Introduction
In modern production equipment, there is a large amount of switching, digital, pulse, and analog signal control. Using relays or discrete components is increasingly inadequate for the requirements of modern production control. Programmable logic controllers (PLCs) have become a more advanced, rapidly growing, and increasingly widely used control device. They are characterized by extremely high reliability, ability to withstand harsh environments, powerful functionality, ease of maintenance, economy, and compliance with modern control requirements, making them the preferred product for industrial automation control.
In industrial production, the packaging of powdery or granular finished products is a frequent problem. This paper uses a programmable logic controller (PLC) to design the control system of a packaging machine. The packaging machine employs dual weighing scales for weighing the finished product. The finished product is delivered by a conveyor and fed into the weighing hopper via vibrators on scales A and B (the feed amount is controlled by vibrating baffles). When the predetermined weight is reached, feeding stops, the weighing hopper opens, and the product is bagged via a transition hopper and then transported away by a conveyor belt.
2. Main process flow of the control system
The process control flow of the dual-weighing packaging machine is shown in Figure 1. After starting the main unit, if the material in the weighing hopper does not reach the specified value, the photoelectric detection switch a(b)1 of weighing hopper a(b) closes. After a 1-second delay, the oscillator starts vibrating, the baffle opens, and material is fed to the scale. When the material feeding approaches the specified value, the photoelectric switch a(b)2 of weighing hopper a(b) closes, the baffle closes, and small-scale vibration feeding is performed. When the scale is full, the oscillator feeding ends, and the vibration stops. At this time, if the scale meets the following conditions: full, bag clamping signal, and the other weighing hopper is not discharging material, the scale can open its door to discharge material. Two seconds after discharging, if the material in the weighing hopper is less than the specified value, the photoelectric switch a(b)1 of weighing hopper a(b) closes, meeting the requirements for the next vibration. However, in order to prepare the weighing system, after a 1-second delay, the vibration resumes, and the next weighing cycle begins. Among them, a(b)1 is a reflective photoelectric detection switch, and a(b)2 is a regular photoelectric detection switch.
Figure 1. Process control flow of the double-weighing packaging machine
3 Control System Programming
3.1 Input/Output (I/O) Allocation
Based on the above process flow analysis, a packaging machine requires 8 input switches: a) photoelectric switch button for the weighing hopper, a2; b) photoelectric switches for the weighing hopper, b1 and b2; bag clamping button; internal counter reset button; and fault switches for weighing hoppers a and b. It also requires 8 control switches: a or b oscillator; a or b baffle solenoid valve; a or b weighing hopper gate solenoid valve; bag clamping solenoid valve; and external counter. A single FX2N-16MR PLC with 8 inputs and 8 outputs would meet these control requirements. However, considering the expandability of the control system, an FX2N-32MR PLC with 16 inputs and 16 outputs was chosen. The key controller, the PLC, is a programmable controller with a high performance-price ratio. Its compact structure, fast execution speed, powerful functions, and low price meet the requirements of this system. The input/output (I/O) allocation of the FX2N-32MR PLC control system is shown in Table 1.
3.2 Software Program Design
The ladder diagrams of the control system for the dual-scale packaging machine designed by the software program are shown in Figures 2(a) and (b). The design of the ladder diagrams is explained below.
(1) The control schemes for scales a and b are the same.
(2) When scale a or scale b malfunctions, x6 or x7 is turned on, and the program generates the corresponding jump step by the jump step instruction cj.
(3) An external counting function is set up to count the number of bags packaged by the packaging machine. In this trapezoidal diagram, y7 is used for external counting.
(4) In the weighing bucket control, there is an interlock control between weighing bucket a and weighing bucket b (such as m4 and m14) to prevent weighing buckets a and b from opening at the same time.
(5) m4 and m14 are auxiliary control relays for scale buckets a and b, respectively.
(6) x5 is an external input signal, which serves as the counter reset signal.
In this system, m1-m7 are auxiliary relays for scale a, tl-t3 are timers controlled by scale a; m11-m17 are auxiliary relays for scale b, t4-t6 are timers controlled by scale b; m0 is an auxiliary relay for bag clamping control.
4. Conclusion
The system designed using a programmable logic controller (PLC) is simple and easy to operate. It can package products in block, granular, and powder forms. Compared to relay or hardware logic circuit control systems, this system is reliable, flexible, and highly adaptable. It features smooth startup, convenient operation, low failure rate, high stability, and a high degree of automation, meeting on-site technical requirements. It is an effective and economical technical means that ensures smooth production and improved productivity.
