Research and design on new type ultrasonic gas leak detection system

Traditional leakage detecting method such as absolute pressure method,differential pressure method、bubble method ,operation is complex with high requirements on technicists,and without real time,currently ,in industry ,widely taking advantage of principle that leakage generated ultrasonic to do leakage detection， Taking advantage of ultrasonic to detect gas leakage position ,not only simple ,but also accurate,based on this,the article researched and designed a new type ultrasonic gas leakage detecting system

1.  Detecting principle 

1.1  gas leakage generates ultrasonic

If a container full of gas,when its internal intensity of pressure bigger than external one,due to inside and outside differential pressure is bigger,once the container has leakage hole,the gas will come out 。When leakage hole has smaller dimension and reynolds number is higher,the gas will form onflow。   Onflow will generate acoustic wave with certain frequency at the around of leakage hole,like graph 1.the frequency of acoustic wave vibration is related to dimension of leakage hole，when leakage is bigger,ears can hear air flow ,

when leakage hole is very small and acoustic wave frequency is bigger than 20KHZ,ears will not hear，but they can spread in the air ,called  no-load ultrasonic。 Ultrasonic is high frequency short wave signal,its intensity is quickly decreased along with improving distance to sound source。Therefore,ultrasonic is thought as a signal that has strong directivity，use this signal to judge leakage position is quite easy。

The ultrasonic frequency band generated by leakage is wider,normally between 20KHZ to 100KHZ,at different frequency point, ultrasonic energy is different,actually,its frequency spectrum peak value is changed along with leakage hole dimension and pressure,for example：under certain leakage hole diameter and pressure,if frequency spectrum peak value of leakage ultrasonic is 38KHZ Then after increasing hole diameter ,its frequency spectrum peak value can appear at 36khz spot,if hole diameter is not change,increase inside and outside pressure difference of system,frequency spectrum peak value can appear at 43khz spot,but at the same frequency spot, for leakage hole with same  shapes,acoustic intensity of ultrasonic generated by leakage is increasing along with increasing of leakage flux,in addition,if leakage flux is constant,namely leakage area is fixed,then the shape of leakage hole will be closer to round shape,acoustic pressure is higher。 

2  system hardware 

Ultrasonic intensity generated by Small hole gas leakage is extremely weak,and in industrial occasions ,environmental noise is relative heavy。 So detect ultrasonic generated by gas leakage under bad environment,it is necessary to do well designed on system signal amplifying parts ,this system just detect 40khz spot leakage ultrasonic  intensity,the reason is according to experiment,at 40khz spot ,leakage ultrasonic energy is bigger,and leakage sound  and  Its noise energy difference value is also the most,this choice can increase system sensitivity。

System principle like graph 4,the system is divided into analog and digital parts, analog part includes signal amplify and audio processing circuit ,signal amplify circuit is consist of pre-amplification circuit、Band pass filter circuit and twice amplifying circuit,audio processing circuit is consist of frequency mixer、power driven circuit。 Digital part is mainly consist of DSP and LCD、RAM 、keyboard,after sensor signal being amplified,one will be dealt by DSP,another reduces its frequency and convert into sound。 

Pre-amplification circuit chooses AD company specialized high accuracy instrument three on-amp AD620。AD620 is a differential specialized instrument on-amp integrate with three precise opamp，it has low excursion、high gain 、especially suitable for amplifying sensor signal,due to sensor received much low frequency noise intensity is much bigger that its received ultrasonic signal,so it must connect a passive high pass filter between sensor and AD620,although improve power consumption of sensor,but can be made up of by increasing magnification in the later .The second level is band pass filter circuit,in this level can filter front most background noise and the noise generated by device or circuit the pass band is 38KHZ--42KHZ,the second level and third level opamp are adopt AD company OP777,it is a low noise opamp with super precise ,equipped with lowest voltage and current excursion and very high gain stability,the third level is same phase amplifying circuit。After the third level amplified,signal range is -3.3V--+3.3V,and then pass two 20KΩ resistance in the graph, and connect to +3.3V bias voltage ,then can change AD sampling signal that  input DSP into 0--3.3V。

Although chosen device is low noise,but for detecting extremely weak leakage ultrasonic signal,it still can not ignore its own noise。After signal entering DSP do digital smoothing on it again，strain away front device and direct current voltage and noise generated by circuit。 This can get enough high accuracy leakage ultrasonic signal。

2.3  DSP

Main function of DSP is responsible for transforming A/D,analyzing and dealing on signal that after A/D transformation 、and  management on LCD and power source。Here adopting TMS320LF2407A, DSP chip is a microprocessor with special structure,inside chip adopts harvard structure that procedure    and data are divided,possessing specialized hardware multiplying unit,widely adopt flow line operation,and provide special DSP instruction,can fast speed realize digital signal processing algorithm。TMS320LF240X is a fixed low power consumption digital signal processor series that base on C2*LP16 introduced by TI company 。2407A type processor is the newest product in this series,40M instruction /second processing speed can provide properties that far surpass traditional 16 bit microcontroller and microprocessor, the built-in ten bit analog/digital switching circuit can simplify circuit。