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Types of NC Servo-Drive Systems

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Types of NC Servo-Drive Systems --- Aheadmold

In a NC machine, the MCU accepts information in the form of punched, magnetic tape codes or stored program. These input data must be transformed by the MCU into specific output codes in terms of voltages, or pulses per second. The transformed data, called output, is used to drive the motors to position the machine slides to the programmed position. These slides, or table drives, are commonly known as servo-drives. The principal function of NC is the positioning of the tool or the machine table in accordance with the programmed data. Industry has developed three different types of drives based on how the NC system accomplishes positioning. These are the open-loop, the closed-loop and half closed-loop drive system.
1.    Open-Loop Servo-Drive
An open-loop control system is the simplest and least cost form of servo-drive. It is characterized as a system that lacks feedback as in Fig. 6-6; that is, once an input control signal is sent, there is no sensing device to confirm the action of the control signal.
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Fig. 6-6   An open-loop control system

In the open-loop control NC machine, the servomotor is usually stepping motor. The stepping motor output shaft rotates in direct proportion to pulses received. It has the advantages of high accuracy, easy implementation and compatible with digital signals, but it has the disadvantages of low torque, limited speed and risk of missed pulse under load. So the open loop control system is used in the economic NC machine. With an open loop system, there is always the risk that the actuator will not have the intended effect on the process, and that is the disadvantage of an open loop system. Open-loop systems are usually appropriate when the following conditions apply: (1) The actions performed by the control system are simple, (2) the actuating function is very reliable, and (3) any reaction forces opposing the actuation are small enough to have no effect on the actuation. If these characteristics are not applicable, then the feedback control system may be appropriate. There are two kinds of feedback control system, one is a closed-loop control and another is half-closed-loop system.
The open-loop application in general, is restricted to smaller machines because of the limited power output availability with the stepping motors (a typical maximum is 4~5 kW and a torque of 200 N?m). Again the pulses per second restrict the speed of the drive. A typical maximum for stepping motors is 16,000 pulses per second. When this is applied to a system requiring 0.001 mm accuracy, the resultant maximum speed would be 0.96 m/min. Again for high-precision application like jig boring where an accuracy of 0.001 mm is to be maintained, an open-loop system does not serve the purpose. There is, of course, no doubt that on light duty machinery where the problems of instability are absent and also the requirements are not of high precision, open-close servo control does offer some cost saving solution. Usually the open-loop NC system is called economical NC system.
2.    Half-Closed-Loop Servo-Drive
A half-closed-loop control NC system is one of the feedback control system as illustrated in Fig. 6-7, uses feedback measurements to ensure that the worktable is moved to the desired position. It is characterized as a system that the indirect feedback monitors the output of servomotor. Although this method is popular with NC systems, it is not as accurate as direct feedback. The haft-closed-loop system compares the command position signal with the drive signal of the servomotor.
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Fig. 6-7    A half-closed-loop control system

In operation, the half-closed-loop system is directed to move the worktable to a specified location as defined by a coordinate value in a Cartesian system. Most positioning system have at least two axes with a control system for each axis, but our diagram only illustrates one of these axes. A servomotor connected to a leadscrew is a common actuator for each axis. A signal indicating the coordinate value is sent from the MCU to the motor that drive the leadscrew, whose rotation is converted into linear motion of positioning table. As the table moves closer to the desired coordinate value, the difference between the actual position and the input value is reduced. The actual position is measured by a feedback sensor, which is attached to servomotor axis or leadscrew. This system is unable to sense backlash or leadscrew windup due to varying loads, but it is convenient to adjust and has a good stability.
3.    Closed-Loop Servo-Drive
A closed-loop control system is another feedback control system as illustrated in Fig. 6-8. It is characterized as a system that the direct feedback monitors the output of servomotor. A feedback sensor directly measures the position of worktable. The closed-loop control system, with its drive signal originated by the worktable, is the preferred system because it monitors the actual position of the worktable on which the part is mounted. It is more accurate; however, its implementation costs are higher.
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Fig. 6-8    A closed-loop control system

A half  closed-loop  or closed-loop  system uses  conventional variable-speed  AC or  DC motors, called servos, to drive the axes.

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