In optoelectronic stabilization gimbals, robotic universal joints, airborne detection equipment, and portable exoskeletons, equipment often requires drive units capable of multi-angle movement, hollow cable routing, extreme lightweight design, and smooth, vibration-free operation at low speeds.
Many overseas R&D engineers often confuse conventional frameless robot joint motors with gimbal frameless motors. However, these two motor types are optimized for very different operating conditions.
What Is a Gimbal Frameless Motor?
Basic Definition
A gimbal frameless motor, also known as a gimbal frameless torque motor, is a specialized type of permanent-magnet brushless frameless motor designed specifically for gimbal stabilization platforms and multi-axis universal joints.
Like standard frameless motors, the motor is supplied only as a stator winding assembly + permanent-magnet rotor. It does not include a housing, bearings, or output shaft. It cannot operate as a standalone motor and must be integrated directly into the mechanical structure of the gimbal.
The biggest difference between a gimbal frameless motor and a conventional robotic joint frameless motor is its thin, flat magnetic circuit design, which is optimized for low-speed holding torque and extremely low cogging torque.
The primary goal is not high-speed rotation, but rather:
Small-angle reciprocating motion
Precise attitude holding
Smooth low-speed positioning
Vibration suppression
Stable operation in gimbal mechanisms
The stator is fixed to the gimbal base housing, while the rotor is directly mounted to the gimbal's main pivot shaft. The gimbal's own crossed-roller bearings or precision miniature bearings provide rotational support.
A large central hollow bore is reserved for signal and power cables, preventing repeated cable bending and wear during multi-angle movement.
Encoders, temperature sensors, brakes, and motor drivers are not normally included with the motor itself. These components are selected and integrated by the system manufacturer according to the requirements of the complete equipment.
Two Main Structural Types of Gimbal Frameless Motors
Outer-Rotor Gimbal Frameless Motor
The rotor surrounds the outside of the stator, making this the most widely used configuration for gimbal stabilization systems.
Its flat, pancake-like structure provides higher holding torque at the same outer diameter, while maintaining moderate rotor inertia and smooth low-speed movement.
It is suitable for:
Camera stabilization gimbals
Optoelectronic detection platforms
Small robotic universal joints
Compact stabilization systems
Most of HOLRY's gimbal motor series use an outer-rotor configuration.
Inner-Rotor Gimbal Frameless Motor
In an inner-rotor configuration, the rotor is positioned inside the stator.
This design provides:
It is commonly used in:
Gimbal Frameless Motors vs. Conventional Robotic Frameless Joint Motors
Many engineers attempt to use conventional robotic frameless motors in gimbal platforms. This can often result in low-speed vibration and unstable positioning.
The fundamental reason is that the two motor types are optimized around different magnetic circuit and operating requirements.
Conventional Robotic Frameless Joint Motors
Design objective: High continuous torque, large-angle continuous rotation, with moderate-speed operation.
Cogging torque: The cogging torque requirements are relatively relaxed because the motor is primarily intended for continuous robotic rotation.
Form factor: Generally thicker, with a strong focus on high torque output.
Operating conditions: Continuous reciprocating or large-angle rotation, with inertia-dominated loads.
Typical applications: Single-axis shoulder, elbow, hip, and knee joints in humanoid robots.
Gimbal Frameless Motors
Design objective: Low-speed small-angle movement, attitude holding, disturbance rejection, and vibration suppression.
Magnetic circuit optimization: Skewed magnets and fractional-slot windings are used to minimize cogging torque and ensure smooth micro-angle movement.
Form factor: Flat and thin construction reduces the overall size and weight of the gimbal.
Operating conditions: Frequent small-angle reciprocating motion and static position holding against external disturbances. High rotational speed is generally unnecessary.
Typical applications: Optoelectronic gimbals, robotic wrist joints, detection and scanning mechanisms, and portable exoskeleton joints.
Gimbal Frameless Motors vs. Conventional Fully Enclosed Gimbal Motors
Many commercially available gimbal motors are supplied as complete units with housings and bearings.
Their main advantage is plug-and-play convenience. However, their fixed housing dimensions and thickness can limit deep integration into equipment cavities. Their hollow bore size may also be restricted, making extensive system-level customization difficult.
A gimbal frameless motor, by contrast, is integrated directly into the mechanical structure of the final equipment.
This provides greater design flexibility and makes frameless motors particularly suitable for OEM customization and mass-production projects.
Key Advantages of Gimbal Frameless Motors
Extremely Low Cogging Torque for Smooth, Vibration-Free Low-Speed Motion
One of the biggest challenges in gimbal platforms is low-speed vibration and sudden attitude changes.
Gimbal frameless motors use technologies such as skewed permanent magnets and fractional-slot windings to significantly reduce cogging torque.
This enables:
Smooth micro-angle movement
Stable attitude holding
Better resistance to external vibration
Stable optical imaging and detection
These characteristics are particularly important for optoelectronic stabilization systems, where even small motor-induced disturbances can affect image quality.
Flat and Thin Design for Lightweight, Compact Equipment
By eliminating unnecessary components such as housings and end covers, the motor adopts a compact pancake-style structure that significantly reduces axial thickness.
Compared with fully enclosed gimbal motors providing similar torque, frameless motors can achieve substantially lower weight.
This makes them particularly suitable for space-constrained applications such as:
Large Hollow Bore for Reliable Internal Cable Routing
A large central hollow bore allows signal and power cables to pass directly through the motor.
During multi-angle gimbal movement, cables can remain routed through the center rather than repeatedly bending around the mechanism.
This helps:
Direct Drive with Zero Backlash for High-Precision Positioning
The rotor is directly mounted to the gimbal's main pivot shaft, eliminating couplings and reduction gears.
This removes transmission backlash and enables:
These characteristics are particularly valuable in precision detection and optical tracking equipment.
Flexible Thermal Management for Static Holding Applications
Gimbal mechanisms often spend significant periods in a static position-holding state. During this time, the windings may remain energized continuously and generate heat.
Because the stator of a frameless motor can be directly mounted to the aluminum-alloy gimbal base, heat can be transferred directly into the housing.
Depending on the operating environment, potting and thermal encapsulation can also be applied to improve heat transfer.
This helps prevent overheating during long-duration attitude holding.
High Customization for Global OEM Projects
HOLRY gimbal frameless motors support customization of:
The motors can be configured for 24 V / 48 V low-voltage DC systems and adapted to demanding overseas environments, including high-temperature, low-temperature, and high-humidity conditions.
HOLRY can also provide comprehensive English-language technical documentation to support international OEM projects.
Limitations of Gimbal Frameless Motors
Despite their advantages, gimbal frameless motors also have several inherent limitations.
1. Higher integration requirements
Because the motor does not include bearings or a housing, the system engineering team must design the mechanical cavity, select precision miniature bearings, develop assembly tooling, and match the encoder.
Therefore, these motors are not ideal for customers without sufficient electromechanical integration capabilities.
2. Highly sensitive to assembly concentricity
Gimbal mechanisms are often compact and feature very small rotor-stator air gaps.
Assembly misalignment can cause rotor-stator contact, increased vibration, and even motor failure.
For overseas customers without precision assembly capabilities, the risk of failure can increase significantly.
3. Not suitable for high-speed operation
The motor is primarily optimized for low-speed movement and holding torque rather than continuous high-speed rotation.
Main Applications of Gimbal Frameless Motors
Optoelectronic Detection Gimbals
Typical applications include:
These systems must resist external vibration and maintain stable optical orientation.
The thin motor structure saves valuable installation space, while the hollow bore simplifies routing for optical, signal, and power cables.
Robotic Universal Wrist Joints
Gimbal frameless motors can be used in:
They provide multi-degree-of-freedom movement while reducing end-effector weight and enabling smooth micro-adjustment during gripping and manipulation.
Portable Exoskeletons and Rehabilitation Equipment
Applications include:
Wearable exoskeleton joints
Rehabilitation training mechanisms
Compact assistive motion systems
Weight is a critical consideration for wearable equipment. The lightweight design of frameless motors, combined with smooth low-speed movement, helps improve user comfort and safety.
Specialized Industrial and Medical Precision Gimbal Mechanisms
Potential applications include:
Medical imaging rotation modules
Laboratory precision positioning platforms
Specialized inspection equipment
Precision detection systems
These applications typically require:
Applications not recommended: High-speed continuous rotation mechanisms, rapid prototypes without sufficient mechanical integration capabilities, and simple single-axis high-torque applications. Conventional frameless joint motors are generally more suitable for these applications.
Five Key Factors When Selecting a Gimbal Frameless Motor
Many overseas customers select motors based primarily on torque ratings while overlooking gimbal-specific parameters such as cogging torque, holding torque, motor thickness, and rotor inertia.
This can result in difficulties during system integration and commissioning.
Distinguish Holding Torque from Peak Torque
Most gimbal applications involve static attitude holding and small-angle reciprocating movement.
Therefore, the key parameter is continuous holding torque, which determines the motor's ability to resist external disturbances.
Peak torque is mainly relevant to short-duration acceleration and rapid movement.
For high-temperature overseas environments, it is recommended to maintain a 20–30% torque safety margin rather than selecting a motor based solely on its peak torque rating.
Pay Close Attention to Cogging Torque
Cogging torque is one of the most important parameters for gimbal motors because it directly affects low-speed smoothness and vibration.
For optoelectronic and precision gimbal applications, it is recommended to prioritize motors with:
Cogging torque ≤ 2% of rated torque
Conventional robotic frameless motors generally have less stringent cogging torque requirements. Using them directly in gimbal platforms can easily result in vibration and positioning drift.
Verify Motor Thickness, Outer Diameter, and Hollow Bore
Internal space in gimbal mechanisms is often extremely limited.
Axial thickness is therefore a critical design constraint.
The hollow bore diameter must also be checked to ensure sufficient space for signal and power cables.
HOLRY supports customized flat and thin motor designs for compact mechanical cavities.
Match Rotor Inertia
Gimbal systems need to respond quickly to external disturbances.
Rotor inertia should therefore be carefully matched to the mechanical system.
If the inertia is too high or poorly matched, the platform may oscillate and PID tuning may become difficult.
When selecting a motor, the supplier should provide the rotor inertia value so that the system manufacturer can perform dynamic simulations and motion-control calculations.
Consider Overseas Environmental and Electrical Requirements
For high-temperature regions, winding potting may be required to improve thermal conductivity.
For low-temperature environments, low-temperature-resistant magnets should be selected to reduce the risk of demagnetization.
The motor's bus voltage must also be compatible with the system power supply.
For international OEM projects, suppliers should ideally provide:
Common Integration Mistakes
Mistake 1: Replacing a Gimbal Frameless Motor with a Conventional Robotic Frameless Motor
Conventional frameless motors are generally optimized for large-angle rotation.
Their cogging torque may be too high for small-angle gimbal movement, resulting in continuous vibration and unstable attitude holding.
Even if the torque rating appears to match, direct replacement is generally not recommended.
Mistake 2: Treating It Like a Complete Gimbal Motor
A gimbal frameless motor typically consists only of the stator and rotor.
It does not include the bearings, mechanical support, or encoder required for a complete gimbal system.
It cannot simply be powered as a standalone finished gimbal motor.
Direct energization without proper mechanical integration may cause rotor-stator contact and permanent-magnet damage.
Mistake 3: Ignoring Concentricity and Air-Gap Requirements
Gimbal mechanisms are compact and often have very small rotor-stator air gaps.
Poor concentricity can lead to:
Vibration
Noise
Rotor-stator contact
Increased wear
Motor failure
The supplier should provide clear English assembly documentation specifying the required air gap and concentricity tolerances.
Mistake 4: Focusing Only on Torque and Ignoring Cogging Torque
A motor may provide sufficient torque while still having excessive cogging torque.
This can cause low-speed vibration, unstable gimbal movement, and image shake in optical systems.
When purchasing a gimbal frameless motor, buyers should request actual cogging torque test data from the supplier.
Mistake 5: Ignoring Thermal Management During Static Holding
Gimbal systems may remain locked in a specific attitude for extended periods, causing the windings to remain energized and generate heat.
Because the frameless motor does not have its own complete cooling structure, heat dissipation must rely heavily on the gimbal housing.
For applications in hot climates, thermal design should be evaluated at the beginning of the project.
Otherwise, overheating and permanent-magnet demagnetization may occur.
FAQ
Q1: Does a gimbal frameless motor require a gearbox?
A: Most gimbal platforms use a direct-drive configuration and do not require a gearbox.
A miniature precision gearbox may only be necessary for heavy-duty or high-load gimbal mechanisms.
Q2: Does a gimbal frameless motor include an encoder and brake?
A: Standard products generally do not include an encoder or brake.
These are system-level components selected according to the requirements of the complete equipment.
HOLRY can reserve sensor mounting interfaces to facilitate encoder integration by the equipment manufacturer.
Q3: What technical documents should overseas buyers request from the supplier?
A: Recommended documentation includes:
Motor datasheet
Torque and holding torque curves
Cogging torque test report
Rotor inertia data
Rotor-stator air-gap requirements
Concentricity and assembly tolerances
Winding resistance and inductance parameters
English assembly manual
Relevant export and certification documentation
Q4: How should I choose between an outer-rotor and inner-rotor gimbal frameless motor?
A: For applications requiring higher holding torque and a flat, thin gimbal structure, an outer-rotor design is generally preferred.
For miniature mechanisms where extremely low inertia and fast dynamic response are the priorities, an inner-rotor design may be more suitable.
Q5: What are the most common failure causes of gimbal frameless motors?
A: Common failure causes include:
Excessive cogging torque causing vibration
Poor assembly concentricity causing rotor-stator contact
Insufficient thermal management during static holding
Overheating and permanent-magnet demagnetization
Improper motor-driver parameter matching
Incorrect control parameters causing attitude oscillation
Conclusion
HOLRY specializes in the R&D and manufacturing of gimbal frameless motors and frameless torque motors.
HOLRY provides both standard and customized gimbal frameless motors, including inner-rotor and outer-rotor flat motor series, for applications such as optoelectronic gimbals, robotic wrist joints, portable exoskeletons, and precision stabilization equipment.
With comprehensive English-language technical documentation and OEM engineering support, HOLRY supports global customers throughout the entire product lifecycle, from prototype development to mass production.
If you are currently selecting a motor for a gimbal drive system, contact the HOLRY technical team with your load conditions, available installation space, and target export market to receive a free motor selection evaluation.
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