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The application of servo control system on CNC machine tool

From:www.joyoucnc.com  Classification:Technical Support  Released:2017-8-11 15:07:45  Reading:(523)
Abstract:

  The floorboard of the nc machine tool is digital control machine tools, is the collection of modern precision mechanical design and manufacturing technology, computer technology, communication technology, automatic control technology, detection technology, power electronics and microelectronics technology, electrical and new material technology, hydraulic and pneumatic technology, photoelectric technology, such as the latest achievement of senior electromechanical integration of typical products, as a processing and manufacturing of machine tools, and combines a high precision, high flexibility, high efficiency, high automation features of modern equipment.
  Numerical control machine tool is the important basic equipment, national defense and national economy of its application in almost every field of social economy, is the mechanical, electronic, automobile, petrochemical, construction and other departments of pillar industry, is also the basic industries such as energy, transportation, materials, communication modernization important tool.
  Machine tool
  In the defense, aerospace, aviation, military equipment, ships, space technology, the sea and other fields are irreplaceable, even become indispensable critical equipment. The numerical control machine tool, especially the high-end CNC machine tool, embodies the national economic and scientific and technological comprehensive strength, has the strategic position, in some cases, even affects the survival of the nation and the nation.
  Therefore, all countries attach great importance to it, and our country is no exception, taking a series of measures to support the development of the CNC machine tool industry. But for a variety of reasons, China''s technological level has not caught up with the advanced advanced countries. In particular, CNC machine tool is a very complicated electromechanical integration system, which involves many technical fields. It is very difficult for a few departments or units to accomplish such a difficult task in the short term.
  At present, not only the servo control system in industrial and agricultural production and daily life has been widely used, and in many high-tech fields, such as laser processing, robot, nc machine tools, large scale integrated circuit manufacturing, office automation equipment, satellite attitude control, radar, and various military weapons servo system, the flexible manufacturing system and the application of the automatic production line, etc is also developing rapidly.
  Servo system is an important part of automatic control system, its performance directly decide and affect the quickness, stability and accuracy of automatic control system, the combination of machine, electricity, liquid become the main technical basis of industrial automation.
  The servo control system is used to accurately follow or reproduce the feedback control system of a process. Servo system in many cases, the amount of points to be controlled (system) of the output is the mechanical displacement or displacement velocity and acceleration feedback control system, its role is to make the output of the mechanical displacement (or Angle) to accurately track the input displacement (or Angle).
  The structure of servo system and other forms of feedback control system have no principle difference. Is the main task of the servo system, in accordance with requirements of the control command, the signal conversion, control and power amplification, such as processing, driving device, the output torque, speed and position can be flexible and convenient control.
  1. Design of control scheme of CNC machine tool
  1.1 machining accuracy
  Accuracy is one of the performance indicators that the machine must guarantee. The position accuracy of position servo control system determines the machining accuracy of CNC machine. So location accuracy is a very important index. In order to ensure sufficient location accuracy, on the one hand, it is the correct selection of the size of the open loop magnification in the system, and on the other hand, the requirement for the accuracy of the position detection element.
  Because in a closed loop control system, for the tested value of the error of the component itself and the deviation is difficult to distinguish, feedback the precision of the detecting element to the precision of the system often plays a decisive role. It can be said that the machining accuracy of nc machine is mainly determined by the accuracy of the detection system.
  The minimum displacement measured by the displacement detection system is called resolution. The resolution depends not only on the detection element itself, but also on the measuring circuit. When designing CNC machine tools, especially high-precision or large and medium-sized CNC machine tools, the detection components must be carefully selected.
  The resolution or pulse equivalent of the selected measurement system is one order of magnitude higher than the machining precision. In short, high - precision control system must have high - precision detection elements as guarantee. For example, the precision of the linear induction synchronizer commonly used in nc machine tools can reach the accuracy of 0.0001 mm, i.e. 0.1 mu m, the sensitivity is 0.05 mu m, and the repetition accuracy is 0.2 mu m; The accuracy of the circular induction synchronizer can reach 0.5 N, sensitivity 0.05 N, and repeat accuracy 0.1 N.
  1.2 open loop control magnification
  In a typical second-order system, the damping coefficient x = 1/2 (KT) -1/2, and the velocity steady state error e (infinity) = 1 / K, where K is the open-loop magnification, which is called the open-loop gain. Obviously, the open-loop magnification of the system is one of the important parameters affecting the static and dynamic parameters of the servo system.
  In general, the amplification multiple of CNC machine tool servomotor is 20 ~ 30 (1 / S). The K20 system is often referred to as a high magnification or hard servo system, which is applied to the contour machining system.
  If the surface roughness and accuracy of the machining parts are not affected, the expected step response does not oscillate, which is that the requirement is larger, and the open loop amplification multiple K is smaller. If the speed of the system starts, we hope that the x choose a smaller one, which means that the expansion of the open loop will be increased, and the increase of K will improve the steady-state accuracy of the system. Therefore, the selection of K values is a comprehensive consideration. In other words, the higher the magnification of the system, the better.
  When the input speed is mutated, the high magnification can result in drastic changes in output, the mechanical devices are subject to greater impact, and some may also cause the stability of the system. This is because system stability has a range of values for K values in higher-order systems. The low magnification system also has some advantages, such as system adjustment is easy, simple structure, not sensitive to disturbance, and the surface roughness of machining is good.
  1.3 reliability of control system
  Numerical control machine tool is a kind of high precision, high efficiency automation equipment, if occurrence failure the loss is bigger, so increase the reliability of numerical control machine tool to appear particularly important. Reliability is one of the main quantitative indexes of reliability evaluation, which is defined as the probability of the product to complete the specified function under the specified conditions and the specified time.
  For numerical control machine tools, its requirements refer to its environmental conditions, working conditions and working methods, such as temperature, humidity, vibration, power supply, interference intensity and operating procedures. The function of the main index control machine tool, such as the various functions of nc machine tools, servo performance, etc.
  Average failures (failure) time interval (MTBF) refers to the failure after the repair or replacement parts still can continue to work to repair equipment or system, from the average time of a fault to the next, nc machine tools it is commonly used as a quantitative index of the reliability.
  The reliability of the servo system is relatively prominent because of the improved reliability of the CNC device. Its fault is mainly from servo element and mechanical transmission part. Generally, the reliability of the hydraulic servo system is worse than the electrical servo system. The reliability of solenoid valve, relay and other electromagnetic components is poor, and the non-contact element should be used instead.
  The reliability of nc machine tools is not high because of the quality of components, process conditions and cost. In order to get the nc machine to get the factory''s welcome, it is necessary to improve its reliability, thus improving its use value. In the design of servo system, the components must be chosen according to the technical requirement and reliability of the design, and according to the strict inspection screening test, the mechanical interlock device, etc., must pay close attention to and minimize the failure caused by mechanical components.
  1.4 speed range
  In the process of nc machine tool, the servo system can satisfy the speed shift and the single step movement at the same time, so that the drive has a wide range of adjustable speed.
  A single step action is used in the adjustment of the workbench for an auxiliary working mode. The servo system can achieve the steady feed at low speed, and the speed must be greater than the "dead zone" range. The so-called dead zone refers to the presence of the static friction force which causes the system to overcome the friction without being able to turn the system under very small input. In addition, due to mechanical clearance, the motor, while rotating, does not move, and these phenomena can be expressed as "dead zones".
  The range of the dead zones is a, and the minimum speed Vmin should meet Vmin greater than or equal to a, since a is less than or equal to dK, and d is the pulse equivalent (mm/pulse). K for open loop magnification, : if Vmin dK or take d = 0.01 mm/pulse, K = 30 x 1 / S, the minimum speed Vmin acuity 30 x 0.01 mm/min = a = 18 mm/min Max speed servo system should consider the choice of mechanical processing allows boundaries and the actual requirements of machine and high speed is can increase productivity, but also to drive requirement is higher. In addition, from the perspective of system control, there is a problem of detection and feedback, especially in computer control system, whether the software processing time is sufficient.
  Because fmax = fmax/d mode: fmax is the pulse frequency of the highest speed, kHz; Vmax is the highest feed speed, mm/min; D is the impulse equivalent, mm. D = vmax/vmin, D = vmax/vmin
  Fmax = Dvmin/d = DKd/d = DK is the minimum interval time tmin, i.e., tmin = 1 / DK. Obviously, the system must complete the location detection and control operation through hardware or software within tmin. For maximum speed, the value of vmax is constrained by tmin.
  A better servo system, the speed range D can be up to 800 ~ 1000. The most advanced level today is the continuous adjustable speed from 0 ~ 240m/min under the conditions of pulse equivalent d = 1 mu m.
  2. Hardware design of CNC machine tools
  2.1 motion control card
  Motion control card is A kind of power control unit, can control the servo motor, which is based on PC bus, using the high performance micro processor (DSP) and large-scale programmable devices to realize multiple multi-axis coordinated control of servo motor of A high performance stepping/servo motor movement control CARDS include pulse output, pulse counting, digital input, digital output, D/A output, and other functions, it can be A continuous, high frequency pulse sequence, by changing the pulse frequency to control motor speed, change the number of pulses to control the position of the motor, its pulse output mode including pulse, pulse/pulse way/direction.
  The pulse count can be used for the position feedback of the encoder, providing the correct position of the machine and correcting the errors generated during the transmission. Digital input/output points can be used for language limit, origin switch, etc. The product is widely used in the field of industrial automation control, which requires precise positioning, fixed-length position control system and PC - based NC control system. In particular, the underlying software and hardware that will realize the motion control are integrated together, so that it has the various speed and position control functions required for servo motor control. These functions can be easily invoked by computer.
  The motion control card not only sends pulse to the motor drive, but also receives the pulse number of the servo motor encoder, and also accepts the grating ruler feedback signal to control the speed of the servo motor. The servo driver is connected to the motor control card with the cable, and it also has to connect the socket power.
  If your motion control card is better, the servo refresh rate can meet the requirements, and the encoder feedback can be directly received by the motion control card to form an overall closed loop. If there is a high requirement for accuracy, double closed loop is used, and the motion control card is based on the position of the x-y platform, which controls the motor movement to the correct position.
  2.2 PC bus
  The existing open CNC system implementation scheme mainly by using the combined method of PC and CNC system, the PC as a PC to realize more complex network communication, interactive features, such as numerical control system as the machine PC input of running parameters processed to perform parts, at the same time detection system of feedback information to PC to achieve real-time monitoring, coordination mutual interference between modules, brought convenience to system upgrade.
  Open type dynamic controller is the core of DSP system, it has a fast calculation speed, to support the characteristics of complex movement algorithm can meet the requirements of high precision motion control, therefore, with DSP as the core of multi-axis dynamic control card is more and more widely used in motion control system, the dynamic control multi-axis card in the PC expansion slots, can form high precision motion control system, the position feedback signal collection, closed-loop control calculation and control the amount of output are performed by dynamic control card, greatly improves the operation speed and control response speed, industrial resources from solve troublesome in data acquisition and calculation, in order to better implement the management of the whole control system.
  2.3 drive
  The servo actuator is a controller used to control the servo motor, which ACTS like a frequency converter for ordinary ac motors. At present, the mainstream servo drives adopt the digital signal processor (DSP) as the control core, which can realize more complex control algorithms, and the items are digitized, networked and intelligent.
  Power devices are widely used in intelligent power module (IPM) as the core design of drive circuit, IPM internal integrated drive circuit, at the same time has the overvoltage, overcurrent, overheating, under-voltage protection circuit, such as fault detection in the primary loop to join the soft start circuit, to reduce the impact to drive during startup. The power - driven unit first passes through the three-phase full bridge rectifier circuit to the input three-phase power or the municipal electricity to the rectifier, and gets the corresponding direct current.
  The three phase permanent magnet synchronous ac servo motor is driven by three - phase sine PWM voltage inverter inverter. The whole process of power drive unit can be simply described as the process of ac-dc-ac. The main topological circuit of the rectifier unit (ac-dc) is a three-phase full-bridge uncontrolled rectifier circuit.
  The servo drive can be used in three kinds of control methods: location, speed and torque. It is mainly used in high-precision positioning system and is currently the high end of transmission technology. Encoder is a device that compiles signals or data into signals that can be used for communication, transmission, and storage.
  The actuator is a drive amplifier that amplifies the signals from the upper computer, such as motion control CARDS, to make the motor work. The MAC series motion control card is a bus-based motor motion control card.
  Adopt special control chip as the core device, input and output signals are photoelectric isolation, can be connected with various types of stepper motor drives, driving a stepper motor, a high precision position control system and speed regulating system.
  The PC can be controlled by the PC: PC machine is responsible for the management and other management of man-machine interface; The control card is responsible for all the details of movement control. Through the dynamic link library we provide, the user can quickly and quickly develop the motion control function that he needs.
  3. Design of servo control system
  Electromechanical integration of the structure of servo control system, the type is various, but from the perspective of automatic control theory analysis, the servo control system generally includes controller, the controlled object, perform link, the testing link, more links, etc. 5

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