1. Introduction

In the field of logistics automation equipment, under-ride AGVs (Automated Guided Vehicles) have become one of the most widely used solutions for material transportation. The drive system of an AGV plays a decisive role in determining its motion capability, application scenarios, operational efficiency, and long-term maintenance costs.

At present, two major drive configurations are commonly used in under-ride AGVs: differential drive and steering drive. These two approaches differ significantly in structural design, motion control principles, system integration, and engineering performance.

This article provides a technical analysis of both drive configurations from the perspectives of structural composition, motion principles, key performance indicators, and practical application limitations. The aim is to provide useful references for AGV system design, component selection, and engineering implementation.

2. Structure and Motion Principles of the Two Drive Systems

2.1 Differential Drive Unit: A Modular Motion Architecture

A differential drive unit typically consists of independent driving modules that generate vehicle movement through the coordinated control of multiple wheels. Steering is achieved through the speed difference between the left and right drive wheels, which follows the classical differential steering principle used in many mobile robotic platforms.

When a single differential drive unit is used, it generally consists of a pair of drive wheels along with the corresponding motor, transmission mechanism, and structural support. Due to the relatively large wheel spacing in this configuration, the AGV can usually perform forward motion and basic turning maneuvers, but the motion capability remains limited. This configuration is therefore mainly used in simple one-directional material transportation tasks.

When two differential drive units are installed on the AGV, coordinated control between the front and rear modules allows the vehicle to achieve bidirectional movement and turning. However, steering is still generated by wheel speed differences, meaning that the vehicle always follows a curved trajectory. As a result, lateral movement or omnidirectional motion cannot be achieved.

The turning behavior of a differential drive system is determined by the difference in linear velocity between the left and right wheels. With a fixed wheelbase, a larger speed difference results in a smaller turning radius. While this principle is simple and reliable, it places higher demands on speed control accuracy, especially at higher operating speeds.

2.2 Steering Drive Unit: An Integrated Mechatronic Solution

A steering drive unit integrates both traction and steering functions into a single mechatronic module. Unlike differential systems, steering drive units utilize independent motors for driving and steering, allowing the wheel orientation to be actively controlled.

This design eliminates the need for steering through wheel speed differences. Instead, the wheel itself rotates to the required orientation before generating traction force. As a result, motion control becomes more direct and precise.

Steering drive AGV systems typically follow the three-point support principle, which ensures stable vehicle structure and load distribution. In most designs, this configuration removes the need for additional suspension systems.

When a single steering drive unit is used, the AGV can already achieve forward and backward movement as well as turning. Compared with differential drive systems, steering response is more direct because the wheel orientation is actively controlled rather than passively generated through speed differences.

When two steering drive units are installed, coordinated control of wheel orientation and speed enables the AGV to perform omnidirectional movement, including forward motion, backward motion, in-place rotation, and lateral translation. This greatly enhances maneuverability in narrow aisles and high-density warehouse environments.

The steering accuracy of such systems is typically determined by the encoder resolution and gear transmission ratio used in the steering mechanism. Through precise encoder feedback and mechanical reduction systems, high-precision steering angle control can be achieved, significantly improving the positioning accuracy of the AGV.

3. Comparison of Core Technical Characteristics

From an engineering perspective, differential drive and steering drive systems exhibit notable differences in several key performance aspects.

In terms of structural size, differential drive systems rely on multiple independent modules and additional mounting structures, which generally leads to larger installation space requirements. Steering drive units, on the other hand, integrate the drive motor, steering mechanism, gearbox, and wheel assembly into a single compact module, resulting in a more compact overall design.

Regarding bidirectional movement capability, differential drive systems often require two drive modules to enable forward and reverse motion efficiently. Steering drive units achieve this simply by reversing the rotation direction of the traction motor, simplifying the control architecture.

For omnidirectional motion, differential drive systems are inherently limited by their steering principle. Because turning is generated by speed differences, the AGV must follow a curved path. Steering drive units can actively change the wheel direction, enabling true omnidirectional movement including lateral motion.

When considering maintenance and reliability, differential drive systems consist of multiple mechanical and electrical modules connected together. The higher number of mechanical interfaces may increase the likelihood of wear or electrical connection issues over time. Steering drive systems reduce the number of components through integrated design, which generally improves system reliability and simplifies maintenance.

In terms of positioning accuracy, differential drive AGVs are affected by cumulative wheel speed errors and mechanical backlash in the transmission system. Steering drive systems utilize encoder feedback for both driving and steering motors, enabling closed-loop control and improved positioning precision.

For traction performance, differential drive systems distribute power across multiple modules, which may introduce transmission losses. Steering drive units use a centralized traction structure, allowing more efficient power transmission and higher load capacity.

Finally, in terms of maximum travel speed, differential drive systems may encounter stability challenges at higher speeds due to the reliance on precise wheel speed control. Steering drive systems maintain stable motion even at higher speeds because steering and traction are controlled independently.

4. Application Status of Drive Systems in Under-Ride AGVs

4.1 Differential Drive as the Traditional Mainstream Solution

From a historical perspective, many early under-ride AGV systems used in China were introduced from Japan, where differential drive has long been the dominant drive configuration for AGVs.

In addition, early AGV applications in the automotive manufacturing industry also relied heavily on differential drive technology. This historical development created a strong technological path dependency within the industry, resulting in widespread adoption of differential drive in under-ride AGVs.

Although steering drive systems are widely used in heavy-duty AGVs developed by companies such as SIASUN, those platforms typically target large-payload industrial vehicles, which differ significantly from the low-profile and lightweight design requirements of under-ride AGVs.

4.2 Limitations of Steering Drive Adoption

Despite their performance advantages, steering drive systems historically faced several obstacles in under-ride AGV applications.

The first limitation is physical size. Traditional steering drive units were designed primarily for heavy-duty AGVs and therefore had relatively large installation heights. Under-ride AGVs, however, typically require extremely low chassis heights, making early steering drive products difficult to integrate.

The second limitation is cost. In the past, high-performance steering drive units were mainly imported products, with prices significantly higher than modular differential drive systems. For light-load AGVs deployed in large quantities, such cost differences greatly affected economic feasibility.

A third factor is industry perception. Due to the long-term dominance of differential drive systems, many AGV manufacturers initially assumed that differential drive was the most suitable solution for under-ride AGVs, which slowed the adoption of steering drive technologies.

5. Emerging Application Trends of Steering Drive Systems

With continuous technological progress in the AGV industry and the rapid development of domestic drive components, steering drive systems are gradually becoming more practical for under-ride AGVs.

One important breakthrough is the development of low-profile steering drive units. Products such as the Plutools PLT120 horizontal steering drive wheel represent a new generation of compact steering drive solutions specifically designed for low-height AGV platforms.

The PLT120 adopts a compact integrated structure optimized for under-ride AGV applications. The module integrates the traction motor, steering mechanism, gearbox, wheel assembly, and encoder system into a single compact unit while maintaining high traction performance and precise motion control.

With this design, a single steering drive unit can already support forward motion, reverse motion, and turning functions for an under-ride AGV. When two units are installed, the AGV can achieve full omnidirectional mobility, including lateral translation and in-place rotation, which significantly improves operational flexibility in dense warehouse environments.

At the same time, many AGV manufacturers have strengthened their independent design capabilities, enabling them to integrate steering drive systems more effectively into low-profile AGV platforms and develop optimized motion control algorithms.

As a result, steering drive technology is gradually overcoming the previous limitations related to size and cost.

6. Technical Summary

Differential drive and steering drive systems represent two different engineering approaches to AGV motion systems: modular drive architecture and integrated mechatronic drive architecture.

Differential drive remains a widely used solution due to its technological maturity and long history of industrial applications. However, its limitations in terms of motion flexibility, positioning accuracy, and system integration make it less suitable for highly dynamic logistics environments.

Steering drive systems offer advantages such as integrated design, omnidirectional mobility, higher positioning accuracy, and lower long-term maintenance requirements. With the emergence of compact products such as the Plutools PLT120 horizontal steering drive wheel, the previous barriers related to installation height and cost are gradually being eliminated.

For AGV developers and system integrators, the choice of drive technology should be based on a comprehensive evaluation of operating environment, payload requirements, space constraints, and economic considerations. Differential drive may remain suitable for simple transportation tasks, while steering drive systems provide clear advantages in applications requiring high maneuverability, dense layouts, and flexible operation.

With continued technological advancements, steering drive solutions are expected to play an increasingly important role in the next generation of logistics automation equipment.