Why Are Coreless Motors Used in Robotic Dexterous Hand Applications?

Designing a robotic hand that mimics human dexterity is a major engineering challenge. Actuators must be compact, responsive, and precise, but traditional motors often fall short in these tightly integrated systems.

Coreless DC motors are used in robotic dexterous hands because their low rotor inertia enables rapid acceleration, while their lack of cogging torque provides the smooth, precise control needed for delicate grasping tasks.

The real challenge isn't just finding a small motor. The difficulty lies in achieving system-level performance where multiple finger joints move with speed, precision, and coordination. This requires a deeper look at the motion dynamics, where the motor's internal design directly impacts the hand's ability to grasp, hold, and manipulate objects effectively¹. Let's break down the specific engineering hurdles and why coreless technology is a go-to solution².

What Motion Challenges Exist in Robotic Dexterous Hands?

Developing human-like robotic hands presents unique motion control problems. The space inside each finger is incredibly limited, yet the system must deliver complex, multi-axis movements smoothly and quickly.

Actuators in robotic fingers must overcome extreme space constraints, manage multiple degrees of freedom simultaneously, and deliver the fast, smooth acceleration needed to replicate natural human hand movements.

Unlike the large, powerful joints in an industrial robot arm, finger actuators operate with different priorities. The challenge with dexterous hands is balancing competing requirements in a miniature package. From my experience integrating these systems, the core motion challenges are consistently the same.

• High Degrees of Freedom (DOF): A single hand can have 15-20 independent joints. Each requires its own actuator, sensor, and control loop, all packed into a structure the size of a human hand.
• Extreme Space Constraints: There is virtually no spare room within a robotic finger. The motor, gearbox, and encoder must be integrated into a tiny volume, forcing engineers to prioritize power density (torque per volume) and thermal dissipation from day one.
• Need for Human-like Motion: Natural movement isn't just about speed; it's about fluidity. This demands an actuator that provides smooth motion at very low speeds for delicate tasks and high acceleration for quick reactions. Traditional iron-core motors often struggle with "cogging"—a jerky motion that ruins fine control.³
• Rapid Acceleration & Deceleration: Robotic fingers need to replicate the speed of human reflexes, requiring a motor with very low mechanical inertia to start and stop almost instantly.
• Simultaneous Control: Coordinated movement of all fingers places significant demands on the control system. This requires actuators that respond predictably and consistently, allowing the master controller to manage grasping patterns and force feedback effectively.

System-Level Observation: Honestly, this is where many early-stage robotic hand designs start running into trouble. A team might find a motor that fits, but it either overheats, responds too slowly, or lacks the fine control needed for manipulation. The problem isn't just the motor; it's the entire motion system.

Why Is Low Rotor Inertia Critical for Finger Motion Control?

A robotic finger needs to react almost instantly to grasp an object or adjust its grip. If the motor's rotor is heavy, it resists changes in motion, creating a noticeable lag in the system's response.

Low rotor inertia is critical because it allows the motor to accelerate and change direction with minimal delay, enabling the fast, responsive movements required for dynamic grasping and manipulation in robotic fingers.

In robotics and medical devices, engineers often focus on torque. But for applications with frequent start-stop and directional changes, inertia is frequently the more critical limiting factor. The relationship T = Jα (Torque = Inertia × Angular Acceleration) shows that to achieve high acceleration (α), you must minimize inertia (J).

Coreless motors excel here. By eliminating the heavy iron core in the rotor, their inertia is an order of magnitude lower than that of a conventional motor of the same size. This has direct, practical consequences for a robotic hand:

• Faster Grasping Reactions: A low-inertia motor begins moving almost instantaneously upon command, reducing the time between signal and physical contact.
• Reduced Control Lag: High inertia introduces a mechanical delay that can lead to overshoot and instability in feedback systems. Low inertia simplifies the control algorithm and allows for more aggressive, responsive tuning.
• Improved Energy Efficiency: Every time the motor accelerates its own rotor, it consumes energy. In the rapid movements of a dexterous hand, a low-inertia rotor directs more power toward moving the actual load—the finger mechanism.

Real Integration Challenge: I've seen teams struggle for weeks trying to tune a control loop for a robotic finger, fighting oscillation and slow response times. Often, the root cause wasn't the software; it was a motor with too much rotor inertia. The datasheet rarely tells the full story of how that inertia will behave in a real-world dynamic system.

How Do Coreless Motors Improve Precision Grasping Performance?

When a robotic hand needs to handle a delicate object like a test tube or a piece of fruit, brute force is the enemy. The system needs to apply just enough pressure without crushing the object, requiring incredibly smooth control.

Coreless motors improve precision grasping by eliminating cogging torque. This allows for exceptionally smooth rotation and precise force modulation, even at very low speeds, which is essential for handling fragile objects.

The key difference is in the motor's construction. Conventional DC motors have an iron core with windings that cause small torque ripples known as "cogging." This results in jerky motion, especially at low speeds. A coreless motor features a self-supporting winding with no iron, completely eliminating this effect.

This characteristic has a profound impact on grasping performance:

• Smooth Low-Speed Operation: For tasks like tracing a surface or slowly closing a grip, the absence of cogging allows a coreless motor to rotate without shudder, enabling stable micro-positioning.
• Better Force Modulation: Since motor current is proportional to torque, cogging acts as "noise" that makes it hard to control small forces. With a coreless motor, the relationship between current and torque is linear and predictable, allowing for high-fidelity force control.
• Stable Handling of Delicate Objects: The smooth control afforded by coreless motors enables a robotic hand to pick up objects like an egg or semiconductor wafer with a firm but gentle grip.

Performance Factor	Iron-Core Motor	Coreless Motor	Impact on Robotic Hand
Cogging Torque	High	None	Enables smooth, judder-free finger movement and fine force control.
Torque Linearity	Fair; affected by magnetic saturation	Excellent	Allows for predictable and precise control of grasping force.
Low-Speed Stability	Poor; prone to jerky motion	Excellent	Critical for delicate tasks like assembly or medical procedures.
Mechanical Vibration	Higher due to torque ripple	Very Low	Reduces noise and improves the stability of onboard sensors.

How Do Encoders and Gearboxes Affect Dexterous Hand Performance?

Selecting a great motor is only half the battle. Without the right gearbox and encoder, the performance potential of a coreless motor in a robotic hand can be completely wasted.

The gearbox translates motor speed into usable torque, while the encoder provides critical position feedback. The backlash and resolution of these components directly dictate the hand's final positioning accuracy and control stability.

Motor performance alone does not determine grasping accuracy. The entire actuator module—motor, gearbox, and encoder—must be treated as a single integrated system. A mismatch in any one of these components will bottleneck the entire design.

Gear Reduction Strategy

Coreless motors are high-speed, low-torque devices. A gearbox converts their speed into the force needed to actuate a finger.

• Planetary Gearboxes: These are common in robotic hands due to their high torque density and coaxial design⁴.
• Gear Ratio: Choosing the right ratio is a critical trade-off between grasping force and finger speed.
• Backlash: This is the "slop" or lost motion in a gearbox when reversing direction. In a robotic finger, high backlash can ruin positioning accuracy and make stable force control nearly impossible.

Encoder Resolution and Feedback

The encoder tells the controller exactly where the joint is and how fast it's moving, enabling precise closed-loop control.

• Resolution: A high-resolution encoder can detect minuscule movements⁵, which is essential for micro-positioning and smooth velocity control.
• Closed-Loop Control: The encoder allows the controller to compare the desired position with the actual position and adjust the motor to minimize error. This is what allows a robotic hand to hold a precise position under load.

Common OEM Mistake: A frequent mistake is selecting the motor, gearbox, and encoder as separate components. This often leads to integration headaches and subpar performance. A successful design defines the system requirements—force, speed, accuracy—and then selects an integrated actuator solution designed to work together.

What Thermal and Reliability Challenges Exist in Humanoid Robot Hands?

A dexterous hand is a dense, enclosed system packed with heat-generating components. When these hands operate continuously, heat becomes a silent killer of performance and reliability.

The compact, enclosed structure of a robotic hand severely restricts airflow, causing motor heat to accumulate. This can lead to motor damage, reduced performance, and long-term reliability failures.

A prototype might work perfectly for a short demo but fail after an hour of continuous use. From my experience, many robotic hand failures originate from thermal limitations rather than insufficient motor power.

The core of the problem lies in a few key areas:

• High Duty Cycles: A dexterous hand constantly adjusting its grip generates continuous heat (I²R losses) in the motor windings.
• Enclosed Structure: With no cooling fans or large vents, heat generated by the motors gets trapped, and the internal temperature rises.
• Cascading Thermal Effects: As a motor heats up, its winding resistance increases.⁶ To produce the same torque, the driver must supply more current, generating even more heat and risking thermal runaway.
• Impact on Magnets: The high-performance magnets in coreless motors can be permanently demagnetized if their maximum operating temperature is exceeded⁷, irreversibly degrading performance.

Thermal Risk	System-Level Impact	Mitigation Strategy
Motor Overheating	Reduced torque output, potential burnout.	Select a motor with higher thermal resistance or lower winding resistance.
Magnet Demagnetization	Permanent loss of motor torque.	Implement current limiting and perform thermal analysis during design.
Heat Transfer to Components	Damage to sensors, plastics, or lubricants.	Use thermal interface materials to conduct heat to the frame.
Performance Inaccuracy	Motor parameters change with heat.	Implement temperature monitoring and use thermally compensated control algorithms.

Design Trade-Off: There's a constant trade-off between power density and thermal management. A smaller motor fits easily but has less surface area to dissipate heat. Choosing a motor with higher efficiency (less heat generation) can often lead to a more reliable system.

Which Coreless Motors Are Recommended for Robotic Dexterous Hand Applications?

Knowing the requirements is one thing; choosing a motor that fits the design is the next step. For robotic hands, selecting a motor based on a single parameter can lead to weak grip, poor control, or thermal issues.

To provide a practical starting point, here are five BODENMOTION coreless motors often considered for robotic hand and compact actuator projects. This range covers various applications, from ultra-compact finger joints to larger service robot grippers.

Successful selection requires matching motor size, torque, and speed to the specific function of the finger or gripper. The key is to evaluate the entire actuator module, including the gearbox and encoder.

Model	Motor Size	Voltage / Power	Rated Speed	Rated Torque	Best For
BDMT1645CL	Ø16 × 45 mm	24V / 32W	36,200 RPM	9 mN·m	Ultra-compact finger joints: Where low weight, fast response, and minimal space are critical.
BDMT1656CL	Ø16 × 56 mm	24V / 40W	35,900 RPM	10.6 mN·m	Compact fingers: Requiring higher torque than the 45mm model within the same 16mm diameter.
BDMT2250CL	Ø22 × 50 mm	30V / 101W	36,226 RPM	27 mN·m	High-performance actuators: For demanding grasping cycles and precision manipulation.
BDMT3064CL	Ø30 × 64 mm	24V / 67W	7,600 RPM	83.5 mN·m	Thumb or palm joints: Needing high torque for stable load-holding and stronger actuation.
BDMT3260CL	Ø32 × 60 mm	18V / 78W	12,155 RPM	60 mN·m	Service robot grippers: For larger hand modules requiring robust torque capacity.

For final selection, the entire system must be considered: gearbox ratio, encoder feedback, backlash, thermal limits, and duty cycle. A compact motor with a well-matched transmission and feedback solution will outperform a larger motor chosen for torque alone. At BODENMOTION, we help robotics teams evaluate these factors at a system level to ensure a practical and reliable motor selection.

What Common Motor Selection Mistakes Occur in Robotic Hand Projects?

Many performance issues in robotic hands trace back to early motor selection decisions. An error on paper can lead to weeks of frustrating debugging with the physical prototype.

Most selection mistakes come from focusing on a single specification like torque while ignoring system-level factors like inertia, thermal buildup, and the performance of the integrated actuator.

I've seen the same patterns of mistakes emerge in many underperforming robotic hand projects. These issues usually originate from system-level oversights rather than a fault in the motor itself.

Here are the most common pitfalls to avoid:

1. Selecting Based on Stall Torque: Stall torque is a peak rating that is not sustainable. A motor running near stall will overheat and fail quickly.⁸ The continuous torque rating is far more important for reliability.
2. Ignoring Rotor Inertia: Inertia dictates responsiveness. A high-torque motor with high inertia can feel sluggish and make a control loop unstable. For dynamic finger movements, low inertia is non-negotiable.
3. Using Excessive Gearbox Reduction: Using a very high gear ratio to get massive torque from a tiny motor can make the system feel "spongy" and imprecise by multiplying backlash and reflected inertia.⁹
4. Underestimating Thermal Buildup: A motor that works on an open bench test can easily fail inside a sealed robotic finger. Failing to perform a thermal analysis for the expected duty cycle is a primary cause of long-term failure.
5. Ignoring Backlash: Pairing a high-precision motor with a low-cost, high-backlash gearbox is a classic mistake that negates the motor's precision and leads to poor positioning accuracy.
6. Finalizing Mechanics Before Actuator Selection: It's better to define the grasping force first, which informs the required motor/gearbox size, and then design the mechanical structure around the actuator.

Avoiding these traps requires a shift in thinking—from selecting a "part" to designing an "integrated motion system."

Conclusion

Choosing the right motor for a robotic hand is a system design challenge. Success depends on balancing inertia, control, thermal limits, and mechanical integration, not just torque.

If your team is navigating the complex trade-offs of actuator selection for a robotics project, our engineers can help. Contact us at info@bodenmotion.com to discuss your application and find a motion solution that delivers the performance and reliability you need.

FAQ: Coreless Motors in Robotic Dexterous Hands

Can a small coreless motor provide enough torque for a strong grip? Yes, when paired with an appropriate planetary gearbox. The coreless motor provides high speed and fast response, while the gearbox multiplies this into high output torque. The key is selecting the correct gear ratio to meet the application's force requirements without sacrificing too much speed. For a humanoid hand, it's common to see ratios from 50:1 to over 200:1.

What is the typical lifespan of a coreless motor in a robotic hand? The lifespan is primarily determined by the brushes and bearings. For brushed coreless motors operating within their specified load and temperature ratings, a lifespan of 1,000 to 3,000 hours is typical. For maximum life and reliability, brushless DC (BLDC) coreless motors are often preferred, as their lifespan is limited only by the bearings and can exceed 10,000 hours.

How does gearbox backlash affect finger control? Backlash is lost motion in the geartrain, creating a dead zone that harms precision. It makes accurate positioning difficult, as the finger will have "slop" at each joint. More importantly, it can cause instability in force control loops, leading to oscillations and an inability to maintain a steady grip. This is why selecting a low-backlash gearbox is as important as selecting the motor.

Why not use a direct-drive motor in a robotic finger? Direct-drive motors eliminate the need for a gearbox, removing backlash. However, to generate enough torque directly, they need to be much larger and heavier. In the extremely constrained space of a robotic finger, a large-diameter direct-drive motor is not feasible. The combination of a small, high-speed coreless motor and a compact planetary gearbox remains the most effective solution for high torque density in miniature applications.

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