Multi axis synchronous control method of NC machin

2022-05-19
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Multi axis synchronous control method of NC machine tool

in machine tool control, double axis or multi axis synchronous control is a common control method, such as beam lifting control of moving beam gantry milling machine, gantry frame movement control of gantry frame mobile machining center, etc. Although in these cases, a single motor can drive two sides through mechanical mechanisms such as bevel gears, the complex transmission mechanism and large clearance are easy to cause the instability of the closed-loop control system, high operation noise and difficult maintenance. In addition, if it is used in the occasion with large load moment of inertia, due to the low transmission efficiency, the motor with large power must be selected. This scheme is not ideal only from the perspective of economy. Therefore, the bilateral drive of two motors is an ideal scheme, which leads to the problem of double shaft synchronous control

1 Comparison of multi axis synchronous control schemes of machine tools

1.1 synchronous control of ordinary machine tools

generally, there are the following solutions for the synchronous control of two axis motors of ordinary machine tools. ① Two DC motors are driven by a set of DC speed regulating device. The armature of the two motors are connected in series and the excitation coil is connected in parallel. In order to maintain rigid connection, the two shafts must be synchronized. Only one of the two motors provides speed feedback signal, and its control diagram is shown in Figure 1. ② Two sets of AC variable frequency speed regulation systems respectively control two AC variable frequency or servo motors as the master-slave shaft, and the two motors provide their own speed feedback signals. In order to keep the speed synchronous, the two motor shafts must also be rigidly connected. See 2 for the control diagram

Figure 1 synchronous control of DC speed regulation system

Figure 2 synchronous control of AC speed regulation system

these two control systems belong to position ring open-loop system, which can only rely on the rigid connection of the shaft to maintain the synchronization of motor speed or position, with simple structure and high reliability. The x-axis drive of b2063 milling machine we transformed for users adopts the control method shown in Figure 2. These two control systems do not detect the actual position of the moving parts connected to the motor, and can not compensate the errors caused by the lead screw pitch, coupling clearance, lead screw torsion, lead screw axial deformation and other factors. The control accuracy is poor, so they can not be applied to NC machine tools

1.2 synchronous control of NC machine tools

the difference between NC machine tools and ordinary machine tools lies in the strong control function of NC system, which can control different parameters such as position and torque. Due to the introduction of position detection device, a double closed-loop system of position and speed is formed to realize position synchronous control. The synchronous control method of NC machine tool can be summarized as follows: after comparing the given positioning parameters of synchronous motor with the adjustment amount of the position feedback difference between the two motors, it can be used as the position reference of the synchronous motor, so as to complete the position synchronous control. The control diagram is shown in Figure 3

Figure 3 synchronous control of NC machine tool

at present, most of the processors of NC system use 32-bit processors such as 68020 and 68040, and now they are transitioning to 64 bit processors. It is precisely because CNC system has stronger and stronger computing power, so the synchronous control of machine tool will have better performance. At present, most domestic CNC systems have the function of servo axis synchronization. The measuring mechanism of synchronous shaft can be either a semi closed-loop system with photoelectric encoder as the measuring mechanism or a full closed-loop system with grating ruler as the measuring mechanism

in the shaft synchronization function, due to different control objects and parameters, there are different types of position synchronization, speed synchronization, torque synchronization and so on, which are suitable for different occasions. In different systems, although the control methods are different from those of many people, the control mechanism is the same. Therefore, the appropriate scheme should be selected according to the characteristics of NC system and machine tool

2 double axis synchronous control scheme of CNC gantry milling and boring machine

2.1 control scheme

xk2520 × 40 CNC gantry milling and boring machine is a new type of CNC machine tool developed by ourselves with the introduction of the advanced technology of French forest line company. The machine adopts the bed height frame beam moving structure. The reciprocating movement of the beam along the guide rail of the bed is x-axis, the left and right movement of the milling head ram along the beam is y-axis, and the up and down movement of the milling head ram along the slide plate is z-axis. The numerical control system adopts num 1040m system of French num company. The system can be equipped with 8 NC axes, including two spindles, with four axis interpolation function. Siemens SIMODRIVE 611A AC servo system is selected as the servo feed system. The x-axis of the machine tool adopts two 5.3kw Siemens 1ft5 AC servo motors, which drive the ball screw nut pair after being decelerated by the primary synchronous belt. The x-axis guidance mode is rolling guide block, and Heidenhain precision grating ruler is used for position detection

as for the transmission of x-axis, on a CNC machine tool with the same structure, we once adopted the transmission structure that a servo electric sample placing machine slows down through the gearbox, and the bevel gear drives the ball screw nut pairs on both sides. This transmission structure has high requirements for the installation position of the encoder: if it is installed at the lead screw end, the position ring will be continuously adjusted due to the gap, resulting in the extremely unstable control system. Generally, the oscillation caused by the static error of the control system can be obviously observed at the shaft end of the motor; If it is installed at the motor end, although the system is stable, it is difficult to ensure the control accuracy due to the transmission gap, so a compromise scheme must be adopted. The final scheme is to install the encoder on the middle stage of the three-stage gear reducer to obtain better accuracy and stability, but this scheme reduces the operation performance of the NC machine tool. In order to make this xk2520 achieve due accuracy and efficiency, we decided to transform its X-axis transmission scheme

the transformation scheme we adopted is to change the x-axis into double motor bilateral drive, and realize the x-axis bilateral synchronization by using the axis synchronization function of num 1040m system. A Heidenhain precision grating ruler is installed on the sides of the two guide rails of the x-axis. The position measurement data read out by the two reading heads enter the x031 signal amplifier respectively, and then feed back to the NC system. As two CNC axes, axis X occupies two axis ports of the CNC system, which are defined as X1 and X2 respectively, and X1 is defined as the synchronous axis, X2 is the synchronized axis of x1, and the two axes form a synchronous axis pair. The specific control and connection methods are limited by space and will not be introduced in detail

2.2 debugging methods and precautions

(1) debugging methods the machine parameters related to synchronous control in the num 1040m NC system used by the machine tool include p27/p64/p28/p24

p27: define the NC axis as the synchronization axis, and assign the synchronized axis to the synchronization axis to form an axis pair

p64: define the synchronization axis pair

p28: synchronization enable bit of synchronization shaft pair. When the synchronization enable bit is 1, the synchronization function takes effect

p24: define the characteristics of synchronous shaft

synchronized axis physical address @

N0} synchronization threshold 1 (1 word per axis) default value 160 m N31

defines the maximum allowable error when the synchronization function is valid. When this threshold is exceeded, the system generates an error, but still allows the synchronization axis to synchronize with its synchronized axis

synchronized axis physical address @ + 32

N32} correction factor 1 (1 word per axis) default value 250 m N63

defines the correction factor of synchronization error, which is the thousandth ratio of synchronization error (1/1000)

N64

synchronization threshold 2 (each axis shares one word)

default value 600 m

defines the maximum allowable error when the synchronization function is effective. When this threshold is exceeded, the system cancels the synchronization function and forcibly stops

in synchronous control, this parameter is very important for adjusting synchronous characteristics. BASF material is important in the overall design of vehicle body, and it is easy to use. By adjusting the synchronization threshold, the machine tool can achieve ideal performance. However, when setting this parameter, it must be estimated according to the mechanical rigidity of the machine tool. If the threshold is too high, it may damage the accuracy of the machine tool; If the threshold is too low, the machine tool may exceed the synchronization error threshold during normal operation, resulting in shutdown and affecting the normal use of the machine tool

the debugging steps of the dual axis synchronous control part are as follows: before the NC system is connected, the servo system must be adjusted first, that is, the inner loop is composed of speed loop and current loop to make the inner loop achieve the ideal performance. The method is: connect the servo system with the motor and connect the enabling signal through short line and other methods to meet the conditions for the system to drive the motor; After the feed is enabled, connect the given signal with the battery; Add the given signal, observe the motor steering and adjust the jumper to make the motor steering correct; Gradually increase the given voltage and adjust the servo parameters to make the motor achieve the ideal performance. This part can be completed when commissioning the electrical control cabinet

after connecting the CNC system, connect the position loop, and the system forms a double closed-loop system of position and speed. At this time, the position feedback polarity of the system has not been determined, which can only be confirmed by experiment. In this step, the matching problem of synchronous motor steering should be considered, which must be paid attention to in synchronous control. After the feedback direction of the single shaft motor is determined, it should also be considered that the rotation direction of the motor after connecting the given signal is the same as the expected value, which needs to comprehensively consider the given signal polarity, feedback polarity, motor rotation direction, motor rotation direction setting in the servo system, ball screw rotation direction and other factors in the NC system. If these factors are ignored during debugging, it may cause devastating damage to the machine tool

as for, the setting of double axis synchronization is preliminarily completed, and then the prepared automatic control program is introduced into the NC system for comprehensive debugging, and the synchronization parameters are carefully adjusted to obtain ideal performance

(2) precautions during commissioning, the following aspects should be paid attention to:

① ensure that the hydraulic and lubrication systems work normally, which is a necessary condition before the machine tool moves

② when moving the shaft, low speed is preferred. At this time, the synchronization characteristics have not reached the best. It is necessary to fine adjust the acceleration, deceleration, positioning bandwidth and amplification factor of the servo system until the system works stably and can achieve the ideal positioning accuracy and repeated positioning accuracy

③ during the commissioning of double axis synchronization, the commissioning plan must be arranged to avoid blind driving as far as possible to avoid accidents. For example, when we debug the servo system, the mechanical personnel move the beam from one side of the x-axis to the other side. At this time, the electrical commissioning personnel add a given signal to the servo system through the battery to move the x-axis when the NC system is not connected. Since no position detection is added when the x-axis moves, the beam tilts during the movement, resulting in the overload of the first axis module and the motor stops; The continuous rotation of the other axis motor caused the accuracy of the x-axis guide rail to change. Due to the timely discovery, the accuracy was restored after adjustment by the mechanical personnel

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