Abstract: A four-story elevator model based on Mitsubishi PLC control was developed. The overall structure and basic functions of the elevator model are introduced, along with the PLC programming implementation of its basic functions. The developed elevator model features small size, light weight, low manufacturing cost, and on-site programming capability. This elevator model can be used for PLC programming training, providing an effective platform for verifying the correctness of PLC programming, and has significant practical value. Keywords: Elevator model; Programmable Logic Controller; Control System 1 Introduction Programmable Logic Controller (PLC) is a highly practical professional course in electromechanical engineering, and it is receiving increasing attention. To strengthen laboratory construction, further improve experimental methods, highlight practical teaching, especially embodying the characteristics of combining mechanics and electronics, cultivate high-quality engineering talents, and familiarize students with PLC applications, a four-story elevator model was developed. 2. General Introduction to the Elevator Model The elevator model consists of a PLC, aluminum alloy columns, transparent acrylic panels, and a printed circuit board. The entire model is mounted on a 30×22cm² base plate. The elevator section is fixed to the front of the base plate, and the PLC is fixed to the rear. The elevator and PLC are connected via a ribbon cable. The total height of the model is approximately 45cm, and the total weight is approximately 4.5kg, as shown in Figure 1. [align=center]Figure 1: Actual Elevator Model[/align] The elevator model adopts a common traction elevator structure, using a pulley system consisting of one fixed pulley and two movable pulleys to reduce the required traction force. The drive motor is a 5V, 1W DC motor. To better showcase the internal structure and operation of the elevator, except for the front which is a printed circuit board, the other sides are made of transparent acrylic panels, making the internal structure of the elevator clearly visible. The elevator model possesses the basic functions of a real elevator and can be considered a miniaturized version of a real elevator. The elevator section measures 12×12×40cm³, and the car measures 10×10×8cm³. Due to the small size, connecting the electronic components using traditional wires is difficult and prone to errors. Therefore, printed circuit boards (PCBs) are used instead of traditional wires. Specifically, a single PCB is used on the front of both the model and the car. Signals between the two PCBs are connected via ribbon cables. All call buttons and indicator lights are soldered onto the PCBs. This not only solves the electrical connection problem of the model but also improves its reliability and makes the model appear neater and more aesthetically pleasing. 3. Control System Hardware Design 3.1 PLC Selection and Input/Output Point Allocation The PLC selected is the Mitsubishi FX2N-32MR, which has 16 input points and 16 output points. The PLC input signals include 14 points: car call, up call, down call, and floor proximity switch; the PLC output signals include 16 points: indicator lights, motor forward rotation, and motor reverse rotation. The address allocation of the input/output points is shown in Figure 2. [align=center]Figure 2 Address Allocation Diagram of Input/Output Points[/align] 3.2 Intelligent Judgment Control When the elevator arrives at a floor and the elevator door opens, but passengers outside the elevator have left for some reason, no passengers enter the elevator car, affecting the elevator's operating efficiency. This situation often occurs in real elevator operation. To address this, the elevator model in this paper adds an automatic cancellation function for invalid calls (due to the small size of the elevator model, this function is only added for the second floor), which can effectively improve the elevator's operating efficiency. The implementation method is as follows: a human body sensing module is added to the elevator model, and the output signal of the human body sensing module is connected to the input point X13. The normally open contact of X13 is connected in series with the holding coil of the second-floor call signal. When the passenger waiting for the elevator on the second floor leaves, X13 is reset, thereby canceling the second-floor call signal, and the elevator will no longer stop at this floor, thus improving the elevator's operating efficiency. 3.3 Elevator Lifting and Lowering Control The elevator car is driven by a 5V DC motor, and the lifting and lowering of the elevator car is controlled by PLC Y13 and Y17 respectively. Although Y13 and Y17 are interlocked during PLC programming, an external hardware interlock design is implemented to prevent accidental power short circuits. This is achieved using spring relays J1 and J2, as shown in Figure 3, thus avoiding power short circuit accidents. The specific implementation process is as follows: When output point Y13 is closed, the coil of relay J1 is energized, connecting the 5V DC power supply to the DC motor. At this time, the DC motor rotates in the forward direction, causing the elevator car to rise. When output point Y17 is closed, the coil of relay J2 is energized, connecting the 5V DC power supply to the DC motor in the reverse direction. At this time, the DC motor rotates in the reverse direction, causing the elevator car to descend. [align=center] Figure 3 Elevator Lifting Interlock Control Principle Diagram[/align] 4 Model Functions and Software Design 4.1 Introduction to Elevator Model Functions The elevator model can achieve the following main functions: ① Memorize call signals; ② Determine the elevator running direction; ③ Respond to forward call signals and eliminate the corresponding call signals after reaching the floor; ④ When the car reaches the designated floor, the floor indicator light flashes, indicating that the elevator door is open, and the flashing time is 5s; ⑤ Interlock and upper and lower limit protection; ⑥ Display the floor position of the car and display call signals. 4.2 Software Design Based on the functions of the elevator model, the corresponding ladder diagram is compiled. 4.2.1 Processing of Call Signals Taking the "first floor up call signal (input point X10)" as an example. When X10 has a signal input, coil Y10 is turned on and self-locked, thus memorizing this call signal and making the "first floor up call indicator light" light up until the response ends, as shown in Figure 4(a). Figure 4(b) shows the response processing of the "first floor up call signal X10". When the car reaches the first floor, the first-floor proximity switch activates, energizing X4, which in turn activates and self-locks M110, causing Y10 in Figure 4(a) to disconnect and the "first-floor up call indicator" to turn off, meaning the "first-floor up call signal" is responded to. Three timers (T20, T21, and T22) are used to make the floor indicator flash for 5 seconds. M310 indirectly stops the motor, and M410 controls the flashing of the "first-floor indicator". [align=center] Figure 4 Memory and Clearing of Call Signals[/align] 4.2.2 Control of Elevator Running Direction The elevator running direction is determined based on the order of the call signals and the car's floor position. Once the elevator running direction is determined, during upward movement, the elevator only responds to internal and external call signals greater than or equal to the current floor; during downward movement, it only responds to internal and external call signals less than or equal to the current floor. Figure 5 is the ladder diagram for elevator upward operation control. Auxiliary relays M204, M205, M206, and M207 are used to remember the floor position of the car, and M130 and M131 are used to remember the forward and reverse operation states of the elevator. Corresponding output relays are used to remember and indicate the corresponding input signals. For example, when the "Second Floor Call Signal" button is pressed, input terminal X11 has an input. Output relay Y11 remembers this input signal, and output point Y11 causes the corresponding diode to light up to indicate that there is a "Second Floor Call Signal". When the car is on the first floor, M204 is powered on, and M205, M206, and M207 are de-powered. At this time, the external call signals X16, X15, X14, X12, X11, and X10, as well as the internal call signals X3, X2, and X1, may all be in an upward running state. Similarly, when the car is on the second floor, M205 is powered on, and M204, M206, and M207 are de-powered. At this time, the external call signals X16, X15, X12, and X11, as well as the internal call signals X3 and X2, may all be in an upward running state. When the car is on the third floor, M206 is powered on, and M204, M205, and M207 are de-powered. At this time, the external call signals X16 and X12, as well as the internal call signal X3, may all be in an upward running state. As shown in Figure 5, all output points Y are self-locked. Therefore, once an upward running state is generated (M130 is energized), the upward running state can only be released (M130 is de-energized) after all upstairs call signals are responded to. [align=center] Figure 5 Elevator Upward Running Control[/align] The elevator downward running control principle is similar to the upward running control. Other functions are relatively simple and will not be elaborated here. 5 Conclusion Compared with other existing elevator models, the elevator model in this paper has the following innovations: ① Using printed circuits instead of traditional wire connections not only reduces the size of the elevator model but also improves reliability; ② The elevator model can automatically cancel invalid call signals, improving the elevator's operating efficiency; ③ Due to its small size, light weight, low cost, and portability, the elevator model is very suitable for classroom teaching, experiments, on-site programming and debugging, and is of great help to students in understanding and mastering programmable logic controllers (PLCs) and their applications as well as industrial process automation control. It is precisely because of these innovations that this elevator model won the third prize in the "Third Advanced Manufacturing Technology Internship Teaching and Innovation Production Competition of Jiangsu Provincial Engineering Colleges" sponsored by the Jiangsu Provincial Department of Education. References [1] Zhong Zhaoxin (ed.). Programmable Controller Principles and Applications. Guangzhou: South China University of Technology Press, 1999 [2] Yang Lixin, Chu Jian, Ding Ru. Design of Elevator Model for Teaching Based on SPLC [J]. Microcomputer Information, 2005, 11-1: 25-27