Abstract : This paper discusses the implementation of servo synchronization function using the Mitsubishi M64S CNC system, the problems encountered during debugging, and the solutions. Keywords : Servo synchronization, origin adjustment, gantry milling. For large machines like gantry milling machines, two motors are needed to simultaneously drive the crossbeam or worktable. Therefore, these two motors must work in perfect synchronization. In CNC systems, this function is called "servo synchronization". The Mitsubishi CNC system has this function. Recently, the author used the Mitsubishi M64S system + MDS-RV type driver on a customer's medium-sized gantry milling machine, and adopted the "servo synchronization" function to form a semi-closed-loop system, achieving good machining results. 1. Implementation of servo synchronization function [align=center] Figure 1. PLC program for servo synchronization[/align] To implement the servo synchronization function, the relevant program must be written on the PLC ladder diagram: In the interface of the Mitsubishi M64CNC, R435 is a data register that determines the servo synchronization function. By setting different values ​​for R435, any two axes can be specified to enter the "servo synchronization" mode. In this paper, the synchronization of the 1st and 4th axes is specified. The first axis is the reference axis (X), and the fourth axis is the driven axis (A). Correction mode is frequently used in the initial debugging phase. For a semi-closed-loop system, when mechanical precision affects the imbalance between two axes, it will trigger a "synchronization error too large" alarm. At this time, it is necessary to enter "correction mode" to adjust one of the axes. In "correction mode," only "handwheel mode" can be used. The interface for correction mode is Y22A. [align=center]Figure 2[/align] 2. Related Parameters Parameter #1068————————- This parameter specifies the "axis number of the driven axis." (Must be set under the reference axis name) Parameter #2024——————————Sets the synchronization error value (only set under the reference axis name) [align=center]Figure 3[/align] 3. Origin Setting For two servo axes in servo synchronization, should the origin be set separately for each axis or only one origin? Theoretically, each axis should have its own origin. However, in practice, with two servo motors mounted at both ends of the crossbeam of the gantry milling machine, during operation, due to mechanical precision errors, if the actual travel distance of one axis is 2mm ahead or behind the other axis, it will trigger an alarm (excessive current) on one of the axes. Setting a separate origin for each axis would greatly complicate the installation and adjustment of the two origin switches. Furthermore, for a gantry milling machine, when both servo axes move forward and backward simultaneously, the position reached by one axis when it reaches the machine tool origin can also be considered the origin position of the other axis. If the travel error between the two axes exceeds a certain value, an "excessive current" alarm will be generated. Therefore, the problem of one axis "falsely returning to the origin" will not occur. Therefore, during actual debugging, only one origin signal was set for the two synchronous servo axes, and this origin signal was set on the reference axis side. 4. Problems encountered during the return-to-origin process Even with only one origin set, the following situation still occurred during the return-to-origin operation: after the reference axis returned to the origin, the driven axis repeatedly triggered an "excessive current" alarm. It was impossible to complete the operation of both axes returning to the origin simultaneously. Of course, it cannot enter automatic mode. After activating the servo synchronization function, during jogging and handwheel operations, due to the influence of mechanical installation and lead screw accuracy, an alarm (overcurrent) will occur after only 30-50 mm of movement. The solution during on-site debugging is to increase parameter #2213 (current limit value) (the maximum value is 500% of the static rated current). However, if this parameter is too large, it may damage the mechanical system, especially for newly assembled machine tools. Parameter #2213 should be adjusted cautiously. The fundamental solution is to immediately perform mechanical accuracy error compensation after the homing operation is completed, so that the electrical accuracy matches the actual accuracy. However, now even homing is problematic… Upon careful observation on the display screen, the alarm phenomenon during the homing process occurs as follows: after the reference axis (X) returns to the origin, the driven axis (A) continues to move 1.6-1.8 mm before the alarm occurs. This indicates that the difference between the electrical origins of the two axes is at least 2-3 mm. After carefully analyzing the parameters for returning to the origin, it can be seen that parameters #2028 (shield amount) and #2027 (origin offset) are the most critical. Parameter #2027 (origin offset) refers to the distance between the electrical origin and the actual origin. Currently, there is a deviation between the origins of the two axes. By adjusting parameter #2027, they can be made consistent. The unit of parameter #2027 is 1/1000 mm. Setting #2027 of the reference axis (x) to 3000 and then performing the return-to-origin operation, both axes returned to the origin simultaneously. This proves that the above analysis is correct. [align=center]Figure 4[/align] To make the origins of the two axes relatively consistent, the current value of the reference axis (x) is monitored using the "servo monitoring" screen on the monitor. Parameter #2027 is repeatedly adjusted. When the current value of the reference axis (x) is <20%, the basic requirement can be considered met. 5. Compensation for Mechanical Precision Errors For servo-synchronized dual-drive systems, "mechanical precision error compensation" must be performed immediately after the homing operation. Clearly, without this compensation, "excessive current" alarms will frequently occur due to mechanical errors. This can be considered an initial compensation; after the break-in period, another compensation should be performed. 6. Problems Caused by Soft Limits Another problem encountered in servo-synchronized dual-drive systems is that when one axis reaches its soft limit, it stops while the other continues, generating another "excessive current" alarm (even though the soft limit values ​​for both axes are the same). This situation is obviously caused by accumulated mechanical precision errors. Once an alarm occurs, the alarm axis must be adjusted manually using the handwheel mode, which is cumbersome for the operator. How can this be avoided? One solution is to install a hard switch to cut off automatic and manual operation. However, this increases the potential for failure. Another solution is to use the DDB function to take a point before the soft limit and use that signal to cut off automatic and manual operation. This method does not increase cost or the number of potential failure points. The method is as follows: [align=center] Figure 5. Implementation of DDB function[/align] The current position data (D200/D204) of the first and fourth axes are compared with a fixed point before the soft limit. When the current position of the first and fourth axes exceeds the fixed point, automatic and manual operation are cut off. This avoids the machine from hitting the soft limit, effectively adding another layer of protection. After the above processing, the two synchronous axes can stop normally without alarms. This is also an effective way to solve the problem of mechanical cumulative error. Mailing address: Room 3805, Taihe Plaza, Wusheng Road, Wuhan, Hubei Province, China. Postcode: 430033. Tel: 027-85712820, 13607177391. Email: [email protected]