Deepak Trivedi

Soft robotics: Biological inspiration, state of the art, and future research

Traditional robots have rigid underlying structures that limit their ability to interact with their environment. For example, conventional robot manipulators have rigid links and can manipulate objects using only their specialised end effectors. These robots often encounter difficulties operating in unstructured and highly congested environments. A variety of animals and plants exhibit complex movement with soft structures devoid of rigid components. Muscular hydrostats e.g. octopus arms and elephant trunks are almost entirely composed of muscle and connective tissue and plant cells can change shape when pressurised by osmosis. Researchers have been inspired by biology to design and build soft robots. With a soft structure and redundant degrees of freedom, these robots can be used for delicate tasks in cluttered and/or unstructured environments. This paper discusses the novel capabilities of soft robots, describes examples from nature that provide biological inspiration, surveys the state of the art and outlines existing challenges in soft robot design, modelling, fabrication and control.

Geometrically Exact Models for Soft Robotic Manipulators

Unlike traditional rigid linked robots, soft robotic manipulators can bend into a wide variety of complex shapes due to control inputs and gravitational loading. This paper presents a new approach for modeling soft robotic manipulators that incorporates the effect of material nonlinearities and distributed weight and payload. The model is geometrically exact for the large curvature, shear, torsion, and extension that often occur in these manipulators. The model is based on the geometrically exact Cosserat rod theory and a fiber reinforced model of the air muscle actuators. The model is validated experimentally on the OctArm V manipulator, showing less than 5% average error for a wide range of actuation pressures and base orientations as compared to almost 50% average error for the constant-curvature model previously used by researchers. Workspace plots generated from the model show the significant effects of self-weight on OctArm V.

Geometrically exact dynamic models for soft robotic manipulators

Unlike traditional rigid-linked robots, soft robotic manipulators can bend into a wide variety of complex shapes due to control inputs and gravitational loading. This paper presents a new approach for modeling the dynamics of soft robotic manipulators that incorporates the effect of material nonlinearities and distributed and payload weight and is geometrically exact for the large curvature, shear, torsion and extension that often occur in these manipulators. The model is based on the general Cosserat theory of rods and a fiber reinforced model of air muscle actuators. The model is validated experimentally on the OctArm V manipulator, showing less that 5% average error for a wide range of actuation pressures and base orientations as compared to almost 50% average error for the constant curvature model previously used by researchers.

Optimal, Model-Based Design of Soft Robotic Manipulators

Soft robotic manipulators, unlike their rigid-linked counterparts, deform continuously along their lengths similar to elephant trunks and octopus arms. Their excellent dexterity enables them to navigate through unstructured and cluttered environments and to handle fragile objects using whole arm manipulation. This paper develops optimal designs for OctArm manipulators, i.e., multisection, trunklike soft arms. OctArm manipulator design involves the specification of air muscle actuators and the number, length, and configuration of sections that maximize dexterity and load capacity for a given maximum actuation pressure. A general method of optimal design for OctArm manipulators using nonlinear models of the actuators and arm mechanics is developed. The manipulator model is based on Cosserat rod theory, accounts for large curvatures, extensions, and shear strains, and is coupled to the nonlinear Mooney-Rivlin actuator model. Given a dexterity constraint for each section, a genetic algorithm-based optimizer maximizes the arm load capacity by varying the actuator and section dimensions. The method generates design rules that simplify the optimization process. These rules are then applied to the design of pneumatically and hydraulically actuated OctArm manipulators using 100 psi and 1000 psi maximum pressures, respectively.

Model-Based Shape Estimation for Soft Robotic Manipulators: The Planar Case

Soft robotic manipulators are continuum robots made of soft materials that undergo continuous elastic deformation and produce motion with a smooth backbone curve. In many applications, these manipulators offer significant advantages over traditional manipulators due to their ability to conform to their surroundings, and manipulate objects of widely varying size using whole arm manipulation. Theoretically, soft robots have infinite degrees of freedom (DOF), but the number of sensors and actuators are limited. Many DOFs of soft robots are not directly observable and/or controllable, complicating shape estimation and control. In this paper, we present three methods of shape sensing for soft robotic manipulators based on a geometrically exact mechanical model. The first method uses load cells mounted at the base of the manipulator, the second method makes use of cable encoders running through the length of the manipulator, and the third method uses inclinometers mounted at the end of each section of the manipulator. Simulation results show an endpoint localization error of less than 3% of manipulator length with typical sensors. The methods are validated experimentally on the OctArm VI manipulator.

Vortex Induced Flutter in Compliant Plate Seals

This paper investigates the possibility of vortex induced flutter in GE compliant plate seals (CPS). The CPS consists of a ring of slotted compliant plates attached circumferentially around the rotor, with an intermediate plate seated in the slot. Experiments show that the compliant plates vibrate in the flow field with amplitude that is a function of the differential pressure applied across the seal. The vibrations are caused by potentially multiple flow induced vibration mechanisms operating during different flow regimes. This work focuses on the dynamic instabilities that may be caused by the vortices shed by individual compliant plates and the intermediate plate. We model the compliant plate seal as a ring of a large number of locally coupled oscillators, with nonlinear stiffness arising from hydrostatic feedback. A two-way coupling exists between the structural and wake dynamics, leading to the phenomenon of “lock-in” between the wake and successive modes of seal vibration as the flow velocity is increased. Using eigenvalue analysis, we obtain the transition boundaries that divide the parameter space into sets of regions with positive and negative damping, corresponding to boundaries of onset and end of vortex induced flutter. Based on averaged equations, the amplitude of the limit cycles of the structure and wake dynamics and the phase between them is determined. It is found that the nonlinearities in seal stiffness and the nature of coupling between the compliant plates have a significant effect on the stability boundaries.Copyright

Shape Sensing for Soft Robotic Manipulators

Soft robotic manipulators are continuum robots made of soft materials that undergo continuous elastic deformation and produce motion with a smooth backbone curve. These manipulators offer significant advantages over traditional manipulators due to their ability to conform to their surroundings, move with dexterity and manipulate objects of widely varying size using whole arm manipulation. Theoretically, soft robots have infinite degrees of freedom (dof), but the number of sensors and actuators are limited. Many dofs of soft robots are not directly observable and/or controllable, complicating shape sensing and controlling. In this paper, we present two methods of shape sensing for soft robotic manipulators based on a geometrically exact mechanical model. The first method use s load cells mounted at the base of the manipulator and the second method makes use of cable encoders running through the length of the manipulator. Simulation results show an endpoint localization error of less than 3% of manipulator length.© 2009 ASME

Experimental Characterization of Variable Bristle Diameter Brush Seal Leakage, Stiffness and Wear

Brush seals are used in a wide variety of turbomachinery for sealing rotor-stator and stator-stator clearances. Application of traditional brush seals is limited by their life and performance at high differential pressures. GE’s patent-pending Variable Bristle Diameter (VBD) brush seal overcomes the limitations of the traditional brush seal by sandwiching a layer of fine bristles, with better sealing capability, between adjacent rows of stiffer bristles capable of withstanding larger differential pressure and flow disturbance. The General Electric VBD design uses thick bristles both in front and back rows. In addition to leakage performance, for successful design it is important to understand the force interactions between a brush seal bristle pack and the rotor. The important failure mechanisms to avoid include overheating and rotor dynamic instabilities caused by excessive brush seal forces. Brush seal stiffness, defined as brush seal force per unit circumferential length per unit incursion of the rotor, depends on the complex interaction of the pressure-dependent inter-bristle forces, the blow-down forces and the friction forces between the backplate and the bristle pack. Furthermore, brush seals exhibit different hysteresis and wear behavior under different pressure loading conditions. In this article, we present experimentally measured leakage, stiffness and wear characteristics of three different VBD brush seal designs subjected to a wide range of pressure loading.© 2013 ASME

Static Testing of Compliant Plate Seals

Self–correcting Compliant Plate Seals are being developed for various turbomachinery sealing applications including gas turbines, steam turbines, aircraft engines and oil & gas compressors. These seals consist of compliant plates attached to a stator in a circumferential fashion around the rotor. The compliant plates have a slot that extends radially inwards from the seal outer diameter, and an intermediate plate extends inwards into this slot from stator. This design is capable of providing passive hydrostatic feedback forces acting on the compliant plates that balance at a small tip–clearance. Due to this self–correcting behavior, this seal is capable of providing high differential pressure capability and low leakage within a limited axial span, and robust non–contact operation even in the presence of large rotor transients. In this paper we have described the testing of Compliant Plate Seals in a static leakage test rig (“shoebox” rig) to study the impact of different design parameters on leakage and vibration. A novel high–speed visualization set–up is described and the high–speed videos demonstrate robust non–contact operation for different assembly clearances, bridge–gaps and bridge–heights, for various differential pressure and pressure ratio conditions. The reported leakage results indicate that the leakage is relatively insensitive to assembly clearances due to the self–correcting behavior.Copyright

Dexterity and Workspace Analysis of Two Soft Robotic Manipulators

Soft robotic manipulators are continuum robots made of soft materials that undergo continuous elastic deformation and produce motion with a smooth backbone curve. These manipulators offer significant advantages over traditional manipulators due to their ability to conform to their surroundings, move with dexterity and manipulate objects of widely varying size using whole arm manipulation. Soft robotic manipulators are complex and difficult to design, model and fabricate. In this paper, we present a cost effective design for a pneumatic air muscle based soft robotic manipulator in which the actuators for the distal section extend from the base to the tip of the arm, thereby simplifying the pneumatic design and eliminating the need for endplates. We compare the workspace and dexterity of continuous tube (CT) design with a previously developed OctArm type manipulator and conclude that although the two designs have comparable workspace area, the OctArm workspace has better dexterity characteristics.Copyright