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Planetary gearbox

Planetary gearbox – definition

A planetary gearbox is a compact gear transmission in which one or more planet gears rotate around a central sun gear inside an internally toothed ring gear. The planet gears are mounted on a carrier, which can act as an input or output member. The mechanism is also called an epicyclic gearbox.

In mobile robotics, a planetary gearbox is commonly installed between a DC, brushless DC, or servo motor and a driven wheel. Its primary purpose is to reduce motor speed, increase wheel torque, and increase the drivetrain’s ability to overcome external loads. This is relevant to differential-drive UGVs such as Leo Rover and larger payload-oriented platforms such as Raph Rover, where wheel motors must overcome rolling resistance, slopes, payload mass, and transient loads during acceleration.

A planetary gearbox does not provide navigation, odometry, or ROS 2 functionality by itself. It is a mechanical component. Its practical effect on a ROS-controlled UGV is indirect: its reduction ratio, backlash, friction, and encoder location affect the relation between commanded motor motion and measured wheel motion.

Operating principle and gear ratio

A simple planetary gearset contains three fundamental members: a sun gear, a ring gear, and a planet carrier. A common arrangement for mobile robot wheel drives uses the sun gear as the input, the ring gear fixed to the housing, and the carrier as the output.

For this arrangement, the ideal speed reduction ratio is:

i = 1 + (Nring / Nsun)

In this equation, i is the reduction ratio, Nring is the number of teeth on the ring gear, and Nsun is the number of teeth on the sun gear. The output carrier speed is:

ωout = ωmotor / i

The theoretical output torque can be approximated as:

Tout = Tmotor × i × η

Here, η is gearbox efficiency. Actual efficiency depends on gear geometry, lubrication, load, bearing losses, temperature, and the number of reduction stages.

Planetary gearboxes are often selected instead of simple spur-gear reductions because several planet gears can share transmitted load. The coaxial input and output arrangement is also useful when a wheel must be mounted close to the motor axis.

Key parameters for UGV wheel drives

A gearbox specification must be evaluated together with the motor, wheel diameter, encoder, vehicle mass, and expected terrain. A high reduction ratio increases available wheel torque, but it also reduces maximum wheel speed and can make the drivetrain less backdrivable.

Parameter Meaning in a mobile robot Integration impact
Reduction ratio Ratio between motor revolutions and output revolutions. Required for converting encoder counts into wheel rotation.
Rated output torque Continuous torque permitted by the gearbox manufacturer. Must exceed expected rolling, slope, and acceleration loads.
Peak torque Short-duration allowable torque. Relevant during starts, obstacle contact, and abrupt braking.
Backlash Angular free movement between input and output when torque direction changes. Can reduce low-speed odometry accuracy and control precision.
Efficiency Mechanical power retained after transmission losses. Affects battery runtime and motor thermal loading.
Radial and axial load limits Permitted loads on the output shaft. Important when the wheel is directly supported by the gearbox output.

Rated torque and peak torque are not interchangeable. A UGV that repeatedly operates near peak torque can overheat the motor, damage gear teeth, or shorten bearing life. Wheel-side loads should also be checked separately from transmitted torque, especially on uneven terrain.

Planetary gearboxes, encoders, and ROS 2 odometry

ROS 2 does not define a dedicated planetary gearbox message type. The gearbox is represented through drivetrain parameters in motor firmware, a hardware interface, or a controller configuration. For a differential-drive robot, wheel encoder measurements are usually converted into wheel position and velocity before being passed to a controller such as diff_drive_controller.

If an encoder is mounted on the motor shaft, the gearbox ratio must be included in the conversion:

wheel_revolutions = motor_encoder_revolutions / gear_ratio

For a wheel with radius r, linear travel is calculated as:

distance = 2 × π × r × wheel_revolutions

Incorrect gear ratio values cause systematic odometry errors. A robot may report a longer or shorter travelled distance than its physical motion. Backlash can additionally create a dead zone when changing direction. This is most visible during slow maneuvers, path tracking, and closed-loop docking.

ROS coordinate conventions should follow REP 103, which defines SI units and standard orientation conventions. Mobile robot frame relationships should follow REP 105. In a typical UGV configuration, wheel odometry contributes to the transform between odom and base_link, while localization or SLAM can estimate the relationship between map and odom.

diff_drive_controller:
  ros__parameters:
    left_wheel_names: ["left_wheel_joint"]
    right_wheel_names: ["right_wheel_joint"]
    wheel_separation: 0.30
    wheel_radius: 0.075
    publish_rate: 50.0
    enable_odom_tf: true

The wheel separation and wheel radius values must be measured on the assembled vehicle; the values shown above are examples only. The gearbox ratio is normally handled below this controller level, where encoder ticks are converted into joint states by motor firmware or a ROS 2 hardware interface.

Use on Leo Rover and Raph Rover

Leo Rover is a four-wheel differential-drive UGV with native ROS 2 support on its integrated Raspberry Pi compute unit. When modifying its drive system or integrating external wheel modules, engineers should preserve consistent wheel radius, encoder scaling, motor direction, and left-right drivetrain conventions. A planetary gearbox with a different reduction ratio requires corresponding firmware or hardware-interface changes before odometry can be considered valid.

Raph Rover is intended for larger payload-oriented applications. For this class of platform, gearbox selection should account for higher wheel loads, payload center of gravity, terrain-induced shock, and output-shaft bearing capacity. Increasing gear ratio alone is not a substitute for verifying continuous torque, thermal limits, wheel traction, and battery current capability.

Limitations and engineering trade-offs

Planetary gearboxes introduce non-ideal mechanical behavior. These effects should be included in the error budget of a mobile robot rather than treated as software faults.

  • Backlash introduces position uncertainty after a reversal of wheel torque.
  • Gear friction reduces efficiency and increases motor current at low speed.
  • High reduction ratios reduce maximum vehicle speed.
  • Output shaft overload can damage bearings even when transmitted torque is within limits.
  • Encoder placement before the gearbox measures motor motion, not necessarily true wheel motion.

Normative references and technical sources

The following sources define the relevant ROS conventions and provide gear transmission reference material.

See also