Abstract: Based on a brief introduction to the principles of multi-axis CNC welding, this paper focuses on the use of Delta's 20PM series PLC with embedded motion control functions to achieve three-axis helical eccentric circular interpolation platform control of the welding trajectory, and provides the software programming.
Keywords: DVP20PM00D motion control, three-axis helical interpolation, CNC welding
1 Introduction
Welding is the most common metal joining process. CNC welding of steel pipes and fittings is used for welding single steel pipes and branch steel pipes, realizing the automation of the process and is of great significance for the precision machining of welding processes.
The core issue in CNC welding of steel pipe fittings is controlling the motion trajectory of the welding point, as shown in Figure 1. The CNC welding torch achieves two-axis motion control (up/down and forward/backward) according to the process requirements. The CNC pipe fitting fixture platform is used to fix the steel pipe to be welded. It can rotate horizontally on a single axis, and simultaneously perform horizontal (X-axis), vertical (Y-axis), and circular motion in the XY plane, thus constituting three-axis motion control. This paper focuses on the three-axis motion control design of the CNC pipe fitting fixture platform; the horizontal and vertical welding of the welding torch will not be discussed in detail.
Figure 1 CNC welding machine
2. CNC process analysis
2.1 Welding of single steel pipes
Welding a single steel pipe is relatively simple; it only requires the intermediate clamp to make a circular motion, which is called concentric circle, as shown in Figure 2.
Figure 2 Welding of a single steel pipe
2.2 Branch pipe welding
Figure 3 shows the welding of branch steel pipes. During the welding process, not only must the intermediate fixture perform circular motion, but the base (XY) and platform must also rotate synchronously in opposite directions to complete the welding of the branch steel pipes. This motion is called eccentric circle, which requires the CNC system to achieve three-axis helical interpolation, as shown in Figure 4. The control complexity of the eccentric circle welding trajectory is far greater than that of the concentric circle trajectory.
Figure 3 Welding of branch steel pipes
Figure 4. Three-axis helical interpolation trajectory of CNC welding torch
3 20PM Motion Control PLC Solution
3.1 Introduction to Motion Control PLC
The Delta DVP-20PM00D is a programmable controller with dedicated motion control functions. The most significant feature of the DVP-20PM00D is that the PLC host directly provides electronic cam functionality; in other words, the DVP-20PM00D is a PLC with built-in electronic cam functionality. Therefore, in some applications, the DVP-20PM00D is simply referred to as the Delta 20PM motion controller.
The 20PM features two 500kHz inputs and outputs. In its electronic cam function, the X-axis is defined as the slave axis, and the encoder input axis as the master axis. Once the CAM table is defined, the slave axis follows the master axis according to the defined curve. Employing a high-speed dual-CPU structure, it utilizes an independent CPU to process motion control algorithms, effectively achieving various motion trajectory controls, logic action controls, linear/circular interpolation controls, etc. Multi-axis CNC pipe welding machines utilize the electronic cam function of the 20PM motion controller to effectively solve complex trajectory control problems. Key features of the 20PM:
(1) The 20PM is suitable for high-speed, high-precision, and highly complex motion control applications;
(2) Multi-speed execution and interrupt location;
(3) 64K large capacity, built-in Flash storage;
(5) Two sets of differential pulse outputs, with the highest pulse output reaching 500KHz;
(6) Controlled by two sets of hand cranks;
(7) Built-in electronic cam function, easily realizing applications such as wire winding, flying shear, and tracking shear;
(8) Supports PLC sequential logic control and NC control (G code and M code).
3.2 Motion Control Solutions
The overall control scheme uses two Delta 20PM series motion control PLCs, among which the 20PM00M is a PLC with three-axis motion control function, which realizes the complex three-axis spiral interpolation eccentric circle welding trajectory of the workpiece fixture platform, which is completely unimaginable for conventional PLCs.
Figure 5 Wiring principle of 20PM00M
3.3 Motion Control Programming
The G-code functions supported by 20PM are as follows: G0 High-speed positioning; G1 Dual-axis or three-axis linear interpolation; G2 Clockwise circular interpolation (set center position); G3 Counterclockwise circular interpolation (set center position); G2 Clockwise circular interpolation (set radius); G3 Counterclockwise circular interpolation (set radius); G4 Dwell time; G90 Set absolute coordinate system; G91 Set relative coordinate system.
Features of the DVP20PM: 64K steps program capacity, 100 motion programs to meet various processing needs; up to 100 motion trajectories can be set in the 20PM. Utilizing the 64K program capacity, G-codes for various running curves to be executed are pre-stored in the PLC. When processing a specific specification, they can be recalled using a text display, touchscreen, etc. Direct import of G-codes is supported. After generating text-format G-codes using CAM software, they can be directly imported into the PLC motion program using the PMSOFT import menu. Text-format G-codes can also be imported via HMI using a USB flash drive. Manual adjustment is supported via a handwheel. An electronic origin return mode is included; the 20PM's memory incorporates an origin memory function. Once the electronic origin is set, it can easily find its position after a power outage. It can connect to the EH2 main unit and all expansion modules. The 20PM is flexibly configurable; it can be connected to the EH2 main unit as a dedicated positioning expansion module, or it can serve as the main unit to connect other analog and other special function modules. It supports PLC sequential language and positioning language (G-code and M-code), achieving a perfect combination of general-purpose PLC and CNC technology. It features offline motion trajectory simulation, which can be used to check for problems in the motion program before actual machining, allowing for timely resolution and reducing errors (see the figure below).
4. Programming of eccentric circle welding trajectory
4.1 Power-on parameter initialization
Fill the mechanical parameters of X, Y, and Z, as well as the mechanical parameters of the upper and lower axes and front and rear axes of the welding torch, into the relevant setting registers such as (D1818, D1820, D1898, D1900, etc.), as shown in Figure 6.
Figure 6 Power-on parameter initialization
4.2 Manual Programming
Manually control the X, Y, and Z axes, as well as the welding torch's up/down and forward/backward axes. First, write the manual speed and other parameters for each axis into the relevant registers, as shown in Figure 7. Figure 8 shows the manual commands placed into the command registers related to the five axes. It's important to note that the 20PM00M three-axis motion controller is the main unit, and the 20PM00D two-axis motion controller is an expansion module. Communication between them is achieved through FROM and TO instructions for reading and writing. The main unit's CR0-CR199 correspond to the expansion modules D1500-D1599.
Figure 7 Manual speed parameters
Figure 8 Manual command parameters
4.3 Automatic Control
Using the recipe function, such as recipe 1, for workpiece 1, which is a concentric circle, with a speed of 4, a radius of 20, and an angle of 30, that is, D2000=1, D2002=40, D2004=200, D2006=300.
For example, in formula 2, workpiece 2 is an eccentric circle with a speed of 4.8, a radius of 21, an angle of 40, and an eccentricity of 58.
D2000=2,D2002=48,,D2004=210,D2006=400,D2008=580
According to the O100 loop procedure
The content value of D2000 calls different motion subroutines respectively.
There are four types of motion subroutines.
D2000=1, calls OX1, for concentric circle control, G00 ZD3000 FD3002 (D3000 is calculated based on the angle radius, D3002 is the speed).
D2000=2, call OX2, for eccentric circle control, G02 XD3010 YD3012 ZD3014 RD3016 FD3020 (D3010, D3012 are coordinates calculated based on the angle radius, D3020 is the velocity)
D2000=3, call OX3, welding torch vertical control
D2000=4, call OX4, platform moves horizontally.
The recipe function can be used to create the desired workpiece. For example, recipe 10 creates workpiece 10, which is an eccentric circle with a speed of 5.8, a radius of 31, an angle of 60, and an eccentricity of 50.
D2000=2,D2002=48,,D2004=310,D2006=600,D2008=500
If the program starts running, it will automatically call the relevant motion subroutines and related parameters, as shown in Figure 9.
Figure 9 Automatic Control
5. Conclusion
The Delta DVP20PM00D is a powerful PLC with both logic control and position control capabilities. It can flexibly, quickly, and accurately perform planar two-axis and three-axis motion control. The DVP20PM00D's excellent control functions have good development prospects and can be widely used in woodworking machinery, welding machinery, cutting equipment, and semiconductor processing equipment, such as LCD slicing machines, dual-axis vertical lathes, welding trajectory control, dispensing trajectory control, gantry CNC drilling machines, flame cutting machines, and other electromechanical control fields.
