Preface

The performance of an Automated Guided Vehicle (AGV) or an Autonomous Mobile Robot (AMR) is largely determined by the design of its drive system. An integrated drive system consists of four key components: the AGV drive wheel, low voltage servo motor, gear reducer, and servo drive. Together, they determine vehicle traction, acceleration, climbing capability, positioning accuracy, motion stability, and long-term operational reliability.

In practical engineering, many drive system issues are not caused by insufficient motor power, but by incomplete vehicle dynamics calculations. Selecting a motor solely based on payload while overlooking rolling resistance, slope resistance, acceleration inertia, and wheel-ground adhesion often leads to poor starting performance, motor overload, wheel slippage, and reduced operating efficiency.

Plutools is one of China's leading manufacturers of mobile robot drive systems, specializing in the development and production of AGV drive wheels, steering drive wheels, differential drive wheels, low voltage servo motors, servo drives, and integrated drive solutions for AGVs and AMRs. Drawing on extensive engineering experience and the principles presented in the Mechanical Design Manual, this guide explains the complete dynamic calculation process for AGV drive system selection, providing practical references for engineers designing high-performance mobile robot drive systems.

1. AGV Vehicle Driving Resistance Analysis

For industrial AGVs operating at speeds below 1 m/s, aerodynamic drag can generally be ignored. The drive system primarily needs to overcome three types of resistance:

Rolling resistance

Slope resistance

Acceleration resistance

The driving force must satisfy:

Fdrive ≥ Ff + Fθ + Fa

Where:

Only when the available traction exceeds the total driving resistance can an AGV start smoothly, maintain stable operation, and climb slopes safely.

2. Rolling Resistance Calculation

Rolling resistance is generated by the elastic deformation between the wheel and the floor surface, making it the most fundamental resistance encountered during AGV operation.

Formula

Ff = (f × G) / r

Where:

Typical Engineering Values

Engineering Tip

Some engineering references express the rolling resistance coefficient in centimeters (cm) instead of meters (m). Always convert the unit before calculation to avoid errors that may exceed 100 times.

3. Slope Resistance Calculation

Industrial AGVs are typically designed with a climbing capability of approximately 2%.

For small slopes:

sinθ ≈ tanθ ≈ slope ratio

Therefore:

Fθ = 0.02 × G

Example

For a 500 kg AGV:

Vehicle weight:

G = 500 × 9.81 = 4905 N

Slope resistance:

Fθ = 98.1 N

For steeper slopes, the actual trigonometric functions should be used for more accurate calculations.

4. Acceleration Resistance Calculation

Frequent acceleration and deceleration generate inertial loads that must be considered during drive system design.

According to Newton's Second Law:

Fa = M × a

Where:

Recommended Values

Lower acceleration reduces peak loads and improves the long-term reliability of the drive system.

5. Vehicle Resistance Calculation Example

Design Parameters

Calculation Results

Design Recommendation

The drive system should provide a minimum traction force of approximately 499 N. In engineering practice, a 20–50% safety margin is recommended to compensate for startup impact, uneven floor conditions, and long-term mechanical wear.

6. Drive Wheel Output Torque Calculation

The output torque of an AGV drive wheel is obtained after the rated motor torque is amplified by the gearbox.

Formula

Twheel = Tmotor × i × η

Where:

Typical Gearbox Efficiency

Different gearbox types have significantly different transmission efficiencies. Using an incorrect efficiency value may result in inaccurate torque calculations and improper motor selection.

7. Traction Force Calculation of the Drive Wheel

The torque output from the drive wheel can be converted into traction force using the following relationship:

F = T / r

Where:

F = traction force (N)

T = drive wheel output torque (Nm)

r = wheel radius (m)

For dual-drive AGV systems:

Ftotal = 2 × F

For multi-drive configurations, the total traction force is the sum of all driving wheels.

Example

Given:

Motor rated torque: 0.4 Nm

Gear ratio: 30

Gearbox efficiency: 0.85

Wheel radius: 65 mm (0.065 m)

Then:

Output torque:

Twheel = 0.4 × 30 × 0.85 = 10.2 Nm

Single wheel traction force:

F = 10.2 / 0.065 ≈ 157 N

This parameter directly determines whether the AGV can overcome the total system resistance.

8. Maximum Operating Speed Calculation

The theoretical maximum speed of an AGV is determined by motor speed and gear ratio:

V = (2 × π × r × n) / i

Where:

V = linear speed (m/min)

r = wheel radius (m)

n = motor speed (rpm)

i = gear ratio

Example

Motor speed: 2500 rpm

Gear ratio: 30

Wheel radius: 65 mm

V ≈ 34 m/min (≈ 0.57 m/s)

If the required speed is not achieved, the motor speed can be increased or the gear ratio reduced. However, torque and traction force must be re-verified accordingly.

9. Motor Power Verification

After torque calculation, motor power must also be verified.

Formula

P = (T × n) / 9550

Where:

P = power (kW)

T = torque (Nm)

n = speed (rpm)

Example

Torque: 0.4 Nm

Speed: 2500 rpm

P ≈ 0.105 kW

Engineering Recommendation

Torque safety factor: 1.2–1.5×

Power safety margin: 20–50%

This ensures reliable operation under continuous and peak-load conditions.

10. Wheel Adhesion and Preload Force Calculation

To avoid wheel slip, the following condition must be satisfied:

μ × FN ≥ F

Thus:

FN ≥ F / μ

Where:

μ = friction coefficient between wheel and floor

FN = normal preload force

Example

Single wheel traction force: 157 N

Friction coefficient: 0.54

Then:

FN ≈ 291 N

In practical design, a 10% safety margin is recommended, so a spring preload of approximately 320 N is selected to compensate for fatigue over time.

Typical Friction Coefficients

11. AGV Drive System Selection Procedure

A complete AGV drive system design typically follows these steps:

Define vehicle parameters: weight, speed, acceleration, slope, wheel diameter

Calculate rolling, slope, and acceleration resistance

Obtain total driving resistance

Determine single wheel traction force

Calculate required output torque

Select gearbox ratio

Match low voltage servo motor

Verify servo drive capacity

Check speed performance

Validate wheel adhesion and preload force

12. Engineering Design Considerations

In AGV system development, drive performance is influenced by multiple factors rather than motor power alone.

Key considerations include:

Rolling resistance coefficient and friction coefficient must not be confused, as they represent different physical meanings.

Gearbox efficiency varies significantly between planetary and worm gear systems and must not be treated as a constant value.

Motor and servo drive selection must consider continuous duty cycles, frequent start-stop conditions, and peak load impacts.

Multi-wheel systems must ensure sufficient ground contact force to prevent wheel slip and positioning deviation.

Conclusion

The design of an AGV drive system is not simply motor selection, but a comprehensive engineering process involving vehicle dynamics, mechanical transmission, electrical control, and system integration.

From resistance modeling and traction force calculation to servo motor selection, gearbox configuration, servo drive matching, and drive wheel design, every parameter directly impacts system performance and operational reliability.