Liu Simin，Chao Xianzhi

(School of Mechatronics Engineering,North Minzu University,Yinchuan City,750021)

Abstract:The ecological and environmental protection as well as social development in the western region of China has been restricted by the long-term encroachment of the drifting dunes.Since straw checkerboard sand barriers paving has been taken as an effective method to combat desertification,this paper mainly focuses on the vast research of paving mechanism design.The paving mechanism is the main executive part of straw checkerboard sand barriers paving machines,which consists of horizontal paving mechanism and longitudinal paving mechanism.The longitudinal paving mechanism is designed,in the direction of the tractor traveling,to press the straw into the sand to pave a continuous straw sand barriers at one meter intervals,and then the horizontal paving mechanism is responsible for inserting the straw once every one meter by using an inserting device,so that we can build a new straw checkerboard sand barrier of 1m×1m with each 1m advance of the machine,so as to achieve a good sand fixation effect.This paper,by employing Pro/E technology,establishes the solid model of straw checkerboard sand barriers paving machines,makes modifications on the original design of mechanism of conveying straw and inserting within the horizontal paving mechanism,and then perform kinematic analysis to ensure the feasibility.

Keywords:Straw Checkerboard;Horizontal Paving Mechanism;Longitudinal Paving Mechanism;Pro/E Technology

Introduction

At present,the desert and desertification land in China is mostly distributed in the West and Northwest between 37°N and 42°N,namely six provinces included, Xinjiang, Gansu, Qinghai,Ningxia, Shanxi and Inner Mongolia.Across the country,there are 1/3 of the land area is suffering from the drifting-sand dune[1].Approximately 400 million people in China are directly or indirectly affected by desertification,that's why the government of State level attaches great importance to anti-desertification program,which has introduced relevant laws,regulations and policy plans,and has vigorously supported the incorporation of technological innovation into desertification control.After years of unremitting efforts,China has taken the lead in achieving zero growth in land desertification around the world.

Based on the study of desertification and the law of it together with the comparison of technologies in wind-proof and sand-fixation at home and abroad,it is found that the most effective method to combat the erosion is the straw checkerboard method in engineering of sand control.The raw materials used in the checkerboard barriers come from a wide range of sources,mostly straw,stalk,and etc.,which could be used to increase the content of organic matter in dunes after decomposing.Concurrently,the barriers could help to intercept the sparse rainfall in the desert with the vertical fence formed on the surface of dunes,indirectly conserving water for the growth of plants.By doing that, it is more scientific and effective to control desertification with the combination of straw checkerboard barriers method and the biological technology.In the harsh weather of desert and sandy land,it is of huge disadvantage to pave the barriers artificially, such as anti-desertification being much slower than desertification, high labor intensity and low efficiency.Having been unable to win the fight with desertification,it is inevitably a reasonable choice to stem the problem by introducing the method of paving the straw checkerboard sand barriers by using the modern mechanical equipment to replace the artificial paving.Therefore,it is of great significance to set about research on straw checkerboard sand barriers paving machines.

Ⅰ.Major Technical Indicators

The size of the straw checkerboard barrier is determined by local wind,and the most reasonable size on the undulating sand dunes is 1 m×1 m,which is also the most common and effective specification,thus we can get some theoretical basis on that[2].The straw barriers can be properly paved more densely along the contour lines on some windward slopes.The straw is pressed and inserted to the sand with depth of 20-30 cm by using the inserting mechanism,with only 20-30cm straw left above the surface enough to fix the drifting dunes.For safety reasons,every 10 meters of straw barriers is paved,a“Z”shaped fire break with width of 1 meter should be set aside,within which no straw is paved.

The machine is mainly composed of the wheeled tractor and the straw checkerboard barrier paving mechanism.The wheeled cross-country tractor travels in the desert, with the wheeled straw checkerboard sand barriers paving machine towed at its rear,at a set speed of 0.5m/s.Under the control of the computer(central controller),the longitudinal and horizontal paving mechanism installed in the machine work together to pave straw checkerboard barriers on the sand dunes[3].

Table 1 Major Technical Indicators

Ⅱ.The Modification of Horizontal Paving Mechanism and Longitudinal Paving Mechanism

（i）The Modification of the Straw Conveying Mechanism

The longitudinal paving mechanism and the horizontal paving mechanism are installed at the rear of the tractor.Using the straw roller as raw material,the straw curtain on the roller is conveyed from the electric motor to the bottom of the horizontal straw inserting device and paved on the sand surface for 1 m on each one meter advance of the tractor.Then the horizontal straw inserting device cuts off the straw curtain,and inserts into the sand,which constitutes one time inserting.The longitudinal paving mechanism is set in front of the horizontal mechanism,of which only the straw pressing part is different from the horizontal mechanism.The inserting mechanism of the longitudinal paving mechanism is realized by a set of wheels that would be in the same direction with the machine.In this way,when the straw curtain is conveyed,all set of wheels,along the direction of the machine,inserts the spreading straw into the sand from the middle of the folding.The straw checkerboard barrier is finished after the insertion of the horizontal and longitudinal paving.When the straw curtain on the roller is used up,the refilling will be realized with the hydraulic driving technology by using horizontal straw changing roller[4].

Although it is quite convenient to install straw roller within the above paving mechanism,the curtain needs to be woven on the curtain weaving machine with a thickness of 8 mm,and tied up by two pieces of nylon cords of 0.5 m long.Each roller,about 1-1.2m in diameter,is rolled up with straw curtain of 100 m long.It is inevitably increasing the paving cost,difficulty of control and precision with such kind of straw barriers,because the straw rollers of this kind have to be produced by a specialized manufacturer,a special cutting mechanism has to be set to cut the nylon cords on the roller and used to cut the curtain just when it's conveyed to the surface of sand for being pressed.The thickness of the straw curtain has been decided at the time of weaving,making it unable to meet the requirements for thickness in the various practical conditions.Paving directly with the scattered straw, it is though convenient to adjust the thickness of the straw paving in practical conditions,would make it difficult for conveying the scattered straw,in which there may be blocking,discontinuous and uneven conveying,and etc.

Therefore,on a basis of various studies and the practical desertification conditions in China, this paper presents the modified design of the horizontal and longitudinal paving mechanism in straw checkerboard sand barriers paving machines.

The new mechanism mainly consists of scattered straw tank,intermittent conveying device,scattered straw carding device, inclined straw discharging mouth, sand track wheel, the pneumatic rotary inserting part and so on.The straw tank,before operation,is full loaded with relatively neat scattered straw,which will be conveyed successively with the intermittent conveying device to the track wheels driven by motor for carding.After carding by the scraper, the processed straw will be sent to the inclined discharging mouth through the double-sized tooth-shaped synchronous belt conveying mechanism.After the tractor arrives in the given position within one meter,the rotary device driven by gear meshing,the conveying board will be inclined at a specific angle so that the scattered straw could be conveyed downward to the surface of sand and paved a one meter wide area within the longitudinal checkerboard barrier.Subsequently,the pneumatic inserting device will insert the straw at the middle folding line to the sand and be retracted after this operation.The control of the conveying quantity,intermittent period and thickness is operated by the intermittent conveying mechanism by the chain drive,at the bottom of straw tank.As the scattered straw inserted usually spreads out,it is necessary to use sand track wheels to round up the sand from both sides to strengthen and protect the insertion,in case of being uprooted by the drifting dunes.Fig.1 shows the three-D model of the horizontal paving mechanism.

Fig.1 Three-Dimensional Model of the Modified Horizontal Paving Mechanism

Consistent with the modification of the horizontal mechanism,the longitudinal paving will similarly process the scattered straw,convey and pave successively to the surface of the sand.With the traveling of the tractor, the pneumatic inserting device will insert the straw at the middle folding line to the sand,and then use sand track wheels to round up the sand from both sides to strengthen the insertion.

（ii）Modification of the horizontal inserting mechanism

In order to protect the scattered straw paved from being blown away by wind,it is necessary to insert the straw into sand as soon as paving.In addition,the inserting device has to be retracted quickly in case of bringing out the inserted straw and destroying the barrier finished.In order to improve the speed of inserting,the inserting device,driven by cylinder,is set to insert into the sand vertically,with displacement and speed amplified by the connecting rod.While the tractor moves forward for paving,there will be an instantaneous rigidity impact to the inserting device when inserting vertically to the surface of sand,which would lead to the vibration of the overall horizontal paving system,even the entire tractor.It is likely to deform or bend the connecting rod as a result of the inserting device unable to bear the relatively huge resistance of the sand,when the inserting device pressing downward to the sand.Based on a great deal of simulation experiments,it is concluded that the rotary horizontal inserting device could be help to accelerate the insertion speed and reduce the rigid impact caused by sudden increase of the speed, greatly reducing the vibration in the working process,so as to extend the service life of the machine and minimize the cost.

Ⅲ.Monitoring Devices

In order to ensure the accuracy of the operation,the monitoring system is set up to take charge of the function status of each mechanism,so as to timely find problems and minimize mistakes.This monitoring system consists of four cameras,one quad multi-plexer set and one monitoring display.The four cameras are installed in the designated position,so that the signals of video captured are transmitted back to the cab's display,with which,the worker monitors the performance of each mechanism.The two of them is placed in the longitudinal mechanism,with the one monitoring the processing and conveying of the scattered straw, the other monitoring the inserting procedure and the surface of sand;and the other two is set in the horizontal mechanism to monitor the operation of carding,conveying,inserting and the sand track.The horizontal inserting is set to insert once every one meter in the traveling of the entire tractor,during which,the traveling sensor is set to collect signals and transmit to the computer for calculating a reasonable inserting time, so as to control the speed of horizontal straw feeding motor,the conveying speed of the horizontal straw feeding motor as well as the operation of horizontal inserting device.

With the undulating ground in the desert,the height of the horizontal paving mechanism varies as the machine travels,which eventually affects the quality of paving.If it is too low,the straw inserted will be scraped and brought out,while if it is too high,the inserting device won't reach deep enough,then the straw would be easily uprooted by the drifting dune.Therefore,it is of great importance to adjust the distance between the horizontal paving mechanism and the surface of sand.In order to solve the problem,it is necessary to install the ranging radar at the bottom of the horizontal inserting mechanism and feedback the signals to the computer to compare it with the setting distance.By operating the cylinder lifting, the height of the horizontal inserting device can be adjusted,so as to ensure the depth and quality of inserting.

Ⅳ.Pro/E Technology Aided Design

（i）Pro/E Software Employed for Three-D Modeling

In the design of straw barriers paving machines,with Pro/e software we built 3-D model and analyzed the simulation experiments.In other words,we built models of each part and connected them with the corresponding relationship to form an assembly.After analyzing the geometric parameters, quality and mechanical properties which can be easily reflected by the setting materials,we modified the design and get the optimal design.

（ii）Design of the Major Parts

Except for the straw carding, the inserting device would be the most important part,of which the adaptable straw discharging board is of certain flexibility.In the process of inserting,the straw will be conveyed to the horizontal suspended board,after which the inserting device is retracted and the supporting board rotates 60° in the negative direction along the Y axis,making that the straw slides down to the sand surface by itself.When the sand track wheel operates,the bottom of it will press the surface of the sand ridges slightly,following the pavement with traveling of the machine, the horizontal suspended board will be retracted.Figure 2 shows the three-dimensional model of the inserting part.

Fig.2 three-D Model of Inserting Part

Figure 3 shows a serrated two-dimensional profile for the shape of the inserting device,of which the blades was set to be left a gap within.On the one hand,it reduces the chance of breaking of straw;on the other hand,it reduces the sand resistance as the inserting device presses into the sand,extending the service life of device.

Fig.3 Dimension Figure of the Serrated Inserting Device

Ⅴ.Conclusion

Based on the original horizontal paving mechanism,this paper presents the amendment from the straw roller to the scattered straw paving,the improvement of the inserting device and the sand track mechanism,which would bring down the cost of paving and improve the precision of inserting;The straw roller is replaced by the scattered straw as the raw material,which makes it possible to adapt the thickness of barriers as well as conduct paving in various conditions;The mechanism of cutting cords of roller is removed,which improves the interoperability and controllability of the whole system.Among all of the modifications,the pneumatic horizontal inserting device is added to accelerate the insertion and minimize the rigidity impact caused by the instantaneous increase of the speed,which greatly reduces the vibration of the paving mechanism in the working process,so as to extend the service life of the machine and lower the cost.Furthermore, a three-dimensional solid model of horizontal paving mechanism is established through Pro/E technology,by which the kinematics simulation and mechanical analysis can be carried out on the assembly model of the mechanism,so that the problems could be found and solved in time.