The automation of material handling in textile manufacturing, particularly in circular weft knitting, presents a significant challenge due to the variety of bobbin specifications used in production. The manual replacement of exhausted yarn bobbins on creels is a labor-intensive, repetitive, and inefficient task, often requiring workers to access high shelves. While robotic solutions exist, a critical bottleneck has been the lack of a versatile end effector capable of reliably gripping the diverse range of cylindrical and conical bobbins common in the industry. This article details the design, analysis, and verification of a novel bobbin change end effector engineered to overcome this limitation. The core innovation lies in its dual-mode gripping mechanism, which can actively adjust both its diameter and taper angle to securely handle bobbins with inner diameters from 20 mm to 65 mm and masses up to 6 kg. The following sections provide a comprehensive, first-person perspective on the key technologies encompassing the mechanical design, kinematic and static analysis, control logic, and experimental validation of this adaptive end effector.
Introduction and System Requirements
In a typical circular knitting workshop, a single machine may require nearly 200 active bobbins arranged on multi-level creels. The bobbins are not uniform; they vary in geometry (straight cylinder vs. cone) and dimensions. The primary challenge for automation is the gripping process. Unlike external grasping, which could damage the yarn tail needed for splicing, the end effector must grip from inside the bobbin’s core. This internal gripping necessitates an expanding mechanism that must accommodate a wide range of inner diameters and, for conical bobbins, a specific taper angle. Furthermore, the creel layout often has symmetric bobbin positions (A and B sides) requiring the gripper to reorient itself by 180°.
Based on an analysis of common bobbin types, the functional requirements for the end effector were formalized, as summarized in Table 1.
| Parameter | Requirement |
|---|---|
| Gripping Diameter Range | 20 mm – 65 mm (inner diameter) |
| Gripping Depth Range | 40 mm – 170 mm |
| Gripping Taper Adaptation | 0° – 5° (cone angle) |
| Maximum Payload Mass | ≤ 6 kg |
| Reorientation Capability | 180° flip for A/B side access |
| Additional Function | Bobbin pushing for final placement |
| Positioning Accuracy | ± 30 mm |
Design and Architecture of the Adaptive End Effector
The designed bobbin change end effector is a modular system mounted on the Z-axis of a gantry robot. Its architecture integrates several key subsystems to meet the requirements outlined above.
1. Overall System Architecture
The end effector consists of four primary modules: the Gripping Mechanism, the Flipping Mechanism, a Bobbin Emptiness Detection System, and a Vision-based Positioning System. The workflow begins with the detection system identifying a near-empty bobbin and its type during routine patrols. Upon receiving a change signal, the gantry robot moves the end effector to the approximate location. The vision system then provides precise coordinates of the bobbin core. The gripping mechanism extracts the empty bobbin, the flipping mechanism reorients if necessary for the target position, and the process is reversed to install a full bobbin from the storage rack.
2. Gripping Mechanism Design
This is the core module of the end effector, responsible for the actual engagement with the bobbin. It comprises four coordinated sub-assemblies: the Jaw Assembly, Brake Assembly, Clutch Assembly, and Push Assembly.
Jaw Assembly: Three gripper jaws are circumferentially arranged at 120° intervals. Their movement is governed by two independent actuators:
- Radial Expansion: A servo motor rotates an “Indexing Plate” against a stationary “Brake Plate.” Guide pins from the jaw carriers follow spiral slots (Archimedean spirals) in the Indexing Plate, converting rotational motion into precise radial expansion/contraction of the jaws. This mode adapts to different inner diameters of cylindrical bobbins.
- Angular Dilation (Tapering): A pneumatic cylinder actuates a central push rod. When extended, the rod contacts the base of the jaws, forcing them to pivot outwards at their tips, creating a conical gripping profile to match tapered bobbins. A return spring restores the parallel jaw configuration when the cylinder retracts.
The kinematic relationship for radial expansion is derived from the Archimedean spiral. The radial position \( r \) of a jaw is a linear function of the rotation angle \( \theta \) of the Indexing Plate:
$$ r(\theta) = r_0 + k \theta $$
where \( r_0 \) is the initial radius (closed position) and \( k \) is the spiral constant (radial displacement per radian). The design ensures a maximum expanded diameter of 65 mm.
Brake and Clutch Assemblies: These work in tandem to control the state of the Brake Plate.
- During radial expansion, the Brake Assembly (a pneumatic clamp) locks the Brake Plate. The servo motor drives only the Indexing Plate, causing the jaws to expand.
- When the gripped bobbin needs to be rotated (e.g., for yarn tail search), the Clutch Assembly engages. The brake is released, and a clutch wheel is extended to couple the Brake Plate and Indexing Plate. The servo motor then drives both plates together, rotating the entire jaw assembly and the bobbin as one unit.
Push Assembly: A simple pneumatic cylinder with a forked push plate is mounted adjacent to the jaws. After the bobbin is placed onto the creel peg, this assembly pushes it to its final seated position, ensuring the gripper jaws have clearance to retract without collision.
3. Flipping Mechanism Design
To address the symmetric A/B positions on the creel, a 180° flipping mechanism is incorporated. A servo motor drives a worm gear set via a timing belt. The worm gear provides high reduction ratio and self-locking. The entire Gripping Mechanism is mounted on the output turntable of the worm gear. Activating the servo motor rotates the gripper precisely by 180°, allowing the same end effector to approach bobbins from either side. The flip angle \( \phi \) is directly controlled by the servo motor rotation \( \theta_m \) and the gear ratio \( N \):
$$ \phi = \frac{\theta_m}{N} $$
The worm gear ratio ensures precise and stable positioning at the 0° and 180° endpoints.
Gripper Mechanism Analysis and Optimization
The gripper jaws are critical load-bearing components. Selecting optimal dimensions (diameter and length) is essential for balancing strength, stiffness, and functional constraints (minimum bobbin inner diameter, required engagement depth).
1. Material and Static Load Analysis
The jaws are fabricated from Aluminum Alloy 6060-T6. Its material properties are listed in Table 2.
| Property | Value |
|---|---|
| Yield Strength (\( \sigma_y \)) | 259.2 MPa |
| Tensile Strength (\( \sigma_{uts} \)) | 313.1 MPa |
| Young’s Modulus (\( E \)) | 69 GPa |
| Poisson’s Ratio (\( \nu \)) | 0.33 |
| Density (\( \rho \)) | 2.71 g/cm³ |
With a safety factor \( n = 1.5 \), the allowable stress \( [\sigma] \) is:
$$ [\sigma] = \frac{\sigma_y}{n} = \frac{259.2}{1.5} \approx 172.8 \text{ MPa} $$
The primary load on each jaw is the frictional force required to hold a 6 kg bobbin against gravity. Assuming a friction coefficient \( \mu \) and three jaws sharing the load, the normal force \( F_n \) per jaw and the resulting bending stress must be analyzed.
2. Finite Element Analysis and Parameter Selection
Static structural simulations were performed using ANSYS Workbench. A distributed load equivalent to the bobbin’s weight (58.8 N) was applied to the inner surface of the jaw models. The goal was to minimize deformation while keeping stress below the allowable limit and respecting the minimum functional diameter (20 mm).
A parametric study was conducted, varying jaw diameter \( d \) and length \( L \). Key findings are summarized conceptually below:
- For a constant length \( L \), maximum von Mises stress \( \sigma_{max} \) and total deformation \( \delta \) decrease as diameter \( d \) increases.
- For a constant diameter \( d \), \( \sigma_{max} \) increases approximately linearly, while \( \delta \) increases exponentially with length \( L \).
- The constraint \( d < 20 \) mm is imperative to fit inside the smallest bobbin.
Simulation results for a promising candidate with \( d = 18 \) mm and \( L = 58 \) mm showed excellent performance:
$$ \sigma_{max} \approx 29.7 \text{ MPa} \quad (\ll [\sigma] = 172.8 \text{ MPa}) $$
$$ \delta_{max} \approx 0.1 \text{ mm} \quad \text{(at the jaw tip)} $$
This minimal deformation is crucial for ensuring the jaws can fully retract to a closed cylindrical shape after gripping a tapered bobbin. Therefore, \( d = 18 \) mm and \( L = 58 \) mm were selected as the optimal dimensions, providing a robust balance between strength, stiffness, and functional adaptability for the end effector.
Control and Operational Sequence
The operation of the end effector is governed by a state machine integrated with the higher-level gantry robot controller. The sequence for a standard bobbin replacement cycle is detailed in Table 3.
| Step | Action | Actuators Engaged |
|---|---|---|
| 1. Approach | Gantry positions end effector near target bobbin based on vision coordinates. Jaws are closed. | Gantry Robot Axes |
| 2. Align & Insert | Fine alignment. End effector moves forward, inserting closed jaws into bobbin core. | Gantry Z-axis |
| 3. Grip | Based on bobbin type: Cylinder: Servo motor expands jaws radially. Cone: Servo motor expands jaws to min diameter, then cylinder angles jaw tips. | Servo Motor, Pneumatic Cylinder (for cone) |
| 4. Extract | End effector retracts, removing bobbin from creel peg. | Gantry Z-axis |
| 5. Flip (if needed) | If destination is opposite (B) side, flipping mechanism rotates 180°. | Flip Servo Motor |
| 6. Transport & Place | Gantry moves to new bobbin location (storage or creel). Jaws are inserted into new position. | Gantry Robot Axes |
| 7. Release & Push | Jaws retract to closed position. Push cylinder extends to seat bobbin fully. | Servo Motor, Push Cylinder |
| 8. Reset | Push cylinder retracts. End effector moves to standby or patrol position. | Push Cylinder, Gantry |
Experimental Verification and Results
A functional prototype of the end effector was manufactured and integrated with a 3-axis gantry robot for testing. The experiments aimed to validate the gripping adaptability, stability, and overall functionality.
1. Gripping Range and Stability Tests
The end effector successfully demonstrated its dual-mode gripping. The radial expansion achieved the designed range from 18 mm (closed) to 65 mm. The angular dilation produced a conical profile suitable for tapered bobbins. The prototype was tested with the full spectrum of bobbin types: long cylinders (290 mm height), short cylinders (175 mm height), and cones (5° taper, 175 mm height). All were gripped securely from their inner core and transported without slippage or instability, confirming the end effector‘s adaptive capability. The flip mechanism also operated reliably, performing the 180° reorientation as required.
2. Performance Summary
The key performance metrics derived from the experimental validation are consolidated in Table 4.
| Metric | Tested Performance | Status vs. Requirement |
|---|---|---|
| Max Gripping Diameter | 65 mm | Meets (Req: 20-65 mm) |
| Min Gripping Diameter | 20 mm (from inside) | Meets (Req: 20-65 mm) |
| Taper Adaptation | 0° to 15° (capability) | Exceeds (Req: 0-5°) |
| Max Payload Mass | 6 kg | Meets (Req: ≤ 6 kg) |
| Flip Function | Stable 180° rotation | Meets |
| Cycle Time (Grip-Place) | < 15 seconds | Efficient for application |
| Positioning Reliability | 100% successful grip in controlled tests | Validates design |
Conclusion
This article has presented a comprehensive technical overview of an adaptive bobbin change end effector developed for automated circular weft knitting lines. The core challenge of handling multiple bobbin types was solved through a sophisticated gripper design featuring independently controlled radial expansion and angular dilation. The integration of complementary mechanisms for braking, clutching, pushing, and flipping resulted in a fully functional and versatile end effector. Rigorous static analysis using FEA guided the optimal selection of gripper jaw dimensions, ensuring mechanical reliability under load. Experimental tests on a physical prototype confirmed that the end effector meets and exceeds the initial requirements, capable of reliably gripping cylindrical and conical bobbins across the specified size and weight range. The successful development of this end effector addresses a significant gap in textile automation, paving the way for fully unattended creel management. The modular and adaptive principles demonstrated here are not limited to knitting and hold potential for application in other textile processes and industries where handling of varied tubular or conical objects is required.