CNC system configuration and function selection system

The configuration and function selection system of the numerical control system is an important part of the numerical control machine tool. What kind of numerical control system is configured and which numerical control functions are selected are the concerns of the machine tool manufacturers and end users.

Open-loop control system: Stepper motor is used as the driving component. There is no position and speed feedback device, so the control is simple and the price is low. However, their load capacity is small, the position control accuracy is poor, and the feed rate is low, which is mainly used for economics. Type numerical control device;

Semi-closed loop and closed-loop position control system: DC or AC servo motor is used as the driving component. The pulse encoder built in the motor can be used. The rotary transformer is used as the position/speed detecting device to form the semi-closed loop position control system. It can also be installed directly. A grating or inductive synchronizer on the table serves as a position detecting device to form a high-precision full-closed position control system.

Due to the presence of the pitch error, the amount of change in the angle of rotation of the screw fed back from the position detector of the semi-closed loop system does not accurately reflect the position of the linear motion of the feed axis. However, after the CNC system compensates for the pitch error, they can also achieve a fairly high position control accuracy. Compared with the full-closed system, they are less expensive, and the position feedback device installed inside the motor has good sealing performance, is more stable and reliable, and requires almost no maintenance, so it is widely used in various types of CNC machine tools.

The control of the DC servo motor is relatively simple and the price is low. The main disadvantage is that the motor has a mechanical reversing device inside, the carbon brush is easy to wear, and the maintenance workload is large. It is easy to spark during operation, which makes it more difficult to increase the speed and power of the motor.

The AC servo motor is a brushless structure, requires almost no maintenance, and has a relatively small volume, which is beneficial to the improvement of the speed and power. At present, the DC servo motor has been replaced in a large range.

[Types of servo control unit]

The separate servo control unit is characterized in that the numerical control system and the servo control unit are relatively independent, that is, they can be used with various numerical control systems, and the commands given by the NC system are DC voltages related to the rotational speed of the shaft (for example, 0). -10V), and returning from the machine tool is the axis motion position detection signal (such as encoder / inductive synchronizer output signal) that matches the NC system. The setting and adjustment of the servo data are performed on the servo control unit side (adjusted with a potentiometer or digitally).

The serial data transmission type servo control unit is characterized in that the data transmission between the NC system and the servo control unit is bidirectional. The command data, servo data and alarm signals related to the axis motion are transmitted through the corresponding clock signal line, strobe signal number, transmission data line, receiving data line, and alarm signal line. Information such as the actual position and state of the NC axis of the NC device is returned from the position encoder.

The network data transmission type servo control unit is characterized in that the shaft control units are densely installed together and are powered by a common DC power supply unit. The NC device is connected in series with the SR and ST points of the network data processing module of each axis control unit (substation) through the connection points SR and ST of the network data processing module on the FCP board to form a servo control loop. The position encoder of each axis and the axis control unit are connected by two high-speed communication lines, and the feedback information has the motion axis position and related state information.

The servo parameters of the serial data transmission type and network data transmission type servo control unit are digitally set in the NC device, and are loaded into the servo control unit during initialization, which is very convenient for modification and adjustment.

The network data transmission type servo control unit (such as the Otsuka OSP-U10/U100 system) has real-time adjustment capability with the corresponding control software. For example, in the Hi-G type positioning acceleration/deceleration function, it can be based on the speed and torque of the motor. The characteristic is obtained by finding the corresponding function, and then the function is used to control the acceleration and deceleration at the time of high-speed positioning, thereby suppressing vibration that may be caused by high-speed positioning. The increase in positioning speed can shorten the non-cutting time and improve the processing efficiency. In the Hi-Cut type feed speed control function, the system can automatically recognize the shape of the part (circular arc, edge, etc.) required by the CNC command after reading the part machining program, and automatically adjust the machining speed to make it the most. Jiahua, and then achieve high-speed and high-precision processing.

After the emergence of the all-digital AC servo system using high-speed microprocessor and dedicated digital signal processor (DSP), the hardware servo control becomes software servo control, and some advanced algorithms of modern control theory are realized, which greatly improves the servo system. Control performance.

The servo control unit is a component directly related to the machine in the numerical control system. Their performance has a great relationship with the cutting speed and position accuracy of the machine tool, and the price also accounts for a large part of the numerical control system. Relatively speaking, the failure rate of servo components is also high, accounting for more than 70% of electrical faults, so it is very important to choose a servo control unit.

In addition to the reliability of the servo control unit, the servo fault is closely related to the machine environment, mechanical conditions and cutting conditions. For example, if the ambient temperature is too high, it may cause the device to overheat and be damaged; if the protection is not strict, the motor may enter the water and cause a short circuit; if the guide rail and the lead screw are not well lubricated or the cutting load is too heavy, the motor may be over-current. The mechanical transmission mechanism is stuck, which may cause damage to the power device. Although the servo control unit itself has a certain overload protection capability, if the fault condition is serious or occurs many times, the device may still be damaged. Some CNC systems have real-time load display functions for the spindle and feed axis (for example, the “Current Position” page of the Ooki OSP system not only displays the real-time position data of the axis motion, but also displays the real-time load percentage of each axis, which the user can use. This information, take measures to prevent accidents.

[Selection of feed servo motor]

The output torque is an indicator of the load capacity of the feed motor. In continuous operation, the output torque decreases as the speed increases. The better the performance of the motor, the smaller the reduction. When configuring the motor for the feed axis, the output torque at the highest cutting speed should be met. Although the cutting is not performed during rapid traverse and the load is small, the starting torque at the highest rapid feed rate should also be considered. Excessive drop in output torque at high speeds also affects the control characteristics of the feed axis.

[Selection of spindle servo motor]

The output power is an indicator of the load capacity of the spindle motor. It can be seen from Fig. 3 that the rated power of the spindle motor refers to the output power when operating in the constant power zone (speed N1 to N2). When the speed is lower than the basic speed N1, the rated power is not reached. The lower the speed, the smaller the output power. In order to meet the power requirement of the spindle at low speed, the gearbox shift is generally adopted, so that the motor speed at the low speed of the spindle is also above the basic speed N1. At this time, the mechanical structure is complicated and the cost is correspondingly increased. In the CNC machine where the spindle and the servo motor are directly connected, there are two methods to meet the power requirements of the spindle at low speed. One is to select a spindle motor with a lower basic speed or a higher rated power, and the other is to use a special winding. Switching spindle servo motor (for example, YMF type spindle motor in Otsuka, Japan), the three-phase winding of this motor is connected to a star shape at low speed operation, and is connected to a triangle at high speed operation, thereby improving the low-speed power characteristics of the spindle motor. , reduce the cost of the spindle mechanical components.

It should be especially pointed out here that although high-speed machining is an effective way to improve the production efficiency of CNC machine tools, high-speed, high-precision cutting will bring higher requirements to servo drives and computer components, which will inevitably increase the cost of CNC systems, while high-speed machining Another important application area is the processing of light metal and thin-walled parts. Therefore, the speed of the spindle and the feed motor should be selected according to the actual needs of the machine.

[Selection of position detection device]

The machine origin is the reference point of all coordinate systems of CNC machine tools. The stability of the machine origin is an extremely important technical index of CNC machine tools, and it is also the basic guarantee for stable machining accuracy. There are two ways to establish the machine origin:

In a numerically controlled machine tool using a relative position encoder, an inductive synchronizer or a grating as a position feedback device, the numerical control system sets the first zero mark signal generated by the position feedback device after the zero return deceleration switch (or mark) of each feed axis. As a benchmark. This type of machine must restart the zero return operation of each feed axis after each power failure or emergency stop. Otherwise, the actual position may be offset, and the relative position of the zero return deceleration switch and its collision block may not be adjusted properly. Causes the instability of the mechanical origin position, which should be taken seriously;

In CNC machines with absolute position encoders as position feedback devices, the absolute position encoder automatically remembers every point in the full stroke of each feed axis without the need for a zero return switch. It is not necessary after each power failure or emergency stop. Re-set the reference point. The position of the reference point is permanently unchanged and is stored by the memory for the absolute position encoder. It is especially suitable for the setting of the zero position of the rotary table for the ratchet positioning. It is not only stable but also operates and adjusted. Bring great convenience.

[Selection of mechanical design plan]

The machine tool is composed of two parts: mechanical and electrical. In the overall design of the machine, the implementation of various functions of the machine tool should be considered from the aspects of electromechanical. The mechanical requirements of the CNC machine tool and the functions of the numerical control system are very complicated, so it is more important to communicate with each other. . The following is an example.

The adjustment of the spindle speed has two methods: automatic stepless speed regulation using servo motor or variable frequency motor, driving with ordinary three-phase asynchronous motor, stepping shift of mechanical gear, and manual shifting.

    The machining center machine uses a variety of tools for continuous machining of different types (milling, drilling, boring and tapping, etc.), so the spindle speed is constantly changing and must be automatically realized by the S command of the machining program. The spindle orientation must be performed, so an automatic stepless speed regulation with orientation function must be used.

For ordinary CNC milling machines with low spindle speed requirements, the tool replacement is performed manually, and during the machining process, the chances of selecting the same speed for the same tool are rare. Manual shifting is performed while manually changing the tool. The impact of production efficiency is not large, so the mechanical gear step-by-step shifting and manual shifting control methods are often used. Compared with the scheme of using the servo motor for stepless speed regulation, it can significantly reduce the production cost, save energy, and repair is simple, which is a very practical choice.

When using a horizontal machining center to perform multi-face machining on parts, it is often necessary to change the fixture and install the card multiple times. It must occupy valuable machine running time. The horizontal machining center with double-station automatic pallet exchange (APC) device can be used. The operation time of the component loading is greatly saved, thereby improving the production efficiency of the machine tool, and the control of the function is completed by the PLC control program. In addition to using a few input/output control points, the cost of the numerical control system is not increased much. It is a choice of features/price ratios.

The tool change time of the machining center machine has a great influence on the production efficiency, and the tool change speed has a great relationship with the mechanical structure. For example, a manipulator controlled by a cylinder generally has a tool change time of more than 10 seconds. A robot that can complete a tool change in 2 to 3 seconds is generally driven by a servo motor, and is equipped with a cam and an inner and outer cylinder loosening mechanism. A tool change speed that is not comparable to the mechanism may increase the failure rate. The selection of a reasonable cutting path, the use of high-quality tools, and the optimization of cutting conditions are also important means to improve production efficiency and should be considered comprehensively.

[Selection of CNC function]

In addition to the basic functions, the CNC system also provides users with a variety of optional functions. The basic functions of the CNC systems of various well-known brands are not much different. Therefore, it is reasonable to choose an optional function suitable for the machine tool, giving up the optional or not practical. The optional features are great for improving the functionality/price ratio of the product. Here are a few examples for reference.

[Animation / Track Display Function]

This function is used to simulate the part machining process and display the cutting path of the real tool on the blank. It can select the simultaneous display of two different planes in the Cartesian coordinate system, or select the three-dimensional display of different viewing angles. It can be displayed in real time while processing, or it can be quickly drawn in the mechanical locking mode. This is an effective tool for verifying part program, improving programming efficiency and real-time monitoring.

[floppy disk drive]

Through this data transfer tool, the programmed program in the system can be archived in a floppy disk and archived. It can also save the processing program generated by other computers into the NC system, thereby reducing the input time of the processing program. It can be used for backup or storage of various machine data, which brings great convenience to the programmers and operators.

[DNC-B communication function]

It is well known that the machining of parts consisting of non-circular curves or curved surfaces is very difficult. The usual method is to subdivide them into tiny three-dimensional straight segments and then write the machining program by means of a general-purpose computer, so the program capacity is extremely large. In the mold processing, such a processing program of several hundred Kbytes is often encountered, and the basic program storage capacity provided by the general numerical control system is 64 to 128 Kbytes, which brings great difficulty to the mold processing.

The DNC-B communication function has two working modes, one of which is two-way program transfer between the processing program storage area of the personal computer and the numerical control system, and the other is that the processing program of the personal computer is transmitted to the numerical control system one by one. The buffer runs the memory and transfers while machining until the end of processing. This completely solves the problem of processing large-volume program parts. Although the use of this feature requires a certain increase in cost, it is indeed an option with a high feature/price ratio.

Of course, choosing to expand the memory capacity is also an effective way to solve the surface mold processing. For example, the maximum operating buffer memory capacity of the Otsuka OSP system is 512K bytes. The program memory capacity can be expanded to 4096K bytes, which can meet the needs of a large part of the mold processing. Compared with the DNC-B method, it has the advantage of eliminating the personal computer and making the operation more reliable and operation. More convenient.

[Simplified programming function]

In order to improve the efficiency of programming, shorten the length of the machining program, and exploit the potential of program memory, the numerical control system provides some methods for simplifying programming.

Fixed cycle

The commonly used machining processes (such as drilling, boring, tapping, cavity and peripheral machining) are programmed into a parametric fixed-loop program. The user fills in the corresponding data during programming (such as base surface, hole depth, and per The number of times of the cut, the spindle speed and the feed rate, etc., can complete the predetermined machining process and can be reused many times.

Coordinate calculation function

Using the real-time computing power of the numerical control system, the hole machining processes (such as slashes, circumferences, and grids) distributed in various rules are programmed into a parametric fixed-loop program, and the user fills in the corresponding data (such as an angle during programming). The predetermined machining process can be completed by the radius, the number of holes, the number of rows, and the number of columns.

Subroutine function

User can place parts

The same machining process used in many places is programmed into subroutines and called at the corresponding parts, thereby shortening the length of the machining program.

User macro

Users can use the various arithmetic, logic and function operators provided by the system and various branch statements to form a mathematical table describing the shape of the machined part.

In the program execution process, the CNC system calculates the results and outputs the results. The machining of special curves and surfaces can be realized with a short program.

Rigid tapping function

The rigid tapping function must use a servo motor to drive the spindle. It is not only required to add a position sensor to the spindle, but also has strict requirements on the clearance and inertia of the spindle drive mechanism, so the cost of this function cannot be ignored. For the user, if there are no special requirements (such as high-speed high-precision, special materials or large-diameter hole processing, etc.), the elastic telescopic chuck can be used, and flexible tapping on the general spindle can meet the processing requirements, so it is not necessary to use rigidity. Tapping function.

Tool life management function

Whether to use the tool life management function in the machining center, we must consider the batch size of the machined parts, the consistency of the tool and the blank quality, and the capacity of the tool magazine. Otherwise, it will not only cause many human errors, but also affect the normal production. Moreover, the position of the spare tool will also greatly reduce the effective capacity of the tool magazine, making some complex parts unable to be processed due to insufficient tool positions.

Automatic tool radius/length and workpiece measurement

The tool path in the machining program is usually programmed according to the tool center and the tool tip, so the corresponding tool radius and length must be entered before the program is executed, which is especially important for machining centers.

The tool radius and length can be measured manually using conventional gages or with a special tool gauge. The operator can compensate for the length offset value by the position difference of the tool tip of each knife in the Z-axis direction with respect to the same "face-to-face" on the machine tool. The "semi-automatic tool length measurement" function provided by the numerical control system is used. The length compensation value relative to the "standard tool".

Automatic tool radius/length and workpiece measurement functions require dedicated contact sensors and laser probes and signal receivers. The following points should be clarified when using this feature:

The contact sensor and signal receiver are installed in the working area of the machine tool. Its protection is very important. The cutting amount is large. It is not suitable to install the machine using spray rinsing. The above measurement needs to take up the machining time of the machine, which may affect the efficiency of the machine tool. The general purpose of the function is to measure the position of the reference hole center or other reference points on the workpiece blank as the origin of the program. Instead of the manual "tool setting", its accuracy is not higher than the positioning accuracy of the machine itself.