Evaluation of measurement uncertainties of virtual instruments

Abstract

This paper deals with measurement uncertainty of virtual instruments (VIs). First the main uncertainty sources of transducer, signal conditioning, A/D conversion and digital signal processing (DSP) are analyzed in detail. Two approaches to evaluate uncertainties of direct and indirect measurements are presented. The first approach deals with measuring an objective variable directly by application of a physical law. Its procedure includes: Step 1: the combined measurement uncertainties of transducer, signal conditioning, A/D conversion, and DSP are estimated respectively according to Type B evaluation of “guide to the expression of uncertainty in measurement (GUM)” based on Gram-Chariler series. Step 2: their corresponding relative measurement uncertainties are calculated, moreover the overall relative uncertainty of the direct measurement is evaluated in rms value. Step 3: the combined uncertainty of the direct measurement is estimated according to the measurement result and the value of overall relative uncertainty of the direct measurement. The second approach involves measurement of an objective variable that is a function of several independent variables; however these variables could be determined by direct respective measurements. The measurement uncertainty of the objective variable could be estimated by applying the “uncertainty propagation law” of GUM. Finally a case study is given to illustrate the application of these approaches.