# Position Sensing with a Compact 0.1 THz Amplitude Modulated Source
## 3 Results
### Distance Measurement  
The distance between the source and the aluminium plate was about 75 cm, which also corresponds to the distance between the sensor and the aluminium plate. The THz wave's angle of incidence on the plate was 37\({}^{\circ}\). The value of the angle is a compromise between the desired resolution and measurement range. As a result of the triangulation, a bigger angle of incidence means a better measuring resolution but smaller measuring range, and vice versa. The incidence angle was defined by some preliminary measurements to satisfy both the measurement range and resolution. A distance manipulator was used to manually change the height of the plate. The measurement protocol consisted of 40 measurements per height with a sensor scanning step of 1 mm. The sampling rate of the system was 50 kHz with 5000 samples per position for data acquisition, so the minimal requirement was to stay at a position for at least 100 ms.  
Figure 5: Demonstration of the Scheimpflug principle. Since a stable measurement was desired after the movement, a 500 ms delay was applied after each movement of the sensor to minimize vibrations. Therefore, additional noise in the signal caused by sensor vibrations was avoided. All the data acquisition and signal processing were conducted on a 1 kHz signal. The signal acquisition time was 100 ms, which is enough for FFT calculations to obtain an accurate signal representation. Thus, the entire scanning process takes about 20 s for the whole 40 mm scanning range, with the acquisition of one sample per 1 mm step. For the measurement concept test, long acquisition time was not an issue. It was assumed that the system will be used in applications where the observed surface distance to the THz system will not change drastically in the few seconds of measurement. After the completed scanning process, the vertical plane of the plate was changed for 5 mm and the measurement was repeated. This protocol was repeated 12 times, resulting in 12 different vertical aluminium plate levels. Additional measurements were taken with additional 5 cm Styrofoam covers and with the same measurement protocol. As expected, Styrofoam did not influence the measurement. The results of the starting vertical height, maximum vertical height and minimal vertical height measurement with and without Styrofoam covering can be seen in Figure 6.  
In Figure 6, the signal labelled 0 mm represents the initial plate level (as shown in Figure 1a). The signal labelled with \(-\)30 mm represents a lifted plate (as shown in Figure 1b) for 30 mm and the signal labelled 30 mm represents a lowered plate for 30 mm (as shown in Figure 1c). The \(x\)-axis represents the distance travelled by the sensor, 0 mm represents the closest point and 40 mm represents the point furthest from the source. Each marked point on the resulting curve represents a measurement during scanning. From the results shown in Figure 6, how the level of the plate affects the measurement and that the maximum amplitude of the THz illuminations changes along the scan line can be understood. The results confirm the presentation of the measurement setup described in the previous section. When the plate was vertically higher and the travelled distance of the THz signal was shorter (label \(-\)30 mm), the lens focused the signal on the point of distance travelled by the sensor of about 26 mm. When the plate was lower and the travelled distance of the THz signal was longer (label 30 mm), the lens focused the signal on a point of the distance travelled by the sensor of about 14 mm. When the plate was at the initial reference position (label 0 mm), the lens focused the signal on the sensor whose scanning distance was at about 20 mm. Figure 6: Results of the vertical level detection of the aluminium plate with or without Styrofoam covering. The displacement of the maximum vertical height and minimal vertical height measurement corresponds to about 12 mm scanning distance difference.