Slipping in AGVs and AMR mobile robots is commonly related to the friction conditions between the drive wheels and the floor, the torque output of the drive system, and the vehicle's load distribution. Common causes include worn drive wheel treads, oil or water on the floor, incorrect track tension, unstable motor control, and brake failures.
It is important to distinguish wheel slip during operation from vehicle rollaway after stopping. Wheel slip mainly involves insufficient traction between the wheels and the floor, while rollaway is generally related to inadequate braking or parking holding force. Identifying the cause requires checking the robot's operating condition and inspecting the relevant mechanical components.
1. Key Components in the Traction Drive System
1. Drive Wheels and Track Assemblies
Drive wheels are the components that make direct contact with the floor. Their tread material, wear condition, and surface condition all affect the traction available to the vehicle.
Industrial mobile robots commonly use polyurethane or rubber drive wheels. After extended use, the tread may become worn, hardened, cracked, or unevenly flattened. When the friction conditions between the tread and floor deteriorate, the wheels may no longer provide sufficient traction. This is more likely to occur during starting, acceleration, turning, and uphill travel.
Oil, water, cleaning fluid residue, and small debris on the floor can also affect wheel traction. Even when the drive wheels show no obvious wear, oil or water on a smooth epoxy floor may cause insufficient traction and wheel slip.
For tracked mobile robots, the condition of the track lugs, track links, and drive sprockets should also be checked. Worn track patterns reduce grip on loose or uneven surfaces. Wear at the engagement points between the track and drive sprocket may cause tooth skipping or unstable movement.
2. Track Tensioning Components
In tracked mobile robots, the tensioning assembly maintains the required track tension. Common components include tension springs, hydraulic tensioning cylinders, idler wheels, and associated connecting mechanisms.
If a tension spring loses its elasticity or a hydraulic tensioning mechanism develops a leak, the track may become loose. This can affect engagement between the track and drive sprocket. In more severe cases, the track may skip teeth, derail, or run unevenly on the two sides of the vehicle.
Excessive track tension also needs to be avoided. Over-tensioning increases operating resistance and places additional loads on the tracks, rollers, bearings, and drive components, accelerating wear.
Track tension should therefore be adjusted according to the equipment manufacturer's specifications. Repeated track loosening should be investigated rather than addressed simply by increasing tension. The tracks, idler wheels, rollers, and drive sprockets should also be inspected regularly, with debris removed from the moving components.
3. Travel Gearboxes
The travel gearbox transfers power from the motor to the drive wheel. Its gear engagement condition, transmission efficiency, and internal lubrication all affect drive system performance.
After prolonged operation, gearbox gears may develop pitting, wear, tooth surface damage, or excessive backlash. Severe wear can reduce transmission efficiency and cause unstable torque output, abnormal noise, or vibration. In some cases, the gearbox may no longer transmit power properly.
However, gear wear does not necessarily cause wheel slip directly. Serious transmission faults are more likely to result in abnormal power transmission or unstable operation. Actual wheel slip must be assessed by considering both the torque delivered to the drive wheels and the friction available between the wheels and the floor.
For non-standard AGV drive systems, the dimensions and mating relationships of gearbox mounting bases, connecting shafts, bearing housings, and other metal transmission components also affect assembly accuracy. We can manufacture these metal parts through CNC precision machining according to customer-supplied engineering drawings.
4. Travel Motors and Motor Bearings
Travel motors provide the power required to rotate the drive wheels. Servo motors and other motor types suitable for mobile robot drive systems are commonly used.
If motor bearings become worn, develop excessive clearance, or suffer from inadequate lubrication, the motor may experience abnormal noise, vibration, temperature rise, and increased rotational resistance. Severe bearing damage can affect normal motor operation and may trigger drive protection.
However, bearing wear is generally not a direct cause of wheel slip. When the left and right drive wheels operate at different speeds, the motor controller, encoder feedback, and power output on both sides should also be inspected. Wheel slip alone is not sufficient evidence that a motor bearing has failed.
2. Braking Components
1. Electromagnetic Brakes
Some AGVs and AMRs use electromagnetic brakes installed inside or outside the servo motor. These brakes are primarily used to hold the vehicle after it stops.
A common design applies braking force through springs and releases the brake electromagnetically. When power is disconnected, the springs apply the brake to generate holding torque. When the electromagnetic mechanism is energized, the brake is released and the motor can rotate normally.
If the friction surfaces become worn, the brake clearance is incorrect, the springs fail, or the electromagnetic release mechanism malfunctions, the brake may lose its holding capacity. This can cause the robot to roll on a slope while parked.
It is important to note that some electromagnetic brakes built into servo motors are designed for static holding rather than frequent dynamic braking during operation. Repeated use for dynamic braking may cause premature wear or failure. During maintenance, the friction surfaces, holding torque, and release condition should be checked according to the specific brake design and manufacturer's specifications. A fixed wear limit should not be applied to all brake models.
2. Hydraulic Braking Components
Some heavy-duty mobile equipment uses hydraulic braking systems. These systems require inspection of the brake lines, seals, and braking actuators.
If a hydraulic line leaks or a seal becomes damaged, the system may lose pressure. The braking actuator may then be unable to generate sufficient braking force, affecting the vehicle's stopping and parking performance on slopes.
This type of failure is mainly associated with reduced braking performance or vehicle rollaway after stopping, rather than ordinary wheel slip during travel.
Hydraulic system maintenance involves risks from residual pressure and high-pressure fluid injection. It should be carried out by personnel with the appropriate technical qualifications and according to the equipment manufacturer's maintenance procedures. Electrical work should also be performed by personnel holding the required qualifications.
3. Wheel-to-Floor Contact and Environmental Components
1. Drive Wheel Treads and Anti-Slip Structures
Some mobile robots use special tread designs, replaceable wheel treads, or other anti-slip structures according to their operating environment. These components must remain securely connected to the drive wheel and maintain consistent contact with the floor.
If the tread bonding fails, mounting bolts become loose, or the tread develops local separation or uneven wear, the drive wheel may become eccentric or lose consistent floor contact. This can reduce traction and affect vehicle movement.
For polyurethane-coated drive wheels, the coating should be checked for cracking, delamination, and localized wear. These defects can cause bonding failure between the polyurethane layer and the metal wheel core.
2. Bearing Seals and Dust Protection Components
Drive wheels are generally equipped with dust covers, sealing rings, and bearing protection structures. These components help prevent dust, metal chips, and other debris from entering the transmission and bearing areas.
If a dust cover is damaged or a seal fails, contaminants may enter the hub and bearing area. This can cause bearing wear, increased rotational resistance, or localized jamming.
However, a blocked dust cover does not normally reduce the friction coefficient between the wheel tread and the floor directly. Reduced wheel-to-floor friction is more commonly associated with oil, dust, or other contaminants on the tread itself, or debris trapped between the wheel and floor.
During inspection, the tread condition, bearing rotation, and accumulation of foreign objects around the wheels should therefore be checked separately.
4. Load Detection and Control Feedback Components
1. Load Sensors
Some AGVs and AMRs are equipped with load detection devices to monitor cargo weight and determine whether the vehicle has exceeded its rated payload.
If a load sensor fails or provides inaccurate signals, the control system may not correctly identify an overload condition. The robot may continue operating with a payload that exceeds its design limits.
Overloading increases the running resistance and traction demand on the drive system. If the cargo is distributed unevenly, the vertical load carried by the drive wheels may also become insufficient. This can reduce the maximum traction force available between the wheels and the floor, increasing the risk of wheel slip.
The total vehicle weight alone is therefore not enough to determine whether the drive wheels have sufficient traction. Cargo position, center of gravity, drive wheel location, and actual load distribution must also be considered. Research has also shown that changes in AGV payload and load distribution can affect wheel operating conditions and positioning errors.
2. Motor Drives and Encoders
Although motor drives and encoders do not make direct contact with the floor, they affect the speed and torque control of the left and right drive wheels.
Abnormal encoder feedback, incorrect drive parameters, or significant differences between the speed control of the two motors may cause uneven traction during starting, turning, or changes in vehicle load.
For example, if the acceleration command is too high, the torque delivered by the drive motor may exceed the traction available under the current wheel-to-floor contact conditions. The drive wheel may then spin without producing the expected vehicle movement.
For AGVs using a two-wheel differential drive system, inaccurate speed feedback from either wheel may also cause the vehicle to deviate from its intended path or turn incorrectly.
When investigating wheel slip, inspectors should therefore check the wheel treads and transmission components as well as the motor drive status, encoder feedback, and relevant control parameters.
5. Inspection Points for AGV and AMR Wheel Slip
When a mobile robot experiences wheel slip, the failure should not immediately be attributed to a single damaged component. The symptoms can first be used to narrow down the possible causes:
The drive wheels continue rotating, but the robot moves slowly: Check tread wear, oil, water, floor friction conditions, and the actual load carried by the drive wheels.
The wheels slip suddenly during starting or acceleration: Check motor torque output, acceleration settings, speed coordination between the left and right wheels, and wheel-to-floor contact conditions.
The robot moves after stopping on a slope: Focus on the electromagnetic brake, braking actuator, and the vehicle's overall parking holding capacity.
A tracked robot skips teeth or moves unevenly: Inspect track tension, drive sprockets, idler wheels, and track wear.
The robot continuously deviates from its intended path during straight-line travel: Check the actual diameters of the left and right drive wheels, encoder feedback, load distribution, and control parameters.
When wheel slip and positioning errors occur together, it is also important to consider the effect of wheel slip on encoder-based odometry. Position calculations based only on wheel rotation data cannot always accurately represent the actual distance traveled by the robot.
Wheel slip in mobile robots generally results from the combined effects of mechanical components, control parameters, and floor conditions. Replacing the drive wheels or motors alone will not resolve every type of slip-related problem.
Hansheng Automation provides CNC precision machining of hubs, connecting shafts, bearing housings, mounting brackets, and other metal structural components for AGV and AMR equipment according to customer-supplied engineering drawings. Our drawing-based machining services also cover precision components for yacht equipment, tobacco packaging machinery parts, RC racing car structural components, and filling machinery parts. We manufacture these components according to the required dimensions and assembly specifications of each application.
