## 2 Approach Description
### Hardware Design  
In this section, the hardware design of the proposed measuring device is described in detail, and the hardware interrelationship is explained. From a structural point of view, the device is divided into three parts: the outer shell, the inner shell, and the probe, as shown in Figure 4. The laser transmitter, the CCD sensor, the beam splitters, and the nut are in the inner shell. The MCU, the motor, the coupling, and the screw are in the outer shell. The fixed ruler and movable ruler of the capacitive grating displacement sensor are, respectively, installed in the inner shell and the outer shell. The laser transceiver unit comprises of the laser transmitter, the CCD and the beam splitter. The displacement measuring unit comprises of the capacitive grating displacement sensor, the MCU, the outer shell, and the inner shell. The driving unit comprises of an electric, a coupling, a screw, and a nut. The CCD is packaged with a convex lens and forms a miniature camera. The camera is fixed by the two sets of fixing frames above the beam splitter. The outer shell and the inner shell are nested structures, the motor is fixed to the end of the outer shell, and the output shaft of the motor is connected to the screw through the coupling. The nut is fixed on the inner casing and is threaded on the lead screw. The MCU for logic control is also disposed at the end of the outer shell. Figure 4: Internal structure of the measuring device; MCU: microcontroller unit. From the perspective of electrical hardware, the measurement system is divided into three units: laser transceiver unit, displacement measuring unit and driving unit. In the laser transceiver unit, the laser is projected through the designed optical path to the hole wall and finally returned to the CCD. This unit is used for state determination during probe measurement. The drive unit is used to implement the telescopic movement of the probe, and during this process, the inner shell and the outer shell are relatively displaced. The function of the displacement measuring unit is to measure the relative displacement between the inner shell and the outer shell. The core component of this unit is the capacitive grating displacement sensor, the function of which is to measure the displacement of the probe. Finally, the hole depth parameter is obtained by intercepting the displacement recorded in the process of laser scanning the hole wall. The structure design of the device is shown in Figure 5. The probe and the inner shell are hollow structures, and the diameter of the probe is only 3 mm, which enables the probe to penetrate into holes with smaller diameters. Even more, the probe can be further miniaturized by using smaller optical lenses. Thus, the device has the ability to measure the depth of holes with a large depth-to-diameter ratio. The designed optical system is placed in the inner shell and the probe. The laser transmitter cannot be placed in the small probe because of its large volume. Therefore, using the designed optical system, the laser emitted from the laser transmitter can be transferred from the inner shell of the device to the probe tip. The laser reflected from the hole wall is received by the CCD located in the inner shell. The outer shell and the inner shell of the device form a retractable structure through the screw and the nut. The inner shell moves forward or backward along the common axis of the outer shell and the inner shell is driven by the rotation of the motor. The capacitive grating scale (fixed ruler) and the capacitance grid ruler (movable ruler) are combined to become a capacitive grating displacement sensor [30], wherein, the movable ruler is arranged in the inner shell, and the fixed ruler is mounted on the outer shell. When the relative displacement between the inner shell and the outer shell occurs under the drive of the motor, the movable ruler slides on the fixed ruler and get the relative displacement data. Figure 5: Schematic diagram of equipment structure design.