the four-bar mechanism actuated by an electric motor to flap two wings at 18 Hz. An instrumented test stand was designed to easily replace the upper part of the four-bar flexure mechanism and wings, and it was used to experimentally determine the power savings associated with flexures of different stiffnesses. While the measured power savings (maximum of 20%) may seem modest, they were nevertheless significant, considering that the use of the rubber-based flexures produced approximately 0.3 g added thrust at a less than 1% cost in weight (0.02 g).

Semi-automated micro assembly for rapid prototyping of a one DOF surgical wrist

We have developed new methods for the automated assembly of prototype structures and we illustrate them with the construction of a simple one DOF 5mm surgical wrist employing polyester flexures instead of revolute joints. The first step in the structural assembly involves the construction of hollow stainless steel triangular beams that are used for the rigid elements of the structure. It includes the development of a folding fixture to bend stainless steel sheets and the determination of a folding angle sequence by static analysis using a compliant mechanism model. The semi-automatic process of using a millirobot (orthotweezers) to manipulate and assemble the beams and attach the flexures is described in detail. The paper ends with a description of the procedure used to design the wrist.

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.

A flapping-wing micro air vehicle with interchangeable parts for system integration studies

This paper describes the development of a unique flapping-wing micro air vehicle (FWMAV) whose major components, i.e. the motor, transmission mechanisms, and wings, are rapidly interchangeable. When coupled with a test stand that includes a 6-axis force sensor, encoder, power-recording capabilities, and high speed video, the result is a highly versatile experimental platform on which system integration studies can be conducted. This paper provides a detailed description of the design and fabrication of this FWMAV whose interchangeability of parts is mostly accomplished through a novel system of tabs, slots, and retaining rods. Results of a study on energy saving elements in the transmission mechanism as well as an exploration of this effect for different wing sizes are also presented. Finally, the implications of interchangeable parts on the creation of customizable flyers are discussed.

A thermal slip sensor for biorobotic applications

This paper presents the design of a novel sensor for slip detection. It consists of an easily fabricated miniaturized thermal probe that senses the additional convective heat transfer associated with the occurrence of mechanical slip. The fabrication procedures and the operating principle for the device are described in detail. A simple experimental setup was used to test the effectiveness of the proposed device. Tests were performed with varying velocities on four materials of differing thermal properties and surface roughnesses. The results show that slip can be effectively detected by the proposed sensor with a response times which can be as low as 6.3 ms. The performance of the device can be further improved when used in conjunction with a separate pressure sensor and by using more accurate methods of electrical resistance measurement.

Carbon Fiber Components with Integrated Wiring for Millirobot Prototyping

We are developing a process to quickly prototype millirobotic systems in which the approach is to identify and develop a construction kit for fabricating almost any design, similar to the kits that are available for larger-scale robots. Two of the basic elements of the kit, the links and flexure joints, have been identified, and an assembly method has been developed. This paper deals with the problem of integrating the wiring in these parts, a significant task on this size scale. This novel feature is achieved through the use of a flexible ribbon cable consisting of three wires made out of patterned copper foil and polyimide. Low melting point solder has been tested successfully to make the electrical interconnect between the parts. We discuss the issues that must be addressed in designing the flexure-wiring combination. In addition, the paper presents the methodology for fabricating structures with integrated wiring using a simple four bar mechanism as an example. Finally, the tests show that the wiring loop over a flexure connecting a distally located sensor on the mechanism maintains both its electrical and mechanical integrity even during large motions. Future work will include the automated assembly of the parts with a low cost assembly tool.

Development of a 3.2g untethered flapping-wing platform for flight energetics and control experiments

This paper presents a biologically inspired, 3.2g untethered vehicle capable of both active (flapping) and passive (gliding) flight. We discuss the overall vehicle design, as well as its validation with thrust data from benchtop testing, simulation, and flight test results. The vehicle has one pair of flapping wings for thrust generation, making it a good analogue for insects of the same scale. Flight energetics and control can be thoroughly explored through the array of simulation and testing that have been implemented. Integrated electronics provide wireless communication, sensing, and basic open-loop flight control, making flight test iteration fast and providing additional dynamics data. All of the testing setups and the physical vehicle working together have created a robust development environment for future iterations on the vehicle. The successful flight of the vehicle, including the data collection from onboard sensors and an external motion capture arena, show that this platform is ideal to study flight energetics and control schemes at an insect scale.