[Robot Hardware 02] - Actuators (1): BLDC Motors
Robot hardware from a Physical AI perspective - BLDC motors
What Is an Actuator?
An actuator is the drive component that physically makes a robot move. If sensors perceive the environment and a controller makes decisions, the actuator converts those signals into physical force and motion.
The Dictionary Meaning vs. Robot Hardware
In the dictionary sense, an actuator is any device that generates physical movement in a system. Under this definition, the following are all actuators:
- hydraulic and pneumatic cylinders
- piezoelectric elements
- a pure electric motor without a reducer
In other words, even a standalone motor can be called an actuator. In the robot-hardware context, however, the term is used somewhat differently.
Actuators in Robots: Motor Plus Reducer
In multi-jointed robots, especially manipulators and legged robots, it is uncommon for a motor alone to drive a joint directly. Most electric motors are optimized for high-speed rotation, so they need a reducer to lower speed and amplify torque before they can support a robot joint.
In a multi-jointed robot, actuator can narrowly mean the combination of motor and reducer. In an actual product or system, it can also refer to an integrated joint-drive module that includes the encoder, bearings, motor driver, temperature sensor, and braking mechanism.
Why This Distinction Matters
This combination largely determines the robot’s output impedance, backdrivability, and control bandwidth.
Even with the same control algorithm, the robot’s motion and contact response can differ greatly depending on how the motor and reducer are configured. Reducer friction and the reflected inertia of the motor and reducer directly affect the mechanical impedance and backdrivability visible at the joint.
Pneumatic and soft actuators are also important in robotics, but this series will focus on the most common type: electric-motor-based actuators.
The complex world of reducers will be covered in the next post. This post first looks at the structure and characteristics of the motor, the heart of robot actuation.
Motors: A Controllable Source of Torque
When discussing robot hardware, people often ask, “How good is this motor?”
For a general machine, maximum output or maximum rotational speed might be the main criteria. From a robotics perspective, the question is different. A “good motor” is not simply one that produces a large force; it is one that can produce force predictably, quickly, and consistently.
From the perspective of a robot actuator, a motor is therefore not merely a rotating component. It is a controllable source of force that accepts current as an input and generates the desired torque at the desired time.
Operating Principle: Magnets and Coils
A motor fundamentally converts electrical energy into mechanical rotation. Inside it are two key elements:
- Permanent magnets: create a fixed magnetic field.
- Coils: become electromagnets when current flows through them.
The principle is simple. When current flows through a coil placed between permanent magnets, a magnetic field is generated around the coil according to Ampère’s law. The magnetic field created by the current and the magnetic field of the permanent magnets push or pull on one another through the Lorentz force, producing torque around the shaft.
The magnitude and direction of the force are determined by the magnitude and direction of the current. By controlling the current through the coils, we can precisely control the torque produced by the motor.
BLDC and PMSM: Names and Actual Structure
There are many motor types, including stepper motors and induction motors. But for high-performance multi-jointed robot actuators, brushless three-phase permanent-magnet motors have become the de facto standard. The component market and robotics industry often refer to these motors colloquially as BLDC (Brushless DC) motors.
Strictly Speaking, PMSM
The name BLDC can make it sound as if direct current continuously flows inside the motor, but that is not how it actually operates. The DC voltage supplied by a battery or power supply is converted by an inverter into three-phase current whose magnitude and direction vary over time. This current creates a rotating magnetic field in the stator, and the permanent-magnet rotor rotates synchronously with it.
From the perspective of motor structure and operating principle, the motors called BLDC in robotics therefore belong strictly to the PMSM (Permanent Magnet Synchronous Motor) family.[6,7] Here, “DC” is less a statement that only direct current flows through the windings than a name describing the lineage that replaced brushed DC motors with a DC power source and electronic commutation.
The terms can still be distinguished in a narrower sense:
- BLDC: a motor with approximately trapezoidal back-EMF, designed for six-step or block commutation
- PMSM: a motor with approximately sinusoidal back-EMF, designed for sinusoidal current control
In practice, a motor’s back-EMF is often somewhere between these two forms, and a motor sold as BLDC can also be driven with sinusoidal current or FOC. It is therefore more accurate to distinguish the motor’s electromagnetic design from the inverter’s drive method.
Robot Joints Are Usually Driven with FOC
In high-performance multi-jointed robots that require precise torque control, such as manipulators and legged robots, motors sold as BLDC are usually driven with an encoder and a three-phase inverter using FOC (Field-Oriented Control).
FOC transforms the three-phase currents into $i_d$, the flux-producing component, and $i_q$, the torque-producing component, in a coordinate frame based on the rotor electrical angle. This reduces torque ripple and vibration and makes the generated torque fast and consistent.
In the normal operating region of a surface-mounted permanent-magnet motor (SPMSM), $i_d$ is kept close to zero and torque is controlled by adjusting $i_q$. Under these conditions, robot-joint torque control can be understood in practice as making the $i_q$ current track its reference quickly and accurately. In an interior permanent-magnet motor (IPMSM) with saliency, reluctance torque is added, so $i_d$ also contributes to torque under MTPA (Maximum Torque per Ampere) operation. In the high-speed region, where back-EMF becomes large, field weakening with negative $i_d$ may also be used.[8]
The important point is that FOC does not change the type of motor. FOC is a drive and control method, while BLDC and PMSM describe motor structure and back-EMF characteristics. In this series, we use “BLDC motor” in the common market sense, while keeping in mind that it is strictly a three-phase permanent-magnet synchronous motor driven by an inverter and usually controlled with FOC in a robot joint.
Structure and Operation: Removing Brushes, Adding Electronic Control
A brushed DC motor needs physical brushes and a commutator to supply current to the rotating coil and reverse its direction. The BLDC/PMSM motors discussed here instead place the coils in the stator and the permanent magnets in the rotor. The inverter electronically controls the direction and magnitude of current instead of using a mechanical commutator.
The key is current distribution. The controller and inverter take over the role previously performed by the brushes and commutator. By distributing current through the stator phases with precise timing, they create a rotating magnetic field, and the permanent-magnet rotor rotates synchronously with it.
Why a Brushless Permanent-Magnet Motor?
This structural difference provides several important benefits for robot control:
- High efficiency and durability: With no brushes to create physical friction, friction loss is lower and brush wear does not require maintenance.
- High power density and heat dissipation: The stator windings, where losses are concentrated, are stationary and can transfer heat to the housing easily. The permanent-magnet rotor also needs no separate excitation current.
- Precise torque control: Inverters and current sensors can adjust the current in each phase quickly, enabling the high-bandwidth torque control required at robot joints.
Permanent-Magnet Synchronous Motor Structure: Inner vs. Outer Runner
The BLDC/PMSM motors discussed here can be broadly divided into a stator and a rotor.
- Stator: the stationary structure. Its coils create a magnetic field that rotates over time when current flows. This is where the current distribution central to motor control takes place.
- Rotor: the rotating structure. Its permanent magnets experience attraction and repulsion from the stator’s magnetic field and rotate toward the direction of lowest magnetic energy.
The motor’s characteristics change depending on whether the rotor is inside or outside the stator.
The comparison below describes general tendencies under the assumption that other design conditions—flux density, current density, pole count, stack length, winding, and cooling—are similar. Actual performance cannot be determined from rotor placement alone.
1) Inner Runner
The rotor is at the center and the stator surrounds it.
Characteristics:
- Small rotational inertia: The rotor has a small diameter, so its moment of inertia is low and acceleration and deceleration are fast.
- Good for high-speed rotation: The structure is mechanically stable and suitable for thousands or tens of thousands of RPM.
- Relatively small torque radius: Compared with an outer-runner motor of the same outer diameter, the air-gap radius may be smaller. Actual torque also depends on flux, winding, active length, and other factors.
Typical use:
Inner runners are often used in the combination of high-speed rotation and a high reduction ratio. The motor itself is small and fast, while a high-ratio reducer such as a Harmonic Drive amplifies the joint torque. This is a representative configuration for industrial robot arms and cobots that require high position accuracy and compact joints.
2) Outer Runner
The rotor is on the outside and the stator is inside.
Characteristics:
- Large torque radius: The air-gap radius is easy to make large, which is advantageous for producing more torque when other design conditions are similar.
- Simple structure: There is more room for the windings, and multi-pole designs are easier.
- Large rotational inertia: The rotating part is large and heavy, making the motor more suitable for low-speed, high-torque operation than for high-speed rotation.
Typical use:
Outer runners are particularly effective in the combination of low-speed, high-torque operation and a low reduction ratio (or direct drive). Lowering the reduction ratio can reduce reflected inertia and reducer friction, which helps achieve backdrivability. But a low reduction ratio alone does not guarantee backdrivability: motor cogging, bearing and seal friction, reducer efficiency, and the control method also matter. For these reasons, an outer-runner motor combined with a low-ratio reducer is a representative configuration for QDD actuators in legged robots, where dynamic control is important.
RC Motor Naming and Geometry
In outrunner motors for drones, it is common to use four digits in the model name to indicate the stator’s diameter and stack height.
The convention is usually:
- First two digits: stator diameter in millimeters
- Last two digits: stator height in millimeters
For example, 3305 usually means a stator with a 33 mm diameter and 5 mm stack height. This is not an international standard followed by every robot, industrial, or frameless-motor manufacturer. Depending on the manufacturer and product family, the name may instead refer to outer diameter, frame size, winding, or simply a series number. Actual dimensions must be checked in the manufacturer’s drawing and datasheet. Even within one manufacturer’s product family, naming systems such as AT2308, U8, and U10 may coexist.[11]
These dimensions matter because the motor’s external geometry is a key variable in its torque–speed characteristics.
Diameter: Torque Capacity
Physically, torque is $\tau=F\times r$ (force times radius), and the simplest relationship to current is $\tau\approx K_t I_q$. If electromagnetic shear stress at the air gap and other design conditions are similar, a motor with a larger air-gap radius is better suited to producing higher torque. The actual torque constant $K_t$, however, also depends on flux density, number of windings, pole count, stack length, and current density.
For low-speed, high-torque operation, a larger-diameter motor is therefore advantageous. The pancake motor takes this characteristic to an extreme. These motors have a very large diameter and a thin height, saving axial space while producing high instantaneous torque, so they are often used in robot-leg joints and gimbals.
Height: Continuous Output and Heat
Motor stack height, or active length, is a design variable that affects torque capacity and thermal behavior.
As the motor becomes longer:
- More flux path: The interaction area between magnets and coils increases, tending to increase the torque constant $K_t$.
- Greater thermal capacity, in general: More active material and mass tend to increase thermal capacitance.
Stack height alone cannot determine continuous torque, however. The allowable continuous current and torque can vary greatly with the cooling path from the windings to the housing, housing mounting conditions, heat sinks, and ambient temperature.[9]
BLDC Motor Physics: Voltage, Current, and Back-EMF
When engineers say “turn on the motor,” they mean controlling the magnitude and direction of a magnetic field. In this process, voltage and current play different roles.
Current Is Torque
In a normal operating region where magnetic saturation and saliency can be neglected, torque can be approximated as
\[\tau\approx K_t I_q\]Here, $K_t$ is the torque constant and $I_q$ is the q-axis current that produces torque. Thus, under the SPMSM condition described above, $i_d\approx0$ and motor torque is nearly proportional to $i_q$.
Voltage, Current, and Back-EMF
The electrical behavior of one motor phase can be simplified as follows:[10]
\[v=Ri+L\frac{di}{dt}+K_e\omega\]- $Ri$: voltage needed to overcome winding resistance
- $L\,di/dt$: voltage needed to change current quickly
- $K_e\omega$: back-EMF generated by rotation
Current determines torque, but voltage limits how quickly that current can be established and how much current can be maintained at high speed. As the motor rotates, back-EMF opposing the applied voltage appears in the coils, with magnitude $E=K_e\omega$ proportional to speed.
- At low speed: back-EMF is small, so most of the battery voltage can be used to push current into the windings. High torque is possible.
- At high speed: back-EMF is large, reducing the effective voltage across the coils.
As speed increases and back-EMF approaches the DC-bus voltage, the voltage margin available for current control decreases. This boundary is the base speed. Beyond it, the motor usually enters a constant-power region by reducing torque, or uses field weakening to increase speed further.
A motor as a generator: Back-EMF also appears when an external force drives the motor. If the motor terminals are connected in a closed circuit, the motor acts as a generator, produces current, and creates resistance to motion. This principle is used for damping control and regenerative braking in robots.
What to Check in a Motor Datasheet
When selecting a motor for a robot, do not look only at its external dimensions or peak torque. Check the following together:
- torque constant $K_t$
- speed constant $K_v$ or back-EMF constant $K_e$
- phase resistance $R$ and phase inductance $L$
- continuous and peak current
- continuous and peak torque
- rotor inertia
- number of pole pairs
- maximum rotational speed
- winding-to-housing and housing-to-ambient thermal resistance
- winding and motor thermal time constants
- maximum winding temperature and allowable magnet temperature
The core relationships can be summarized by four equations:
\[\tau\approx K_t I_q,\qquad E=K_e\omega\] \[P_{\mathrm{copper}}\approx I^2R,\qquad P_{\mathrm{mechanical}}=\tau\omega\]Datasheets may specify voltage and current on a phase or line-to-line basis, and their RMS and peak definitions may differ. Check the basis of every quantity before comparing numbers or entering them into a controller. Actual manufacturer datasheets often list not only torque and speed constants, but also resistance, inductance, rotor inertia, thermal resistance, and thermal time constants.[9]
Heat: The Real Limit of Motor Performance
Peak torque is intermittent performance that is allowed only for a short time. The allowable duration depends on the winding thermal time constant, initial temperature, cooling conditions, driver current limit, and repeated duty cycle. A single number is therefore not enough to judge the actual performance of a robot task.
When a motor gets hot, the problem is not merely that it can burn someone. Its physical properties change.
- Resistance increase: As the copper windings heat up, resistance $R$ increases and more voltage is required.
- Reversible flux reduction: Even within the normal allowable temperature range, permanent-magnet flux can decrease reversibly as temperature rises.
- Risk of irreversible demagnetization: If the magnet’s allowable temperature is exceeded or it is exposed to a strong opposing magnetic field, its flux may not recover after cooling.
- Insulation degradation: Repeated high winding temperatures reduce the life and reliability of the insulation.
These effects happen gradually, so they are easy to miss in a short experiment. During long operation, however, they are a major cause of degraded control performance and motor failure.
Stall torque, peak torque, and continuous torque must therefore be distinguished. Continuous torque (or rated torque) is the torque an actuator can produce continuously without exceeding the allowable winding temperature under specified ambient temperature, mounting, and cooling conditions. In a robot’s repeated motion, review not only the continuous operating point but also RMS torque over the complete duty cycle and the duration of each peak.
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/80732f8a46655aa4a1037a7fbdc154f4ceb33c50
[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/
[6] https://www.microchip.com/en-us/solutions/technologies/motor-control-and-drive/motor-control-algorithms
[7] https://www.st.com/en/applications/industrial-motor-control/pmsm-bldc-motor-control.html
[8] https://software-dl.ti.com/msp430/esd/MSPM0-SDK/latest/docs/english/middleware/motor_control_pmsm_sensorless_foc/doc_guide/doc_guide-srcs/MSPM0_Sensorless_FOC_Software_Users_Guide.html
[9] https://www.maxongroup.com/medias/sys_master/root/9414557433886/EC-frameless-DT-65-M-EN-05-2025.pdf
[10] https://developerhelp.microchip.com/xwiki/bin/view/applications/motors/control-algorithms/bldc/position/
[11] https://store.tmotor.com/product/at2308-long-shaft-fixed-wing-motor.html, https://store.tmotor.com/product/u8-lite-u-efficiency-kv150.html, https://store.tmotor.com/product/u10-2-u-efficiency.html