Velocity measuring system
A laser Doppler velocimeter uses self-mixing amplification of backreflections from scatterers below the surface of a flow. A time domain signal is divided into segments that are roughly equal to a transit time of particles through a focus of a laser beam. The segments are connected to a frequency domain through the use of an FFT algorithm to produce frequency domain data segments. Signal-to-noise ratio is enhanced through signal processing techniques using the segments to produce a final enhanced signal spectrum.
1. A method of estimating volumetric flow executable by a laser Doppler velocimeter, comprising the steps of:
directing a laser beam of the laser Doppler velocimeter to measure velocity at several points in the volumetric flow;
estimating the volumetric flow using volumetric measurements at the several points; and
eliminating signals from surface reflections, the eliminating signals from surface reflections includes the step of eliminating signals that are more intense and have sharper velocity profiles than signals from scattering in a bulk of a flowing stream.
2. A method in accordance with claim 1 , wherein the step of eliminating surface reflection signals that are more intense and have sharper velocity profiles than signals from scattering in the bulk of the flow stream includes the step of eliminating surface reflection signals that have higher or narrower frequency peaks than normal in a Doppler beat frequency spectrum before a flow velocity determination is made.
3. A method in accordance with claim 2 wherein the step of eliminating surface reflection signals that have higher or narrower frequency peaks than normal includes the step of eliminating peaks that appear as spikes in spectra.
4. An apparatus for noncontact open channel fluid flow measurement comprising:
a laser Doppler velocimeter using self-mixing amplification of backreflections from scatters below a surface of flow for enhanced detection, the laser Doppler velocimeter including:
a laser diode module configured to transmit light to a focus drive motor, receive backscattered light, and mix transmitted light frequency and the backscattered light to provide a Doppler beat signal to a Doppler beat frequency subsystem.
5. The apparatus in accordance with claim 4 , wherein the laser Doppler velocimeter further includes
a focus drive motor configured to position the laser diode module.
6. The apparatus in accordance with claim 5 , wherein the laser diode module includes a photodetector configured to receive backscattered light.