Abstract: the anti-interference measures of steam flow meter
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anti-interference measures of the introduction in numerous traffic detection solutions, steam flowmeter due to their high measurement precision, small pressure loss, easy installation, not affected by the physical properties of measured medium, signal facilitate remote transmission characteristics, application technology.
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Steam flowmeter anti-interference measures of the introduction in numerous traffic detection solutions, steam flowmeter due to their high measurement precision, small pressure loss, easy installation, not affected by the physical properties of measured medium, signal facilitate remote transmission characteristics, application technology has become increasingly perfect, especially in the big diameter and the flow of water, oil and other liquid medium measurement used more widely.
Steam flowmeter is made on the basis of the principle of fluid dynamics of the karman vortex street.
In a certain range of Reynolds number, fluid velocity or volume flow and vortex frequency is proportional to the physical properties of the fluid (
Pressure, temperature, density, etc. )
Has nothing to do, i. e. , Q = k f type: Q is a volume flow;
K is constant instrument;
F vortex frequency.
According to the principle of the measurement, a feature of steam flowmeter is vulnerable to electromagnetic interference and mechanical vibration, the resulting restrictions on several occasions that the normal use of the steam flowmeter, which is also the working conditions of steam flowmeter.
Solve the problem of anti-jamming is extended range lower limit, the effective ways to improve the steam flowmeter.
2 working conditions of steam flowmeter bluff body with piezoelectric crystals to detect vortex frequency, piezoelectric signal amplified by communication and trigger, the vortex frequency into pulse signals.
Pulse signal to secondary instrument after conversion according to the measured flow.
Among them, the ac amplifier amplification factor K and the trigger threshold voltage can be adjusted, as shown in figure 1.
In figure 1 for E, interference signal voltage signal is converted to the input V, threshold voltage U convert to input for U, ac magnification for K.
Because u = the UK, the adjusting effect of K or u are the same.
In order to make the threshold voltage can prevent interference signals in order to make sure the output can effectively, must make jamming signal V is less than the threshold voltage u, E is greater than the threshold voltage and effective signal voltage u, that is, the working conditions of steam flowmeter is: E & gt;
V is the size of the jamming signal V decided to steam flow meter range limit.
Expand the range of steam flowmeter lower limit must therefore start from reducing interference signal.
Adjust the magnification factor K of the output signal only strengthen communication, and the range limit and cannot be extended.
3 the main disturbance signal anti-jamming measures steam flowmeter are two kinds of electromagnetic interference and mechanical vibration interference, how to solve these two kinds of anti-interference problem becomes the key to improve the steam flowmeter.
Steam flow meter is usually adopts metal case, shell shielding effect can prevent electric field and radio frequency interference;
For the interference of magnetic field, can be in the internal circuit design through the reasonable selection of magnetic components, printed circuit board wiring solution, with the development of electronic technology and the improvement of the manufacturing process is not a problem.
So the resistance to electromagnetic interference are mainly ground current interference.
Steam flowmeter of piezoelectric crystal is installed on the bluff body structure, one end of the piezoelectric crystal shell, so the preamplifier signal grounding.
Steam flow meter output signal to the secondary instrument, the dc power required to signal amplification and provided by the secondary instrument.
Piezoelectric crystals between the ground and secondary instrument of ground form current step is likely to exist.
The current flow in the ground of amplifier will have the pressure drop, the pressure drop and the effective signal superposition together, not separation, is the interference of ground current.
Interference of ground current solution is to reduce or eliminate the ground current, zui radical idea is to turn the secondary instrument to dc power supply isolation.
The dc power supply after isolation transformer rectifier again into the dc supply steam flowmeter, make secondary instrument ground with no electrical connection between the ground plane piezoelectric crystal.
At the same time effective measure after preamplifier signal into pulse signal, the pulse transformer output to the secondary instrument, fundamentally eliminate the influence of ground current, is an extremely effective anti-interference measures.
Transformer isolation method costs are relatively high, however, big volume, manufacturing process is not easy to implement, greatly reduces the practicality.
Optical isolation current-limiting anti-interference measures, can effectively reduce the interference of ground current.
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