## 2 Principle of the Measurement Mechanism  
### Measuring Mechanism for Measuring Two Horizontal Parallel Surfaces  
Figure 2 shows the measurement mechanism for measuring the positional relation between two horizontal parallel surfaces. The laser displacement sensor (triangulation) ILD-2300 is selected as the detection element and the technical parameters of the sensor are shown in Table 1.  
Figure 2: Measurement mechanism for measuring two horizontal parallel surfaces.  
The measurement mechanism consists of slider, guideway, spring limit block, laser receiving plate, counterweight block, spring, contact line \(A\) and \(B\), and detection line \(C\). The length of contact line \(A\), \(B\) and detection line \(C\) are 50 mm, and the distance between contact line \(A\) and detection line \(C\) is 60 mm. The contact line \(A\), \(B\) and detection line \(C\) are ground after processing, and the processing accuracy grade is IT3.The measurement mechanism is in equilibrium with the counterweight, and contact lines \(A\) and \(B\) are fixed contact lines. Detection line \(C\) is connected with the measurement mechanism through micro-guideway and springs, the slider block is fixed on the measurement mechanism, the laser receiving plate is integrated with the micro-guideway and detection line \(C\). In the process of measurement, the measurement mechanism is manually driven to move in a fixed direction, under the function of the spring, the detection line \(C\) fluctuates up and down with the change of shape contour the measured surface, the fluctuation is transmitted to the laser receiving plate through the micro-guideway. At this time, the laser displacement sensor detects the fluctuation and the computer records the measurements.  
It is known that the surface contour consists of roughness contour, waviness contour and surface shape contour. The surface shape contour is macroscopic. In this paper, the three-dimensional shape of the measured surface is projected on a plane perpendicular to it, and the macroscopic contour curve obtained is the surface shape contour, as shown in Figure 3. Therefore, the motion trajectories of the contact lines \(A\), \(B\), and detection lines \(C\) are surface shape contour.  
As shown in Figure 4a, according to Table 1, the initial measuring range is 50 mm. When the detection line \(C\) and the contact line \(A\) are collinear, the detection data should be a fixed value, defining it as the initial value \(X_{0}\). When considering the shape contours of the two surfaces, the position of the  
\begin{table}
\begin{tabular}{c c} \hline
**Model** & **Parameter** \\ \hline Frequency & 20 kHz \\ Resolution & 0.3 μm \\ Absolute error & 4 μm (\(\leq\)\(\pm\)0.02\%) full scale \\ Starting measurement range & 50 mm \\ Light source & semiconductor laser (1 mW, 670 nm red) \\ Impact resistance & 15 g/6 ms \\ Operating temperature & 0–50 \({}^{\circ}\)C \\ Power requirements & 24 VDC (11 \(\dots\) 30 VDC), Max 150 mA \\ Security Level & Class 2 according to DIN EN 60825-1: 2001-11 \\ Spot diameter & SMR140 \(\times\) 200 μm, MMR46 \(\times\) 45 μm, EMR140 \(\times\) 200 μm \\ \hline \end{tabular}
\end{table}
Table 1: Laser displacement sensor parameters.  
Figure 3: Detection path.  
detection line \(C\) may change, as shown in Figure 4b. The difference between the measurements and the initial value is the displacement of the detection line \(C\) relative to the contact line \(A\).  
The advantage of this mechanism is that it easy to operate, and suitable for field measurement. It can continuously obtain the stored data which change with the shape contour of the measured surface, instead of some discrete points on the same straight line along the measuring direction. Meanwhile, it can separate the shape contours of the two surfaces, which cannot be achieved by ordinary testing equipment.