Efficient Model Learning for Human-Robot Collaborative Tasks

We present a framework for learning human user models from joint-action demonstrations that enables the robot to compute a robust policy for a collaborative task with a human. The learning takes place completely automatically, without any human intervention. First, we describe the clustering of demonstrated action sequences into different human types using an unsupervised learning algorithm. These demonstrated sequences are also used by the robot to learn a reward function that is representative for each type, through the employment of an inverse reinforcement learning algorithm. The learned model is then used as part of a Mixed Observability Markov Decision Process formulation, wherein the human type is a partially observable variable. With this framework, we can infer, either offline or online, the human type of a new user that was not included in the training set, and can compute a policy for the robot that will be aligned to the preference of this new user and will be robust to deviations of the human actions from prior demonstrations. Finally we validate the approach using data collected in human subject experiments, and conduct proof-of-concept demonstrations in which a person performs a collaborative task with a small industrial robot

Transport properties of the layered Rh oxide K_0.49RhO_2

We report measurements and analyses of resistivity, thermopower and Hall coefficient of single-crystalline samples of the layered Rh oxide K_0.49RhO_2. The resistivity is proportional to the square of temperature up to 300 K, and the thermopower is proportional to temperature up to 140 K. The Hall coefficient increases linearly with temperature above 100 K, which is ascribed to the triangular network of Rh in this compound. The different transport properties between Na_xCoO_2 and K_0.49RhO_2 are discussed on the basis of the different band width between Co and Rh evaluated from the magnetotransport.Comment: 3 figures, submitted to PR

Memory function approach to the Hall constant in strongly correlated electron systems

The anomalous properties of the Hall constant in the normal state of high-$T_c$ superconductors are investigated within the single-band Hubbard model. We argue that the Mori theory is the appropriate formalism to address the Hall constant, since it aims directly at resistivities rather than conductivities. More specifically, the frequency dependent Hall constant decomposes into its infinite frequency limit and a memory function contribution. As a first step, both terms are calculated perturbatively in $U$ and on an infinite dimensional lattice, where $U$ is the correlation strength. If we allow $U$ to be of the order of twice the bare band width, the memory function contribution causes the Hall constant to change sign as a function of doping and to decrease as a function of temperature.Comment: 35 pages, RevTex, 3 ps figures include

Collapse of the vortex-lattice inductance and shear modulus at the melting transition in untwinned YBa2Cu3O7\rm YBa_2Cu_3O_7

The complex resistivity $\hat{\rho}(\omega)$ of the vortex lattice in an untwinned crystal of 93-K $\rm YBa_2Cu_3O_7$ has been measured at frequencies $\omega/2\pi$ from 100 kHz to 20 MHz in a 2-Tesla field $\bf H\parallel c$, using a 4-probe RF transmission technique that enables continuous measurements versus $\omega$ and temperature $T$. As $T$ is increased, the inductance ${\cal L}_s(\omega) ={\rm Im} \hat{\rho}(\omega)/ \omega$ increases steeply to a cusp at the melting temperature $T_m$, and then undergoes a steep collapse consistent with vanishing of the shear modulus $c_{66}$. We discuss in detail the separation of the vortex-lattice inductance from the `volume' inductance, and other skin-depth effects. To analyze the spectra, we consider a weakly disordered lattice with a low pin density. Close fits are obtained to $\rho_1(\omega)$ over 2 decades in $\omega$. Values of the pinning parameter $\kappa$ and shear modulus $c_{66}$ obtained show that $c_{66}$ collapses by over 4 decades at $T_m$, whereas $\kappa$ remains finite.Comment: 11 pages, 8 figures, Phys. Rev. B, in pres

Vanishing Hall Constant in the Stripe Phase of Cuprates

The Hall constant R_H is considered for the stripe structures. In order to explain the vanishing of R_H in LNSCO at x = 1/8, we use the relation of R_H to the Drude weight D as well as direct numerical calculation, to obtain results within the t-J model, where the stripes are imposed via a charge potential and a staggered magnetic field. The origin of R_H ~ 0 is related to a maximum in D and the minimal kinetic energy in stripes with a hole filling ~ 1/2. The same argument indicates on a possibility of R_H ~ 0 in the whole range of static stripes for x < 1/8.Comment: RevTeX, 4 pages, 5 figure