Study of a Small Robot for Mine Hole Detection

Abstract

China is rich in coal resources, but complex hydrogeological conditions lead to difficulties in coal mining, including coal mine collapses, roof and water damage, and other accidents that occur frequently, resulting in many casualties and property losses. The use of coal mine hole detection technology to detect and analyze the internal environment of the coal mines in advance helps to reduce safety hazards and prevent coal mine accidents; however, the operation of existing coal mine hole detection technology is cumbersome, difficult to control, and encounters problems due to an insufficient depth of jacking. This paper designs a new type of small robot for mine hole detection. Firstly, we analyzed the function and structural design of the mine hole detection robot, designed a variable diameter function according to the characteristics of narrow and uneven mine holes in coal mines, and analyzed the mechanics of the critical parts using theoretical calculations. Secondly, using three-dimensional modeling software (Solidworks 2019), we established a structural model of the small robot for mine hole detection. After that, we designed a hardware circuit and control program for the robot and emphasized the safety design of the circuit, considering the presence of water and gas inside the coal mine. Finally, to verify the feasibility of the design program, the basic parameters and function tests of the mine hole-detection small robot were carried out. The experimental results show that the developed mine hole-detection small robot can adapt to working hole diameters from 65 mm to 100 mm and has a maximum working power of only 12 W and a maximum crawling speed of 3.96 m/min. The maximum crawling slope reaches 90°, which can meet existing mine hole inspection needs. This research provides theoretical and design guidance for developing mine hole-detection robots with substantial engineering practical reference values.

1. Introduction

Coal is a vital fossil energy source and the foundation of China’s energy system. China is rich in coal resources, but the complex geological conditions of coal mines and the difficulties in mining have led to frequent coal mine production accidents in China, and there is a significant gap in the safety situation of coal mines compared with that of developed countries [1,2,3,4,5,6]. According to relevant statistics, between 2008 and 2021, the total number of coal mine accidents in China reached 1157, causing a large number of casualties and property losses, of which roof accidents and water damage accounted for 44.43%, resulting in 514 deaths; this represents a severe hazard [7,8]. These accidents are closely related to local hydrogeological conditions, and the use of mine hole-detection technology can determine the stratigraphic conditions of coal mines, which can effectively reduce the risk of coal mining. A coal mine borehole is usually up to 60 m long, with a particular slope, slight and uneven borehole diameter, slippery borehole, and the presence of flammable and explosive gas. The existing mine borehole-detection technology uses the manual push rod jacking probe method, using the internal environment of the borehole to shoot and subsequent image processing recognition and other work to assess the condition of the stratum. The manual advancement of this method of operation is cumbersome, the jacking depth is insufficient, the speed is difficult to regulate, and the imaging results are poor. Therefore, the development of a small mobile robot integrating various detection functions is of great engineering significance for safeguarding safe production in the mineral industry.

The mine hole-inspection robot belongs to the pipeline robot category. Pipeline robots represent electromechanical integrated systems capable of navigating through and operating inside or outside boreholes. Pipelines constitute critical components of modern industrial transportation, necessitating regular inspections as a crucial safety measure. Scholars worldwide have designed various pipeline-inspection robots catering to different pipeline structures and usage environments, laying the foundation for developing mine borehole-inspection robots [9,10,11,12]. Different driving methods divide pipeline robots into two categories: external load-driven and self-driven robots.

External load-driven robots mainly refer to PIGs, which are now widely used in the oil and gas industry and are based on the principle of utilizing rubber discs that move under the pressure of fluids in the pipeline. Xiong Yi and colleagues (2019) designed a novel foam smart PIG for oil and gas pipelines with good pass-through capability to detect pipeline defects while performing pipeline cleaning. However, such robots’ particular power source limits their application environments to sealed pipelines filled with flowing media [13,14,15,16,17].

Self-driven pipeline robots have their own power source and are categorized into serpentine robots, crawler robots, wheeled robots, and other types, depending on how they move [18,19,20]. Serpentine pipe robots have strong crawling ability and can adapt to complex environments. Paulo Debenest designed a serpentine pipe robot named PipeTron, whose main body is divided into several parts; each part has an active wheel and uses the wall-pressing method of walking. The parts are connected with each other via deflector joints, which possess good crawling and traction ability; however, the robot’s energy consumption is large, and it is complicated to operate [21]. Mariko and colleagues improved the structure of the snake robot to allow for traveling in a rolling fashion, which grants the robot a strong obstacle avoidance capability; however, this robot has limiting requirements concerning the pipe diameter, meaning it cannot adapt to small caliber pipes [22,23]. Tracked pipeline robots have a large support area, slight ground-specific pressure, resistance to slip, good traction adhesion performance, and good passability. Bogadan designed a tracked pipeline robot in 2021 that innovatively used a flexible articulated permanent magnetic track to generate local adhesion force, which allowed the robot to be adsorbed to the pipe wall for movement and solved the passability problem of non-clearable pipes. However, this kind of magnetic track only works in ferromagnetic pipes, and the robot’s crawling performance in non-magnetic orifices is significantly weakened [24]. Zhao and colleagues designed a tracked pipeline robot with three tracks uniformly distributed on the outside of the body 120 degrees apart and an electric actuator, which can independently adjust the radius of the tracks to adapt to different pipeline sizes and adjust the pressure between the tracks and the pipe wall to regulate the traction force and pressure wall. It can adjust the traction force by adjusting the pressure between the tracks and the pipe wall, and the method of pressing the wall makes it suitable for pipes made of various materials; however, the tracks of the pressure-wall tracked pipeline robot cannot be retracted inside the body, which makes the space utilization rate low and difficult to miniaturize, and it is not suitable for pipes under 100 mm [25]. Wheeled pipeline robots are the most widely used because of their excellent maneuverability and high efficiency. A simple wheeled pipeline robot only relies on the crawling wheel and the lower side of the pipe wall contact to travel. However, the structure is simple, the traction force is insufficient. Cao designed a six-wheeled pressure-wall pipeline robot with each of the two ends of the three wheels distributed at 120°. The ball screw forms the two sides of the wheels, which can be retracted and released to regulate the pressure between the crawling wheels and the pipe wall, thus allowing the robot to adapt to the diameter of a pipe from 100 mm to 200 mm. The robot is flexible and stable, but the structure is too exposed to adapt to a waterlogged environment [26,27]. Sawabe designed an articulated wheeled pipeline robot that is flexible in its movement, strong in obstacle avoidance, and easy to miniaturize. However, its structure and control method are too complex, and its performance is unstable [28].

In summary, although the pipeline robot technology has matured, the existing pipeline robots are often designed for specific environments, and simple modifications cannot adapt these robots to new environments. In 2006, the rescuers for the U.S. West Virginia Pressure Coal Mine Rescue Operation used conventional rescue robots to enter the underground to detect the situation, but the robot could not continue to move forward in the wells stuck in the mire. In 2010, on the New Zealand island west of the Atyrau Pike River, the rescuers for a coal mine gas explosion accident attempted to detect the underground situation with a simple spark-proof modification to a pipeline robot. However, the robot short-circuited underground due to waterproofing problems. Therefore, pipeline robots in particular environments must be specialized according to specific conditions [29]. The main task of the coal mine hole-detection robot is to carry a camera with various types of sensors to photograph the coal mine stratum and detect the environmental parameters in the hole. Considering the task requirements and the actual mine hole environment, the robot’s diameter should not be too large, and it should have a specific adaptability to changes in the hole diameter and an excellent crawling ability. The robot should also be waterproof and explosion-proof and should be able to run smoothly in the mine hole in order to ensure the effect of filming and detection.

To address the above needs, a small-wheeled robot is proposed in this paper. Firstly, the functional analysis and structural design of the robot were carried out according to the engineering requirements, the key components were mechanically modeled, and their specific structural parameters were calculated. Secondly, a three-dimensional model was constructed, and the design of the hardware circuit and control program was carried out. Finally, the robot was machined with 3D printing, and a practical test was conducted to test the basic parameters and crawling ability of the robot. In this paper, the design of the mine hole-inspection small robot included a cable power supply, a minimum body diameter of 65 mm, and a variable diameter function able to adapt to 65 mm to 100 mm pipe. The gear drive form was used in order to ensure low power consumption and, at the same time, to provide sufficient traction to meet the need for dragging the cable and ensure the smooth operation of the robot. The waterproof and explosion-proof design can adapt to the unique environment of coal mine holes containing water and gas. Compared with manual propulsion, the mine hole robot can better adapt to small-diameter mine holes. The speed is stable and adjustable, with a long moving distance. It is safe and reliable and can significantly improve the effect of mine hole detection and promote the intelligence and safety of the production of the coal mining industry.

2. Functional Analysis and Mechanical Modeling

The design of mine hole-inspection small robots faces two challenges to ensure the robots work properly in coal mine holes. The first challenge is designing the robot to navigate swiftly and smoothly within a mine hole; the second challenge is ensuring that a robot can maintain its mobility even when encountering steep slopes or variations in hole diameter.

The functional analysis of a robot is shown in Figure 1. For the first problem, the crawler wheel is used as the walking mechanism of the small robot during mine hole inspection. The motor’s rotation is conveyed to the crawler wheel via the drive arm, providing a stable driving force for the robot. For the second problem, a new type of adjustment mechanism is designed, in which the drive arm of the robot can be extended and retracted with the cooperation of the stepper motor, the screw nut, and other components. This design enables the robot to adjust the positive pressure, stabilize its body, and navigate through mine holes with significant inclines.

2.1. Mechanics Modeling

Figure 2 shows a force analysis diagram of the crawling wheel of a small robot used for mine hole detection. The rotary motion of the motor on the left is transferred to the ball screw, and the nut is driven to move back and forth via the rotation of the screw to convert the rotary motion of the motor into the linear motion of the nut. The sleeve moves back and forth with the nut, thus driving the push rod and the drive arm up and down, realizing the crawler wheel’s extension and retraction and the reducer’s function and regulating the positive pressure. Exploring the relationship between the pushrod thrust and the positive pressure on the crawler wheel can help with stepper motor selection and traction evaluation.

To model the mechanics, the axial thrust provided by the sleeve actuator is determined by the spring elasticity and the fixed position of the screw nut, which is held in place by the stepper motor turning the screw nut to the proper position. The gravity of the rod is ignored in the calculations due to the light weight of the rod and the fact that only the relationship between thrust and positive pressure is discussed here.

According to the geometric relationship of the structure, Equation (1) can be established to indicate the initial position in the horizontal and vertical directions.

$$\left\{\begin{array}{c}x=L_1\cos {\alpha }_1+L_2\cos {\alpha }_2\\ y=L_1\sin {\alpha }_1=L_2\sin {\alpha }_2\end{array}\right.$$

(1)

The variation in both sides of Equation (1) gives Equation (2):

$$\left\{\begin{array}{c}\Delta x=L_1\sin {\alpha }_1\times \Delta {\alpha }_1+L_2\sin {\alpha }_1\times \Delta {\alpha }_2\\ \Delta y=L_1\cos {\alpha }_1\times \Delta {\alpha }_1=L_2\cos {\alpha }_2\times \Delta {\alpha }_2\end{array}\right.$$

(2)

Integrating Equation (2) yields:

$$\Delta x=(\tan {\alpha }_1+\tan {\alpha }_2)\times \Delta y$$

(3)

Based on the principle of imaginary work, that is, assuming an imaginary displacement, the force $F$ multiplied by the imaginary displacement is equal to the imaginary work, and the total imaginary work is 0.

$$N\times \Delta y=F_2\times \Delta x$$

(4)

Substituting Equation (3) into Equation (4), the relationship between thrust and positive pressure is obtained as follows:

$$F_2=\frac{N}{\tan {\alpha }_1+\tan {\alpha }_2}$$

(5)

$F_1$ can be calculated using Equation (6):

$$F_1=\mu \times N$$

(6)

In Equation (6), $\mu$ is the coefficient of friction, and substituting Equation (5) into Equation (6) yields the relationship between the traction force $F_1$ and $F_2$:

$$F_1=\mu \times F_2\times (\tan {\alpha }_1+\tan {\alpha }_2)$$

(7)

2.2. Shaft Strength Calibration Calculation

The shaft, a crucial transmission component of the robot, is tasked with supporting the rotating parts and facilitating motion and power transfer. To achieve the design goals, it is necessary to consider both the structural dimensions and the operational capacity. The precise structural dimensions of the shaft are defined by its specific mounting position and its designated function. The operational capacity of the shaft is calculated taking into account factors such as strength, stiffness, and vibration stability. Typically, the operational capacity of the shaft relies primarily on its strength, particularly with regard to the conditions of torsional strength. Based on the mechanical design manual, it is known that the torsional strength of the shaft should be satisfied:

$${\tau }_T=\frac{T}{W_T}\approx \frac{\mathrm{9,550,000}\frac{P}{n}}{0.2d^3}\le [{\tau }_T]$$

(8)

In Equation (8), ${\tau }_T$ is the torsional shear stress, MPa; $T$ is the torque applied to the shaft, Nmm; $W_T$ is the torsional shear factor of the shaft, mm; $n$ is the speed of the shaft, r/min; $P$ is the transmission efficiency of the shaft, kW; $d$ is the diameter of the shaft at the calculated section, mm; and $\left[{\tau }_T\right]$ is the allowable torsional shear stress, MPa.

We limit the output power of the motor to 8 W, so the power allocated to a single-driven bevel gear is 4 W. Then, taking $P=4\mathrm{W}$, the traction force required by the mine hole-inspection robot is 150 N. Then, taking $F=200\mathrm{N}$, based on $P=FV$, it can be deduced that the robot’s moving speed is 0.06 m/s. The crawling wheel radius is 22.5 mm, as calculated using Equation (9):

$$1 \mathrm{m}/\mathrm{s}=\frac{30}{\pi R} \mathrm{r}/\min$$

(9)

It can be deduced that the speed of the shaft is 25.5 r/min, and the shaft diameter condition can be obtained using the torsional strength condition of the shaft:

$$d\ge \sqrt[3]{\frac{9550,000}{0.2[{\tau }_T]}}\sqrt[3]{\frac{P}{n}}=A_0\sqrt[3]{\frac{P}{n}}$$

(10)

where 45 steel is chosen as the material of the shaft. Then, $\left[{\tau }_T\right]$ is taken as 30 and $A_0$ is taken as 110. Substituting into Equation (10), we obtain $d\ge 5.93$, and thus, the design dimension of the shaft should be 6 mm.

Finite element simulation software (Abaqus 2021) can effectively verify the force of the components in the limit state. When the robot is obstructed and the crawler wheel stops, the motor’s output torque reaches its maximum, and at this time, the drive shaft will be subjected to a considerable load and reach the limit state.

The gear motor’s maximum output torque is 8.8 Nm, and the bevel gear ratio is 1. Neglecting unfavorable factors such as friction, i.e., the maximum torque the drive shaft is subjected to is also 8.8 Nm, the post is modeled in finite element software, the torque is added at the right end of the stick, and the constraints are fixed at the left end to simulate the case of a blocked stop of the crawler wheel.

Figure 3 shows the strain cloud on the left side and the stress cloud on the right. The analysis results show that the portion where the maximum strain and the maximum stress appear is near the end where the load is added. In addition, the shaft holes on the bevel gears should not be oversized during subsequent machining and assembly to improve the tightness and stiffness of the mounting area and reduce deformation.

2.3. Analysis of the Effect of the Regulating System on Positive Pressure

In Figure 2, it can be seen that a spring is used as a buffer in the adjustment system. When there is a small change in the diameter of the hole, the buffering effect of the spring can ensure reliable contact between the wheel and the wall of the hole, as can be seen from the geometric relationship in the figure:

$$\left\{\begin{array}{l}OA=L_1\cos {\alpha }_1+L_2\cos {\alpha }_2\hfill \\ L_1\sin {\alpha }_1=L_2\sin {\alpha }_2\hfill \\ D=L_1\sin {\alpha }_1+R\hfill \end{array}\right.$$

(11)

By differentiating Equation (11), Equation (12) is obtained:

$$\left\{\begin{array}{l}\Delta OA=-L_1\sin {\alpha }_1\Delta {\alpha }_1-L_2\sin {\alpha }_2\Delta {\alpha }_2\hfill \\ L_1\cos {\alpha }_1\Delta {\alpha }_1=L_2\cos {\alpha }_2\Delta {\alpha }_2\hfill \\ \Delta D=L_1\cos {\alpha }_1\Delta {\alpha }_1\hfill \end{array}\right.$$

(12)

Simplifying Equation (12) yields:

$$\Delta OA=-\Delta D(\tan {\alpha }_1+\tan {\alpha }_2)$$

(13)

The tension angles ${\alpha }_1$ and ${\alpha }_2$ can be determined using Equation (14):

$$\left\{\begin{array}{c}\sin {\alpha }_1=\frac{D-R}{L_1}\\ \sin {\alpha }_2=\frac{D-R}{L_2}\end{array}\right.$$

(14)

Given that the inner diameter of the mine hole changes, it is evident that the adjustment system also experiences fluctuations in spring compression during its operation regardless of changes in the inner diameter. Thus, it is imperative to maintain a certain level of spring compression so that the crawler wheel always has contact with the hole wall and provides a certain positive pressure.

3. Mechanical Structure Design of Small Robots for Mine Holes

Figure 4 shows the overall structure of the robot. The small robot, designed to operate within mine holes, must maintain compact body dimensions in both diameter and length. It also requires significant climbing and adaptive diameter capabilities, as well as an adjustable speed. Based on the design concept of adaptability, the overall structure of the mine hole-inspection small robot is divided into a power system, an auxiliary system, and a detection system. The power system furnishes the driving force propelling the small robot’s movement; the auxiliary system manages the positive pressure and the robot’s movement status, while also interconnecting various modules to ensure operational stability; and the detection system is tasked with capturing images and gathering environmental parameters within the mine hole.

3.1. Power System Design

The power system is a pipeline robot’s core and provides the robot’s direct drive. When the power system is too complex, it becomes an essential factor that causes the robot’s size to increase. In order to reduce the radial size of the robot to adapt to narrow mine holes while ensuring the stability of the robot power transmission, this paper carries out a unique design of the power system that abandons the use of motors to drive the crawler wheel directly and uses motors, drive shafts, and gears that cooperate to transmit power to the crawler wheel. As shown in Figure 5, The radial rotation of the motor is converted into tube rotation with bevel gears, and the bevel gears are connected coaxially with cylindrical spur gears so that the two rotate concentrically and at the same speed. The transmission arm should not only function in power transmission but should also have a certain length of stretching space to meet the needs of the mine hole-detection robot to change the size of the function. It is designed as a cabin: the four cylindrical spur gear transmission structure constitutes the internal structure of the extension arm, and the power is transmitted to the crawler wheel via the mesh transmission of the cylindrical spur gears. The geared motor is used to drive the two crawler wheels, which reduces power consumption and guarantees the safety of the robot’s work.

At the crawling wheel and transmission arm coaxial connection, surface knurling treatment is used to increase friction. The crawling wheel diameter size is directly related to the working life of the bearings. For the same crawling speed, when the crawling wheel diameter is smaller, the rotational speed is higher and the more severe the wear. When the traveling wheel is too large, the revolution will not be able to be wholly retracted inside the shell, and this will lead to the rest of the structure being too compact. After careful consideration, the crawling wheel diameter was designed as 50 mm.

3.2. Auxiliary System Design

The auxiliary system is capable of realizing the radius change function of the small robot for mine hole detection. It holds the body upright, adjusts the travel state, connects multiple modules, and uses the screw nut structure to realize the radius change function, which can effectively control the radial size of the robot. As shown in Figure 6, the pusher is initially in a contracted state. When the robot starts to work, the stepping motor works on the action sleeve via the screw nut, and the spring in the action sleeve pushes the connecting rod after accumulating force, which pushes the crossbar to slide in the slide groove and holds up the pusher, forming a stable triangular structure with the drive arm. By controlling the forward and reverse rotation of the filament nut, the auxiliary system can also adjust the positive pressure between the crawler wheel and the pipe wall, thus changing the size of the friction force. The spring design of the action sleeve plays a cushioning role. When the crawling wheel encounters a minor bump or depression, the deformation of the spring will drive the slider to slide left and right, enhancing the robot’s passability.

Suppose the pipeline inspection class robot deviates from the pipeline axis during the working process. In that case, it will not only affect the robot’s crawl performance but also the shot’s quality. In order to ensure that the robot coincides with the axis of the pipe during travel, and to ensure that the robot is stable and fixed on the inner surface of the pipe, the front and rear ends of the body are supplemented with a support device, as shown in Figure 7. The tripod structure ensures that the positive pressure on each group intersects at one point; the spring structure of each foot plays a cushioning role and is adapted to the diameter-varying function of the mine hole inspection robot so that the robot maintains balance in the body when moving.

The design of a small robot for mine hole inspection has certain requirements for the size of the traction force, and in order to obtain a sufficiently large traction force, the friction between the crawler wheel and the hole wall needs to be increased. The friction force can be improved by increasing the friction coefficient between the crawler wheel and the hole wall and by increasing the positive pressure between the crawler wheel and the pipe wall in two ways. The first approach requires knurling the surface of the crawler wheel to increase the friction. The second approach requires the robot parts to have enough strength and stiffness to prevent the excessive positive pressure from destroying the structure of the robot, but this will result in the structure of the robot being too large to meet the size requirements of the robot. In order to increase the traction force of the robot as much as possible, the two approaches are used in combination. As shown in Figure 8, the robot is designed modularly, and multiple groups of crawling modules are connected in series to solve the problem of dimensional parameters.

The robot system is divided into crawling modules and functional modules by module, and each robot is equipped with multiple sets of crawling modules and one set of functional modules. The functional modules are used to test the environment in the coal mine borehole, and the crawling modules are used to drive the robot to move. The crawling modules are connected to each other in series with universal joints. As depicted in Figure 9, a universal joint has two degrees of freedom, allowing the connected modules to rotate in both the horizontal and vertical planes, thereby enhancing the robot’s flexibility. The required traction force determines the number of crawling module groups, with the load distributed across each group. This setup reduces the size requirement for individual modules and ensures that the mine hole-inspection robot can adapt to diverse conditions, thereby broadening its utility.

4. Hardware Circuit and Program Design

4.1. Hardware Circuit Design

The small robot for mine hole inspection also needs to detect the environment inside the coal mine hole, mainly including methane content, temperature, humidity, mine hole depth, and stratigraphic conditions, under the condition that the motion state is controllable. Figure 10 shows the hardware circuit composition of the robot; the hardware circuit of the robot consists of a master control module, a motion control module, a detection module, and an image module. The main control module is composed of an STM32F101R6 microcontroller, a power supply circuit, a crystal circuit, a Micro-USB circuit, etc., which form the control core of the circuit. The motion module is responsible for the robot’s movements. It contains MY36GP-3626 DC brushless (MYDJ, Shenzhen, China) planetary gear motors to provide power, which can provide a maximum torque of 100 kg, and 42HD1403GT891 stepping motors to execute the variable diameter movements, which can provide a maximum thrust of 130 N and smooth and controllable movements. The detection module contains a methane detection sensor, a temperature and humidity sensor, and an optical encoder. The sensor detects the methane concentration, temperature, and humidity, and the optical encoder records the number of motor drive revolutions, deduces the robot displacement, and derives the depth of the mine hole. The image module consists of a camera and LED auxiliary light source, which takes pictures of the hole wall and passes the picture to the upper computer via the main control module, which analyzes the stratigraphic condition.

To account for the phenomenon of the presence of gas and water inside coal mine boreholes, the circuitry of the mine borehole-detection small robot needs to be safely designed.

In order to avoid an explosion, the robot’s power must be strictly limited, and the total power should be limited to 12 W according to the requirements. The design uses a cable to power the robot, and the power consumption of the robot is mainly from the driver module, the STM32, and the light bulb. The maximum power of the selected driver module is 9.12 W, and the power of the lighting lamp is 0.5 W. The operating current of all peripherals is selected as 36 mA, so the maximum power of the STM32 is 0.12 W, and the total power of the robot is less than 9.74 W. Low power prevents explosions caused by sparks due to sudden changes in current, leading to glow discharges in the event of a circuit breakage fault. At the same time, the circuit is sealed and fixed to isolate the circuit from external gases, thus reducing the risk of explosion.

In order to avoid short-circuiting the robot due to the accumulation of water, the robot needs to be designed to be waterproof. The electrical equipment of the mine hole-detection small robot is placed in a functional compartment for sealing and treated with sealant. The exposed equipment is waterproofed with IP68-rated products. The functional compartment inside the robot is fully sealed and treated with rubberized waterproof mats and sealant at the places where the wires pass through.

4.2. Control Program Design

The control program of the small robot for mine hole inspection is designed to facilitate the real-time management of the robot’s operational status and to collect environmental parameters within the mine hole. The process of designing the control program is shown in Figure 11: first, the communication protocol is configured and the corresponding mapping serial port and baud rate are selected. Upon completing the configuration, commands can be dispatched, and data can be obtained with the host computer. After receiving instructions from the host computer, the small robot will analyze and recognize the instructions and adjust its motion posture. It provides feedback when motion control instructions are received, and when data query instructions are received, it transmits data—including parameters such as the crawling speed, methane concentration, temperature, and humidity—to the host computer, which the host computer then utilizes for subsequent processing.

5. Robot Testing and Analysis

5.1. Robot Routine Parameter Testing

The design approach for the small robot used for mine hole inspection involves an initial modular design phase followed by an assembly process. The robot needs to work in a specific environment and, therefore, has strict requirements on its specification parameters.

Figure 12 shows the robot’s overall experimental system. According to the program design requirements, the mine hole detection small robot needs to complete work in the 70 mm inner diameter of a pipe, so its body size must be manageable. After measurement, the robot’s drive arm completely retracts when the outer diameter is the smallest, about 65 mm, and the robot’s drive arm fully extends when the outer diameter is up to 100 mm of the total length of 300 mm so that the robot can fit in a pipe with a diameter of 65 mm to 100 mm and complete regular work. A PC pipe with an inner diameter of 70 mm is chosen as the test environment, powered by a 24 V DC/0.4 A power supply, and UM242-type drivers drive the stepper motor.

The mine hole-detection small robot uses a cable for power supply and information transmission, and it needs to have a specific traction ability to drag the cable to work. As is shown in Figure 13, the PC pipe is fixed horizontally on the desktop. The diameter of the robot is adjusted to the appropriate size with the variable diameter function. The spring force gauge is connected to the tail of the robot. The spring force gauge is fixed horizontally when the body of the robot is entirely inside the pipe, and the crawling state is stable. The spring force gauge is fixed horizontally and the display of the spring force gauge is the maximum traction force of the robot when the crawling wheel is stagnant or when there is obvious slipping after many measurements. The results show that the average maximum traction force of the robot can reach 40 N.

Flammable gases such as methane are present in coal mine boreholes, so the current and power of the small robot for mine hole detection cannot be too large.

Table 1. Specification parameters of the single-group module of the small robot for mine hole inspection.

Name	Parameters
Outer diameter	65 mm
Length	300 mm
Weight	0.925 kg
Maximum drive current	0.4 A
Maximum power	12 W
Horizontal traction	40 N

shows the parameters of the robot. The maximum drive current of the robot is 0.4 A, and the maximum power is 12 W, which meet the actual demand.

5.2. Robot Routine Parameter Testing

Given that mine holes are not always horizontal, a small robot designed for mine hole inspections must be able to climb and maintain an appropriate speed. Both the crawling speed and climbing ability of this mine hole-inspection robot were tested. As shown in Figure 14, a vertical scale is affixed to one end of the PC pipe. We calculated the height to which the PC pipe can be lifted at various angles and recorded these measurements on the vertical scale. Starting from the horizontal position, we gradually lifted the PC pipe, timing how long it took for the robot to traverse the pipe at each angle. This allowed us to indirectly measure the robot’s crawling speed on varying slopes. After the measurement, we determined that the maximum crawling speed of the robot is 3.96 m/min and the maximum crawling slope is 90°. Crawl times are hand-recorded with a maximum error of 0.3%.

Figure 15 shows the fitting analysis of the experimental results. The speed of the robot increases with the angle, and the attenuation is only 0.592%; thus, it can be considered that the crawling speed of the robot is more stable and has good adaptability. There is no strict standard for the propulsion speed of the coal mine hole-detection instrument, and it needs to be changed flexibly according to the actual situation in different mines. It is generally required that the propulsion speed of the instrument should be at most 6 m/min. Therefore, a robot of this design can meet the actual requirements.

5.3. Robotic Diameter Variation Test

The diameter-varying function of the robot was tested by selecting two pipes with internal diameters of 70 mm and 85 mm and connecting the two with a joint.

The test process is shown in Figure 16. The robot first starts traveling from the 70 mm pipe; when the robot advances to the pipe interface, due to the sudden increase in pipe diameter, the robot’s crawler wheel cannot be tightly attached to the pipe wall. There is a phenomenon of skidding and idling, and the robot cannot perform normally. At this time, the stepper motor starts to act by reducing the diameter, holding open the crawler wheel until it is tightly attached to the pipe wall. The robot obtains enough friction and passes through the interface to enter the 85 mm pipe. After that, the robot starts crawling from the 85 mm pipe; when the robot reaches the pipe interface, due to the pipe diameter suddenly becomes smaller, the robot becomes stuck in the joint and cannot move. At this time, the stepping motor is activated to retract the crawling wheel inward until the crawling wheel can enter the pipe, and the robot enters the 70 mm pipe smoothly.

6. Conclusions

With the increase in coal mining resources, mining operations in complex areas have increased. However, these areas were characterized by complicated hydrogeological conditions, which result in issues such as water damage, collapses, roof collapses, and other mining-related accidents. The deployment of mine hole-detection technology proved effective in minimizing such accidents. In this study, we aimed to examine the shortcomings of existing mine hole-inspection technologies and propose a configuration for a small mine hole-inspection robot specifically designed to operate in the complex environment found in coal mines. The robot had a radial diameter of 65 mm, making it suitable for small bores ranging from 65 mm to 100 mm in size. Notably, it exhibited excellent crawling performance and a low power consumption of only 12 W. Additionally, it was equipped with waterproof, explosion-proof, and variable-diameter capabilities, allowing it to adapt to the complex conditions encountered in coal mine boreholes. This robot provides essential technical support for the detection of coal mine boreholes. The main research work of this paper is as follows:

(1)

The functional analysis and structural design of a small robot for mine hole inspection were carried out according to engineering requirements. The variable-diameter function was designed to adapt to the unevenness of mine holes. The force situation was theoretically analyzed, and the optimal parameters of the critical structure were calculated to ensure the working life of the robot.

(2)

Using three-dimensional modeling software, the structural model of the small robot for mine hole detection was established. The gear structure was used to reduce the radial size of the robot to adapt to the small diameter of a mine hole; the buffer structure was set to improve the shooting quality and passability; and the gimbal structure was designed to facilitate the expansion of the robot.

(3)

The small robot’s hardware circuit and control program for mine hole detection were designed to focus on the waterproof and explosion-proof design to address the problem of water and gas inside the mine hole. The power of the whole machine was limited to no more than 12 W, which met the engineering requirements.

(4)

The mine hole detection small robot was tested, and the test results showed that the mine hole detection small robot worked within a hole diameter of 65 mm to 100 mm and had a maximum power of 12 W, a top crawling speed of 3.96 m/min, and a maximum crawling slope of 90°. The experimental results demonstrated that the mine hole-detection robot could adapt to the coal mine hole environment and meet engineering needs.