As a core component of flexible material handling, differential drive AGVs are widely used in various logistics scenarios due to their compact structure, mature control, and high flexibility. A deep understanding of their technical details is crucial for proper selection and design.
1. Drive Method and Wheel System Structure
The core principle of differential drive is achieving steering by independently controlling the speed difference between two fixed drive wheels. Based on the number of drive wheels and their functional integration, they are mainly divided into three types:
Dual-Wheel Differential Drive
Wheel System Composition: 2 independently driven drive wheels (often with damping or swing structures) + 2 or more passive caster wheels.
Motion Characteristics: Possesses the most complete mobility, capable of forward, reverse, arbitrary curved paths, and zero-radius in-place rotation, offering extremely high flexibility.
Load Adaptation: When drive wheels have spring damping, sufficient counterweight is needed to prevent slipping. If a swing balance beam design is used for the drive wheels, adaptability to load changes is stronger without needing extra weight.
Unidirectional Differential Steering Drive
Wheel System Composition: 1 integrated differential steering wheel (combining drive and steering, with damping) + 1 fixed directional wheel + 1 caster wheel.
Motion Characteristics: The motion mode is similar to a car, supporting only forward movement and turning while moving forward, cannot reverse. Suitable for fixed-path, unidirectional logistics loops.
Bidirectional Differential Steering Drive
Wheel System Composition: 1 reversible differential steering wheel (with damping) + 2 caster wheels.
Motion Characteristics: Expands upon the unidirectional steering wheel's functionality, enabling forward, reverse, and lateral translation, enhancing maneuverability in confined spaces.
2. Key Parameter Calculations: Traction Force and Turning Radius
Stable AGV operation relies on sufficient traction force and appropriate turning capability. Here are the core calculation methods.
Traction Force Calculation
Ensuring the drive system can overcome the total resistance during operation is critical. The total traction force required (F_traction) must satisfy:
F_traction ≥ F_resistance = F_rolling + F_slope + F_acceleration
Rolling Resistance (F_rolling): F_rolling = μ_rolling × m × g
μ_rolling: Rolling resistance coefficient (0.01-0.02 for high-quality floors)
m: Total mass (AGV tare weight + rated load) in kg
g: Gravitational acceleration (9.8 m/s²)
Gradient Resistance (F_slope): F_slope = m × g × sin(θ)
θ: Maximum incline angle of the path
Acceleration Resistance (F_acceleration): F_acceleration = m × a
a: Maximum acceleration/deceleration of the AGV in m/s²
Motor Torque Verification: Based on the total traction force, verify if a single motor's torque is sufficient.
Single Motor Torque T ≥ (F_traction × R_wheel) / (2 × η)
* R_wheel: Drive wheel radius in meters
* η: Transmission efficiency (typically 0.8~0.9)
Turning Radius Calculation
For Dual-Wheel Differential AGVs: Their kinematic model allows for in-place rotation, thus the theoretical minimum turning radius is 0. In practical applications, a reasonable turning path is planned considering stability and efficiency.
For Differential Steering Drive AGVs: Their turning radius is determined by the wheelbase and the maximum steering angle, calculated as:
Minimum Turning Radius R_min = L / tan(α_max)
L: Wheelbase between the steering wheel center and the follower axle
α_max: Maximum steering angle of the steering wheel
It follows that shortening the wheelbase and increasing the steering angle effectively improve turning flexibility.
3. Core Component Selection Considerations
Drive Motor: Must meet both the rated torque (ensuring continuous running traction) and peak torque (meeting startup, acceleration, and gradeability demands) requirements. The torque value calculated from the aforementioned traction force is the direct basis for motor selection.
Spring Damping System: Its primary role is to maintain continuous contact between the drive wheel and the ground to provide stable traction. The spring's preload and stiffness coefficient need precise calculation and selection based on the AGV's tare weight, rated load, and floor flatness, ensuring the drive wheel does not slip due to lifting off the ground under varying loads.
4. Application Scenario Summary
Differential drive systems cover a spectrum from high flexibility to cost-effective applications.
Dual-Wheel Differential AGVs, due to their superior flexibility, are the preferred choice for automobile welding shops, flexible component assembly lines, and "goods-to-person" picking warehouses, especially suitable for high-frequency, small-batch transport tasks in space-constrained or complex-path scenarios.
Differential Steering Drive AGVs are more often used for unidirectional or bidirectional material transport where paths are relatively fixed but still require some maneuverability, excelling in scenarios like line-side material supply in general assembly workshops.
Conclusion: Selecting a differential drive AGV is a systematic process starting from scenario requirements (flexibility), verifying power through traction force calculations, and then validating feasibility through turning radius and spatial analysis. Precise calculation and reasonable matching are the foundation for ensuring the efficient and stable operation of the AGV system.
