With the acceleration of the transformation and upgrading of China's manufacturing industry, mechanical automation instead of artificial labor has become the focus of today. Compared with the past, the current form of logistics and material handling has gradually changed from traditional manual handling to intelligent automatic handling. AGV has been one of the most widely used handling robots in the domestic manufacturing industry.
AGV originated from warehousing logistics, in which the application environment is relatively good, and the application site of AGV also has relevant standard definitions. With the rapid development of intelligent logistics, AGV has been involved in manufacturing, port handling, security inspection and other fields. The expansion of the application scope means that the complexity of the operation scenario is improved. Therefore, the adaptability of AGV should also be improved. As one of the adaptive structures of AGV, the structure of shock-absorbing floating structure is various at present. For different types of AGV chassis and bearing capacity, the structure of shock-absorbing floating structure is also different. In this paper, the common shock absorption floating structure forms of AGV are listed, and the characteristics of its shock absorption are studied and analyzed, so as to provide the characteristic analysis and theoretical reference for the suspension floating system design of AGV.
1. Function of floating structure
In general, the shock absorption floating structure of AGV is to enable AGV to achieve the performance of driving on the complex road, and its specific role is to:
(1) The gear train lands together. In the multi wheel arrangement of AGV, in order to ensure the driving wheel landing, the general idea is to install the driving unit protruding out compared with other auxiliary wheels to ensure the first landing of the driving wheel. But so it is. In fact, the auxiliary wheel is not close to the ground, resulting in more load applied to the driving unit, reducing the bearing capacity of AGV, at the same time, the driving stability of AGV will also be reduced.
Then, on the basis of the above, the damping floating structure makes the driving unit have the freedom of up and down compression. As shown in Figure 1, under the convex of the driving wheel, the driving wheel is pressed to be flush with the auxiliary wheel by the self weight of AGV. The floating structure is used to realize the problem of multiple wheels landing together, which ensures that the driving force of AGV and the landing of auxiliary wheels share a part of the load.
(2) Adapt to uneven road surface. In the working environment of AGV, the uneven road surface will lead to the suspension of the driving wheel, which will cause the AGV to lose power or be jacked up. The spring in the damping floating structure will make the driving wheel always close to the ground. When encountering the raised road surface, because of the floating property of the driving unit and the compressibility of the spring, it can avoid the driving unit driving the AGV to be jacked up as a whole. The reaction force of the spring keeps the driving wheel close to the ground all the time, and the ground also provides the supporting force of the driving wheel at all times to ensure sufficient adhesion, so as to ensure that AGV will not lose power due to uneven road surface.
(3) Slow down the impact. The uneven road surface and the obstacles in the direction of the road will impact the driving unit, and the shock absorption spring will absorb the impact, effectively alleviate the damage of the impact on the driving unit, and extend the service life of the driving unit.
2 design requirements of floating structure with damping
In order to ensure that the floating structure can play the above-mentioned specific functions, the design of its structure should meet certain conditions, otherwise, there will be functional failure caused by too large or too small floating stiffness.
Now, it is assumed that the required stiffness of the spring is k, the road roughness is ± δ, and the camber of the driving wheel is λ. So, in the analysis of the floating structure, the working road conditions of AGV should be divided into three kinds
(1) Flat surface. Flat road is the longest working condition of AGV. At this time, AGV should ensure that all wheels touch the ground together, the load of each wheel is within its rated load range, and the adhesion of driving wheel is enough to prevent wheel slipping.
When the AGV is on a flat road surface, i.e. the driving wheel is flush with other auxiliary wheels, the damping spring at this time is equivalent to the external convex value λ compressed. At this time, the force F N1 between the driving wheel and the ground is:
FN1 = (Δ+λ)·nk
In the formula, Δ is the installation preload of spring; n is the number of spring.
The bearing capacity shall meet the following requirements:
FN1 ≤ Fmax1
FN2 ≤ Fmax2
FN2 = f(FN1,G)
In the formula, f Max1 rated load of driving wheel; F N2 supporting force of auxiliary wheel on flat road; F max2 rated load of auxiliary wheel; g = AGV working
The calculation equation of F (f N1 , g) for F N1 and G is different for different gear train structure.
On the adhesion of driving wheel, f F , the following requirements shall be met:
Ff ＞ Fq
Ff = FN1·μ1
Fq = G·μ2
In the formula, f Q is the traction force required for AGV to walk; μ 1 is the coefficient of adhesion between the driving wheel and the ground; μ 2 is the rolling friction coefficient of AGV.
(2) Sunken pavement. In the concave Road, in order to make the driving wheel close to the ground, the damping spring will push the driving wheel against the ground. At this time, the shape variation of the spring compared with the flat road and the pressure of the driving wheel are smaller, while the pressure of other auxiliary wheels is larger.
It can be seen from the geometric relationship in Figure 3 that when the AGV is located on a concave Road, the compression amount of the damping spring is actually the difference between the camber and the road roughness, which shows that the camber of the driving wheel must be greater than the road roughness,
Otherwise, the drive wheel will be suspended when the road surface is depressed.
If it is ensured that all the wheels of AGV are on the ground together in the flat ground and the camber of the driving wheel is greater than the road roughness, all the wheels must also be on the ground together when AGV is on the depressed road surface. Therefore, it is necessary to ensure that the bearing range of each wheel and the adhesion of the driving wheel are enough to prevent the wheel from slipping.
At this time, the force between the driving wheel and the ground f N1 'is:
FN1 = (Δ+λ-δ)·nk
λ > δ
Compared with flat road and concave Road, when the spring shape variable is reduced, the driving wheel load becomes smaller and the auxiliary wheel load becomes larger. Because the working frequency of the concave road is lower than that of the flat road, that is to say, the large load working time of the auxiliary wheel is shorter,
At this time, the load of the auxiliary wheel is within its limit load range (if the working frequency is high, it must be within the rated load range), and the bearing capacity of the shock absorption floating structure must meet the following requirements:
FN1' ≤ Fmax1'
FN2' ≤ Fmax2'
FN2' = f(FN1',G)
In the formula, f N2 'is the supporting force of the auxiliary wheel on the depressed road; F max2 ' is the ultimate load of the auxiliary wheel; f (f N1 ', g) equations about f N1 ' and G, the calculation equations are different for different wheel system structures.
On the adhesion of driving wheel f F ', it must meet the following requirements:
Ff' > Fq
Ff' = FN1'·μ1
(3) Raised pavement. In the convex road surface, the damping spring of the driving unit is compressed by the convex road surface. Theoretically, the compression amount of the damping spring is larger than that of the flat spring. However, if the spring force in the compression process is enough to support the weight of the whole AGV, then the spring will no longer compress, but will jack up the whole AGV as a rigid connection. As the above analysis, at this time, the spring compression is the largest, so the driving wheel load is the largest.
In order to ensure that all wheels touch the ground together, it should be ensured that when the spring is raised to compress, the spring force will not support the AGV as a whole, so the force between the driving wheel and the groundFN1"it must meet the following requirements：
FN1" = (Δ+λ+δ)·nk
2FN1" < G
In the raised Road, at this time, the driving wheel load is the largest and the auxiliary wheel load is the smallest. Because the working frequency of raised road is still lower than that of flat road, the driving wheel can only bear large load for a short time, and the load is within its limit load range. The bearing capacity of the floating structure shall meet the following requirements:
FN1" ≤ Fmax1'
FN2" ≤ Fmax2'
FN2" = f(FN1",G)
In the formula, f Max1 'is the ultimate load of driving wheel; F N2 "raised road surface
The supporting force of the upper auxiliary wheel; the equation of F (f N1 ", g) about f N1 " and G, the calculation equation of the gear train structure is different.
(4) Comprehensive conditions. According to the above conditions (1), (2) and (3), the comprehensive conditions to be met for the floating structure are as follows:
For the above comprehensive conditions, each condition can be analyzed as above to build the equations and range inequalities of the relevant spring stiffness. Through multiple range conditions of the stiffness, it can be determined that the spring stiffness meets all conditions
The value range under. Then, the stiffness of the spring used for damping floating structure should be within this value range.
3 common shock absorption floating structure of AGV
(1) Hinged swing type. Articulated swing floating structure is a widely used damping structure. As shown in Figure 6, the driving wheel is fixed with the mounting base and is hinged with the vehicle body. Then the driving unit and the vehicle body can rotate and swing around the hinge point 1 to realize the floating in the up and down directions. By setting the spring damping device between the driving unit and the vehicle body, the swing amplitude of the driving unit is determined by the spring force.
There is a force arm relationship between the driving wheel support force and the spring reaction force of this kind of structure (as shown in Figure 7). When a certain driving wheel support force is needed, the actual spring force required is smaller than the driving wheel support force. However, the amount of floating is just the opposite. When the driving unit needs to obtain a certain amount of floating, the compression amount of the spring needs to be greater than that of the driving unit.
Based on the above characteristics, the articulated swing floating structure is more suitable for the layout of AGV gear train with large load and sufficient space. The force arm can effectively reduce the stiffness required by the spring, but it has certain requirements for the swing space.
When the AGV travels in the ramp, its ramp direction is shown in the figure 8 above, and its driving wheel supporting force is shorter than the arm length between the swing hinge points (Figure 8 below). When the compression amount of the spring is fixed, that is, when the spring reaction force is fixed, the driving wheel bearing force shown in Figure 8 above is greater. When the AGV load is large, it should be noted to check whether the driving wheel load is within the rated range.
(2) Vertical guide post type. The vertical guide pillar floating structure is a kind of shock absorption structure which is fixed by the driving wheel and the mounting base. The mounting base is provided with a guide sleeve and a guide rod to form a moving pair, and the guide rod is provided with a pressure spring. The driving unit floats up and down through the guide pillar guide sleeve pair, and the pressure spring provides the driving unit with vertical reaction force in the vertical direction.
The structure shall reasonably arrange the position relationship between the guide post and the driving wheel, as shown in Figure 10. To avoid the moment between the guide post and the guide sleeve due to the uneven force distribution, the two guide posts shall be arranged in the middle relative to the contact location of the driving wheel. If the guide post is not placed in the middle, the spring reaction force on both sides is not equal, resulting in more compression at the end with larger reaction force and less compression at the end with smaller reaction force. At this time, the moment between the guide post and the guide sleeve will inevitably cause the moving pair to be stuck.
In order to further prevent the binding between the guide post and the guide sleeve, as shown in Figure 11, the central connecting line of the two guide posts shall be at the center of the drive wheel width. As shown in FIG. 12, when the central connecting line of the two guide pillars deviates from the wide center of the driving wheel, there is a moment arm between the supporting force of the driving wheel and the spring reaction force, which must produce a counter force on the mating surface of the guide sleeve and the guide pillar, so that the moving pair will be stuck.
As a whole, the structure of vertical guide column floating occupies a small volume and has a simple structure. In terms of cost, it is a more economical shock absorption structure, which is more suitable for the layout of light and medium load gear trains with limited space.
In order to solve this problem, the relative position relationship between the guide post and the driving wheel should be reasonably arranged. At the same time, increasing the matching length of the guide pillar and the guide sleeve can effectively reduce the jacking force caused by torsion, reduce the probability of the guide pillar guide sleeve sticking, and avoid the possibility of the guide pillar bending deformation.
(3) Scissor type. The shear fork floating structure is a kind of damping structure based on the shear fork lifting structure. The utility model comprises an upper and a lower bracket of a scissor type lifting structure, the middle of which is connected by a scissor and a damping spring is arranged in the middle of the two brackets.
The structure has the same shock absorption floating type as the scissor lift. When the road surface is uneven, the lower bracket will compress vertically upward and close to the upper bracket. At the same time, the horizontal direction between the lower bracket and the upper bracket will also displace.
Because the scissor structure occupies a large space in height, this damping structure is more suitable for the differential unit module. Among them, the space of the middle part of the scissor fork structure can be effectively used. Besides the road adaptation function, the differential drive module also has the steering function of rotation relative to the vehicle body to improve the steering performance of AGV. Therefore, the steering structure can be placed in the middle space of the scissor structure, so as to save more space while having the function of shock absorption and steering.
Compared with the damping module, the shear fork floating structure takes up a large volume, which is more combined with the differential steering structure, combining the two structural spaces. Its structure is not suitable for the layout of the steering wheel with high space requirements and steering function.
In the adaptability of pavement, the scissor structure has some limitations. As shown in Figure 14, when the road surface height of the two driving wheels is inconsistent, because the scissor structure does not have more freedom to adapt to the uneven height on both sides, the AGV is tilted up as a whole.
(4) Swing bridge. The swing bridge structure connects the two wheels through the whole bridge, and the center of the bridge is used as the swing center to hinge with the car body. The pavement adaptive structure of swing bridge is commonly used in loaders and related construction machinery, which can adapt to the uneven ground by releasing the rotational freedom of the whole bridge. In practical application, if the road surface is only uneven and does not bring more impact to the driving unit, the floating structure of the swing bridge can not need to be equipped with a spring. The uneven terrain makes the distance between the supporting force of the two wheels and the swing center different, so the wheel with a far arm has a small supporting force, and the wheel with a short arm has a large supporting force, so the floating structure can adapt to the uneven road surface.
For a swing bridge structure, two wheels always adapt to uneven road surface through swing, which can be regarded as changing two wheels on the bridge into one big wheel of the whole bridge. So, for the four-wheel layout of the oscillating bridge (as shown in Figure 16), that is to say, the four-wheel layout is changed into three-wheel layout. Theoretically, the three wheels must be grounded in three points to determine a plane, so as to solve the problem that all wheels are grounded together.
For the six wheel layout and other multi gear layout, more groups of swing bridges are needed to realize the pavement adaptation through the swing bridge structure. From the above analysis, a oscillating axle can be regarded as changing two wheels into one wheel. Because of three
The wheels must be grounded, and the six wheel layout must be changed into three wheels, that is, three groups of oscillating bridge structures are required.
(5) Four sided form. The four side floating structure is based on the swing principle of four-bar linkage, on which the damping spring is added to compress the damping spring when the structure swings.
The vibration absorption type of the four sides floating structure is similar to the hinged swing floating structure. Both of them compress the shock absorption spring by rotating around the hinge point, so as to achieve the shock absorption effect. However, they are not the same in the moving structure and the force.
As shown in Figure 19, the up and down floating mode of four-way floating structure is the swing principle of four-bar linkage, while the up and down floating mode of hinged swing floating structure is the principle of circular motion around the hinge joint.
The swing principle of the four-bar linkage can realize that the attitude of the driving unit will not change when it floats, while the inclination of the driving unit of the articulated swing structure will change gradually during the floating process. The change of the angle of inclination causes the arm between the supporting force of the driving wheel and the supporting force of the mounting base, which makes the driving unit twist.
Table 1 Analysis of common floating structure characteristics of AGV
|Type of damping structure||adaptability||Space occupation||advantage||Limitations||Scope of application|
|Hinged swing type||excellent||secondary||The spring with small stiffness can provide greater ground tightening force and better shock absorption adaptability.||There are two-way force differences in the direction of blocked driving wheel torsion, it is necessary to check its structural strength||Gear train layout with large load and enough space|
|Vertical guide post type||secondary||Small||Small occupation space and simple structure||The guide post is easy to be stuck due to torsion. Lubrication and anti torsion measures shall be taken||Gear train layout with light medium load and high space requirements|
|Scissor type||difference||more||Easier to integrate with differential steering module||Large occupation volume, poor shock absorption due to structure||Differential drive layout with damping and relative body rotation|
|Oscillating Bridge||secondary||large||The structure is simple and the adaptability of multi bridge combination is good||In order to adapt to the uneven road surface, the multi wheel system must be combined with multi bridges.||Gear train layout with multiple gear trains and sufficient height space|
|Quadrilateral form||excellent||secondary||The spring with small stiffness can provide higher ground tightening force, better shock absorption adaptability, and the driving unit's attitude is unchanged during the floating process, which can eliminate the torsional problem of the driving wheel of the articulated swing structure||The structure is complex, and the space occupation is larger than that of hinged swing type||Gear train layout with sufficient load and space|
The attitude of the four sided floating structure will not change during the floating process, and the force between the driving unit and the mounting base is always collinear.
The four side floating structure requires more space in the vertical direction, and its structure is more complex than the hinged swing structure. This kind of structure is generally used in the vertical steering wheel and differential drive of forklift AGV.
4 comparative analysis of the advantages and disadvantages of common floating structures
See Table 1 for the analysis of the characteristics of the common damping floating structure of AGV. From the current domestic AGV damping type, the large load steering wheel layout is more of the hinged swing floating structure, and the small load steering wheel layout is the vertical guide post structure. For differential drive, the layout with higher requirements for road adaptability generally adopts the independent suspension damping type, which includes hinged swing type, vertical guide pillar type and four side type.
The main gear system layout of AGV includes differential layout and steering wheel layout. According to the different types of layout, the structural mode of shock absorption should also be analyzed.
 Sun Jianmin. Discussion on Key Technologies of vehicle damping system [J]. Road building machinery and construction mechanization, 2011, (6): 80-82
 Ma Yue, Wang yong'en, Ma Rui. Mechanical structure design of heavy-duty AGV [J]. Mechanical research and application, 2018 (2)
 LV Wangbiao, Liu Hao, Wu Yonghai. Floating drive and AGV [P]. Hangzhou: cn207257828u, April 20, 2018
 Zhou Zhengjun, Liu Qixin, Lu Huiwen. Driving suspension device and automatic navigation vehicle of an automatic navigation vehicle [P]. Guangdong: cn106494257a, 2017-03-15
 Peng Huaming, Zhu Zhong, Peng Qinghua, Cao Rui. AGV steering wheel drive damping mechanism and AGV steering wheel drive device [P]. Guangdong: cn206327106u, 2017-07-14
 Zhao Huadong, Xu Yicun. AGV scissor type damping unit rotation limit mechanism [P]. Guangdong: cn206187160u, 2017-05-24