Application of Servo Technology in Slow Feeding Machine

1 Introduction

Wirecut Electrical Discharge Machining (WEDM) is sometimes referred to as wire cutting. The basic working principle is to use a continuous moving thin metal wire (called electrode wire) as the electrode, and the workpiece is subjected to pulsed spark discharge to remove metal and cut into shape. Wire-cutting is mainly used to process a variety of complex and delicate workpieces, such as punches, dies, dies, fixture plates, stripper plates, etc. for punching dies, forming tools, templates, and metals for EDM machining. Electrodes, all kinds of micro-holes, narrow slits, arbitrary curves, etc., have the outstanding advantages of small processing margin, high processing accuracy, short production cycle, low manufacturing cost, etc., have been widely used in production, the current domestic and foreign electricity Spark wire cutting machine tools have accounted for more than 60% of the total number of electric processing machines.

2 Slow wire processing machine

2.1 Wire Cutting Machine Classification

According to the different operating speed of the electrode wire, the WEDM-HS machine tools are usually divided into two types: one is WEDM-HS, and the electrode wire is used for high-speed reciprocating motion. The general wire speed is 8 ~10m/s, electrode wire can be reused, the processing speed is high, but rapid wire walking can easily cause electrode wire jitter and pause in the reverse direction, so that the quality of processing is declining. It is the main model of production and use in China, is also our country's original Wire-cut WEDM-LS; the electrode wire is used for low-speed unidirectional movement, and the wire speed is generally lower than 0.2m/s. After use, the work is smooth, uniform, small jitter, and good processing quality, but the processing speed is low, and it is the main model for production and use abroad. This article describes the application of Delta ASD-AB servos on the slow-wire machine tool.

2.2 wire cutting machine structure

The main components of the machining machine, such as slow wire walking and wire walking, include:

(1) Mainframe: Including bed, coordinate table, wire walking mechanism, etc.;

(2) Pulse power supply: a unidirectional pulsed current that converts alternating current into a certain frequency;

(3) The control system controls the movement of the machine tool, which is the focus of this article.

(4) Working fluid circulation system: Provides a clean working fluid with a certain pressure.

3 Motion Control Based on Delta Servo Technology

3.1 Hardware Introduction

The control part is the upper computer system and servo system:

(1) PC: It is a special NC controller based on the PC architecture for the wire walking machine.

(2) Servo system: Two sets of Delta ASD-AB10212MA servo controller and Delta ECMA-G31309ES large inertia servo motor.

(3) Mechanical transmission: The servo motor is directly connected to a screw with a pitch of 6 mm.

3.2 Technical difficulties

The main technical difficulty of the servo application on the wire walking machine tool is to maintain a stable follow-up error during the ultra-low speed operation from 0.5 to 0.2 rpm, and the follow-up error must be less than 2 pulses. Such ultra-low-speed technical requirements are in fact closely linked to the processing characteristics of the slow-moving machine tools.

The diameter of the electrode wire of the wire walking machine is usually 0.03~0.35mm, and the total area of ​​the centerline cut on the workpiece is up to about 350mm2/min when the workpiece is machined. Usually, both the processing precision and the large processing thickness are required. This has a high requirement on the feed speed of the slow-threading machine tool. The speed must be within a certain range. Too fast a speed will cause incomplete machining, leaving a rough trace of roughness on the surface of the workpiece to make the surface roughness. Deterioration, but also will cause the electrode wire to increase vibration, easy to cause broken wire! As for the servo system is less than 2 pulse follow-up error, because the cutting principle of wire cutting is the constant discharge between the positive and negative electrodes burn the surface of the workpiece, That is to maintain a certain distance between the two electrodes, can not be contacted together to cause a short circuit, but can not be too far away to cause a trip!

4 Large Inertia Servo System Debugging 4.1 System Debugging It can be said that the debugging process is difficult because the overall load of the customer's machine is very large, and the process tank is filled with water during the machining process to make the load inertia and load. The greater quality increases the difficulty of servo control. In order to ensure the tracking error of 2 pulses, the gain of the servo controller must be maintained at a high level, but since almost no vibration is allowed, the gain of the servo control system cannot be too large, that is, A balance between a stable response level and a stable control balance is the key to our debugging.
Since the operating speed of this equipment can't exceed the running speed 200rpm required by our debugging software to estimate the load inertia, we can only set the parameters through continuous tests to find the best gain parameters, that is, to match the servo parameters. Gain related 2-00 (position control gain), 2-02 (position feedforward gain), 2-03 (position feedforward gain smoothing constant), 2-04 (speed control gain), 2-06 (speed integral compensation) ), as well as resonance-related 2-25 (resonance-suppressed low-pass filtering), 2-23 (resonance-suppressed Notchfilter), and 2-24 (resonance-suppressed Notchfilter decay rate) for continuous test setup and running tests, 2-00 mainly The role is to maintain the position loop control response level, 2-04 main role is to maintain the speed loop control response level, these two values ​​are the basis of the servo response, and 2-02 and 2-06's main role is to reduce the position control For errors and speed control errors, adjusting these two values ​​to very high positions can significantly reduce follow-up errors. The role of 2-03 is to greatly reduce the operating vibration of the transmission.
Immediately after the start of the commissioning, the response level of the servo was adjusted to the highest level of control that does not cause the servo motor to vibrate. The operation effect of the servo motor at this gain level is that the servo system runs at a speed of 0.5 rpm. The error is basically kept within 2 pulses, but it will jump to 3~4 pulses at certain points of operation. This is not allowed in the operation of the slow-moving equipment. The operating effect must be improved by adjusting the gain of the servo because the jump of such follow-up error is due to a change in the load condition of the servo system at a certain operating point. The response of the servo system is not fast enough, so there will be a situation where the following error becomes larger. Through the above analysis, what needs to be realized is to increase the response level of the servo system and not to cause any vibration during the operation of the servo motor. In order not to allow the motor to vibrate, but also to increase the gain level, only by increasing the vibration of the mechanism to reduce the vibration of the 2-03 can improve 2-04 and 2-00 to improve the servo system overall gain level position higher response, at the same time, You can't adjust the value of 2-03 too high because it affects the position tracking error.
In the case where the adjustment has been very satisfactory, start the workpiece test cut. A very unusual phenomenon was found during the trial cutting process. When the x-axis feeds at a speed of 0.3 rpm, the y-axis tracking error will have a constant jump. When the y-axis has no signal input at all, there should not be any movement. of.
There are only two possible reasons for this situation. One is signal interference and the other is mechanical vibration. The y-axis will have resonance when judging that the x-axis is moving at a certain speed! Use our debugging software ASDA_A_SW to find the resonance frequency. Elimination of resonance, commissioning completed.
4.2 Workpiece Testing As a processing machine tool, we must prove through the machining test cut that our servo can meet the application of the wire walking machine tool.
5 Conclusion Delta's ASD-AB servo is mainly applied to the feed control in the control system. The servo motion performance is directly related to the workpiece's machining accuracy and surface roughness. The customer originally used a servo system of a well-known foreign brand. The servo motor uses a 17-bit high-line encoder, while the Delta ASD-AB servo system uses an ECMA motor supporting the ASD-B series servo system. The encoder is only a low number of 2500 line servo encoders. The machining accuracy and surface roughness of the wire feeders using Delta ASD-AB servo system are almost the same as those of the wire feeders that used the servo system with a 17-bit high-line encoder. As a result, the customer's manufacturing cost is greatly reduced under the premise of no decrease in performance, and the market competitiveness of its products in the slow-wire processing machine tool is improved.