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Configurable high-performance bias current reference circuits are useful in complex mixed-signal chips. This paper presents the design of a configurable current reference array with ultra wide dynamic range (DR). A coarse-fine architecture using octal coarse current spacing and 8 bits of fine resolution increases the overall current DR with less area compared with the prior work. Compact current multipliers and dividers also save chip areas. Shifted-source current mirrors and an off-current suppression technique improve the accuracy of generated low currents. A buffer with dual-threshold source followers is used to generate the output biasing voltage with a wide DR input current. Biases are individually addressable and configurable. Measurement results of this design in UMC 0.18μm 1P6M CMOS process suggest that over 170dB DR is achieved at room temperature. Each additional bias occupies an incremental area of 360×22μm2, which is smaller by a factor of 4 compared to the previous design.
Posted on: August 27, 2012
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In this work we apply optimal control to create running gaits for the model of an electrically driven one legged hopper, and compare the results obtained for five different objective functions. By using high compliant series elastic actuators, the motions of joint and motor are decoupled, which allows the exploitation of natural dynamics. Depending on the cost function, this exploitation varies. Energy is injected at different points of time, the amplitude of actuator action changes significantly, and the optimal gear ratios differ by a factor of two. Variations are, however, comparable over a wide range of hopping heights and running velocities. Purely force-based cost functions prove to be ill-suited for such non-conservative systems, and it is shown that thermal electrical losses, in contrast to common belief, do not dominate energy expenditure. The numerical results are corroborated by detailed analytical considerations which give general insights into optimal excitation with electric actuators.
Posted on: August 27, 2012
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This paper introduces the concept of hybrid operational space control, a method that unifies kinematic tracking of individual joints with an inverse dynamics task space controller for the remainder of the robot. The proposed control strategy allows for a hierarchical task decomposition while simultaneously regulating the inner forces between the contact points. At the same time it improves fast tracking for compliant systems by means of appropriate low level position controllers. Introducing StarlETH, a compliant quadrupedal robot, the applicability of the controller and the hardware is demonstrated in realtime simulations and hardware experiments. We perform static walking in challenging terrain and show how the controller can combine precise and fast position control with robust and compliant interaction with the environment.
Posted on: August 27, 2012
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This paper introduces a state estimation framework for legged robots that allows estimating the full pose of the robot without making any assumptions about the geometrical structure of its environment. This is achieved by means of an Observability Constrained Extended Kalman Filter that fuses kinematic encoder data with on-board IMU measurements. By including the absolute position of all footholds into the filter state, simple model equations can be formulated which accurately capture the uncertainties associated with the intermittent ground contacts. The resulting filter simultaneously estimates the position of all footholds and the pose of the main body. In the algorithmic formulation, special attention is paid to the consistency of the linearized filter: it maintains the same observability properties as the nonlinear system, which is a prerequisite for accurate state estimation. The presented approach is implemented in simulation and validated experimentally on an actual quadrupedal robot.
Posted on: August 27, 2012
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This paper introduces StarlETH, a compliant quadrupedal robot that is designed to study fast, efficient, and versatile locomotion. The platform is fully actuated with high compliant series elastic actuation, making the system torque controllable and at the same time well suited for highly dynamic maneuvers. We additionally emphasize key elements of a powerful real time control and simulation environment. The work is concluded with a number of experiments that demonstrate the performance of the presented hardware and controllers.
Posted on: August 27, 2012
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We describe a process for enabling quadrocopters to perform and improve upon aerobatic maneuvers. We describe such maneuvers as a set of desired keyframes and a parametrized input trajectory. The full state trajectory of the vehicle is left unspecified – only predefined partial-state keyframes are used to measure errors and to refine the primitive. A first-principles model is used to find nominal trajectory parameter values and a first-order correction matrix. We apply this method to extending previous work on vertical-plane 2D adaptive flips to a fully 3D adaptive maneuver. We also show how this method can be applied to finding trajectories for flips with matching non-zero initial and final velocities. Preliminary results are presented from simulation and from quadrocopters in the ETH Flying Machine Arena.
Posted on: August 23, 2012
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Swimming microrobots have the potential to be used in medical applications such as targeted drug delivery. The challenges for navigating microrobots in the human body lie not only in the viscosity of body fluids but also in the existence of different types of fibers and cells such as blood cells or protein strands. This paper investigates artificial bacterial flagella (ABFs), which are helical microrobots actuated by an external magnetic field, in methyl cellulose solutions of different concentrations. It can be shown that the microrobots can be propelled in these gel-like heterogeneous solutions and successful swimming was demonstrated in solutions with a viscosity of more than 20 times that of water. Furthermore, results indicate that the existence of fibers can help ABFs swim more effectively, which agrees with previous experimental results reported for natural bacteria.
Posted on: August 23, 2012
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A simple and general fabrication method for helical swimming micromachines by direct laser writing and e-beam evaporation is demonstrated. The magnetic helical devices exhibit varying magnetic shape anisotropy, yet always generate corkscrew motion using a rotating magnetic field. They also exhibit good swimming performance and are capable of pick-and-place micromanipulation in 3D. Cytotoxicity of the devices was investigated using mouse myoblasts. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Posted on: August 23, 2012
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We present a conceptually and computationally lightweight method for the design and iterative learning of fast maneuvers for quadrocopters. We use first-principles, reduced-order models and we do not require nor make an attempt to follow a specific state trajectory-only the initial and the final states of the vehicle are taken into account. We evaluate the adaptation scheme through experiments on quadrocopters in the ETH Flying Machine Arena that perform multi-flips and other high-performance maneuvers.
Posted on: August 23, 2012
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During natural locomotion, the stiffness of the human knee is modulated continuously and subconsciously according to the demands of activity and terrain. Given modern actuator technology, powered transfemoral prostheses could theoretically provide a similar degree of sophistication and function. However, experimentally quantifying knee stiffness modulation during natural gait is challenging. Alternatively, joint stiffness could be estimated in a less disruptive manner using electromyography (EMG) combined with kinetic and kinematic measurements to estimate muscle force, together with models that relate muscle force to stiffness. Here we present the first step in that process, where we develop such an approach and evaluate it in isometric conditions, where experimental measurements are more feasible. Our EMG-guided modeling approach allows us to consider conditions with antagonistic muscle activation, a phenomenon commonly observed in physiological gait. Our validation shows that model-based estimates of knee joint stiffness coincide well with experimental data obtained using conventional perturbation techniques. We conclude that knee stiffness can be accurately estimated in isometric conditions without applying perturbations, which presents an important step toward our ultimate goal of quantifying knee stiffness during gait.
Posted on: August 23, 2012
