Dynamic Stability Analysis of Linear Time-varying Systems via an Extended Modal Identification Approach

The problem of linear time-varying(LTV) system modal analysis is considered based on time-dependent state space representations, as classical modal analysis of linear time-invariant systems and current LTV system modal analysis under the ‘‘frozen-time' assumption are not able to determine the dynamic stability of LTV systems.Time-dependent state space representations of LTV systems are first introduced, and the corresponding modal analysis theories are subsequently presented via a stabilitypreserving state transformation. The time-varying modes of LTV systems are extended in terms of uniqueness, and are further interpreted to determine the system's stability. An extended modal identification is proposed to estimate the time-varying modes, consisting of the estimation of the state transition matrix via a subspace-based method and the extraction of the time-varying modes by the QR decomposition. The proposed approach is numerically validated by three numerical cases, and is experimentally validated by a coupled moving-mass simply supported beam experimental case. The proposed approach is capable of accurately estimating the time-varying modes, and provides a new way to determine the dynamic stability of LTV systems by using the estimated time-varying modes.     

MECHANICAL PROPERTIES OF TOMATOES AS RELATED TO PUNCTURE INJURY SUSCEPTIBILITY

The relation between mechanical properties of tomato ( Lycopersicon esculentum Mill.) and puncture injury susceptibility was investigated using two cultivars: Tradiro, which is moderately resistant and Blitz, which is very susceptible. Mechanical properties were determined using a universal testing machine and an acoustic firmness sensor. The susceptibility to puncture injury was measured by a pendulum test. By means of Partial Least Squares analysis (PLS), a relationship with a coefficient of determination (R²) of 0.78 was found between the mechanical properties and the puncture injury susceptibility of the tomato cultivar. High loadings were found for the force required to puncture the tomato with and without skin, the elasticity of the fruit, the toughness of the skin and the acoustic firmness of the tomato fruit, indicating that these mechanical properties affect the susceptibility to puncture injury. The obtained results directly imply that these mechanical properties should be used to screen for genotypes with lower susceptibility to puncture injury.