A frameless inner rotor permanent magnet motor integrated in a robot joint module

Abstract

The application discloses a frameless inner-rotor permanent magnet motor integrated in a robot joint module, which comprises a stator, a rotor provided radially inside the stator and carrying permanent magnets, and a shell, an inner wall of the shell is provided with a cylindrical sleeve for receiving the stator, the sleeve has a small wall thickness and is provided with a plurality of axial grooves penetrating the wall thickness and extending along the axial direction of the motor, so that the sleeve has radial flexibility and allows the stator to produce limited radial displacement relative to the shell; the stator can be radially moved relative to the shell under the action of electromagnetic force during initial debugging of the motor, so that self-alignment of the air gap between the stator and the rotor is realized; the motor also has a locking structure for fixing the position of the stator after the self-alignment is completed, so that the stator is converted from the alignment state of being capable of moving radially relative to the shell to the locked state of preventing further relative displacement between the stator and the shell during normal operation.

Description

Technical Field

[0001] This invention relates to the field of motors for robots, and in particular to a frameless internal rotor permanent magnet motor integrated into a robot joint module. Background Technology

[0002] Robot joints typically employ frameless motors to achieve compact integration with reducers, bearings, and hollow shafts. In a typical frameless assembly, the stator is rigidly press-fitted or bolted to a metal housing, while the rotor, with surface permanent magnets mounted on its outer periphery, rotates around the stator. However, robot joint housings suffer from machining tolerances, roundness errors, coaxiality deviations, and tolerance accumulation issues due to the assembly of multiple components. These factors can cause air gap eccentricity between the rotor magnets and stator teeth. Air gap eccentricity increases cogging torque, torque pulsation, vibration, and acoustic noise, and can also lead to localized magnetic saturation and winding hotspots.

[0003] Existing solutions rely on strict machining tolerance control, shim adjustment, precision alignment fixtures, or sensor-based active control technologies. These methods not only increase costs and production cycles but also struggle to guarantee consistent performance in mass production. Therefore, there is an urgent need for a frameless motor structure capable of self-correcting installation errors and reliably maintaining its corrected position throughout the motor's entire lifespan, without requiring expensive alignment procedures. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides a frameless internal rotor permanent magnet motor integrated into a robot joint module, which can solve the following technical problems: Existing solutions rely on strict machining tolerance control, shim adjustment, precision alignment tooling, or sensor-based active control technology. These methods not only increase costs and production cycles, but also make it difficult to guarantee performance consistency in mass production.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a frameless internal rotor permanent magnet motor integrated into a robot joint module, comprising a stator, a rotor radially disposed inside the stator and equipped with permanent magnets, and a housing. The inner wall of the housing is provided with a cylindrical sleeve for receiving the stator. The sleeve has a small wall thickness and is provided with multiple axial grooves that penetrate its wall thickness and extend along the motor axis, so that the sleeve has radial flexibility and allows the stator to generate limited radial displacement relative to the housing. The stator can move radially relative to the housing under the action of electromagnetic force during the initial commissioning of the motor, realizing self-alignment of the air gap between the stator and the rotor. The motor also has a locking structure for fixing the position of the stator after self-alignment is completed, so that the stator is transformed from an aligned state that can move radially relative to the housing to a locked state that prevents further relative displacement between the stator and the housing during normal operation.

[0006] As a preferred embodiment of the present invention, the sleeve includes an axial central region and an axial end region. The wall thickness of the axial end region is greater than that of the axial central region, so that the axial central region provides radial flexibility while the axial end region maintains structural strength.

[0007] As a preferred embodiment of the present invention, the sleeve includes 8 to 12 axial grooves, which are arranged at approximately equal angular intervals along the circumference of the sleeve. The axial grooves are distributed along the circumference of the sleeve, while at least one continuous annular portion is retained at the axial end of the sleeve to maintain circumferential integrity.

[0008] As a preferred embodiment of the present invention, the housing further includes an axial positioning shoulder and an anti-rotation structure. The axial positioning shoulder is used to define the axial position of the stator, and the anti-rotation structure is used to prevent relative rotational movement between the stator and the housing.

[0009] A frameless motor assembly method for robot joints, the assembly method being used to assemble a frameless internal rotor permanent magnet motor as described in any one of claims 1 to 4, the assembly method comprising the following steps: S1, assembling the stator into a grooved sleeve inside the robot joint housing, the grooved sleeve having temporary radial flexibility, and restricting the axial displacement of the stator by an axial positioning structure and restricting the circumferential rotation of the stator by an anti-rotation structure, so that the stator retains a controlled radial adjustment degree of freedom while being constrained in the axial and circumferential directions; S2, assemble the rotor carrying the permanent magnet to the radial inner side of the stator, so that the rotor and stator form a motor assembly state; S3, after the rotor and stator are assembled, the motor controller enters the alignment mode and actively runs the motor at a preset low speed and low torque, so that the stator is radially adjusted relative to the housing within the radial displacement range allowed by the slotted sleeve. S4, In alignment mode, monitor at least one parameter used to characterize the air gap state between the stator and rotor, the monitored parameter including at least one of phase current pulsation, back electromotive force harmonic content, vibration amplitude and torque fluctuation. S5, When at least one operating parameter meets the preset alignment judgment condition, the motor controller determines that the alignment is complete and initiates the stator locking operation; S6, lock the stator relative to the housing, changing the stator from an aligned state that allows it to move radially relative to the housing to a locked state that prevents further relative displacement between the stator and the housing during normal operation, and exiting the alignment mode after locking is completed.

[0010] As a preferred embodiment of the present invention, the step of locking the stator includes at least one of the following methods: bonding with adhesive through an injection port on the housing, plastically deforming part of the housing to clamp the sleeve, or mechanically clamping through a wedge structure or annular structure.

[0011] A robot joint actuator includes the aforementioned frameless internal rotor permanent magnet motor. The housing constitutes a structural part of the robot joint and cooperates with a reducer or bearing assembly. Through the self-alignment and locking of the stator, the torque smoothness and positional stability of the robot joint are improved.

[0012] Compared with the prior art, the beneficial effects that this invention can achieve are: 1. By self-aligning the stator after installation, torque smoothness is improved, vibration and acoustic noise are reduced, and the force control accuracy required for collaborative robots, industrial robotic arms and humanoid robot joints is improved without increasing the number of sensors or control complexity. 2. By achieving self-alignment and permanent locking of the stator within the housing during the commissioning phase, this invention reduces the sensitivity to machining tolerances, coaxiality errors, and roundness deviations during robot joint integration. Improved air gap uniformity reduces torque pulsation, cogging torque, and acoustic noise, improves control smoothness, and reduces localized winding heating. This solution reduces or eliminates expensive alignment fixtures, shim adjustments, and precision pressing processes, thereby increasing assembly yield and reducing manufacturing costs. 3. Since this invention mainly relies on passive mechanical structures (grooved sleeve and locking features), it avoids the verification burden and failure modes involved in sensor-based active air gap control, while achieving long-term stable performance suitable for robot work cycles. Attached Figure Description

[0013] Figure 1 A schematic diagram of the traditional assembly of a frameless brushless DC motor; Figure 2 A schematic diagram of the new assembly of a frameless brushless DC motor; Figure 3 For the present invention Figure 2 Enlarged view of a portion of point A in the middle; Figure 4 This is a diagram of the sleeve with axial groove of the present invention; Figure 5 This is a flowchart of the self-alignment assembly process of the present invention; Figure 6 This is a flowchart of the self-alignment and locking control software of the present invention; The components are: 1. Housing; 2. Stator; 3. Winding; 4. Stator slot; 5. Permanent magnet; 6. Rotor; 7. Bearing; 8. Shaft; 9. Sleeve; 10. Air gap; 11. Axial slot. Detailed Implementation

[0014] To make the technical means, creative features, objectives, and effects of this invention easier to understand, the invention is further described below with reference to specific embodiments. However, the following embodiments are merely preferred embodiments of this invention and not all of them. Other embodiments obtained by those skilled in the art based on the embodiments described herein without creative effort are all within the protection scope of this invention.

[0015] Example Please refer to Figures 1-6 As shown, this invention provides a frameless internal rotor permanent magnet motor integrated into a robot joint module. This motor is an internal rotor frameless permanent magnet motor used in robot joint actuators. The rotor 6 is a ring-shaped component with a large inner diameter. Depending on the design, surface permanent magnets 5 are mounted on its inner surface or outer cylindrical surface, and it rotates around a central axis supported by a joint structure integrated bearing 7. The stator 2 includes a laminated iron core and windings 3, and is installed in a metal stator housing 1. The housing 1 also serves as a structural component of the robot joint. The inner wall of the housing 1 has a stator seat for supporting the stator 2.

[0016] Unlike traditional rigid press-fit seats, the stator seat area is designed as a flexible sleeve 9, with a reduced wall thickness compared to adjacent housing areas. The sleeve 9 includes an axial central region and axial end regions. The wall thickness of the axial central region is approximately 1.0–1.2 mm, while the wall thickness of the axial end regions is greater than that of the axial central region. This provides radial flexibility in the axial central region while maintaining structural strength in the axial end regions. Multiple axial grooves 11 are formed on the sleeve 9, extending approximately parallel to the motor axis. These axial grooves 11 divide the sleeve 9 into multiple arc-shaped segments, allowing for controlled radial elastic deformation of the sleeve 9 under lateral forces. The number of axial grooves 11 can be 8 to 16, evenly distributed along the circumference. The length of each axial groove 11 can cover most of the length of the stator laminations, while retaining continuous material at one or both ends to maintain circumferential integrity. This grooved sleeve acts as a spring ring or elastic clamp, allowing for localized small opening or contraction, permitting limited radial displacement of the stator 2 relative to the housing 1, while providing stable support.

[0017] To prevent undesirable degrees of freedom during the temporary flexible phase, housing 1 also includes an axial positioning shoulder and an anti-rotation structure. The axial positioning shoulder limits the axial position of stator 2, and the anti-rotation structure prevents relative rotational movement between stator 2 and housing 1. The end faces of the stator laminations fit against the shoulder, ensuring that stator 2 cannot move axially during operation. In addition, anti-rotation structures (such as keys, grooves, planes, splines, or pins) are provided to prevent stator 2 from twisting relative to housing 1 under electromagnetic torque. Through this design, stator 2 is constrained in both the axial and rotational directions, while maintaining controlled flexibility primarily in the radial direction.

[0018] After the rotor 6 is assembled around the stator 2, the motor needs to undergo a break-in / adjustment process. During this process, the motor operates at low speed and limited torque, allowing the magnetic field of the rotor 6 to interact with the teeth of the stator 2. If there is an eccentricity in the air gap 10, the side with the smaller air gap 10 will generate a larger magnetic attraction and a smaller magnetic reluctance, forming a net radial force component that pulls the stator 2 towards a more concentric position. Due to the temporary flexibility of the stator base, the sleeve 9 will elastically deform, allowing the stator 2 to shift slightly until the magnetic force in the circumferential direction reaches equilibrium, thereby achieving uniform air gap.

[0019] The debugging process terminates when the measured indicators reach the target values. The indicators are as follows: phase current pulsation decreases, back EMF harmonic distortion rate decreases, vibration measured by the accelerometer built into the joint module decreases, or the no-load current stabilizes within the threshold range.

[0020] After self-alignment, the housing 1 permanently locks the position of the stator 2 through the following structure (and can be locked using at least one of the following methods, or other methods can be used to lock and fix the stator 2): 1. First locking method: The housing 1 has a pressable edge or a thin deformable collar at one axial end of the grooved sleeve 9. After commissioning, the edge is plastically deformed (pressed) to clamp the sleeve 9 and the stator 2, converting the previous flexible interface into a rigid interference fit fixing structure.

[0021] 2. Second locking method: Multiple adhesive injection ports are provided on the outer side of the housing 1, extending radially from the outer surface of the housing 1 to the stator seat interface. Shallow circumferential or spiral distribution grooves can be selected to promote uniform flow of adhesive. After commissioning, adhesive is injected through the injection ports to fill at least part of the gap space. Subsequently, a solid adhesive layer is formed by heating or ultraviolet curing, locking the stator 2 in the aligned position. An exhaust structure can be provided to allow air to escape and avoid the formation of gaps.

[0022] 3. Third locking method: The housing 1 is provided with threaded holes and wedge screws or set screws. After alignment, tighten the screws to evenly clamp the sleeve section through the pressure pad; anti-loosening measures (riveting, thread locking agent, locking plate or self-locking screws) can be adopted to ensure the vibration resistance of the robot during the working cycle.

[0023] The assembly and alignment process is assisted by control software executed by the motor controller. For example... Figure 6 As shown, after motor assembly, the control software initiates alignment mode, driving the motor at a preset low speed and low torque, and monitoring at least one alignment parameter related to air gap uniformity. When the monitored parameter meets the preset standard, the control software determines that self-alignment is complete and outputs a control signal to initiate stator locking operation. After the locking operation is completed, the control software exits alignment mode and switches the motor to normal operation mode.

[0024] In a further embodiment, the flexible and locking structures are parametrically optimized so that the permissible radial displacement during the commissioning phase is sufficient to correct for typical tolerance accumulation in the robot joint, while being small enough to avoid instability. For example, the flexible sleeve can be designed to allow radial displacement in the range of ±0.15mm to ±0.30mm under magnetic centering force, while maintaining sufficient stiffness to prevent oscillation. Once locked, the interface exhibits a rigid installation, effectively eliminating radial, axial, and rotational displacements throughout the motor's entire lifespan, even after temperature cycling and vibration.

[0025] This invention can be applied to robot joints that employ harmonic reducers, cycloidal pinwheel reducers, or direct drive joints, and is particularly suitable for designs with large hollow shafts and compact radial dimensions.

[0026] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A frameless internal rotor permanent magnet motor integrated into a robot joint module, comprising a stator (2), a rotor (6) radially disposed inside the stator (2) and equipped with permanent magnets (5), and a housing (1), characterized in that: The inner wall of the housing (1) is provided with a cylindrical sleeve (9) for receiving the stator (2). The sleeve (9) has a small wall thickness and is provided with a plurality of axial grooves (11) that penetrate its wall thickness and extend along the motor axis, so that the sleeve (9) has radial flexibility and allows the stator (2) to generate a limited radial displacement relative to the housing (1). The stator (2) can move radially relative to the housing (1) under the action of electromagnetic force during the initial debugging of the motor, thereby realizing the self-alignment of the air gap (10) between the stator (2) and the rotor (6); The motor also has a locking structure for fixing the position of the stator (2) after self-alignment is completed, so that the stator (2) is changed from an aligned state in which it can move radially relative to the housing (1) to a locked state during normal operation that prevents further relative displacement between the stator (2) and the housing (1).

2. The frameless internal rotor permanent magnet motor integrated into a robot joint module according to claim 1, characterized in that: The sleeve (9) includes an axial central region and an axial end region. The wall thickness of the axial end region is greater than that of the axial central region, so that the axial central region provides radial flexibility while the axial end region maintains structural strength.

3. A frameless internal rotor permanent magnet motor integrated into a robot joint module according to claim 2, characterized in that: The sleeve (9) includes 8 to 12 axial grooves (11) arranged at equal angular intervals along the circumference of the sleeve (9), while at least one continuous annular portion is retained at the axial end of the sleeve (9) to maintain circumferential integrity.

4. A frameless internal rotor permanent magnet motor integrated into a robot joint module according to claim 1, characterized in that: The housing (1) further includes an axial positioning shoulder and an anti-rotation structure. The axial positioning shoulder is used to define the axial position of the stator (2), and the anti-rotation structure is used to prevent relative rotational movement between the stator (2) and the housing (1).

5. A frameless motor assembly method for robot joints, characterized in that: The assembly method is used to assemble the frameless internal rotor permanent magnet motor according to any one of claims 1 to 4, and the assembly method includes the following steps: S1, the stator (2) is assembled into the sleeve (9) inside the robot joint housing (1). The sleeve (9) has temporary radial flexibility and restricts the axial displacement of the stator (2) through the axial positioning structure and restricts the circumferential rotation of the stator (2) through the anti-rotation structure, so that the stator (2) retains the controlled radial adjustment degree of freedom while being constrained in the axial and circumferential directions. S2, the rotor (6) with permanent magnet (5) is assembled to the radial inner side of the stator (2) so that the rotor (6) and the stator (2) form a motor assembly state; S3, after the rotor (6) and stator (2) are assembled, the motor controller enters the alignment mode and actively runs the motor at a preset low speed and low torque, so that the stator (2) is radially adjusted relative to the housing (1) within the radial displacement range allowed by the sleeve (9); S4, in alignment mode, monitor at least one parameter for characterizing the air gap state between the stator (2) and the rotor (6), the parameter including at least one of phase current pulsation, back electromotive force harmonic content, vibration amplitude and torque fluctuation; S5, when at least one parameter meets the preset alignment judgment condition, the motor controller determines that the alignment is complete and starts the locking operation of the stator (2); S6, lock the stator (2) relative to the housing (1), so that the stator (2) is changed from an aligned state in which it can move radially relative to the housing (1) to a locked state in which the stator (2) and the housing (1) are prevented from further relative displacement during normal operation, and exit the alignment mode after locking is completed.

6. A frameless motor assembly method for robot joints according to claim 5, characterized in that: The steps of locking the stator (2) include at least one of the following methods: adhesive bonding through the injection port on the housing (1), plastic deformation of part of the housing (1) to clamp the sleeve (9), or mechanical clamping through a wedge structure or annular structure.

7. A robot joint actuator, comprising a frameless internal rotor permanent magnet motor as described in any one of claims 1 to 4, characterized in that: The housing (1) constitutes the structural part of the robot joint and cooperates with the reducer or bearing assembly. After the stator (2) self-aligns and locks, the torque smoothness and position stability of the robot joint are improved.

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