- Detail

The rapid development of high-speed machining technology has been widely concerned by academia and industry. Now high-speed CNC machine tools are gradually replacing ordinary CNC machine tools and become the mainstream of the development of CNC technology. In general, when discussing high-speed CNC machine tools, high spindle speed and high fast moving speed are mentioned, and the indicators related to actual machining are rarely mentioned, especially the important role of high acceleration on machining accuracy is less discussed. For example, in the high-speed machining of complex surfaces, for the NC code composed of a large number of micro segments (0.1~0.5mm), how much can the feed speed of the machine tool be achieved under the condition of ensuring the machining contour accuracy? High speed and high precision machining has considerable requirements for machine structure, functional components, feed system, cutting tools, etc. because there are many literatures and introductions in these aspects, we will not discuss them here. This paper mainly discusses the basic requirements of high-speed CNC machine tools for CNC system and the role, characteristics and requirements of high-speed CNC system in actual high-speed machining

2. A sufficiently high feed acceleration is the guarantee of high-speed machining accuracy.

high speed machining mainly refers to the high speed, high feed speed and high feed acceleration of the spindle. The relationship between the first two is expressed by the following formula:

spindle speed: n=vc/d π

feed speed: vt=fzzn

fz -- the cutting thickness of each blade in one revolution, unit: mm

z -- the number of edges of the milling cutter

vc -- linear speed of the tool, unit: mm/min

d -- diameter of the tool

substitute n into the above formula to obtain the feed speed: VT = fzzvc/d π

that is, when the tool and cutting parameters are selected, the feed speed is directly proportional to the speed of the spindle. Therefore, the high-speed machining machine tool should not only have a high spindle speed, but also have a high feed speed matching the spindle speed (not only a high idle travel speed). In addition, in order to ensure the high accuracy of the machining contour, the machine tool must also have a high feed acceleration. If a high-speed machine tool does not have a high enough feed acceleration, it cannot process the contour of a high-precision complex surface at high speed, because it cannot meet the need to constantly adjust the feed speed in the shortest time according to different curvature radii when machining complex surfaces

3. High precision interpolation is the basis of high-speed and high-precision CNC system

cnc servo system executes the data of NC code dispersed by the CNC system. The first requirement of high-speed and high-precision processing is extremely short interpolation cycle and high calculation accuracy. For example, fanu16i uses nano level position instructions for calculation and data exchange

interpolation cycle △ t= △ l/f

if △ l remains unchanged, f will be doubled, and the interpolation cycle △ T will be doubled. In high-precision contour machining, chord height error should be reduced ε， It is also necessary to reduce △ L, which requires a shorter interpolation period △ t (Figure 1)

when the sampling period △ t becomes smaller, if the calculation accuracy is not high enough, errors will occur, and the stability and continuity of servo speed will be affected. For example, a straight line is interpolated on the XY plane (Figure 2), the interpolation cycle is 0.5ms, the feed speed is 6m/min, and vy=6 × sin2=0.209m/min; Y-axis position increment of each interpolation cycle △ ly=vy × △T=1.74 μ m. Obviously, if the system interpolation calculation accuracy is 1 μ m. It not only affects the contour error, but also causes the theoretical speed instability and discontinuity in the operation of y-axis

the interpolation accuracy of sky2003n CNC system of Sikai company is 1 nm (0.001 μ m. ), the sampling period and interpolation period are 0.4ms ~ 0.1ms

4. Feedforward control reduces the lag of the servo system, and the acceleration and deceleration before compensation eliminate the theoretical difference of acceleration and deceleration output after interpolation. The CNC servo system is a complex control system. The traditional servo control system is mainly used to PID adjust and control the servo position deviation and speed deviation. Because the known subsequent interpolation output conditions, inertia of machine tool moving parts, friction damping lag and other information are not used, In high-speed machining, the dynamic following error will be relatively large. In modern CNC systems, feedforward control is generally used to reduce the lag of servo system, such as speed feedforward and torque feedforward tracking error compensation technology adopted by Siemens 840Di CNC system

4.1 servo feedforward control reduces the following error introduced by friction and system inertia

because the speed of each axis changes at a high speed in the high-speed machining of complex surfaces, in order to reduce the error of the dynamic process of the machine tool system with complex surfaces, the dynamic characteristics can be improved through effective friction feedforward and acceleration feedforward. Generally, the response of servo drive system to torque or thrust command is fast, while the response of speed loop and position loop lags behind. Therefore, in modern numerical control system (such as sky2000 numerical control system), in order to speed up the response speed of speed loop and position loop of servo driver, the speed and position closed loop of electric machine is completed by control system (see Figure 3), and servo driver only controls current loop. Figure 3 is the speed loop and position loop control block diagram of sky2000 CNC system. Friction feedforward FC can compensate the friction resistance and vertical weight imbalance of the mechanical system in advance, and acceleration feedforward Kaff can compensate the mechanical inertia in advance. In practical application, the error of dynamic response of the machine tool can be close to zero

Figure 3

fc - friction feedforward Kaff - acceleration feedforward KP proportional gain t motor torque or thrust command

kvff - speed feedforward Ki - error integral kD differential gain

4.2 acceleration and deceleration processing before interpolation makes the acceleration and deceleration output synthetic trajectory unchanged

after interpolation, each axis accelerates and decelerates respectively to make the actual output trajectory deviate from the interpolation trajectory (Figure 4b), In high-speed machining, no matter which mode of acceleration and deceleration (exponential, linear acceleration and deceleration) is adopted after interpolation, greater contour error will be generated. Due to the greatly improved computing speed and ability of the computer CPU, the pre compensation acceleration and deceleration calculation and prediction processing are generally realized by software in modern CNC systems (Fig. 4C), so that the spatial synthetic trajectory and theoretical trajectory output after acceleration and deceleration are basically unchanged

figure 4

5. The clamping force of the continuous sample will change with wear. Look ahead

in high-speed machining, the optimization pretreatment of acceleration and deceleration of the ahead path is just like driving a car on various roads. The road surface is good, and there is no sharp turn in front. You can increase the throttle and drive faster. If there is a turn in front, you have to reduce the throttle in advance and drive slower. In high-speed machining, G code is the road surface, and the motor is your car. In order to ensure the accuracy and stability of the machine tool under high-speed motion, the system must see a series of spatial paths to be processed, and see far enough according to the speed. During multi axis linkage control, according to the G code in the program preprocessing buffer (the advance processing program section of sky2000 high-speed machining system allows 2500 lines), whether to reduce the current speed or increase to the theoretical speed can be determined by comparing the theoretical acceleration and deceleration of each axis with the actual allowable acceleration and deceleration of each axis, that is, dynamically adjust the feed speed according to the radius of curvature of the circular arc, Its working principle is: first, set a maximum allowable feed speed for circular arcs with different radii. When the CNC system finds that the maximum allowable feed speed of a circular arc to be processed is less than its programming speed, it will automatically reduce the feed speed to the maximum allowable feed speed of the circular arc. If the NC system finds that the path to be processed is relatively flat, it will immediately increase the feed speed to the maximum allowable theoretical feed speed. The NC system of the machine tool will make the machine tool work at the maximum theoretical speed as far as possible under the condition of ensuring the machining accuracy. It can change the feed speed 2000 ~ 10000 times per second to achieve the above purpose

in the high-speed machining of complex surfaces, due to the dense NC data and the short vector distance of data segments, only processing the pre compensation acceleration and deceleration between two segments of data will produce excessive deceleration, and only using impact smoothing will have large contour error

as shown in Figure 5a, the theoretical speed during high-speed machining is V, and the speed of PI at the corner is related to the dynamic acceleration and deceleration characteristics of the machine tool, that is, it is related to the allowable acceleration a of the machine tool spindle and the allowable acceleration and deceleration change rate j=da/dt (that is, the change rate of motion force reduces the impact). To ensure high contour accuracy, it is necessary to establish the deceleration point PK and the acceleration and speed of each section from PI to PK in advance according to the speed V, the allowable acceleration a of each axis and the acceleration change rate J, so as to ensure the contour accuracy of each point. However, the general system only calculates the acceleration and deceleration of PI-1 to PI in advance, and the error is large in high-speed machining (Fig. 5b)

5.1 predict the reasonable acceleration and deceleration program segment

under the condition of certain motion quality, the thrust is proportional to the acceleration, that is, f=ma. For the machine tool driven by linear motor, the acceleration Amax that can be achieved is related to the mass of the moving parts of the machine tool and the thrust of the linear motor. For the machine tool driven by rotary servo, it is related to the moment of inertia and motor torque converted to the motor shaft

in the acceleration and deceleration pretreatment of the NC system, the maximum acceleration that the machine tool can achieve must be considered. At the same time, the acceleration change rate of the smooth operation of the machine tool must be considered in order to ensure the dynamic accuracy of the machine tool. In sky2000 CNC system, the forward-looking processing steps of continuous contour of multi segment NC code are as follows:

(1) during debugging, determine the maximum acceleration aimax of each axis and the maximum acceleration change rate jimax (j=da/dt) of stable machine operation on the premise of ensuring tracking accuracy

(2) the maximum speed of large and small arcs is determined by the aimax and jimax of each axis

(3) the aimax and jimax of each axis determine the allowable speed and acceleration between program segments during continuous operation of the micro segment, and recursively predict the reasonable deceleration program segment (the multi segment program buffer should be large enough to ensure that the forward search range meets the requirements)

figure 5C is the actual contour track after the forward-looking optimization processing of multiple NC codes. In the sky2000 CNC system, the high-speed and high-precision contour control mode is shown in Figure 6

figure 6

5.2 General requirements for forward-looking processing of high-speed machining

(1) due to the complexity of multi-stage prediction calculation, it is best to have two CPUs for parallel processing of interpolation and preprocessing to ensure data continuity and real-time performance

(2) feedforward control reduces errors caused by acceleration and friction changes during interpolation

(3) in the structural design of the machine tool, the coating has reliable fire resistance, light self weight (dry density of 514 kg per cubic meter), simple structure, and long hydrocarbon fire resistance time (the coating thickness is 23 mm, the workpiece is best not to move, and the inertia of each axis is certain, which simplifies the control and optimizes the parameters.

(4) reasonably adopt new transmission elements (such as linear motors) to increase the allowable acceleration and acceleration change rate of each axis, which can reduce the number of prediction program segments, Improve operation efficiency

Treat small things as big things(5) adopt high-capacity NC code memory (above 40g) or high-speed transmission mode (such as Ethernet with speed greater than 10m, using tcp/ip communication protocol) to avoid data hunger caused by general transmission

6. The system is required to accurately predict the truncation error of high-speed sampling to ensure the stability of the system operation

in multi coordinate high-speed sampling interpolation, because the sampling interpolation cycle is very short (in sky2000 CNC system, the speed loop and position power frequency are 50Hz; the three loop sampling cycle is 0.1ms), and the resolution of the feedback grating is limited, so some coordinate axes may have a position pulse after dozens of sampling cycles during low-speed operation, Regardless of the running speed, the sampling truncation error of pulse at any time of high-speed sampling

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