Open-loop Measurements Research Articles

Nonparametric frequency response function estimators based on nonlinear averaging techniques

Nonparametric frequency response function (FRF) estimators based on nonlinear averaging techniques are proposed. The suitability of such estimators for FFT based signal analyzers is studied. Under mild assumptions, it is shown that their bias is a function of the fourth-order moments of the perturbing errors on the input-output Fourier coefficients while the bias of the classical H/sub 1/ estimator is a function of the second-order moments. The performances of these estimators are analyzed in different situations: uncorrelated input-output perturbing errors (i.e., open-loop measurements), mutually correlated input-output disturbances (i.e., closed-loop measurements) and in presence of outliers. Analytical expressions are given for the bias when the input-output errors are zero-mean normal distributed (and mutually correlated). >

On the inadequacies of loop gain in broad-band, multiloop feedback amplifiers

Experience in communications engineering has repeatedly shown that loop gain can be an unreliable basis for the assessment of stability margins in feedback amplifiers. There are both theoretical and practical reasons for this. A simple identity provides a sounder theoretical basis, and provides a novel way of looking at old problems. The presence of any unstable modes of the closed-loop amplifier can be predicted with more assurance from open-loop measurements by examining a product of functions derived from the identity rather than a single-loop gain. Further improvements in stability assessment are possible by using the data base of the embedding network method [1]. The loop gains computed from the data base can be compared with those derived directly from two-port s-parameter measurements at the corresponding nodes, thus avoiding the use of circuit diagrams. Results on several practical amplifiers are quoted, which show that plots of loop gains can fail to predict closed-loop instability. In another actual case, a loop gain failed to predict conditional stability. The correct predictions in each case were obtained using the method described in [1].

Investigation of the stability of linear feedback amplifiers by means of open-loop scattering-matrix measurements

This letter suggests a method for studying the stability of linear feedback amplifiers with a single external loop. The method is based on scattering-parameter measurements of the open loop. No simulating networks are needed, even if the internal feedback of the amplifying part of the complete ioop cannot be neglected. A simple calculation has to be performed for obtaining a stability criterion.

An experiment to stabilize an electromechanical continuum

An experiment is presented which demonstrates the stabilization of a distributed electromechanical interaction by means of feedback. A conducting elastic string, stressed by a transverse electric field, is shown to exhibit a threshold electric pressure for instability. The transverse displacements of the spring are sampled and these signals used to induce a feedback field which tends to retain the static equilibrium. Open loop measurements are used to determine the distributed parameters. The experimental and theoretical stability limits of a two station system are compared. Using two sampling stations, the electric pressure consistent with stability is increased by a factor of five. The experiment has implication for the stabilization of more complicated electrostatic levitation devices and for the stabilization of certain hydromagnetic equilibria.