In many cases, natural irregularity waveform
W(ξ) of the surfase of a strip is considered to be a sample function from some stationary stochastic process. Suppose the strip is moving at speed υ, signal
X(t) obtained by observing it at ξ=0 is given by
X(t)=
W(υ
t), so,
X(t) is a stochastic process whose simultaneous probability distribution contains υ as one of it's parameters.
Therefore, when observation data
X={
X(t);
t∈
T}is given, we_can estimate υ statistically from
X and non-contact speed measurement is made possible through it.
In this paper, the author studied on this problem in case where
W(ξ) is stationary Gaussian process, whose autocorrelation function is σ
2e
-α|ξ|, and the observation is time-discrete and subject to diserete white noise, that is,
X={
xi=
W(iυΔ)+
ni;
i=1, …,
N},
E[
ni;
nj]=ρ
2δ
ijMaximum likelihood estimate is adopted as υ, estimate of υ, and Cramer-Rao bound is used to evaluate estimation error assuming sample size
N is very large.
From this analysis it is made clear that υ is a fairly good estimate in the ordinary measurement conditions.
As for the effect of observation noise {
ni}, increase of estimation error ratio (to noiseless case) due to the noise is almost equal to noise-to-signal ratio, therefore, noise is not so harmfull if it is small.
To the contrary, if parameters of the probability distribution of irregularity and noise are unkown, estimation error becomes many times larger than in case where they are known.
It is also painted out that determination of sample interval Δ is very important in the actualsituation. That is, if observation time
NΔ is constant, decrease of Δ makes the estimation error small but at the same time it also makes our mathematical model of the irregularity not appropriate for the actual one whose waveform is differentiable.
Although our analysis is made on some special model and therefore not necessarily applicable to actual cases, some insight into the statistical method of non-contact speed measurement is obtained.
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