Arvind Ananthanarayanan

Towards a bio-inspired leg design for high-speed running

High-speed terrestrial locomotion inevitably involves high acceleration and extensive loadings on the legs. This imposes a challenging trade-off between weight and strength in leg design. This paper introduces a new design paradigm for a robotic leg inspired by musculoskeletal structures. The central hypothesis is that employing a tendon-bone co-location architecture not only provides compliance in the leg, but can also reduce bone stresses caused by bending on structures. This hypothesis is applied to a leg design, and verified by simulations and the experiments on a prototype. In addition, we also present an optimization scheme to maximize the strength to weight ratio. Using the tendon-bone co-location architecture, the stress on the bone during a stride is reduced by up to 59%. A new foam-core prototyping technique enables creating structural characteristics similar to mammalian bones in the robotic leg. This method allows us to use lighter polymeric structures that are cheaper and quicker to fabricate than conventional fabrication methods, and can eventually greatly shorten the design iteration cycle time.

Integrated Product and Process Design for a Flapping Wing Drive Mechanism

Successful realization of a flapping wing micro-air vehicle (MAV) requires development of a light weight drive mechanism that can convert the continuous rotary motion of the motor into oscillatory flapping motion of the wings. The drive mechanism should have low weight to maximize the payload and battery capacity. It should also have high power transmission efficiency to maximize the operational range and to minimize weight of the motor. In order to make flapping wing MAVs attractive in search, rescue, and recovery efforts, they should be disposable from the cost point of view. Injection molded compliant drive mechanisms are an attractive design option because of manufacturing scalability and reduction in the number of parts. However, realizing compliant drive mechanism using injection molding requires use of multipiece multigate molds. Molding process constraints need to be considered during the design stage to successfully realize the drive mechanism. This paper describes an approach for determining the drive mechanism shape and size that meets both the design and molding requirements. The novel aspects of this work include (1) minimizing the number of mold pieces and (2) the use of sacrificial shape elements to reduce the impact of the weld-lines on the structural performance. The design generated by the approach described in this paper was utilized to realize an operational flapping wing MAV.

A SYSTEMATIC METHODOLOGY FOR ACCURATE DESIGN-STAGE ESTIMATION OF ENERGY CONSUMPTION FOR INJECTION MOLDED PARTS

Today’s ubiquitous use of plastics in product design and manufacturing presents significant environmental and human health challenges. Injection molding, one of the most commonly used processes for making plastic products, consumes a significant amount of energy. A methodology for accurately estimating the energy consumed to injection-mold a part would enable environmentally conscious decision making during the product design. Unfortunately, only limited information is available at the design stage. Therefore, accurately estimating energy consumption before the part has gone into production can be challenging. In this paper, we describe a methodology for energy estimation that works with the limited amount of data available during the design stage, namely the CAD model of the part, the material name, and the production requirements. The methodology uses this data to estimate the parameters of the runner system and an appropriately sized molding machine. It then uses these estimates to compute the machine setup time and the cycle time required for the injection molding operation. This is done by appropriately abstracting information available from the mold flow simulation tools and analytical models that are traditionally used during the manufacturing stage. These times are then multiplied by the power consumed by the appropriately sized machine during each stage of the molding cycle to compute the estimated energy consumption per part.© 2010 ASME

A compact two DOF magneto-elastomeric force sensor for a running quadruped

This paper presents a novel design approach for a two-DOF foot force sensor for a high speed running quadruped. The adopted approach harnesses the deformation property of an elastomeric material to relate applied force to measurable deformation. A lightweight, robust and compact magnetic-field based sensing system, consisting of an assembly of miniature hall-effect sensors, is employed to infer the positional information of a magnet embedded in the elastomeric material. Instead of solving two non-linear models (magnetic field and elastomeric) sequentially, a direct approach of using artificial neural networks (ANN) is utilized to relate magnetic flux density (MFD) measurements to applied forces. The force sensor, which weighs only 24.5 gms, provides a measurement range of 0 - 1000 N normal to the ground and up to ± 125N parallel to the ground. The mean force measurement accuracy was found to be within 7% of the applied forces. The sensor designed as part of this work finds direct applications in ground reaction force sensing for a running quadrupedal robot.

Fabrication of Highly Articulated Miniature Snake Robot Structures Using In-Mold Assembly of Compliant Joints

Snake inspired robots have promising applications in minimally invasive surgery. However one of the most significant challenges, which presents itself as a roadblock to large scale exploitation of this technology, is the high manufacturing costs associated with fabricating highly articulated miniature structures. We present a method to fabricate such highly articulated miniature snake robots using a modular mold design. This design combines the benefits of in-mold assembly and insert molding to fabricate highly articulating miniature structures. The experimental results demonstrate the feasibility of the modular mold design for making snake robot chains. This paper also discusses a mathematical framework which can be used to optimize the size of the compliant links and thereby the overall cross section of the snake robot chain that can be manufactured using the modular mold design.

Design of Revolute Joints for In-Mold Assembly Using Insert Molding.

Creating highly articulated miniature structures requires assembling a large number of small parts. This is a very challenging task and increases cost of mechanical assemblies. Insert molding presents the possibility of creating a highly articulated structure in a single molding step. This can be accomplished by placing multiple metallic bearings in the mold and injecting plastic on top of them. In theory, this idea can generate a multi degree of freedom structures in just one processing step without requiring any post molding assembly operations. However, the polymer material has a tendency to shrink on top of the metal bearings and hence jam the joints. Hence, until now insert molding has not been used to create articulated structures. This paper presents a theoretical model for estimating the extent of joint jamming that occurs due to the shrinkage of the polymer on top of the metal bearings. The level of joint jamming is seen as the effective torque needed to overcome the friction in the revolute joints formed by insert molding. We then use this model to select the optimum design parameters which can be used to fabricate functional, highly articulating assemblies while meeting manufacturing constraints. Our analysis shows that the strength of weld-lines formed during the in-mold assembly process play a significant role in determining the minimum joint dimensions necessary for fabricating functional revolute joints. We have used the models and methods described in this paper to successfully fabricate the structure for a minimally invasive medical robot prototype with potential applications in neurosurgery. To the best of our knowledge, this is the first demonstration of building an articulated structure with multiple degrees of freedom using insert molding.

Design of Revolute Joints for Insert Molding: A Step Towards Realizing Low Cost Highly Articulated Robot Structures

Creating highly articulated miniature robotic structures requires assembling a large number of small parts. This is a very challenging task and increases cost of the robots. Insert molding presents the possibility of creating a highly articulated structure in a single molding step. This can be accomplished by placing multiple metallic bearings in the mold and injecting plastic on top of them. In theory, this idea can generate a multi degree of freedom robot in just one processing step without requiring any post molding assembly operations. However, the polymer material has a tendency to shrink on top of the metal bearings and hence jam the joints. Hence, until now insert molding has not been used to create articulated structures. This paper presents a theoretical model for estimating the extent of joint jamming that occurs due to the shrinkage of the polymer on top of the metal bearings. The level of joint jamming is seen as the effective torque needed to overcome the friction in the revolute joints formed by insert molding. We then use this model to select the optimum design parameters which can be used to fabricate functional robotic assemblies while meeting manufacturing constraints. Our analysis shows that the strength of weld-lines formed during the in-mold assembly process play a significant role in determining the minimum joint dimensions necessary for fabricating functional revolute joints. We have used the models and methods described in this paper to successfully fabricate the structure for a minimally invasive medical robot prototype with potential applications in neurosurgery. To the best of our knowledge this is first demonstration of building an articulated structure with multiple degrees of freedom using insert molding.Copyright

Modeling and Characterization to Minimize Effects of Melt Flow Fronts on Premolded Component Deformation During In-Mold Assembly of Mesoscale Revolute Joints

In-mold assembly can be used to create mesoscale articulating polymeric joints that enable the miniaturization of devices, reduction in production costs, and increase in throughput. One of the major challenges in miniaturizing devices using the in-mold assembly is to develop appropriate characterization techniques and modeling approaches for the interaction between polymer melt flow fronts and premolded components. When a high speed, high temperature second stage melt comes in contact with a premolded mesoscale component that has similar melting temperatures, the premolded component can experience a significant plastic deformation due to the thermal softening and the force associated with impingement of the melt flow front. In our previous work, we developed methods to inhibit the plastic deformation by supporting the ends of the mesoscale premolded components. In this paper, we present an alternative strategy for controlling premolded component deformations. This involves a mesoscale in-mold assembly strategy that has a multigate mold design for bidirectional filling. This strategy permits in-mold assembly using polymers with comparable melting points. This paper demonstrates the technical feasibility of manufacturing in-mold-assembled mesoscale revolute joints using this bidirectional filling strategy. An experimental technique was developed for characterizing the transient impact force of the melt flow front on premolded components inside of a mold. The experimental data were used to validate a new computational model for predicting the effects of the melt flow front position in order to minimize the plastic deformation of premolded component using the bidirectional filling strategy. This paper also investigates the effects of the flow front position on the force applied on the premolded component and its corresponding plastic deformation.

Investigation of Revolute Joint Clearances Created by an In-Mold Assembly Process

Revolute joints are frequently used in articulated structures. Traditionally, such a joint is formed by assembling two components. As an alternative, revolute joints can be created inside the mold using an in-mold assembly process. This process eliminates the need for post-molding assembly, thus significantly reducing the cycle time and part count. The functional performance of a revolute joint depends on the clearance in the joint. The clearance in turn depends on the part shrinkage and the mold deformation during the molding process. The presence of a polymer part during the second molding stage makes an in-mold assembly process significantly different from the traditional molding process due to the difference in heat transfer and deformation characteristics. This paper presents experimental data and a preliminary model to explain the differences in clearance produced by an aluminum mold and an Aluminum mold with an Acrylonitrile butadiene styrene (ABS) insert. Our data indicates that there is a significant difference between the clearances observed from these two different types of molds. We believe that clearances produced depend strongly on the thermal history of the parts.