Electrical Control of Motion Coordinates of CNC Machine Tools

The electrical control of a moving coordinate of a CNC machine tool is composed of a current (torque) control loop, a speed control loop and a position control loop in series.

(1) The current loop is a circuit that provides torque to the servo motor. In general, the matching adjustment between the motor and the motor is already done by the manufacturer or corresponding matching parameters are specified. The feedback signal is also connected in the servo system, so no wiring and adjustment are needed.

(2) The speed loop is a circuit that controls the speed of the motor, that is, the speed of the axis. The speed regulator is a proportional-integral (PI) regulator. The P and I adjustment values ​​are completely dependent on the load magnitude of the driven coordinate axes and the mechanical characteristics such as the transmission stiffness and transmission clearance of the mechanical transmission system (guide rails, transmission mechanisms). When the characteristics change significantly, the mechanical transmission system must first be repaired and then the speed loop PI regulator must be readjusted.

The best adjustment of the speed loop can be done only when the position loop is open-loop, which is easier for the horizontal axis of rotation and the rotation axis, and for the vertical axis of motion, the position will be automatically dropped when the loop is open. If danger occurs, you can take off the no-load adjustment of the motor first, and then install the motor together with the position ring to adjust or adjust directly with the position ring. At this time, it needs a certain amount of experience and care.

See the "Speed ​​Measurement" section above for the feedback loop of the speed loop.

(3) The position loop is the control link that controls the precise positioning of each coordinate axis according to the command position. The position loop will ultimately affect the position accuracy and working accuracy of the coordinate axes. There are two aspects of this work:

One is the matching between the accuracy of the position measuring element and the pulse equivalent of the CNC system. The number of pulses emitted by the measuring element per unit movement distance is matched with the resolution specified by the numerical control system after the frequency multiplication of the external frequency multiplication circuit and/or the internal frequency multiplier of the CNC. For example, when the position measuring element is 10 pulses/mm, and the resolution of the numerical control system is 0.001 mm, the pulse sent from the measuring element must be 100 times frequency matched.

The second is the correct setting and adjustment of the position loop gain coefficient Kv. Usually, the Kv value is set as the machine data. In the numerical control system, the Kv value setting addresses and numerical units are respectively specified for each coordinate axis. The setting of the Kv value after optimization of the speed loop becomes an important factor reflecting the performance of the machine tool and affecting the final accuracy. The Kv value is a direct expression of the performance of the machine's motion coordinates and cannot be arbitrarily enlarged. About the setting of Kv value should pay attention to two problems, we must first meet the following formula:


Where v is the coordinate running speed, m/min

Δ - tracking error, mm

Note that the units used by different CNC systems may be different. When setting up, pay attention to the units specified by the CNC system. For example, if the unit of coordinate operation speed is m/min, then the unit of Kv value is m/(mm·min). If the unit of v is mm/s, the unit of Kv should be mm/(mm·s).

Secondly, the Kv values ​​of the linkage axes must be the same to ensure the accuracy of the composite motion. Usually based on the axis with the lowest Kv value.

There are three kinds of position feedback: one is no position measuring element, which is position open loop control that is no position feedback, stepping motor drive is generally open loop; one is semi-closed loop control, that is, the position measuring element is not in the final coordinate axis movement Parts, that is to say there are part of the transmission link in the position outside the closed-loop control, this situation requires the ring outside the transmission part should have a considerable transmission stiffness and transmission accuracy, adding backlash compensation and pitch error compensation, you can get very High position control accuracy; the third is full-closed-loop control, in which the position measuring element is mounted on the final moving part of the coordinate axis. Theoretically, the position accuracy of this control is best, but it has more requirements for the entire mechanical transmission system. High rather than low, otherwise, it will seriously affect the dynamic accuracy of the two coordinates, and the machine tool can only work under the condition of reducing the speed loop and position accuracy. Another important issue that affects the accuracy of full-closed-loop control is the precise installation of measuring components. Do not underestimate it.

(4) Feedforward control is contrary to feedback. It is pre-adding the command value extraction part to the following regulation circuit. Its main function is to reduce the tracking error to improve the dynamic response characteristics and thus improve the position control accuracy. Because most of the machine tools do not have this function, this article does not elaborate, but it should be noted that the feedforward must be added after all three control loops have been optimally commissioned.

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