Papers by Keyword: Hysteresis

Abstract: Piezoelectric actuator (PEA) is widely applied in micro/nanopositioning system. However, its inherent hysteresis limits its application. Modeling of hysteresis plays an important role in solving this problem. Linear play operators (LPO) adaptive hysteresis model is introduced in this paper. LPO operators are used to replace delay operators of adaptive transversal filter to compose a new serial structure of adaptive transversal filter model, and LMS (Least Mean Square) algorithm is used to adjust the weight values. As hysteresis loop of piezoelectric actuator is asymmetric and LPO operator is symmetric, a modified LPO (MLPO) adaptive filter is proposed for asymmetric hysteresis effect. At last, the two LPO filters are applied to model hysteresis characteristic of Piezoelectric actuator, and the modeling effect is verified via a micro-positioning system experiment platform based on Piezoelectric actuator. Experimental results show that the modified LPO filters can achieve better accurate hysteresis modeling.

Abstract: A nonvolatile memory based on an organic thin-film transistor (OTFT) with biopolymer of DNA-OTMA as the gate dielectric is fabricated. The device prepared by DNA-OTMA show a very large and metastable hysteresis in the transfer characteristics. In order to obtain the organic thin film transistor memory device with high electronic performance, one of the most widely used method such as post annealing have been applied to improve the quality of gate dielectric layer. In conclusion, the post-annealing at elevated temperatures plays a very important role in the performance of the OTFT memory device.

Abstract: A piezoceramic actuator is widely employed in micropositioning and MEMS. However, the piezoceramic actuators are limited due to the natural hysteresis nonlinearity which affect the accuracy of the actuators in applications. In order to revise the hysteresis nonlinearity, lots of hysteresis models have been proposed such as the Preisach model, the classical Prandtl—Ishlinskii model and so on. While some drawbacks still exist with these models, a generalized hysteresis model for asymmetric hysteresis basing on the classical Prandtl—Ishlinskii model is devised. In the modified model, the exponential functions which contain the amplitude and the frequency of the input voltage and its gain factor are introduced into the NLPO (nonlinearity play operator). As a result, the generalized model in this paper applies to modeling asymmetric hysteresis. This model was identified and simulated using the experimental data by other researchers. At last, the validity and the accuracy of the given model were tested through the experiment of the piezoceramic control.

Abstract: Hysteresis curves were calculated according the Stoner-Wohlfarth (SW) model using values of K₂/K₁ from zero up to 0.25 (where K₂ is the 2^nd order anisotropy constant and where K₁ is the 1^st order anisotropy constant). The SW calculations assume Nd₂Fe₁₄B nanocrystalline magnets with single domain particle size (grain diameter less than 150 nm). The effect of K₂ on several index of merit of magnets as BH_max and squareness are discussed. As a general result, increasing K₂, the intrinsic coercivity increases. However, the increase of K₂ almost does not improve the BH_max of isotropic magnets. It is found that the effect of the K₂/K₁ ratio is coupled with the effect of texture, i.e., the effect of K₂ is more significant for pronounced texture.

Abstract: The coercivity in soft and hard magnetic materials has different origin. The high coercivity of barium ferrite, SmCo₅, Sm₂Co₁₇ or Nd₂Fe₁₄B is due to high magnetocrystalline anisotropy, and the processing aims very small grain size (nanocrystalline). In the case of soft magnetic materials, the coercivity has origin in defects that are able to stop domain wall movement, as for example grain boundaries, inclusions or dislocations. Soft magnetic materials in general present large domain wall thickness (thousands of Angstroms for pure iron), whereas domain wall thickness is ~ 50 Angstroms for SmCo₅ and Nd₂Fe₁₄B. The differences between hard and soft magnetic behavior are commented and discussed. The domain wall energy and thickness can be used as parameters for classifying soft and hard magnetic behavior. Other examples of soft magnetic materials are the amorphous alloys and the nanocrystalline soft magnetic materials with grain size very below the single domain particle size. The soft behaviour in amorphous and soft nanocrystalline materials is also discussed.

Abstract: Hysteresis loops were calculated according the Stoner-Wohlfarth model. Using as values for constants of magnetocrystalline anisotropy K₁ =4.5 and K₂=0.66 (J m⁻³), and 1.61 T for magnetization of saturation of Nd₂Fe₁₄B, the maximum coercivity for isotropic Nd₂Fe₁₄B was predicted as mi0 H = 2.95 T (29.5kOe). For a very well aligned magnet, with M_r/M_s=0.96, following the f (alpha)=cosⁿ(alpha) distribution, the theoretical coercivity limit was estimated as mi0 H = 3.6 T (36 kOe). These estimates are valid for the ternary Nd₂Fe₁₄B alloy. It is predicted the upper limit for the coercive field as function of grain size for NdFeB and PrFeB magnets. Addition of Praseodymium is an effective method for increasing coercivity of NdFeB magnets.

Abstract: The high coercivity region of the Nd-Fe-B and Sm-Co phase diagrams is discussed. Slow cooling heat treatments may eliminate lattice defects, which are responsible for nucleation of reverse magnetization. The concept of diffusion length can be used for the design of heat treatments. Analytical formulas for calculation of the diffusion length as function of the cooling rate of the heat treatment are presented.

Abstract: The aim of this work is the development of an electronic device for the rapid characterization of soft ferromagnetic materials for electrical purposes named single sheet tester (sst). The basis of this study consists in determining the magnetic induction, B and the magnetic field, H, by using a simplified circuit. Results are compared with classical epstein test and the obtained losses present an error less than 10%.

Abstract: The volume fraction of the single domain size particles can be directly estimated from the initial magnetization of thermally demagnetized magnets. Multi-domain grains present initial magnetization curve with high initial susceptibility, whereas single-domain grains present low susceptibility initial magnetization curve. In the case of single domain size particles, the coercivity mechanism is coherent rotation and the Stoner-Wohlfarth (SW) model can be applied. From the initial magnetization curve of magnets, the volume fraction of grains with diameter less than 0.3 micrometers can be estimated in NdFeB magnets. This is possible because the Nd₂Fe₁₄B phase is single domain below 0.3 micrometers.

Abstract: Micromagnetics and its particular case, the Stoner-Wohlfarth model, are reviewed. The possibility of occurrence of curling is discussed. The use of the Felix Bloch approximation for exchange energy description should be avoided. A tensor 3x3 has to be used for appropriate evaluation of the exchange energy in phases with less symmetry, as Nd₂Fe₁₄B, Sm₂Co₁₇ and SmCo₅. The assumptions used in classic Brown micromagnetics make it less suitable for phases with high magnetocrystalline anisotropy. The role of the exchange term in the Stoner-Wohlfarth Callen-Liu-Cullen model is clarified.