# Research on Data Transmission of Laser Sensors for Reading Ruler
## 4 Sensor Technology and Code Reading Error Analysis
### Laser Ranging Sensor  
A laser ranging sensor also has two main parts, a laser transmitter and a photoelectric receiver, but the transmitter and receiver of the ranging sensor are mounted on the same side as the correlative laser sensor, and the laser transmitter emits a laser beam that strikes the target surface and is reflected. The photodetector receives the reflected light signal and calculates the distance of the target object through measurement, which gives the laser sensor extremely high measurement accuracy and stability. (1) Pulse laser ranging  
The pulse method is also known as the laser echo method. Its core principle is to use the flying time difference of the laser for accurate ranging. This method makes full use of the characteristics of the laser pulse; that is, the pulse duration is very short, and the energy is relatively concentrated in time so as to achieve instantaneous, high-power output. In the case of a cooperative target, the pulse method can be used to perform accurate, long-distance measurements. Even in a relatively close range (several thousand meters), this method can lead to effective ranging in a low-accuracy scenario even if there is no cooperative target [17]. The operating principle of pulsed laser ranging, as shown in Figure 3, is as follows: The laser emission system emits a pulse laser with a very short duration, which is reflected by the target object after passing through the distance L to be measured. The reflected pulse laser signal is then captured by a photodetector in the laser receiving system. The time difference t between the laser emission and the arrival of the echo signal is accurately calculated by the time interval circuit, and the accurate distance L between the target object and the emission source can be calculated according to the principle of constant light velocity. (2) Phase laser ranging  
The phase-type laser range finder uses the frequency of the radio wave band to conduct amplitude modulation on the laser beam and measure the phase delay generated by the modulation light back and forth through the measuring line. The distance represented by the phase delay is then converted according to the wavelength of the modulation light [18]. Indirect methods are used to determine the time required for light to travel back and forth through the measuring line. If the angle frequency of modulation light is \(\omega\) and thephase delay generated by one round trip on the distance to be measured \(D\) is \(\phi\), the time \(t\) corresponding to one round trip can be expressed as follows:  
\[t=\frac{\phi}{\omega} \tag{1}\]  
If the propagation velocity of light in the atmosphere is \(c\)  
\[D=\frac{c\cdot t}{2} \tag{2}\]  
Substituting Equation (1) into Equation (2), the distance \(D\) can be expressed as follows:  
\[D=\frac{c\cdot t}{2}=\frac{c\cdot\phi}{2\omega}=\frac{c}{4\pi f(N\pi+\Delta N )}=\frac{c}{4f(N+\Delta N)} \tag{3}\]  
where \(\phi\) is the total phase delay generated by the signal round-trip survey line at one time; the angular frequency of the modulation signal \(\omega=2\pi\cdot f\); \(N\) is the number of modulation half-wavelengths included in the measuring line; \(\Delta\phi\) is the phase delay less than \(\pi\) generated once by the signal round-trip measuring line; and \(\Delta N\) is the fractional part of the modulation wave of less than half a wavelength included in the measuring line, \(\Delta N=\phi/\omega\). Under the given modulation and standard atmospheric conditions, the frequency \(c/(4\pi\cdot f)\) is a constant. At this time, the measurement of distance becomes the measurement of the number of half-wavelengths included in the measuring line and the measurement of the decimal part less than half-wavelength, i.e., measuring \(N\) or \(\phi\). The measurement of \(\phi\) has reached a very high level of accuracy due to the development of modern precision machining and radio phase measurement technology. In order to measure the phase angle \(\phi\) that is less than \(\pi\), different methods can be used.