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Fictionlab > Glossary > Payload capacity

Payload capacity

Payload capacity – definition

Payload capacity is the maximum additional mass that a mobile robot can carry while remaining within its specified mechanical, electrical, stability, traction, and thermal operating limits. For an unmanned ground vehicle (UGV), payload includes every item added to the base platform: sensors, compute hardware, batteries, brackets, protective enclosures, cables, antennas, and research instruments.

Payload capacity is not only a mass value expressed in kilograms. A payload that is below the nominal mass limit can still make a rover unsafe or unreliable if it shifts the centre of gravity upward or outward, overloads a power rail, reduces wheel traction, obstructs sensors, or exceeds the available computing and thermal budget.

In mobile robotics, payload assessment must therefore consider the complete configured system rather than the mass of a single sensor. This is particularly important for four-wheel differential-drive platforms operating on uneven terrain, where acceleration, braking, slopes, wheel slip, and obstacles create dynamic loads.

Payload capacity in UGV system design

A payload specification published by a platform manufacturer is usually based on defined operating conditions. These conditions may include surface type, speed, battery configuration, centre-of-gravity position, and environmental constraints. The stated value should not be treated as a universal limit for every terrain or mission profile.

For practical integration, the payload mass can be expressed as:

m_payload = m_sensors + m_compute + m_mounts + m_cables + m_power + m_enclosure

The integrator should also calculate the total vehicle mass:

m_total = m_rover + m_payload

The payload mass alone does not describe stability. The payload location matters because it changes the system centre of mass. A simplified centre-of-mass calculation along one axis is:

x_COM = Σ(m_i × x_i) / Σm_i

Here, m_i is the mass of each component and x_i is its position relative to a defined robot reference frame. For ROS-based robots, coordinate-frame conventions should follow REP 103 and REP 105. REP 103 defines standard units and coordinate conventions, including metres, kilograms, radians, and right-handed frames. REP 105 defines common mobile-robot frame relationships such as map, odom, and base_link.

Key payload parameters

Payload capacity must be evaluated through several connected parameters. A mass-only calculation is insufficient when integrating perception hardware, onboard computers, or scientific instruments on a UGV.

Parameter Why it matters for a UGV Typical verification method
Payload mass Determines whether the configured rover remains within the manufacturer’s load specification. Weigh all installed components, including brackets and cabling.
Centre of gravity A high or offset payload can reduce stability during turns, slopes, and obstacle crossing. Use CAD mass properties or measure component positions and calculate the centre of mass.
Mounting position Changes sensor field of view, mechanical vibration, cable routing, and rollover risk. Review CAD assembly, sensor frustums, and wheel-envelope clearance.
Power demand Can reduce runtime or overload available voltage rails. Measure current under startup, nominal, and peak compute loads.
Thermal load Compute modules and enclosed electronics may throttle or shut down at high temperature. Log CPU, GPU, and enclosure temperatures during representative missions.
Dynamic load Terrain impacts and acceleration produce forces higher than static weight. Test at the intended speed, terrain type, incline, and payload configuration.

Payload capacity on Leo Rover

Fictionlab specifies a maximum payload of approximately 5 kg for Leo Rover 1.9 on standard tires. Leo Rover is a four-wheel, differential-drive mobile platform with an integrated Raspberry Pi-based compute unit. It supports ROS 2; compatibility depends on the installed operating system, ROS 2 distribution, and available platform packages. Its IP55-rated enclosure and compact form factor make it suitable for laboratory, educational, and field experiments, but autonomy is not provided by default. A navigation stack, sensors, calibration, and mission-specific safety logic must be integrated separately.

On Leo Rover, payload planning is particularly relevant when adding a LiDAR, depth camera, IMU, GNSS/RTK receiver, external computer, or sensor mast. A tall LiDAR mast may improve scan coverage but can increase the centre-of-gravity height and introduce vibration. An external NVIDIA Jetson module may provide additional compute capacity for perception, but it adds mass, power consumption, heat, and cable-routing requirements.

A typical ROS 2 sensor payload can include the following components:

  • A 2D LiDAR publishing sensor_msgs/msg/LaserScan.
  • An IMU publishing sensor_msgs/msg/Imu.
  • A depth camera publishing sensor_msgs/msg/Image, sensor_msgs/msg/CameraInfo, and optionally sensor_msgs/msg/PointCloud2.
  • A GNSS receiver publishing sensor_msgs/msg/NavSatFix.
  • A fused odometry source publishing nav_msgs/msg/Odometry.

The frames of rigidly mounted devices should be represented by fixed transforms in the robot URDF or TF tree. A sensor frame should be attached to base_link, directly or through intermediate frames, using fixed transforms. Incorrect frame placement can create mapping and navigation errors even when the mechanical payload remains within the platform’s rated capacity.

<joint name="lidar_joint" type="fixed">
  <parent link="base_link"/>
  <child link="laser_frame"/>
  <origin xyz="0.12 0.00 0.28" rpy="0 0 0"/>
</joint>

Payload capacity on Raph Rover

Fictionlab specifies a maximum payload of approximately 10 kg for Raph Rover 1.0. Raph Rover is a larger and heavier mobile platform intended for applications that require a higher payload capability than Leo Rover. It is relevant where the payload includes larger sensor suites, protected compute enclosures, multi-battery systems, environmental monitoring equipment, or inspection instruments.

A higher payload capacity does not remove integration constraints. Raph Rover still requires verification of centre of gravity, wheel load distribution, electrical power budget, braking behaviour, terrain conditions, and emergency-stop access. It should be selected when the payload and mission requirements exceed the practical integration envelope of a smaller platform, not as a replacement for Leo Rover in standard laboratory or educational work.

Monitoring payload-related system limits in ROS 2

ROS 2 does not define a universal payload-capacity message type. Payload state is usually monitored indirectly through battery data, motor-controller telemetry, temperature sensors, diagnostics, odometry, and IMU measurements. The standard diagnostic_msgs/msg/DiagnosticArray message can aggregate warnings from multiple hardware drivers.

payload_monitor:
  ros__parameters:
    battery_voltage_warning: 11.0
    cpu_temperature_warning_c: 75.0
    imu_roll_limit_rad: 0.35
    imu_pitch_limit_rad: 0.35
    publish_diagnostics: true

The thresholds above are example integration values, not Leo Rover or Raph Rover manufacturer limits. Actual thresholds must be derived from the rover documentation, battery specification, motor-controller specification, and validated operating tests.

Verification procedure

Payload validation should be performed before autonomous operation. The process should use the final mechanical configuration, software stack, battery setup, and intended sensor data rates.

  1. Measure the mass of the complete payload, including mounting hardware and cables.
  2. Confirm that the payload mass, and the total vehicle mass where a gross-mass limit is specified, remain within the platform manufacturer’s limits.
  3. Calculate or measure the centre of mass after installation.
  4. Inspect wheel clearance, cable strain relief, enclosure sealing, and emergency-stop access.
  5. Measure electrical current during boot, compute peaks, and motor operation.
  6. Run controlled driving tests on representative terrain, slopes, and turning radii.
  7. Log odometry, IMU, battery, temperature, and diagnostic data in a ROS 2 bag.

Normative references and technical sources

Payload capacity is primarily defined by platform-specific mechanical and electrical specifications. ROS standards are relevant for representing the resulting robot model, coordinate frames, sensor data, and diagnostics.

See also