The φ3500 vertical machine tool is a highly automated electromechanical device, generally used in the metallurgical industry to machine various large workpieces. It typically employs relay logic control, resulting in a high failure rate and long maintenance cycles for its electrical control system. With technological advancements, this type of control system has shown increasing drawbacks. In recent years, PLCs have been increasingly widely used in industrial automation control. Their comprehensive advantages in control performance, setup time, and hardware cost are unmatched by other industrial control products. Therefore, upgrading these systems using PLC control technology has broad technical and economic significance.
1. Solution Selection
The original equipment was manually operated. Based on the suggestion of the user department, the modification plan will maintain the original human-machine interface of the equipment before the modification. This can effectively reduce the misoperation of the equipment by the users and prevent the accident rate of the equipment from increasing after the technical modification.
The system's table speed change unit relies on hydraulic solenoid valves, with mechanical transmission components continuously altering the transmission ratio to change the table speed. The speed change operation is quite cumbersome, requiring the main unit to be stopped first, the speed selected, a micro-motion to re-engage the gears, then the gear shifting, and finally a restart. Based on on-site load calculations and theoretical analysis, the main drive method is retained, using an FRN15G95-4JE voltage-type general-purpose frequency converter for speed commutation control of the table motor. The FRN15G954JE voltage-type general-purpose frequency converter features a series of advanced functions such as torque vector control, slip compensation, electric AVR self-tuning, and load torque self-adaptation. Under sensorless open-loop operation, with flux vector control and automatic motor parameter testing, its speed regulation performance reaches or even surpasses that of traditional thyristor-powered dual-closed-loop DC speed regulation systems. The original system also had two tool holders on the left and right, capable of longitudinal and lateral movement, connected to the left and right drive motors via electromagnetic clutches. The left and right drive motors have Y/Δ conversion, thus providing two levels of working feed speed: Level I 0.29-45.6 m/min, and Level II 0.58-91.2 m/min. During the modification, the original method will be maintained.
2. PLC Model Selection
2.1 Number of Input/Output Points
The input signals of the PLC control system include the operator console control input, the speed signals of the worktable and tool post, the limit switches distributed throughout the machine tool, and the inverter engagement signals. There are a total of 64 input points.
The PLC's control load is mainly divided into three categories: first, the main contactors for forward and reverse rotation of 10 AC motors; second, the clutch coils for speed regulation of the left and right tool holders (the electromagnetic clutch is directly driven by the PLC); third, the display and alarm loads (including indicator lights, audible and visual alarms, etc.); and fourth, the workbench speed regulation output (to the frequency converter). There are a total of 56 output points.
2.2 PLC Selection
After determining the input/output points, the PLC selection is also required. Except for the analog signal required for connection with the frequency converter, all other signals in this system are digital signals. Considering the performance-price ratio and the number of input/output points, the ATCS series PPC31 model from Devison is selected. The PPC31 has a modular structure, flexible system configuration, strong programming function, and high performance-price ratio.
