[Robot Hardware 02] - Actuators (1): BLDC Motors
Robot hardware from a Physical AI perspective - BLDC motors
Robot Hardware from a Physical AI perspective
What Is an Actuator?
An actuator is the part of a robot that turns commands into physical motion. If sensors observe the environment and the controller decides what should happen next, the actuator is the piece that converts that decision into force and movement.
The Dictionary Meaning vs. Robot Hardware
In the broadest sense, an actuator is any device that creates physical motion in a system. Under that definition, all of the following qualify:
- Hydraulic and pneumatic cylinders
- Piezoelectric devices
- Bare electric motors without reducers
So yes, a standalone motor can be called an actuator. But in the context of robot hardware, the word usually means something more specific.
Actuators in Robots: Motor Plus Reducer
In multi-jointed robots, especially manipulators and legged systems, motors rarely drive joints directly. Most electric motors are designed to spin fast, but robot joints need lower speed and much higher torque. That is why a reducer is usually added: it trades speed for torque so the joint can support and move the robot.
From the perspective of robot development and hardware design, an “actuator” usually means an integrated drive module: electric motor + reducer.
Why This Distinction Matters
That motor-reducer pairing determines the robot’s output impedance, backdrivability, and control bandwidth.
Even with the same control algorithm, the robot’s motion and contact response can change completely depending on how the motor and reducer are built. In many robots, half of the physical performance is already decided at the actuator level.
There are many other actuator types in robotics, including pneumatic systems and soft actuators. In this series, I will focus on the most common case: electric-motor-based actuators.
We will go deeper into reducers in the next post. Here, I want to start with the heart of robotic actuation: the motor.
Motors: A Controllable Source of Torque
When discussing robot hardware, people often ask, “How good is this motor?”
For ordinary machines, that question might mean maximum power or maximum RPM. In robotics, the answer is a little different. A good motor is not just one that produces a large force. It is one that can produce force predictably, quickly, and consistently.
From the actuator perspective, a motor is not just a rotating part. It is a controllable source of force: it takes current as an input and produces the desired torque at the desired time.
Operating Principle: Magnets and Coils
A motor converts electrical energy into mechanical rotation. Internally, it relies on two basic elements:
- Permanent magnets: They create a fixed magnetic field.
- Coils: When current flows through them, they become electromagnets and create another magnetic field.
The principle is simple. When current flows through a coil placed near permanent magnets, the current produces a magnetic field. That magnetic field interacts with the permanent magnet field, creating Lorentz force. When this force acts around a rotation axis, it becomes torque.
The magnitude and direction of that force depend on the magnitude and direction of the current. By controlling the current in the coils, we can control the torque produced by the motor.
BLDC Motors
There are many types of motors, including stepper motors and induction motors. For high-performance multi-jointed robots, however, the BLDC motor has become the practical standard.
Structure and Operation: Removing Brushes, Adding Electronic Control
A BLDC motor is a DC motor without brushes.
Traditional brushed DC motors use physical brushes and a commutator to deliver current to the rotating shaft. A BLDC motor removes those contact parts and reverses the layout: coils are placed on the stator, and permanent magnets are placed on the rotor.
The key idea is current distribution. The driver or inverter takes over the job that brushes used to perform. By precisely distributing current across the stator phases over time, it creates a rotating magnetic field. The rotor magnets follow that rotating field.
Why BLDC?
This structural difference gives BLDC motors several advantages for robot control.
- High efficiency and durability: There are no brushes rubbing against the commutator, so friction losses are lower and brush wear is eliminated.
- Precise torque control: Because the magnetic field is controlled electronically rather than through mechanical contact, the response can be fast and the torque can be regulated accurately.
BLDC Motor Structure: Inner Runner vs. Outer Runner
A BLDC motor is usually divided into two main parts:
- Stator: The stationary part. Coils are wound here, and the time-varying current distribution creates a rotating magnetic field.
- Rotor: The rotating part. Permanent magnets are attached to it, and the rotor turns as it aligns with the magnetic field created by the stator.
The motor’s behavior changes depending on whether the rotor sits inside or outside the stator.
1. Inner Runner
In an inner-runner motor, the rotor is at the center and the stator surrounds it.
Characteristics:
- Low rotational inertia: The rotating part has a small diameter, so its moment of inertia is low. This makes acceleration and deceleration fast.
- Good high-speed performance: The structure is stable at thousands or tens of thousands of RPM.
- Lower torque: Because the radius is small, the same electromagnetic force produces less torque.
Use case: Inner-runner motors are well suited to the combination of high speed + high reduction ratio. The motor spins fast, and a high-ratio reducer such as a Harmonic Drive amplifies the torque.
This makes them common in industrial robot arms and collaborative robots, where compact joints and high positioning accuracy matter.
2. Outer Runner
In an outer-runner motor, the rotor is on the outside and the stator sits inside it.
Characteristics:
- Higher torque: The air-gap radius is larger. Like a longer lever arm, a larger radius produces more torque from the same electromagnetic force.
- Simple packaging: There is more room for windings, and multi-pole designs are easier.
- Higher rotational inertia: The rotating body is larger and heavier, so the motor is better suited to low-speed, high-torque operation than high-speed rotation.
Use case: Outer-runner motors shine in the combination of low speed, high torque, and low reduction ratio.
When the gear ratio is kept low, usually below about 10:1, the joint’s mechanical impedance from friction and reflected inertia becomes much smaller. This improves backdrivability, allowing the robot to absorb impacts and regulate contact forces more naturally. This is why outer-runner motors are widely used in QDD actuators for legged robots.
Motor Naming and Appearance
Off-the-shelf motors for drones and robots often include numbers in the model name. These are usually not arbitrary serial numbers. They often describe the motor’s diameter and height.
The convention is usually four digits:
- First two digits: stator diameter in millimeters
- Last two digits: stator height in millimeters
For example, a 3305 motor is roughly 33 mm in diameter and 5 mm tall.
These dimensions matter because the external shape of a motor is closely tied to its torque-speed characteristics.
Diameter: Torque Capacity
Motor diameter is directly connected to torque.
Physically, torque is $\tau = F \times r$. A larger motor diameter means a larger air-gap radius, so the same electromagnetic force can generate more torque.
For low-speed, high-torque actuation, a larger-diameter motor is advantageous. This is the idea behind pancake motors: they are wide and thin, which saves axial space while still allowing high peak torque. They are common in robot leg joints, gimbals, and compact actuators.
Height: Continuous Output and Heat
Motor height or length is more closely related to continuous power and thermal behavior than to instant peak torque.
When the motor is longer:
- More magnetic interaction area: The interaction area between magnets and coils increases, improving the torque constant $K_t$.
- More thermal capacity: The motor has more volume and surface area to store and dissipate heat.
Even at the same diameter, a taller motor can usually tolerate high current for longer. Long motors are therefore useful in continuous-duty applications where heat management matters, or in linear actuators where a narrow but long package is easier to fit.
BLDC Motor Physics: Voltage, Current, and Back-EMF
People often say they “turn on” a motor, but from an engineering point of view, motor control means controlling the magnitude and direction of a magnetic field. In that process, voltage and current play different roles.
Current Is Torque
This is the first rule of BLDC motor control. The magnetic field strength is determined by the current through the coils, and that field produces torque.
Before magnetic saturation, motor torque is almost proportional to current. So when a robot controller performs torque control, it is really performing current control.
Voltage Sets the Speed Limit
Current cannot flow by itself. Voltage is the pressure that pushes current into the windings.
- Current: determines torque
- Voltage: determines how quickly and how much current can be driven into the motor, which sets the speed limit
Back-EMF: Why Speed Eats Torque
When a motor spins, it generates back-EMF inside the coils due to Lenz’s law. Back-EMF is a voltage that opposes the voltage you apply.
- At low speed: back-EMF is small, so most of the battery voltage can be used to push current into the motor. High torque is possible.
- At high speed: back-EMF becomes large, reducing the effective voltage across the windings.
Eventually, as speed rises, the motor hits a voltage ceiling. The drive can no longer push enough current into the windings, and torque drops sharply.
A motor as a generator: When an external force spins the motor, back-EMF is generated as well. If the motor terminals are connected in a closed circuit, the motor acts as a generator, produces current, and creates resistance. This principle is used in damping control and regenerative braking.
Heat: The Real Limit of Motor Performance
The “peak torque” listed on a motor datasheet is often something the motor can sustain only for a second or two. For a robot engineer, the more important performance limit is usually heat.
When a motor gets hot, the issue is not just that it can burn someone. Its physical properties begin to change.
- Resistance increases: As copper windings heat up, their resistance $R$ increases, so more voltage is needed.
- Magnetic flux weakens: Permanent magnets lose magnetic strength as temperature rises.
- Torque constant $K_t$ drops: For the same current, the motor produces less torque than before.
These changes happen gradually, so they are easy to miss in short experiments. During long operation, however, they are a major cause of degraded control performance and motor failure.
When selecting an actuator, we should therefore look beyond stall torque and focus on continuous torque or rated torque: the torque the motor can sustain indefinitely at thermal equilibrium.
Next post: [Robot Hardware 03] - Actuators (2): Reducers
References
[1] https://docs.espressif.com/projects/esp-iot-solution/en/release-v2.0/motor/bldc/bldc_overview.html
[2] https://www.gian-transmission.com/a-comprehensive-guide-to-brushless-dc-motor/
[3] https://www.semanticscholar.org/paper/A-low-cost-modular-actuator-for-dynamic-robots-Katz/80732f8a46655aa4a1037a7fbd154f4ceb33c50
[4] https://things-in-motion.blogspot.com/2019/05/understanding-bldc-pmsm-electric-motors.html
[5] https://www.kebamerica.com/blog/how-a-3-phase-ac-induction-motor-works/