Abstract: This paper introduces the hardware structure of the CNC system of a multi-point positioning tester, which is based on the Mitsubishi FX2N-20GM position control unit, the development of multi-process motion programs, and the key technologies in the development and debugging of communication programs between the PLC and the FX2N-20GM.
Keywords: FX2N-20GM position control unit multi-motion process, handwheel motion command, M command
1. Technical requirements for the control system of the multi-point positioning tester
The "multi-point positioning tester" is a testing instrument used in specialized processing industries, and its operating requirements are as follows:
① It can achieve multi-point positioning. The first generation of products is required to achieve 10-point positioning; the number of positioning points can be expanded in the future.
② The "positioning distance" and "positioning speed" can be set arbitrarily, with a positioning accuracy of 0.01mm.
③ It can arbitrarily select the "location point";
④ The number of tests can be set arbitrarily.
⑤ It can realize functions such as inching, handwheel, automatic one-button start/stop, emergency stop, and interruption of operation.
2. Basic configuration of the control system
2.1 Composition of the control system:
Based on the technical requirements of the "multi-point positioning tester" and after comprehensive analysis, the configuration of its control system is shown in Figure 1:
2.2 Functions of each part of the control system
① The motion positioning system is composed of an FX2N-20GM2 axis positioning unit and an MR-J2S3 servo system. The FX2N-20GM2 axis positioning unit can control two servo axes to perform interpolation or independent operation.
②The FX2N-20GM2 shaft positioning unit can be connected to a handwheel to achieve manual precise positioning.
③ The main control system adopts FX2N-80MR. This controller mainly controls the input and output signals of the "multi-point positioning tester" and serves as the interface for connecting to the touch screen (hereinafter referred to as GOT). At the same time, the PLC communicates with the positioning control unit FX2N-20GM to realize the control of the positioning unit FX2N-20GM.
④ Use GOT as the user interface, where you can set the "positioning distance" and "positioning speed". You can also select the "positioning location" and arbitrarily set the "number of loops".
3. Motion logic analysis and motion program development for multi-point positioning testing instruments
3.1 Positioning motion requirements of the multi-point positioning tester
The positioning motion requirements for the multi-point positioning tester are as follows:
There are 10 positioning points in total, as shown in Figure 2:
Figure 2. Schematic diagram of the positioning motion of the multi-point positioning tester
The tester is required to operate from the "current point" to "any point". Furthermore, the number of cycles can be set arbitrarily.
3.2 Analysis of motion logic:
The tester has 10 positioning points, and the workbench can be positioned at any of these points. If the requirement is to move from the "current point" to "any point," there are 9 possible motion paths. If we add the "same number selection" judgment, the entire system has 10 * 10 = 100 motion path choices. This means the motion program would need at least 100 judgment conditions and 90 motion procedures. Such a motion program is far too complex.
Could we simplify its movement procedure?
Carefully analyze the movement mode of the tester's worktable: from the "current position" to the "next position", it must first return to the Y0 position, then move along the "X" axis, and move to the "next position" according to the selected conditions.
As shown in Figure 2, an origin is set on the X-axis. Regardless of the current position of the worktable, when it is instructed to move to the "next point," a "return to origin" action is performed first, and then the movement proceeds from the origin to the "next point." This simplifies the entire motion process from 90 steps to 9, greatly simplifying the programming and improving safety. Since the origin is chosen in the middle position, statistically, the impact on work efficiency is minimal.
3.3 Development of Exercise Programs
The Mitsubishi FX2N-20GM positioning unit is a CNC system that can perform motion control independently.
Its motion program can be programmed using a programmer or specialized software. The specialized instructions for programming the motion program are similar to general G-code instructions.
3.3.1 Dedicated Instruction-Type Motion Program
The following is a portion of the "positioning motion program" compiled using motion commands specific to the FX2N-20GM:
Exercise program (exercise flow selection section)
N1 LD M701-----------Selection criteria: If bit #1 is selected
N2 FNC00(P1)-----------Jump to the “P1” pointer program segment
(………….)
N22 LD M710-----------Selection criteria: If selecting bit 10#
N23 FNC00(P10) ----------- Jump to the “P10” pointer program segment
………
N30 P1--------------------- Return to bit 1# program segment
N31 cod00 Y0 f2000-----------Return to the "Y-axis origin"
N32 cod00 X0 f2000---------- Return to the "X-axis origin"
N33 cod00 xD106 fxD108;---- --- Return to position 1, "X position"
(Set the X position using D106, and the running speed using D108)
N34 cod00 yD110 fyD112; ---- --- Return to position 1, "Y position"
(Set the Y position using D110, and the running speed using D112)
N35 M02(END) ----- Program End
3.3.2 Motion program developed using the dedicated programming software "FXVPS":
The dedicated programming software "FXVPS" is a graphical programming software specifically designed for the FX2N-20GM. It features easy and clear motion flow construction, easy creation of multi-flow motion programs, easy parameter setting, and convenient debugging and monitoring.
Before developing a "movement program," it's essential to thoroughly understand the client's requirements, focusing on the big picture and establishing a solid "movement flow" first. Then, refine the details. Do not create the details before developing the flow.
Figure 3. Movement Flow
3. Online communication between the positioning unit FX2N-20GM and the PLC
4.1 Dedicated instructions and PLC program for online communication between the positioning unit FX2N-20GM and the PLC
Another feature of the Mitsubishi FX2N-20GM positioning unit is its ability to connect with a Mitsubishi PLC. By utilizing the communication function between the PLC controller and the FX2N-20GM, relevant control programs can be programmed on the PLC side to select operating modes, issue program start commands, and set data such as positioning distance, positioning speed, and number of cycles, thus achieving more flexible system control. This control system employs a "PLC + FX2N-20GM" operating mode. The communication program between the PLC and the "FX2N-20GM" is the key to the entire PLC program.
Communication between the PLC and the "FX2N-20GM" is mainly achieved through the PLC's "FROM" and "TO" instructions.
The “FROM” instruction reads information from the “FX2N-20GM” into the “PLC”.
The "TO" instruction writes information from the PLC to the "FX2N-20GM" PLC.
Figure 4 shows the communication section of the PLC program:
In Figure 4, step 0 writes instructions to the "Buffer Memory BFM" in the FX2N-20GM. Step 10 writes the value set on the touchscreen to the "Buffer Memory BFM" in the FX2N-20GM (D130 in Figure 3). Step 20 reads information from the "Buffer Memory BFM" in the FX2N-20GM indicating the controller's operating status. Step 30 reads information from the "Buffer Memory BFM" in the FX2N-20GM indicating the M instruction.
5. Problems and solutions during debugging
Use of M commands
Like conventional CNC systems, the FX2N-20GM also provides M-instruction functionality to control the actions of peripheral devices. The M-instructions issued in the motion program are stored in its buffer memory BFM--#3, and can be read using the PLC's FROM instruction. Step 30 in Figure 4 reads the M-instruction information from the FX2N-20GM's buffer memory BFM. Step 46 uses the M-instruction to drive peripheral device Y10.
The key to using M instructions is determining their "completion condition"—that is, whether the action of the object driven by the M instruction has been completed. The "completion condition" triggers the "M instruction completion signal." Only then can the motion program proceed to the next line. In Figure 4, step 46 uses the M instruction to drive peripheral device Y10, followed by "M5+Y10".
M103, M103 is the "M instruction completion signal" specified in this program.
During debugging, it is often encountered that after executing the M instruction, it stops at the "current line" and cannot proceed to the next line. The key reason is that the "M instruction completion signal" is not driven in the relevant PLC program.
6. Conclusion:
This control system has been used by the customer for one year and is operating well, meeting all of the customer's operational requirements.
