Feed servo system for most popular CNC machine too

2022-07-30
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In order to improve the performance of CNC machine tools, high requirements are put forward for the feed servo system of CNC machine tools. Due to the different processing tasks completed by various CNC machine tools, the requirements for the feed servo system are also different, but can be summarized as the following four aspects

(1) high precision in order to ensure the machining quality of parts and improve efficiency, it is necessary to ensure the positioning accuracy and machining accuracy of CNC machine tools. Therefore, high positioning accuracy is required in position control, such as 5m, 1M, etc; In speed control, it is required to have high speed regulation accuracy and strong ability to resist load disturbance, that is, the static and dynamic speed drop should be as small as possible

(2) fast response in order to ensure the profile cutting shape accuracy and machined surface roughness, in addition to high positioning accuracy, the system is also required to have good fast response characteristics, that is, the response of tracking command signal is required to be fast, and the position tracking error (position tracking accuracy) is required to be small

(3) wide speed regulation range refers to the ratio of the maximum speed to the minimum speed that the motor can provide under rated load. For general CNC machine tools, the feed servo system is required to work normally at 0 ~ 24m/min

(4) low speed high torque according to the machining characteristics of machine tools, most of them carry out heavy cutting at low speed, which requires that the feed servo system has a large torque output at low speed

in order to meet the above four requirements, corresponding requirements are also put forward for the actuator of the feed servo system - servo motor. They are:

1) the motor can run smoothly within the whole speed range, and the torque fluctuation should be small. Especially at low speed, it should still have a stable speed without crawling

2) the motor shall have certain overload capacity to meet the requirements of low speed and large torque

3) in order to meet the requirements of rapid response, the motor must have small moment of inertia, large basic operating torque of locked rotor tension testing machine, and as small as possible electromechanical time constant and starting voltage

4) the motor shall be able to withstand frequent starting, braking and reversing

the machine tool feed servo system is generally composed of five parts: upper position control, speed control, servo motor, detection components and mechanical transmission mechanism. However, in practice, the control part is usually made up of two basic parts, loading and force measurement, together with the numerical control device, and does not include the mechanical transmission mechanism. Therefore, the conventional feed servo system only refers to three parts: speed control, servo motor and detection part. Moreover, the speed control section is referred to as a servo unit or a driver. According to the structural characteristics of servo system, it usually has four basic structural types: open-loop, closed-loop, semi closed-loop and hybrid closed-loop. The semi closed-loop structure is the most widely used in machine tools, because it has few nonlinear factors in the loop and is easy to set. It can easily improve the position control accuracy through compensation, and the electrical control part is relatively independent of the executive machinery, so the system has strong universality. Its structure block diagram is shown in figure 19-14

figure 19-14 semi closed loop servo system

the machine tool feed servo system has entered the AC servo system stage after going through two stages of open-loop stepping motor system and DC servo system. This is because the AC motor has the characteristics of simple structure, ruggedness and durability. With the miniaturization and high-performance of power electronic devices and the rapid development of computer technology, the problems of AC motor control that were difficult to realize technically and economically in the past have been solved, so that the AC servo system has achieved a dominant position

at present, in the medium and small power range, the AC motors of High-Performance AC servo systems mainly adopt asynchronous motors and permanent magnet synchronous motors. Generally speaking, asynchronous motors are mostly used in occasions with high power, low precision requirements and low investment costs; Permanent magnet synchronous motor (PMSM) is widely used in the field of high precision and small capacity. Therefore, permanent magnet synchronous motor is widely used in machine tool feed servo system

permanent magnet synchronous motor can be divided into two types according to its internal structure, working principle, driving current waveform and control mode: permanent magnet motor driven by rectangular wave current, namely brushless DC motor (BDCM) and permanent magnet motor driven by sine wave current (PMSM). Among them, BDCM has high power density and low system cost, but it has large torque ripple at low speed. At high speed, rectangular wave current is distorted and torque drops. Therefore, it is generally used at low speed and places with low performance requirements; PMSM is more often used in situations requiring higher speed or position servo. At present, the permanent magnet materials used in permanent magnet synchronous motors have developed from ferrite to samarium cobalt (SmCo) with high Curie point and neodymium iron boron (NdFeB) with high coercivity, high magnetic energy product and low relative price

AC servo unit can be divided into analog type and digital type. The early servo units were all analog, but at present, most foreign countries use digital analog hybrid or all digital, while China is still in the laboratory stage and has not yet achieved real commercialization

analog and digital servo units have their own advantages and disadvantages: analog servo units generally work very fast, and the frequency of the system can be very wide, which makes the system have fast dynamic response performance and wide speed regulation range. Its disadvantage is that it is difficult to realize complex control methods, and there are many devices, large volume, difficult to debug, and there are also some problems such as zero drift. The advantage of digital servo unit is that it is easy to realize complex algorithm by software programming, and it has good flexibility. Sometimes, the change between several control methods can be realized only by changing the software, without changing the hardware. The hardware circuit is generally simple and can be designed to be relatively compact. Since the setting and adjustment of parameters do not need to be carried out by adjusting the potentiometer, the repeatability is good and it is easier to produce in batch. However, due to the large amount of calculation of high-performance motor control algorithm, the execution speed of single-chip microprocessor is not fast enough, and the low-cost analog-to-digital (A/D) conversion time is long, the current loop response frequency band of practical full digital servo unit is generally difficult to be wide enough, that is, high-speed digital signal processor (DSP) or dual CPU structure is used to improve the calculation speed, The current loop sampling period of the system is about 100 s, which can not be compared with the analog system. Of course, with the development of microelectronics technology and the improvement of control algorithm, these shortcomings can be overcome. In the digital analog hybrid system between the two, the current loop (inner loop) is realized by hardware circuit, while the speed loop and position loop (outer loop) are realized by software. At the same time, the microprocessor can also be used to realize the functions of system operation monitoring, receiving digital/analog given signals and communicating with external equipment

figure 19-15 shows a simple and practical control scheme for digital brushless DC motor

the CPU in figure 19-15 receives the speed setting command, detects the motor speed, and completes the speed adjustment function. The generated current setting command is output through D/A; The DC bus current of the inverter is introduced by the Hall current sensor as the current feedback signal, and the pulse width modulation signal (PWM) is obtained through the current regulator; The inverter switch vector table formed according to the signals of steering, operation/blocking and magnetic pole position is stored in EPROM. By stroking the address line of EPROM, the corresponding switch vector is output to the data line. After being driven and amplified, six drive signals can be obtained to control the inverter. The inverter in the block diagram is a part of the electromechanical system that converts DC power into AC power: the excellent electronic universal experimental motor adopts the exchange servo speed regulation system. The most widely used is the pulse width modulation (PWM) inverter, which actually controls the on-off sequence and time distribution law of the inverter switching devices, and obtains a rectangular wave with equal amplitude and adjustable width at the output end of the inverter. According to its formation mode, it can be roughly divided into four categories: sinusoidal pulse width modulation (SPWM), adaptive current control PWM, phase-shift PWM and PWM with anti bending equipment under the measurement of harmonic suppression principle. Among them, SPWM is the most widely used one

the circuit of the above control scheme is very simple. Different from the control scheme of three-phase current regulation (three Hall current sensors and three current regulators are required), it only needs one current sensor and one current regulator to effectively realize the current closed-loop control. Not only the hardware implementation is very convenient, the cost is low, but also the switching times of the inverter can be greatly reduced

at present, most of the full digital permanent magnet synchronous motor control methods being researched and developed in China are based on space voltage vector control method. Figure 19-16 shows a block diagram of a full digital permanent magnet synchronous motor control system

figure 19-16 block diagram of full digital permanent magnet synchronous motor control system

the key of them is to select the voltage vector applied by the inverter according to the position of the motor stator voltage vector or the deviation value of the current vector. The calculation of the action time of the zero vector and the non-zero vector is the key of these control methods. Moreover, it is a contradiction for improving the performance of the control system to accurately calculate the action time of the voltage vector and shorten the sampling period of the current loop. To accurately calculate the action time of the vector requires complex calculation, and the sampling period of the current loop will increase with the increase of the calculation time; On the contrary, if the sampling period is shortened and the action time of the vector is roughly calculated, the system performance will also be degraded

the detection components used in the feed servo system of the machine tool mainly include rotary transformer and pulse encoder. Grating is also used for position detection in the full closed loop. Their performance directly affects the performance of the servo system

resolver is an induction micromotor whose output voltage is a continuous function of angular displacement. From the physical essence, the resolver is a kind of transformer that can rotate. It is composed of stator and rotor. The primary and secondary windings are placed on the stator and rotor respectively. The electromagnetic coupling degree between the primary and secondary windings is related to the rotation angle of the rotor. Therefore, when the primary winding is excited by a single-phase AC voltage, the amplitude of the output voltage of the secondary winding will be related to the rotor angle. There are many classification methods for rotating Transformers: if they are classified according to whether there is a brush or not, they can be divided into contact type and non-contact type; If divided according to the number of pole pairs, it can be divided into single pole pairs and multi pole pairs; According to the purpose, it can be divided into resolver for calculation and transformer for data transmission; According to the functional relationship between output voltage and rotor angle, it can be divided into sine cosine resolver, linear resolver, proportional resolver and special function resolver

pulse encoder, also known as photoelectric encoder, is a sensor that converts the mechanical geometric displacement on the output shaft into pulse or digital quantity through photoelectric conversion. This is the most widely used sensor in machine tools at present. According to the different ways it generates pulses, it can be divided into three types: incremental, absolute and hybrid. The incremental encoder directly uses the photoelectric conversion principle to output three groups of square wave pulses a, B and Z

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