1 Introduction
With the increasing demands for precision and efficiency in machining, PLCs are being rapidly applied to the automated control systems of machining equipment [1-5]. PLCs offer high reliability, strong anti-interference capabilities, flexible and convenient system combinations, simple and easy-to-understand programming languages, and strong adaptability to production processes.
DU-type combination machine tools are high-efficiency, automated, specialized machining equipment designed for specific workpieces. Their relay-contactor control systems suffer from complex wiring, high failure rates, and poor reliability. Upgrading these machines using PLCs can significantly enhance their functionality, efficiency, and flexibility, improving product quality, production efficiency, and reducing equipment failure rates, resulting in substantial economic benefits.
2. System Process Introduction and Operation
The DU-type combination machine tool consists of a hydraulic power head and a hydraulic rotary table. Its vertical power head has four stations: drilling, reaming, boring, and workpiece loading and unloading, as shown in Figure 1.
The automatic working cycle of this machine tool is as follows: rotary table lifts – rotary table rotates – rotary table reverses – rotary table clamps – power head fast advance – power head working advance – delayed pause – power head fast retraction.
2.1 Rotary Table Hydraulic System
As shown in Figure 2, the hydraulic rotary table achieves its indexing action by controlling the oil circuit of the hydraulic system. The indexing action is as follows: the self-locking pin disengages and the rotary table lifts – the rotary table rotates and buffers – the rotary table reverses – the rotary table clamps.
(1) Self-locking pin disengagement and rotary table lifting: When the power head is in the original position (limit switch sq1 is pressed), press the rotary button sb4, and the electromagnet ya6 is energized. Push the valve stem of the solenoid valve yv1 to the right end, and send the pressure oil from the hydraulic pump to the clamping hydraulic cylinder 1g, causing its piston to move upward and lift the rotary table. At the same time, the pressure oil through valve yv1 is also sent to the self-locking hydraulic cylinder 2g, and the piston moves downward to disengage the self-locking pin.
(2) Rotation and buffering of the rotary table: After the rotary table is raised, the pressure switches SQ5 and YA7 are energized, and the valve stem of the solenoid valve YV3 is pushed to the right. The pressure oil is sent to the left chamber of the rotary hydraulic cylinder 3G, while the oil discharged from the right chamber flows back to the oil tank through valves YV2 and YV3. Therefore, the piston moves to the right, and the rotary table rotates through the transmission mechanism. When it rotates close to the positioning point, the positioning block 1 presses down the slider 2, thereby energizing SQ6 and YA9, so that the return oil of the hydraulic cylinder 3G can only flow back to the oil tank through the throttle valve L, and the rotary table rotates at low speed.
(3) Rotary table reversal: The rotary table continues to rotate, causing the positioning block 1 to move away from the slider 2. The limit switch sq6 returns to its original position, ya7 is de-energized, and ya8 is energized at the same time. The valve stem of yv3 moves to the left. The pressure oil is sent to the right chamber of the rotary hydraulic cylinder 3g through yv1 and throttle valve l, causing the rotary table to reverse at low speed. At this time, the right end face of the positioning block will be pressed against the left end face of the stop block through the slider, achieving accurate positioning.
(4) Rotary table clamping: After the reverse clamping, the channel lever presses the limit switch sq7, energizing ya6, causing the yv1 valve stem to move to the left, and the hydraulic cylinder 1g presses the rotary table down onto the base. Since the hydraulic cylinder 2g is connected to the return oil circuit, the self-locking pin 4 is lifted, locking the positioning block 1. When the rotary table clamping force reaches a certain value, the pressure relay kp is activated, ya8 and ya9 are de-energized, and valve yv3 returns to the middle position. At this time, the left and right oil chambers of 3g are connected to the return oil circuit, causing the rotary hydraulic cylinder to depressurize. Ya10 is energized, causing the yv4 valve stem to move to the right, disengaging the clutch 7 through the hydraulic cylinder 4g.
(5) The state after the clutch is disengaged: the piston rod of the hydraulic cylinder 4g presses the limit switch sq8, ya9 is de-energized, ya8 is energized, the piston of the rotary hydraulic cylinder returns to its original position, the lever presses the limit switch sq9, all the electrical appliances that are activated are de-energized, ya10 is de-energized so that the clutch is re-engaged, and the system returns to the initial state in preparation for the next rotation cycle.
2.2 Power Head Hydraulic Circuit
The power head is a power component that can perform both the feed motion and the cutting motion of the tool at the same time. Its hydraulic system is shown in Figure 3.
Its automatic working cycle is as follows: power head fast advance - working feed - delay - fast return to original position.
(1) The power head stops in place: the hydraulic cylinder 5g drives the power head to move back and forth. When the electromagnets ya1, ya2, and ya3 are all de-energized, the solenoid valve yv1 is in the middle position, and the power head stops in place. At this time, the limit switch sq1 is pressed by the stop iron.
(2) Power head fast forward: Select the automatic position for the selector switch S1. When the rotary table is clamped, the piston of the rotary hydraulic cylinder returns to its original position, and the electromagnets YAL and YA3 are energized. The power head moves forward rapidly.
(3) Power head working feed: During the rapid advance of the power head, when the stop block presses the limit switch sq3, ya3 is de-energized and the power head works and feeds.
(4) Power head rapid retraction: When the power head reaches the end of its working feed, the stop switch sq4, ya1 and ya3 are de-energized, and the power head stops feeding. At the same time, a timer starts. When the timer expires, ya2 is energized, the electromagnetic valve yv1 moves to the left, and the power head rapidly retracts. After the power head returns to its original position, sq1 is pressed, ya2 is also de-energized, and the power head stops moving.
(5) Jogging adjustment of the power head: Set the selector switch S1 to the jogging position, press button SB5 to energize electromagnets YA1 and YA3, and release SB5 to de-energize YA1 and YA3, stopping the power head immediately. When the power head is not in its original position and needs to retract quickly, press button SB6 to energize YA2, causing the power head to retract quickly until it returns to its original position, at which point SQ1 is pressed down and the power head stops. When stopping, press stop button SB1 to de-energize contactors KM1, KM2, and KM3, stopping the main motor M1, hydraulic pump motor M2, and coolant pump motor M3, thus stopping the automatic working cycle.
3 System Design
The main circuit of the control system is shown in Figure 4. M1 is the main motor, M2 is the hydraulic pump motor, and M3 is the cooling pump motor. The start and stop of M1 and M2 are controlled by buttons SB2 and SB1. Switches SA3 and SA4 are selection switches for starting M1 and M2 individually. When the rotary switch S is in the automatic position, M3 starts automatically during the power head's working feed; when S is in the manual position, it can be started by button SB3.
The PLCI/O allocation is shown in Figure 5. After the power is turned on, the indicator light HL illuminates and only turns off after the hydraulic pump motor M2 starts. The motor start and stop procedure is shown in Figure 6.
The hydraulic worktable driver program is shown in Figure 7. The power head driver program is shown in Figure 8. When the normally open contact Y001 closes, the program between MCN0 and MCRN0 is executed. When the normally open contact X013 closes, Y010 is energized, and the worktable slightly lifts. When it reaches its final position, the normally open contact X016 closes, Y012 is energized, and the worktable rotates. When the normally closed contact X022 closes, the program between MCN1 and MCRN1 is executed. The worktable then sequentially performs reverse alignment and clamping. When the normally closed contact X012 closes, the program between MCN2 and MCRN2 is executed. The power head performs rapid advance, working advance, and slow retraction. The program scan ends when the instruction "end" is reached.
4. Conclusion
The programmable logic controller (PLC) used in this system upgrade is a new type of controller specifically developed for industrial control. It inherits the advantages of relay contactor control systems—simple, easy to understand, convenient to operate, and inexpensive—while combining the comprehensive, flexible, and versatile features of computers. The DU-type combined machine tool control system, with this PLC as its core controller, has been greatly simplified, featuring fewer hardware components, a simpler system, convenient maintenance, and high reliability, thus significantly improving product quality and production efficiency.
