With the transformation and upgrading of manufacturing and the rapid development of intelligent logistics, the application of AGVs (Automated Guided Vehicles) has expanded rapidly from traditional warehouses with controlled environments to increasingly complex scenarios such as manufacturing workshops, port terminals, and outdoor inspection areas. The expansion of application scenarios and frequent transitions between environments, especially indoor-to-outdoor operation, impose much higher requirements on AGV environmental adaptability. Among these factors, road surface adaptability is particularly critical.
As a core mechanical structure that ensures smooth vehicle motion, reliable load bearing, and long service life of the drive unit, the rational design and selection of shock-absorbing floating structures play a decisive role. To meet different chassis layouts and load requirements, various types of floating suspension structures have been developed. This article systematically reviews common AGV shock-absorbing floating structures, analyzes their working mechanisms, design constraints, and performance characteristics, and provides theoretical references and practical guidance for suspension system design and selection.
1. Core Functions of Shock-Absorbing Floating Structures
The fundamental objective of a shock-absorbing floating structure is to ensure stable AGV operation on uneven and complex road surfaces. This objective is achieved through three closely related mechanisms.
(1) Ensuring coordinated ground contact of the wheel system
In multi-wheel AGV configurations, if the drive wheel is installed in a more protruded position than auxiliary wheels to guarantee traction, auxiliary wheels may lose ground contact. This leads to excessive load concentration on the drive unit, reducing effective payload capacity and significantly affecting driving stability.
By introducing elastic freedom through suspension springs, the shock-absorbing floating structure allows the drive unit to move vertically. Under the self-weight of the AGV, the drive wheel can be pressed back to the same height as the auxiliary wheels, enabling all wheels to contact the ground simultaneously. This ensures sufficient traction for the drive wheel while allowing auxiliary wheels to share part of the load, resulting in optimized load distribution across the vehicle.
(2) Adapting to road irregularities and obstacles
When operating on uneven road surfaces without shock absorption, the drive wheel may lose traction in depressions or be rigidly lifted by obstacles, causing vehicle vibration, deviation, or instability. With a floating suspension, the spring allows the drive wheel to continuously follow the road surface profile.
When encountering a protrusion, spring compression prevents the drive unit from rigidly lifting the entire vehicle. When passing over a depression, the spring restoring force pushes the drive wheel downward to maintain ground contact. This ensures continuous traction and stable driving behavior under varying road conditions.
(3) Buffering impact loads and protecting the drive unit
Road irregularities and obstacles generate transient impact loads that are transmitted directly to the motor, gearbox, bearings, and other critical components. Over time, these loads accelerate wear and failure.
The suspension spring absorbs and buffers impact energy through elastic deformation, converting sudden shock loads into gradually released elastic energy. This significantly reduces peak loads transmitted to the drive unit, extending component service life and reducing maintenance costs.
2. Design Constraints and Mathematical Modeling (Plain-Text Format)
To reliably achieve the above functions, shock-absorbing floating structures must satisfy a series of mechanical constraints. The core design variable is the accurate matching of spring stiffness k. Based on three typical operating conditions-flat ground, depressions, and protrusions-key design relationships are established below using engineering-friendly plain-text expressions.
Key parameter definitions
k : stiffness of a single suspension spring
lambda : protrusion height of the drive wheel relative to auxiliary wheels
delta : road surface unevenness (bump = +delta, depression = -delta)
Delta : spring preload
n : number of springs per drive unit
G : total AGV weight at full load
mu1 : friction coefficient between drive wheel and ground
mu2 : rolling resistance coefficient of the AGV
Fmax1 , Fmax1_limit : rated and ultimate load of the drive wheel
Fmax2 , Fmax2_limit : rated and ultimate load of auxiliary wheels
(1) Flat ground condition (baseline case)
This is the most common operating condition. All wheels must maintain ground contact, loads must remain within rated limits, and drive wheel slip must be avoided.
Drive wheel normal load:
FN1 = (Delta + lambda) * n * k
Load constraint for the drive wheel:
FN1 <= Fmax1
Auxiliary wheel load FN2 must satisfy:
FN2 <= Fmax2
(Note: FN2 is obtained from static force equilibrium of the wheel system as a function of FN1 and total vehicle weight G.)
Anti-slip condition:
FN1 * mu1 > G * mu2
(2) Depressed road condition
In a road depression, the spring extends further, reducing the drive wheel load and increasing the auxiliary wheel load. To prevent loss of drive wheel contact, the following geometric condition must be satisfied:
lambda > delta
Drive wheel normal load:
FN1_depressed = (Delta + lambda - delta) * n * k
Load constraints (short-term limits allowed):
FN1_depressed <= Fmax1_limit
FN2_depressed <= Fmax2_limit
Anti-slip condition:
FN1_depressed * mu1 > G * mu2
(3) Protruding road condition
When the AGV encounters a protrusion, the spring is further compressed and the drive wheel load reaches its maximum value. The spring force must not lift the entire vehicle and cause auxiliary wheels to lose contact.
Drive wheel normal load:
FN1_bump = (Delta + lambda + delta) * n * k
Common ground-contact constraint
(for a typical four-wheel AGV configuration):
2 * FN1_bump < G
Load constraint (short-term limit allowed):
FN1_bump <= Fmax1_limit
(4) Comprehensive stiffness range determination
By combining all inequality constraints from flat, depressed, and protruding road conditions, a feasible range for spring stiffness k can be obtained.
Within this feasible range, appropriate values of spring preload Delta and drive wheel protrusion lambda should be selected.
In engineering practice, the following guideline is commonly adopted:
lambda = (1.5 to 2.0) * delta
This provides sufficient safety margin for road surface unevenness.
3. Common Types of AGV Shock-Absorbing Floating Structures
(1) Articulated swing type
The drive unit is connected to the chassis via a pivot joint and can swing under spring-generated restoring torque. This structure provides mechanical amplification, allowing a relatively small spring force to generate a large ground contact force. However, the relationship between floating travel and spring compression is nonlinear.
Although adaptability is strong, bidirectional load differences exist. During uphill operation, the drive wheel load increases significantly, requiring careful structural strength verification. This type is widely used in heavy-duty AGVs where installation space is sufficient.
(2) Vertical guide column type
The drive unit floats vertically along linear guide columns or guide sleeves, with compression springs providing shock absorption. The structure is compact, cost-effective, and easy to maintain.
A critical design requirement is that the guide columns must be symmetrically arranged and centered relative to the wheel-ground contact point. Improper alignment can generate additional moments, leading to jamming or abnormal wear. This type is suitable for light- to medium-load AGVs with strict height constraints.
(3) Scissor-link type
Floating motion is realized through a scissor linkage mechanism and is often integrated with differential steering modules to save installation space. However, when left and right drive wheels encounter different road heights, the structure lacks self-adaptability and may cause diagonal lifting of the chassis.
This type is mainly used in specific integrated differential steering drive modules and offers relatively poor adaptability to general uneven road surfaces.
(4) Swing-axle type
Two wheels are rigidly mounted on a single axle that can pivot around a central hinge. Road unevenness is accommodated by swinging the entire axle, effectively treating the two wheels as a single virtual large wheel.
In multi-wheel systems, multiple swing axles can be combined to reduce the wheel system to an equivalent three-point ground contact configuration, fundamentally solving co-grounding issues. This structure is simple and robust, making it highly suitable for multi-wheel, heavy-duty, and outdoor AGVs.
(5) Four-linkage type
Based on the parallelogram linkage principle, the four-linkage structure allows vertical floating while maintaining constant orientation of the drive unit. Compared with articulated swing types, forces remain collinear, eliminating torsional loads during floating motion.
Although structurally more complex and space-consuming, this design provides superior stability and is well suited for heavy-duty AGVs with strict wheel attitude requirements, such as forklift-type AGVs using vertical AGV drive wheels.
4. Comparison and Selection Guide for Shock-Absorbing Floating Structures
Comparison of Common Floating Structure Types
| Structure Type | Road Adaptability | Space Requirement | Main Advantages | Limitations | Typical Applications |
|---|---|---|---|---|---|
| Articulated Swing Type | Excellent | Medium | High mechanical gain, strong adaptability, mature technology | Bidirectional load difference; potential torsional load on drive unit | Heavy-duty steering drive wheels; layouts with sufficient space |
| Vertical Guide Column Type | Good | Small | Compact structure, low cost, easy maintenance | Highly sensitive to guide column alignment; risk of jamming | Light- to medium-load AGVs; applications with strict height constraints |
| Scissor-Link Type | Relatively Poor | Large | Easy integration with differential steering modules | Poor adaptability to uneven left-right road conditions; large space occupation | Integrated differential steering drive units |
| Swing-Axle Type | Excellent (multi-wheel) | Large | Simple and robust principle; strong multi-wheel ground-contact capability | Bulky structure; large vertical and lateral space requirements | Multi-wheel heavy-duty outdoor AGVs; construction machinery–type AGVs |
| Four-Linkage Type | Excellent | Medium to Large | Constant wheel attitude during floating; no additional torsional load; stable performance | More complex structure; higher cost | High-precision, heavy-duty forklift AGVs; applications with strict wheel attitude requirements |
Selection Recommendations Summary
Differential drive layouts:
When compact structure and low cost are primary objectives, the vertical guide column type is a suitable choice. If steering integration is required and installation space allows, the scissor-link type may be considered. For applications with high requirements on road adaptability and motion accuracy, the articulated swing type or four-linkage type is recommended.
Steering drive layouts:
Vertical guide column structures are widely used in light- to medium-load applications. In heavy-load scenarios, the articulated swing type is the mainstream solution. For forklift-type AGVs where strict vertical alignment of the drive wheel is required, the four-linkage type offers clear advantages.
Special multi-wheel heavy-duty or outdoor layouts:
The swing-axle type, or combinations of multiple swing axles, represents one of the most effective solutions for ensuring reliable ground contact on complex and uneven terrain.
5. Conclusion
Shock-absorbing floating structures form the critical interface between an AGV and the ground. Their performance directly determines the vehicle's operational capability and reliability in complex environments. The core of suspension design lies in accurately matching spring parameters to specific operating conditions-including road profiles, load levels, and vehicle speed-while simultaneously satisfying multiple constraints such as multi-wheel ground contact, load balance, anti-slip performance, and impact buffering.
At present, articulated swing and vertical guide column structures dominate both differential-drive and steering-drive AGVs due to their respective advantages. Four-linkage structures demonstrate outstanding performance in high-end heavy-duty applications, while swing-axle structures provide unique and effective solutions for multi-wheel heavy-duty outdoor AGVs.
Looking ahead, as AGV application scenarios continue to expand and deepen, active and semi-active suspension technologies, as well as intelligent adaptive suspension systems integrated with road perception, are expected to become key development directions to address higher dynamic performance requirements and more extreme operating environments.
