The equipment shown in Figure 2 is designed with integrated mechanisms, including a material clamping and feeding unit, a cotton clamping and cutting unit, a winding unit, a cotton blocking unit, a pinning unit, a threading unit, and a cotton control unit. The system’s main technical parameters are as follows: an overall size of 2000 mm × 1150 mm × 1900 mm, a total power consumption of 2 kW, a supply voltage of 220 V AC, a supply air pressure range of 0.5 - 0.7 MPa, and a production efficiency of 120 bundles per day (calculated based on an 8-hour daily operation). The entire strapping process of solid propellants is fully automated by the equipment, requiring operators to only perform material loading and unloading tasks, thereby achieving a high level of automation.

Figure 2. The automated solid propellant strapping equipment.

As shown in Figure 3, after manual loading, the material clamping and feeding mechanism clamps the grouped solid propellants together and moves them to the winding position. Through the coordinated movement of the material clamping and feeding mechanism and the winding mechanism, the solid propellants are wound. The cutting mechanism then cuts the cotton thread, followed by the completion of tying knots using the line blocking mechanism, pinning mechanism, threading mechanism, and line control mechanism. This completes the first binding. The process repeats until three bindings are finished. Finally, the material clamping and feeding mechanism moves the solid propellants to the loading and unloading positions for manual unloading.

Figure 3. The automatic strapping process for solid propellants.

PLC (Programmable Logic Controller) is widely used in various industrial equipment due to its ability to operate stably in harsh industrial environments, its rich Input/Output (I/O) interfaces for convenient integration with industrial sensors and actuators, and its support for multiple communication protocols. As shown in Figure 4, the control system topology of the solid propellant automatic strapping

Figure 4. System topology diagram

equipment is designed with a Siemens PLC as the core controller. Operators use a touchscreen HMI (Human-Machine Interface) to start or stop the equipment. The PLC receives signals such as “material in-position” and “compartment door closed” from sensors, and under the condition that no alarm signals are triggered by any mechanisms, it sequentially controls the operation of each mechanism according to the predefined workflow to complete the automatic strapping of solid propellants.

Figure 6 illustrates the material clamping & feeding mechanism of the equipment. The mechanism includes a gripping base, spring-loaded gripper blades, quick-change tooling, finger cylinders, a pushing motor, linear guides, and follower cylinders. The clamping process operates as follows: First, adjust the position of the spring-loaded gripper blades according to the diameter specifications of the solid propellant and install the corresponding quick-change tooling on both sides of the finger cylinders. Then, place seven solid propellants onto the gripping base. At this stage, the two lower solid propellants are constrained by the base and the spring-loaded blades to maintain their correct positions, while the remaining five are held in place by gravity and the lower propellants. Finally, the finger cylinders retract, driving the quick-change tooling to clamp the sides of the grouped solid propellants. The resulting constraints on the grouped propellants are illustrated in Figure 7.

The feeding mechanism operates as follows: When the pushing motor rotates forward, the clamping & feeding mechanism drives the solid propellant toward the winding mechanism. At this point, the follower cylinder installed on the side of the winding mechanism retracts under applied force. When the pushing motor reverses, the clamping & feeding mechanism moves the solid propellant away from the winding mechanism. After the follower cylinder’s constraint is released, it extends outward, ensuring the solid propellant remains securely constrained at both ends and moves with the clamping & feeding mechanism away from the winding mechanism.

Figure 6. Structure of the material clamping & feeding mechanism.

Figure 7. Schematic diagram of bundled solid propellant held in place.

Figure 8 illustrates the cotton clamping & cutting mechanism of the equipment, which includes a telescoping cylinder, a clamping cylinder, a cutting cylinder, and a blade. The operational sequence is as follows: Before the operation begins, the clamping cylinder grips the cotton thread end of the spool. Prior to winding, the telescoping cylinder extends, the clamping cylinder opens, and the cotton thread end is transferred to the cotton control mechanism at the base of the equipment, after which the telescoping cylinder retracts. After winding, the telescoping cylinder extends again, and the clamping cylinder grips the cotton thread at one end of the spool. Once the cotton control mechanism’s claw securely holds the cotton thread, the cutting cylinder extends, driving the blade to cut the thread between the clamping cylinder and the cotton control mechanism’s claw. Subsequently, the cutting cylinder retracts, and the telescoping cylinder retracts, returning the mechanism to its initial position. At this point, the two ends of the cut cotton thread are gripped by the two claws of the cotton control mechanism, while the spool’s cotton thread end remains clamped by the clamping cylinder.

Figure 8. Structure of the cotton clamping & cutting mechanism.

Figure 9 illustrates the winding mechanism of the equipment, which includes a winding motor, winding gear set, tension rod, spool holder, and protective cover. The winding operation works as follows: The spool is installed on the spool holder, and the cotton thread is passed through the spring-loaded tension clamp on the

Figure 9. Structure of the winding mechanism.

tension rod, ultimately being clamped by the clamping cylinder of the cotton clamping and cutting mechanism. After the cotton control mechanism’s claw grasps the cotton thread from the clamping cylinder, the winding motor rotates forward, driving the winding gear set to move the tension rod and spool in a circular motion around the solid propellant. By coordinating the speed of the winding motor with that of the feed motor, precise alignment of the cotton thread is achieved.

Figure 10 illustrates the cotton blocking mechanism of the equipment, which includes cotton cylinder 1, cotton cylinder 2, cotton rod 1, and cotton rod 2. The function of the cotton blocking mechanism is as follows: When the winding mechanism reaches the last turn, cotton cylinder 1 and cotton cylinder 2 extend, moving cotton rod 1 and cotton rod 2 to the center between the winding mechanism and the solid propellant. This action increases the length of the cotton thread end after winding, facilitating the subsequent knotting process.

Figure 10. Structure of the cotton blocking mechanism.

Figure 11 illustrates the pinning mechanism of the equipment, which includes an overall telescopic cylinder, linear guide, needle punch, needle telescopic cylinder, needle lifting cylinder, threading assist cylinder, and threading stopper. The mechanism serves two primary functions:

Knotting Support: When the two ends of the cotton thread are gripped by the cotton control mechanism’s claws and prepared for knotting, the overall telescopic cylinder extends, moving the entire pinning mechanism along the linear guide. Subsequently, the needle telescopic cylinder and needle lifting cylinder extend, causing the needle punch to press down on the cotton thread between the two lowermost solid propellant rods. This action prevents the thread from loosening during knotting and ensures the knot is precisely constrained at the needle punch location when the thread ends are tightened.

Threading Assistance: Before threading, the threading assist cylinder extends, driving the threading stopper outward. This motion, combined with the rotational movement of the line control mechanism, creates a diamond-shaped area in the cotton thread, facilitating the subsequent threading operation by the threading mechanism.

Figure 11. Structure of the pinning mechanism.

Figure 12 illustrates the threading mechanism of the equipment, which includes an overall telescopic cylinder, linear guide, threading motor, threading rod, and threading rod telescopic cylinder. The threading mechanism operates as follows: Under the coordination of the pinning mechanism and the cotton control mechanism, the cotton thread forms a diamond-shaped threading area. The overall telescopic cylinder extends, moving the entire threading mechanism along the linear guide. The threading motor reverses direction, while the threading rod remains horizontal and rotates clockwise towards the cotton control mechanism’s claws. The threading rod telescopic cylinder retracts, closing the gripping claws at the end of the threading rod to retrieve the cotton thread from the cotton control mechanism’s claws. Then, the threading motor rotates forward, keeping the threading rod horizontal and rotating counterclockwise through the upper diamond-shaped threading area. After the cotton control mechanism’s claws grip the cotton thread, the threading rod telescopic cylinder extends again, opening the gripping claws at the end of the threading rod to pass the cotton thread to the line control mechanism’s claws. Finally, the threading motor reverses direction, returning the threading rod to its initial position. The overall telescopic cylinder retracts, and the threading mechanism returns to its initial position along the linear guide.

Figure 12. Structure of the threading mechanism.

Figure 13 illustrates the cotton control mechanism of the equipment, which includes jaw 1, jaw 2, jaw cylinder 1, jaw cylinder 2, jaw lifting cylinder 1, jaw lifting cylinder 2, jaw rotating motor 1, jaw rotating motor 2, jaw translational module 1, jaw translational module 2, base telescopic cylinder, base rotating motor, and base translational cylinder. The action flow of the cotton control mechanism is as follows:

Figure 13. Structure of the cotton control mechanism.

Before threading, jaw lifting cylinder 1 extends, jaw rotating motor 1 rotates forward, jaw translational module 1 moves outward, and base telescopic cylinder extends. Jaw cylinder 1 closes, gripping the end of the cotton thread at the thread cutting cylinder of the cotton cutting mechanism, then returns to its initial position. The base rotating motor rotates the entire cotton control mechanism 180˚, initiating the threading process.

After threading, jaw lifting cylinder 2 extends, jaw rotating motor 2 rotates forward, jaw translational module 2 moves outward, and base telescopic cylinder extends. Jaw cylinder 2 closes, gripping the cotton thread at the thread cutting cylinder of the cotton cutting mechanism. After the blade of the cotton cutting mechanism cuts the cotton thread, jaw cylinder 2 returns to its initial position, with both ends of the cotton thread held by jaw 1 and jaw 2.

When the pinning mechanism’s threading stopper extends, the base rotating motor rotates the cotton control mechanism 180˚, forming a diamond-shaped threading area. After the threading rod of the threading mechanism grips the cotton thread at jaw 1, jaw 1 releases.

Once the threading rod passes through the diamond-shaped area, the base translational cylinder retracts, jaw lifting cylinder 1 extends, jaw translational module 1 moves outward, and base telescopic cylinder extends. Jaw cylinder 1 closes, gripping the cotton thread at the threading rod, then returns to its initial position.

Finally, jaw lifting cylinder 1 and 2 extend, jaw rotating motors 1 and 2 rotate forward, and jaw translational modules 1 and 2 move outward. The jaws pull the thread ends sideways to form a knot at the needle punch location. After the needle telescopic cylinder retracts the needle, the motors continue rotating forward to position the final knot between the two lowest solid propellant rods. Jaw cylinders 1 and 2 open, and the line control mechanism returns to its initial position.