In systems employing PLCs to achieve electrical automation control of packaging machinery such as automatic die-cutting and creasing machines, most interlocking controls in classic relay-contactor control systems can be transferred to the PLC. This greatly simplifies the relay-contactor control system, transforming it into an MCC (Motor Control Centre) system containing only essential electrical interlocks, acting as a conditional executor of PLC system instructions. Relay-contactor interlocks should be retained . In the integrated design of PLC and relay-contactor systems, these interlocks primarily refer to important safety interlocks, encompassing both personal safety and equipment safety. From a personal safety perspective, interlocks crucial to personal safety should be retained, such as fault switches and emergency stop devices. These switches generally require the use of non-semiconductor electromechanical components. From an equipment safety perspective, it is often necessary to retain important limit switches and protective measures. PLC and MCC system interface : There are two common methods for interfaceing PLC and MCC systems: one is using relay isolation, and the other is where the PLC directly receives signals from the MCC system and directly drives the contactors in the MCC. When using the first method, the signal entering the PLC must first pass through an opto-isolated relay, and then its contacts connect to the PLC's input module. The PLC's output signal also first drives the relay coil, and then the relay contacts participate in the MCC's interlocking control. The advantages of this method are: it simplifies the selection of the PLC module, potentially reducing the PLC's cost, and essentially eliminates the possibility of external high-voltage intrusion into the PLC, effectively ensuring the safe operation of the PLC system. Its disadvantage is the increased number of relays, thus increasing potential new fault points. In practical applications, high-quality relays should be selected, preferably those with light-emitting devices indicating engagement. The advantage of using a direct interface between the PLC and MCC systems is its simplicity and clarity, resulting in fewer fault points in the entire control system. The disadvantage is that the system's security is not as high as the first method. In engineering practice, both methods are widely used, and many systems combine both. The selection of the interface should be flexibly determined based on the environment, specific conditions, and component capabilities. Hot Standby for I/O Substations In system designs with extremely high reliability requirements for PLC systems, not only is hot standby required for the main unit, but sometimes it's also necessary to consider hot standby for certain I/O substations in the system, enabling automatic or manual switching of I/O channels in case of failure. To achieve this functionality, the system design should establish two identical I/O substations with parallel inputs and switchable output signals. For automatic switching applications, the switching device should be controlled by the PLC. Local Operation Local operation refers to stand-alone operation at the machine. It has two design principles: one is to control electrical equipment directly through an MCC panel without going through the PLC; the other is to go through the PLC, which is essentially another form of manual operation. Regardless of the method used, the necessary safety interlocks for personnel and equipment are still implemented in the relay-contactor system. The advantage of using local control without going through the PLC is that during local operation testing, simply switching the selector switch allows disconnection from the PLC, completely eliminating concerns about the PLC system's hardware and software, allowing focus on testing the mechanical performance. This method is generally used in international designs. The disadvantage is that it increases the complexity of interlocking on the MCC panel. If local operation via PLC is used, the local switch can be connected to the PLC module. This simplifies the external cables and the MCC panel. In long-distance situations, the reliability of the input signal can be ensured by selecting a high-voltage input module. The output is controlled directly by the MCC panel, eliminating the long cables of the relay circuit used in the non-PLC local operation method and ensuring the relay operating voltage. Therefore, this method also has advantages. Reasonable and scientific PLC programming: PLC programming abandons the expression methods of commonly used computer programming languages ​​and uniquely forms a visual programming language and modular software structure based on relay ladder diagrams. Users can directly program according to the relay ladder diagrams and logic algebra expressions based on the PLC manual. However, in the process of programming PLCs, the safety and reliability of the equipment must be ensured, the most important of which is to eliminate the impact of misoperation. Misoperation mainly includes human error and errors caused by the system itself. 1. Human error 1) Finger tremor caused the error. The solution is to use the differential instruction DIFU (13) to retrieve the rising edge of the button's input signal. The PLC executes it only once in one execution cycle, thus avoiding such errors. As shown in Figure 1, 00005 is the high-pressure pump stop button, and HR0005 is the low-pressure pump start flag. When the low-pressure pump start button 00003 is pressed, the signal is converted into the differential instruction HR0005. HR0005 receives only one rising edge pulse in one program scan cycle, thus filtering out the extra pulses caused by finger tremor, ensuring that the timer TIM000 delays normally for 1 minute, and ensuring that the high-pressure pump starts on time. 2) There are two ways to solve unintentional errors. One is to optimize the display function through the program to reduce human error. In the design, an indicator light is used to display various working states. Flat light - indicates that the system is in the running state. High frequency flash - indicates that the system is in the test state, flashing once every 1 second; low frequency flash - indicates that the system is in the stepping state, flashing once every 3 seconds. Secondly, interlocking between input signals is a more complex method, requiring comprehensive consideration; otherwise, mutual interference between input signals can occur, leading to adverse effects. Figure 2 is a simplified ladder diagram. 00003 is the start button for low-pressure pump 1; 00005 is the start button for low-pressure pump 2; HR0400 is the indicator for pump 1 after 24 hours of shutdown; HR0401 is the indicator for pump 2 after 24 hours of shutdown. The working principle of Figure 2 is that either low-pressure pump 1 or pump 2 can operate, with pumps 1 and 2 serving as backups to ensure that at least one is working. The normally closed contacts (00004, 00006) of the stop buttons for low-pressure pumps 1 and 2 are interlocked. When the operator accidentally presses the stop button 00004 (00006), the program will automatically start the other low-pressure pump (pump 2 or pump 1), thus preventing serious accidents caused by low-pressure pump shutdown. HRO100 is the low-pressure pump stop/start indicator. Its normally open (closed) contacts are widely connected in series with various related circuits, especially the high-pressure pump control circuit. This ensures that starting the high-pressure pump is ineffective if the low-pressure pump is not running, thus preventing accidental operation of the high-pressure pump. After low-pressure pump 1 or 2 starts, HRO100 is energized. Technically, HRO100 is de-energized in only three situations: 1 and 2 main units (00000 and 00001) stop simultaneously, and the system main stop button (00002) is pressed; 25315 is sent to the PLC power-on reset signal; 3 or HR0400 and HR0401 are sent after 24 hours of simultaneous shutdown of main units 1 and 2. Except for these three situations, it should remain energized to ensure the stability of the entire control system. Accidentally pressing the system main stop button is also acceptable. 2. Errors arising from the system itself: Due to the complexity of PLC input signals, and the fact that the PLC's response time is much shorter than that of a relay contact control system, momentary contact jumps, which are less noticeable in relay contact control systems, can lead to malfunctions in PLC control systems. TIM000 is used to eliminate momentary closure caused by mechanical vibration when contact 00104 opens; TIM001 is used to eliminate momentary opening caused by jumps and interference when contact 00104 closes. CNT020 is used to hold the input signal. HR0410 and HR0411 are related operating devices. When 00104 (lower oil level limit switch) opens, it may momentarily close due to the above reasons, starting timer TIM000. If 00104 opens within the set time, the system determines this closure as a malfunction and does not execute the following procedure. If 00104 remains closed within the set time, the system determines this closure as a normal command, maintains the input signal through counter CNT020, and starts the relevant operating equipment. When 00104 closes and momentarily opens due to the above reasons, the method is the same. The time settings for TIM000 and TIM001 are set to #0002 (0.2s), which will not have any impact on the control system. Generally speaking, the time settings for TIM000 and TIM001 are based on the time required for the input relay to reliably engage and immediately disengage, approximately 0.2 to 0.5s. Within this range, the purpose of eliminating contact jump interference can be achieved. If the time setting is too large, the system action will be delayed; if it is too small, the interference filtering effect will not be achieved.