Configuration and function selection of the hottes

2022-10-01
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Configuration and function selection of numerical control system

numerical control system is an important part of numerical control machine tools. What kind of numerical control system to configure and which numerical control functions to choose are the concerns of machine tool manufacturers and end users

selection of servo control unit

1 The position control mode of CNC system

open loop control system: the stepping motor is used as the driving part, and there is no position and speed feedback device, so the control is simple and the price is low, but their load capacity is small, the position control accuracy is poor, and the feed speed is low, which is mainly used for economic CNC devices

semi closed loop and closed loop position control system: DC or AC servo motor is used as the driving part, pulse encoder installed in the motor and resolver as the position/speed detection device can be used to form a semi closed loop position control system, or grating or inductosyn directly installed on the workbench can be used as the position detection device to form a high-precision full closed loop position control system

due to the existence of pitch error, the change of lead screw rotation angle fed back from the position detector of the semi closed loop system cannot accurately reflect the linear motion position of the feed shaft. However, after the CNC system compensates the pitch error, they can also achieve quite high position control accuracy. Compared with the full closed-loop system, their price is lower, the position feedback device installed in the motor has good sealing, works more stably and reliably, and requires almost no maintenance, so they are widely used in various types of CNC machine tools

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

AC servo motor is a brushless structure, which hardly needs maintenance, and its volume is relatively small, which is conducive to the improvement of speed and power. At present, it has replaced DC servo motor in a large range

2. The type of servo control unit

separate servo control unit, which is characterized by the relative independence of the NC system and servo control unit, that is, they can be used with a variety of NC systems. The command given by the NC system is the DC voltage related to the shaft motion speed (such as 0? 10V), and the return from the machine tool is the shaft motion position detection signal matched with the NC system (such as encoder? Inductosyn and other output signals). The setting and adjustment of servo data are carried out at the side of servo control unit (adjusted by potentiometer or input by digital mode)

serial data transmission servo control unit is characterized by bidirectional data transmission between NC system and servo control unit. Command data, servo data and alarm signals related to axis motion are transmitted through corresponding clock signal lines, strobe signal numbers, sending data lines, receiving data lines and alarm signal lines. The actual position and status of the moving shaft of the NC device are returned from the position encoder

network data transmission servo control unit is characterized by dense installation of shaft control units, which 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 (sub station) 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 is connected with the axis control unit through two high-speed communication lines, and the feedback information includes the position of the moving axis and related status information

the servo parameters of serial data transmission and network data transmission servo control units are set digitally in the NC device, and are entered into the servo control unit during startup initialization. It is very convenient to modify and adjust

the network data transmission type servo control unit (such as Okuma osp-u10/u100 system) has the ability of real-time adjustment with the cooperation of the corresponding control software. For example, in the hi-g positioning acceleration and deceleration function, the corresponding function can be obtained according to the speed and torque characteristics of the motor, and then the acceleration and deceleration speed during high-speed positioning can be controlled with its function, so as to suppress the vibration that may be caused during high-speed positioning. The improvement of positioning speed can shorten the non cutting time and improve the machining efficiency. For another example, in the hi cut feed speed control function, the system can automatically recognize the part shape (arc, edge, etc.) required by the NC command after reading the part processing program, and automatically adjust the processing speed to optimize it, so as to realize high-speed and high-precision processing

after the emergence of the full digital AC servo system using high-speed microprocessor and special digital signal processor (DSP), the hardware servo control has changed into software servo control, and some advanced algorithms of modern control theory have been realized, which greatly improves the control performance of the servo system

servo control units are components directly related to machinery in the CNC system. Their performance is closely related to the cutting speed and position accuracy of machine tools, and their price also accounts for a large part of the CNC system. Relatively speaking, the failure rate of servo components is also high, accounting for more than 70% of electrical failures, so it is very important to select servo control units

servo failure is not only related to the reliability of servo control unit, but also closely related to the operating environment, mechanical conditions and cutting conditions of machine tools. For example, if the ambient temperature is too high, it is easy to cause overheating and damage of devices; Lax protection may cause water ingress into the motor and cause short circuit; Poor lubrication of guide rail and lead screw or heavy cutting load will cause motor overcurrent. The mechanical transmission mechanism is stuck, which will cause damage to the power device. Although the servo control unit itself has a certain overload protection capacity, the device will still be damaged when the fault condition is serious or occurs many times. Some CNC systems have the function of real-time load display of spindle and feed axis (such as "current position" of Okuma OSP system The page can not only display the real-time position data of axis movement, but also display the real-time load percentage of each axis at the same time. Users can use this information to take measures to prevent accidents

3. Selection of feed servo motor

output torque is an indicator of the load capacity of the feed motor. It can be seen from Figure 2 that under continuous operation, the output torque decreases with the increase of speed. The better the performance of the motor, the smaller the reduction. When configuring the motor for the feed shaft, the output torque at the maximum cutting speed should be met. Although there is no cutting during rapid feed and the load is small, the starting torque at the highest rapid feed speed should also be considered. Excessive reduction of output torque at high speed will also affect the control characteristics of the feed shaft

4. Selection of spindle servo motor

output power is an indicator of the load capacity of spindle motor. It can be seen from Figure 3 that the rated power of the spindle motor refers to the output power when running in the constant power area (speed n1 to N2). When it is lower than the basic speed n1, the rated power cannot be reached. The lower the speed, the smaller the output power. In order to meet the power requirements of the main shaft at low speed, the gearbox is generally used to change the speed, so that the motor speed of the main shaft at low speed is also above the basic speed n1. At this time, the mechanical structure is more complex, and the cost will increase accordingly. In numerical control machine tools where the spindle is directly connected with the servo motor, there are two methods to meet the power requirements of the spindle at low speed. One is to select the spindle motor with low basic speed or one gear higher rated power. The other is to use a special winding switching spindle servo motor (such as Ymf spindle motor of Okuma, Japan). The three-phase windings of this motor are connected into a star when running at low speed and a triangle when running at high speed, Thus, the low-speed power characteristics of the spindle motor are improved and the cost of the spindle mechanical parts is reduced

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

5. Selection of position detection devices

the mechanical origin is the reference point of all coordinate systems of the NC machine tool. The stability of the mechanical origin is an extremely important technical index of the NC machine tool and the basic guarantee of stable machining accuracy. There are two ways to establish the mechanical origin:

1) in the NC machine tool that uses the relative position encoder, inductosyn or grating as the position feedback device when the screw is baked and cooled by acetylene flame, The CNC system takes the first zero mark signal generated by the position feedback device after the zero return deceleration switch (or mark) of each feed axis as the reference point. After each power failure or emergency shutdown, this kind of machine tool must be reset for the zero return operation of each feed shaft, otherwise, the actual position may be offset, and the relative position of the zero return deceleration switch and its impact block may be improperly adjusted, which will also cause the instability of the mechanical origin position, which should be paid attention to

2) in the CNC machine tool using the absolute position encoder as the position feedback device, the absolute position encoder can automatically memorize the position of each point within the full stroke of each feed shaft, and there is no need to return to the zero switch. After each power failure or emergency shutdown, there is no need to reset the reference point. The position of the reference point is permanently fixed after setting, and is memorized by the memory specially used for the absolute position encoder. It is especially suitable for the setting of the zero point position of the rotary workbench for the positioning of the mouse tooth disc. It is not only stable, but also brings great convenience to the operation and adjustment

6. Selection of mechanical design scheme

machine tool is composed of mechanical and electrical parts. When designing the overall scheme, the implementation scheme of various functions of the machine tool should be considered from both mechanical and electrical aspects. The mechanical requirements of NC machine tool and the functions of NC system are very complex, so it is more necessary to communicate between mechanical and electrical parts to develop strengths and avoid weaknesses. The following is an example

example 1

there are two methods to adjust the spindle speed: automatic stepless speed regulation by using servo motor or variable-frequency motor, and manual gear shift by using ordinary three-phase asynchronous motor drive, mechanical gear step-by-step speed change, hr=130-e

the machine tool of the machining center uses a variety of tools to carry out continuous cutting of different types (milling, drilling, boring, tapping, etc.), so the speed of the spindle is often changed, and must be automatically realized by the s command of the machining program. When changing the tool automatically, the spindle orientation must also be carried out, so the automatic stepless speed regulation mode with orientation function must be adopted

for ordinary CNC milling machines with low spindle speed requirements, tool replacement is carried out manually, and in the processing process, there are few opportunities for the same tool to choose different speeds. Manual speed change at the same time of manual tool change has little impact on production efficiency, so the control mode of mechanical gear grading speed change and manual shift is often used. Compared with the scheme of stepless speed regulation with servo motor, it can significantly reduce production costs, save energy, and simple maintenance. It is a very practical choice

example 2

when using a horizontal machining center for multi-faceted machining of parts, it is often necessary to replace fixtures and load cards for many times, which must occupy valuable machine tool running time. Selecting a horizontal machining center with a double station automatic pallet exchange (APC) device can greatly reduce

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