1 Introduction
A factory's extrusion equipment uses a vacuum feeding system imported in the 1980s, which is a drying and feeding system. The electrical control system is outdated, and some maintenance drawings are lost, making it difficult to maintain. Therefore, a project was initiated to upgrade the original electrical control system using a PLC.
2 System Design
This material extraction system utilizes atmospheric pressure to draw solid plastic granules into a vacuum tank, and then blows them back into the lower tank to transport the raw materials. The system uses a single air pump to transport materials to three tanks, each with its own function: the first tank is a storage tank for storing raw materials; the second tank is a drying tank used by the raw material drying system to remove moisture from the raw materials to meet production process requirements; and the third tank is placed on the extruder and directly transported to the extruder chamber for heating before proceeding to the next process, as shown in Figure 1.
The original equipment used an air pump to sequentially pump and blow materials from three tanks, transporting the raw materials between them. The control system controlled the amount of material transported each time by adjusting the pumping time, and the material level was manually monitored. This original control system was inefficient, and the equipment had to be constantly running to meet the production speed of material delivery. This not only increased the working time of the air pump but also hindered efforts to reduce electricity consumption and save energy.
3. Automation Hardware and Process Automation
3.1 PLC Hardware Variable Definition
The original control system was replaced by Siemens S7-200. The hardware variable definitions are shown in the attached table.
3.2 Process Automation
The PLC controls the opening and closing of the air valves and receives signals from the material level switch. K4 and K5 jointly control the air pump's blowing and suction actions on the main pipeline. When K4 is connected to the main pipe and K5 is disconnected from the main pipe and connected to the atmosphere, the main pipeline experiences backflow airflow; conversely, it experiences suction airflow. K1, K2, and K3 control the suction and blowing actions of the three material tanks respectively. XWK1, XWK2, and XWK3 detect whether the material in the tank is full; this detection can only collect signals during backflow. XWK4 detects the minimum material level in the extruder's material tank. When the material in the extruder's material tank reaches XWK4, the air pump starts working. When XWK3 detects that the extruder's material tank is full, the air pump stops operating and resets all air valve states. If the storage tank is detected to be full, the "suction for the storage tank" action is skipped, and the backflow continues to the next material tank. If the drying tank is detected to be full, the "suction for the drying tank" action is skipped, and the backflow continues to the next material tank. This process of repeated suction and feeding is used to meet material handling requirements. Because the old wire-wound pipes had a short service life and were prone to aging and leaking, they were replaced with seamless steel pipes. The joints should be properly sealed, as shown in Figure 2.
4 Software Design
4.1 Control Flowchart Design
The software control flowchart is shown in Figure 3. The initialization process sets all valves to their initial states: k1, k2, and k3 are all connected to the main pipeline, and k4 and k5 are connected to the atmosphere. Then, it checks if the raw material in the extruder's hopper is below the xwk4 level. If so, it starts the air pump to backflush the storage tank. Specifically, k1 is connected to the main pipeline, k2 and k3 are disconnected, k4 is connected to the main pipeline, and k5 is connected to the atmosphere for 3 seconds to complete the backflush. Simultaneously, the xwk1 value is collected to determine if the storage tank is full. If not, material is pumped from the storage tank: k1, k2, and k3 remain unchanged, k4 is connected to the atmosphere, and k5 is connected to the main pipeline for 10 seconds. This is followed by actions such as backflush the drying tank.
4.2 Ladder Diagram Programming Design
The following is a partial PLC program:
network1 // Start and stop interlock
ldi0.0 // Start button
om10.0
ani0.1 //Stop button
=m10.0//interlock
network2 // Ready state
ldnm10.0 // Startup flag
rq0.0, 1 // Air pump stops
sq0.1, 3 // Gas valves 1, 2, and 3 connect to the main pipeline
rq0.4, 2 // Gas valves 4 and 5 are connected to the atmosphere.
rm10.1,3
movw0,t37
network3 // Determines if the extruder material level has reached the low level. If so, starts the air pump and begins timing.
ldm10.0 // Boot flag
ai0.5 // Has the material level reached a low level?
sq0.0, 1 // Start the air pump
tont37, 1 // Start timer
network4 // Backflush the storage tank for 3 seconds to determine if it is full.
ldm10.0 // Boot flag
aw>=t37, 1 // When timer is greater than 1
aw <= t37, 30 // When the timer is less than 3 seconds
The system automatically detects the material level in the extruder's hopper. When it reaches a certain low point, the air pump starts working and automatically completes the material delivery. At the same time, it detects the status of each hopper. If a hopper is full, it automatically stops sucking material from that hopper to avoid accidents caused by sucking material from a full hopper, effectively ensuring the smooth operation of the equipment.
5. Conclusion
After the modification of this material extraction system, a series of drawbacks such as low efficiency and high energy consumption were avoided. It can efficiently and accurately control the conveying volume and speed of solid granular raw materials, reduce the working time of motors and air pumps, thereby reducing the frequency of equipment failures, accurately control the extraction time, avoid unnecessary energy waste, improve work efficiency, and make the equipment more intelligent. At the same time, it saves the factory a significant amount of labor and maintenance costs.
