High-precision self-sensing micro steering engine device

Abstract

The application discloses a high-precision self-sensing micro steering engine device and belongs to the technical field of precise driving intelligent control of micro robots. In the application, a micro groove is processed in the tooth root of a high-load gear of a reduction gear train of a steering engine, and a PVDF piezoelectric film is directionally embedded as a built-in torque sensing unit. When a gear transmits a torque, the tooth root generates elastic strain, and the PVDF outputs a charge signal linearly related to the torque through a piezoelectric effect. Two dynamic signal leading schemes of a slip ring and a passive wireless device are matched. A control end collects the charge signal to convert a real-time torque, and torque-position double-loop force-position hybrid control is realized in combination with an angle signal. The application does not need an external sensor, does not additionally increase the volume and weight of the steering engine, has high torque detection linearity, fast dynamic response and anti-electromagnetic interference, is suitable for micro precise driving scenes such as dexterous hands, micro air vehicles and micro manipulators, and can realize joint force feedback, collision detection and lossless grabbing control.

Description

Technical Field

[0001] This invention relates to the fields of micro-robots, precision drive and intelligent control technology, and specifically to a high-precision self-sensing micro servo motor device. Background Technology

[0002] A servo motor is a position (angle) servo actuator suitable for control systems that require continuous angle changes and maintenance. It is now widely used in model aircraft and industrial automation. As the application range of servo motors expands, higher demands are being placed on their size, weight, and control precision in high-precision manufacturing, high-end toys, and robotics. When using miniature servo motors with self-sensing capabilities, areas for improvement include: Dexterous hands and other end effectors typically require multiple miniature servos to drive the finger joints. To achieve precise grasping and force control, the output torque of each servo must be known in real time. Traditional methods suffer from the following problems: External torque sensors are bulky, costly, and difficult to install, making them hard to integrate into micro servos with a diameter of only about 10mm.

[0003] Motor current detection method: Affected by factors such as friction, temperature rise, and nonlinear back electromotive force, the torque estimation error is very large at low speed and when stalled, which cannot meet the precise force control requirements of dexterous hands.

[0004] Torque detection based on strain gauges: Strain gauges are attached to gears or output shafts, but require independent power supply and signal conditioning, and the micro strain gauges are difficult to attach and position on gears, resulting in low reliability.

[0005] Existing technologies lack a solution for directly sensing torque using piezoelectric thin films on tiny transmission gears, thus failing to achieve "self-sensing of the gear itself". Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides the following technical solution: a high-precision self-sensing micro servo motor device, comprising: Miniature DC motor (diameter ≤ 6mm); Multi-stage reduction gear system (reduction ratio 50:1 to 200:1); The servo motor output shaft is fixedly connected to the final stage gear. The control circuit board integrates motor drive, signal processing, and communication modules. The feature is that a PVDF piezoelectric thin film torque sensing unit is integrated on a gear that transmits a large torque at a certain stage of the gear system (preferably the last stage metal gear).

[0007] 4.2.2 Core Technology – Gear Microgroove Machining and PVDF Embedding (1) Gear parameters and microgroove structure The gears are made of metal (preferably beryllium copper or stainless steel), with a pitch circle diameter D=8mm, a module of 0.3~0.5, and 20~26 teeth.

[0008] Microgrooves are machined in the root circle region of the gear along the direction of the greatest bending stress at the root (i.e., perpendicular to the tooth surface and along the tangent of the root arc).

[0009] Microgroove width: 0.3mm; Microgroove depth: 0.12~0.18mm (slightly larger than the PVDF film thickness); Microgroove length: Covers the root portion of 1 to 3 consecutive teeth, typically a segment of an arc in the circumference of the gear (e.g., covering a 90° central angle range).

[0010] The microgrooves are achieved through precision micromilling or laser processing, with a surface roughness Ra≤0.4μm at the bottom of the groove to ensure good adhesion of the PVDF film without damage.

[0011] (2) Oriented embedding of PVDF films PVDF film thickness: 28μm or 50μm (selected according to sensitivity requirements), cut into thin strips that match the shape of the microgroove.

[0012] Key orientation requirements: PVDF films must exhibit piezoelectric anisotropy, and their mechanical stretching direction (i.e., piezoelectric coefficient d) must be within the specified range. 31 The maximum orientation direction must be aligned with the principal strain direction of the force applied to the gear tooth root.

[0013] When a gear transmits torque, the tooth root undergoes bending deformation, and the principal strain direction is the tangent of the tooth root arc (that is, along the tangent direction of the gear circumference, perpendicular to the radial direction of the tooth root arc).

[0014] Therefore, the orientation of the PVDF film should be embedded in the microgroove along the tangent direction of the tooth root arc.

[0015] During embedding, an extremely thin layer of conductive silver paste is coated on the bottom of the microgroove, and then the PVDF film is placed in according to the orientation requirements. Air bubbles are gently pressed out, and then conductive silver paste or gold electrodes are sputtered onto the surface of the film to form the upper and lower electrodes.

[0016] Finally, cover the surface with an insulating protective layer (such as parylene C, 5-10 μm thick) to prevent short circuits and wear.

[0017] (3) Signal extraction method Since the gears rotate continuously during operation, the problem of dynamic signal transmission needs to be solved. This invention provides the following two preferred solutions: Option 1 (Conductive Slip Ring Type): Install a miniature conductive slip ring at the gear shaft center, and connect the two electrode leads of the PVDF to the charge amplifier at the fixed end through the slip ring.

[0018] Option 2 (Passive Wireless): A miniature flexible circuit board is integrated onto the gear, containing an ultra-low power charge amplifier, an analog-to-digital converter, and an RFID radio frequency transmitter chip; it is powered by an external coil and wirelessly transmits the torque signal to the servo motor's main control board. This solution avoids slip ring friction loss and contact noise.

[0019] 4.2.3 Self-sensing principle When the micro motor drives the servo motor output shaft to rotate and output torque through the gear system, the root of the gear embedded in PVDF (such as the last stage gear) produces micron-level elastic bending deformation (typical deformation of 1 to 10 μm, which is proportional to the output torque).

[0020] The PVDF film deforms along with the gear tooth root. Since its orientation direction is completely consistent with the principal strain direction, the piezoelectric effect is significant, generating a charge Q between the upper and lower electrodes of the film that is proportional to the strain magnitude.

[0021] According to the piezoelectric equation: Q=d31·σ·A Where d31 is the piezoelectric coefficient of PVDF (approximately 23 pC / N), σ is the average stress at the tooth root, and A is the effective area of ​​the film.

[0022] Since σ is linearly related to the torque M transmitted by the gear (within the elastic range), the charge Q is equal to the output torque M.

[0023] The charge signal is converted into a voltage signal Vout=Q / Cf (where Cf is the feedback capacitor) by a charge amplifier, and then the torque value is obtained through calibration. M = k·Vout, where k is the calibration coefficient (unit: N·m / V).

[0024] 4.2.4 Control Methods This invention also provides a self-sensing control method based on the above-mentioned micro servo motor, comprising the following steps: Initialization and calibration: After power-on, perform zero torque calibration on the servo motor (no-load rotation, record zero drift) and store the calibration coefficient k.

[0025] Real-time signal acquisition: The output voltage of the PVDF charge amplifier is read at a sampling rate of ≥1kHz, and high-frequency noise is removed by low-pass filtering (cutoff frequency 500Hz) to obtain the real-time torque value M_raw.

[0026] Torque-position hybrid control: In position control mode, M_raw is used as the feedback value. When the torque exceeds the set threshold (such as a sudden change in force when contacting an object), it automatically switches to torque closed-loop mode or force-position hybrid mode.

[0027] In torque closed-loop mode, the difference between the desired torque M_ref and M_raw is used by the PID controller to adjust the motor drive voltage, thereby achieving precise force output.

[0028] At the same time, it retains the angle signal read by traditional potentiometers or magnetic encoders to achieve a dual closed loop of "angle-torque".

[0029] Application in collaborative control of dexterous hands: Torque signals from multiple micro servos are uploaded to the dexterous hand's main controller via a bus (such as I2C or CAN). The main controller determines the gripping force distribution based on the torque information of each joint and adjusts the target torque of each servo to achieve stable and non-destructive gripping. Beneficial effects

[0030] This invention discloses a high-precision self-sensing micro servo motor device, which has the following beneficial effects: 1. Truly "self-sensing": No external torque sensor is required. The sensing function is integrated directly into the transmission gear inside the servo motor, without adding extra size and weight. It is especially suitable for micro servos with a diameter of 8 to 12 mm.

[0031] 2. High sensitivity and high linearity: The PVDF film directly senses the micron-level deformation of the tooth root, and the charge output has a good linear relationship with the torque (nonlinear error <3%), which is far superior to the current detection method. 3. Fast dynamic response: The piezoelectric response of PVDF is in the microsecond range, which can detect torque fluctuations up to 1kHz and above, meeting the needs of dexterous hands for rapid grasping and tactile feedback.

[0032] 4. Strong anti-interference capability: The signal is a charge quantity, which is transmitted through high-quality cables or wirelessly and is not severely affected by the electromagnetic noise of the motor.

[0033] 5. Feasibility of the process: The current micro-machining capabilities are sufficient to create a 0.3mm microgroove on an 8mm diameter gear, and both PVDF directional embedding and electrode lead-out are supported by mature MEMS packaging technology.

[0034] 6. Enhance the intelligence of dexterous hands: Directly obtain the torque of each joint to realize functions such as force closed-loop control, grasping pattern recognition, and collision detection, significantly improving the dexterity and safety of dexterous hands. Attached Figure Description

[0035] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1This is an exploded view of the overall structure of the miniature servo motor of the present invention; Figure 2 A magnified view of the final stage gear with integrated PVDF film, with microgrooves, PVDF orientation, and principal strain direction at the tooth root marked. Figure 3 A schematic diagram of the cross-sectional structure of a PVDF thin film embedded in a microgroove; Figure 4 Block diagram of PVDF torque signal conditioning circuit; Figure 5 This is a flowchart of the torque-position dual closed-loop control of the present invention; Figure 6 This is a schematic diagram of the servo motor arrangement and signal interaction of the present invention applied to a three-finger dexterous hand.

[0036] In the diagram: 1-Miniature DC motor; 2-Multi-stage reduction gear system; 3-Final stage metal gear; 4-Microgroove; 5-PVDF piezoelectric film; 6-Conductive silver paste electrode; 7-Parylene insulating protective layer; 8-Miniature conductive slip ring / wireless flexible PCB; 9-Servo output shaft; 10-Control circuit board; 11-Charge amplifier; 12-ADC analog-to-digital converter module; 13-Main control MCU; 14-Angle detection unit; 15-Motor drive module. Detailed Implementation

[0037] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0038] Example 1 Please see Figures 1-5 , This invention provides a high-precision self-sensing micro servo motor device. A high-precision self-sensing micro servo motor device, comprising a micro DC motor 1, a multi-stage reduction gear system 2, a servo motor output shaft 9, and a control circuit board 10, wherein the micro DC motor 1 has a diameter ≤ 6mm, the multi-stage reduction gear system 2 has a reduction ratio of 50:1 to 200:1, the servo motor output shaft 9 is fixedly connected to the final stage metal gear 3 of the reduction gear system 2, and the control circuit board 10 integrates a motor drive module 15 and a signal processing and communication module; characterized in that: The final stage metal gear 3 in the reduction gear system 2, which bears the maximum transmitted torque, has an arc-shaped microgroove 4. A PVDF piezoelectric film 5 torque sensing unit is embedded in the microgroove 4. The piezoelectric sensitive orientation of the PVDF piezoelectric film 5 is consistent with the direction of the principal strain of the gear tooth root. Conductive silver paste electrodes 6 are prepared on the upper and lower surfaces of the PVDF piezoelectric film 5. The conductive silver paste electrodes 6 are connected to a dynamic signal lead-out structure 8 to output a charge signal that is proportional to the transmitted torque of the gear.

[0039] The final stage metal gear 3 is a beryllium copper or stainless steel metal gear with a pitch circle diameter of 8 mm, a module of 0.3 to 0.5, and 20 to 26 teeth. The micro-groove 4 is formed in the root circle region of the final stage metal gear 3, extending tangentially along the root arc. The micro-groove 4 has a width of 0.3 mm, a depth of 0.12 to 0.18 mm, and a length covering 1 to 3 consecutive tooth roots. The surface roughness Ra of the groove bottom is ≤0.4 μm.

[0040] The PVDF piezoelectric film 5 is a uniaxially stretched film with a thickness of 28 μm or 50 μm; the bottom of the microgroove 4 is coated with conductive silver paste electrode 6 to bond the PVDF piezoelectric film 5 substrate, and the upper surface of the PVDF piezoelectric film 5 is coated with conductive silver paste or sputtered with gold layer to form the upper electrode; the sensing area of ​​the PVDF piezoelectric film 5 is completely covered with a 5-10 μm parylene insulating protective layer 7.

[0041] The dynamic signal output structure 8 is a miniature conductive slip ring; the PVDF piezoelectric film 5 and the conductive silver paste electrode 6 are connected to the slip ring moving ring, and the slip ring stator leads are connected to the fixed-end charge amplifier 11.

[0042] The dynamic signal output structure 8 is a passive wireless transmission module; the side of the final stage metal gear 3 is fixed with a flexible PCB, which integrates a charge amplifier 11, an ADC analog-to-digital converter unit 12 and an NFC radio frequency chip; the servo housing is equipped with an induction coil, which is passively powered by a 13.56MHz radio frequency field and wirelessly transmits torque signals.

[0043] The PVDF piezoelectric film 5 generates a charge under force that satisfies the piezoelectric equation Q=d_{31}·σ·A\. The charge is converted into a voltage signal by the charge amplifier 11, and the servo motor output torque M=k·V_{out}\ is obtained by conversion through calibration coefficients.

[0044] The control method of the self-sensing micro servo motor includes the following steps: S1 Power-on initialization and zero-point calibration: Drive the servo motor to rotate reciprocally under no-load conditions, collect the zero-torque voltage of the PVDF piezoelectric film 5 as the zero-point reference, and store the torque calibration coefficient; S2 Signal acquisition and noise reduction: Read the output voltage of the charge amplifier 11 at a sampling rate of ≥1kHz, remove the high-frequency noise of the micro DC motor 1 by 500Hz low-pass filtering, and obtain the real-time raw torque value; S3 Dual closed-loop signal fusion: Synchronously acquire the angle signal output by the servo motor angle detection unit 14, and construct the angle-force... Torque dual closed-loop control architecture; S4 adaptive mode switching: In position control mode, torque is monitored in real time. When the torque exceeds the preset threshold, it automatically switches to torque closed-loop or force-position hybrid mode; In torque closed-loop mode, the difference between the expected torque and the measured torque is adjusted by the PID motor drive module 15 to output voltage; S5 multi-servo cooperative grasping control: The torque data of multiple servos is uploaded to the dexterous hand main control MCU13 through I2C or CAN bus. The main control MCU13 adjusts the target torque of a single servo according to the torque distribution of each joint to achieve stable and non-destructive grasping, sliding, and collision detection.

[0045] The working principle of the above technical solution is explained below: Miniature servos for the finger joints of dexterous hands Servo specifications: Dimensions 12mm×15mm×8mm, rated voltage 3.7V, stall torque 0.15 N·m. The reduction gear system is a four-stage reduction gear, with the final stage gear made of stainless steel, pitch circle diameter 8mm, number of teeth 24, and module 0.33.

[0046] Production steps: An arc-shaped micro-groove with a width of 0.30 mm, a depth of 0.15 mm, and a length covering three tooth roots was machined in the root circle region of the final stage gear using a precision micro-milling machine.

[0047] A 28 μm thick uniaxially stretched PVDF film (d31 = 23 pC / N) was laser-cut into strips with the same shape as the microgrooves. The microgrooves were cleaned with alcohol, and conductive silver paste (thickness < 5 μm) was coated on the bottom of the grooves.

[0048] Under a microscope, align the stretching direction of the PVDF film (the orientation marked by the manufacturer) with the tangent of the gear tooth root arc, place it in the microgroove, and press gently. Bake at 80°C for 30 minutes to cure the silver paste.

[0049] Conductive silver paste was coated onto the surface of the PVDF film to form the upper electrode, and an enameled wire with a diameter of 0.05 mm was led out. Then, a Parylene C protective layer (8 μm thick) was coated over the entire sensing area.

[0050] Two leads are connected to the moving ring of the miniature conductive slip ring at the gear shaft, and the slip ring stator leads are connected to the charge amplifier (using OPA129 op-amp, feedback capacitor 10pF, feedback resistor 100GΩ).

[0051] Assemble the servo motor and perform torque calibration: Using a precision torque sensor, apply load torques of 0, 0.02, 0.04, 0.06, 0.08, 0.10, and 0.12 N·m to the servo motor respectively, record the output voltage of the charge amplifier, and fit the linear relationship M = 0.012·V_out (unit: N·m, V_out unit: V).

[0052] The torque signal is connected to the servo control MCU (STM32G031) and used together with the potentiometer angle signal for force-position hybrid control.

[0053] Test results: Within the range of 0–0.12 N·m, the torque detection error is < ±0.003 N·m, with a bandwidth of 800 Hz. When this servo is installed near the knuckle of the index finger of a dexterous hand, it can stably grasp an egg (with a set grasping force of 0.05 N·m) without breaking it, and can detect slippage tendency through torque feedback.

[0054] Example 2 Passive wireless signal extraction solution To avoid slip ring wear, another implementation employs wireless power supply and transmission. A ring-shaped flexible PCB is fixed to the side of the gear, integrating a charge amplifier (MAX4239), a 12-bit ADC (AD7091), and an NFC RF chip (NT3H2111). An external read / write coil is mounted inside the servo housing, powering the circuitry on the gear and reading torque data via a 13.56MHz RF field. Due to the slow gear rotation speed (maximum 10 rpm), the wireless communication update rate can reach 500Hz, fully meeting the control requirements of a dexterous hand.

[0055] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the scope of the invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0056] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A high-precision self-sensing micro servo device, comprising a micro DC motor (1), a multi-stage reduction gear system (2), a servo output shaft (9), and a control circuit board (10), wherein the micro DC motor (1) has a diameter ≤ 6 mm, the multi-stage reduction gear system (2) has a reduction ratio of 50:1 to 200:1, the servo output shaft (9) is fixedly connected to the final stage metal gear (3) of the reduction gear system (2), and the control circuit board (10) integrates a motor drive module (15) and a signal processing and communication module; characterized in that: The final stage metal gear (3) in the reduction gear system (2) that bears the maximum transmitted torque has an arc-shaped microgroove (4), and a PVDF piezoelectric film (5) torque sensing unit is embedded in the microgroove (4); the piezoelectric sensitive orientation of the PVDF piezoelectric film (5) is consistent with the direction of the principal strain of the gear tooth root; conductive silver paste electrodes (6) are prepared on the upper and lower surfaces of the PVDF piezoelectric film (5), and the conductive silver paste electrodes (6) are connected to a dynamic signal lead-out structure (8) to output a charge signal that is proportional to the transmitted torque of the gear.

2. The high-precision self-sensing micro servo motor device according to claim 1, characterized in that: The final stage metal gear (3) is a beryllium copper or stainless steel metal gear with a pitch circle diameter of 8 mm, a module of 0.3 to 0.5, and 20 to 26 teeth. The micro-groove (4) is opened in the root circle area of ​​the final stage metal gear (3) and extends tangentially along the root arc. The micro-groove (4) has a width of 0.3 mm, a depth of 0.12 to 0.18 mm, and a length covering 1 to 3 consecutive tooth roots. The surface roughness Ra of the groove bottom is ≤0.4 μm.

3. The high-precision self-sensing micro servo motor device according to claim 1, characterized in that: The PVDF piezoelectric film (5) is a uniaxially stretched film with a thickness of 28 μm or 50 μm; the bottom of the microgroove (4) is coated with a conductive silver paste electrode (6) to bond the PVDF piezoelectric film (5) substrate, and the upper surface of the PVDF piezoelectric film (5) is coated with conductive silver paste or sputtered with a gold layer to form an upper electrode; the sensing area of ​​the PVDF piezoelectric film (5) is completely covered with a 5-10 μm parylene insulating protective layer (7).

4. The high-precision self-sensing micro servo motor device according to claim 1, characterized in that: The dynamic signal output structure (8) is a miniature conductive slip ring; the PVDF piezoelectric film (5) conductive silver paste electrode (6) leads are connected to the slip ring moving ring, and the slip ring stator leads are connected to the fixed end charge amplifier (11).

5. The high-precision self-sensing micro servo motor device according to claim 1, characterized in that: The dynamic signal output structure (8) is a passive wireless transmission module; the final stage metal gear (3) has a flexible PCB fixed on its side, and the PCB integrates a charge amplifier (11), an ADC analog-to-digital converter (12) and an NFC radio frequency chip; the servo housing is equipped with an induction coil, which is passively powered by a 13.56MHz radio frequency field and wirelessly transmits torque signals.

6. The high-precision self-sensing micro servo motor device according to claim 1, characterized in that: The PVDF piezoelectric film (5) generates charge under force, which satisfies the piezoelectric equation \ (Q=d_{31}·σ·A\). The charge is converted into a voltage signal by the charge amplifier (11), and the servo motor output torque \ (M=k·V_{out}\) is obtained by conversion through calibration coefficient.

7. A control method based on the self-sensing micro servo motor according to any one of claims 1 to 6, characterized in that, Includes the following steps: S1 power-on initialization and zero-point calibration: drive the servo motor to rotate back and forth under no-load conditions, collect the zero torque voltage of the PVDF piezoelectric film (5) as the zero-point reference, and store the torque calibration coefficient; S2 Signal Acquisition and Noise Reduction: The output voltage of the charge amplifier (11) is read at a sampling rate of ≥1kHz, and the high-frequency noise of the micro DC motor (1) is removed by a 500Hz low-pass filter to obtain the real-time original torque value; S3 Dual Closed-Loop Signal Fusion: The angle signal output by the servo angle detection unit (14) is acquired synchronously to construct the angle-torque dual closed-loop control architecture; S4 Adaptive Mode Switching: In position control mode, torque is monitored in real time. When the torque exceeds the preset threshold, it automatically switches to torque closed-loop or force-position hybrid mode. In torque closed-loop mode, the difference between the expected torque and the measured torque is adjusted by the PID motor drive module (15) to output voltage. S5 Multi-servo Collaborative Grabbing Control: The torque data of multiple servos is uploaded to the dexterous hand main control MCU (13) through I2C or CAN bus. The main control MCU (13) adjusts the target torque of the single servo according to the torque distribution of each joint to achieve stable and non-destructive grasping, sliding, and collision detection.

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