■ What Generates Heat in Robots?
There are many heat sources inside a robot.
Typical ones include
● Servo motors
● Motor drivers
● AI computing boards
● CPUs and GPUs
● Batteries
● DC/DC converters
● Power supply circuits
● Cameras
● LiDAR
● Communication modules
In other words, a robot is not a single large heat source.
It is a system with many distributed heat sources.
Moreover, each heat source is placed in a different location, such as
the head,
torso,
arms,
joints,
legs,
and battery section.
That is why robots need different thermal management for each part.
■ Motors Become Large Heat Sources
Robots use many motors.
Moving joints.
Moving arms.
Turning wheels.
Holding heavy objects.
The greater the operating load, the more current flows to the motors and the more easily they generate heat.
Heat generation increases especially with
● High torque
● Long-term holding
● High-speed operation
● Repetitive operation
In other words, the more a robot works, the hotter the areas around its joints tend to become.
■ Even When a Robot Is Stopped, Its Motors May Generate Heat
Suppose a robot is stopped while holding a heavy object.
Because it is not moving, it seems to generate little heat.
However, to maintain its posture, current may continue to flow to the motors.
Then
heat is generated even when it is not moving.
In other words, the heat problems of robots change not only with operating speed, but also with torque and holding time.
■ Motor Heat Is Also Transferred to Surrounding Components
Looking at the motor temperature alone is not enough.
Motor heat is transferred to
● Reduction gears
● Bearings
● Frames
● Wiring
● Sensors
As a result,
● Grease properties may change
● Sensor accuracy may change
● Resin parts may degrade
● Cable life may shorten
In other words, what matters is not only
cooling the motor, but
where to release the motor’s heat.
■ Using Metal Frames as Heat Sinks
Robots often have metal structures.
So it is possible
to use the frame itself as an escape route for heat.
For example,
Motor
↓
TIM
↓
Aluminum frame
releases the motor’s heat into a wide frame.
This is a good match for robots,
because adding large heat sinks increases weight.
If existing structural parts can be used as heat paths,
structural materials can double as heat dissipation components.
■ However, Heat Will Not Escape to the Frame If the Contact Surface Is Poor
If there is
● A gap
● Warping
● Paint
● Surface roughness
between the motor case and the frame, heat may not transfer sufficiently.
So
● TIM
● Thermally conductive sheets
● Thermally conductive adhesives
are used.
In other words,
to make the frame a heat sink, too, the interface is important.
■ Motor Drivers Also Generate a Lot of Heat
To move motors, motor drivers are needed.
They may use power semiconductors such as
● MOSFETs
● IGBTs
● SiC
● GaN
These also generate heat.
Especially when miniaturized,
high-output components are gathered on small boards,
and heat generation density becomes high.
That is why thermal management using
● TIM
● Insulating heat dissipation sheets
● Metal bases
● Heat sinks
is needed.
■ In Power Semiconductors, "Pass Heat, Stop Electricity"
Motor drivers handle high voltages and large currents.
That is why, while you want to release heat to cooling components,
you want to avoid electrical contact with metal enclosures.
In other words, there is the challenge that
heat should pass through.
Electricity should not pass through.
Here,
● Insulating TIM
● Ceramic-based heat dissipation materials
● Insulating films
● Thermally conductive adhesives
become important.
■ In AI Robots, Computing Units Also Become Large Heat Sources
In future robots, there will be more situations where
● Image recognition
● Speech recognition
● Autonomous movement
● Motion planning
● Object recognition
● Generative AI
are processed inside the robot.
That is why more edge AI computing units will be installed.
If GPUs or AI accelerators are installed, heat problems similar to those of AI servers arise inside the robot.
■ Robots Cannot Hold Large Cooling Equipment Like AI Servers
In a data center,
large fans,
liquid cooling equipment,
and large heat sinks
can be installed.
In robots, however, this is difficult,
because
● They become heavier
● They become larger
● Power consumption increases
● Noise increases
● Range of motion is affected
In other words, in robots, the contradiction arises that
you want to install high-performance AI.
But you want the cooling equipment small and light.
■ In AI Computing Sections, Spreading Heat Becomes Important
When large heat sinks are hard to use,
local heat is spread with
● Graphite
● Copper foil
● Aluminum foil
● Vapor chambers
For example, a structure of
AI chip
↓
TIM
↓
Graphite
→→→ Metal enclosure
In other words, rather than
strongly cooling just one part,
the idea of
dispersing heat throughout the robot’s entire structure
becomes important.
■ Heat Tends to Concentrate Especially in the Head
In humanoid robots and service robots,
the head may concentrate
● Cameras
● AI computing
● Communication
● Sensors
● Motors
However, one does not want to make the head larger.
Furthermore, when used near people,
loud fan noise is also to be avoided.
That is why it becomes the severe thermal condition of small size, quietness and high density.
■ Cameras Are Also Affected by Temperature
Robots carry many cameras.
Inside camera modules there are
● Image sensors
● ISPs
● Communication circuits
which generate heat.
Furthermore, they may also receive heat from surrounding AI boards and motors.
When the temperature changes,
● Noise
● Optical characteristics
● Sensor characteristics
may change.
In other words, in robots, not only
cooling the camera
but also
stabilizing the temperature around the camera
is important.
■ LiDAR and Optical Sensors Also Have Heat Problems
LiDAR and optical sensors use
● Lasers
● Photodetectors
● Computing ICs
These also generate heat.
Furthermore, in optical components, dimensions and optical characteristics may change with temperature.
That is why heat can affect measurement accuracy.
This problem is shared with semiconductor manufacturing equipment.
■ Batteries Are Also Important Heat Sources
Mobile robots and humanoids require batteries.
Through
high-output operation,
fast charging,
and long operation,
batteries also generate heat.
In other words, inside a robot, there are three large heat sources:
motors
AI computing
batteries
■ Heat from Batteries and AI Computing Interferes
If an AI board is placed near the battery,
their heat may affect each other.
AI load is high.
Battery temperature rises.
Because battery temperature is high, even more cooling is needed.
In other words, component layout itself becomes thermal design.
■ In Robots, Heat Sources Move
This is a major difference from fixed equipment.
Robot arms.
Legs.
Joints.
These move.
In other words, heat sources also move.
Furthermore, depending on posture,
● Airflow
● Contact
● Direction of gravity
also change.
That is why there are cases where
thermal design cannot be done based on a single fixed state alone.
■ Movable Parts Need Flexible Thermal Management
It is difficult to place a large metal heat sink across a movable joint.
So there is the idea of moving heat with
● Thin graphite
● Flexible metal foil
● Flexible thermally conductive materials
However, repeated bending may cause
● Cracks
● Peeling
● Breakage
● Performance degradation
In other words, in robots, not only thermal performance but also bending durability is important.
■ There Are Also Problems with Cables and Heat
Through robot joints pass
● Power lines
● Communication lines
● Sensor wiring
Cables carrying large currents generate heat themselves.
Furthermore, the inside of joints is narrow, and air does not flow easily.
In other words,
wiring also becomes a heat source.
The more things are miniaturized, the more one wants to make cable diameters thinner.
However, making them thinner can increase resistance and heat generation.
Here too there is a design trade-off.
■ Summary
As we have seen, robots are no longer mechanical devices consisting only of motors and control panels.
With added heat sources such as AI computing units, cameras, LiDAR and batteries, they are changing into moving computers.
Moreover, the heat sources of robots are distributed in different places, such as the head, torso, arms, joints and legs.
Countermeasures are needed such as releasing motor heat into the frame and spreading AI computing heat with graphite or metal foil, but large cooling equipment like that of data centers cannot be installed.
Furthermore, the decisive difference from fixed equipment is that the heat sources themselves move.
When joints bend, thermally conductive materials are also required to have bending durability, and even cables passing through narrow joints become heat sources.
In Part 2, we look at how robots carrying such distributed, constantly moving heat sources make thermal management work within real constraints such as weight reduction, safe coexistence with people, waterproofing and dustproofing, and volume production.
Column Supervisor: Koji Kakumoto (President, OTIS Co., Ltd.)
After studying abroad and working in planning and development at a trading company, he joined OTIS Co., Ltd. in 2011. While primarily working in the Corporate Planning Department, he has also served concurrently in manufacturing and technical divisions, and since 2018 has served as President, working to drive business growth and strengthen the organization.
This article is a general technical explanation intended for educational purposes and does not refer to any specific company, product, or technology.
