vortex frequency detection
Published on：2015-3-12 14:54:44 Hits：
Vortex flow signal is reflected by the frequency of the vortex, so how vortex frequency detection, is an important topic vortex flowmeter developed.
Eddy current signals from the detection principle points, generally have two categories:
① detect changes in frequency after generation of vortex flow occurs near the body of the vortex, primarily through thermal elements,
② detect changes in frequency after the vortex generated by the force of the body occurs in the whirlpool, which is mainly done by the piezoelectric sensing element.
Below, we will discuss specific examples of each of the two detection methods.
The first detection method with a cylindrical vortex shedder hot wire, for example, the structure can be seen in Figure 3-9. Cylinder surface pressure guide hole opened, and the cylinder internal cavity connected. Divided into two parts by the cavity wall, in the central portion of the partition wall there - holes, detected in the wells containing the fluid flow resistance of the platinum wire.
When the vortex generated at the downstream side of the cylinder, due to lift, so that the pressure is higher than the bottom of the cylinder above a few. The fluid at the bottom of the vertical cylinder pressure difference, from the bottom of the cylinder into the pressure guide hole empty plastic, through the aperture of the central portion of the partition wall, flows through the platinum resistance wire, the outflow pressure guide hole from above. If the platinum resistance wire heating to a temperature above the temperature of the fluid, when the fluid flows through the platinum resistance wire, it will take away the heat, its temperature change, i.e. change its resistance value. When the top of the cylinder to produce a vortex, then fluid from the pressure guide hole into the effluent by the pressure guide hole, and again by the platinum resistance wire, once again changed its resistance value. It can be seen: the resistance value changes correspond to changes in the flow, it corresponds to the frequency of the vortex. Therefore, by detecting the change in resistance of platinum resistance wire vortex frequency is the frequency, and then get the flow value.
Converting the resistance change of platinum resistance wire into electrical circuit shown in Figure 3-11.
Figure 3-11 Vortex heat detection mode circuit diagram
Fig A1 The output of the bridge arm of the platinum resistance wire differentially amplified by output current feedback amplifier A2 of the green bridge, the temperature of the platinum resistance wire is higher than the fluid temperature at a constant value.
The second detection method with a triangular column vortex shedder example. The structure can be seen in Figure 3-10. Both sides of the triangular prism fitted two elastic metal film, they pull and capacitor pole, which is equipped with an electrode plate. Pull between the metal electrode film and is full of oil, in order to transmit pressure.
So that when the triangle below Zhu produce a vortex, while the pressure is higher than the top of the pressure on the bottom of the metal film to the inside bottom of the triangle Zhu pushed, and the metal film on the top of the pop-out, changing the capacitance of each of the two capacitors . Thus, corresponding to the lift generated alternately, the capacitance of two capacitors to differentially change. Thus, the capacitance changes and the corresponding changes in the lift, and it corresponds to the frequency of the vortex. Thus, you can get by the vortex frequency capacitance change frequency and then flow value.
Converting the capacitance variation to an electrical signal circuit block diagram shown in Figure 3-22. Vortex sensor consists of two metal film and the electrode plates differential capacitor change, put it in the capacitance detection circuit bridge excitation generated by the RF oscillator circuit. When the vortex generation, the electrostatic capacitance changes, resulting in the bridge unbalance, the unbalance potentials amplified by RF amplifying circuit, obtained by the detector after the frequency corresponding to the vortex signal. The signal is amplified and shaped into a square wave, from the constant current circuit as a given power
Pulsed output. Feedback loop for bridge imbalance compensation caused by temperature changes.
Two detection methods as compared to the first method with high sensitivity for a wide frequency range, so that the corresponding flow rate range is wider. But it fine structure, susceptible to fluid impurities. The second method rugged, reliable, and in the wider industrial measurement applications. However, as the lift is proportional to the square of the flow rate, i.e., the flow rate increased by 10 times, 100 times will vary the lift. So, if the requirements of both flowmeters for large flow range, but also to ensure that it has a certain degree of spiritual enough, this method is a certain difficulty.
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