Basic motion of the hottest digital gear machining

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Basic motion of digital gear machining at present, it is more and more common to use numerical control method to process gears. The so-called "numerical control machining" refers to the use of computer-controlled servo system to drive and control the movement of the machine tool when machining gears. The gears processed by this method are called digital gears. There are two basic motion modes of NC machining: 1) accurate tooth division. Precise synchronous motion is required. It is mainly the synchronous movement between workpiece and tool. 2) Different gears have different shapes, which are formed by interpolation and linkage of different moving shafts. Therefore, the machining of gears also requires multi axis linkage interpolation

I. synchronous motion

1 Synchronization concept

there are two types of synchronization: mechanical synchronization and electromechanical synchronization. In the electromechanical synchronization system, the concept of synchronization refers to that the system has two or more control objects composed of an electronic controlled automatic regulation system and a servo motor, one of which is the main control object and the other one or more slave control objects. The control quantity is the displacement or speed of the machine (for the rotating motion, it is the rotation angle or speed). The controller keeps a certain proportional relationship between the output control quantity of the slave control object and the master control object, This kind of motion system is called synchronous system. The synchronous system is a position control system in gear machining. For simplification, the control device in the synchronization system is simplified as shown in Figure 1. Where K1 is the gain of the position controller of the simplified control device; K2 is the gain of the simplified speed controller; K=k1k2 is called the open-loop position gain of the system. X1, x0 and D are input, output and load disturbance converted to speed controller input. △ is the error between input and output

simplified control device block diagram in Figure 1

the control device in Figure 1 can be used to form two synchronization systems:

a. active synchronous system: the output of the master control device is used as an instruction to control the slave control device to ensure that its output maintains a strict proportional relationship with the output of the master control device. It is called the electronic gearbox function in gear processing. As shown in Figure 2A. Wherein, xamo is the output of the main control device of the self synchronizing system, and xaso is the output of the slave control device of the self synchronizing system. Its output automatically follows the output change of the main control device, which is called self synchronization system

Fig. 2 two types of synchronous systems

b. passive synchronous system: in the synchronous system, the controller sends instructions to the master control device and the slave control device at the same time to synchronize the outputs of the two control devices, as shown in Fig. 2B. Xpmo is the output of the main control device of its synchronization system, and xpso is the output of the slave control device of its synchronization system. In this synchronous system, if the output of the master control device changes for some reason, the output of the slave control device is not controlled. This system is called its synchronous system. Due to its lack of self synchronization ability, it is not used as synchronization in gear processing

2. Improve the synchronization performance

a. for the self synchronization system, to improve the synchronization performance of the system, it is necessary to reduce the following and synchronization errors of the control device. In order to reduce the errors, on the premise of system stability, it is necessary to maximize the position gain K of the system and reduce the load disturbance

b. for its synchronization system, to improve the performance of the synchronization system, first of all, it is necessary to reduce the following error of the control device, improve the position gain K of the control device, and reduce the load disturbance. In addition, to reduce the synchronization error, the parameters of the master and slave control devices in the system should be as close as possible; The load disturbance of master and slave control devices shall be minimized and the same load shall be distributed

c. in addition to the above methods, the feedforward method can also be used for control to reduce the following error and improve the synchronization performance. As shown in Figure 1, part a is the feedforward control part. When a=s/k2: △ =0, that is, theoretically, the following error caused by input can be reduced to zero. This is a feedforward compensation method to compensate the error caused by the input. In addition, the motion command shall be accelerated or decelerated to smooth the motion command. During acceleration and deceleration processing, for its synchronous system, special attention should be paid to the symmetry of acceleration and deceleration parameters of master and slave control devices

3. The synchronization reference point and the reference point of the compensation

synchronization system represent the starting point of the position of the master control device and the slave control device when the system enters the synchronization. Return the master and slave control devices to the starting point before synchronization

error caused by "pitch error" of mechanical transmission chain (i.e. different distance of motor per revolution) and clearance of mechanical transmission chain. When the semi closed loop is adopted, the compensation link should be added to improve the accuracy. If the system is digital, the compensation can be carried out in each sampling time interval. Compensation is required for both master and slave control devices

II. Interpolation movement

in order to process a certain tooth profile and tooth direction, such as hobbing helical gears, in addition to the rotary motion of the hob, the tooth splitting rotary motion of the workpiece and the hob and the vertical feed motion of the hob, additional rotation of the workpiece is also required. These movements must satisfy certain relations before helical gears can be machined. This requires interpolation of each axis of motion, which is called interpolation motion

The core technology of numerical control system of machine tool is interpolation technology. Interpolation is the movement process that the numerical control system determines the position coordinate values of multiple intermediate points between two known points on the required path or contour line according to a certain mathematical function. According to the coordinate values of these positions, the movement of the tool or workpiece is controlled to realize NC machining. According to different machining parts, there are single coordinate and multi coordinate interpolation. The mathematical functions of interpolation can be straight lines, arcs, various plane and space conics, helices, involutes, free curves, etc. Because straight lines and arcs are the basic lines that constitute the contour shape of the workpiece, and various complex curves can be decomposed into many small segments of straight lines or arcs, general CNC devices have the interpolation of straight lines and arcs. The high-grade CNC system also has the interpolation function of conic curve and spiral line

general NC machining process requirements: 1) the cutter or workpiece moves according to the determined straight line or curve, so as to process the required shape. 2) In order to ensure the roughness and accuracy of the parts that have been set up by the national power battery innovation center of the people's Republic of China, and to extend the service life of the tool, the tangential speed between the tool and the workpiece is required to remain unchanged during the machining process

the device that completes interpolation is called an interpolator. The interpolator in the hardware NC system is generally composed of digital circuits, which is called hardware interpolation. In CNC system, the function of interpolator is completed by software or software and hardware together, which is called software interpolation or mixed interpolation. Since there are basically two types of output from NC device to servo drive device: pulse train and digital increment value, there are two different interpolation methods according to different output forms: reference pulse interpolation and data sampling interpolation. According to the programmed track and the selected interpolation method (line, arc or others), the system calculates the number of pulses or values equivalent to the displacement of each coordinate within the sampling time and sends them to the servo system to control the movement of the system. In general, the disorderly situation of private mining, over mining, over production and disorderly competition in the rare earth industry should be ended as soon as possible. It is said that the shorter the sampling time, the higher the accuracy of the system

according to the different calculation methods of interpolation value, three interpolation methods are mainly used at present: 1) digital pulse multiplier interpolation; 2) Point by point comparison interpolation; 3) Digital integral interpolation

there is a great relationship between the accuracy of NC machining and interpolation, especially in the dynamic process (refers to the acceleration process) and the parts where the servo axis feed rate changes greatly during machining, such as sharp corners, corners, etc. At this time, special measures should be taken to ensure the accuracy of interpolation. In order to carry out high-speed and high-precision machining without interrupting the processing speed, the system is required to have the function of look ahead for multi program segments. In addition, when a corner is encountered during machining, the speed of the moving axis will change sharply, and the machine tool will produce a great acceleration. In order to smooth the motion, automatic acceleration and deceleration are required. In order to ensure the smooth movement of the actuator during start and stop without strong impact, the CNC system must control the speed of the servo motor by acceleration and deceleration. Acceleration and deceleration can be conducted before or after interpolation; The acceleration and deceleration control before interpolation only controls the synthetic speed, that is, the programmed command speed, so it will not affect the position accuracy of the actual interpolation output. After interpolation, the acceleration and deceleration of each moving axis are controlled respectively, so the position error may occur after acceleration and deceleration

to realize rapid and high-precision machining on NC machine tools, the following problems must be solved: 1) large memory capacity of NC part programs; 2) Improve the running speed of transferring NC part programs to NC; 3) Improve the performance of converting small line segments into motor operation. The first two problems can be solved by adding a data server in front of the system and transmitting data through Ethernet. The latter problem can be understood as follows: read the NC part program of one program segment and make necessary data processing for the interpolation operation of motor movement instructions, which is called "program segment processing". The time required for processing is called "block processing time (BPT)". The shorter the BPT, the faster the transfer of small movements. In order to shorten BPT, special microprocessor can be used; The 64 bit RISC chip can also be used for high-speed operation to realize high-speed performance

III. synthetic motion

take the gear hobbing machine as an example. The CNC gear hobbing machine has 6-axis control and hob spindle (b); Workpiece spindle (c); Radial feed (x); Tangential feed (y); Axial feed (z); Hob swivel seat (a). The movement between the hob and the workpiece is performed by the function of the electronic gear, as shown in Figure 3

Fig. 3 working principle of electronic gear

within the sampling time, the system reads the feedback pulse number of hob spindle B, then calculates the synchronization pulse number strictly according to the synchronization ratio R, and then outputs it to the workpiece spindle. For the system with good performance, the sampling time is completed within 1ms. The horizontal tensile testing machine adopts mature testing machine technology. When the system needs to process helical gears, it also needs to add an additional rotation to the movement of the workpiece. When the length of the hob moving in the vertical direction is equal to the helix lead of the gear, "the shaft should rotate additionally. This requires interpolation. Add an additional movement to the" shaft. The increased angle is bp:

bp= (360/p) ztmsinp

where Z is the movement of Z axis; P is the helix angle; T is the number of teeth; M is modulus

according to the formula, various helical gears can be processed by interpolation

in addition to helical gears, these two movements can also be properly matched with the mechanical structure to process gears of different shapes

IV. summary

this paper discusses two basic movements of digital gears, namely, synchronous and interpolation movements. The systems used in these two movements are the position control system, which is steadily promoted by the intelligent networked vehicle. With the cooperation of these two movements, various digital gears can be machined

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