| With the rapid development of the national economy,the power supply is also experiencing unprecedented tremendous development as the industrial lifeline.The scale of power system grows larger,the voltage level of the transmission lines improves continuously and the transmission distance gets farther and farther.Therefore,as the result of growing number of cables and complicated laying path after a long time operation,a variety of faults including disconnected fault appeared and made it difficult to keep in repair.And both huge economic losses and much inconvenience to everyone are caused.Presently in the oil field electronics technology service companies and project departments of China National Petroleum Corporation equipments which used to maintain the data-transmission cable are updated annually,but only to deal with some common hard faults and powerless in soft faults such as codes missing,crosstalk and attenuation in data-transmission.Situations occurred frequently such as the repaired cables suddenly call impassability arrangement in the field in construction and production of seismic crews in the field.Back to overhaul with existing instrument to check that everything is normal,but can’t detect the soft faults.In electrical equipment trading wholesalers and suppliers have to measure a lot of cables into a disk everyday.Although there are existing measuring instruments abroad, they are usually too expensive to be widely used in domestic.To solve these problems,the paper aims to design an effective and low-cost device to measure the cable length.And with the device,we can detect the disconnected fault and determine the fault location.The cable length measuring has a wide application range including measuring the building height,the well depth,distance and detecting the disconnected fault of communication cables,power cables and telephone lines,in addition to measuring the length of the transmission lines.If effective cable length measuring device can be used when cable fault occurs,the fault location will be immediately determined and solved as soon as possible.That can greatly reduce the labor intensity of maintenance staff and minimize the physical and economic losses.Therefore,the paper introduced and compared the existing cable length measuring methods first and focused on bridge method and low-voltage pulse reflection method,then studied the low-voltage pulse reflection method as an emphasis.The paper discussed the wire properties about pulse reflection method,theories about electromagnetic waves spreading in transmission lines and then summarized the measuring principle and several related parameters of pulse reflection method.The basic principle of pulse reflection method is that a voltage pulse is transmitted into the wire under test and reflects on the point which impedance mismatched.After the reflected pulse is received by the receiving device and based on the principle that the time difference between the transmitted pulse and reflected pulse is directly proportional to the distance the pulse travelled,we can determine the location of the point which impedance mismatched thus measure the length of uniform conductor or detect the location of disconnected point.The proportionality coefficient is the speed that the pulse travels in the wire.Based on the measuring principle,the paper discussed the equivalent model of single uniform conductor and derived equivalent circuit by mathematical analysis.After this mathematical model was established by MATLAB/Simulink mathematical simulation software and a simulation experiment was successfully carried out.Result from the experiment simulation waveforms of the incident pulse and the reflected pulse were obtained and it laid a theoretical foundation for program design and hardware design.Based on the microcontroller combined with high-performance FPGA chip,the paper designed a set of cable length measuring solution adopting the method of transmitting a incident pulse into the wire under test and receiving the reflected pulse reflected from the destination or disconnected point calculating the round-trip time difference between the two pulses and then measuring the length according to the speed that the pulse travels in the wire.In the specific implementation of the program,the paper finished the designation of the micro-controller module,FPGA measuring and displaying module,pulse transmission and reception circuit,pulse shaping circuit,high frequency counter module and power circuit of the system and then described the function of each part in detail.At the end of the designation part,the program algorithm and flow charts of system software and FPGA measuring module followed the applications of the SDRAM and serial configuration device.After the theoretical design completed,physical hardware test platform based on pulse reflection theory and micro-controller and FPGA chip was established.Then transmission lines with different lengths and different insulating materials were tested.The data result from the test was analyzed and the causes of errors and the methods and directions to improve the designation were discussed,too.The analysis showed good repeatability and high precision of the test device and it can be used to measure the lengths and detect the disconnected points of transmission lines.It has a certain practical value.Followed achievement were obtained according to the designation and test.(1) Based on the development status of cable length measuring of the domestic and foreign,the paper introduced and analyzed the basic principle characteristic and insufficiency of each measuring method,discussed how to calculate various parameters of the transmission line and derived the long-line module and the equivalent circuit of transmission line.The length measurement principle and related conception of pulse reflection method were introduced and regarded as the foundation of simulation and designation.(2) Simulation model was established by MATLAB/Simulink and a simulation experiment was successfully carried out.That proved the correctness of the design idea.(3) The width of the incident pulse influences the measuring precision a lot.Wider the width,greater the measuring blind spot,then lower the precision.But much too narrow pulse contains too much high frequency components and increases the loss in dissemination process.It results in severely distorted reflected wave and affects the long-distance wire measuring effect.Therefore precision can be improved by choosing pulse of suitable width.In addition, in case of reflected pulse of much too low amplitude,incident pulse of higher amplitude should be adopted if the transmission line under test is too long.Otherwise,the precision of time difference will be severely affected when it passes through the comparing and shaping circuit.Extremely,the amplitude is too low to get the shaped pulse to measure because it’s lower than the threshold voltage of the comparator.(4) By4-fold the original clock signal and phase-shift,4-way high-speed counting pulses were derived to improve the precision through averaging the4-way counting value.At the same time,the maximum operating frequency of the system as a whole remains unchanged.(5) The paper designed a set of devices to measure the length of transmission line and tested transmission lines of different lengths and insulating materials. The data result from the test was analyzed and the analysis showed good repeatability and high precision of the test device.Bu the experimental device was still in its infancy,and there were a number of shortcomings,such as the precision,could be improved.For the reason that some methods were put forward as the reference directions of future research. |
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