DISTANT System for Planetary Rovers

• The paper presents the DISTANT system that relocates actuation and steering mechanisms to a central, thermally controlled compartment to enhance durability.
• It employs a hybrid transmission chain—including bevel gears, dual Cardan joints, and capstan-driven steering—to optimize mechanical performance in harsh conditions.
• The design integrates advanced dust protection and rigorous thermal regulation, enabling rovers to traverse up to 50 km without maintenance.

The DISTANT (Distant Transmission and Steering Systems) design is an integrated mechanical and thermal architecture for planetary rovers, intended to address the challenges of actuator longevity, environmental resilience, and performance across extended traverses in extreme extraterrestrial environments. The distinguishing feature of DISTANT is the relocation of all traction and steering actuators from the wheel vicinity to a centralized, thermally protected "warm box" inside the rover chassis, with mechanical power transmitted externally via a hybrid system of bevel gears, dual Cardan joints, and capstan-driven cable steering. This architecture aims to decouple actuator performance and lifetime from exposure to harsh thermal cycles and dust contamination, extending operational endurance for planetary exploration rovers (Luna et al., 7 Oct 2025).

1. Centralized Actuation and Transmission Architecture

DISTANT relocates all wheel-traction and wheel-steering electric motors inside a single, thermally managed compartment within the rover’s main body. This "warm box" enables precise environmental control using active heating, multi-layer insulation (MLI), and minimized thermal bridging, maintaining internal actuator temperatures between –20 °C and +60 °C despite ambient ranges from –180 °C to +120 °C.

Mechanical power generated within the warm box is routed to each wheel assembly through a tripartite transmission chain:

• Bevel-Gear Stage: Transfers rotational motion from the motor axis (aligned with the chassis) to the articulated suspension axis.
• Dual Cardan Joint Linkage: Facilitates power transfer through the parallelogram suspension while permitting up to ±30° suspension travel and ±90° of steering rotation.
• Terminal Drive Method: For wheel traction, a short output shaft directly engages the hub. For wheel steering, a capstan drum and high-tensile steel cable transmit actuation to the wheel’s steering knuckle.

All motorization is thus contained and protected within the rover body, with only mechanical power links crossing into the external environment (Luna et al., 7 Oct 2025).

2. Suspension, Kinematics, and Steering Implementation

2.1 Double Wishbone Suspension

The system employs a parallelogram-based double wishbone suspension, expressed as a four-bar linkage with link vectors $$\mathbf{r}_1, \mathbf{r}_2, \mathbf{r}_3, \mathbf{r}_4$$ connecting chassis-fixed ($A$, $B$) to knuckle-located points ($C$, $D$). System configuration is governed by the vector loop-closure equation:

$$\mathbf{r}_1 + \mathbf{r}_2 = \mathbf{r}_3 + \mathbf{r}_4$$

with componentwise closure in all spatial coordinates.

This relation allows the ride height and wheel path to be prescribed by selecting link angles (e.g., lower arm angle $\theta_s$), with dependent variables such as the upper arm angle $\theta_u$ solved from the closure scalar form:

$$r_1\cos\theta_s + r_2\cos\phi = r_3\cos\theta_u + r_4\cos\psi$$

2.2 Dual Cardan Joint Transmission

To accommodate large articulation angles while minimizing velocity non-uniformity, each drive line includes two equal-angle Cardan (universal) joints arranged sequentially with a crossover shaft. For a cardan joint between shafts at angle $\alpha$:

$A$0

A paired equal-angle configuration cancels first-order harmonic velocity modulation, yielding an output angular velocity ideally matching the input ($A$1).

2.3 Capstan-Driven Steering

Steering control is enacted via a capstan cable drive:

• Tension $A$2 in a steel cable is translated into torque on a steering drum of radius $A$3 as $A$4.
• The frictional capstan relation, with coefficient $A$5 and cable wrap angle $A$6, is:

$A$7

$A$8

This arrangement enables precise control and strong mechanical advantage while permitting the actuator to remain isolated inside the warm box (Luna et al., 7 Oct 2025).

3. Environmental Protection: Thermal and Dust Management

3.1 Internal Thermal Regulation

Heat transfer pathways are quantitatively modeled:

• Structural conduction:

$A$9

where $B$0 is the thermal conductivity of structural material, $B$1 is area, and $B$2 the temperature gradient.

• Radiative exchange:

$B$3

$B$4 is emissivity, $B$5 the Stefan–Boltzmann constant, $B$6 the environmental temperature.

• Active heating is servo-regulated to stabilize internal temperatures.

3.2 Dust Ingress Protection

Critical moving interfaces are isolated using:

• Stainless-steel bellows, achieving dust ingress less than 0.1 mg per 50 km traverse.
• PTFE lip seals (6×16 mm) on all rotary shafts.
• Felt filters on ventilation, blocking >99.9% of particles $B$7 µm.

These methods together effect an ingress reduction factor exceeding $B$8, compared to unsealed arrangements (Luna et al., 7 Oct 2025).

4. System Performance and Design Trade-Offs

Design selection was informed by a trade analysis over criteria including mass, torque density, transmission efficiency, reliability (MTBF, failure-mode count), and environmental protection. The selected architecture—double wishbone with dual cardan and capstan elements—offered:

• Unsprung mass: 10.25 kg per leg.
• Drive torque: 30 Nm nominal, 86 Nm peak.
• Steering torque: 26–32 Nm.
• Steering step resolution: $B$90.1$C$0 with encoder feedback.
• Transmission efficiency: 43–99% (torque path, articulation-dependent), $C$191% (steering).
• Maximum traverse distance: 50 km without lubrication/servicing.
• Power consumption per leg: 150 W continuous (traction, 30 Nm at 50 rpm); 30 W continuous (steering, 26 Nm at 10 rpm).

A summary table of performance metrics is provided below.

Metric	Value/Range	Notes
Unsprung Mass (per leg)	10.25 kg
Drive Torque (Nm)	30 (nom); 86 (peak)
Steering Torque (Nm)	26–32
Step Resolution (steering)	$C$20.1$C$3	With encoder feedback
Transmission Efficiency	43–99% (traction); ~91% (steering)	Dependent on articulation
Max Traverse Before Service	50 km	No relubrication or seal replacement
Power Consumption (leg)	150 W (drive), 30 W (steering)	At continuous nominal torque/speed

[Table content from (Luna et al., 7 Oct 2025)]

5. Validation and Testing Regimen

A 1:3 scale breadboard of a single wheel-suspension-steering assembly is designated for multi-stage testing in Q1 2026. The validation plan includes:

1. Characterization of torque vs. speed for both cardan-bevel drive and capstan steering, identifying nonlinearities and slip thresholds.
2. Empirical mapping of transmission efficiency across all suspension and steering configurations.
3. Thermal-vacuum cycling to verify actuator temperature control under environmental extremes from –180 °C to +120 °C.
4. Dust ingress testing under simulated regolith exposure, measuring particulate incursion post 50 km equivalent articulation.
5. Suspension kinematics verification: laser track 3D motion compared against four-bar parallelogram loop-closure predictions.
6. Durability testing: 10,000-cycle endurance runs examining wear in bellows and bearings.

These activities are designed to quantitatively validate mechanical, thermal, and environmental robustness metrics, supporting future adaptation of DISTANT to flight applications (Luna et al., 7 Oct 2025).

6. Context and Prospects

By consolidating actuation within the protected rover body, the DISTANT system directly addresses limitations inherent in traditional hub-mounted solutions—thermal cycling-induced actuator degradation, lubrication breakdown, and dust-compromised mechanical assemblies. The modular design facilitates independent control of traction, steering, and suspension, supporting long-range traversal objectives (minimum 50 km) without scheduled maintenance. Planned breadboard validation is expected to provide a foundation for further evolution toward qualification for future planetary surface mobility platforms (Luna et al., 7 Oct 2025).