1. Introduction Paper cutting machinery is one of the most commonly used pieces of equipment in the printing and packaging industry. The most basic action of a paper cutter is to deliver the material to be cut to a designated position and then cut it. Its core control mechanism is a single-axis positioning control. Our company imported two paper cutting machines from a European company. Their propulsion positioning system is implemented using a microcontroller. When the pulse signal received from the encoder reaches the set value, the microcontroller system outputs a signal, disconnecting the contactor of the feed motor. Simultaneously, the electromagnetic clutch and brake disengage, braking the inertia of the propulsion system, thereby achieving precise positioning. Due to the aging of the microcontroller control system, inaccurate positioning and erratic paper cutting actions occur, preventing normal production. However, this control system is an early product, and suitable replacement parts are unavailable, so modification is the only option. Currently, there are two main methods for modifying the feed positioning system of paper cutting equipment in China: one is to use a microcontroller combined with a frequency converter, and the other is to use a microcontroller combined with a servo system. However, both of these modification schemes cost over 20,000 yuan. Furthermore, the microcontroller system was designed by a professional development company with conservative technology. If a fault occurs, it can only be repaired or replaced by the original company, resulting in long repair cycles and high costs, which is detrimental to the maintenance and use of the modified equipment. Based on the characteristics of our equipment, we proposed a new modification scheme: using the high-speed counter function of the PLC combined with the multi-speed function of the frequency converter to achieve positioning control, and using HMI (Human Machine Interface) for setting cutting parameters and completing manual operation. 2. Feasibility Analysis of the Modification Most PLCs nowadays have high-speed counter functions, which can handle pulse signals with frequencies of tens or hundreds of kHz without adding special function units. The paper cutter does not have very high requirements for the accuracy and response speed of the feeding system. By calculating the relevant parameters of the paper cutter's feeding system and reasonably selecting the encoder, the pulse frequency can be made to be within the PLC's processing range while meeting the feeding accuracy requirements. During the feeding process, the PLC compares the number of received pulses with the set value. Based on the comparison result, it drives the corresponding output point to control the output frequency of the frequency converter, so that the feeding speed slows down when approaching the set value, thereby reducing system inertia and achieving precise positioning. Furthermore, the technology of frequency converters has made great strides, significantly increasing the torque of motors at low speeds, thus ensuring the feasibility of low-speed propulsion during feeding and positioning. 3 Selection of Main Control Components 3.1 Selection of PLC The input and output signals required by the equipment are shown in Table 1. [align=center] Table 1 PLC Input/Output Allocation Table[/align] For these required input points, the FX1S-30MR PLC was selected. Because a human-machine interface was chosen, other manual actions, such as forward, backward, and tool changing, are all implemented through the human-machine interface, without occupying PLC input points. This makes it possible to select the low-priced FX1S series PLC, as the FX1S series PLC has a maximum of 16 input points. In addition, the high-speed counter of this series of PLCs has the ability to process pulses with a frequency of up to 60 kHz, which is sufficient to meet the accuracy requirements of the paper cutter. 3.2 Selection of Encoder The selection of encoder must meet two aspects: the highest pulse frequency received by the PLC and the accuracy of the feed. We selected an encoder with a resolution of 500 p/r (500 pulses per phase per revolution). Verification confirms that this resolution meets the two conditions mentioned above. The required parameters for verification are: a maximum motor speed of 1500 rpm (25 revolutions per second) and a feed screw lead of 10 mm/revolution. Verification results are as follows: Maximum pulse frequency of this system = 25 revolutions per second × 500 pulses/revolution × 2 (A/B phases) = 25 kHz. Theoretical feed resolution = 10 mm/500 = 0.02 mm. From the above data, we know that the encoder emits 50 pulse signals for every 1 mm of feed system movement (this data is crucial and used in PLC program data processing). Since this project counts the A/B phase pulses of the encoder separately, using two high-speed counters, and applies high-speed positioning instructions in the program, the highest pulse frequency that this PLC can handle is 30 kHz, thus meeting the first condition. Our paper cutter's cutting accuracy requirement is 0.2 mm, and the theoretical accuracy fully meets this requirement. 3.3 Selection of Inverter and HMI For these two components, we selected Mitsubishi products, namely FR-E540-0.75K-CH and F920GOT-BBD-KC. The F920GOT is a button-type HMI, which is very simple and convenient to use and program. It has the following features: (1) It can easily realize data exchange with the PLC; (2) Through its own 6 function button switches, it can control the soft relays inside the PLC, thereby reducing the use of PLC input points; (3) It has two communication ports, one RS232C (for communication with a personal computer) and one RS422 (for communication with the PLC). After connecting the computer to the F920GOT, it can not only read and upload the program of the HMI, but also directly upload, download, adjust and monitor the program of the PLC. 4 Design of PLC and HMI Programs The main difficulty of the program in this project lies in the data processing. In addition to manually controlling the feed positioning mechanism to move forward and backward during the paper cutter's operation, it also needs to perform equal-division cutting and specific-position positioning functions, and the HMI must display the current position of the positioning mechanism in real time. To simplify the calculations in the program, we used two high-speed counters, C235 and C236. C236 calculates the number of pulses for forward and backward movement, converts the result, and then uses it to display the current position of the feed mechanism; C235 is used for precise positioning. The positioning process is as follows: each time the feed mechanism needs to be positioned, the required number of pulses is calculated and sent to C235. Regardless of whether the feed mechanism moves forward or backward, C235 counts down and compares the values ​​in C235. Based on the comparison result, the corresponding output point is driven to control the output frequency of the frequency converter, so that the feed speed slows down when it approaches the set value, thereby achieving precise positioning. Because any system has inertia and time lag, the inverter stops outputting not when the count value in C235 decreases to 0, but by comparing C235 with a data register D130. When the value in C235 decreases to the set value in D130, the PLC controls the inverter to stop outputting. The value of D130 can be modified and set through the HMI. During debugging, this value is modified to achieve accurate positioning. 1) Program to display the current position of the positioning mechanism; 2) Program segment to implement positioning control; 3) Decimal point issue when setting parameters. In actual work, the position needs to be accurate to 0.1mm. This problem is often encountered in some microcontroller systems. The common solution is to increase the value by an order of magnitude, that is, when setting the data, use 1 to represent 0.1mm and 10 to represent 1mm on the HMI. However, when we dealt with this problem, we achieved a WYSIWYG effect by setting the data in the HMI and writing the PLC program. The main thing in the HMI is to set the format of the values ​​correctly. The HMI setting screen is shown in Figure 1. For example, to cut 10.5mm paper into equal parts, the HMI can be set to 10.5 instead of 105 as required on other similar devices in the company. However, the PLC's D128 register contains 105 instead of 10.5. Therefore, when calculating the required number of pulses, the following command should be used: `mul d128 k5 d10` (Although D11 doesn't appear during programming, it's actually occupied and cannot be used elsewhere, otherwise problems will occur). Instead of `mul d128 k50 d10`. 4) Other issues to note in programming: ● Dual coil problem. This project avoids the dual coil problem by using conditional jumps and step instructions. ● Error signal problem. Encoders are relatively precise photoelectric products, and error signals are inevitable when subjected to vibration. The paper cutter causes significant vibration during the cutting action. If this phenomenon is ignored, the positioning accuracy and the display of the current position of the actuator will be inaccurate. The processing method in this project is shown in the example program diagram 1 above. Only when Y3 and Y4 are connected, that is, only when the feed mechanism moves forward and backward, will C236 count. This effectively shields the false signals caused by vibration during cutting. 5. Inverter Parameter Settings The main parameters of the inverter are shown in Table 2. During the debugging process, in order to achieve a perfect combination of positioning speed and accuracy, the three-speed setting values, acceleration/deceleration time, and the values ​​of D130, D200, and D202 in HMI should be adjusted accordingly. [align=center]Table 2 Overview of Main Inverter Parameter Settings[/align] 6. Conclusion Through the modification process, the function of our paper cutter was completely restored, and it has been running very stably for three months since its trial use. This application example shows that combining the high-speed counter function of the PLC with reasonable application can replace the high-cost positioning control system in certain situations, achieving the optimal cost-effectiveness of the control system. Furthermore, the use of a universal and open PLC-inverter integration solution brings long-term benefits to the company's later independent equipment management.