How to choose a BLDC motor for robotic arms and mobile robots

There are several factors to choosing brushless DC or BLDC motors for robots. Source: Faisal Mahmood

Selecting a brushless DC, or BLDC, motor for robotic arms or mobile robots starts with the mechanical system, not the controller catalog. A motor rated for a particular voltage and wattage does not tell an engineer whether it can deliver the required torque at low speed, regulate position accurately, absorb regenerative energy, or survive the thermal duty cycle.

The actuator is a system. The motor, inverter, current sensors, rotor position feedback, gearbox, brake, output encoder, thermal path, communications interface, and higher level robot software all influence the result.

An industrial or collaborative robotic arm joint generally prioritizes controllable torque, low-speed smoothness, position accuracy, collision response, backdrivability, and thermal performance. A mobile robot drive motor generally puts more emphasis on efficiency, starting torque, braking, traction, battery utilization, speed regulation, and synchronization between drive channels.

The terminology also deserves care. Commercial robotics literature often uses “BLDC” and “PMSM” (permanent magnet synchronous motor) somewhat interchangeably because both are three-phase, permanent-magnet machines driven by electronic inverters. Their back EMF waveform, winding design, controller algorithm, and feedback method still determine how a particular motor behaves.

For example, STMicroelectromics treats three-phase BLDC and PMSM control as related application areas and describes both six-step and field-oriented control architectures.

Start with the mechanical load, not the controller catalog

The first selection exercise is to establish what the motor actually has to do. For a geared robotic joint, a useful first order relationship is: τ_joint = (τ_motor × gear ratio × η_gear) − τ_losses

Motor speed follows the transmission ratio: ω_motor = ω_joint × gear ratio

Mechanical power is: P_mech = τ × ω

And a first approximation of battery side electrical power is: P_elec ≈ P_mech / η_total

These equations do not replace a dynamic model, but they expose the parameters that matter before a controller is selected. Considerations include continuous torque, peak torque, peak duration, acceleration torque, gravity torque, and reflected inertia. In addition, regenerative braking, stall or near stall operation, duty cycle, thermal resistance, battery voltage range, and cable and connector current limits should all be established.

For robotic arms, the load calculation has to account for link inertia, payload, gravity compensation, gearbox ratio, backlash, compliance, counterbalance, and encoder location. A motor side encoder can report the rotor position accurately while the joint output still contains transmission error.

For mobile robots, the equivalent exercise starts with vehicle mass, wheel radius, rolling resistance, slope, acceleration, terrain, wheel slip, and drive configuration. Differential-drive platforms impose different synchronization requirements from four wheel drive vehicles. An outdoor uncrewed ground vehicle ( UGV ) can also spend much more time in high load operation than a lightweight indoor delivery robot .

Regenerative energy deserves particular attention. During deceleration or downhill travel, the motor can return energy to the DC bus. The battery, controller, braking strategy, and bus capacitance all have to accommodate that energy. A controller that handles acceleration correctly can still create an over-voltage problem during aggressive regenerative braking.

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Choose the commutation method according to motion quality

Six-step, sinusoidal, and field-oriented controls are not interchangeable labels for the same behavior.

Six-step, or trapezoidal, commutation switches the inverter through six electrical states according to rotor position. It can be attractive where implementation simplicity and cost matter or torque ripple is acceptable. It can also be suitable when the motor spends much of its operating time at moderate or high speed and precise low speed torque is not a central requirement. Hall sensors can provide the rotor position information needed for straightforward sensored implementations.

Sinusoidal control drives the phases with continuously varying current or voltage references rather than the discrete commutation states associated with basic six-step operation. It can reduce torque ripple and acoustic effects compared with a simple trapezoidal strategy, although actual performance depends on motor construction, feedback, current regulation, and tuning.

Field-oriented control, or FOC, takes the control problem further by transforming the measured phase currents into a rotating reference frame. The Clarke transformation converts the three phase quantities into a stationary two-axis representation, while the Park transformation rotates that representation into d axis and q axis components.

The controller can then regulate the components independently through current loops before transforming the command back into phase voltages and generating pulse width modulation (PWM).

In a typical PMSM application, q axis current is closely associated with torque production, while d axis current can be used for flux related control and, at higher speeds, field weakening. TI ‘s motor control reference material illustrates the practical structure with Clarke and Park transformations, PI regulators, position and speed loops, and space vector PWM.

FOC is commonly attractive when the application requires smooth low-speed motion, fast torque response, or accurate speed regulation. It can also help with reduced torque ripple, regenerative braking, field weakening, or torque control for impedance and force-related behavior. FOC can therefore be a natural fit for many robotic joint and traction applications.

FOC does not automatically create a…

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