Foreword:
The numerical control system divides the position control system, the linear control system, and the continuous control system according to its control method. In machining, the point control of the CNC system is generally used in hole processing machines (such as drilling, reaming, boring machine), which is characterized by the fact that the machine moving part can achieve precision from one position to another. Move, ie accurately control the end position of the moving part, but does not consider its trajectory. The usual methods for achieving point control of numerical control systems can include:
1, the use of full-function numerical control device. This kind of device function is very perfect, but its price is very expensive, and many functions are superfluous to the position control;
2, using single-chip control. In addition to software development, this method also needs to design hardware circuits, interface circuits, and drive circuits, especially considering the anti-jamming problems in the industrial field.
3, using programmable logic controller control.
Because the programmable controller (PLC) is an industrial control computer designed specifically for use in industrial environments, it has the advantages of strong anti-interference ability, high reliability, and small size, and is an ideal control for realizing mechatronics integration. Device.
PLC (Programmable Logical Controller) is often called a programmable logic controller. It is a general-purpose industrial automatic control device based on a microprocessor and integrated with modern computer technology, automatic control technology, and communication technology. Because it has the advantages of small size, strong functions, simple programming, and easy maintenance, especially its ability to adapt to harsh industrial environments and its high reliability, it makes its application more and more widely, and it has been called modern industry. One of the three pillars (ie, PLC, Robotics, and CAD/CAM).
The man-machine interface is a bridge between the operator and the machine equipment. The user can freely combine words, buttons, graphics, numbers, etc. to handle or monitor management and cope with the multi-functional display screen that may change at any time. With the rapid development of mechanical equipment, the previous operating interface was operated by skilled operators, and the operation was difficult and the working efficiency could not be improved. However, the use of the human-machine interface can clearly indicate and inform the operator of the current condition of the machine and equipment, making the operation simple and vivid, and can reduce operational errors. Even a novice can easily operate the entire machine and equipment. Using the man-machine interface can also standardize and simplify the wiring of the machine. At the same time, it can also reduce the number of I/O points required by the PLC controller and reduce the production cost. At the same time, due to the miniaturization and high performance of the panel control, the added value of the entire set of equipment is relatively increased.
Stepper motors are open-loop control elements that convert electrical pulse signals to angular or linear displacements. In the case of non-overloading, the speed of the motor, the position of the stop depends only on the frequency and number of pulses of the pulse signal, and is not affected by the load change, ie, a pulse signal is applied to the motor, and the motor rotates through a step angle. The existence of this linear relationship, plus the stepper motor only periodic errors without cumulative error and other characteristics. It is very simple to use a stepper motor to control the speed and position.
1, the structure of the control system:
The block diagram of the system is shown in Figure 1 below. The complete electrical system consists of three parts: (1) Operation part (human machine interface): used to operate, set the machine operation status, fault alarm; (2) control part PLC): main Control the positioning of the machine; (3) Execution (step motor and step motor drive).
2, the choice of hardware:
1) The principle of man-machine interface selection: The man-machine interface on the market can basically achieve the following functions:
Indicators (PLCI/O display, internal node display, multi-segment indicators, etc.)
Switch (bit status switch, multi-section switch, switching window switch, etc.);
Various dynamic charts (sticks, meters, moving elements, trends, etc.)
Data display (numeric display, ASCII display, text display, etc.);
Data input (numeric input, ASCII input, text input, etc.)
Abnormal alarm (alarm display, marquee display, event display, etc.);
Static display (straight, round, rectangular, text, etc.)
Based on the characteristics of the price/performance ratio, the author of this machine selected the MT506T/C of Taiwan Weiluntong.
2) The principle of PLC selection: When selecting the type of PLC, it is mainly based on the required function and capacity, and considering the convenience of maintenance, the versatility of spare parts, whether it is easy to expand, and whether there are special functional requirements.
(1), PLC input / output points to determine: I / O points to choose to leave a suitable margin;
(2) PLC storage capacity: When the system has an analog signal or a large amount of data processing, the capacity should be larger;
(3), the output relay type must be a transistor (only the transistor output type PLC can output high-speed pulses);
(4) Positioning and pulse output function: Each phase in a PLC unit can output 2 100KHz pulses at the same time.
(5) PLC is equipped with special positioning instructions, including zero return, absolute value readout, absolute or relative address expression and special pulse output control.
On this machine, the author chose Mitsubishi's small PLC. The model number is: FX1S-14MT.
3) Principle of selection of stepper motor and stepper motor driver:
(1) Determine the torque required by the stepper motor to drag the load;
(2) Determine the maximum operating speed of the stepper motor;
(3) In the end, it is also necessary to consider that a certain amount (such as 30%) of the amount of moment and the amount of rotation should be considered.
(4) According to the above indicators, the moment-frequency characteristics can be selected to suit their own stepper motor.
(5) Select the step motor driver and it must be matched with the motor.
(6) Select as much as possible
The motors of different manufacturers are very different in designing, using materials and processing technology. The selection of stepping motors should focus on reliability and light performance, heavy quality and light price. It is best to use the same manufacturer's controllers, drives, and motors. This facilitates final maintenance.
On this machine, the author chose Stardart's motor and driver. The motor model is: 110BYG350A; the driver is its model number MS-3H110M.
3, the software design of the control system
1) Man-machine interface configuration screen design
Different brands of human-machine interface software operations have their own characteristics, so be sure to use the software features. The software generally provides a variety of controls and graphics library files, can configure a variety of display and control functions, to achieve system operation status, the current process value and fault visualization. The man-machine interface is used to operate the monitored system to display and record the real-time data of the PLC, and a dynamic picture can also be designed to simulate the control process so as to meet the monitoring requirements.
Machine tools (such as drilling, reaming, boring machines) have a different operation method for each type, so the design of human-machine interface is different, but generally there are several interfaces:
(1), the main interface: the main operation of the machine tool or other interface transfer. As shown in Figure 2 below:
figure 2
(2) Parameter setting interface: The user obviously wishes that when the on-site conditions change, certain control parameters of the system can be modified accordingly, such as the distance of the workpiece advancement, speed adjustment, and the like. In order to meet the continuity of production, it is required to modify the variable parameters of the control system online.
(3) Manual operation interface: When the user debugs the machine tool, the operation interface when a certain part of the machine tool moves alone.
(4) Fault alarm interface: When the machine tool fails, the corresponding fault code is displayed on the man-machine interface, so that the user can quickly determine the cause of the fault.
(5) Help page design: Considering the needs of the industrial site, a large number of help pages were added during page design, including the maintenance of the machine tool, the use of the system, and the setting of parameters.
In order to save space, only the main screen is introduced here. Refer to the references for programming ideas of other contents.
After the design is completed, it is compiled correctly and downloaded from the personal computer to the man-machine interface. If the communication with the PLC can be performed normally and the corresponding program on the PLC side is correct, you can use it.
2), PLC program design:
According to the control and process requirements, use Mitsubishi's simple and powerful PLC editing software to develop corresponding applications for input and output. (Achieve all logic actions of the whole machine in a stepping manner) Simultaneously connect the PLC with the external equipment; write the completed program into the PLC, simulate the operation, and debug and modify; after successful simulation debugging, connect Into the actual control system in the field to re-commission until fully passed.
In this project, the author divided the PLC program into several parts. Figure 3 shows the system structure of the PLC program for readers' reference.
image 3
The engineering software system uses the PLC program and human-machine interface screen design to cooperate to complete the system's functions. In the entire system software design process, the most obvious feature is that the PLC program uses a flag to separate the parts of the program to achieve better results.
4, offline debugging
To shorten the on-site commissioning time, perform offline debugging before installation. First, download the program to the PLC and the man-machine interface using the download cable, and then use the communication cable RS-422 to connect the PLC and the human-machine interface, and check whether the PLC and the man-machine interface can communicate normally. In the debugging, the situation of communication error was encountered. By changing the communication parameters of the PLC and human-machine interface, they were finally allowed to communicate normally. And can correctly display the value of the function code. Finally, put the program according to the system requirements into the PLC and the man-machine interface. The input of the PLC is connected to the switch for debugging and debugging.
5, on-site debugging
After off-line commissioning, load is carried out, that is, commissioning after motorization. Look at whether the stepper motor is operating normally, whether there is any out-of-step and error during operation, positioning accuracy, and how accurate the machine tool is when the positioning speed is increased.
Conclusion
The system test shows that the method proposed in this paper using man-machine interface and PLC control stepping motor to realize the point control function of the numerical control system can meet the control requirements. The developed control system has clear programming ideas, simple and practical hardware circuit, and high reliability. Strong anti-interference ability, with good performance and price ratio and other significant advantages, its hardware and software design ideas can be applied to the transformation of the relevant industrial and mining machine tools, and is practical and feasible in the actual operation. After long-term operation, it shows that the control precision is high and the operation is reliable, the automation level of production is improved, and the labor intensity of the operator is reduced.
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