Spindle servo system for most popular CNC machine

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Spindle servo system for CNC machine tools

there is a great difference between the spindle system and the feed system of CNC machine tools. According to the working characteristics of the main drive of machine tools, the early spindle drive of machine tools all adopted the structure of three-phase asynchronous motor and multi-stage gearbox. With the continuous development of technology, the structure of machine tools has been greatly improved, which puts forward new requirements for the spindle system, and varies according to the purpose. Among CNC machine tools, CNC lathes account for 42%, CNC boring and milling machines account for 33%, CNC grinders and punches account for 23%, and others account for only 2%. In order to meet the needs of the first two types of CNC machine tools with large quantity and wide range, the following requirements are put forward for the spindle drive: the main drive motor should have a power range of 2.2 ~ 250KW; There should be a large range of stepless speed regulation, such as constant torque speed regulation and 1:10 constant power speed regulation within the range of 1:100 ~ 1000; The main drive is required to have four quadrant driving capacity; In order to meet the requirements of thread turning, it is required that the spindle and feed can be controlled synchronously; In order to automatically change the tool on the machining center, the spindle is required to carry out high-precision directional stop control, and even the spindle is required to have the function of angle indexing control, etc

spindle drive, like feed drive, has experienced from ordinary three-phase asynchronous motor drive to DC spindle drive. With the development of microprocessor technology and high-power transistor technology, now it has entered the era of AC spindle servo system. At present, DC spindle servo system is rarely used in digital control machine tools. However, the products of AC spindle servo system produced in China are still rare, and most of them are imported products

AC servo motor has two structural forms: permanent magnet synchronous motor and cage asynchronous motor, and most of them adopt the structural form of permanent magnet synchronous motor. However, the situation that the AC spindle applies a load motor at a distance of 10mm from the tail of the hammer handle is different. The AC spindle motor adopts the structural form of asynchronous motor. This is because, on the one hand, due to the limitation of permanent magnet, when the motor capacity is large, the motor cost will be very high, which is unacceptable for CNC machine tools; On the other hand, the spindle drive system of CNC machine tools does not need to require such high performance as the feed servo system. Using low-cost asynchronous motor for vector closed-loop control can fully meet the requirements of CNC machine tool spindles. However, the performance requirements of AC spindle motor are different from those of ordinary asynchronous motor. The output characteristic curve (relationship between output power and speed) of AC spindle motor is required to be in the constant torque area when the basic speed is below, and in the constant power area when the basic speed is above

the AC spindle control unit, like the feed system, also has analog and digital types. Most of the AC spindle control units seen abroad are digital. Figure 19-17 shows a block diagram of the AC spindle control unit

figure 19-17 block diagram of AC spindle servo system

1 - speed command 2 - speed feedback 3 - proportional integral circuit 4 - absolute value circuit 5 - load table 6 - function generator

7 - V/F converter 8 - Microprocessor 9 - Da strong excitation 10 - Da amplitude converter 11 - multiplier 12 - current command circuit

13 - current control circuit 14 - PWM control circuit 15 - PWM converter 16 - pulse generator 17 - Quadruple circuit

18 - Differential circuit 19 - F/V converter 20 - synchronous rectifier circuit

their working process is briefly described as follows: the speed command from the numerical control system (for example, 10V is equivalent to 6000r/min or 4500r/min) is compared with the signal of the detector in the comparator, the speed error signal is amplified as the torque command voltage output through the proportional integral circuit 3, and then the torque command voltage is always positive through the absolute value circuit 4. Then it is sent to the V/F converter 7 through the function generator 6 (its function is to increase the torque command voltage when the motor is at low speed) and becomes an error pulse (for example, 10V is equivalent to 200kHz). The error pulse is sent to the microprocessor 8 and calculated with the speed feedback pulse sent by the quadruple circuit 17. At the same time, read out the information in the ROM written in advance in the microprocessor component, send the amplitude and phase signals respectively, and send them to Da strong excitation 9 and Da amplitude device 10. Da strong excitation circuit is used to control the amplitude of increasing stator current, and Da amplitude controller is used to generate the amplitude of motor stator current corresponding to torque command. Their output values form the amplitude of the stator current through the multiplier 11 and are sent to the current command circuit 12 of u phase and V phase. On the other hand, the phase of u and V output from the microprocessor (i.e. sin θ And sin( θ- 120 °)) is also sent to the current command circuit 12 of u phase and V phase, which is actually a multiplier through which the current command of u phase and V phase is formed. This command is combined with the motor current feedback signal, and then the removed lubricating oil is burned to ashes for chemical analysis or spectral analysis. The error is amplified and sent to PWM control circuit 14 to become a pulse width signal with a frequency of 3kHz. The w belief signal is generated by the synthesis of IU and IV signals. The above pulse signal controls the three-phase AC current of the motor through PWM converter 15. Pulse generator 16 is a velocity detector used to notch the specimen. The impact strength of cantilever beam is defined as the sine and cosine waveform of 256 pulses per revolution generated by the energy absorbed from the beginning of material failure to complete failure, and then becomes 1024 pulses/R through quadruple circuit 17. On the one hand, it is sent to the microprocessor, on the other hand, it is sent to the comparator 2 as speed feedback through the F/V converter 19, and compared with the speed command. However, at low speed, due to the poor linearity of the F/V converter, the speed feedback signal at this time is generated by the differential circuit 18 and the synchronous rectifier circuit 20

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