Introduction
Towing AGVs are widely used in intelligent manufacturing and warehouse logistics systems. One of the core challenges in their design lies in whether the traction capability of the drive system matches the total vehicle load, and whether insufficient torque occurs during startup and acceleration.
In practical engineering applications, many customized AGV designs still rely heavily on empirical motor selection methods, using only motor power or rated torque as references. This often leads to the following problems:
Motor overload during startup
Insufficient traction causing failure to tow carts properly
Slow acceleration response
Excessive impact or failure of buffering structures
Therefore, it is necessary to establish a unified modeling and calculation method for drive load, acceleration resistance, and spring buffering systems based on classical mechanics principles.
2. AGV System Load Definition
The foundation of towing AGV power calculation is the total system mass:
M = m_AGV + m_load
Where:
m_AGV: AGV self-weight
m_load: Towed cart or payload mass
M: Total system mass
The total gravitational load of the vehicle is:
W = M × g
Where g = 9.8 m/s².
3. Driving Resistance Analysis (Core Design Basis)
During straight horizontal movement, the primary resistances acting on an AGV consist of rolling resistance and acceleration inertial resistance.
3.1 Rolling Resistance (Primary Steady-State Resistance)
Rolling resistance is generated by deformation between the wheel and ground contact surface:
Ff = f × M × g
Where f is the rolling resistance coefficient, typically ranging from 0.03 to 0.06.
It should be noted that under turning conditions or uneven floor surfaces, this resistance usually increases by 5%–10%. Therefore, sufficient design margin must be reserved in engineering applications to avoid insufficient power during cornering.
3.2 Air Resistance (Negligible at Low Speed)
In indoor AGV systems, operating speed is generally low, so air resistance has minimal influence:
Fw = 0.5 × rho × Cd × A × v²
In most engineering calculations, this term is usually neglected.
3.3 Acceleration Inertial Resistance (Critical During Startup)
During startup or acceleration, the AGV must overcome the inertia of the entire system mass:
Fj = M × a
If the rotational inertia of motors, reducers, and other rotating components is considered, the equation can be expanded as:
Fj = M × a + Σ(Ji × alpha_i / ri)
However, in practical engineering design, the simplified model is commonly adopted:
Fj ≈ M × a
3.4 Total Required Driving Force
Therefore, the total traction force required for AGV operation is:
F_total = M × a + f × M × g
This equation serves as the core basis for drive system selection.
4. Relationship Between Drive Wheel Force and Torque
The drive wheel is the key component responsible for converting motor torque into ground traction force.
4.1 Basic Mechanical Relationship
F = T / r
Where:
F: Ground traction force
T: Drive wheel output torque
r: Drive wheel radius
It can be seen that a smaller wheel radius generates greater traction force under the same torque condition, which is an important optimization direction in lightweight AGV design.
4.2 Multi-Drive Wheel Systems
For a system with n drive wheels:
T_total = n × T_wheel
Considering transmission efficiency and gearbox reduction:
T_wheel = (T_motor × i × eta) / n
Where:
i: Gear reduction ratio
eta: Transmission efficiency (typically 0.9–0.95)
5. Acceleration and Motor Torque Verification
AGV design should not only determine whether the vehicle can move, but also verify whether it can achieve the target acceleration performance.
5.1 Acceleration Formula
Substituting the driving force into Newton's Second Law:
a = (F_total - f × M × g) / M
Further expanded:
a = (n × T / r - f × M × g) / M
This formula is used to evaluate the actual acceleration capability of the system.
5.2 Starting Torque Requirement
The startup phase is the most critical because the system must simultaneously overcome static friction and inertia:
T_start = ((M × a + f × M × g) × r) / n
5.3 Steady-State Running Torque
Under constant-speed operation, only rolling resistance needs to be overcome:
T_steady = (f × M × g × r) / n
6. Engineering Example Analysis (Corrected Calculation)
Taking a typical AGV operating condition as an example:
M = 100 kg
r = 0.015 m
f = 0.05
n = 2
Target acceleration:
a = 0.5 m/s²
6.1 Rolling Resistance
Ff = 100 × 9.8 × 0.05 = 49 N
6.2 Total Driving Force
F_total = M × a + f × M × g
F_total = 100 × 0.5 + 49 = 99 N
6.3 Single Wheel Torque Requirement
T = (F_total × r) / n
T = (99 × 0.015) / 2 = 0.7425 N·m
Engineering Conclusion
The above result demonstrates that:
AGV power system design must be based on actual acceleration targets
Motor selection cannot rely solely on motor power or rated torque
Otherwise, situations may occur where the design appears theoretically feasible but fails under real operating conditions
7. Buffer Spring Selection Design
During docking or anti-collision processes, towing AGVs require spring systems to absorb impact energy.
7.1 Basic Model (Hooke's Law)
F = k × x
Where:
k: Spring stiffness
x: Compression displacement
7.2 Multi-Spring Load Distribution Design
If the system uses n_s springs:
Single spring load:
F_spring = (M × g) / n_s
Spring stiffness design:
k = F_spring / x
7.3 Design Principles
The core principle of spring system design is not "the stiffer, the better," but rather:
The load must be evenly distributed
Sufficient compression stroke must be guaranteed
Rigid impact transmission to the vehicle structure must be avoided
8. Core Design Summary
The design of towing AGV power systems must follow the principles below:
Drive system design is essentially a balance between traction force, resistance, and inertia
Wheel radius r, motor torque, and reduction ratio jointly determine system performance
Acceleration must be included as a design target rather than merely a verification result
Power cannot replace torque analysis
Buffer systems must be structurally designed based on total load distribution
