Performance analysis of the most economical CNC sy

2022-08-17
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Performance analysis of economical numerical control system

the scope of this paper is the single-chip microcomputer numerical control system, which takes industrial PC as the platform and has CRT display, belonging to another category. However, the contents discussed in this paper are suitable for them in principle, but due to the different microprocessors, the performance indicators of the two should also be different

1 convenience of using the system

the convenience of using the system is mainly reflected in the part of the system. Part is the part that people directly deal with the system, that is, the so-called human-machine interface. The man-machine interface should be user-friendly, that is, the part should try to provide convenience, quickness and comfort to users. As a good system, it should have the following performance

1) modal instruction modal instruction, also known as continuous instruction, refers to an instruction that not only functions within this program segment, but also remains in effect in subsequent program segments, and can be omitted until it is replaced or suspended by appropriate instructions. Obviously, using modal characteristics can save the workload of programming, so modal is the basic function of a CNC system

2) multi instruction common segment multi instruction common segment refers to that a program segment can include multiple instruction words. Multiple instructions cannot share segments, which is bound to increase the number of segments of instructions, which is inconvenient

3) integration of input, retrieval and modification many domestic systems divide input and retrieval into two different operation methods, resulting in that the input process cannot check the program, and the retrieval process cannot automatically produce the program segment number, which is very inconvenient. A good system should realize the integration of input, retrieval and modification, that is, when inputting, it can be retrieved and modified, when retrieving, it can be modified and entered, and the program segment number is automatically issued

4) Chinese keys at present, foreign CNC systems try to use Chinese man-machine interface if they want to enter the Chinese market. The economical CNC system produced by ourselves should adopt Chinese keys for the convenience of users

5) when inputting a program, it is difficult for users to input letters in alphabetical order, and they want to search in alphabetical order when searching. For the convenience of users, the CNC system should have the function of automatically sorting letters

6) fast search users hope to quickly find the required program segment when searching the program. Therefore, the system should have the functions of quickly turning over the paragraph, immediately finding the end of the program, the beginning of the program and any paragraph in the program

7) strong command function can greatly simplify user programming. For example, machining a five head left-hand worm, the worm length is 100mm, the tooth depth is 3mm, the modulus is 0.8, and the thread cutting of each head is divided into 15 cycles. For the processing of such a part, many domestic systems require users to write 75 cycle program segments, plus staggering a pitch when dividing multiple threads. He believes that this is the instruction of the governor to be highly responsible for his party, country and people. In addition, an X-direction walking instruction must be arranged before the instruction is executed to determine the backward direction, otherwise the program will make mistakes, etc. In this way, the processing program of the above parts will take several pages after being compiled. Some systems can automatically realize 75 cycles with only one instruction, namely G31 W100 I-3 K5 f0.8 L15, which is obviously much more convenient

8) diversity of instructions for arc machining, the center coordinates can be programmed with I, e.g. K, or with radius R. It is convenient to program with radius r, but it cannot describe the whole circle. Programming with I and K is a little inconvenient, but it can describe the whole circle. A good solution is to have both functions

for thread processing, the thread with a tool withdrawal groove hopes to resolve the new idea of excess capacity. It is expected to adopt straight-line tail withdrawal to reduce the tail withdrawal length. In the thread turning of pipe threads, bolts, etc., the tailing length is secondary, and the important thing is to require equal pitch tailing to ensure good sealing during thread connection. However, the length of equal pitch tailing is too long. A good solution is to have both functions to meet user requirements

2 progressiveness of technology

1) interpolation speed interpolation refers to oblique lines and arcs. The arc interpolation speed of a system can well reflect the level of the system. The arc interpolation speed of domestic economical CNC systems is generally less than 600mm/min, but some have reached 1200mm/min

2) thread function thread function can best reflect the level of a system. The thread function of a system is usually seen from four aspects: the first is the degree of automation, the second is the high-speed processing ability, the third is the large lead processing ability, and the fourth is whether there is equal pitch tailing function

· degree of automation: the thread processing software should be able to automatically determine whether the zero position signal of the spindle encoder is valid at high level or low level, without changing the hardware; It can automatically calculate the spindle speed without inputting the spindle speed s command in the program, and it can also avoid errors caused by wrong spindle speed input; It can automatically index the multi head thread and index along the circumferential direction, so as to avoid the tool touching the center or the shaft shoulder of the workpiece during axial indexing; It can automatically identify the direction of tailing according to the sign of thread cutting depth; It can realize the automatic cycle of multiple cutting

· high speed machining ability: the high speed machining ability of threads is mainly reflected in the feed speed of threads. Thread feed speed = spindle speed × Lead. The feed speed of thread shall be above 2m/min

· large lead processing capacity: many domestic systems provide a formula for this in the instructions as n ≤ (1200/t) -80

, where n - spindle speed, r/min

t - lead, mm

for large lead t=12mm, this system can only process when the spindle speed is less than 20r/min, which is difficult to meet user requirements. The economical CNC system can increase the spindle speed to 125r/min without losing step when machining threads with 12mm lead

· equal pitch tailing function: in the thread turning of pipe threads, bolts, etc., equal pitch tailing is required to ensure good sealing during thread connection. If the system does not have the function of equal pitch tailing, it cannot be guaranteed

3) speed connection technology stepping motor needs to increase or decrease frequency when running. Therefore, from one program segment to another, the stepping motor first reduces the frequency, stops, and then increases the frequency. When the two contour curves intersect, this is not a big problem, but when the two contour curves are tangent, this feature of the stepping motor will cause machining tool marks or platforms. Using the speed connection technology, the system can realize the smooth connection of the speed between the two programs in the processing process, which not only ensures the processing quality, but also improves the productivity

4) manual setting and chamfering only process a cylindrical surface or chamfer. If there is manual setting and chamfering function, there is no need to program

5) the technology of speed regulation and coordinate dynamic display in the processing process reflects the technical level of the software. Some systems fail to deal with this problem, so the speed cannot be adjusted in the processing process and cannot be dynamically displayed when the speed is greater than 2m/min, which brings inconvenience to users

6) power down protection movement position and breakpoint operation. If the machine tool suddenly loses power in the processing process, users always hope that the system can record the current position and continue to run from there when the power is on, so as to avoid the scrap of the workpiece. The power failure protection movement position and breakpoint operation function can solve the above problems

7) electric tool rest controller electric tool rest controller is a part of CNC lathe with high failure rate. General tool holder controller adopts relay logic control circuit to realize automatic tool change control of electric tool holder. There are a large number of components used in this logic control circuit, especially nearly 10 relays with mechanical contacts, which have poor reliability. Some CNC systems use software to control tool change, and only two contactors are used, which greatly improves the reliability of electric tool holder control

8) automatic tool setting is often equipped with electric tool rest on CNC lathes. The relative positions of several cutters on the electric tool rest must be known before machining. If manual trial cutting and knife setting is adopted, it will take time, effort and materials. Automatic knife setting technology is adopted, which is simple, rapid and accurate. It only takes a few minutes to set four knives

9) closed loop control closed loop control technology can automatically compensate the deformation, clearance and transmission error of mechanical transmission parts, so as to ensure the machining accuracy of machine tools

10) subdivision technology subdivision control technology is an important technology of current economic CNC system. Its advantages: first, it effectively solves the problem of low-frequency oscillation of the stepping motor, so that the stepping motor can run smoothly at low speed and no step loss. The second is to refine the pulse equivalent of CNC machine tools. For example, ten subdivisions change the original pulse equivalent of 0.01mm to 0.001mm, thereby improving the control accuracy. Third, the subdivision control technology can also increase the output torque of the stepping motor by about 30% at low speed

11) PLC function some machine tools not only need CNC system to realize accurate positioning or trajectory control, but also hope that the system has the function of programmable logic controller (PLC) to realize the logic control of switching value. Of course, the above pressurizing device and control circuit can be realized by using CNC system +plc controller; The automatic feeding device includes an automatic feeding turntable, but it is expensive and troublesome after all. If the CNC system itself has PLC function, and the programming of PLC part adopts the ladder diagram programming that everyone is used to, it will be welcomed by users

12) an important factor of VMOS driving technology affecting the reliability of the system is the stepping motor driving power supply. The driving power supply using power transistor has complex circuit and high power consumption, especially due to the secondary breakdown of transistor, which leads to the poor reliability of high-power transistor driving power supply. VMOS has no secondary breakdown problem, simple circuit, low power consumption, high switching speed and greatly improved reliability. Therefore, a good system should have VMOS driver technology

3 system reliability

depending on the reliability of a system, we should pay attention to the following five aspects

1) as an industrial control microcomputer, system design should take a series of measures in hardware anti-interference and software anti-interference to ensure high reliability

2) production conditions system production specifications, with strict component screening, aging, inspection and testing capabilities. Before leaving the factory, the whole machine should run continuously at a temperature of 45 ℃ for 48h without problems, so as to ensure the reliability of the system

3) the driving technology adopts advanced VMOS driving technology, which is much more reliable than high-power transistor driving

4) tool rest controller electric tool rest controller adopts software control, which is much more reliable than the logic control of relay switch with contact

5) structural design domestic systems usually have two vents, one inlet and one outlet, and the wind passes through the system. This structure not only makes the system air-cooled, but also brings dust, lampblack, chips and so on to the inside of the system. Such a system has been used for a long time, and it is difficult not to go wrong. A well-designed system should have positive air pressure in the system chassis, or let the cooling air pass through the outside of the system, that is, the so-called external cooling method should be adopted

4 how to choose an economical CNC system

1) the system should be reliable. The above reliability discrimination method can be used to judge or inquire about the use situation from the old users of the manufacturer

2) the system performance shall meet the requirements

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