(2) Closed loop CNC machine tools
The closed loop servo system works on a closed loop principle. The actual displacement directly detected by the displacement detecting device mounted on the feed executing component is fed back to the numerical control device via the feedback loop, and the numerical control device compares the displacement command with the actual displacement, and according to the difference and the command feed speed requirement, press After a certain regular conversion, the speed command of the feed servo system is obtained. At the same time, the speed measuring component measures the rotation speed of the servo motor as a speed feedback signal, and after comparing with the speed command signal, its speed error adjusts and corrects the speed of the servo motor. Thereby controlling the worktable to accurately move according to the command displacement and speed.
The position feedback loop, the speed feedback loop and the main control loop form two closed loops, so it is called closed loop control. The double closed loop control consisting of position control and speed control can obtain higher accuracy, faster speed and higher driving power than the open loop feed system.
(3) Semi-closed loop CNC machine tools
If the position detecting device is installed at the end of the servo motor or the drive screw, the actual displacement of the actuator is indirectly measured and controlled according to the closed loop principle. Then constitute a semi-closed loop control. Because the control loop does not contain the actual components that need to be controlled, it is a semi-closed loop. Since the actual displacement is not directly controlled, the accuracy is lower than the closed loop control, but it is still higher than the open loop. Moreover, the stability of the control system is easier to obtain than a closed loop system. Most CNC machines now use a semi-closed-loop feed servo system.
three. Classified according to the type of numerical control device used
(1) Hardwire CNC (NC) machine tool
Use a hard CNC system. Its input processing, interpolation operations and control functions are all implemented by dedicated fixed combination logic circuits. Different functions of CNC machine tools, combination logic circuits are also different. Therefore, when changing or increasing or decreasing the control and calculation functions, it is necessary to change the hardware circuit. Therefore, the versatility, the flexibility is poor, the manufacturing cycle is long, and the cost is high.
(2) Computer numerical control (CNC) machine tools
Use a computer numerical control device (soft wire numerical control device). The hardware circuit consists of a small or micro computer, plus a general purpose or dedicated large scale integrated circuit. The main numerical control functions such as input processing and interpolation calculation are almost all realized by system software. Machines with different functions are only different from the system software. Modify or increase or decrease the function, no need to change the hardware circuit, just change the system software. Therefore, it has high flexibility, and since the hardware is basically versatile, it is advantageous for mass production to improve quality and reliability, shorten manufacturing cycle and reduce cost.
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