Abstract: This paper introduces the master-slave control principle of the SIEMENS 840D CNC system and the shifting principle of the French REDEX reducer. It also focuses on analyzing the problems and solutions that occur during shifting in the master-slave control structure.
Keywords: Master-slave control, REDEX reducer, gear shifting
Shift control of master-slave drive structure spindle
1. Introduction
Our company's heavy-duty vertical (horizontal) turning centers mostly adopt a dual-drive (or multi-drive) master-slave structure for the spindle (or worktable), that is, two (or more) servo motors drive one spindle (or worktable). This greatly simplifies the mechanical structure and improves the indexing and positioning accuracy of the machine tool. The CHX61315 is a heavy-duty horizontal turning and milling machining center designed and manufactured by our company for a large domestic enterprise. It can process workpieces with a maximum diameter of 3.15m and bear a maximum workpiece weight of 150 tons. The main drive adopts a master-slave dual-drive structure, as shown in Figure 1. The master (slave) servo motors drive gears 1a(1b), 2a(2b), 3a(3b) and gear 4 through a two-stage reduction gearbox from REDEX (a French company) (consisting of a cycloidal pinwheel planetary gearbox with two gears and a worm gearbox). Finally, gear 4 drives the spindle to rotate.
Main hardware components and spindle configuration:
Master (slave) servo motor: Siemens 1PH7 224-2ND03-0CA0, 71Kw;
Two-stage gearboxes: REDEX MSR656.FP5S.2286M1.60K75.H.5T, i=1:1 and 1:3.83;
Control system: Siemens 840D, NCU572.5, software version V7.4;
Spindle encoder: Heidenhain ROD280 angle encoder, 18,000 pulses/revolution, measurement step 0.0001 degrees.
The active servo motor is set as the first spindle, and the driven servo motor is set as the third spindle (the milling head main motor is the second spindle). Other axis parameters, including gear ratio, speed, gain, acceleration, etc., are set exactly the same for both servo motors.
Figure 1. Schematic diagram of the master-slave drive structure of the spindle.
2. Introduction to Master-Slave Control Principle
Master-slave drive involves two or more servo motors simultaneously driving a single axis, establishing a master-slave control relationship between them. The slave servo motor moves according to the speed set by the master servo motor. The master servo motor typically uses position control, while the slave servo motor always uses speed control. First, a PI torque controller ensures that the two servo motors produce a certain amount of output torque. Additionally, in axis control (or positioning) mode, an additional torque can be specified to generate tension between the two servo motors in the master-slave drive, thus eliminating backlash in the transmission system. The main axis parameter settings for master-slave control are as follows (slave axis):
MD37250 3 Main axis number for speed coupling (active servo motor axis number)
MD37252 3 Main axis number for torque control (active servo motor axis number)
MD37254 1 Torque Compensation Control Activated
MD37256 2 Torque Compensation Control Gain Coefficient
MD37262 0 Fixed effective master-slave drive control
MD37264 10% Master-Slave Tension Torque
MD37268 50% Driven Shaft Torque Coefficient
3. Shifting principle of REDEX reducer
Figure 2 Electrical schematic diagram of REDEX gearbox shifting
Figure 3 Logic diagram of gear shifting process
REDEX's two-stage gearbox has two mechanical gears, 1:1 and 1:3.83, achieved by changing the star gear. The electrical principle of the gear shifting is shown in Figure 2. M is the gear shift drive motor, and KA1 and KA2 are intermediate relays for switching voltage polarity. When shifting to the 1:1 side, KA2 is energized, KA1 is de-energized, M rotates forward, driving the shift fork to swing, and simultaneously the active servo motor starts swinging (the shifting logic is shown in Figure 3). The shift fork drives the star gear to move. When the detection switch detects the gear position reached, KA2 is de-energized, and the main motor stops swinging. When shifting to the 1:3.83 side, KA1 and KA2 are energized simultaneously, M reverses, and the shifting process is similar to the previous one.
The wiring of the two reducers is exactly the same, and the system considers the gear shifting process to be complete only when the gear position detection switch signals of both reducers arrive.
4. Debugging
During actual on-site debugging, we found that the driven-side reducer could not complete gear shifting in most cases. Even when gear shifting was occasionally completed, the servo motor load current was extremely high when the spindle was rotating, at least 80% of the rated current. After rotating continuously for a few seconds, the servo power module would trigger an overheating alarm. After repeated tests and analysis, we found that there were two main reasons for the above problems:
- As can be seen from the spindle drive structure and the master-slave control principle, the driven servo motor rotates along with the active servo motor. Because a certain tension needs to be generated between the master and slave servo motors when they begin to rotate, there is a time difference between them; that is, when the master servo motor starts rotating, the driven servo motor has not yet rotated. However, the gear shifting process of REDEX's reducers is very short. The active-side reducer has already shifted gears before the driven servo motor starts to rotate. At this time, the rotation of the master servo motor is transmitted in reverse along the drive chain to the reducer on the driven servo motor side. At this point, the two planetary gears that need to mesh in the driven-side reducer rotate at the same speed and in the same direction, making meshing difficult.
- According to the master-slave control principle of the Siemens 840D CNC system, the driven axis always follows the drive axis. Therefore, the M-code executing the spindle shift instruction in the program is output to the drive servo motor (i.e., the first spindle). The PLC determines the current actual gear by detecting the status of the two reducer gear switches. When the actual gear matches the required gear, the shift is completed. Since the NC always processes the shift signal of the drive servo motor, even if the driven servo motor and the drive servo motor are in the same gear, the NC does not process the shift signal of the driven servo motor (i.e., the third spindle), but considers the driven servo motor to still be in the original gear. When processing the S-value in the program, the NC selects different parameter groups (transmission ratios) according to different gears. If the selected parameter groups are inconsistent, different motor speeds will be output, leading to severe asynchrony and reverse transmission, resulting in a large current and causing the power module to overheat.
In response to the causes of the above problems, we have proposed corresponding solutions:
(1) Cancel the master-slave relationship during gear shifting. Before the spindle starts shifting gears, first disconnect the master-slave relationship and delay the shutdown of the control enable signal of the driven servo motor, so that the driven servo motor is in a free state. Then start shifting gears. The active servo motor oscillates left and right intermittently to facilitate gear meshing. When the active side reducer is in gear, it drives the star gear of the driven side reducer to rotate through reverse transmission, and the driven side reducer starts shifting gears. When both reducers are in gear, the master servo motor stops oscillating. Finally, activate the control enable signal of the driven servo motor to activate the master-slave control. At this point, the entire gear shifting process is completed. The following is the PLC program for this process:
...
AN DB33.DBX82.3 //Main servo motor shift signal
= M13.3
A M13.3
= DB34.DBX24.7 //Signal for master/slave control of the slave servo motor (activation/deactivation)
AN M13.3
L S5T#4S
SD T36
AN T36
= DB34.DBX2.1 // Enable signal for slave servo motor control
...
(2) Change the parameter group of the driven servo motor. After each spindle gear shift, select the parameter group of the driven servo motor according to the actual gear signal of the active servo motor so that the current parameter groups of the two are consistent.
The axis parameter of the driven servo motor that needs to be modified is: MD35590, value 2 (parameter group settings can be changed).
PLC program for assigning parameter groups:
A DB33.DBX16.0 //Actual gear stage A of the active servo motor
= DB34.DBX9.0 // Slave servo motor parameter group A
A DB33.DBX16.1 // Actual gear stage B of the active servo motor
= DB34.DBX9.1 // Slave servo motor parameter group B
A DB33.DBX16.2 // Actual gear stage C of the active servo motor
= DB34.DBX9.2 // Slave servo motor parameter group C
...
Through the above measures, the problems mentioned above were solved, and the machine tool operated stably and reliably during the long-term follow-up service.
5. Conclusion
Since this type of main drive structure is being used for the first time in China, corresponding measures were taken to solve the problems that occurred during the debugging process by analyzing the master-slave control principle of the CNC system and the gear shifting principle of the reducer. This has accumulated experience for the application and promotion of this type of transmission structure.
