At present, digital AC servo system is widely used in CNC machine tools. The digital servo drive module and servo motor constitute a high precision angle closed-loop servo system. Its input is the command pulse given by the CNC system, and the output is the motor angle. Under the closed-loop control with optical encoder as feedback link, the rotation angle of motor shaft will strictly follow the change of command value, and the angular displacement of motor will be converted into the required linear displacement of worktable through gear pair and screw nut pair.
Although the digital servo system with rotation angle as output is a closed-loop system, the position control system composed of it is an open-loop system (or semi closed-loop system) from the perspective of taking the displacement of machine tool table as the final controlled value. Therefore, the position control accuracy is not only related to the performance of the control system, but also largely depends on the mechanical structure of the machine tool. In this way, the error of information transmission link, the error of mechanical transmission link and the influence of various non-linear factors in the system will make the table displacement deviate from the command value, and the open-loop system can not effectively correct it. Therefore, even if the conventional CNC machine and the machine use high-performance digital servo system, it is difficult to achieve high machining accuracy.
2、 Composition of digital full closed loop position control system
In order to solve the above problems, this paper proposes a digital full closed-loop position control method. Its basic idea is: on the basis of the digital drive of the moving parts of the machine tool, the digital measurement link which directly detects the final displacement of the moving parts is introduced to fully obtain and use the system information to count the motion of each coordinate of the machine tool from the perspective of feedback control The digital closed-loop system consists of various error sources and nonlinear links. In this way, the system can not only make the positioning accuracy of moving parts determined by the measurement accuracy of the detection link, but also make effective dynamic correction for the influence of various interference and non-linear factors on the displacement of moving parts, so that the actual displacement of the moving parts at any time always follows the change of the command value, so as to ensure the high dynamic and steady-state accuracy of the movement of the machine tool coordinates Degree.
The basic composition of the digital closed-loop position control system is shown in Fig. 2. The system uses grating as the linear displacement detection device to directly obtain the displacement information of the machine tool table. After preprocessing, the two-way displacement pulse signals with phase difference of 90 degrees are obtained. The frequency is directly proportional to the displacement speed of the worktable, and the number is the actual displacement of the worktable divided by the pulse equivalent. The displacement pulse is sent to the reversible counter for counting, and the count value in the counter represents the current actual position of the worktable. The function of the position controller in the system is to control the operation of the whole system according to the difference between the given position value and the position feedback value according to the digital control law designed in advance, so as to ensure that the displacement of the worktable strictly follows the instruction value.
3、 Dynamic structure and controller design of the system
The generalized object is composed of zero order holder, pulse generator, digital AC servo system and mechanical moving parts. Among them, the zero order holder plays a bridge style of connecting discrete link (digital controller) and continuous link. The task of pulse generator is to generate command pulse to control the operation of AC servo system according to the control signal given by the position controller, so this link is a proportional link.
AC servo system is a digital driving device in the system. From the macroscopic point of view, there is an integral relationship between the angle θ of servo motor and the command pulse frequency f, but the inertial characteristics between θ and F should be further considered from the microscopic point of view. The function of mechanical moving parts is to convert the rotation angle of the motor into the linear displacement of the worktable. If the influence of transmission error and nonlinear factors is treated as the dynamic disturbance to the system, the link can also be regarded as a proportion link. In this way, the transfer function of the generalized object can be expressed as a feedback channel after proper processing, although it includes detection device, pre-processing, information transfer, reversible counting and many other links, involving more complex information processing process.
But from the point of view of taking the actual position of the worktable as the input and the position feedback value (the count value in the reversible counter multiplied by the pulse equivalent) as the output, the feedback channel can be regarded as a proportional link, and its transfer function GF (s) = l can be made by proper design. Considering that the generalized object is a third-order system with an integral part, in order to make the closed-loop system composed of it have fast dynamic performance and no steady-state error of tracking slope input, PID controller is used as position controller
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