Thermal Management and Protection

Introduction

Robots generate significant heat during operation. Without effective thermal management, performance degrades or hardware may be damaged. Additionally, robots often work in complex environments requiring appropriate dust and water protection. This section discusses thermal management and environmental protection design.

Heat Source Analysis

Major Heat Sources in Robots

Heat Source	Typical Power	Thermal Challenge	Temperature Limit
Compute board (Jetson AGX Orin)	15–60W	Concentrated heat, small chip area	Junction <105°C
Motor drivers (MOSFET)	1–10W/channel	Multiple channels accumulate	Junction <150°C
DC motors	10–30% efficiency loss	Sealed motors dissipate poorly	Winding <130°C
BLDC motors	5–15% efficiency loss	High power density	Magnet demagnetization temperature
DC-DC converters	5–15% efficiency loss	Inductor/MOSFET heating	<125°C
LiDAR motor	1–3W	Usually self-cooled	—

Heat Calculation

Heat generated equals power loss:

\[Q = P_{loss} = P_{input} - P_{output} = P_{input} \times (1 - \eta)\]

For example, a 90%-efficient DC-DC converter outputting 30W:

\[P_{input} = \frac{30W}{0.9} = 33.3W\]

\[Q = 33.3W - 30W = 3.3W\]

Thermal Resistance Model

Heat conduction is analogous to Ohm's law in electrical circuits:

\[\Delta T = Q \times R_{th}\]

Where \(\Delta T\) is temperature difference (°C), \(Q\) is heat flow (W), and \(R_{th}\) is thermal resistance (°C/W).

Series thermal resistance:

\[R_{th,total} = R_{th,junction-case} + R_{th,case-sink} + R_{th,sink-ambient}\]

Passive Cooling

Heat Sinks

Heat sinks reduce thermal resistance by increasing the dissipation surface area.

Natural convection cooling capacity estimate:

\[Q = h \cdot A \cdot \Delta T\]

Where \(h\) is the convective heat transfer coefficient (natural convection ~5–25 W/(m²·K)), and \(A\) is the surface area.

Common Heat Sinks:

Type	Thermal Resistance	Suitable Power	Features
Small aluminum fin (20x20 mm)	15–30°C/W	<3W	MCU/small ICs
Medium aluminum fin (40x40 mm)	5–15°C/W	3–10W	Motor drivers
Large aluminum fin (60x60 mm)	2–8°C/W	10–30W	SBC/GPU
Heat pipe cooler	1–3°C/W	30–100W	Jetson AGX

Thermal Interface Materials

Type	Thermal Conductivity (W/m·K)	Thickness	Suitable For
Thermal paste	1–12	<0.1 mm	Flat intimate contact
Thermal pad	1–8	0.5–5 mm	Gap filling
Thermal tape	0.5–3	0.1–0.3 mm	Mounting + heat transfer
Graphite sheet	In-plane 400–1500	0.025–0.1 mm	Planar heat spreading
Phase change material	3–8	—	High performance

Heat Pipes

Use evaporation-condensation cycle of working fluid for heat transfer
Effective thermal conductivity can be 50–100x that of copper
Suitable for transferring heat from confined spaces to remote heat sinks
Jetson AGX Orin developer kit uses heat pipe cooling

Active Cooling

Fans

Type	Size	Airflow	Noise	Suitable For
Axial fan	25–120 mm	Medium-high	Medium	Blowing on heat sinks
Centrifugal fan	30–80 mm	Medium	Low	Thin designs
Blower	40–60 mm	High pressure, low flow	Medium-high	Directed airflow

Forced convection coefficient: \(h = 25-250\) W/(m²·K), 5–10x improvement over natural convection.

Fan Selection:

\[\dot{Q} = \dot{m} \cdot c_p \cdot \Delta T_{air}\]

Where \(\dot{m}\) is air mass flow rate, \(c_p = 1005\) J/(kg·K), and \(\Delta T_{air}\) is inlet-outlet temperature difference.

Liquid Cooling

Suitable for >100W high-power systems
Circulation path: Water pump → Cold plate → Radiator → Reservoir
Cold plate contacts heat source directly
Used in large robots or high-performance computing clusters

Unitree Go2 Thermal Design

The Go2 quadruped robot's thermal solution:

Dual-fan design: Two small axial fans
Aluminum heat sink: Mated to the Jetson compute module
Thermal pads: Connect chip to heat sink
Body vents: Front and rear openings form an airflow channel
Conductive shell: Aluminum alloy body also participates in heat dissipation

Airflow Design Principles

graph LR
    A[Inlet<br/>Filter] --> B[Cool air passes<br/>sensitive components first]
    B --> C[Flows past motor drivers<br/>DC-DC]
    C --> D[Flows past compute board<br/>main heat source]
    D --> E[Exhaust outlet]

    style A fill:#bbdefb
    style E fill:#ffcdd2

Cool air passes temperature-sensitive components first: Sensors, MCU
Short hot air path: Exhaust as quickly as possible
Avoid dead zones: Ensure airflow covers all areas
Filter on inlet: Prevent dust from clogging

Thermal Simulation

Simplified Calculation

For preliminary design, a thermal resistance network can be used for estimation:

\[T_{junction} = T_{ambient} + Q \times (R_{jc} + R_{cs} + R_{sa})\]

Example: Jetson Orin NX (25W mode), ambient temperature 35°C

Thermal Resistance Link	Value	Description
\(R_{jc}\)	0.5°C/W	Chip to case (Datasheet)
\(R_{cs}\)	0.5°C/W	Thermal pad
\(R_{sa}\)	1.5°C/W	Heat sink + fan
Total	2.5°C/W

\[T_j = 35 + 25 \times 2.5 = 97.5°C\]

Below the 105°C junction temperature limit, but with limited margin — optimization needed.

FEA Thermal Simulation

Fusion 360 Thermal: Basic steady-state thermal analysis
ANSYS Icepak: Professional electronics thermal simulation
FloTHERM: Industry standard for electronics thermal management
SimScale: Online CFD thermal simulation (free tier available)

IP Protection Design

Dust Protection

IP First Digit	Protection Level	Design Measures
4	Protection from >1 mm objects	Gaps <1 mm
5	Protection from harmful dust	Sealing + labyrinth structure
6	Completely dust-tight	Fully sealed

Water Protection

IP Second Digit	Protection Level	Design Measures
4	Splash-proof	Drain holes + shielding
5	Water jet proof	O-ring sealing
6	Powerful water jet proof	Double sealing
7	Brief immersion proof	Full-perimeter O-rings + waterproof connectors
8	Continuous immersion proof	Face sealing + potting

Sealing Component Selection

O-Rings:

Materials: NBR (oil-resistant), silicone rubber (heat-resistant), EPDM (water-resistant)
Compression ratio: 15–25%
Groove design: Per standard dimensions

Sealed Connectors:

Connector	Protection Rating	Suitable For
GX12/GX16	IP65	Power/signal
M12 circular	IP67	Industrial sensors
USB Type-C (with seal cap)	IP67	Data/charging
RJ45 (industrial)	IP67	Ethernet

Environmental Testing

Common Test Items

Test	Standard	Conditions	Purpose
High-temperature storage	—	70°C / 48 h	Material heat tolerance
Low-temperature storage	—	-20°C / 48 h	Low-temperature performance
Temperature cycling	IEC 60068	-20 to 60°C / 100 cycles	Thermal fatigue
Vibration	IEC 60068	10–500 Hz / 2G	Structural reliability
Drop	—	0.5–1 m	Impact resistance
Waterproofing	IEC 60529	IP rating test	Water protection verification
Salt spray	IEC 60068	5% NaCl / 48 h	Corrosion resistance

Design Checklist

[ ] Identify all heat sources and their power dissipation
[ ] Build thermal resistance model, estimate junction temperatures
[ ] Select cooling approach (passive/active)
[ ] Size and verify heat sinks/fans
[ ] Determine required IP protection rating
[ ] Design sealing structures (O-rings, gaskets)
[ ] Select waterproof connectors
[ ] Design inlet filter
[ ] Plan environmental testing

References

NVIDIA Jetson Thermal Design Guide
Aavid Thermalloy: Heat Sink Design Guide
IEC 60529: IP Protection Rating Standard
"Electronic Equipment Thermal Design"