1 Introduction
PLC is widely used in equipment control of machinery manufacturing. However, in the transformation of ordinary lathes, the use of PLC as the core component of CNC system is still a new topic. With the continuous improvement of PLC technology and functions, this will be a development trend.
This article discusses this.
2 Lathe's PLC Numerical Control System Control Principle Design
2.1 Lathe operating requirements
Lathes are usually machined with rotary surfaces, threads, etc. Requirements of its action is generally X, Z fast forward, work into, retreat. In the process of processing, automatic, manual, exterior and vehicle threads can be converted; and single-step operation can be performed.
2.2 PLC numerical control system needs to solve the problem
The operation of the lathe is more complicated, and the PLC is generally only suitable for sequential control of the movement. To use the PLC to control the lathe motion, three problems must be solved:
Figure 1 CNC system schematic
1) How to generate the signal of servo drive and the coordination of X and Z movements;
2) How to change the speed of the feed system;
3) How to realize the internal contact transmission and thread lead change of the thread.
Combining the PLC and its control modules with the corresponding actuators, these problems can be solved.
2.3 Control Principles of Numerical Control System
The transformation of ordinary lathes in numerical control is to change the tool holder, X, Z feed to numerical control. According to the characteristics of the transformation, the stepping motor is used as a servo element and the open loop control system can meet the requirements. The Z-direction pulse equivalent is taken as 0.01mm, and the X-direction pulse equivalent is taken as 0.005mm. The transistor output type PLC is selected. Drive the pulse signal of stepping motor to produce by programming, produce the different frequency pulse through the procedure and realize the speed change. X and Z movements can be controlled automatically by inputting manual operations or programs. The thread signal of the thread is generated by the spindle pulse generator. The gate circuit is connected to the input of the PLC via the AND gate circuit. The pulse of the required lead is obtained by the frequency conversion of the PLC program. Tool holder indexing and turning tool advance and retraction can be controlled by manual or automatic program. Figure 1 shows the schematic diagram of the CNC system. [1]
3 PLC input and output (I/O) points are determined
The designed lathe operations are: starting point total stop, Z, X forward, work forward, retreat; tool rest, reverse; manual, automatic, single step, thread conversion. Therefore, the input needs 14 points. According to Figure 1, the output needs 9 points. The I/O connection diagram is shown in Figure 2 (using the Mitsubishi F1S-30MT) as an example.
Figure 2 I/O connection diagram
4 Driver (Ladder) Design
4.1 General Program Structure Design
Manual, automatic, single-step, threading program selection is implemented using jump instructions. Figure 3 is a block diagram of the overall program structure. If you close X12 (X13, X14, X15 disconnected), it normally closes and disconnects, and executes the manual program; if X12 is disconnected, X13 is full, the program skips the manual program, and the pointer goes to P0 to execute the automatic program.
Figure 3 total program block diagram
4.2 Manual Program Ladder Design
The manual program and automatic program need to be designed according to specific parts. Here only the Z-direction fast forward, forward, and backward motions are used as an example. Its ladder diagram is shown in Figure 4.
Figure 4 Z-manual program ladder
In the state of executing the manual program, press X0, Y1 to turn on, ready to start. Press X2 to turn on auxiliary relay M0. Through the combination of T63 timing and Y2 contact, a pulse signal with a frequency of 103/2i is generated (i is the timing time and is set in units of ms as required) to drive Z-direction fast forward. When X3 is pressed (M0 is off), M1 is turned on, and M1 is combined with timer T32 to generate a pulse of frequency 103/2j (j>i) from Y2, which is output from Y2 to realize the progress. When pressing X4, M0 and Y3 are turned on at the same time, and the motor quickly reverses to achieve fast retreat. Due to space limitations, other programs are slightly omitted. [2]
5 Concluding remarks
The application of CNC lathes in China's machinery manufacturing industry is rapidly developing. However, high-precision CNC machine tools are expensive, and there are a large number of parts with less complex shapes and general requirements in terms of actual production. This requires precision CNC lathe machining.
At the same time, there are a large number of common lathes available in China, and the numerical control of these lathes is an effective way to increase production efficiency and increase efficiency with less investment. In the past, the transformation of CNC lathes used the Z80 and 8031 ​​chips as the core components of the CNC system. Its price was relatively expensive and the system was more complicated. Using CNC as the lathe's numerical control system has the advantages of low cost, simple system and convenient adjustment, which will surely be widely used.
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