The automation of material handling in traditional, labor-intensive industries like textiles presents a unique set of engineering challenges. One such critical and repetitive task is the loading of bobbin yarns—the final product from winding machines—onto storage or transport racks. This process, traditionally performed manually, is characterized by low efficiency, high physical strain, and inconsistent placement. To address these issues, I have developed a specialized robotic end effector designed specifically for the automated grasping and placement of bobbin yarns. The design prioritizes non-damaging contact, high cycle efficiency, and adaptive handling to accommodate natural variations in the work environment.
The core challenge lies in the object’s characteristics. A bobbin yarn consists of yarn wound onto a hollow, conical or cylindrical paper or plastic tube. The external dimensions are typically within a range of 210-275 mm in diameter, with the internal tube diameter being significantly smaller, often around 65 mm. The mass can vary between 1.6 to 3.8 kg. Any end effector must interact with this object without damaging the delicate yarn surface on the outside. Furthermore, to maximize efficiency in a high-throughput setting, the end effector should handle multiple units per cycle.
The proposed robotic system consists of a standard six-degree-of-freedom industrial robot arm providing dexterous positioning, paired with the custom-designed end effector responsible for the actual grasping and release functions. The end effector’s design is a direct response to the bobbin’s geometry and handling requirements. Its primary design objectives were: to grip from the inside to protect the yarn, to handle two bobbins simultaneously, to ensure secure grip during high-speed moves, and to reliably release the bobbins onto fixed-position pegs on a rack.
The overall architecture of the end effector can be decomposed into three main subsystems working in concert:
1. The Dual Internal-Expansion Grasping Mechanism.
2. The Pneumatic Yarn-Pushing (Release) Mechanism.
3. The Inter-Gripper Spacing Adjustment Module.
| Grasping Method | Advantages | Disadvantages | Suitability for Bobbin Yarn |
|---|---|---|---|
| External Pinch Grip | Simple mechanism, common | High risk of yarn damage/crushing, requires precise alignment | Poor |
| Vacuum Suction | Non-contact, fast | Requires flat, clean surface; unreliable on porous/curved yarn surface | Poor |
| Mechanical Fork/Support | Simple, robust | Cannot lift, only support; requires specific orientation for placement | Fair (for transport only) |
| Internal Expansion Grip | Zero contact with yarn surface, secure hold, adapts to inner diameter | More complex mechanism, requires access to tube interior | Excellent |
1. Detailed Design of the End Effector Subsystems
1.1 Dual Internal-Expansion Grasping Mechanism
This is the heart of the end effector. Each of the two grippers is designed to be inserted into the hollow core of a bobbin. The gripping action is achieved not by clamping from the outside, but by expanding radially from within the tube. Each gripper unit comprises a three-jaw pneumatic cylinder as the actuator. The jaws of this cylinder are connected to custom-designed, lightweight contact fingers. When pressurized, the cylinder drives the three fingers outward synchronously, pressing them against the inner wall of the bobbin tube with uniform force, creating a secure frictional hold.
The fingers are a critical component. They are designed with a specific curved profile to match the common inner taper of a bobbin tube, maximizing the contact area and distributing pressure evenly to prevent local deformation or damage to the tube. To minimize the inertia of the moving parts—a key factor for high-speed robotic cycles—the fingers are fabricated from carbon fiber reinforced polymer (CFRP). This material offers an excellent strength-to-weight ratio. The relationship between the pneumatic pressure (P), the cylinder’s effective area (A), and the resulting normal force (F_n) per jaw is given by:
$$ F_n = \frac{P \cdot A}{3} $$
The total gripping force (F_grip) for one bobbin is the sum of the frictional forces from all three fingers:
$$ F_{grip} = 3 \cdot \mu \cdot F_n = \mu \cdot P \cdot A $$
where $\mu$ is the coefficient of friction between the finger material and the bobbin tube interior.
| Parameter | Symbol | Value / Specification | Notes |
|---|---|---|---|
| Number of Fingers per Gripper | n | 3 | Ensures stable, centered grip |
| Finger Material | – | Carbon Fiber Reinforced Polymer | Low density, high stiffness |
| Actuator Type | – | Pneumatic Three-Jaw Cylinder | Compact, simultaneous jaw motion |
| Max Jaw Stroke | S_max | 15 mm | Accommodates tube ID variance |
| Typical Operating Pressure | P_op | 0.4 – 0.6 MPa | Adjustable for grip security |
1.2 Pneumatic Yarn-Pushing (Release) Mechanism
Simply retracting the gripper fingers to release the bobbin onto a vertical peg is unreliable. Gravity alone may not overcome static friction, or the bobbin might tilt and jam. Therefore, a positive release mechanism is integral to the end effector. A pneumatic linear guide cylinder is mounted centrally on the main frame. Attached to its piston rod is a large, flat push plate. During the grasping and transport phases, this plate is retracted. When the robot has positioned the bobbin accurately over its target peg, the gripper fingers remain expanded, and the push cylinder is activated. The plate extends, applying a controlled force to the top flange of the bobbin, positively shearing it off the stationary gripper fingers and onto the peg. This ensures a clean, reliable release cycle independent of friction coefficients or minor misalignments.
1.3 Inter-Gripper Spacing Adjustment Module
A significant challenge in automation is accommodating variability. While the pegs on the destination rack are at a fixed center-to-center distance (C_rack), the bobbins presented for pickup (e.g., in a bin or on a conveyor) may not be perfectly spaced at that same distance. A rigid dual-gripper end effector would require perfect pre-alignment of the bobbins, adding system cost and complexity.
To create a more flexible and robust end effector, one of the two gripper assemblies is mounted on a movable carriage. This carriage is driven by a separate compact pneumatic cylinder—the spacing adjustment actuator. During the pickup operation, the robot approaches the bobbins with the grippers at the nominal rack spacing. If the bobbins are closer or farther apart, the adjustment actuator can extend or retract, allowing the movable gripper to center itself on its target bobbin. This passive compliance, controlled by simple pneumatic logic, greatly increases the system’s tolerance to pickup position errors without needing complex sensory feedback for every cycle.
| Function | Actuator | Type | Control Signal | Purpose |
|---|---|---|---|---|
| Gripper 1 Jaw Motion | Cylinder 1 | Pneumatic 3-Jaw | Digital (Open/Close) | Expand/retract fingers inside bobbin |
| Gripper 2 Jaw Motion | Cylinder 2 | Pneumatic 3-Jaw | Digital (Open/Close) | Expand/retract fingers inside bobbin |
| Bobbin Release | Cylinder 3 | Pneumatic Linear Guide | Digital (Extend/Retract) | Push bobbin off gripper onto peg |
| Gripper Spacing | Cylinder 4 | Pneumatic Linear | Digital (Extend/Retract) or Proportional | Adjust distance between the two grippers |
2. Mathematical Modeling and Analysis of the End Effector
2.1 Static Force Model of the Grasping Mechanism
To ensure the end effector provides a secure grip without excessive force that could damage the bobbin tube, a static model of the finger linkage mechanism is developed. The goal is to relate the input torque/force from the pneumatic jaw to the resulting contact force applied normally to the tube wall. The finger mechanism can be simplified as a planar linkage system.
Using the principle of virtual work, the relationship between the input and output forces can be derived. For a mechanism in static equilibrium, the total virtual work done by all forces is zero. For our gripper finger linkage, we consider the input virtual work ($\delta W_{in}$) done by the actuator torque T through a virtual angular displacement $\delta\theta$, and the output virtual work ($\delta W_{out}$) done by the contact force F at the finger tip through a virtual linear displacement $\delta$r.
$$ \delta W_{in} + \delta W_{out} = 0 $$
$$ T \cdot \delta\theta + \mathbf{F} \cdot \delta\mathbf{r} = 0 $$
Assuming the force F is applied perpendicular to the tube wall (normal direction) and the linkage geometry defines a velocity relationship, we can express the virtual displacement $\delta$r in terms of $\delta\theta$ via a Jacobian relationship. For a simplified symmetric four-bar linkage representation of one finger section, the velocity of the contact point v_c is related to the angular velocity of the input link $\omega_{in}$ by:
$$ v_c = \omega_{in} \cdot L_1 \cdot \sin(\phi) $$
where $L_1$ is a link length and $\phi$ is an instantaneous transmission angle. Consequently, the magnitude of the force relationship becomes:
$$ F = \eta \cdot \frac{T}{L_{eff}} $$
Here, $\eta$ is a force transmission efficiency factor (typically <1 due to friction and non-ideal geometry), and $L_{eff}$ is an effective lever arm length that depends on the instantaneous configuration of the linkage. This model confirms that the contact force is directly proportional to and controllable by the input actuator torque, which in turn is set by the pneumatic pressure. This allows for tuning the grip force by adjusting the system pressure.
2.2 Kinematic and Dynamic Simulation
Prior to fabrication, the end effector’s motion and forces were analyzed virtually. The 3D model created in SolidWorks was used for kinematic simulation of the grasping sequence. The simulation tracked the displacement, velocity, and acceleration of the gripper fingers during a typical expansion cycle to ensure smooth, stable motion without excessive peaks that could cause shock or vibration.
The simulation results showed that from a starting diameter of 35 mm (fully retracted), the fingers contact the target inner wall of 65 mm diameter in approximately 0.5 seconds under a controlled pneumatic profile. The velocity profile was trapezoidal: an initial period of constant acceleration to a peak velocity, a period of constant velocity, and a period of constant deceleration to zero speed at the point of contact. This profile minimizes jerk and ensures stable, predictable motion. The governing equation for the displacement during the constant acceleration phase is:
$$ s(t) = \frac{1}{2} a t^2 $$
where $s$ is the radial displacement of the finger tip, $a$ is the radial acceleration, and $t$ is time. The peak radial velocity $v_{max}$ achieved was approximately 80 mm/s, which was deemed suitable for a fast yet controlled cycle.
3. Structural Integrity Validation via Finite Element Analysis
The carbon fiber fingers, as the primary load-bearing contact elements, must withstand the operational forces without excessive deformation or failure. A static structural Finite Element Analysis (FEA) was performed using ANSYS Workbench.
Boundary Conditions and Loads: The finger model was fixed at its base (connection to the jaw cylinder). A worst-case load scenario was applied: a concentrated force of 20 N (representing the reaction force from the bobbin tube during a high-pressure grip or during a dynamic acceleration of the robot arm) was applied normal to the contact surface at the fingertip.
Material Properties: The orthotropic properties of the carbon fiber composite were defined:
– Density ($\rho$): $1.8 \times 10^3$ kg/m³
– Young’s Modulus along fiber direction (E1): 120 GPa
– Ultimate Tensile Strength: 1.5 GPa (Safety factor applied in analysis)
Results: The FEA yielded the following key results:
1. Maximum Deformation: The maximum total deformation was located at the free end (fingertip), with a value of 0.041 mm. This is negligible relative to the part dimensions and would not affect grip positioning accuracy.
2. Stress Distribution: The von Mises stress distribution showed that the highest stress concentrations occurred at the root of the finger, near the connection pin holes, as expected. The maximum equivalent stress was found to be 18.5 MPa.
Safety Factor Calculation: Using a conservative allowable stress ($\sigma_{allow}$) based on the material’s yield strength (approximately 300 MPa for the composite laminate), the safety factor (SF) is calculated as:
$$ SF = \frac{\sigma_{allow}}{\sigma_{max}} = \frac{300 \text{ MPa}}{18.5 \text{ MPa}} \approx 16.2 $$
This high safety factor confirms the structural reliability of the finger design under the specified operational loads, ensuring the end effector’s long-term durability.
| Component | Material | Max. Deformation | Max. Von Mises Stress | Safety Factor | Conclusion |
|---|---|---|---|---|---|
| Gripper Finger | Carbon Fiber Composite | 0.041 mm | 18.5 MPa | > 16 | Design is safe with significant margin. |
4. Operational Sequence and Advantages of the End Effector
The operational cycle of the complete end effector demonstrates its integrated functionality:
- Approach & Adjust: The robot moves the end effector over two bobbins. The spacing adjustment actuator positions the movable gripper to align with the second bobbin.
- Insertion: The robot lowers the end effector, inserting both gripper assemblies into the hollow cores of the bobbins.
- Grasping: The three-jaw pneumatic cylinders are pressurized, expanding the fingers to grip the inner walls securely.
- Lift & Transport: The robot lifts the bobbins and moves to the target rack.
- Alignment & Release: The robot positions each bobbin over its respective peg. The push cylinder is activated, extending the plate to positively detach the bobbins from the gripper fingers onto the pegs.
- Retract & Reset: The gripper fingers retract, the push plate retracts, and the robot moves away, ready for the next cycle. The spacing actuator may reset to the nominal position.
The designed end effector offers several key advantages:
– Non-Damaging Handling: Internal gripping completely avoids contact with the valuable yarn surface.
– High Efficiency: Dual-bobbin handling doubles the cycle productivity.
– Inherent Flexibility: The spacing adjustment module allows for robust pickup from semi-structured presentations.
– Reliable Release: The positive push mechanism ensures consistent placement, critical for automated systems.
– Lightweight & Robust: Use of composites and aluminum keeps inertia low while maintaining strength, suitable for fast robotic arms.
In conclusion, this end effector design presents a tailored, effective solution for automating a specific but common industrial handling task. Through a combination of appropriate mechanism choice (internal expansion), system integration (dual grippers, pusher, adjuster), and thorough engineering analysis (kinematic simulation, FEA), the end effector meets the requirements of safety, speed, reliability, and adaptability. This approach serves as a template for developing specialized end-of-arm tooling that bridges the gap between general-purpose robots and the nuanced demands of real-world industrial applications.