Spring (device)

Summary

A spring is a device consisting of an elastic but largely rigid material (typically metal) bent or molded into a form (especially a coil) that can return into shape after being compressed or extended. Springs can store energy when compressed. In everyday use the term often refers to coil springs, but there are many different spring designs. Modern springs are typically manufactured from spring steel. An example of a non-metallic spring is the bow, made traditionally of flexible yew wood, which when drawn stores energy to propel an arrow. When a conventional spring, without stiffness variability features, is compressed or stretched from its resting position, it exerts an opposing force approximately proportional to its change in length (this approximation breaks down for larger deflections). The rate or spring constant of a spring is the change in the force it exerts, divided by the change in deflection of the spring. That is, it is the gradient of the force versus deflection

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Towards Dynamic Trot Gait Locomotion—Design, Control and Experiments with Cheetah-cub, a Compliant Quadruped Robot

Mostafa Ajallooeian, Auke Ijspeert, Alexandre Tuleu, Massimo Vespignani

We present robot design and results from locomotion experiments with a novel, compliant quadruped robot: Cheetah-cub. The robot's leg configuration is based on a spring-loaded, panthograph-mechanism with multiple segments. A dedicated open-loop, joint-space locomotion controller was derived and implemented. Experimentally, we found high speed and dynamic gaits, and evaluated the robot's locomotion characteristics. Experiments were run in simulation and in hardware on flat terrain, and at a step-down obstacle. The robot reached a running trot with maximum speed of 1.42m/s, in the hardware experiments. This speed corresponds to a Froude number of 1.3, or 6.9 body lengths per second. Besides typical control parameters, also morphological parameters such as the leg design played a role for maximum robot speed. Our robot platform has several advantages, especially when compared to larger and stiffer quadruped robot designs. 1) It is the fastest (Froude number) trotting quadruped robot of its kind, light-weight, compact, electrically powered, and made from many off-the-shelf components. 2) The robot shows self-stabilizing behavior at increasing robot speed, with open-loop control. 3) It is cheap, easy to reproduce, robust, and safe to handle. This makes it an excellent tool for research of multi-segment legs in quadruped robots.

2013

In this thesis, the development, design, fabrication and characterization of three innovative laser beam shaping, homogenizing and speckle reduction devices are reported. Typical beam shaping devices are static and not easily adaptable to a large variety of optical setups. Furthermore, the main issues when shaping coherent light are diffraction effects, speckle patterns, interferences and wavelength dependency. The devices presented in this work solve these issues. The main subjects discussed are beam shaping of coherent light and speckle reduction. Beam shaping and speckle reduction are important in many applications like laser machining, illumination, laser displays and photolithography. The first device presented is a dynamic diffuser and homogenizer for line generation. The 1D linear diffuser consists of a large single crystal silicon membrane of 5x5, 10x10 or 15x15 mm2 areas and having a thickness of 5 μm to 10 μm. This optical surface has a 100 % fill factor. The goal is to deform the membrane in one dimension only so that the light is also diffused in one dimension. The free standing continuous silicon membrane has a bridge type configuration. The actuation of the device is made electromagnetically at resonance by the use of the Lorentz force. Stiffening beams are fabricated below the membrane to decouple the 1D and 2D modes and to prevents 2D dynamic deformations. The device is fabricated on SOI wafer using standard microfabrication technologies. The devices are characterized using a laser doppler vibrometer (LDV) and a goniometer. The LDV measurements have shown that 1D and 2D modes can be excited separately. The diffusion angle is tunable between 0 to 22 mrad. The device is used to smooth out the interference effects created by a static 1D beam shaping optical element. The relative optical power efficiency of is 99.8 %. The device is capable to sustain high optical load of at least 140 W/cm2. It is now being tested in an industrial environment with high power CO2 laser for sealing glass capillaries. The second device presented is a 2D dynamic diffuser. It has to fulfill several characteristics, namely to diffuse light with small and tunable angle with high efficiency, reduce the speckle contrast and interference patterns, have a simple driving and packaging scheme and handle high optical power. A device with a 5x5 mm2 optical surface is designed, fabricated and characterized. The mirror is made of a thin and large deformable a-Si membrane which is supported by an array of posts. The disposition of the posts determines how the membrane is deformed in smaller sub-reflecting elements. A periodic and a pseudo random arrangements of the post arrays are designed. Electrostatic actuation is used to deform the membrane. The whole v deformable mirror is fabricated over a scanning stage. Out of plane resonant comb drive actuators are used to actuate the scanning stage. The diffusion of the light is performed by the deformation of the membrane while the homogenization of the beam profile is accomplished by the simultaneous scanning of the whole deformed membrane. Optical simulations are successfully used to predict the optical properties of the diffuser and to set the requirements for the electro-mechanical part of the device design. The fabrication process is using parylene-C as a refilling material and as a sacrificial layer to enable the fabrication of a 100 % fill factor deformable mirror. The membrane is continuous and release hole free. It is aluminum coated and showed an absolute optical power efficiency of 81 % in the actuated state. The high optical power handling of the device could not be tested. Nevertheless , the material of the device is silicon and the stage is connected with large springs to the frame for high heat conductivity. The pseudo random devices showed ability to shape an incoming beam regardless of its intensity profile into a output beam having a Gaussian profile. Besides, the diffusion angle is small and tunable between 0 to 10 mrad FWHM. The periodic devices is able to transform an incoming Gaussian beam into a super Gaussian intensity profile. Finally, the dynamic diffuser is capable of reducing the speckle contrast of a laser beam projected on a diffusing screen. The speckle contrast is reduced from 0.7 to 0.21 using the device alone and it is reduced to 0.11 when the dynamic diffuser is used in combination with a static diffuser. Tunable diffusion angle, smoothing of interferences, tunable beam shaping and speckle re- duction is realized while keeping the degree of freedom of the device to a minimum. By taking advantage of the optical design of the pseudo random diffuser, only one control electrode is needed to deform the membrane and one signal is used to scan the diffuser. The result is a very compact device, simple actuation and a straightforward packaging schemes. The last device presented is a very small footprint speckle reduction device. A simple actuation scheme using comb drive actuators and four folded springs enable large in plane displacement of a random phase plate. 180 μm of in plane displacement is achieved with a driving voltage as low as 29 V. The speckle contrast is reduced to 0.21 when the device is translating at 314 Hz. A comprehensive study of the diffuser characteristics is made to understand the surface parameters of the diffuser that influence directly the performances on the speckle reduction. The surface profile of the binary height has to be well defined to insure a phase shift of π. This very small device is a good candidate for integration in any portable device where speckle reduction is needed.

EPFL2012

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In this paper, we present a simple control framework for online push recovery on biped robots with dynamic stepping properties. Owing to relatively heavy legs in our humanoid robot COMAN, we use a linear model called 3LP, which is composed of three pendulums to take swing and torso dynamics into account. Based on 3LP equations, we formulate discrete linear quadratic regulator (LQR) controllers and use a particular time-projection method to adjust footstep locations during the motion continuously. This process, which is based on pelvis and swing foot tracking errors, naturally considers swing dynamics and leads to leg-retraction properties. Suggested adjustments are added to the Cartesian 3LP gaits and converted into joint-space trajectories through inverse kinematics. Fixed and adaptive foot lift strategies are also used to ensure enough ground clearance in perturbed walking conditions. The proposed control architecture is robust, yet uses very simple state estimation and basic position tracking. We rely on series elastic actuators to absorb impacts while introducing simple laws to compensate for spring compressions. Extensive experiments on COMAN (real) and Atlas (simulated) robots demonstrate the functionality of different control blocks and prove the effectiveness of time-projection in extreme push recovery scenarios. We also show self-produced and emergent walking gaits when the robot is subject to continuous dragging forces. These gaits feature dynamic walking robustness with minimal reliance on the ankles and avoiding any active zero moment point (ZMP) control. The proposed architecture is therefore generic, computationally very fast and yet with no critical parameter to tune.

SAGE PUBLICATIONS LTD2019

EPFL2012