Mechanical–electrical–pneumatic systematic design exploration of hexapod robot experimental prototype

: Aiming at the problem of the detect and rescue in the coal mine with the leaking vas, a type of pneumatic hexapod explored robot which uses a high pressure nitrogen cylinder as the power producer is designed. The leg mechanism is designed with three degrees of freedom, the straight line walking and turning gaits are planned, which are testified by the virtual prototype. The control loop of the cylinder is designed with the magnetic valve with three positions and five paths. The air bottle, the reducing valve, the general follow control part (GFC) the magnetic valve with three positions and five paths, the cylinder are selected and the air supply loop of the hole machine is designed, the 3D model of the hole machine is built which is used to confirm the installing form of the hole body. The experimental prototype is obtained at last. The power supply and control circuits of the magnetic valves are designed, the control series is written on the Arduino single chip. The experiment of the leg swing of the robot on the stage is proceeded which verifies the correctness of the design.


Introduction
After the leakage of vas gas in the underground coal mine, the flammable and explosive gas is fully filled in the coal mine, the danger coefficient of manual detection and rescue work is extremely high. Therefore, the use of robots to carry detection equipment instead of manually entering the disaster environment for detection and collection of effective data firstly can greatly improve the rescue efficiency and safety. At present, crawler robots are usually used for rescue detection at home and abroad. However, there are also problems with high maintenance quality and poor terrain adaptability. Foot robots have the characteristics of light weight and good terrain adaptability, and pneumatic robots use air or inert gas as a driving force. Its leg mechanism can be completely exposed, and all charged control systems can be concentrated together for packaging, which facilitates the explosion-proof design.
At present, the research studies on coal mine rescue robots include the following: Li [1] designed a joystick crawler walking mechanism for coal mine disaster relief robots. Based on Li's research, Liu [2] explored the joystick crawler robot walking simulation and trajectory tracking research, the control and communication system of robot is established by using Ethernet. Zhou [3] studied the design and control of a coal mine wheeled mobile robot. Wang [4] studied the navigation technology of Lidar and inertial sensor of coal mine robot.
The research on pneumatic robots mainly includes the following: Verrelst and Vanderborght [5] designed a pneumatic bipedal robot, which confirmed the feasibility of pneumatic systems as power sources. Lavoie and Desbiens [6] designed a pneumatic six-foot robot. Morimoto and Aliff [7] used a flexible cylinder to design a flexible tactile manipulator for medical rehabilitation treatment and achieved high action accuracy, Qiu and Dohta [8] designed a pipeline detection robot using a flexible cylinder; Diez and Badesa [9] designed a pneumatic robot for nerve rehabilitation treatment. Low and Tan [10] studied a flexible gas massage machine for assisting joint movement; Ramsauer and Kastner [11] studied a pneumatic Stewart platform for error detection.
From the status quo of the above studies, there is no research on pneumatic rescue detection robots. At present, most of the research on pneumatic robots at home and abroad is applied to the medical field. This also shows that pneumatic robots do not have control accuracy problems.
Taking into account the stability of the whole machine, this study chooses the mechanism form of the hexapod robot, and carries out the initial mechanism design and the planning of the straight walk and turning gait. A complete pneumatic hexapod robot includes a mechanical system, a pneumatic system, a control system, and a detection system. The main role of the mechanical system is to carry three major systems, and the pneumatic system determines the structure of the entire mechanical system. For this purpose, the pneumatic endurance ability and driving ability are estimated, and the pneumatic components, including the gas source, relief valve, two-piece, three-piece five-way electromagnetic valve, and the cylinder selection, are determined. The overall design of the mechanical and pneumatic system is carried out using three-dimensional modelling software. The main dimensions of each mechanical structure are determined, and finally the test prototype is installed. The action of the cylinder is realised by the ordered sequence opening and closing of electromagnetic valve. In order to verify the rationality of the pneumatic system and mechanism, the control circuit of the electromagnetic valve is designed, the control timing is compiled using Arduino microcontroller, and the pneumatic hexapod robot is initially used. The stand swing test, according to the movement characteristics of the legs, the correctness of the pneumatic loop is verified, and the basic response characteristics of the cylinder under the opening and closing timing of the solenoid valve are obtained, which provides the basis for the next step of walking experiments.

Mechanism design and gait planning
The design of the foot robot leg mechanism should fully consider the road conditions it walks on. The ground environment under the mine can be roughly divided into flat road surface, slope road surface, slope road surface, ladder road surface, convex platform and concave road surface, as shown in Fig. 1. In addition to the slope road surface, the other five road conditions require only two degrees of freedom for the legs to achieve. When the robot appears in a laterally unstable condition, it is necessary to step sideways and look for ground contact points on the outside of the trunk to restore stability to the robot. Therefore, the legs need at least three degrees of freedom. Its mechanism diagram and preliminary threedimensional modelling design are shown in Fig. 2 At the beginning of the design, the pneumatic hexapod robot needs to meet the two basic walking conditions, that is, straight and turning walking. The walking process of the foot robot is accomplished by rotating the leg joints according to a certain time series, and the rotation of the joints is realised by the telescopic effect of the cylinder. Therefore, the feasibility of pneumatic engineering should be considered when carrying out gait planning of the pneumatic hexapod robot. The expansion of the cylinder is achieved by changing the direction of the electromagnetic valve. Taking into account the pressure-holding effect, a three-piece fiveway medium-seal electromagnetic valve is selected here as the control element. The basic principle is shown in the left figure in Fig. 3. When the solenoid valve is in the left position, the left end of the cylinder is supplied with air and the air rod is extended. When the solenoid valve is in the right position, the right end of the cylinder is supplied with air, the left end is discharged, and the air rod is retracted. When the solenoid valve is in the middle position, the cylinder remains stationary. No movement occurs.
Based on the analysis of the above pneumatic principles, it can be seen that the time series of the straight walking gait of the pneumatic hexapod robot can be planned as shown in the right figure in Fig. 3. As shown in Fig. 2, 145 is replaced with ground support to achieve a straight line walk for the robot.
After completing the linear gait planning of the pneumatic hexapod robot, it is necessary to plan its turning gait. In order to reduce the design difficulty of pneumatic execution system, the design of turning gait must be based on the straight air path. The turn of the robot is a turning process with a fixed radius. The process is to make the trunk rotate at a certain angle first, and then follow the process of walking in a straight line. That is, within a turning cycle, the robot's trunk rotates at a certain angle first, and then walks in a straight line. Its principle is shown in Fig. 4.
According to the characteristics of the robot's straight walking gait, the robot's turn can be seen as a turning motion by the movement of the front and rear legs of the other side under the support of one side of the middle leg. This turning gait can be achieved by adding a one-way valve on the basis of the original air supply path of the middle leg. The aerodynamic principle is shown in Fig. 4. When walking in a straight line, the one-way valve is stationary. When turning, the one-way valve supplies power to cut off the air supply to the middle leg joint.
Based on the above analysis, the gait time series of pneumatic hexapod robots under turning conditions is shown in Fig. 5. Unlike the linear walking gait, the movement of the middle leg in each group of turning gait changes, and the rest remains unchanged.
In order to verify the correctness of the linear walking and turning gait mentioned above, a virtual prototype analysis was introduced into ADAMS after modelling the leg mechanism using three-dimensional modelling software. The variation curves of the forward displacement of the pneumatic hexapod robot under a straight line walk and the XYZ direction displacement curve of the centre of mass under a turn walk are shown in Fig. 6.
From the linear movement of the dry body in Figs. 6 and 7, it can be seen that in 15 s, the robot moves forward by 450 mm. During the process of moving forward, the left and right amplitude oscillates within 3 mm, and there is a 3.5 mm amplitude fluctuation up and down. From the displacement curve of the trunk walking on the turn of Fig. 6, it can be seen that during the turn, the robot moves forward 300 mm, while the lateral displacement is 80 mm, and the upper and lower directions are waves with a amplitude of 4.5 mm, with a total displacement of 500 mm.

Pneumatic system design
The pneumatic system is designed to determine the cylinder diameter and the volume of the cylinder to ensure that the robot has a certain bearing capacity and endurance. The general calculation formula for cylinder thrust is given by: where F is the thrust of the cylinder, D is the effective operating diameter of the cylinder, and P is the air supply pressure. The MAL cylinder is a low-pressure cylinder with a rated working pressure of 0.8 MPa. The cylinder diameter series is 16, 20, 25, 32, 40 mm. The pressure working range of the cylinder is 0.15-0.8 MPa.
According to the pressure resistance standard of the cylinder, if the medium is nitrogen, it can be filled with about 10 MPa. The expression of robot's maximum load-bearing capacity is given by Maximum forward driving force is given by where θ is the angle between the thigh drive cylinder and the trunk. Using MATLAB, the variation curves of the total carrying capacity of the hexapod robot under different cylinder diameters are drawn according to the angles of the cylinder and trunk at 30° and 45°, respectively, as shown on the left of Fig. 7.
During the operation of the pneumatic hexapod robot, the process of releasing gas from the cylinder to supply the cylinder is an isothermal process, which is known by the formula pV = MRT: Among them, p 1 is the gas supply pressure of the cylinder, V 1 is the gas storage capacity of the cylinder, and p 2 is the gas supply pressure. From this, the actual usable gas volume of the system can be obtained under a certain amount of gas storage and pressure of the cylinder: The amount of gas consumed by cylinder during each line is where h is the cylinder stroke, taking into account that the cylinder and the trunk have a certain degree of angle in the horizontal direction, so in the case of a certain amount of storage capacity, the total stroke of the cylinder: S = V 2 (π/4)D 2 cos θ = 1.274 It can be seen that the endurance of the pneumatic hexapod robot S is a function of p 1 , V 1 , D, and p 2 . Using MATLAB, the endurance of different cylinders under different mounting angles and pressure is analysed (Fig. 7, right side). According to the requirements of force balance and energy conservation, the gas source should be partially pressed, and the branches of the partial pressure at all levels can be converted. The gas flows out of the cylinder, and the pressure is reduced from 10 to 1 MPa by the decompression valve, and then the pressure is initially reduced to 0.8, 0.4, and 0.1 MPa by the binaries, of which 0.8 and 0.4 MPa are used for the air supply of the middle leg and the front hind legs, respectively. 0.1 MPa is used for air supply for each joint swing leg, 0.1 MPa gas path and 0.8, and 0.4 MPa gas path is converted with two five-pass electromagnetic valves. The partial pressure gas path obtained is shown in Fig. 8.

Electric control system design
This design uses the ARDUINO single chip computer and the electromagnetic valve drive plate as the main electronic control elements, as shown in Fig. 9. The power supply voltage is 5 and 24 V, respectively. There are 12 digital IO interfaces and six analogue interfaces on the ARDUINO monolithic machine. Taking into account the design of the subsequent loading sensor, the robot's straight line and turning control system should occupy as few IO ports as possible to meet the requirements of the control system to minimise the design. In the solenoid valve drive plate interface, Y0 to Y17 are connected to the digital IO port of the single machine, and the COM port is connected to the GND. That is, when the Yi port is high, the solenoid valve is turned on, and the solenoid valve is reset at low power. The drive plate adopts the common anode connection method, so the negative electrode of the power supply line is connected to the electromagnetic valve from   In the pneumatic system, there are a total of 12 three-seat solenoid valves and two two-digit solenoid valves, requiring a total of 26 power supply circuits. In order to ensure that the control circuit is minimised, the power supply circuit of the electromagnetic valve with the same action is finally assembled and a total of 10 control circuits are obtained. In the actual wiring, the solenoid valve has a total of 52 leads. In order to facilitate the wiring, the wiring numbers of each solenoid valve are first numbered, and then the number of the solenoid valve line is matched with the number of the connection port of the solenoid valve drive plate, as shown in Table 1. The connection relationship between the single chip microcomputer and the drive plate of the solenoid valve is shown in Table 2.
Based on the above wiring method, the electronic control system test table of pneumatic hexapod robot is shown in the left figure in Fig. 10. In order to make the action of the solenoid valve easy to observe, an LED monitoring lamp is added between the single chip microcomputer and the solenoid valve drive plate, and the opening and closing of the solenoid valve is determined by the flicker of the lamp.
According to the design of the linear walking gait, the opening and closing timing of the electromagnetic valve is designed. In order to facilitate the preparation of the timing, the timing of the robot's linear walking gait is improved to obtain the timing diagram shown in the right figure in Fig. 10. An empty load opening and closing experiment of electromagnetic valve is carried out by using the Arduino microcontroller to write the control timing. Through experiments, the solenoid valve can meet the opening and closing action of 10 times per second, that is, the minimum opening time can reach 100 ms, which can meet the requirements of pneumatic six-foot robot during walking.

Prototype test and experiment
Based on the design results of the above mechanism, pneumatic system and electronic control system, three-dimensional modelling of pneumatic hexapod robots is carried out to determine the size of the main mechanical parts and the installation method. The obtained three-dimensional model of the whole machine is shown in the left figure in Fig. 11. According to the size and model of the main mechanical structure parts in the three-dimensional model, the pneumatic six-foot robot model after processing and assembly is shown on the right side of Fig. 11.
In order to verify the consistent effectiveness of the system, the electronic control system and the pneumatic system, the experimental prototype is fixed on the bracket and the whole machine swing leg experiment is carried out. The gyroscope is used to measure the changing signals of joint rotation angle and angular velocity, as shown in Fig. 12 LFTHIGH  F1  A1  A2  M0  C1  C2  M4  LFSHANK  F2  A7  A8  M1  C7  C8  M5  RFTHIGH  F3  B1  B2  M2  D1  D2  M6  RFSHANK  F4  B7  B8  M3  D7  D8  M7  LMTHIGH  F5  B3  B4  M2  D3  D4  M6  LMSHANK  F6  B9  B10  M3  D9  D10  M7  RMTHIGH  F7  A3  A4  M0  C3  C4  M4  RMSHANK  F8  A9  A10  M1  C9  C10  M5  LBTHIGH  F9  A5  A6  M0  C5  C6  M4  LBSHANK  F10  A11  A12  M1  C11  C12  M5  RBTHIGH  F11  B5  B6  M2  D5  D6  M6  RBSHANK  F12  B11  B12  M3  D11  D12  M7  valve 1 F13 of the swing. It can be seen that the minimum opening and closing time of the electromagnetic valve is 0.1 s. 0.15 MPa air supply pressure still exceeds the actual needs of the thigh joint, and if the air supply pressure is lower, the electromagnetic valve will not work. The peak angular velocity occurs at the vibration position of the joint. The stability of the swing of the calf joint under 0.15 and 0.2 MPa air supply pressure is lower than that of the thigh joint. This is due to the fact that the self-moment of inertia and quality of the thigh joint is significantly higher than that of the calf joint, and the peak angular velocity is greater than the peak of the thigh joint. This shows that the vibration of the calf joint under 0.15 and 0.2 MPa air supply pressure will be more obvious.
Through the above experiments, it can be seen that although the swing of the thigh joint and the calf joint can be achieved under the lowest air supply pressure, the vibration situation still exists. Therefore, in the subsequent engineering prototype design, the vibration can be reduced by increasing the damping of the leg joint. Or from the action time of the solenoid valve, further reduce the minimum action time of the solenoid valve to achieve a stable swing of the leg.

Conclusion
The gait design of pneumatic hexapod robot should take fully into account the characteristics of the electromagnetic valve of pneumatic system, and the turning gait should be based on the straight gait. The simulation of virtual prototype proves that the gait is effective.
Load ability and endurance ability are the prerequisites for the design of pneumatic six-foot robot experimental prototypes. By estimating the load and endurance capability, the main size of the experimental prototype and the type of cylinder can be roughly determined. On this basis, the machinery is carried out. The design of pneumatic and electronic control system ensures the effectiveness of the experimental prototype design.
According to the linear walking experiment under different driving pressures, the stability of the linear walking has a direct relationship with the selection of the air supply pressure. When the air supply pressure is lower than the required pressure, the stability of the entire machine walking will be reduced, and it is higher than the required pressure. While, stability will not change, but it will increase gas consumption.