Introduction to the principle of distance measurement of electromagnetic wave range finder
There are two methods of electromagnetic wave ranging: pulse ranging and phase ranging.
pulse ranging
A part of the light pulse emitted by the instrument at one end of the measuring line directly enters the receiving optoelectronic device from the inside of the instrument as a reference pulse; the rest of the emitted light pulse also enters the receiving optoelectronic device after being reflected by the mirror at the other end of the measuring line. By measuring the time t between the reference pulse and the reflected pulse, the distance D can be obtained from the following formula: , where c is the speed of light. The laser rangefinders used in satellite geodesy to measure the moon and artificial satellites all use the pulse ranging method.
phase ranging
The light wave or microwave modulated by high-frequency current is emitted from one end of the measuring line, and after returning from the other end, the phase difference between the emitted wave and the echo is measured with a phase detector. If the modulation frequency is f, the time for the electromagnetic wave to go back and forth is: where n is
The number of whole weeks in time t. Substituting t into the formula of the pulse ranging method listed above, the distance D is: , where λ is the known wavelength of the modulated wave, which is equivalent to the length of the ruler for measuring the distance, and n is equivalent to the number of full feet on the measuring range. A mantissa that is less than one ruler long.
In order to determine the number n of the whole foot, the variable frequency method and the multi-level fixed frequency method are usually used. The former is to make the modulation frequency of the rangefinder change continuously within a certain range, which is equivalent to continuously changing the length of the measuring ruler, so that it can just use up the energy to measure the distance. When measuring the distance, the frequency is modulated successively so that the mantissa of less than a full foot is equal to zero. According to the number of occurrences of zero and the corresponding frequency value, the number n° of the whole measuring ruler can be determined. When the multi-level fixed frequency method is used, it is equivalent to using several measuring rulers of different lengths to measure the same distance. According to the phase difference measured with different frequencies, the integer number n can be solved to obtain the distance D.
In addition to measuring the phase difference with a phase detector, the variable optical path method can also be used, that is, the optical system inside the instrument is used to change the optical path of the received signal to delay the signal for a period of time. When the electronic instrument indicates that the phase of the transmitted signal is the same as that of the received signal, read the mantissa directly on the scale. In addition, a delay circuit can also be used to change the phase of the received signal, and the circuit can adjust the division on the controller and read the mantissa.
