| 1. IntroductionThe medical splint to cure bone fracture recovery is one of the important instrument.For a long time,there is a firm problem about the application of a small splint that adjustment of cincture's tightness degree is decided by the doctor's feeling and experience. In fact,cincture can't be too loose or too tight using the small clipboard. If the cincture is too loose, then it won't be fixed firmly. In this case, the bone fracture may easily move again so as to influent the concrescence. However,if the cincture is too tight, it will lead to press ya chuang of partial skin. It may even result in severe blood-lacked muscle convulsion and even body gangrene. Therefore developing an automatic constant-pressure controlled small splint to cure bone fracture recovery is one of the important research topics for illness treatments. Currently there is a real-time supervisory system about cincture's tightness degree composed of pressure sensor and MCU. This system has functions of data manifestation and the sound-light alarms. In addition, it has the advantages of briefly operation, convenient usage and the safe credibility. Both doctors and sufferers can operate it. But this kind of small splint can only supply 800 grams pressure to every sufferer. It can't be applied to every sufferer'body condition. The paper is aiming to design an automatic constant-pressure small clipboard system for medical using based on Ultrasound Doppler bloods stream speed theory. The purpose of this equipment is applied to limb fracture sufferers of various ages and use blood stream speed peak value of human body shrink period as the system control variable. It can design a unique and accurate small splint sanction for every sufferer. Therefore the system is more humanlike and cincture's tightness degree is more precise and credible.2. Research contentThe system designed in this paper is based on the ultrasound Doppler blood flow measuring theory and method, studied the relation between the values of peak systolic velocity and the variable of the splint's force. It also pays more attention on the performance requests on concrete applications, designed and constructed automatic constant splint based on Doppler Ultrasound blood flow. Automatic constant splint based on Doppler Ultrasound blood flow. can realize the measurement of blood flow of the artery vessel, the realization of automatic constant manipulative force. Through programme the pwm signal, the realization of automatic constant manipulative force. The main research work of the paper can be divided into several pats as follows:1.Research the relation between the values of peak systolic velocity and the variable of the splint's force(1)To examine the normal upper extremity arteries with the color Doppler flow imaging. The normal values of each segment were obtained successfully. The internal diameter of the upper extremity artery reduced. The values of peak systolic velocity is decreased.Therefore the friction force of the blood vessel wall is invariable,after give the blood vessel a force,its section was reduced(the section of the blood vessel closed to a round,the perimeter is invariable, the round area is the biggest),when give the blood vessel a force, The values of peak systolic velocity is decreased again.This is gist that use blood stream speed peak value of human systolic velocity as the system control variable.(2)The system takes the blood speed of flow peak 90% and 80% does for the system'threshold value(The system'threshold value is obtained through some doctor's experience and test some person, In in this value scope the system can give the bone fracturebody a firm fixup,also can provide enough oxygen to the human body ).2. Blood flow measuring circuit(1) Ultrasound transmission and reception circuit design The system adopts dual-plex transducer to complete ultrasound transmission and reception. The main clock generator generates high frequency sine wave, and the latter is transmitted after a power amplifier. In the receiving channel the ultrasound echoes are received on the receiving transducers, through the isolation stage and into the preamplifier.(2) Controlling signal generating circuit designPulse Doppler system is sensitive to both frequency and phase. Both the occasional phase of emission pulse and the swing of the sampling pulse phase can introduce the noise into the system. So there is only one oscillator in the system, all the other oscillating signals and controlling signals including transmission pulse, reference signal of demodulating signal, or PRF are derived from this oscillator. In the paper this function is realized by the frequency synthesizer designed by the PLL.(3) Doppler frequency shift demodulation circuit designThe purpose of demodulation module is to extract the frequency shift signal from the echo signal. The pre-amplified echo signals are sent to the demodulator, where they are coherently demodulated with the carrier generated by the main oscillator. As a result there will be a series of short output pulse, namely the sampled Doppler frequency shift signal output.(4) Signal shaping and data acquisitionThis part of electric circuit is composed of the mode transformation circuit of the demodulated signal, the band-pass circuit and A/D switching circuit. It first completed the demodulated signal mode from current to voltage and make the demodulated signal further shape and amplified.Then a band-pass filter is used to remove the high frequency of residual PRF component and the low frequency interference of echoes from the blood vascular wall. After these processing, the demodulated signals make preparations for the following A/D(the signal of the PRF control A/D switch opens or close) switching circuit acquisition,the MCU can process more fast.3.Automatic constant manipulative circuit designIn order to give the bone fracture of the patient constant force.The paper designed the device of automatic constant force. The air pump charge aerocyst with air or exhaust,thus the air in the aerocyst is changed,so the force of the splint can be fit. The air pump charge aerocyst must pass through two processes:one is the pump charge aerocyst quickly,the other is the pump charge aerocyst with air slowly and linearly.4. The design of DC-DC transfer circuitIn order to preventing the drain current and the accident by high voltage ,the system adopts battery operation mode to make sure the patient's security. Because the transmitting circuit,amplifying circuit,demodulator circuit,hand-pass filter circuit,A/D converter and PIC16F873 MCU all adopt 5V power supply,and considering the battery's capacity and specification,the system adopts two 1.5V dry batteries power supply. In this way,the DC-DC transfer circuit is the key of the power system design.The system adopts MAX619 to boost DC-DC converter, solving the problem of system power supply design.5. Human-machine interact interface circuit designIn an actual application system, system work according to that user's intention and back the result that system handles to the user at the same time., so an excellently designed human-machine interact interface is necessary. In this paper the control instructions are sent to the system by a key matrix of 4×1, and the results from the system displayed to the operator is given by the LCD.3. ConclusionThe system designed in this paper is based on the ultrasound Doppler blood flow measuring theory and method, studied the relation between the values of peak systolic velocity and the variable of the splint's force. It also pays more attention on the performance requests on concrete applications, designed and constructed Automatic constant splint based on Doppler Ultrasound blood flow. can realize the measurement of blood flow of the artery vessel, the realization of automatic constant manipulative force. Through programme the pwm signal, the realization of automatic constant manipulative force. The experiments indicate that the system designed in the paper has realized the scheduled goals.
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