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Search for Variable Stars in Two Old Open Clusters: NGC 2506 and NGC 2420

Kim S.-L., Chum M.-Y., Park B.-G., Kim S. C., Lee S. H., Lee M. G., Ann H. B., Sung H., Jeon Y.-B., Yuk I.-S.

Acta Astronomica

2001

Vol. 51, No. 1

49--63

We present results of photometric observations of two old open clusters: NGC 2506 and NGC 2420. V-band time-series and UBVI absolute CCD photometric observations were carried out to search for variable stars and to investigate their physical properties. From the UBVI photometry of NGC 2506, we obtain interstellar reddening of E(B-V)=0.04±0.03 mag, distance modulus (V-M_V)0=12.5±0.1 mag, and age log(t/yr)=9.25. From monitoring of 590 stars with 304 CCD frames taken for ten nights, three δ Sct stars and one eclipsing binary star are discovered in NGC 2506. For the other cluster, NGC 2420, we have examined light variations of 505 stars using 347 time-series data obtained for five nights, finding no variable star. It should be noted that we have found no γ Dor star among main-sequence stars near F0 in these two old open clusters, which is consistent with the suggestion that γ Dor-type phenomenon occurs in stars younger than log(t/yr)=8.4. On the other hand, it has been known that color-magnitude diagrams of these two clusters show well-established binary sequences, implying high incidence of binary systems: ≥20% for NGC 2506 and ≈50% for NGC 2420. However, only one eclipsing binary star was found in these two clusters.

Position and force control of a two-link flexible manipulator with piezoelectric actuators

Choi S. B., Kim H. K., Kim S. C.

Applied Mechanics and Engineering

2000

Vol. 5, no 1

75-88

This paper presents a new control strategy for the position and force control of flexible manipulators. The governing equation of motion of a two-link flexible manipulator which features piezoceramic actuators bonded on each flexible link is derived via Hamilton?s principle. The control torques of the motors to command desired position and force are determined by a sliding mode controller (SMC) on the basis of the rigid-mode dynamics. In the controller formulation, the sliding mode controller with perturbation estimation (SMCPE) is adopted to determine appropriate control gains. The SMCPE is then incorporated with the fuzzy technique to mitigate inherent chattering problem while maintaining the robust stability of the system. A set of fuzzy parameters and control rules are obtained from estimated perturbation. During the commanded motion, undesirable oscillation is actively suppressed by applying feedback control voltages to the piezoceramic actuators. These feedback voltages are also determined by the SMCPE. Consequently, accurate position and force control of a two-link flexible manipulator are achieved.