Browsing by Subject "Time varying"

Now showing 1 - 5 of 5

Open Access
Adaptive control of a spring-mass hopper
(IEEE, 2011) Uyanık, İsmail; Saranlı, Uluç; Morgül, Ömer
Practical realization of model-based dynamic legged behaviors is substantially more challenging than statically stable behaviors due to their heavy dependence on second-order system dynamics. This problem is further aggravated by the difficulty of accurately measuring or estimating dynamic parameters such as spring and damping constants for associated models and the fact that such parameters are prone to change in time due to heavy use and associated material fatigue. In this paper, we present an on-line, model-based adaptive control method for running with a planar spring-mass hopper based on a once-per-step parameter correction scheme. Our method can be used both as a system identification tool to determine possibly time-varying spring and damping constants of a miscalibrated system, or as an adaptive controller that can eliminate steady-state tracking errors through appropriate adjustments on dynamic system parameters. We present systematic simulation studies to show that our method can successfully accomplish both of these tasks. © 2011 IEEE.
Open Access
Effects of monetary policy on the long memory in interest rates: Evidence from an emerging market
(Elsevier, 2013) Sensoy, A.
We study the presence of long memory in a variety of interest rates in Turkey by time-varying generalized Hurst exponent. We reveal that adopting inflation targeting cause a sudden and considerable decrease in the long memory in interest rates. The improvement lasts till the collapse of Lehman Brothers in 2008 which is followed with an increased persistence in interest rates. Moreover, degree of long memory increases with maturity which is in contrast to economic theory.
Open Access
Lineshapes, shifts and broadenings in dynamical X-ray photoelectron spectroscopy
(2009) Dâna, A.
We describe in detail a model that can be used to estimate the X-ray photoelectron spectroscopic data of surfaces when a time varying bias or a modulation of the electrical properties of the surface is applied by external stimulation, in the presence of a neutralizing electron beam. Using the model and spectra recorded under periodic sample bias modulation, certain electronic properties related to charging dynamics of the surface can be estimated. The resulting technique is a non-contact impedance measurement technique with chemical specificity. Typical behavior of spectra under a square wave bias is given. Alternative modulation schemes are investigated, including small-signal square wave modulation, sinusoidal modulation and modulation of sample resistivity under fixed bias. © 2009 Elsevier B.V. All rights reserved.
Open Access
A switching control approach to stabilization of parameter varying time delay systems
(IEEE, 2009) Yan, P.; Özbay, Hitay; Şansal, M.
Robust stabilization problem is considered for time varying time delay systems, where the system parameters are scheduled along a measurable signal trajectory. A switching control approach is proposed for a class of parameter varying systems, where candidate controllers are designed for robust stabilization at certain operating regions. A dwell time based hysteresis switching logic is proposed to guarantee the stability of the switched parameter varying time delay system in the whole operating range. It is shown that if the parameter variation is slow enough (upper bound of the time derivative is determined in terms the dwell time for the switched delay system), then the system is stable with the proposed switched controllers.
Open Access
Time-varying lifting structures for single-tree complexwavelet transform
(IEEE, 2012) Keskin, Furkan; Çetin, A. Enis
In this paper, we describe a single-tree complex wavelet transform method using time-varying lifting structures. In the dualtree complex wavelet transform (DT-CWT), two different filterbanks are executed in parallel to analyze a given input signal, which increases the amount of data after analysis. DT-CWT leads to a redundancy factor of 2 d for d-dimensional signals. In the proposed single-tree complex wavelet transform (ST-CWT) structure, filters of the lifting filterbank switch back and forth between the two analysis filters of the DT-CWT. This approach does not increase the amount of output data as it is a critically sampled transform and it has the desirable properties of DT-CWT such as shift-invariance and directional selectivity. The proposed filterbank is capable of constructing a complex wavelet-like transform. Examples are presented. © 2012 IEEE.