10 research outputs found
Contact-Implicit Trajectory Optimization Based on a Variable Smooth Contact Model and Successive Convexification
In this paper, we propose a contact-implicit trajectory optimization (CITO)
method based on a variable smooth contact model (VSCM) and successive
convexification (SCvx). The VSCM facilitates the convergence of gradient-based
optimization without compromising physical fidelity. On the other hand, the
proposed SCvx-based approach combines the advantages of direct and shooting
methods for CITO. For evaluations, we consider non-prehensile manipulation
tasks. The proposed method is compared to a version based on iterative linear
quadratic regulator (iLQR) on a planar example. The results demonstrate that
both methods can find physically-consistent motions that complete the tasks
without a meaningful initial guess owing to the VSCM. The proposed SCvx-based
method outperforms the iLQR-based method in terms of convergence, computation
time, and the quality of motions found. Finally, the proposed SCvx-based method
is tested on a standard robot platform and shown to perform efficiently for a
real-world application.Comment: Accepted for publication in ICRA 201
Plan-Guided Reinforcement Learning for Whole-Body Manipulation
Synthesizing complex whole-body manipulation behaviors has fundamental
challenges due to the rapidly growing combinatorics inherent to contact
interaction planning. While model-based methods have shown promising results in
solving long-horizon manipulation tasks, they often work under strict
assumptions, such as known model parameters, oracular observation of the
environment state, and simplified dynamics, resulting in plans that cannot
easily transfer to hardware. Learning-based approaches, such as imitation
learning (IL) and reinforcement learning (RL), have been shown to be robust
when operating over in-distribution states; however, they need heavy human
supervision. Specifically, model-free RL requires a tedious reward-shaping
process. IL methods, on the other hand, rely on human demonstrations that
involve advanced teleoperation methods. In this work, we propose a plan-guided
reinforcement learning (PGRL) framework to combine the advantages of
model-based planning and reinforcement learning. Our method requires minimal
human supervision because it relies on plans generated by model-based planners
to guide the exploration in RL. In exchange, RL derives a more robust policy
thanks to domain randomization. We test this approach on a whole-body
manipulation task on Punyo, an upper-body humanoid robot with compliant,
air-filled arm coverings, to pivot and lift a large box. Our preliminary
results indicate that the proposed methodology is promising to address
challenges that remain difficult for either model- or learning-based strategies
alone.Comment: 4 pages, 4 figure
A Comparative Analysis of Contact Models in Trajectory Optimization for Manipulation
In this paper, we analyze the effects of contact models on contact-implicit
trajectory optimization for manipulation. We consider three different
approaches: (1) a contact model that is based on complementarity constraints,
(2) a smooth contact model, and our proposed method (3) a variable smooth
contact model. We compare these models in simulation in terms of physical
accuracy, quality of motions, and computation time. In each case, the
optimization process is initialized by setting all torque variables to zero,
namely, without a meaningful initial guess. For simulations, we consider a
pushing task with varying complexity for a 7 degrees-of-freedom robot arm. Our
results demonstrate that the optimization based on the proposed variable smooth
contact model provides a good trade-off between the physical fidelity and
quality of motions at the cost of increased computation time.Comment: 6 pages, 7 figures, 4 tables, IROS 2018 camera-ready versio
Transient performance of a vertical axis wind turbine
A coupled CFD/rotor dynamics modeling approach is presented for the analysis of realistic transient behavior of a height-normalized, three-straight-bladed VAWT subject to inertial effects of the rotor and generator load which is manipulated by a feedback control under standardized wind gusts. The model employs the k-ε turbulence model to approximate unsteady Reynolds-averaged Navier-Stokes equations and is validated with data from field measurements. As distinct from related studies, here, the angular velocity is calculated from the rotor's equation of motion; thus, the dynamic response of the rotor is taken into account. Results include the following: First, the rotor's inertia filters large amplitude oscillations in the wind torque owing to the first-order dynamics. Second, the generator and wind torques differ especially during wind transients subject to the conservation of angular momentum of the rotor. Third, oscillations of the power coefficient exceed the Betz limit temporarily due to the energy storage in the rotor, which acts as a temporary buffer that stores the kinetic energy like a flywheel in short durations. Last, average of transient power coefficients peaks at a smaller tip-speed ratio for wind gusts than steady winds
Hardware-in-the-loop simulations and control design for a small vertical axis wind turbine
Control design plays an important role in wind energy conversion systems in achieving high efficiency and performance. In this study, hardware-in-the-loop (HIL) simulations are carried out to design a maximum power point tracking (MPPT) algorithm for small vertical axis wind turbines (VAWTs). Wind torque is calculated and applied to an electrical motor that drives the generator in the HIL simulator, which mimics the dynamics of the rotor. To deal with disturbance torques in the HIL system, a virtual plant is introduced to obtain an error between the speeds in the HIL system and virtual plant. This error is used by a proportional-integral (PI) controller to generate a disturbance torque compensation signal. The MPPT algorithm is tested in the HIL simulator under various wind conditions, and the results are compared with numerical simulations. The HIL simulator successfully mimics the dynamics of the VAWT under various wind conditions and provides a realistic framework for control designs
Küçük dikey eksenli rüzgâr türbini için basit kontrol tasarımı (Simple control design for a small vertical axis wind turbine)
Bu makalede, küçük dikey eksenli rüzgâr türbinin elde ettiği enerjiyi maksimize edecek basit bir kontrolör tasarlanmıştır. Bu önerilen kontrol algoritmasının amacı mevcut sistemlere kıyasla daha basit bir yapıda olmasıdır. Algoritma kontrol işlemini sisteme uygulanan yük katsayısını önceden belirlenen değer aralıklarında müdahalede bulunarak yapabilmektedir. Bunu yapmak için önceden enerjiyi maksimize eden bir optimizasyon yöntemiyle belirlenmiş olan sınır değerlerinden faydalanmaktadır. Bu makalede, değişik simülasyonlar sonucu elde edilen enerjiyi maksimize ederken, basitleştirilmiş bir dikey eksenli rüzgâr türbini modeli kullanılmıştır
Model predictive control for energy maximization of small vertical axis wind turbines
In this paper, a model predictive control (MPC) approach is presented to maximize the energy generated by a small vertical axis wind turbine (VAWT) subject to current and voltage constraints of electrical and power electronic
components. Our method manipulates a load coefficient and optimizes the control trajectory over a prediction horizon such that a cost function that measures the deviation from the maximum available energy and the violation of current and
voltage constraints is minimized. Simplified models for the VAWT and a permanent magnet generator have been used. A number of simulations have been carried out to demonstrate the performance of the proposed method at step and oscillatory wind conditions. Furthermore, impacts of the constraints on
energy generation have been investigated. Moreover, the performance of the MPC has been compared with a typical maximum power point tracking algorithm in order to show that maximizing the instantaneous power does not mean maximizing the energy; and simulation results have shown that the MPC outperforms the maximum power point tracking algorithm in terms of generated energy by allowing deviations from the maximum power instantaneously for future gains in
energy generation