Trimble X7: Auto-Calibration Eliminates the Guesswork

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Inside the Trimble X7: Deep Dive into Trimble X7 Auto-Calibration 9 9 Automatic Distance Calibration The Electronic Distance Measurement (EDM) is a key component to the performance of a 3D laser scanner. A high quality 3D point cloud is dependent on the EDM performing consistently and reliably over extreme working conditions. In the X7, Trimble has integrated proprietary technology in order to guarantee a survey-grade measurement performance for its customers. One of the challenges to address is to guarantee a range accuracy over the lifetime of the instrument, at all temperatures and for all target reflectivity. In order to achieve this, Trimble has integrated into its EDM an innovative calibration routine that is carried out before every scan. The X7 EDM is based on the principle of pulsed time-of-flight. In figure 5 we show a schematic representation of the EDM. Figure 5: Schematic of EDM The laser is triggered to send a high power laser pulse. At the same time a "Start" signal is sent to the Time-to- Digital Converter (TDC) to start the "stopwatch". The laser pulse travels to the object to be measured at the speed of light. A small amount of the laser pulse is reflected off the object and travels back into the telescope and is detected by a photodetector. The photodetector measures the signal level and sends a stop signal to the TDC, which stops the "stopwatch", and the time-of-flight of the laser pulse is measured. This is then converted to a measurement of distance. The X7 operates between a large temperature range of -20°C to 50°C. In addition it has a very large dynamic range, meaning that it can detect both very weak and very strong optical signal levels. In order for the X7 to measure accurately the absolute distance, it uses an innovative auto-calibration technique. In figure 6 we show the modified schematic of the EDM whereby a copy of the emitted laser pulse is sent through a calibration line. Figure 6: Schematic of EDM with calibration line

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