A hollow dual encoder and angle measurement method

Abstract

This invention relates to the field of encoder technology, and in particular to a hollow dual encoder and an angle measurement method. The PCB stator board has a hollow structure; the PCB rotor board is positioned opposite the PCB stator board; the magnetic encoder unit includes a magnetic sensor mounted on the PCB stator board and a permanent magnet connected to the input shaft, used to detect the angle of the input shaft; the inductive encoder unit includes coils symmetrically distributed on the PCB stator board, used to detect the angle of the output shaft; the magnetic encoder unit and the inductive encoder unit are spatially independent. This invention eliminates magnetic field interference between the two magnetic encoders by employing a combined structure of a magnetic encoder and an inductive encoder; it eliminates gear transmission and directly measures the angles of the input and output shafts, significantly improving system accuracy. The inductive encoder uses a 90° phase difference symmetrical four-coil differential design, enhancing anti-interference capability and error recognition level; combined with the hollow PCB and high integration design, it adapts to the hollow wiring requirements of robot joints.

Description

Technical Field

[0001] This invention relates to the field of encoder technology, and in particular to a hollow dual encoder and an angle measurement method. Background Technology

[0002] In fields such as industrial automation and robotics, high-precision joint modules are core components for achieving precise motion control. Currently, dual-encoder solutions applied to robot joints often employ a combination of two magnetic encoders. Due to the compact internal space of the joint, the sensing elements and magnetic rings of the two encoders must be installed close together, causing mutual interference between their magnetic fields. This results in distortion in the angle measurement outputs of both encoders, severely impacting the system's control accuracy. Secondly, such solutions typically rely on precision gear transmission mechanisms to indirectly transmit the output shaft's motion to the encoders. Machining errors, assembly clearances, and wear after long-term operation of the gears directly contribute to the measurement results as system errors, making it difficult to achieve the theoretically high precision.

[0003] On the other hand, inductive encoders generally use a single coil or simple coil pair design, which has weak resistance to external electromagnetic interference and insufficient measurement stability. Traditional coil designs cannot meet the urgent needs of modern robot joints for miniaturization and ultra-high resolution, and cannot be adapted to the "hollow shaft" design commonly found in robot joints, resulting in difficulties in internal wiring and limiting their integration and application in advanced joint modules.

[0004] Therefore, there is a need for an integrated dual encoder solution that can simultaneously address multiple issues such as magnetic field interference, transmission error, weak anti-interference capability, large size and low resolution, and poor installation adaptability, in order to meet the needs of the current environment. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is that the dual encoder scheme relies on a precision gear transmission mechanism, the dual magnetic encoder is large in size and is prone to mutual interference, the dual inductive encoder has insufficient measurement stability, it is difficult to meet the urgent needs of modern robot joints for miniaturization and ultra-high resolution, and it cannot be adapted to hollow shaft design.

[0006] The above-mentioned technical problems are solved by the following technical solution: This invention proposes a hollow dual encoder, which includes, The PCB stator board adopts a hollow structure; The PCB rotor board is positioned opposite the PCB stator board. The magnetic encoder unit includes a magnetic sensor disposed on the PCB stator and a permanent magnet connected to the input shaft for detecting the angle of the input shaft; An inductive encoder unit includes coils symmetrically distributed on a PCB stator board for detecting the angle of the output shaft; The magnetic encoder unit and the inductive encoder unit are spatially independent.

[0007] In a preferred embodiment of the hollow dual encoder described in this invention: The inductive encoder unit includes four sets of coils symmetrically distributed with a 90° phase difference; The two sets of coils opposite each other form a differential pair, forming two pairs of differential coils: Sin+ / Sin- and Cos+ / Cos-.

[0008] In a preferred embodiment of the hollow dual encoder described in this invention: A decoding chip is installed on one side of the PCB stator board; The decoding chip processes the differential signal output by the inductive encoder unit to identify the angle, rotor eccentricity direction, and tilt error.

[0009] In a preferred embodiment of the hollow dual encoder described in this invention: The magnetic encoder unit and the inductive encoder unit are coaxially connected to the input shaft and the output shaft, respectively, eliminating the need for a gear transmission mechanism.

[0010] An angle measurement method, comprising the aforementioned hollow dual encoder, and, Step 1: The angle of the input shaft is directly detected by the magnetic encoder unit, and the first angle signal is output. Step 2: The angle of the output shaft is directly detected by the inductive encoder unit, and a second angle signal is output. Step 3: Perform signal conditioning and decoding on the first angle signal and the second angle signal, and output the processed input axis angle and output axis angle.

[0011] In a preferred embodiment of the angle measurement method described in this invention: In step 3, Alternating excitation signals are provided to four sets of induction coils, causing eddy currents to be generated in the conductor rotor on the PCB rotor board; An alternating excitation signal is provided to the coil, causing the inductance of each coil on the PCB rotor board to change, and the oscillation frequency of each coil to change. The inductance of the coil changes according to the patterns sinp, sinn, cosp, and cosn, respectively, forming two pairs of differential signals; The oscillation frequency change of the coil is converted into a digital output by the decoding chip. The MCU synthesizes and calibrates the digital signal and analyzes it to obtain the absolute angle value of the output shaft.

[0012] In a preferred embodiment of the angle measurement method described in this invention: The amplitude ratio, phase relationship, and waveform distortion of the two differential voltage signals are analyzed to detect and calculate the eccentricity of the PCB rotor board relative to the ideal position.

[0013] In a preferred embodiment of the angle measurement method described in this invention: The first angle signal and the second angle signal are independently provided to the motion controller to achieve high-precision dual closed-loop position control.

[0014] The beneficial effects of this invention are as follows: by adopting a combined structure of a magnetic encoder and an inductive encoder, the magnetic field interference between the two magnetic encoders is fundamentally eliminated; gear transmission is abandoned, and the input and output shaft angles are directly measured, significantly improving the system's accuracy and reliability. The inductive encoder adopts a 90° phase difference symmetrical four-coil differential design, which greatly enhances the anti-interference capability and error recognition level; combined with a hollow PCB and high integration design, miniaturization and high resolution are achieved while adapting to the hollow wiring requirements of robot joints. Attached Figure Description

[0015] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings of the embodiments of the present invention will be briefly described below. Obviously, the drawings described below only relate to some embodiments of the present invention and are not intended to limit the present invention.

[0016] Figure 1 The hardware framework diagram of the hollow dual encoder in this invention is shown.

[0017] Figure 2 The diagram shows the planar spatial distribution of the PCB stator board of the hollow dual encoder in this invention.

[0018] Figure 3 The circuit schematic of the inductor decoding chip in this invention is shown.

[0019] Figure 4 The circuit diagram of the magnetic decoding chip in this invention is shown.

[0020] Figure 5 The circuit diagram of the external communication and power interface of the dual decoding chip in this invention is shown.

[0021] Figure 6 A diagram showing the connection relationship between the coil and the decoding chip in this invention is provided. Detailed Implementation

[0022] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0023] The terminology used in this invention refers to those general terms currently widely used in the art in consideration of the functionality of the invention; however, these terms may vary according to the intent of those skilled in the art, precedent, or new techniques. Furthermore, specific terms may be chosen independently, and in such cases, their detailed meanings will be described in the detailed description of the invention. Therefore, the terminology used in this specification should not be construed as simple names, but rather based on the meaning of the terms and the overall description of the invention.

[0024] Reference Figures 1-2 This embodiment provides a hollow dual encoder.

[0025] One of the conventional choices in the existing technology is the dual magnetic encoder. This design is simple and easy to lay out, but it is prone to mutual interference or magnetic field interference. The circuit size is small, and the magnetic fields of the two magnets in the traditional dual magnetic encoder interfere with each other, making it difficult to improve accuracy.

[0026] Another common choice is a dual-inductor encoder. This design is symmetrical and has strong anti-interference capabilities, but the coils are large, and the chip circuitry can only be placed on the outside, further increasing the PCB area. Furthermore, the dual-coil structure requires increased board thickness to prevent mutual interference between the coils, further increasing the overall encoder thickness. Because the dual-coil layout forces the chip and other components to be placed outwards, the overall diameter is large, the coils cannot fully utilize the existing diameter, resulting in a large volume and height.

[0027] In this design, a combination of magnetic encoder unit 3 and inductive encoder unit 5 is adopted. The parameters of this structure are designed as follows: low height, small diameter, and smaller overall plate diameter and volume for the same size coil.

[0028] The total height of a traditional dual-inductor encoder is as follows:

[0029] The PCB stator board 1, serving as the fixed base plate of the entire encoder, is characterized by its hollow disc structure with a central through-hole, allowing the robot joint's motor shafts and cables to pass through, meeting the requirements for hollow wiring. The PCB stator board 1 is made of FR-4 material using a multi-layer circuit board process, with wiring circuits and electronic components integrated on the surface and inner layers. The PCB rotor board 2, as the rotating component, corresponds in shape to the PCB stator board 1 and also has a hollow structure. The PCB rotor board 2 is fixedly connected to the output shaft 6 through a central hole and is positioned on a plane parallel to the PCB stator board 1, maintaining a small air gap between them to achieve non-contact measurement.

[0030] The magnetic encoder unit 3 is used to detect the rotation angle of the input shaft 4. It mainly consists of two parts: a permanent magnet 32, which is a single-pole ring-shaped permanent magnet, fixedly sleeved at the end of the input shaft 4 and rotates synchronously with it; and a magnetic sensor 31, which is a TMR angle sensor chip integrating signal processing functions. The magnetic sensor 31 is surface-mounted to a specific position on the PCB stator board 1, with its sensitive surface facing the magnetic pole surface of the permanent magnet 32 ​​and within the effective magnetic field range. During operation, the rotating permanent magnet 32 ​​generates a spatially varying magnetic field. The fixed-position magnetic sensor 31 detects the change in the direction of this magnetic field and directly outputs a digital signal corresponding to the absolute angle of the input shaft 4.

[0031] The inductive encoder unit 5 is used to detect the rotation angle of the output shaft 6. The core sensing component is a coil 51 fabricated on the PCB stator board 1. The coil 51 is etched from copper foil on the PCB and is distributed symmetrically in a ring. In this embodiment, four independent coils 51 are used, which are evenly arranged in the circumferential direction with the encoder center as the center, and adjacent coils are spaced 90 degrees apart. These four coils 51 together form an induction coil array.

[0032] On the surface of the PCB rotor board 2 facing the PCB stator board 1, there is a closed ring conductor made of copper foil serving as the target rotor for inductive induction. During operation, the PCB stator board 1 provides a high-frequency AC signal to the coil 51, generating an alternating magnetic field. This magnetic field induces eddy currents in the conductor of the PCB rotor board 2. When the output shaft 6 drives the PCB rotor board 2 to rotate, the distribution of the eddy current field changes, thereby altering the equivalent inductance of the coil 51 on the PCB stator board 1, and consequently changing the oscillation frequency of the coil 51. The decoding chip 7 measures the corresponding parameters of the coil 51, and after processing, can obtain the digital calculation of the absolute angle of the output shaft 6.

[0033] The magnetic sensor 31 and related circuitry are arranged in one area of ​​the PCB stator board 1, while the coil array 51 of the inductive encoder 5 is arranged in another area. Sufficient distance is maintained between them. The magnetic field of the permanent magnet 32 ​​primarily acts on the magnetic sensor 31 area, having negligible impact on the inductive coil 51 area located far away. Simultaneously, the high-frequency electromagnetic field generated by the inductive encoder 5 during operation is confined to the vicinity of the coil 51, preventing interference with the magnetic sensor 31's detection of the static magnetic field direction. This layout, based on fundamental differences in principle and physical isolation, ensures that the magnetic fields of the two encoder units do not interfere with each other during operation.

[0034] This embodiment provides a compact, low-interference hollow dual encoder. The hollow PCB stator board 1 meets the installation and wiring requirements. The magnetic encoder unit 3 and the inductive encoder unit 5 measure the angles of the input axis 4 and the output axis 6 independently and directly. The spatial separation layout effectively solves the magnetic interference problem between traditional dual magnetic encoders, providing a highly reliable dual-axis angle measurement solution for robot joints and other scenarios.

[0035] As one embodiment provided, such as Figures 1-2 , The rotor shape designed in this invention is mainly based on the angle calculated from the sin+cos signal. The sin and cos signals are used as differential signal pairs, which are divided into sinp, sinn, cosp, and cosn. Therefore, four pairs of coils are designed.

[0036] The design could be optimized to use a 120° phase difference, using three pairs of differential signals, which are then spatially transformed into sin and cos signals to better suppress odd harmonic noise. However, this optimization would increase computational complexity. Currently, the angle analysis algorithm is based on MCU operation, so to pursue simpler and faster computation, a 90° phase difference is chosen instead of a 120° phase difference.

[0037] The coil 51 of the inductive encoder unit 5 consists of four independent fan-shaped coils, which are symmetrically distributed around the hollow center on the PCB stator board 1 with a 90° phase difference.

[0038] The four sets of coils 51 are arranged clockwise as coil A, coil B, coil C, and coil D. The center line of coil A is exactly 180 degrees away from the center line of coil C; similarly, the center lines of coil B and coil D are also 180 degrees apart. At the same time, the angle between the center lines of coil A and coil B is 90 degrees, forming a precise 90-degree phase difference symmetrical relationship.

[0039] In terms of electrical connection, coils A and C, which are positioned opposite each other, are connected in parallel to serve as the Sin+ and Sin- signal terminals, forming the first differential pair. Similarly, coils B and D are connected in parallel to serve as the Cos+ and Cos- signal terminals, forming the second differential pair.

[0040] When the conductor target on the PCB rotor board 2 rotates, the two coils 51 in the same differential pair, being in symmetrical positions, induce frequency signals with nearly equal amplitudes but opposite phases. External common-mode electromagnetic interference will couple to both coils 51 simultaneously and in phase. In subsequent signal processing, by calculating the difference between the signals of the two coils 51, the common-mode interference can be largely canceled out, significantly improving the signal-to-noise ratio and measurement stability.

[0041] Meanwhile, the two pairs of differential signals maintain a 90-degree phase relationship, providing a basis for high-precision angle calculation. A dedicated decoding chip 7 is located in the non-coil area on one side of the PCB stator board 1. The decoding chip 7 is preferably a high-resolution inductive angle decoding dedicated integrated circuit.

[0042] The decoding chip 7 is connected to the output terminals of the two pairs of differential coils 51 and the magnetic sensor 31 of the magnetic encoder unit 3 via fine traces on the PCB.

[0043] For the inductive encoder unit 5, the decoding chip 7 first contains two high-precision differential amplifiers to amplify the weak signals from the Sin and Cos differential pairs, respectively. The two amplified quadrature analog signals are then sampled by a high-speed analog-to-digital converter (ADC) and converted into digital signals.

[0044] The digital signal processor (DSP) inside the decoding chip 7 performs arctangent calculations and angle tracking on the two digital signals, ultimately calculating the high-resolution absolute angle value of the output shaft 6. The algorithm of the decoding chip 7 can monitor the amplitude balance and orthogonality of the two differential signals. When the PCB rotor board 2 is eccentric or tilted due to installation tolerances, it will cause asymmetrical changes in the induction intensity of the four coils 51, which will be reflected in the amplitude or phase relationship of the Sin and Cos signals.

[0045] By analyzing this characteristic change, the decoding chip 7 can identify the rotor's eccentricity direction and estimate the tilt error in real time. This error information can be output together for the control system to compensate.

[0046] The magnetic encoder unit 3 and the inductive encoder unit 5 respectively realize direct measurement with the corresponding shaft, completely eliminating the gear transmission mechanism in the traditional solution.

[0047] The permanent magnet 32 ​​is directly fixed to the end of the input shaft 4, and the magnetic sensor 31 is fixed on the PCB stator board 1. The angle change of the input shaft 4 is directly reflected by the magnetic field change of the permanent magnet 32 ​​and directly picked up by the magnetic sensor 31, without any mechanical transmission links in between.

[0048] The output shaft 6 is directly connected to the PCB rotor board 2. The conductor target on the PCB rotor board 2 and the coil 51 on the PCB stator board 1 form an inductive coupling pair. The rotation angle of the output shaft 6 directly represents the change in the spatial position of the conductor target relative to the coil array, and can be detected directly through electromagnetic coupling, without the need for gear transmission.

[0049] This direct coaxial connection method completely eliminates transmission chain errors caused by gear machining errors, assembly clearances, meshing backlash, and long-term wear. This allows the angle measurement results of the input shaft 4 and the output shaft 6 to directly and accurately reflect the physical position of the shafts, thereby achieving extremely high absolute measurement accuracy and system rigidity.

[0050] As one embodiment provided, such as Figures 1-6 , The angle measurement method is a real-time, cyclical process that mainly includes three logical steps. Step S1 involves acquiring the angle from the magnetic encoder. The magnetic encoder unit 3 directly and non-contactly detects the real-time rotation angle of the input shaft 4 and outputs the corresponding first angle signal.

[0051] Step 2 involves acquiring the angle and error information from the inductive encoder. The inductive encoder unit 5 directly and non-contactly detects the real-time rotation angle of the output shaft 6 and outputs a second angle signal containing this angle information. Step 2 also deeply integrates key aspects of subsequent signal processing.

[0052] Step 3 involves signal processing and output. The first and second angle signals are conditioned, amplified, filtered, and decoded respectively, ultimately outputting the processed, high-precision digital angle values ​​for input axis 4 and output axis 6.

[0053] In steps 2 and 3, the decoding chip 7 and its driving circuit provide an alternating excitation signal of a specific frequency to the excitation coil of the inductive encoder unit 5. The alternating magnetic field passes through the air gap between the PCB stator board 1 and the PCB rotor board 2, and acts on the conductor rotor of the PCB rotor board 2, causing eddy currents to be generated on the surface.

[0054] During differential signal acquisition, the output shaft 6 rotates, and the changing eddy current field modulates the electromagnetic coupling state of the four sets of induction coils 51. The decoding chip 7 synchronously acquires the original induced voltage output by the two pairs of differential coils 51, Sin+ / Sin- and Cos+ / Cos-.

[0055] Due to the 90° symmetrical differential design of the coil, these two signals are essentially two pairs of differential sinusoidal signals whose amplitudes vary with the angle as sinusoidal and cosine, and whose phase difference is 90 degrees.

[0056] The decoding chip 7 contains two high-precision differential amplifiers that amplify the two signals respectively. The core benefit of differential amplification is that it can significantly cancel out common-mode noise and environmental electromagnetic interference contained in the two signals, thereby significantly improving the signal-to-noise ratio and anti-interference capability. The amplified signals can be further low-pass filtered to eliminate high-frequency noise. The two orthogonal analog signals, after conditioning, are converted into digital signals by a high-speed analog-to-digital converter (ADC). The digital signal processor (DSP) inside the decoding chip 7 can perform angle calculation algorithms on these digital signals to calculate the absolute angle value of the output shaft 6 in real time.

[0057] The algorithm module of decoding chip 7 continuously analyzes the characteristic parameters of the two differential voltage signals. When there is no eccentricity, the peak amplitudes of the Sin and Cos channels should be approximately equal. When eccentricity occurs, the coupling of coil 51 in a certain direction will be enhanced, causing the corresponding differential pair signal amplitude to become unbalanced.

[0058] By monitoring the deviations of these parameters and combining them with the pre-stored calibration model, the decoding chip 7 can detect and calculate the eccentricity of the PCB rotor board 2 relative to the ideal rotation center.

[0059] After processing, the angles of input axis 4 and output axis 6 are transmitted separately and independently to the motion controller of the robot joint via a high-speed communication interface.

[0060] The motion controller utilizes these two sets of independent feedback to construct an advanced dual-closed-loop position control system. The inner loop uses data from input shaft 4 to perform precise current and speed control on the motor; the outer loop uses data from output shaft 6 to perform final position control on the load.

[0061] Simultaneously, by calculating the difference between the two in real time—output data minus input data—the controller can accurately sense the actual torsional angle of the joint, enabling advanced functions such as stiffness-based control and collision detection. Independent dual-channel signal outputs ensure both flexibility and precision in control.

[0062] Finally, it should be noted that the methods and devices described in detail above are merely embodiments, and those skilled in the art can modify these embodiments in different ways as long as they do not depart from the scope of the present invention.

Claims

1. A hollow dual encoder, characterized in that: include PCB stator board (1) adopts a hollow structure; The PCB rotor board (2) is positioned opposite to the PCB stator board (1); The magnetic encoder unit (3) includes a magnetic sensor (31) disposed on the PCB stator board (1) and a permanent magnet (32) connected to the input shaft (4) for detecting the angle of the input shaft (4); The inductive encoder unit (5) includes coils (51) symmetrically distributed on the PCB stator board (1) for detecting the angle of the output shaft (6); The magnetic encoder unit (3) and the inductive encoder unit (5) are spatially independent.

2. The hollow dual encoder according to claim 1, characterized in that: The inductive encoder unit (5) includes four sets of coils (51) symmetrically distributed with a 90° phase difference. The two sets of coils (51) opposite each other form a differential pair, forming two pairs of differential coils (51) of Sin+ / Sin- and Cos+ / Cos-.

3. The hollow dual encoder according to claim 1, characterized in that: A decoding chip (7) is set on one side of the PCB stator board (1); The decoding chip (7) processes the differential signal output by the inductive encoder unit (5) to identify the angle, rotor eccentricity direction and tilt error.

4. The hollow dual encoder according to claim 3, characterized in that: The magnetic encoder unit (3) and the inductive encoder unit (5) are coaxially connected to the input shaft (4) and the output shaft (6) respectively, without the need for a gear transmission mechanism.

5. An angle measurement method, employing the hollow dual encoder as described in claim 3, characterized in that, Includes the following steps: Step 1: The angle of the input shaft (4) is directly detected by the magnetic encoder unit (3), and the first angle signal is output; Step 2: The angle of the output shaft (6) is directly detected by the inductive encoder unit (5), and a second angle signal is output; Step 3: Perform signal conditioning and decoding on the first angle signal and the second angle signal, and output the processed input axis (4) angle and output axis (6) angle.

6. The angle measurement method according to claim 5, characterized in that: In step 3, Alternating excitation signals are provided to the four sets of induction coils (51) to generate eddy currents in the conductor rotor on the PCB rotor board (2); An alternating excitation signal is provided to the coil (51), causing the inductance of each coil (51) on the PCB rotor board (2) to change, and the oscillation frequency of each coil (51) to change. The inductance of coil (51) changes according to sinp, sinn, cosp, and cosn patterns, respectively, forming two pairs of differential signals; The oscillation frequency change of the coil (51) is converted into a digital output by the decoding chip (7), and the digital output is synthesized and calibrated by the MCU to obtain the absolute angle value of the output shaft (6).

7. The angle measurement method according to claim 6, characterized in that: The amplitude ratio, phase relationship and waveform distortion of the two differential voltage signals are analyzed, and the eccentricity of the PCB rotor board (2) relative to the ideal position is detected and calculated.

8. The angle measurement method according to claim 7, characterized in that: The first angle signal and the second angle signal are provided independently to the motion controller to achieve dual closed-loop position control.

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