Abstract : This paper mainly introduces the design and application of CNC products in a 7-axis CNC silicon block double-plane grinding machine. The control system adopts a high-end CNC system, servo motors, and electrical products from well-known brands, specifically developed for 7-axis grinding machines. The human-machine interface is user-friendly and intuitive, the product structure is scientific and reasonable, the degree of automation is high, the precision and grinding efficiency are high, the operation is safe and convenient, and the appearance is beautiful and elegant.
Keywords : Solar photovoltaic industry, CNC system, human-machine interface, servo, laser measurement, CAN, Open bus
introduction :
In recent years, due to the enormous demand for energy worldwide, especially the development and utilization of clean energy such as solar, wind, tidal, and biomass power, and with solar power generation (photovoltaic industry) being a major representative of clean energy and capable of large-scale development and utilization, governments around the world have introduced policies to support and develop the solar energy industry. Many solar-related companies have seized the opportunity to develop rapidly and achieve remarkable business results. It is estimated that by 2020, my country's total installed capacity of solar photovoltaic power generation will reach 2200MW, which means that my country's demand for photovoltaic technology and equipment is also very strong.
Solar panels are devices that respond to light and convert light energy into electricity. Taking crystalline silicon as an example, the photovoltaic power generation process can be described as follows: P-type crystalline silicon is doped with phosphorus to obtain N-type silicon, forming a P-N junction. When sunlight shines on the surface of the solar panel, some photons are absorbed by the silicon material. The energy of the photons is transferred to the silicon atoms, causing electrons on the surface of the silicon atoms to migrate and become free electrons, accumulating on both sides of the P-N junction and creating a potential difference. Connecting the two sides of the silicon wafer with wires to the outside forms positive and negative electrodes. When the positive and negative electrodes are connected to a circuit, under the influence of this voltage, a current will flow through the external circuit, generating a certain output power. Essentially, this process converts photon energy into electrical energy.
Materials that can generate the photovoltaic effect mainly include monocrystalline silicon, polycrystalline silicon, amorphous silicon, gallium arsenide, and copper indium selenide. The strong demand for monocrystalline (polycrystalline) silicon for solar panels has driven the development of related equipment manufacturing industries. Commonly used equipment in this industry includes polycrystalline furnaces, cutting saws, wire squaring machines, rounding grinders, end face grinders, chamfering grinders, and multi-wire slicing machines.
Due to the high cost of raw materials such as monocrystalline silicon rods and polycrystalline silicon ingots, coupled with rising prices and increasing labor costs, manufacturing companies are demanding higher levels of automation and production efficiency from their equipment. This article introduces the design and application of the CNC system in our newly developed 7-axis CNC double-plane grinding machine for silicon blocks, model WSK015. The machine's appearance is shown in Figure 1.
Figure 1: Outline drawing of WSK015 CNC silicon block double-plane grinding machine
Technical features and process flow:
Compared to traditional dual-axis, three-axis, and five-axis face grinders, the WSK015 7-axis CNC silicon block dual-plane grinding machine has the following technical features:
1. The machine tool is equipped with a self-centering system to ensure that the silicon block being processed is in the center of the worktable, ensuring uniform and consistent grinding of the two surfaces, minimizing silicon material loss, and reducing the operator's requirements for the position of the silicon block, thus reducing the operator's workload.
2. The machine tool is equipped with an automatic clamping device, which automatically clamps the silicon block after the silicon block completes the automatic centering function, thereby improving work efficiency and ensuring the clamping quality.
3. The machine tool is equipped with an automatic laser measurement system. After the silicon block is clamped, the Z-axis feeding servo starts to work. At this time, the laser measuring instrument automatically and accurately measures the size of the silicon block and transmits it to the CNC system for calculation so as to evenly distribute the grinding allowance of each grinding head.
4. Except for manual loading and unloading, all other functions of this machine tool are automatic. During the grinding advance, the machine automatically measures the dimensions and distributes the grinding allowance to each surface. Dimensions are measured after grinding. The three pairs of brush grinding heads are for rough grinding, fine grinding, and finishing grinding.
5. Servo motors and ball screws are used to control three pairs of grinding heads X1, X2, X3, U1, U2, U3, and the feed axis Z. Each axis can be controlled and operated independently. A handheld electronic handwheel control unit is also provided for tool setting operations on each axis.
6. The machine tool performs precision machining on the two opposing surfaces of the silicon block. After grinding with three brushes of different coarseness, the surface smoothness of the silicon block is improved, and the fine cracks and edge damage formed on the surface of the silicon block during the wire cutting process are removed, thereby improving the yield of silicon block wafers in the next process.
II. The process flow of the WSK015 CNC silicon block double-plane grinding machine is shown in Figure 2:
Figure 2: Process flow of WSK015 CNC silicon block double-plane grinding machine
The structure and principle of the control system:
Figure 3 shows the control architecture of the WSK015 7-axis CNC silicon block dual-plane grinding machine.
Figure 3: Control architecture of WSK015 7-axis CNC silicon block double-plane grinding machine
II. Functions of CNC numerical control system:
The machine tool's control system is a CNC numerical control system jointly developed by our company and a well-known domestic enterprise. This system handles all data processing and logical interlocking for the entire machine tool. Because the functions implemented by the CNC numerical control system are achieved by the user through software programming, and the hardware and software adopt a modular structure, the modification and expansion of system functions are quite flexible. Furthermore, the CNC system offers a rich set of functions, such as interpolation, compensation, advanced human-machine interface, and programming capabilities.
Based on the high-speed computing power and ease of function programming of CNC, the CNC system of this machine tool needs to process the following data:
1. Collect high-speed pulses from the electronic handwheel to the 7-axis servo and control the movement of the corresponding servo motors online in real time. The control accuracy is 1 micrometer, and the servo motor response hysteresis is less than 50 milliseconds. This ensures that when the handwheel is turned, the servo motor moves accordingly, and when the handwheel is stopped, the motor stops immediately. All position data is displayed on the screen.
2. The instrument collects measurement data of the silicon block size from the Keyence laser position sensor and allocates the feed amount of each pair of grinding heads and the feed speed of the feeding motor according to the actual size. The measurement accuracy of this instrument can reach 0.01mm.
3. Collect voltage, current, and coded feedback data from each servo drive and display them on the screen in real time. Compare the current magnitude with the set empirical value; if the value exceeds the set value, an alarm will sound to alert the corresponding mechanical problem. Determine the position of each grinding head and the feeding silicon block based on the coded feedback pulse, set alarms and limit values, and stop the machine and report the corresponding fault information if the position exceeds the limit, facilitating troubleshooting by maintenance personnel.
4. Real-time communication to collect other information from each slave station.
Based on CNC PLC and extended PLC functions, the system performs logical interlocking control on all digital signals of the machine tool, including start/stop control of the six grinding head motors, control of all cylinders and detection of up/down position magnetic switches, automatic oil mist lubrication of all linear guides and ball screws, position detection of inductive proximity switches at the limit positions of all seven servo motors, automatic control of grinding coolant for each grinding head, and online diagnosis of all fault points. The system has 72 digital input points and 48 digital output points. Through PLC functions, these signals are detected and the outputs are controlled, achieving fully automated control of all machine tool movements except for manual handling when loading and unloading silicon blocks, and enabling cyclic operation.
III. Servo Motor
Servo motors are essential actuators in CNC systems, and their high-speed response, high-precision positioning, and high stability play a vital role in the control of CNC machine tools.
The WSK015 7-axis CNC silicon block dual-plane grinding machine uses ESTUN's PRONET high-end AC servo drive for the grinding head servo motor and the feeding motor. It adds functions such as current feedforward control, acceleration feedforward control, speed observer and inertia observer. With the help of these new functions, its response performance is more than three times that of general products. It can also detect the load inertia online in real time and adjust the gain at any time to achieve the best control effect.
The PRONET series servo drives are equipped with a high-resolution 17-bit serial encoder, supporting both incremental and absolute types. The communication rate is 2.5 Mbps, and the encoder resolution is 17 bits/rev, meaning that 131,072 pulses are generated per revolution. This significantly improves positioning accuracy and enhances low-speed smoothness and responsiveness.
By utilizing the absolute value 17-bit serial encoder of the servo motor, the CNC can achieve 1-micron precision control for 6 grinding heads. Furthermore, by using the encoder battery to memorize the position of the servo motor during a power outage, processing can continue from the current position when power is restored. This reduces the damage to the processed parts and the need for operators to reposition the process during factory power outages or power failures, thus saving time and improving production efficiency.
IV. CANopen Bus Protocol
CAN, short for "Controller Area Network," utilizes the CANopen bus protocol for crucial data transmission in machine tool control. Due to its high-speed data transmission rate of up to 1Mbps, CAN easily enables real-time display of the position of each grinding head and real-time monitoring of the current of each servo motor. Therefore, by designing the CANopen network data interface, analyzing its communication characteristics, and confirming the CAN interface parameters, monitoring of various control effects can be achieved. As a technologically advanced, highly reliable, feature-rich, and cost-effective remote network communication control method, CAN has been widely applied in various automated control systems. This can be achieved through bus control.
Firstly, it can save a lot of cable connections compared to other control methods, as it connects various electrical components through communication lines, simplifying the wiring method.
Secondly, it reduces the potential for malfunctions, greatly lowers the machine's failure rate, and ensures the machine's production time.
Thirdly, it facilitates the maintenance and troubleshooting of the machine by engineers.
The bus network connection is shown in Figure 4 (partial).
Figure 4: Bus control network connection
Conclusion:
This article mainly introduces the design and application of CNC products on a 7-axis CNC silicon block double-plane grinding machine. It focuses on the functional characteristics of the CNC system and servo motor, as well as their role in this grinding machine. It reflects that our company's positioning standard in developing this type of machine tool is to meet customers' requirements for high precision, low cost, reliable operation, simple operation and fully automated intelligent control. It provides the market with an advanced solar silicon block processing equipment, while also saving customers a lot of raw material costs and maintenance costs.
