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Application of INVT Servo System in Engraving Machine
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Keywords: servo system engraving machine

1. Preface
With the economic advancement, the efficiency of small engraving machines, which have traditionally been dominated by stepper motor motion axis, has not been able to meet the increasing demand for productivity. As a motion control system with high rotational speed and high precision, AC servo has comparable advantages. And now it slowly developed into the trend of high efficiency engraving machine that takes AC synchronous servo motor as the motion axis.

AC synchronous servo motor is mainly used in the X.Y.Z axis of engraving machine system, driving the engraving machine spindle platform for the role of carving track. Engraving machine system requires a relatively good mechanical rigid platform with high servo drive dynamic response performance.

2. Working principle

General carving and milling machine structure includes the following parts: CNC system, spindle system and sports feed system. The CNC engraving and milling machine of the enterprise is composed of the GE300 motion control card developed by Googoltech Company. The spindle system adopts imported electric spindle, and is driven by INVT CHE200 series inverter. The motion feed system consists of three servo motors.


One for X-axis feed, one for Y-axis feed, and one for Z-axis feed. The X, Y axis movement feed system consists of ball screw, elastic coupling, linear guide, servo motor and other components, servo motor selects INVT DB100 750W servo products. Z-axis movement feed system is also composed of ball screw, flexible coupling, linear guide, servo motor and other components, servo motor selects INVT DB100 400W servo products with brake series.

Figure 1: Control composition diagram


Figure2: GE300 software control interface

3. Introduction of control methods
At present, the control system adopted by related machine of engraving machine industry equipment are mostly CNC and motion control card, the industrial control computer is less used.
Motion control card generally controls the operation of servo driver by sending positioning pulse or analog signal, the commonly used signals between motion control card and servo driver include servo enable signal, left and right limit signal, fault clear signal, fault alarm signal. 
In addition, the pulse signal sent by motion card is generally differential mode, you can select other pulse output mode through the DIP switch on the board, such as open collector. Pay attention to the corresponding connection method when connecting with the servo driver,. The control method described in this paper adopts a semi-closed loop control method with analog given servo feedback.


4. Basic debugging steps
  
Engraving and milling machine has higher requirements for the performance of servo; a fast dynamic response speed is required for circular interpolation. 
1. In accordance with the general principles of debugging, the first step is to conduct debugging and parts processing, measuring linear interpolation and circular interpolation accuracy, the debugging will be completed after meeting the requirements.
2. if the accuracy does not meet the requirements, then continue to increase the speed loop gain and position loop gain, at this time if the motor issues a high frequency sound, you can conduct the mechanical resonance frequency measurement, and then set the mechanical resonance frequency value to notch frequency value;
3. Repeat the above steps until find the best parameter value


In general, the mechanical resonant frequency points of engraving and milling machine won’t exceed two points. When the mechanical resonance frequency is eliminated, the stiffness of servo motor can be adjusted very high, which can meet the dynamic response speed of  circular interpolation of engraving and milling machine, so as to achieve the accuracy of circular interpolation.


Parameter settings:
X axis:
P0.03=0
P0.25=1
P0.26=1
p1.03=17
P1.20=1
P2.10=30
Y axis:
P0.03=0
P0.25=1
P0.26=1
p1.03=17
P1.20=1
P2.10=30
Z axis:
P0.03=0
P0.25=1
P0.26=1
p1.03=17
P1.20=1
P2.10=30


5. Precautions
In order to ensure the reliable, stable and correct operation of servo drive, now combined with our servo application experience, we have summarized a few servo application precautions:


1. Correct wiring: Our servo standard encoder line and motor line, the signal line between the servo drive and controller must be correctly connected, or the servo won’t run normally.
2. Correctly setting the servo control mode: As each machine application is different, so the correct setting of servo control mode is the premise of ensuring the correct operation of servo.
3. Set the electronic gear ratio correctly: Set the servo according to the electronic gear ratio set by control program. In addition, we should not set the electronic gear ratio in the case of servo enable.
Make sure the motor is well grounded and the driver is connected to device housing. On the one hand, it can avoid interference, on the other hand, it can avoid leakage.
4. Take every effort to choose shielded twisted pair as the signal line, the shielding layer is generally connected to the terminal shell
5. Pay attention to the interference problem of engraving and milling machine high-speed spindle inverter. Prevent the encoder signal and control signal from interference, the encoder line, signal line should not be tied with motor line, power line or passing the same trunking, try to keep a certain distance between them.
6. Drive installation environment requirements, because the engraving machine will produce a lot of dander while working, so user should pay attention to the driver installation environment to avoid the internal circuit damage of driver caused by dander processing.


6. Conclusion
We achieve the process requirements through the above process: processing the accuracy of a circular slice within 5 S (1 S = 10um). The stiffness is turned up by improving the notch filter and suppressing the mechanical resonance, as a result, the processing accuracy is within 4 S, best in 2 S same to Panasonic processed slices. Customers even think the effect of some slices is better than the one of Panasonic.


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