Abstract : The PJR-Z type shotcrete robot is a new type of special equipment for shotcreting concrete. Its overall structure and operating principle are introduced, and the overall design scheme and reliability design of the robot's hydraulic and electrical control systems are described. Practice has proven that the PJR-Z type shotcrete robot sprays evenly, with significantly reduced rebound and dust. It has high reliability and stability, strong practicality, and fully meets the requirements of concrete shotcreting technology. Keywords: shotcrete; robot; design; reliability Foreign countries began using robotic shotcrete in the early 1960s. In China, from the mid-to-late 1960s, many units developed shotcrete robotic arms [1, 2]. However, due to various reasons, despite more than 20 years of effort, no satisfactory product has been developed. The main problems are unreasonable structure, low automation level, and poor reliability, mainly manifested in complex operation and incompatibility with the harsh environment of underground engineering. Currently, all shotcrete robotic arms used domestically are imported, which are relatively expensive, and maintenance and spare parts supply are difficult. 1. Shotcrete Operation Principles and Requirements for Shotcrete Robots Shotcrete is divided into dry shotcrete and wet shotcrete. Dry shotcrete involves mixing dry cement, sand, gravel, and quick-setting agent powder in a specific ratio in a rotary mixer (commonly known as a shotcrete machine). The mixture is then blown into a feed pipe by high-pressure air from the mixer's outlet. A spray gun (also called a nozzle) is installed at the outlet of the feed pipe, and a ring-shaped water sprayer is installed at the inlet of the spray gun to continuously spray water into the gun. The water and concrete mix during this approximately 1-meter movement, mostly becoming concrete slurry. Due to the impact force provided by the high-pressure air, after the concrete hits the sprayed surface, some adheres to the surface, while some bounces back. The amount of rebound (i.e., the rebound rate) is mainly related to the angle and distance between the spray gun and the sprayed surface. Wet shotcrete involves using high-pressure air to feed pre-mixed concrete through a feed pipe into a spray gun and spray it onto the sprayed surface. There are several principles behind wet shotcreting. Some rely on a concrete pump to deliver slurry to the spray gun inlet, while others use a wet shotcrete machine (rotor-type wet mixer) and high-pressure air to deliver the slurry to the spray gun inlet. The optimal process principle for shotcreting is to keep the spray gun perpendicular to the surface being sprayed, with a distance of approximately 1 meter between the spray gun nozzle and the surface. Under these conditions, rebound is minimized. The three major technical challenges of the widely used dry shotcreting method in China (dust, rebound, and unstable concrete quality) have long plagued people. Domestic and international construction practices have proven that wet shotcreting is the fundamental way to solve these three problems. Because the entire material pipe is filled with concrete during wet shotcreting, it is difficult for a person to hold the spray gun, so robots and robotic arms are needed. From a development trend perspective, the use of shotcrete robots will be an inevitable trend. 2. Overall Robot Design The design principle of the robot body is that the robot should meet the requirements of the shotcrete process and the working environment as well as possible. The PJR-Z type shotcrete robot (with a spraying height of up to 10 m) was developed based on the research results of the National 863 Program project "Shotcrete Robot Product Prototype". Its structural principle is shown in Figure 1. It consists of a manipulator, hydraulic system, control system, and manipulator. The robot has 6 degrees of freedom: upper arm pitch, lower arm swing, horizontal telescopic arm longitudinal feed, wrist rotation, spray gun swing, and spray gun rotation. The operating principle and structural features are as follows: ① The upper arm 1 moves in a pitching motion driven by the hydraulic cylinder 2; ② The lower arm 3 swings horizontally. Since the lower arm adopts a four-bar linkage mechanism, its end moves in translation during the swing, thus keeping the relative distance and posture of the horizontal telescopic arm 4 fixed at its end and the sprayed surface unchanged; ③ The horizontal telescopic arm 4 can feed in a direction parallel to the tunnel axis, allowing the spray gun to move horizontally and maintain its posture; ④ The wrist 5 can complete the arc of the spray gun along the arch while ensuring that the spray gun is perpendicular to the sprayed surface; ⑤ The hydraulic cylinder 6 can adjust the posture of the spray gun, so that even when encountering large depressions, the spray gun can still be adjusted to be perpendicular to the sprayed surface; ⑥ With the help of the spray gun circular mechanism 7, the spray gun can move along the conical surface, thus making the spray gun nozzle draw a continuous 360° circle. [align=center]1. Boom four-link linkage 2. Boom cylinder 3. Forearm four-link linkage 4. Horizontal telescopic arm 5. Wrist rotation mechanism 6. Spray gun posture adjustment cylinder 7. Spray gun circular motion mechanism 8. Nozzle 9. Feed pipe 10. Horizontal telescopic arm cylinder 11. Forearm cylinder Figure 1 PJR-Z shotcrete robot structural principle diagram[/align] The 6-DOF shotcrete robot shown in Figure 1 can be installed on a vehicle or a rail-mounted chassis, depending on site requirements. This robot can be used with both wet and dry shotcrete machines. Relying on these 6 degrees of freedom, any motion trajectory required for the shotcrete operation can be achieved. A typical shotcrete operation process is briefly described below. The chassis vehicle is generally not in the center of the tunnel, so the forearm can only be swung to position the end of the forearm on the longitudinal symmetry plane of the tunnel. In order to reduce rebound, the shotcreting is carried out from bottom to top. Therefore, at the beginning, the end of the forearm is located at the bottom of the symmetry plane, so that the distance from the nozzle of the spray gun to the sprayed surface is about 1 meter, which is enough to prepare for the operation. Once the shotcreting begins, the spray gun rotates, its nozzle circling, and the sprayed material traces a spiral on the surface, forming a 20 cm wide spray band. The robot's horizontal telescopic arm extends and retracts horizontally while the spray gun circles. After reaching a set distance, the arm rises 20 cm, and this process is repeated until the junction of the slab and the arch is reached. Afterward, the forearm end remains on the center line of the arch's arc surface. By rotating the gun at a set angle and the robot chassis moving across the tunnel, the spray gun nozzle, while circling, also forms a 20 cm wide spray band on the arch's surface, closely connecting with the slab's spray band. The process for spraying the arch is similar to that for the slab, proceeding sequentially until the arch crown. Upon reaching the crown, the robot either returns to the starting position for a second spray, or rotates to the opposite slab base (similar to the previous process) to spray the other half. 2.1 Overall design of the hydraulic system of the robot. Due to the advantages of hydraulic drive, such as large power-to-weight ratio, large torque-to-inertia ratio, easy to realize linear drive and direct drive, and easy to realize explosion protection, it is widely used in work places with large inertia, large load-bearing weight and explosion protection. According to the spraying environment, and considering the technology, economy, volume, adaptability and explosion protection, the spraying robot selected the hydraulic drive system [3]. When designing the structure of the spraying robot, in view of the characteristics of spraying operation, control system and operation process requirements, the upper arm and lower arm of the spraying robot are designed as a multi-bar linkage mechanism. The working principle of the hydraulic drive system is as follows: the upper arm cylinder 2 drives the upper arm four-link 1 to realize the up and down lifting movement; the lower arm cylinder 11 drives the lower arm four-link 3 to swing in the horizontal plane to adjust the distance between the spray gun nozzle and the wall; the horizontal telescopic arm cylinder 10 drives the horizontal telescopic arm 4 to feed longitudinally; the spray gun posture adjustment cylinder 6 drives the spray gun head 8 to make ±45° posture adjustment swing. The maximum pressure of the system is 16 MPa, which is obtained by force analysis and solution of working parameters. There are two working control modes of the hydraulic system: fully automatic mode and master-slave mode. In the fully automatic mode, the hydraulic system can automatically complete each operation in sequence according to the geometric size of the working surface and the set program and spraying process under the control of the computer; in the master-slave mode, the hydraulic system can arbitrarily complete each manual operation according to the signal issued by the operator [4]. 2.2 Overall design of robot electric control system The guiding idea of ​​the electric control system design is to use advanced technology and components as much as possible to ensure the high reliability of the system under harsh environment and the ease and intuitiveness of operation; the control system should be modular and standardized as much as possible to facilitate standardized and serialized production. The PJR-Z type spraying robot adopts a control scheme that integrates fully automatic control and master-slave remote control, and the two control modes can be smoothly switched between each other at will. It can also retain only one control mode according to user requirements (to simplify the system), which can conveniently meet the needs of various users. In the automatic trajectory control mode of teaching and playback, the robot can automatically perform tasks according to the motion trajectory and posture taught to it in advance, without the need for manual operation. There are two teaching methods for this robot: trajectory teaching and feature point teaching. The operator can choose the teaching method according to the tunnel cross-section. In the remote master-slave control mode, the robot is operated via a 15-20 m long cable using a remote control. The remote control can be considered the robot's master hand, and the robot's arm can be considered the slave hand. The slave hand follows the master hand's movements step by step according to the instructions issued by the master hand, which is very intuitive. In particular, through careful design of the hardware and software, the two control methods can be switched at any point on the trajectory while maintaining a smooth and continuous motion trajectory. The design of the control system mainly includes the computer system, servo amplifier, power supply, operator, and system resistance to harsh environments. The computer control system adopts a two-level distributed control, namely, a distributed control system composed of a planning level (host computer), a CAN bus, and a direct control level (multiple slave computers), as shown in Figure 2. The planning level's task is to receive teaching data and control commands from the teach pendant, receive various information from the direct control level, identify the working environment, and thus complete the planning of the motion trajectory and generate control commands for the motion trajectory, instructing the control level to complete the motion specified by the commands. The direct control level directly faces the controlled object. The shotcrete robot has six degrees of freedom, each with its own relatively independent controller. It consists of a controller, power amplifier, and I/O drive circuit. Its tasks are to receive commands from the planning level and complete the motion control specified by the planning level; receive sensor information from relevant degrees of freedom and preprocess it before sending it to the planning level for the next round of motion trajectory planning; perform fault diagnosis on the controller, servo amplifier, etc., and send the diagnostic results to the planning level for appropriate processing. The controller is a key component of the servo control system, its core consisting of an Intel 80c196KC single-chip microcomputer system and a CAN controller. To meet the requirements of controllability and reliability, isolation measures are adopted between the CAN controller and its interface, and high-reliability, high-precision, and long-life rotary transformers and XSZ digital converters are selected as feedback elements. The controller output uses high-precision high-speed output HSO to obtain PWM output. After opto-isolation, it is then sent to the servo amplifier via an active filter to obtain DC control voltage. 2.3 Reliability design in harsh environments Reliability is one of the most critical issues in engineered robots, and it is also a common weakness and difficulty in robot products in my country. The reliability problem of robots in harsh environments is even more difficult [5]. In terms of the whole machine, the most likely to have problems is the electrical control system, followed by the hydraulic system. 2.3.1 Reliability design of electrical control system In order to obtain high reliability of the shotcrete robot, high-reliability components, fault-tolerant technology, three-proof technology, electromagnetic compatibility technology, anti-interference technology and explosion-proof technology are adopted. (1) In order to solve the reliability problem of the control system, redundant configurations are made for the planning level and the control level respectively, and fault-tolerant design is realized. Their structural schematic diagrams are shown in Figure 3 and Figure 4. (2) To address electromagnetic compatibility, heat dissipation and cooling, moisture prevention, corrosion prevention, mildew prevention, and vibration resistance, considering that the working environment of this robot is similar to that of field equipment, it has been fully hardened according to military computer hardening technology. Therefore, this electrical control system is actually a hardened computer control system. (3) To improve the reliability of the circuit itself, strict isolation measures are adopted for both the single-chip microcomputer and the I/O channels. Analog and switching quantities are isolated. Dual-line sampling, differential input, linear isolation, and active filtering are used in the input channels. For the power supply, transformer isolation and interference suppressor filtering are used. (4) In terms of explosion protection, this system is designed as an explosion-proof and intrinsically safe type and has obtained a national explosion-proof inspection certificate. 2.3.2 Reliability Design of Hydraulic Systems The most problematic component in a hydraulic system is the electro-hydraulic proportional valve. To improve the reliability of the hydraulic system, two key aspects are addressed: First, meticulous design. To protect proportional valves and various solenoid valves from explosions and harsh environments, an integrated design is adopted for the oil circuit. All valves are centrally located on the same side of the same distribution block, which is then designed as an explosion-proof enclosure. This ensures that all valves are within the explosion-proof enclosure, achieving both explosion-proof and protective effects—a highly economical and reasonable approach. Second, emphasis is placed on overall quality. Every part is given high priority to prevent any single component or even a single pipe joint from affecting the entire system. References: [1] Wang Huanwen, Wang Jiliang. Anchor spraying support. Beijing: Coal Industry Press, 1989 [2] Li Yunjiang, Fan Binghui, Jiang Hao, et al. Design and implementation of shotcrete robot. Mechanical Science and Technology, 2001, 20 (2): 189-190 [3] Liu Changnian. Analysis and design of hydraulic servo system. Beijing: Science Press, 1985 [4] Li Yunjiang, Rong Xuewen, Fan Binghui, et al. Design of hydraulic system for large tunnel shotcrete robot. China Mechanical Engineering, 2001, 12 (7): 735-737 [5] Su Xuecheng, Fan Binghui, Li Yibin, et al. On the research and development of coal mine robots. Robot, 1995, 17 (2): 123-127 Author's profile: Li Yunjiang, male, Born in 1966. Associate Professor, Robotics Research Center, Shandong University of Science and Technology (Jinan 250031). Main research area: special robots. Recipient of one National Science and Technology Progress Award (Second Class) and one Shandong Provincial Science and Technology Progress Award (First Class). Published two monographs and over 40 papers. Rong Xuewen, male, born in 1973. Engineer, Robotics Research Center, Shandong University of Science and Technology. Fan Binghui, male, born in 1958. Professor, Robotics Research Center, Shandong University of Science and Technology. Jiang Hao, male, born in 1959. Senior Engineer, Robotics Research Center, Shandong University of Science and Technology.