Control System and Servomechanisms of CNC system


Control System

A large variety of components are manufactured in industries. Every component has its own geometric parameters, hence, different machining operations. For meeting the machining operations of different components a single CNC system cannot give optimum results.  So the CNC machines are designed to meet specific requirements to make them cost effective. CNC or NC machines are classified on the basis of feedback control and on the basis of control system features. Based on feedback control, the NC and CNC system is classified as open-loop and closed-loop control systems.
  • Open-Loop Control SystemMachine    tool    controls    in    which there is no provision to compare the actual position of the cutting tool or work piece with the input command value, are called open-loop systems. In the open-loop control system, the electric motor continues to run until the absence of power, from input command signal, indicates that the programmed location has been attained. There is no monitoring of the actual displacement of the machine slide. In the open-loop control system, the control may indicate a movement of 50mm whereas actually the slide may have moved only 49.8 mm. Figure "Block Diagram of an Open-Loop System" below shows the block diagram of an open-loop control system.  In open-loop control system the actual displacement of the slide may vary with change in external conditions and due to the wear of components of the drive mechanism, i.e. backlash errors in lead screw, etc. Since there is no provision of feedback in the control system periodical adjustments are required to compensate for the changes due to various factors.  Although open-loop systems are less accurate compared to a closed-loop system, these are adequate for many applications, e.g. foundry work. Open-loop systems are less expensive than closed-loop systems due to the absence of monitoring devices and their maintenance is not complicated.
    Block Diagram of an Open-Loop System
    Block Diagram of an Open-Loop System
  • Closed-Loop Control System. In a closed-loop control system, the actual output from the system i.e. actual displacement of the machine slide is compared with the input signal. The closed loop control systems are characterized by the presence of feed-back devices in the system. In the closed-loop control system the displacement can be achieved to a very high degree of accuracy because a measuring or monitoring device is used to determine the displacement of the slide. The feedback from the monitoring device is then compared with the input signal and the slide position is regulated by the servo system until it matches with the desired position. Figure "Block Diagram of Closed-Loop System" below shows a closed loop control system with a provision for feedback for the displacement of position of machining slide. In order to measure the speed of the motor and to compare the actual speed with the programmed speed, a velocity feedback system is added to the system.
    Block Diagram of Closed-Loop System
    Block Diagram of Closed-Loop System


In modern usage, the term servomechanism or servo is restricted to feed back control system, in which the controlled variable is mechanical position or time derivatives of position, e.g. velocity and acceleration. A servo system is used to position a load shaft, in which the driving motor is geared to the load to be moved. The output (controlled) and desired (reference) positions are measured and compared by a potentiometer pair whose output voltage is proportional to the error in angular position. The voltage is amplified and is used to control the field current of a DC generator which supplies the armature voltage to the driving motor.

Servomechanism System
Servo System

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