0752-3083349    


About us

ABOUT US

  • Tel0752-3083349

    Fax0752-3083359

    CNC Design 0752-3083369

    E-mailfxwedm@126.com

    AddressChen town Baiyun Mountain Boulevard into the Huicheng District of Huizhou City, 1500 meters (JinBoUue Hotel diagonally opposite)
Current locations : Home > About us

Digital full closed loop control of CNC machine tools

點擊數9482016-04-11 15:32:23 來源: Huizhou Yi Tuo industrial automation equipment Co., Ltd.

High precision position control is the key technology of developing high precision NC machine tool. In recent years, we according to the needs of the development of domestic high precision CNC machine tools, to fully digitized closed loop high accuracy position control technology were studied taking the opportunity of development based on a new digital closed loop position control system, and has carried on the application in a variety of domestic CNC machine tools, in the complex precision machining parts achieved good results. The basic composition, dynamic structure, controller design and application of the new system are introduced in this paper.


High precision position control is the key technology of developing high precision NC machine tool. In recent years, we according to the needs of the development of domestic high precision CNC machine tools, to fully digitized closed loop high accuracy position control technology were studied taking the opportunity of development based on a new digital closed loop position control system, and has carried on the application in a variety of domestic CNC machine tools, in the complex precision machining parts achieved good results. The basic composition, dynamic structure, controller design and application of the new system are introduced in this paper.


Conventional digital position control


At present, the digital AC servo system is getting more and more extensive application in the numerical control machine tool. The basic components of the position control system composed of the system are shown in figure 1. The digital servo drive module and the servo motor are combined together to form a high precision angle close loop servo system, whose input is the instruction pulse given by the numerical control system, and the output is the rotation angle of the motor. In the closed loop to achieve optical encoder as the feedback control, rotation of the motor shaft will strictly follow the instruction value change. The angular displacement of the motor is converted to the required working table linear displacement through the gear pair and the screw nut pair transmission.



From figure visible, although the corner for the output of digital servo system is a closed loop system, but from machine worktable displacement as the ultimately controlled the amount of perspective, composed of the position control system is an open-loop system (or semi closed loop system). Therefore, its position control accuracy is not only related to the performance of the control system, but also to a large extent depend on the mechanical structure of the machine tool. So system information transmission link error, mechanical transmission link error and all kinds of nonlinear factors influence will deviate from the command value of the displacement of the worktable, and open loop system and unable to carry on the effective correction, which makes conventional NC machine, bed even the use of a high performance digital servo system is also difficult to achieve high machining accuracy.


Two, digital full closed loop position control system


In order to solve the above problem. In this paper, a digital closed loop position control method, the basic idea is; on the basis of movement of the machine tool parts of digital drive, the introduction of ultimate displacements of direct detection of moving parts of digital measuring link, to full access to and use of information system, from the angle feedback control of the movement of the machine tool coordinate of digital control, which contains the various sources of error and non-linearity of fully digitized closed loop system. So that the system can not only make the positioning precision of the moving parts is decided by the detection part of the measurement accuracy, but also impact on various kinds of interference and nonlinear factors on the displacement of the moving parts of effective dynamic correction, the actual displacement of any moment of moving parts always strictly follow the command value changes, so as to ensure the movement of the machine tool coordinate has a high dynamic and static accuracy.


According to the above ideas, the basic composition of digital closed-loop position control system is shown in Figure 2. The system using grating as line displacement detection device direct access to the worktable displacement information. This information after preprocessing phase difference 90 degrees of two displacement pulse signal, the frequency and the worktable displacement is proportional to the speed, the number of working table actual displacement divided by the pulse equivalent. The displacement pulse is sent to the counter to count, and the count value of the counter indicates the current actual position of the working table. Position controller in the system, according to the position setpoint and position feedback value difference and control the operation of the whole system according to the pre designed digital control law, to ensure work platform displacement strictly follow the instruction value changes.



Three, the dynamic structure of the system and controller design


The dynamic structure of the system is shown in Figure 3, including digital position controller, generalized object and feedback channel 3 parts. The generalized object consists of zero order holder, pulse generator, digital AC servo system, mechanical moving parts and so on. Among them, the zero order holder plays a link discrete (digital controller) and continuous link bridge style. The task of the pulse generator is to control the operation of the AC servo system based on the control signal given by the position controller.


AC servo system is a system of digital driving device, from a macro perspective, servo motor angle theta and instruction f between the integral relationship between the frequency of the pulse, but from the micro should further consideration of the theta and f the inertial properties. The role of mechanical moving parts is to convert the motor angular and linear displacement, if the transmission error and nonlinear factors as the system dynamic disturbance, the link will be look as a proportional component. In this way, after proper treatment, the generalized object representation for the transfer function of feedback channel though, including detection device, pre processing, information transmission, reversible counting, and so on many aspects, involving more complex information processing process.


But in the actual position from the input, the position feedback value (reversible counter of meter value multiplied by a pulse equivalent) look for the output point of view, the feedback channel watch as a proportional component, through proper design can make the transfer function of GF (s) = L. Considering generalized object to contain a integral part of third order system, to make the closed-loop system has a fast dynamic performance. And tracking ramp input zero steady-state error, PID regulator is used as a position controller, realization algorithm for:



In the UK, the K sampling period of the controller output EK -- the K sampling cycle of the input and output of the system between follow error KP, proportional coefficient of Ti, integral time constant TD, differential time constant of the PID control system for E type system, found the actual debugging, the system dynamic process of ultra tune are often large, sometimes even can not work normally. In order to solve this problem, the integral action of (2) - type is separated and controlled. When the system is in the transition process, tracking error is larger, the cancellation of the integral control, so that the system for the I type system, to ensure stable operation. When the system enters the steady state tracking process, the integral control system is added to the K type system to ensure that the steady state error of the tracking slope input is zero. Thus obtained control algorithm for type in the Delta, the steady-state tracking interval for further improve control performance, using computer to achieve the flexibility of the algorithm for digital, variable parameter control in (3) on the base, namely according to the different stages of the operation of the system and the algorithm parameters KP, Ki and KD were taken different values, in order to obtain better control effect.

Four, experiment and Application


After the system prototype is completed, the motion experiment is carried out on a SK8140 CNC milling machine. The machine tool X, Y and Z are all driven by a digital AC servo system, and a grating ruler with a resolution of 0.005mm is installed as a displacement detection device. Experiments with 50mm pitch on the working table stroke length to determine a certain number of test points, and then let the table in the system drive control on these points positioning and 6 times every point positioning and locating each time for a measurement. Test results are:


The difference between the measured value and the instruction value at each point is not more than 0.01mm. This shows that the digital closed loop position control system has good position control precision. So far, the development of digital closed loop position control system has been applied in dozens of domestic CNC machine tools.


The machining accuracy of the new type of fully closed loop NC machine tool is obviously improved, and the accuracy is very good, and good results have been achieved in the complex die parts and precision hole machining parts.



Five, conclusion


In view of the demand of the development of high precision CNC machine tools, the new digital full closed loop position control system is studied:


(1) digital drive the movement of the machine tool parts, not only can effectively eliminate the influence of the temperature drift, zero drift, and can make the controlled object has ideal dynamic structure, the digital closed loop system of which the dynamic performance is the basic guarantee.


(2) the ultimate displacement of moving parts of digital detection, real-time access to moving parts of the accurate locations information is effectively suppressed mechanical transmission error and nonlinear factors on the movement of the machine tool coordinate precision laid a foundation through the whole closed-loop control,.


(3) is the application of computer digital position controller, can be implemented by software with switching logic of complex control rules, make full closed loop position control system has excellent dynamic and static performance, so as to improve the contour machining accuracy of NC machine tools.


(4) the practical application shows that the numerical control machine tool which is controlled by the digital full closed loop position control system has good effect on the machining of complex precision parts.
【責任編輯(Top) 返回頁麵頂端
<