电子教案统一模板(数控)Unit3

The open-loop control can be used in applications in which there is no change in load conditions，such as the NC ， drilling machine．The advantage of the open-loop control ． system is that it is less expensive，since it does not require ， the additional hardware and electronics needed for positioning feedback．The disadvantage is the difficulty of ． detecting a positioning error (Figure 3-1).
Closed-loop systems are very accurate. Most has an automatic compensation for error, since the feedback device indicates the error and the control makes the necessary adjustments to bring the slide back to the position. They use AC, DC or hydraulic servomotors (Figure 3-2)． ．
Servo motors are connected to the spindle. They are also connected to the machine table through the ball-nut leadscrew. A device called a resolver continuously monitors the amount by which the table and/or spindle has moved and sends this information back to the MCU. The MCU can then adjust its signal as the actual table and/or spindle position approaches the programmed position.[2] Systems that provide feedback signals of this type are called servo systems or servomechanisms. They can position tools with a very high degree of accuracy even when driving motors with highhorsepower ranges.
In the closed-loop control system ， the electronic movement pulses are sent from the control to the servomotor, enabling the motor to rotate with each pulse. The movements are detected and counted by a feedback device called a transducer. With each step of movement, a transducer sends a signal back to the control，which compares the current ， position of the driven axis with the programmed position. When the number of pulse sent and received match, the control starts sending out pulses for the next movement.
Because the machine automatically knows how to move in response to an axis command, the programmer need not be concerned whether it is the spindle or table that moves. For programming purposes, programmers should consider the CNC machine table as locked with only the tool in the spindle moving along the ±X, ±Y, and ±Z axes. The main point is that the tool arrives at the programmed location. Thus, the machine axis will be defined in terms of spindle movement.
3.2 Lesson 6 Closed-loop Control System ClosedA closed-loop system utilizes the servo motor whose types include AC servo motors, DC servo motors, and hydraulic servo motors. A servo motor does not operate like a pulse counting stepping motor. The speed of an AC or DC servo motor is variable and depends upon the amount of current passing through it. The speed of a hydraulic servo motor depends upon the amount of fluid passing through it. The strength of current coming from the MCU determines the speed at which a servo motor rotates. [1]
1. Primary machine axes of movement should follow the right-hand rule (Figure 3-3). 2. Spindle movement is primarily along the Z axis. Movement into the work is along the－Z axis and movement － away from the work is along the ＋Z axis. 3. In a majority of milling machines, motion along the X axis is the longest travel perpendicular to Z. Motion indicated by －X is directly opposite to that indicated by ＋ X. The X axis is parallel to the work holding and is in the horizontal plane. The X axis moves to the right along a plane of the work as the operator looks at that plane.
CNC equipment executes machining operations by performing some form of sliding linear motion and rotary motion. The actual method of movement is designed by the manufacturer and can vary from machine to machine. For example, the table can move in the horizontal plane (XY-axis motion) and the spindle in the vertical plane (Z-axis motion). The system will respond to a command to move the spindle (tool) along the ＋X or ＋Y axis by moving the table in the opposite direction, －X or－Y. －
The open-loop control systems does not provide positioning feedback to the control unit．The movement ． pulses are sent out by the control and they are received by a special type of servomotor called a stepper motor. The number of pulses that the control sends to the stepper motor controls the amount of the rotation of the motor. The stepper motor then proceeds with the next movement command. Since this control system only counts pulses and cannot identify discrepancies in positioning，the machine will ， continue this inaccuracy until somebody finds the error． ．