In modern industrial and agricultural logistics, the loading of bagged materials such as flour, starch, wheat, fertilizer, and corn seeds remains a labor-intensive and inefficient process. Traditional manual loading methods lead to irregular stacking, low efficiency, and potential health hazards from particulate exposure, while also compromising the aesthetic appearance of bags due to footprints. As labor costs rise, automation becomes imperative. The core of an automated loading system lies in the robotic end effector, which directly impacts loading efficiency and quality. In this study, we focus on designing and analyzing a specialized end effector for bagged materials to address these challenges. Our goal is to develop an end effector that ensures symmetric grasping,整齐 stacking, and high efficiency, while reducing energy consumption.
Existing end effectors for bagged materials often employ single-gripper designs with dual pneumatic cylinders, relying on synchronous control valves for symmetric movement. However, this approach suffers from poor symmetry in gripper rotation, leading to irregular loading and high air consumption. To overcome these limitations, we propose a novel double-gripper end effector with a synchronous drive mechanism actuated by a single pneumatic cylinder. This design aims to improve symmetry, enhance loading整齐ness, and lower operational costs. We begin by detailing the structural scheme and working principles, followed by kinematic modeling and optimization to minimize angular deviations. Additionally, we incorporate a pressing mechanism to maintain bag平整ness during transport. A static analysis is conducted to determine the required actuator torque, guiding cylinder selection. Experimental validation confirms the effectiveness of our end effector, demonstrating significant improvements in loading efficiency and整齐ness compared to conventional designs.
The end effector is a critical component in automated loading systems, and its performance hinges on precise mechanical design. Our end effector consists of several key parts: a gripper frame, gripper rotation mechanisms, a synchronous drive mechanism, a driving unit, and a pressing mechanism. The synchronous drive mechanism ensures that both grippers rotate symmetrically, while the pressing mechanism applies downward force to keep bags flat during robot movement. The use of a single cylinder reduces air consumption compared to dual-cylinder setups. The working principle involves the robot positioning the end effector above bags on a conveyor, where the grippers close to grasp the bags. After grasping, the pressing mechanism engages, and the robot moves to the loading site. Upon reaching the target location, the grippers open symmetrically to release the bags, and the pressing mechanism retracts. This cycle repeats for continuous loading.
To achieve symmetric gripper rotation, we model the gripper mechanism as a double-rocker linkage system. Kinematic analysis is performed using the closed-loop vector method, which provides higher accuracy than graphical techniques. The mechanism comprises two closed loops: A-C-D-E and B-M-F-E. By establishing vector equations for these loops, we derive relationships between the angular displacements of the grippers. Let L1 to L8 represent the lengths of various links, and θ1, θ2, θ3, θ4, θ5, θ6 be the corresponding angles. The fixed angles θ11 = 25° and θ22 = 155° are known from the geometry. The vector equations are:
For loop A-C-D-E:
$$
\vec{AC} + \vec{CD} + \vec{DE} = \vec{AE}
$$
Resolving into components:
$$
L_2 \cos(\pi + \theta_1) + L_4 \cos \theta_3 + \frac{1}{2} L_5 \cos(2\pi – \theta_4) = L_7 \cos(2\pi – \theta_{11})
$$
$$
L_2 \sin(\pi + \theta_1) + L_4 \sin \theta_3 + \frac{1}{2} L_5 \sin(2\pi – \theta_4) = L_7 \sin(2\pi – \theta_{11})
$$
For loop B-M-F-E:
$$
\vec{BM} + \vec{MF} + \vec{FE} = \vec{BE}
$$
Resolving into components:
$$
L_3 \cos(2\pi – \theta_2) + \frac{1}{2} L_5 \cos \theta_5 + L_6 \cos(2\pi – \theta_6) = L_8 \cos(2\pi – \theta_{22})
$$
$$
L_3 \sin(2\pi – \theta_2) + \frac{1}{2} L_5 \sin \theta_5 + L_6 \sin(2\pi – \theta_6) = L_8 \sin(2\pi – \theta_{22})
$$
Additionally, the geometry imposes:
$$
\theta_4 + \theta_5 = \pi
$$
Solving these equations analytically is complex, so we use numerical methods and simulation to obtain the relationship between θ1 and θ2. The symmetry of the end effector is quantified by the angular deviation δ1 = θ1 – θ2. Minimizing this deviation is crucial for整齐 loading.
We perform kinematic simulation using Adams software to analyze the gripper movement. The model is built with key points based on initial dimensions, as summarized in Table 1.
| Key Point | X-coordinate (mm) | Y-coordinate (mm) |
|---|---|---|
| POINT_1 | -195 | 81 |
| POINT_2 | 195 | 81 |
| POINT_3 | -250 | 0 |
| POINT_4 | 250 | 0 |
| POINT_5 | 0 | 40 |
| POINT_6 | 0 | -10 |
| POINT_7 | 0 | -60 |
| POINT_8 | 195 | 171 |
| POINT_9 | 39.5 | 171 |
Constraints and drives are added: revolute joints between links, a fixed revolute joint for the cylinder body, and a sliding joint between the cylinder and rod. The cylinder stroke is set to 50 mm with a simulation time of 1 s. The results show that the angular deviation δ1 increases during gripper opening, reaching a maximum of 12.05°, indicating poor symmetry. This necessitates optimization of the linkage dimensions.
To improve symmetry, we optimize the lengths of links DE and EF, denoted as design variables DV1 and DV2. The goal is to minimize the absolute value of δ1. The initial values and ranges are given in Table 2.
| Design Variable | Point | Coordinate | Initial Value (mm) | Range (mm) |
|---|---|---|---|---|
| DV_1 | POINT_5 | Loc_Y | 40 | -5 to 5 |
| DV_2 | POINT_7 | Loc_Y | -60 | -30 to 0 |
After optimization, we evaluate several design points, as shown in Table 3. The optimal configuration is selected based on the minimum δ1.
| Sequence | DV_1 (mm) | DV_2 (mm) | |δ1| max (°) |
|---|---|---|---|
| 1 | 37 | 76 | 0.20 |
| 2 | 38 | 76 | 0.18 |
| 3 | 39 | 77 | 0.15 |
| 4 | 39 | 78 | 0.25 |
| 5 | 40 | 80 | 0.30 |
| 6 | 41 | 83 | 0.40 |
| 7 | 42 | 86 | 0.50 |
The optimal design corresponds to Sequence 3, with DV1 = 39 mm and DV2 = -83 mm (since DV2 is Loc_Y, negative values indicate downward adjustment). The updated key point coordinates are listed in Table 4.
| Key Point | X-coordinate (mm) | Y-coordinate (mm) |
|---|---|---|
| POINT_1 | -195 | 81 |
| POINT_2 | 195 | 81 |
| POINT_3 | -250 | 0 |
| POINT_4 | 250 | 0 |
| POINT_5 | 0 | 41 |
| POINT_6 | 0 | -10 |
| POINT_7 | 0 | -83 |
| POINT_8 | 195 | 171 |
| POINT_9 | 39.5 | 171 |
Simulation of the optimized end effector shows that δ1 remains within 0.15° for 0–0.9 s and increases to 0.55° at 1 s, representing a significant improvement over the initial design. This enhanced symmetry ensures that bags are released evenly, promoting整齐 stacking.
In addition to symmetry, the pressing mechanism is vital for maintaining bag平整ness during transport. The pressing device consists of a pneumatic cylinder and a pressure plate. The plate is I-shaped and made of polyurethane resin to reduce weight. The cylinder parameters are listed in Table 5.
| Cylinder Model | Working Pressure (MPa) | Stroke (mm) | Theoretical Force (N) |
|---|---|---|---|
| TCM32X50S | 0.4–0.6 | 50 | 321–482 |
To ensure reliable operation, we perform a static analysis of the end effector when grasping a bag. The bag mass is 25 kg, and the pressing mechanism applies a downward force. The system is in equilibrium during robot movement. We analyze the forces on individual links to determine the required torque at the driving point B, denoted MB. The pneumatic cylinder output force FR is related to MB by:
$$
M_B = \frac{F_R L \sin \vartheta}{n}
$$
where L = 80 mm is the lever arm, ϑ = 90° for maximum efficiency, and n = 2.5 is the safety factor. From force balance on link AC, we have:
$$
\sum M_A = 0: F_C \cos \beta_1 L_{AC} \cos \alpha_1 – F_C \sin \beta_1 L_{AC} \sin \alpha_1 – F_Y L_1 – F_G L_2 = 0
$$
with α1 = 56°, β1 = 81°, L1 = 40 mm, L2 = 65 mm, LAC sin α1 = 81 mm, LAC cos α1 = 55 mm. Solving yields FC = -248.81 N (compressive).
For link DF, since CD and FG are two-force members, FC = FD and FF = FM. Taking moments about E:
$$
\sum M_E = 0: F_F \cos \theta_1 L_{EF} – F_D \cos \theta_2 L_{ED} = 0
$$
with θ1 = 18°, θ2 = 9°, LED = 51 mm, LEF = 73 mm. Solving gives FF = 180.64 N.
For link BM, the moment equation about B is:
$$
\sum M_B = 0: F_M \sin \beta_2 L_{BM} \cos \alpha_2 + F_M \cos \beta_2 L_{BM} \sin \alpha_2 + F_Y L_3 + F_G L_4 – M_B = 0
$$
with α2 = 56°, β2 = 18°, L3 = 40 mm, L4 = 65 mm, LBM sin α2 = 81 mm, LBM cos α2 = 55 mm. Substituting values yields MB = 33770.91 N·mm.
Thus, the required cylinder force is:
$$
F_R = \frac{M_B n}{L \sin \vartheta} = \frac{33770.91 \times 2.5}{80 \times \sin 90^\circ} = 1055.35 \text{ N}
$$
Based on this, we select a pneumatic cylinder with specifications in Table 6.
| Cylinder Model | Working Pressure (MPa) | Stroke (mm) | Theoretical Force (N) |
|---|---|---|---|
| SMC-CG1ZN63-50FZ1-NW | 0.4–0.6 | 50 | 1246–1869 |
This cylinder provides sufficient force margin, ensuring reliable actuation of the end effector.
We conduct experiments to validate the performance of our end effector. The test material is 25 kg bagged granular物料, with bag dimensions of 77 cm × 44 cm. The loading system includes a conveyor, a bag-lifting mechanism, and a robotic arm equipped with the end effector. We compare our double-gripper end effector with a conventional single-gripper design that uses two cylinders. The working pressure is set to 0.6 MPa. The results are summarized in Table 7.
| Metric | Single-Gripper End Effector | Double-Gripper End Effector (Our Design) |
|---|---|---|
| Loading Efficiency (bags/hour) | 850 | 1200 |
| Stack整齐ness | Irregular | 整齐 |
| Additional Capacity per Vehicle | Baseline | +8% |
| Air Consumption | Higher (two cylinders) | Lower (single cylinder) |
| Bag Appearance | Footprints, wrinkles | Clean, flat |
The double-gripper end effector achieves a loading efficiency of 1200 bags per hour, which is a 41.2% improvement over the single-gripper design. The symmetric gripper movement ensures that bags are stacked整齐ly, allowing an 8% increase in loading capacity for the same vehicle size. Moreover, the use of a single cylinder reduces air consumption, lowering operational costs. The pressing mechanism effectively maintains bag平整ness, resulting in a clean appearance without footprints or deformation.
In conclusion, we have developed and analyzed a novel end effector for automated loading of bagged materials. Through kinematic modeling and optimization, we minimized gripper angular deviation to within 0.55°, ensuring symmetric operation. The incorporation of a pressing mechanism enhances bag平整ness during transport. Static analysis provided a basis for actuator selection, leading to a robust design. Experimental results confirm that our end effector outperforms conventional designs in terms of efficiency,整齐ness, and energy efficiency. This end effector represents a significant advancement in automated loading systems, with potential applications in agriculture, logistics, and manufacturing. Future work may focus on adaptive control for varying bag sizes and weights, further improving the versatility of the end effector.