Asymptotic stabilization of a transformed quadrotor model using energy-shaping method: Theory and experiments.

This note presents a new passivity-based controller that ensures asymptotic stability for quadrotor position without solving partial differential equations or performing a partial dynamic inversion. After a resourceful change of coordinates, a pre-feedback controller, and a backstepping stage on the yaw angle dynamic, it is possible to identify new quadrotor cyclo passive outputs. Then, a simple proportional-integral controller of these cyclo-passive outputs completes the design. The cyclo-passive outputs allow the construction of an energy-based Lyapunov function that includes five out of six quadrotor degrees of freedom and guarantees asymptotic stability of the desired equilibrium. Moreover, the constant velocity reference tracking problem is solved with a slight modification to the proposed controller. Finally, the approach is validated through simulations and real-time experimental results.

Plasma homocysteine levels and folate status in children with sickle cell anemia.

PURPOSE: A sensitive inverse relationship between plasma homocysteine concentration and folate status has been demonstrated. Although children with sickle cell anemia (SCA) are at potential risk for folate deficiency, plasma homocysteine levels have not been reported in such patients. Therefore, a study was designed to assess plasma homocysteine levels as a marker of folate status. DESIGN: Plasma homocysteine concentrations were measured in 120 children with SCA (102 in steady state and 18 during an acute complication) who had never received supplemental folic acid. Folate status was directly assessed in 34 of these patients. RESULTS: Plasma homocysteine levels in the patients with SCA and control subjects were similar. The mean value +/- 1 SD was 5.8+/-2.5 micromol/L (range, 1.6 to 14.1 micromol/L) in the patients with SCA and 6.1+/-2.7 micromol/L (range, 1.7 to 15.3 micromol/L) in 73 pediatric control subjects. In a subpopulation of the study group (34 children), simultaneous serum folate, red cell folate, and total homocysteine concentrations were also measured. Their serum folate and red cell folate concentrations were normal: 12.4+/-10.0 nmol/L (range, 1 to 42 nmol/L) and 604+/-374.7 nmol/L (range, 205 to 1741 nmol/L), respectively. There was no correlation of plasma homocysteine concentration with various clinical or laboratory measures or with red cell folate concentration. CONCLUSION: Folate stores in children with SCA not receiving folic acid supplements are adequate despite an underlying hemolytic anemia.