Application of ppm in measurement
1mm+1ppm is the abbreviation of 1mm+1ppm×D (km), which reflects the nominal ranging accuracy of total station or range finder. Among them:
1mm represents the fixed error of the instrument, which is mainly caused by the measurement error, centering error and phase measurement error of the instrument plus constant, and the fixed error has nothing to do with the measured distance. That is, no matter how far the actual distance is measured, there will be a fixed error of the total station that is not greater than this value.
1ppm×D km represents the proportional error, where 1 is the proportional error coefficient, which is mainly caused by instrument frequency error and atmospheric refractive index error. Ppm means parts per million (a few), and D is the distance actually measured by total station or range finder in kilometers. With the change of the actual measuring distance, the proportional error of the instrument also changes proportionally. For example, when the distance is 1 km, the proportional error is 1 mm.
For a total station or rangefinder with a ranging accuracy of 1mm+1ppm, when the measured distance is 1 km, the ranging accuracy of the instrument is 1mm+1ppm×1 (km) = 2mm, that is to say, when the total station measures 1 km, the maximum ranging error is not more than 2 mm. 。
Three seconds is the time for ranging.
Structure composition of infrared range finder
The infrared rangefinder is mainly composed of modulated light emitting unit, receiving unit, phase measuring unit, counting display unit, logic control unit and power converter. Its light source is usually GaAs semiconductor light emitting diode. When a considerable current passes through the P-N junction of GaAs diode, near-infrared light with the wavelength of 0.72μm and 0.94μm will be emitted in the P-N junction, which is caused by the excess energy released in the form of photons when electrons and holes recombine in doped GaAs semiconductor. And the emitted light intensity will change with the change of injection current. Therefore, if it is used as the light source of the range finder, the emitted light intensity can be directly modulated by changing the size of the feed current, that is, this semiconductor light emitting device has dual functions of "radiation" and "modulation".
Infrared photoelectric detection and conversion devices used to receive modulated light are usually silicon photodiodes or avalanche photodiodes, which have "photovoltaic effect". When external light shines on its P-N junction, due to the effect of photoelectric energy conversion, a potential difference can be generated at the P-N poles, and its magnitude will follow.
The intensity of the emitted light changes, thus playing the role of "demodulation".
