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1. Technical concept and compositional structure servo control technology
Technical concept and compositional structure Servo control technology is an automatic control technology that uses the position and speed of moving parts as a control quantity. The system composed of this technology, the feed servo system, is a typical mechatronic system and consists of the following components: position control unit, speed control unit, drive element (motor), detection and feedback unit, and mechanical execution. part.
Feeds each module (unit) of the servo system. From the perspective of regulation principle, the feed servo system is a precise position tracking and positioning system. According to the opening or not of its position loop, it can be divided into two types, open loop and closed loop. The closed loop system detects the components according to their positions. The installation site can be further divided into two types: full-closed and semi-closed. The full-closed position detection element is mounted at the end of the feed chain and the semi-closed position detection element is mounted on a transmission element in the feed drive chain.
If the CNC device is the "brain" of CNC equipment and is the "command post" that issues "commands," then the feed servo system is the "four limbs" of the numerical control device and is an "executive organization." The feed servo system faithfully executes the motion commands sent from the CNC device to precisely control the movement direction, feed speed, and displacement of the actuator.
Generally, the numerical control machine equipment often needs to control multiple moving parts. For example, a numerical control lathe generally has two feed axes, a numerically controlled milling machine generally has three feed axes, and a machining center has more feed axes (including straight lines). Shaft or rotary shaft). Some of these feed axes move the table with the workpiece, while others drive the tool holder (such as a lathe) or the spindle box (such as a milling machine) equipped with a cutter. Each feed axis is a feed servo system. The coordination and coordination of multiple feed servo systems is achieved through CNC devices (in the form of instructions). Through the coordination of a plurality of feed servo systems, the tool can generate complex curve motions relative to the machining workpieces and machine complex workpieces.
2. Working principle
1) Open-loop feed servo system
The principle of the open-loop feed servo system is similar to Figure 1, except that there is no detection and feedback unit (including the corresponding position and speed detection elements). The drive motor can only use a stepper motor, because the pulse can control its speed (pulse The frequency), in turn, can control its position (the number of pulses).
Figure 1 Schematic diagram of the feed servo system
The open-loop feed servo system works as shown in Figure 2.
Figure 2 Open-loop feed servo system working principle
The system accepts instructions from CNC interpolation, that is, the displacement of the axis within the interpolation period. According to the pulse equivalent of the system, the displacement is converted into the corresponding pulse number n and the pulse frequency, which is output to the pulsed circular distributor. It distributes the pulses one by one to the drive power for each phase of the stepper motor, which drives the stepper motor after amplification.
2) How the closed-loop and semi-closed-feed servo system works
The drive motors used in closed-loop and semi-closed-feed servo systems are usually DC servo motors and AC servo motors. Since this kind of drive motor can only control its speed and cannot control its position, when it is used in a position control system, it can only use closed-loop or semi-closed-loop control, and cannot be used for open-loop control, so the structure of the system is more complex than that of open loop. .
The system accepts the command from the CNC to interpolate, that is, the displacement of the axis within the interpolation period, and uses the displacement and the actual position value of the actuator measured by the detection and feedback unit as the input of the position control unit. Comparator, that is, the position following error:
Following error = position command value - actual position value
The following error is the input of the position control regulator, and the speed command value is calculated according to the given regulation law (P, PI, PID, or other methods) and becomes the speed command voltage after the digital-to-analog conversion (D/A). The voltage is an input to the speed control unit.
The speed control unit inputs the speed command voltage output by the position control unit and the actual speed value measured by the detection and feedback unit, and after comparison by the comparator, the speed following error is obtained:
Speed ​​following error = speed command voltage - actual speed voltage
The speed following error is the input of the speed regulator. According to the given regulation, the servo motor control voltage can be obtained. According to the difference of the servo motor, this voltage will be converted into different forms of power, and after amplification, the output will be controlled to control the motor. The operation.