The Experts below are selected from a list of 309 Experts worldwide ranked by ideXlab platform

Feihu Sun - One of the best experts on this subject based on the ideXlab platform.

  • simulations of reflected sun beam traces over a Target Plane for an azimuth elevation tracking heliostat with fixed geometric error sources
    Solar Energy, 2013
    Co-Authors: Minghuan Guo, Zhifeng Wang, Feihu Sun
    Abstract:

    Abstract For a heliostat with geometric errors, the reflected central solar ray from the mirror surface center forms a curved error trace on the Target Plane during the day rather than staying fixed on one Target point. A general azimuth–elevation tracking angle formula has been developed for a heliostat with a mirror-pivot offset and other typical geometric errors. This tracking angle formula is re-rewritten here as a series of easily solved expressions. This azimuth–elevation tracking angle formula is then used in a new complete geometric model of the sun-beam tracking errors for an azimuth–elevation tracking heliostat to simulate the sun beam tracking error trace on the Target Plane for a heliostat with fixed geometric errors. Here, the analysis is for a point sun and a point heliostat (or the heliostat considered as a small optical flat). The mirror surface center is defined as the orthogonal projection of the heliostat pivot on the mirror surface Plane. The reflected sun-beam centre in the Target Plane is defined as the intersection of the mirror-surface-centre reflected central solar ray with the Target Plane. Due to a position tracking error in the Target Plane depending on the position and the orientation of the specific Target Plane, the position tracking error is further converted to the angular tracking error in the reflection direction to facilitate evaluation of the heliostat tracking performance. Simulations for the artificial #78 heliostat in the Beijing solar tower system on June 21st are shown to illustrate this heliostat tracking error model. This heliostat tracking error model can be used to reveal the effect of various geometrical errors in pedestal tilt etc. on the location of the beam at the Target, and thus is useful in setting limits on the various geometrical errors. Essentially this paper allows one to estimate the offset of the reflected solar beam centre due to specific geometrical tracking errors, once the beam centre is computed by some other means. It also allows one to determine a limit on each error or set of errors which are allowable for a given purpose.

  • Simulations of reflected sun beam traces over a Target Plane for an azimuth–elevation tracking heliostat with fixed geometric error sources
    Solar Energy, 2013
    Co-Authors: Minghuan Guo, Zhifeng Wang, Feihu Sun
    Abstract:

    Abstract For a heliostat with geometric errors, the reflected central solar ray from the mirror surface center forms a curved error trace on the Target Plane during the day rather than staying fixed on one Target point. A general azimuth–elevation tracking angle formula has been developed for a heliostat with a mirror-pivot offset and other typical geometric errors. This tracking angle formula is re-rewritten here as a series of easily solved expressions. This azimuth–elevation tracking angle formula is then used in a new complete geometric model of the sun-beam tracking errors for an azimuth–elevation tracking heliostat to simulate the sun beam tracking error trace on the Target Plane for a heliostat with fixed geometric errors. Here, the analysis is for a point sun and a point heliostat (or the heliostat considered as a small optical flat). The mirror surface center is defined as the orthogonal projection of the heliostat pivot on the mirror surface Plane. The reflected sun-beam centre in the Target Plane is defined as the intersection of the mirror-surface-centre reflected central solar ray with the Target Plane. Due to a position tracking error in the Target Plane depending on the position and the orientation of the specific Target Plane, the position tracking error is further converted to the angular tracking error in the reflection direction to facilitate evaluation of the heliostat tracking performance. Simulations for the artificial #78 heliostat in the Beijing solar tower system on June 21st are shown to illustrate this heliostat tracking error model. This heliostat tracking error model can be used to reveal the effect of various geometrical errors in pedestal tilt etc. on the location of the beam at the Target, and thus is useful in setting limits on the various geometrical errors. Essentially this paper allows one to estimate the offset of the reflected solar beam centre due to specific geometrical tracking errors, once the beam centre is computed by some other means. It also allows one to determine a limit on each error or set of errors which are allowable for a given purpose.

Minghuan Guo - One of the best experts on this subject based on the ideXlab platform.

  • simulations of reflected sun beam traces over a Target Plane for an azimuth elevation tracking heliostat with fixed geometric error sources
    Solar Energy, 2013
    Co-Authors: Minghuan Guo, Zhifeng Wang, Feihu Sun
    Abstract:

    Abstract For a heliostat with geometric errors, the reflected central solar ray from the mirror surface center forms a curved error trace on the Target Plane during the day rather than staying fixed on one Target point. A general azimuth–elevation tracking angle formula has been developed for a heliostat with a mirror-pivot offset and other typical geometric errors. This tracking angle formula is re-rewritten here as a series of easily solved expressions. This azimuth–elevation tracking angle formula is then used in a new complete geometric model of the sun-beam tracking errors for an azimuth–elevation tracking heliostat to simulate the sun beam tracking error trace on the Target Plane for a heliostat with fixed geometric errors. Here, the analysis is for a point sun and a point heliostat (or the heliostat considered as a small optical flat). The mirror surface center is defined as the orthogonal projection of the heliostat pivot on the mirror surface Plane. The reflected sun-beam centre in the Target Plane is defined as the intersection of the mirror-surface-centre reflected central solar ray with the Target Plane. Due to a position tracking error in the Target Plane depending on the position and the orientation of the specific Target Plane, the position tracking error is further converted to the angular tracking error in the reflection direction to facilitate evaluation of the heliostat tracking performance. Simulations for the artificial #78 heliostat in the Beijing solar tower system on June 21st are shown to illustrate this heliostat tracking error model. This heliostat tracking error model can be used to reveal the effect of various geometrical errors in pedestal tilt etc. on the location of the beam at the Target, and thus is useful in setting limits on the various geometrical errors. Essentially this paper allows one to estimate the offset of the reflected solar beam centre due to specific geometrical tracking errors, once the beam centre is computed by some other means. It also allows one to determine a limit on each error or set of errors which are allowable for a given purpose.

  • Simulations of reflected sun beam traces over a Target Plane for an azimuth–elevation tracking heliostat with fixed geometric error sources
    Solar Energy, 2013
    Co-Authors: Minghuan Guo, Zhifeng Wang, Feihu Sun
    Abstract:

    Abstract For a heliostat with geometric errors, the reflected central solar ray from the mirror surface center forms a curved error trace on the Target Plane during the day rather than staying fixed on one Target point. A general azimuth–elevation tracking angle formula has been developed for a heliostat with a mirror-pivot offset and other typical geometric errors. This tracking angle formula is re-rewritten here as a series of easily solved expressions. This azimuth–elevation tracking angle formula is then used in a new complete geometric model of the sun-beam tracking errors for an azimuth–elevation tracking heliostat to simulate the sun beam tracking error trace on the Target Plane for a heliostat with fixed geometric errors. Here, the analysis is for a point sun and a point heliostat (or the heliostat considered as a small optical flat). The mirror surface center is defined as the orthogonal projection of the heliostat pivot on the mirror surface Plane. The reflected sun-beam centre in the Target Plane is defined as the intersection of the mirror-surface-centre reflected central solar ray with the Target Plane. Due to a position tracking error in the Target Plane depending on the position and the orientation of the specific Target Plane, the position tracking error is further converted to the angular tracking error in the reflection direction to facilitate evaluation of the heliostat tracking performance. Simulations for the artificial #78 heliostat in the Beijing solar tower system on June 21st are shown to illustrate this heliostat tracking error model. This heliostat tracking error model can be used to reveal the effect of various geometrical errors in pedestal tilt etc. on the location of the beam at the Target, and thus is useful in setting limits on the various geometrical errors. Essentially this paper allows one to estimate the offset of the reflected solar beam centre due to specific geometrical tracking errors, once the beam centre is computed by some other means. It also allows one to determine a limit on each error or set of errors which are allowable for a given purpose.

Teck-yong Tou - One of the best experts on this subject based on the ideXlab platform.

  • Target-Plane deposition of diamond-like carbon in pulsed laser ablation of graphite
    Applied Surface Science, 2007
    Co-Authors: Seong Shan Yap, Teck-yong Tou
    Abstract:

    In pulsed Nd:YAG laser ablation of highly oriented pyrolytic graphite (HOPG) at 10(-6) Torr, diamond-like carbon (DLC) are deposited at laser wavelengths of 1064, 532, and 355 nm on substrates placed in the Target-Plane. These Target-Plane samples are found to contain varying sp(3) content and composed of nanostructures of 40-200 nm in size depending on the laser wavelength and laser fluence. The material and origin of sp(3) in the Target-Plane samples is closely correlated to that in the laser-modified HOPG surface layer, and hardly from the backward deposition of ablated carbon plume. The surface morphology of the Target-Plane samples shows the columnar growth and with a tendency for agglomeration between nanograins, in particular for long laser wavelength at 1064 nm. It is also proposed that DLC formation mechanism at the laser-ablated HOPG is possibly via the laser-induced subsurface melting and resolidification. (C) 2007 Elsevier B.V. All rights reserved

  • Diamond-like carbon formation for various positions by pulsed laser deposition
    Applied Surface Science, 2005
    Co-Authors: Seong Shan Yap, Teck-yong Tou
    Abstract:

    Pulsed laser ablation of pyrolytic graphite Target was carried out by an Nd-YAG laser with lambda = 1064 nm and fluence in the range of 1-10 J/cm(2). The plume was produced by focusing the laser beam and rastering over a 6.5 mm x 6.5 mm area on the graphite Target. The substrates were placed at two positions: on-axis position facing the Target and off-axis position in the Target Plane with 2 mm offset from the ablation site. Diamond-like carbon was formed on the substrates at both positions and on the ablated area as detected by Raman spectroscopy. Rough and granular surface was observed for the samples placed in the Target Plane and smooth diamond-like carbon films for the samples placed facing the Target as observed by SEM and optical microscopy. (c) 2005 Elsevier B.V. All rights reserved

Giovanni B. Valsecchi - One of the best experts on this subject based on the ideXlab platform.

  • Target Plane Confidence Boundaries: Mathematics of the 1997 XF11 Scare
    Impact of Modern Dynamics in Astronomy, 1999
    Co-Authors: Andrea Milani, Giovanni B. Valsecchi
    Abstract:

    The uncertainty of the close approach distance of a Potentially Hazardous Object (PHO), either an asteroid or a comet, can be represented on the Modified Target Plane (MTP), a modification of the one used by Opik. The MTP is orthogonal to the geocentric velocity at the plosest approach along the nominal orbit, solution of the least square fit to the observations. The confidence regions of this solution in the 6-D space of orbital elements (for an epoch close to the observations) are well approximated by a family of concentric ellipsoids, if the observed arc is not too short. In the linear approximation these ellipsoids are mapped on the MTP into concentric ellipses, which can be computed by solving for the state transition matrix.

  • The Asteroid Identification Problem: II. Target Plane Confidence Boundaries
    Icarus, 1999
    Co-Authors: Andrea Milani, Giovanni B. Valsecchi
    Abstract:

    Abstract The nominal orbit solution for an asteroid/comet resulting from a least squares fit to astrometric observations is surrounded by a region containing solutions equally compatible with the data, the confidence region. If the observed arc is not too short, and for an epoch close to the observations, the confidence region in the six-dimensional space of orbital elements is well approximated by an ellipsoid. This uncertainty of the orbital elements maps to a position uncertainty at close approach, which can be represented on a Modified Target Plane (MTP), a modification of the one used by Opik. The MTP is orthogonal to the geocentric velocity at the closest approach point along the nominal orbit. In the linear approximation, the confidence ellipsoids are mapped on the MTP into concentric ellipses, computed by solving the variational equation. For an object observed at only one opposition, however, if the close approach is expected after many revolutions, the ellipses on the MTP become extremely elongated, therefore the linear approximation may fail, and the confidence boundaries on the MTP, by definition the nonlinear images of the confidence ellipsoids, may not be well approximated by the ellipses. In theory the Monte Carlo method by Muinonen and Bowell (1993, Icarus 104 , 255–279) can be used to compute the nonlinear confidence boundaries, but in practice the computational load is very heavy. We propose a new method to compute semilinear confidence boundaries on the MTP, based on the theory developed by Milani (1999, Icarus 137 , 269–292) to efficiently compute confidence boundaries for predicted observations. This method is a reasonable compromise between reliability and computational load, and can be used for real time risk assessment. These arguments can be applied to any small body approaching any Planet, but in the case of a potentially hazardous object (PHO), either an asteroid or a comet whose orbit comes very close to that of the Earth, the application is most important. We apply this technique to discuss the recent case of asteroid 1997 XF 11 , which, on the basis of the observations available up to March 11, 1998, appeared to be on an orbit with a near miss of the Earth in 2028. Although the least squares solution had a close approach at 1/8 of the lunar distance, the linear confidence regions corresponding to acceptable size of the residuals are very elongated ellipses which do not include collision; this computation was reported by Chodas and Yeomans. In this paper, we compute the semilinear confidence boundaries and find that they agree with the results of the Monte Carlo method, but differ in a significant way from the linear ellipses, although the differences occur only far from the Earth. The use of the 1990 prediscovery observations has confirmed the impossibility of an impact in 2028 and reduces the semilinear confidence regions to subsets of the regions computed with less data, as expected. The confidence regions computed using the linear approximation, on the other hand, do not reduce to subsets of the regions computed with less data. We also discuss a simulated example (Bowell and Muinonen 1992, Bull. Am. Astron. Soc. 24 , 965) of an Earth-impacting asteroid. In this hypothetical case the semilinear confidence boundary has a completely different shape from the linear ellipse, and indeed for orbits determined with only few weeks of observational data the semilinear confidence boundary correctly includes possible collisions, while the linear one does not. Free software is available now, allowing everyone to compute Target Plane confidence boundaries as in this paper; in case a new asteroid with worrisome close approaches is discovered, our method allows to quickly perform an accurate risk assessment.

  • Target Plane Confidence Boundaries: Mathematics of The 1997 XF11 Scare
    International Astronomical Union Colloquium, 1999
    Co-Authors: Andrea Milani, Giovanni B. Valsecchi
    Abstract:

    The uncertainty of the close approach distance of a Potentially Hazardous Object (PHO), either an asteroid or a comet, can be represented on the Modified Target Plane (MTP), a modification of the one used by Öpik. The MTP is orthogonal to the geocentric velocity at the closest approach along the nominal orbit, solution of the least square fit to the observations. The confidence regions of this solution in the 6-D space of orbital elements (for an epoch close to the observations) are well approximated by a family of concentric ellipsoids, if the observed arc is not too short. In the linear approximation these ellipsoids are mapped on the MTP into concentric ellipses, which can be computed by solving for the state transition matrix.For a PHO observed at only one opposition, with a close approach expected after many revolutions, the ellipses on the MTP become extremely elongated and the linear approximation may fail. In this case the confidence boundaries on the MTP, i.e. the nonlinear images of the confidence ellipsoids, may not be well approximated by the ellipses. The Monte Carlo method (Muinonen and Bowell, 1993) can be used to find nonlinear confidence regions, but the computational load is very heavy: to estimate a low probability event the number of test cases must be larger than the inverse of the probability. We propose a new method to compute semilinear confidence boundaries on the MTP (Milani and Valsecchi, 1998), based on the theory developed to compute confidence boundaries for predicted observations (Milani, 1999). This method is a good compromise between reliability and computational load, and can be used for real time risk assessment.

Zhifeng Wang - One of the best experts on this subject based on the ideXlab platform.

  • simulations of reflected sun beam traces over a Target Plane for an azimuth elevation tracking heliostat with fixed geometric error sources
    Solar Energy, 2013
    Co-Authors: Minghuan Guo, Zhifeng Wang, Feihu Sun
    Abstract:

    Abstract For a heliostat with geometric errors, the reflected central solar ray from the mirror surface center forms a curved error trace on the Target Plane during the day rather than staying fixed on one Target point. A general azimuth–elevation tracking angle formula has been developed for a heliostat with a mirror-pivot offset and other typical geometric errors. This tracking angle formula is re-rewritten here as a series of easily solved expressions. This azimuth–elevation tracking angle formula is then used in a new complete geometric model of the sun-beam tracking errors for an azimuth–elevation tracking heliostat to simulate the sun beam tracking error trace on the Target Plane for a heliostat with fixed geometric errors. Here, the analysis is for a point sun and a point heliostat (or the heliostat considered as a small optical flat). The mirror surface center is defined as the orthogonal projection of the heliostat pivot on the mirror surface Plane. The reflected sun-beam centre in the Target Plane is defined as the intersection of the mirror-surface-centre reflected central solar ray with the Target Plane. Due to a position tracking error in the Target Plane depending on the position and the orientation of the specific Target Plane, the position tracking error is further converted to the angular tracking error in the reflection direction to facilitate evaluation of the heliostat tracking performance. Simulations for the artificial #78 heliostat in the Beijing solar tower system on June 21st are shown to illustrate this heliostat tracking error model. This heliostat tracking error model can be used to reveal the effect of various geometrical errors in pedestal tilt etc. on the location of the beam at the Target, and thus is useful in setting limits on the various geometrical errors. Essentially this paper allows one to estimate the offset of the reflected solar beam centre due to specific geometrical tracking errors, once the beam centre is computed by some other means. It also allows one to determine a limit on each error or set of errors which are allowable for a given purpose.

  • Simulations of reflected sun beam traces over a Target Plane for an azimuth–elevation tracking heliostat with fixed geometric error sources
    Solar Energy, 2013
    Co-Authors: Minghuan Guo, Zhifeng Wang, Feihu Sun
    Abstract:

    Abstract For a heliostat with geometric errors, the reflected central solar ray from the mirror surface center forms a curved error trace on the Target Plane during the day rather than staying fixed on one Target point. A general azimuth–elevation tracking angle formula has been developed for a heliostat with a mirror-pivot offset and other typical geometric errors. This tracking angle formula is re-rewritten here as a series of easily solved expressions. This azimuth–elevation tracking angle formula is then used in a new complete geometric model of the sun-beam tracking errors for an azimuth–elevation tracking heliostat to simulate the sun beam tracking error trace on the Target Plane for a heliostat with fixed geometric errors. Here, the analysis is for a point sun and a point heliostat (or the heliostat considered as a small optical flat). The mirror surface center is defined as the orthogonal projection of the heliostat pivot on the mirror surface Plane. The reflected sun-beam centre in the Target Plane is defined as the intersection of the mirror-surface-centre reflected central solar ray with the Target Plane. Due to a position tracking error in the Target Plane depending on the position and the orientation of the specific Target Plane, the position tracking error is further converted to the angular tracking error in the reflection direction to facilitate evaluation of the heliostat tracking performance. Simulations for the artificial #78 heliostat in the Beijing solar tower system on June 21st are shown to illustrate this heliostat tracking error model. This heliostat tracking error model can be used to reveal the effect of various geometrical errors in pedestal tilt etc. on the location of the beam at the Target, and thus is useful in setting limits on the various geometrical errors. Essentially this paper allows one to estimate the offset of the reflected solar beam centre due to specific geometrical tracking errors, once the beam centre is computed by some other means. It also allows one to determine a limit on each error or set of errors which are allowable for a given purpose.