1 Introduction
Programmable Logic Controllers (PLCs) are a new type of industrial automatic control device based on microprocessors, integrating modern technologies such as computer technology, automatic control technology, and communication technology. They possess powerful functions including logic control, timing, counting, data processing, networking, and communication. Furthermore, due to their high reliability and great application flexibility, PLCs have become an inevitable replacement for traditional relay-contact control systems. Many devices that previously used traditional relay-contact control systems have been upgraded to PLC-controlled automation systems, offering advantages such as low upgrade costs, short turnaround times, and high reliability. This paper introduces the design and application of a PLC control system for a double-sided, single-station hydraulic transmission combined machine tool.
2. Motion and control requirements of combination machine tools
A combination machine tool is a machine tool that can perform multiple or multiple processing operations simultaneously. The processing actions of a combination machine tool are often arranged according to predetermined steps, similar to simple program control, which is exactly what PLCs excel at.
The double-sided single-station hydraulic transmission combination machine tool is driven by three electric motors: m1 and m2 are the left and right power head motors, and m3 is the coolant pump motor. Their corresponding control AC contactors are km1, km2, and km3, respectively. sa1 is the independent adjustment switch for the left power head, and sa2 is the independent adjustment switch for the right power head, allowing for individual adjustment of the left and right power heads. sa3 is the coolant pump motor operation selection switch. The working cycle of the left and right power heads of this machine tool is shown in Figure 1, and the electromagnet action sequence is shown in Table 1.
Figure 1 Working cycle diagram of the power head
As shown in Figure 1 and Table 1, when the combination machine tool is in automatic cycle mode, after pressing the start button, the left and right power head motors m1 and m2 rotate simultaneously. Pressing the "fast forward" button energizes solenoid valves yv1 and yv3, causing the left and right power heads to rapidly feed and move away from their original positions. Limit switches sq1, sq2, sq5, and sq6 reset first, followed by limit switches sq3 and sq4. After sq3 and sq4 reset, during the power head feed process, the respective limit valves automatically switch from fast forward to working feed. Simultaneously, limit switch sq is pressed, activating the coolant pump motor m3 to supply coolant. When the left power head finishes machining, it presses down sq7 and presses against the dead stop, increasing the oil pressure in its oil circuit and activating kp1. When the right power head finishes machining, it presses down sq8 and activates kp2, energizing yv2 and yv4. Simultaneously, yv1 and yv3 are de-energized, and the left and right power heads rapidly retract. When the left power head resets SQ, the coolant pump motor will stop. When the left and right power heads quickly retract to their original positions, first press down SQ3 and SQ4, then press down SQ1, SQ2, SQ5, and SQ6 to de-energize the power head motors M1 and M2. Simultaneously, de-energize YV2 and YV4, stopping the power heads and ending the machine tool cycle. During machining, the left and right power heads can be quickly retracted to their original positions and stopped at any time.
Hardware design of 3PLC control system
The control system of a double-sided single-station hydraulic transmission combination machine tool should first meet the basic requirements of the above-mentioned work cycle. Then, based on the number of limit switches required for the work cycle and control, plus the number of working points required for buttons, pressure relays, and thermal relay contacts, a programmable controller is selected. The PLC input/output wiring diagram is shown in Figure 2.
Figure 2 PLC input/output wiring diagram for double-sided single-station combination machine tool
This PLC has 21 input signals (4 buttons, 9 limit switches, 3 thermal relay normally closed contacts, 2 pressure relay contacts, and 3 changeover switches), requiring 21 input points. In practical applications, to save on the number of PLC input points, the input signal wiring can be appropriately modified. For example, connecting SQ8 and KP2 in series as one input signal can reduce one input point. This reduces the number of input points from 21 to 13, lowering equipment costs. However, since some contacts use series and parallel connections, the reliability of the input devices and the overall control system may be reduced.
The output control objects of a programmable logic controller (PLC) are mainly the actuators in the control circuit, such as contactors and solenoid valves. The actuators in a double-sided, single-station hydraulic transmission combination machine tool include AC contactors KM1, KM2, and KM3, and solenoid valves YV1, YV2, YV3, and YV4, requiring seven output points. Based on their operating voltages, the PLC output port wiring diagram can be designed. Since the types and levels of voltage applied to the contactor and solenoid valve coils are different, two sets of output channels of the PLC must be used, and a relay output type PLC should be selected. Through comprehensive analysis of the input and output circuits of the machine tool's PLC control system, the Mitsubishi FX2N 32MR programmable controller from Japan is selected to control this machine tool, which can basically meet the above overall requirements.
Programming of a 4PLC control system
The PLC control program is implemented using ladder diagram programming. Based on the control requirements, a ladder diagram for the cyclic operation of a double-sided, single-station hydraulic transmission combination machine tool is designed, as shown in Figure 3. In the figure, when the combination machine tool is in the automatic cycle position, X013 and X014 are activated. Pressing the start button SB2 activates Y000 and Y001, energizing and locking the KM1 and KM2 coils, and starting the left and right power head motors. Pressing button SB3 activates Y004 and Y006, energizing YV1 and YV3, causing the left and right power heads to rapidly feed and leave their original positions. During the power head feed process, the rapid feed is automatically converted to working feed by their respective stroke valves. Simultaneously, pressing SQ activates Y002, energizing the KM3 coil, and starting the coolant pump motor M3. When the left power head finishes machining, press down SQ7 and place it against the dead stop, increasing the hydraulic pressure and activating KP1, which in turn activates Y005. When the right power head finishes machining, press down SQ8 and activate KP2, activating Y007. YV2 and YV4 will be energized, while YV1 and YV3 will be de-energized, causing both the left and right power heads to retract rapidly. When the left power head resets SQ, Y002 is de-energized, KM3 is de-energized, and the coolant pump motor stops. When both power heads retract to their original positions, press down the limit switches to de-energize Y000 and Y001, de-energize the KM1 and KM2 coils, stop M1 and M2, and simultaneously de-energize YV2 and YV4, stopping the power heads and ending the machine tool cycle. If SB4 is pressed, X003 will be activated, allowing the left and right power heads to retract to their original positions at any time. Additionally, the machine tool is designed with overload and short-circuit protection.
Figure 3. PLC control ladder diagram of the combined machine tool
5. Conclusion
Applying PLC technology to double-sided, single-station hydraulic transmission combination machine tools can fully leverage the advantages of PLCs, such as high reliability, ease of debugging, and flexible use. This significantly shortens product development cycles, reduces design costs, increases success rates and product reliability, and boosts production efficiency.
