Study On Real-time Positioning System For Horizontal Directional Drilling Based On Ground Magnetic Beacon

Horizontal Directional Drilling(HDD)technology,one of the fastest growing engineering application technology of our country in recent years,plays a dominant role in the field of Trenchless technology.It is widely used in the water supply and drainage,electricity,communications,gas,and other fields.In addition,HDD is also widely used in geological exploration and resource exploitation of geological prospecting,metallurgy and petroleum.But during long and depp HDD construction,the drilling trajectory often deviates from the design trajectory because of the influence of the formation,and needs to correct the direction of drilling or redesign the drilling path,or even abandon the drilled hole.These problems delay the construction period and increase the construction cost.The solution of these problems is to track the position of drill bit in real-time during drilling.The traditional positioning method is to use the MWD(Measurement While Drilling)technique to obtain the azimuth and inclination of the drill bit and than calculates the position of drill bit.But this method has a large accumulative error,which causes a serious trajectory deviation.At present,the mainstream orientation positioning methods include wireless guidance system and grid wire frame direction control system.These methods have a high requirement for construction site conditions(smoothness and obstruction etc.),and have the disadvantages of small scope of application,small detection depth,complicated operation and low positioning efficiency.Moreover,these positioning technologies are basically monopolized by foreign countries,which are expensive and poor after-sales service.Aiming at the problem of detecting the drill bit positioning in HDD,a new HDD positioning method based on ground magnetic beacon is proposed.Compared with other HDD positioning method,this method has the advantages of no need of scanning back and forth,low requirement for construction site condition,simple and easy operation,etc.And this method is totally avoiding the impact ofthe environmental magnetic field.The positioning method is researched systematically,and a prototype of the positioning system is designed and manufactured.The specific research contents are carried out as follows:(1)A new HDD positioning method based on ground magnetic beacon is proposed.A permanent magnet or a DC(direct current)solenoid as a magnetic beacon is placed on the ground.And the position of drill bit is detected by measuring the magnetic fild and magnetic gradient tensor(MGT)of the ground magnetic beacon after establishing the reference coordinate system based on the ground magnetic beacon and the carrier body coordinate system based on the drill tool.However,the location algorithm baesd on the MGT is apparently affected by geomagnetic fields and drilling tools.So the differential treatment of the magnetic field gradient tensor positioning algorithm which is totally avoid the impact ofthe environmental magnetic field is put forward.The influence of environmental interference magnetic field on the magnetic field and MGT measurement is largely eliminated by differential treatment method,and a corresponding HDD model is established.As the MGT localization algorithm may have singular value,an improved HDD model based on Tri-? Criterion and differential treatment is proposed.Using a motor controls the rotation of sensors array while positioning measurement is done.Then,many positioning results of the measuring point are obtained by differential treatment of localization algorithm under different posture of the sensors array.Finally,the singular value and the big error can be eliminated by Tri-? Criterion effectively,and the positioning accuracy is further improved.(2)The measurement precision of several MGT measurement structures are analyzed.Considering the characteristics of horizontal directional drilling,the cruciform structure was selected as the most optimal measurement array structure.The proposed positioning model needs to measure the initial attitude of the drilling tool and the attitude of rotating measuring array.The attitude measurement method based on tri-axial accelerometer and tri-axial magnetometer is susceptible to interference magnetic field.The attitude measurement method based on the tri-axial MEMS gyroscope and tri-axial MEMS accelerometer are chosen as it is absolutely totally avoid the impact of the interference magnetic field.And the concrete implementation process and algorithm of the method are studied.Finally,the positioning system measurement array is desigiend by five MEMS tria-xial magnetometer,one MEMS tria-xial accelerometer and one MEMS tri-axial gyroscope.(3)The magnetic field distribution of the permanent magnet and DC solenoid are analyzed,and two kinds of magnetic beacons are designed.The permanent magnet can be used as a magnetic beacon in an electrically inaccessible place.Due to the large size of the permanent magnet is not easy to make and the cost is very high,acombined permanent magnet beacon comprising of NdFeB permanent magnet and DT4 C magnetic core with larger magnetic field strength and low cost is designed.The design parameters such as the size and structure of the solenoid magnetic beacon are given in this paper,and DT4 C pure iron is designed as the solenoid magnetic core.Duing to the uneven distribution of magnetic field in the horizontal direction of the single permanent magnet beacons or the single-solenoid magnetic beacons,the positioning accuracy of each direction in the horizontal plane is inconsistent.In order to ensure the accuracy of each direction of the horizontal plane,the orthogonal double permanent magnet beacon and orthogonal double solenoid magnetic beacon are designed.(4)Based on the numerical simulation of the proposed HDD positioning model conducted by MATLAB,the influence on positioning and related factors including magnetic beacon and measuring array position relationship,number of magnets,number of enameled wire layers,magnetic beacon posture,precision of magnetometer and base line length are analyzed.The results show that the positioning accuracy of the positioning method can meet the requirements of HDD both in horizontal direction and depth direction,theoretically relative positioning error can be controlled within 3%.The resolution of magnetometer has a great influence on tne positioning accuracy.In limited drilling space,the longer the baseline is,the higer the positioning accuracy is.Interference magnetic fields such as environmental magnetic field have very small influence on this method.(5)The relevant error of the positioning system is analyzed and studied,and the corresponding error compensation methods are put forward.Although the difference processing eliminates the influence of the environment interference magnetic field,the existence of the three-axis magnetometer measurement error has influences on the magnetic field and the magnetic field gradient measurement.Therefore,the total error compensation method based on BFGS quasi-newton method is proposed in this paper.The total error parameters give full consideration to the internal error of the three-axis magnetometer and the external environmental error.Considering that the noise in the initial attitude measurement will cause the initial attitude of the measurement to be out of alignment,the discrete wavelet threshold denoising method is used to removal the noise of the signal to reduce the misalignment angle so as to improve the accuracy of the initial attitude.The attitude measurement correction algorithm based on trust region method and complementary filtering is proposed to modify the attitude drift of gyroscope.According to the measurement data of the accelerometer and the magnetometer,the optimal estimation of the quaternions by the trust redion method is carried out.And then,a complementary filtering algorithm is adopted to fuse the data from the quaternion of the gyroscope update and the quaternion of the accelerometer and the magnetometer.The precision and dynamic characteristics of continuity attitude measurement are improved based on the data fusion.(6)The hardware and software of the positioning system is specifically designed.STM32F103ZET6 is used as the MCU(micro controller unit).and stepper motor rotation is controlled by TMS320F28335 DSP.The communication between the slave computer and the host computer is connected through the RS485 protocol.Adn then,the prototype of the measuring system is finally produced.The host computer software was developed with MATLAB GUI.The software implements serial communication.The profile and the plan of the design trajectory can be displayed in the host computer software by inputting the control point information of the design trajectory.The attitude of the drilling tool can be displaied in real time.And the position of the drill bit can be shown in real time in the profile and the plan of the design trajectory,so as easy to correct the direction of drilling.(7)A preliminary positioning test was carried out to verify the effect of the HDD positioning method.The results of the test and the simulation results are coincident approximately.And it show that the HDD positioning method has good practical application effect.