Wide band-reject filter

Figure 12.3. General schematic of fiber-optic Raman sampling, showing the laser-fiber interface and the fiber-spectrometer interface. The bandpass (BP) and band reject (BR) filters are sometimes integrated into the probe head. The fiber-optic (FO) length can vary over a wide range, from less than 1 m to greater than 100 m, depending on the application.

A flow measurement is always accompanied by noise. This noise is attenuated somcwhnt by the wide proportional band of the eontroller and passed on to the valve. If the noise is of any magnitude, the valve may be stroked sufficiently to introduce actual changes in flow. The nonlinear function is an efficient noise filter, in that, it rejects small-amplitude signals. The result is smoother valve motion and a more stable loop. Figure 5.22 shows comparative records for linear and nonlinear control of a noisy flow loop. The nonlinear controller has proven to be quite effective on pulsating flows too, where the disturbance is periodic rather than random. 

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