Mobile platform of rocker-type coalmine rescue robotLI Yun wang *, GE Shirong, ZHU Hua, FANG Haifang, GAO JinkeSchool of Mechanical and Electrical Engineering, China University of Mining rescue robot; rocker suspension; differential; explosion-proof design1 IntroductionIn the rescue mission of a gas and coal dust explosion ,rescuers easily get poisoned in underground coal mines full of toxic gases, such as high-concentrationCH4 and CO, if ventilation and protection are not up to snuff. Furthermore, secondary or multiple gas explosions may be caused by extremely unstable gases after such a disaster and may cause casualties among the rescuers [1]. Therefore, in order to perform rescue missions successfully, in good time and decrease casualties, it is necessary to develop coal mine rescue robots. They are then sent to enter the disaster area instead of rescuers and carry out tasks of environmental detection, searching for wounded miners and victims after the disaster has occurred. The primary task of the robots in rescue work is to enter the disaster area. It is difficult for robots to move into restricted spaces and unstructured underground terrain, so these mobile systems require good obstacle-surmounting performance and motion performance in this rugged environment [2]. The application of some sensors used for terrain identification are severely restricted by low visibility and surroundings full of explosive gas and dust; hence, a putative mobile system should, as much as possible, be independent from sensing and control systems[3].Studies of coal mine rescue robots are just beginning at home and abroad. Most robot prototypes are simple wheel type and track robots. The mine exploration robot RATLER, developed by the Intelligent Systems and Robotics Center (ISRC) of Sandia National Laboratories, uses a wheel type mobile system [4]. The Carnegie Mellon University Robot Research Center developed an autonomous mine exploration robot, called “groundhog [5]. Both the mine rescue robot V2 produced by the American Remote Company and the mine search and rescue robot CUMT developed by China University of Mining and Technology, use a two-track fixed type moving system[6-7]. These four prototypes are severely limited in underground coal mines. Rocker type robots have demonstrated good performance on complex terrain. All three Mars rovers, i.e., Sojourner, Spirit and Opportunity used mobile systems with six independent drive wheels[8-9]. Rocker-Bogie, developed by the American JPL laboratory has landed successfully on Mars. The SRR robot from the JPL laboratory with four independent drive and steering wheels consists of a moving rocker assembly system, similar to the four wheel-drive SR2 developed by the University of Oklahoma, USA [10]. Both tests and practical experience have shown that this type of system has good motion performance, can adapt passively to uneven terrain, possesses the ability of self-adaptation and performs well in surmounting obstacles. Given the unstructured underground terrain environment and an atmosphere of explosive gases, we investigated a coalmine rescue robot with four independent drive wheels and an explosion-proof design, based on a rocker assembly structure. We introduce the composition and operational principles of this mobile system, discuss the design method of its rocker assembly and differential device and carried out motion simulation of the kinematic performance of the robot with ADAMS, a computer software package. In the end, we tested the terrain adaptability and performance of the prototype in surmounting obstacles.2 Mobile platform [11-12]As shown in Fig. 1, the mobile platform of the rocker-type four-wheel coal mine rescue robot includes a main body, a gear-type differential device, two rocker suspensions and four wheels. The shell of the differential device is attached to the interior of the main body. The two extended shafts of the differential device are supported by the axle seats in the lateral plate of the main body and connected to the rocker suspensions installed at both sides of the main body. The four wheels are separately connected to the bevel gear transmission at the terminal of the four landing legs. The four wheels are independently driven by a DC motor, installed inside the landing legs of the rocker suspension. A flameproof design ofthe legs has been developed, which includes a flameproof motor cavity and a flameproof connection cavity. Via a cable entry device, the power and control cables of the DC motor are connected to the power and controller of the main body.2.1 Rocker suspension2.1.1 FunctionThe primary role of the rocker suspension is to provide the mobile platform with a mobile system that can adapt to the unstructured underground terrain, such as rails, steps, ditches and deposit of rock and coal dumps because of the collapse of the tunnel roof after a disaster. By connecting the differential device intermediate between the two rocker suspensions, the four drive wheels can touch the uneven ground passively and the wheels can bear the average load of the robot so that it is able to cross soft terrain. The wheels can supply enough propulsion, which allows the robot to surmount obstacles and pass through uneven terrain.2.1.2 StructureAs shown in Fig. 1, the rocker suspension is composed of a connecting block, landing legs and bevel gear transmissions. The angle between the landing legs on each side of the main body is carefully calibrated. The legs are connected to the connecting block and the terminals, which in turn are connected to the bevel gear transmissions. Fig. 2 illustrates the structure of the landing leg. It is divided into an upper and a bottom section. The bottom section is cylindrical. The DC motor is in the leg and fixed to the connecting cylinder. The motor shaft connects to the bevel gear transmission and the wheel is also connected to the transmission. The upper section has a blind center hole through witch a connection is formed to the bottom section, via a connection cavity. Through the cable entry device of the upper section, the motor power and control cable from the main body of the robot are put into the connection cavity and connect to the wiring terminals which, in turn, connect to the guidance wires in the wire holder. Another end of the guidance wires connects to the motor in the bottom section.2.1.3 Flameproof designA coal mine environment is full of explosive gases; hence, a rescue robot must be designed to be flameproof. The DC motors, for driving each wheel, are installed in the landing legs of the rocker suspensions. At the present low-powered DC motors, available in the market, are of a standard design and not flameproof, hence a flameproof structure for these motors must be designed. Given the structural features of the rocker suspension, it is very much necessary that a flameproof design for the landing legs be carried out.There are two important points to be considered in this flameproof design. First, a flameproof cavity is needed, in which the standard DC motor is installed. Given the flameproof design requirements, a group of flameproof joints should be formed between the motor shaft and the shaft hole. Generally, the motor shaft made by the manufacturer is too short to comply with the requirement of flameproof joints, so the motor shaft needs to be extended. Second, a flameproof connection cavity should be designed to lead the c a- Fig. 1 Rocker-type four-wheel mobile platform Main body Rocker suspension Landing leg Wheel Rocker suspension Connecting block Differential device Bevel gear transmission Axle seat Upper section Wire holder Bottom section DC motor Flameproof joints Flameproof joints Bevel gear transmission Connecting cylinder Shaft sleeve Wheel Cable entry into the connection cavity through a flameproof cable entry device. DC motors, especially brush DC motors, may generate sparks in normal running and when the motor load is high, the working current may be more than 5 A, which exceeds the current limit in Appendix C2 of the National Standard GB3836.2- 2000 of China. Therefore, the motor power and control cable cannot be directly in the connection cavity。Given these requirements, the landing legs have been designed as flameproof units, as shown in Fig. 2. An elongated shaft sleeve has been assembled from the motor shaft, with the same inside radius as that of the motor shaft and this is how the motor shaft is extended. The front flange of the motor is fixed to the intermediate plate of the connecting cylinder. The motor shaft with the shaft sleeve passes through the center hole embedded with a brass bush and then connects to the input gear of the bevel gear transmission at the end of the bottom section of the landing leg. Therefore, flameproof joints are formed between the motor shaft and the shaft sleeve, as well as between the shaft sleeve and the brass bush. The terminal of the bottom section of the leg connects to the connecting cylinder and a flameproof joint is formed between the external cylindrical surface of the terminal and the inner cylinder surface of the connecting cylinder.There is also a flameproof connection cavity in the upper section of the leg. In order to save space, the guidance wire is sealed together with the wire holder using a sealant. The seat of the guide wire is installed in the hole of the upper section of the landing leg. Another flameproof joint is formed between the wire holder and the hole. The cavity of the upper section connects to the rabbet structure of the bottom section, with yet another flameproof joint. There is a flameproof cable entry device at the end of the upper section of the landing leg. Hence, a flameproof connection cavity is formed in the upper section of the leg. Based on the structure described, the standard DC motor was installed in the flameproof cavity of the bottom section of the leg. The power and control cables of the motor connect to the flameproof connection cavity of its upper section through a wire holder. Moreover, the cable from the flameproof main body of the robot connects to the connection cavity via the flameproof cable entry device. Thus, the flameproof design of the landing leg of the rocker suspensionsection was completed.2.2 Differential device[13-15]2.2.1 Characteristics of the differential mechanism The differential mechanism of a rocker-type robot is a motion transfer mechanism with two degrees of freedom, which can transform the two rotating inputs into a rotating output. The output is the linear mean values of the two inputs. If we let 1 ω and 2 ω be two angular velocity inputs, ω the angular velocity Two rotational input components connect to the left and the right rocker suspension of the robot and the output component connects to the main body of the robot. In this way, the swing angles of the left and right rocker suspensions are averaged by the differential mechanism and the mean value, transformed into the swing angle (pitching angle) of the main body, is the output. It is effective in decreasing the swing of the main body and thus reduces the terrain effect. Taking the main swing angle of the main body as input and the swing angles of the left and the right rocker suspension as outputs, the rotational input is decomposed into two different rotational outputs. If the output is the mean value of two inputs, it is helpful to allocate the average weight of the body to each wheel which can adjust its position passively alone in the terrain.Given the characteristics and operating requirements of differential mechanisms, a bevel gear type differential mechanism has been designed. We have analyzed the working principle of the bevel gear differential mechanism and present its detailed structural design.2.2.2 Principle of the bevel gear differential mechanismFig. 3 shows the schematic diagram of the bevel gear differential mechanism. Two semi-axle bevel gears 1 and 2 mesh with the planetary bevel gear 3 orthogonally. Carrier H connects to planetary bevel gear 3 coaxially. Let the angular velocities of gears 1,It can clearly be seen that this bevel gear differential mechanism can be used in the rocker-type mobile robot.2.2.3 Bevel gear differential deviceGiven the above principle of a bevel gear differential mechanism, we designed such a bevel gear differential device, shown in Fig. 4. Fig. 4a is the outline of the differential device, and Fig. 4 bitsinte rnalstructure .This bevel gear differential device is composed of a shell, end covers, an axle base, semi-axle bevel gears, planetary bevel gears, a connecting shaft, etc. The end covers and axle beds connect to the shell by screws. In the shell, two planetary bevel gears are coaxial and symmetrically installed at the connecting shaft, with the shaft terminals supported at the end covers. There are bearings between the connecting shaft and bevel gears. The circlips are installed on the connecting shaft to limit the load on the bearings. Two semi-axle bevel gears are housed in the two axle beds separately, two axle beds are fixed on the shell symmetrically and two semi-axle bevel gears mesh with two planetary bevel gears orthogonally. The twoaxle bases have the same structure. The semi-axle bevel gears are located by the bearings, shaft sleeve and circlips in the axle beds. When the differential device is installed on the robot, the two axles of the left and right semi-axle bevel gears are connected to the left and right rockers. The shell of the differential is fixed on the main body of the robot.3 Mobile platform Test3.1 Simulation testAccurate, simulated 3D model of the robot was imported into the ADAMS software. Using the kinematic pairs in the joints database of the ADAMS/View, the movement of each part of the simulation model is constrained. For simulating the differential action of differential devices acting on the robot body, a revolute joint between the left and right rockers of the model and the “Ground” is established. Random moments of forces are exerted to the left and right rockers to simulate the rough action of the terrain on the rockers. For simulating the movements of the differential device accurately, contact forces are exerted to the pair of gears of the differential device. After corresponding marker points on the robot areestablished, the swinging angles of the left and right rockers and the robot body are measured and the curves of the swinging angles along with the time are obtained via the ADAMS/Post processor module, shown in Fig. 6. Curves 1 and 2 are swing angle curves of the two rockers, while curve 3 is the swing angle curve of the main body.?The bevel gear differential device can average the swing angles of the right and left rockers, and the average value is the swing angle of the main body. The gap between two teeth and other factors cause the return difference of the gear drive, so when the main body is swinging at the early start-up and through the zero angle, there is a slight swinging angle deviation between the simulated and theoretical values.Typical steps, channels, slopes and other complex terrain models are built in the Solid Works software. For testing the traffic ability characteristics and ride comfort of the four wheel robot, all-terrains models are imported into the ADAMS software[16-17]. Then the joints and restraints are rebuilt, Contact Force between the terrain and the wheels is exerted and torque is exerted to each wheel. The running condition of the robot is simulated on the complex terrain, as shown in Fig. 7a. The vertical displacement, velocity and acceleration curves of the centroid of the body and the centers of the four wheels can be obtained, as shown in Figs. 7b~7d. According to the curves, the curve of the centroid displacement of the main body (main body d curve) is very smooth and the velocity and acceleration of the main body is approximately the mean of that of the four wheels. The simulation results show that the mobile platform of the robot has good trafficability and rides comfortably on the complex terrain.3.2 Prototype testIn order to verify the performance of the robot in surmounting obstacles and adapting to a complex terrain, an obstacle-surmounting test of the robot was carried out on a simple obstacle course built in the laboratory and on a complex outdoor terrain bestrewn with messy bricks and stones. Fig. 8 shows the video image of the robot when moving on the complex terrain. The tests indicate that the four drive wheels of the robot can passively keep contact with the uneven ground and the robot performed well in surmounting obstacles. When moving on uneven ground, the swing angle of the main body was small and the differential device could effectively reduce the effect ofthe changing terrain to the main body. One side of the robot can cross a 260 mm-high obstacle. Only large obstacles between the landing legs of the rockers appear to block progress. The performance in surmounting obstacles by the four wheels of the robots is clearly better than that of a track-type robot of the same size.4 Conclusions1) Coal mine accidents, especially gas and coal dust explosions, occur frequently. Therefore, it is necessary to investigate and develop coal mine rescue robots that can be sent into mine disaster areas to carry out tasks of environmental detection and rescue missions after disasters have occurred, instead ofsending rescuers which might become exposed to danger.2) An underground coal mine environment presents a space-restricted, unstructured terrain environment, with a likely explosive gas atmosphere after a disaster. Hence, any mobile system would require a high motion performance and obstacle-surmounting performance on complex terrain.3) Given an unstructured underground terrain environment and an explosive atmosphere, we investigated an explosion-proof coal mine rescue robot with four independent drive wheels, based on a rocker type structure. Our simulation and test results indicate that the robot performs satisfactorily, can passively adapt to uneven terrain, is self-adaptive and performs well in surmounting obstacles.4) In our study, we only investigated the rocker type mobile platform of a coal mine rescue robot. In order to adapt to the underground coal mine environment, we also carried out a flameproof design for the main body. It was necessary to improve the rocker suspensions in order for the robot to be able to adjust the angle between two landing legs automatically, sothat the height of the center of gravity of the robot can be controlled, which should improve the anti-rollover performance of the robot.Acknowledgements The authors thank the National Hi-tech Research and Development Program of China for its financial support (No.2006AA04Z208).References[1] Peng G J. Elementary discussion on the gist and difficultyof technology in processing gas explosion. Safety in Coal Mines, 2004, 11: 29-30. (In Chinese)[2] Xu S F. Applied prospect of the industry robot in mining.Safety in Coal Mines, 1993, 1: 28-31. (In Chinese)[3] Hao J F, Pan W, Li X. Recognition of work space usingmultiple ultrasonic sensor. Journal of China Universityof Mining 懸架搖臂,微分, 防爆設(shè)計(jì)1.簡介在救援任務(wù)的氣體和煤塵爆炸,救援人員很容易中毒煤礦井下充滿有毒的氣體,如 high-concentrationCH4和 CO,如果通風(fēng)和保護(hù)不符合標(biāo)準(zhǔn)。此外,二次或多個氣體爆炸可能是由于極不穩(wěn)定的氣體在這樣的一個災(zāi)難,也可能會造成人員傷亡在救援人員 [1]。因此,為了成功執(zhí)行救援任務(wù),在適當(dāng)?shù)臅r間,減少人員傷亡, 這是必要的,以開發(fā)煤礦救援的機(jī)器人。然后他們被送到進(jìn)入災(zāi)區(qū) ,而不是救援者和執(zhí)行下一個任務(wù)的環(huán)境檢測,尋找受傷的礦工和受害者在這場災(zāi)難之后發(fā)生。機(jī)器人的首要任務(wù)是在營救工作進(jìn)入災(zāi)區(qū)。很難進(jìn)入受限空間機(jī)器人和非結(jié)構(gòu)化的地下的地形,所以這些移動通信系統(tǒng)需要進(jìn)行良好的 obstacle-surmounting 性能和運(yùn)動性能在這崎嶇的環(huán)境[2]。使用傳感器用于地形識別低能見度嚴(yán)格限制和環(huán)境充滿了爆炸性氣體和塵埃;因此,所謂的移動電話系統(tǒng)應(yīng)當(dāng)盡可能地,獨(dú)立于傳感和控制系統(tǒng)[3]。在國內(nèi)還是在國外研究煤礦的救援機(jī)器人是剛剛開始。大多數(shù)機(jī)器人原型是簡單類型和軌道輪機(jī)器人。RATLER 礦山勘探機(jī)器人開發(fā)的智能系統(tǒng)和機(jī)器人學(xué)中心(ISRC)的美國山迪亞國家實(shí)驗(yàn)室,使用一個車輪式移動系統(tǒng)[4]?？▋?nèi)基·梅隆大學(xué)的機(jī)器人研究中心自主機(jī)器人開發(fā)礦藏的開采,被稱為“鼠” [5]。煤礦救災(zāi)機(jī)器人都產(chǎn)生了由美國偏遠(yuǎn) V2公司和礦井搜救機(jī)器人 CUMT…中國礦業(yè)大學(xué)開發(fā)的技術(shù),使用一個 two-track 固定式系統(tǒng)[7]移動。這四個原型是煤礦井下嚴(yán)重限制。搖桿式機(jī)器人在復(fù)雜的地形具有良好的性能。所有三個火星布萊克本,也就是說,寄居的,精神號與機(jī)會使用移動系統(tǒng)有六個獨(dú)立驅(qū)動輪(8 - 9]。Rocker-Bogie、發(fā)達(dá)的由美國領(lǐng)導(dǎo)的實(shí)驗(yàn)室成功登陸在火星上。這個 SRR 機(jī)器人實(shí)驗(yàn)室有四個獨(dú)立傳動領(lǐng)導(dǎo)