An AGV differential drive system consists of two independently controlled drive wheels and one or more passive support wheels. By adjusting the speed and rotation direction of the left and right drive wheels, the vehicle can change its direction without a separate mechanical steering mechanism. This drive arrangement has a relatively simple structure and supports straight-line travel, curved turning, and in-place rotation. It is commonly used in warehouse logistics and industrial material handling equipment.
A typical AGV differential drive system includes drive wheel assemblies, drive motors, gearboxes, caster wheels, encoders, and a motor control system. The wheel arrangement and power configuration vary depending on the vehicle design. The actual configuration should be selected according to the payload, operating speed, and working environment.
1. Main components of an AGV differential drive system
1. Drive wheel assemblies
A differential drive system typically uses two drive wheels, installed on the left and right sides of the AGV chassis. Each wheel is driven by an independent motor. The two drive wheels are generally positioned along the same drive axle, and their speeds and rotation directions can be controlled separately.
A drive wheel assembly generally consists of a wheel body, hub or metal core, connecting shaft, and associated support components. The wheel tread commonly uses polyurethane or rubber to provide the required friction, wear resistance, and load capacity. The hub and internal metal components connect the wheel to the power transmission system and withstand the loads generated during vehicle operation.
The wheel diameter, width, tread material, and internal connection structure should be selected according to the vehicle weight, operating speed, and floor conditions. For non-standard AGV equipment, the hub, connecting shaft, and mounting components can also be custom manufactured according to the vehicle's engineering drawings.
2. Drive motors and gearboxes
The two drive wheels are powered by separate motors. Servo motors are commonly used, although other motor types may be selected according to the application. The motors provide the driving power, while the gearboxes reduce rotational speed and increase output torque to meet the requirements for starting, acceleration, and operation under load.
Planetary gearboxes are commonly used, although other gearbox designs may be suitable depending on the transmission requirements. The gear ratio should be selected based on the motor's rated speed, drive wheel diameter, maximum vehicle speed, and required output torque.
Some AGV drive wheel assemblies integrate the motor, gearbox, and wheel into a single unit to reduce installation space. The actual arrangement depends on the chassis layout and power system design.
3. Passive support wheels
Passive wheels do not directly generate driving force. Their main functions are to support the AGV chassis, distribute the vehicle load, and maintain operating stability. Common designs include swivel casters and other support rollers, which are installed at positions outside the main drive axle.
The number and mounting positions of passive wheels should be determined according to the vehicle's center of gravity, drive wheel locations, and load requirements. Swivel casters can change their orientation according to the direction of vehicle movement, allowing them to follow the turning motion generated by the differential drive system.
Poorly positioned support wheels can cause uneven load distribution and affect the contact pressure between the drive wheels and the floor. The design should therefore account for the vehicle's center of gravity and the actual load carried by each wheel.
4. Encoders and control system
A differential drive system requires independent speed control of the left and right drive motors. Motor drives receive control commands and adjust the operating states of both motors according to encoder feedback.
Encoders are generally installed on the motors or associated transmission shafts. They detect rotational position and speed. The controller uses these signals to calculate the actual speed of each drive wheel and compares the feedback values with the target speeds. Closed-loop control is then used to adjust motor output.
PID control is one commonly used method for closed-loop speed regulation. The specific control algorithm depends on the controller and the vehicle's operating requirements. The accuracy of speed coordination between the two drive wheels affects straight-line tracking, turning performance, and path-following accuracy.
5. Braking devices
Some AGV differential drive assemblies are also equipped with electromagnetic brakes, usually installed at the drive motor or an appropriate position in the transmission system.
The main purpose of the brake is to hold the vehicle after stopping and prevent unintended movement. For AGVs that need to stop on slopes or perform emergency braking, the braking force, response time, and holding capacity must be checked against the complete vehicle braking requirements. The brake should not be selected based on motor power alone.
2. Main parameters and selection criteria for AGV differential drive wheels
The parameters of an AGV differential drive system depend on the total vehicle weight, operating speed, drive configuration, and working environment. The following values can be used as preliminary selection references, but they are not universal specifications for all AGVs.
1. Drive motor power
The 400W to 750W servo motor range mentioned in the original reference can be used as a selection guide for some applications. However, it should not be treated as a standard configuration for all medium-sized AGVs.
Actual motor selection requires consideration of the AGV's total mass, target travel speed, acceleration time, running resistance, and gradient requirements. Insufficient motor power may reduce acceleration performance under full load or prevent the vehicle from meeting its operating requirements on slopes.
The output capacity of both drive motors must also match the gearboxes and drive wheels. Rated motor power alone is not enough to determine whether the complete drive system meets the application requirements.
2. Gearbox and gear ratio
The gear ratio affects the output speed and torque at the drive wheels. The 10:1 to 50:1 range can be used as a preliminary reference, but the final selection should be based on motor speed, drive wheel diameter, vehicle speed, and required load torque.
When selecting a gearbox, engineers should check the rated output torque, peak torque, transmission efficiency, and permissible output speed in addition to the gear ratio. For AGVs that frequently start, stop, and reverse, shock loads and gearbox service life should also be considered.
3. Drive wheel diameter and tread material
The drive wheel diameter should be selected according to the chassis height, floor conditions, operating speed, and load requirements. A diameter range of 200 mm to 300 mm can serve as a reference for some applications, although AGV drive wheels are also available in smaller and larger sizes.
Common tread materials include polyurethane and rubber. Polyurethane offers good wear resistance and load capacity, making it suitable for many indoor material handling applications. Rubber should be selected according to the required elasticity, floor adaptability, and friction characteristics.
The 75A to 85A Shore hardness range mentioned in the original reference is also a selection guideline rather than a fixed requirement. Different polyurethane formulations and tread designs use different hardness values. The actual selection should account for floor material, tread wear, traction, and operating noise.
4. Drive wheel load capacity
The load capacity of each drive wheel should be calculated according to the total vehicle mass, number of drive wheels, caster arrangement, and load distribution.
The original reference specifies a dynamic load capacity of at least 500 kg per wheel. This may be a design requirement for a particular application, but it should not be applied to all medium-sized AGVs. The actual loads on both drive wheels and passive wheels must be checked, with allowance for dynamic loads caused by starting, stopping, turning, and uneven floor conditions.
A sufficient rated load capacity does not necessarily mean that the drive wheel provides adequate traction. The friction conditions between the wheel and floor, together with the vertical load carried by the drive wheel, also affect the available driving force.
5. Minimum turning radius
A differential drive system can theoretically achieve a zero turning radius in its kinematic model. When the left and right drive wheels rotate in opposite directions at equal speeds, the AGV can rotate in place around the midpoint of its drive axle.
However, a zero turning radius refers to the motion radius of the midpoint of the drive axle. It does not mean that the vehicle requires no space to rotate. The chassis dimensions, caster movement, and clearance from surrounding equipment must still be considered when planning the operating space.
3. Working principle of an AGV differential drive system
The operating principle of an AGV differential drive system is based on independent control of the left and right drive wheel speeds and rotation directions. The control system calculates the target speed for each wheel according to the vehicle's motion commands and then controls both motors through their respective motor drives.
1. Straight-line travel
When the AGV needs to travel in a straight line, the control system sends matching speed commands to the left and right drive motors. Both drive wheels rotate in the same direction at approximately equal speeds.
The traction forces generated by the two drive wheels move the vehicle forward or backward. A significant difference between the actual wheel speeds may cause the AGV to gradually deviate from its intended path.
The encoders therefore monitor the operating states of both motors continuously. The controller adjusts motor output according to the difference between the target and actual speeds.
2. Curved turning
When the AGV needs to change direction, the control system adjusts the speeds of the left and right drive wheels to create a speed difference.
For example, when the AGV turns left, the right drive wheel generally moves faster than the left drive wheel. The difference between the linear speeds of the two wheels determines the vehicle's angular velocity and affects its turning radius.
A greater speed difference generally produces a higher turning angular velocity. At the same vehicle centerline speed, this results in a smaller turning radius.
3. In-place rotation
When the AGV needs to change its heading in a confined area, the left and right drive wheels can rotate in opposite directions to produce an in-place turn.
For example, the left drive wheel rotates backward while the right drive wheel rotates forward at the same linear speed. The vehicle then rotates around the midpoint of its drive axle.
This movement does not require a separate mechanical steering mechanism and is useful for indoor material handling equipment that operates in confined spaces. However, floor friction, caster design, and vehicle load distribution can all affect actual performance during in-place rotation.
4. Differential drive calculations
In an ideal differential drive kinematic model, the linear and angular velocities of the AGV are determined by the linear speeds of its left and right drive wheels.
V=\frac{V_R+V_L}{2}
\omega=\frac{V_R-V_L}{L}
Where:
(V) is the linear velocity of the AGV center, in m/s.
(V_R) is the linear velocity of the right drive wheel, in m/s.
(V_L) is the linear velocity of the left drive wheel, in m/s.
(L) is the track width between the left and right drive wheels, in m.
(\omega) is the angular velocity of the AGV around its vertical axis, in rad/s.
Based on these relationships, the control system can calculate the required drive wheel speeds from the target linear and angular velocities:
V_R=V+\frac{\omega L}{2}
V_L=V-\frac{\omega L}{2}
To calculate the angular velocity difference between the two drive wheels, the drive wheel radius must also be considered:
\Delta\omega_w=\frac{VL}{Rr}
Where (R) is the distance from the midpoint of the drive axle to the instantaneous center of rotation, (r) is the drive wheel radius, and (\Delta\omega_w) is the angular velocity difference between the two wheels, in rad/s.
When the speed difference is expressed in RPM, an additional unit conversion is required:
\Delta n=\frac{60}{2\pi}\times\frac{VL}{Rr}
The original speed difference formula can be used directly when the calculated result is expressed in rad/s. It should not be labeled as RPM without the corresponding unit conversion.
In actual operation, the controller continuously adjusts the speeds of the left and right drive wheels using encoder feedback and closed-loop control algorithms. The equations above describe ideal kinematic relationships. Actual vehicle movement can also be affected by wheel slip, differences in wheel diameter, floor friction, and mechanical backlash.
4. Advantages and limitations of AGV differential drive systems
Advantages
A differential drive system has a relatively simple structure. The left and right drive wheels can be controlled independently without requiring an additional mechanical steering mechanism, which helps reduce chassis complexity.
Its control principle is well established. Adjusting the speeds of the two drive wheels allows the AGV to travel in a straight line, follow curved paths, and rotate in place. This arrangement is suitable for AGVs that need to change direction within limited operating spaces.
Limitations
Differential drive requires accurate coordination between the two drive wheels. Differences in motor control parameters, wheel diameters, or actual operating conditions may cause the vehicle to deviate from its intended path during straight-line travel.
Uneven tread wear after extended operation can also change the actual linear speeds of the two drive wheels, affecting positioning and motion accuracy.
Differential drive relies on friction between the wheels and the floor for traction and turning. On slippery floors or surfaces with significant irregularities, the wheels may slip. This can increase odometry errors and cause the vehicle to deviate from its intended trajectory.
A standard two-wheel differential drive system cannot move directly sideways. If an application requires lateral movement, a different wheel arrangement or motion control configuration is necessary.
5. Applications and selection considerations for AGV differential drive wheels
AGV differential drive systems are commonly used in indoor warehousing, production line material delivery, and automated material handling. They are particularly suitable for applications with relatively flat floors and clearly defined operating routes.
Several factors should be checked during the selection process.
First, calculate the required output torque and power for both drive motors based on the vehicle's own weight, maximum payload, operating speed, and acceleration requirements. The gear ratio should then be selected accordingly.
Next, select the wheel diameter and tread material according to the floor material, vertical load on the drive wheels, and traction requirements. For AGVs that need to operate on slopes, calculate the gradient resistance, rolling resistance, and acceleration resistance. The available traction must be sufficient, and the motors, gearboxes, and brakes must all meet the operating requirements.
When an AGV needs to travel on slopes of 5° to 10°, the actual gradient, fully loaded vehicle mass, floor friction conditions, and starting and stopping requirements must be included in the calculations. This gradient range should not be treated as a universal selection standard for all differential drive AGVs.
For outdoor applications or floors with significant irregularities, differential drive should not automatically be considered unsuitable. The wheel diameter, tread material, chassis ground clearance, suspension arrangement, and drive wheel contact conditions should be evaluated according to the actual environment.
Reliable operation of an AGV differential drive system depends on the proper matching of the drive wheels, transmission components, and control system. For non-standard equipment, the hub, shaft components, motor mounting bases, and other metal structural parts may also need to be designed and manufactured according to the actual assembly dimensions.
In addition to components for AGVs and warehouse logistics equipment, Hansheng Automation provides drawing-based custom machining services for precision parts used in various industries. These include yacht components, RC racing car parts, tobacco packaging machinery components, and filling machinery parts. We can arrange CNC precision machining and metal component manufacturing according to customer-supplied engineering drawings.
