## 3 Experiment Results
### The Effect of Feed Speed on Measurement Accuracy and Stability  
The function of laser signal reception of the device is realized by CCD and its signal processing circuit. The speed of image acquisition is limited by the performance of CCD. Moreover, due to the delay of the image processing program, some errors will appear in the process of measurement. When the laser is projected to the upper or lower surface of the hole (Step 2 and Step 4 as shown in Figure 1), displacement data should be recorded immediately. However, due to the response delay of the circuit and image processing algorithms, the starting point of the recorded data will be delayed, which will undoubtedly lead to measurement errors, and the magnitude of the error depends on the feed speed of probe. Therefore, from the above theoretical analysis, the faster the feed speed of the probe is, the greater the error will be. This experiment only uses the simplest way to find out the relationship between feed speed and measurement results, that is, to measure the same hole depth at different feed speeds for many points in time. This paper does not delve into the mechanism of the influence of feed speed on measurement stability and accuracy and how to compensate for it. These issues will be addressed in our subsequent studies. Table 2 lists five groups of actual measured values under a certain feed speed and gives their corresponding average value and standard deviation. In this experiment, the white pixel threshold is set to 100, and the hole with standard depth of 10 mm was measured 10 times under each feed speed (i.e., from 3 mm/s to 12 mm/s), and five groups of measurement data were randomly selected for each feed speed. In order to observe the influence of the feed speed on the measurement accuracy, Figure 10 was drawn, according to the experimental data provided in Table 2. The horizontal axis is the feed speed of the probe, and the vertical axis is the actual measured hole depth. The standard hole depth in this experiment is 10 mm. The speed of the probe is adjusted by controlling the speed of the motor. The feed speed is from 3 mm/s to 12 mm/s. Five groups of measurement data are selected for each speed. Each group of measured data is represented by a curve with certain colour and node shape. Figure 10 shows that the probe has higher stability when the feed speed is slower. As the feed speed increases, the measured data will gradually diverge, the fluctuation will become larger and the stability will be lower. Therefore, it can be concluded that the stability of the measuring device will decrease with the increase of feed speed of the probe. The specific relationship between the accuracy or stability of the probe and the feed speed will be analysed below. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Feed Speed**} & \multicolumn{5}{c}{**Five Groups of Actual Measured Value (mm)**} \\ \cline{2-5}  & **First** & **Second** & **Third** & **Fourth** & **Fifth** \\ \hline
3 & 9.97 & 9.99 & 9.97 & 9.94 & 9.99 \\
4 & 9.95 & 10.01 & 9.91 & 10.00 & 10.04 \\
5 & 9.99 & 10.01 & 9.94 & 10.13 & 10.00 \\
6 & 10.12 & 10.03 & 10.16 & 10.11 & 9.84 \\
7 & 10.14 & 9.94 & 9.91 & 9.94 & 9.85 \\
8 & 10.06 & 9.84 & 10.10 & 9.74 &Figure 11 shows the average value and standard deviation under different feed speeds. The average value of each node is the average of 5 groups of measured data. As can be seen from the figure, the measured value of the probe varies with the increase of feed speed. When the feed speed is less than 5 mm/s, the measured value is close to the actual value. When the feed speed continues to increase, the measured value will fluctuate and deviate from the true value. The standard deviation of the measuring device increases with the increase of feed speed, and the stability of the probe decreases accordingly. As can be seen in Figure 11, the standard deviation can be located within the range of 0.02 mm to 0.07 mm when the feed speed is less than 5 mm/s.