There are many formulations of the concept of servo control, but the formulation of the concept is not important. In order to achieve a certain goal, it is one of the most important activities for us human beings to generate motion and control the motion of objects. The so-called servo control refers to the effective control of the object movement, that is, the control of the speed, position and acceleration of the object movement. This kind of control has become ubiquitous and more and more common.
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New technology in servo control
1, servo motor (as shown)
There are two technologies of servo motor that deserve attention. One is high-density motor, which adopts a design idea called "big pole motor". For example, the stator of a six-pole nine-slot motor is composed of nine independent poles, each of which is wound with a concentrated coil, and then the nine poles are assembled together to form a nine-slot motor core. Because each magnetic pole is formed by independent winding, the slot full rate can be kept above 90% even if automatic winding is adopted. This kind of motor is called high-density motor because of its good manufacturing process and minimal space utilization and volume. In terms of operation principle, this kind of motor does not belong to rotating magnetic field motor, but works under three-phase pulsating magnetic field, so its applicability, design method and operation mode have certain particularity. For example, this kind of motor is not suitable for square wave current drive. So is the vision of the automobile industry. At present, An Chuan, Panasonic, Fuji, Kobe, Delta and other low-power servo motor products all adopt high-density motor design scheme, of course, we are no exception.
The other is embedded magnetic steel speed servo motor, which can make use of the characteristics of AC and DC axis inductance changing with position caused by salient pole effect to form a real sensorless speed servo electric drive system.
2. Sensor
In addition to various photoelectric encoders, magnetic encoders are also worthy of attention. The volume and weight of magnetic encoder are dozens of times smaller than photoelectric encoder, and the temperature range is wider, and it is almost not afraid of shock and vibration. Its working principle is very simple. The stator is a chip embedded with Hall magnetic sensor and DSP, and its volume can be as small as MSOP-24 package. The rotor is a bipolar magnetic steel. The resolution is 10- 12 bits and the accuracy is 8- 10 bits. This kind of magnetic encoder is now on the market.
As a space application, in order to meet the environmental requirements of -350C-800C, it is almost difficult to adopt the traditional photoelectric encoder. Therefore, we developed a magnetic encoder with a resolution of 16 bit and an accuracy of 12 bit. The signal processing circuit of the magnetic encoder is stored in the drive control circuit (FPGA), forming the integration of the sensor and the drive control circuit.
Current sensor is very important for servo control. Resistance sampling can be used in low power systems, and Hall current sensors can be used in general occasions. Both methods need to convert analog signals into digital signals and then participate in digital servo control. The output form of the above A/D conversion is usually serial digital pulse or pulse width modulation signal. IR2 175, a special chip for current feedback, was designed for resistance sampling by American IR company. With 12 bit resolution, 600V primary and secondary withstand voltage, it is very convenient to use. In order to improve the withstand voltage level and effective resolution, we developed a minimal current sensor based on Hall magnetic balance principle. Its resolution is 12 bit, withstand voltage is 2500V, and the frequency of pulse width modulation signal is 168KHz.
3. Servo parts
In 2003, the American IR company introduced a single-chip rate servo control system, including: motor vector FOC controller, current PI regulator, speed PI regulator, SVPWM modulator, sensor interface, SPI and parallel communication interface. The biggest feature of the single-chip rate servo control system introduced by IR company is that it allows users to give hundreds of parameters in real time and initialize them. The rate servo control system shown in the figure below is an application system built with the chip of IR company.
We have accumulated in servo control system for decades. Through cooperation with American IR company and American AD company, we have developed a high-precision motion control system scheme with independent intellectual property rights.
This technology realizes all servo control modules, such as FOC controller, current PI regulator, speed PI regulator, position PID regulator, speed feedforward controller, IIR filter, SVPWM modulator, trapezoidal speed trajectory generator, position instruction processor, monitoring and protection link, communication module, register file and so on. And the CPU is integrated internally, which can complete keyboard, display and external communication control. This is a real digital programmable system on chip (SOPC).
Because all the control algorithms are implemented in hardware, the servo controller can achieve quite high performance, the sampling frequency of its current loop and speed loop can reach 20kHz, and the sampling frequency of its position loop can reach more than 10kHz. The frequency index is mainly limited by the performance of the chip itself. All internal registers can be accessed through the upper computer, which can achieve various control purposes. All parameters can be modified online, including switching frequency, dead time, regulator parameters, filter parameters, etc. It is suitable for driving control of different motors such as PMSM, IM and BLDCM, and is compatible with various sensor interface signals such as Hall sensor, incremental/absolute encoder, magnetic encoder and resolver. It can receive pulse instructions, analog instructions, digital instructions and other input signals, and can complete all operating functions through the upper computer or control panel. With controller identification code interface, multi-axis control can be easily realized.
This single-chip controller greatly reduces the volume of the system, improves the anti-interference ability, and with perfect protection measures, ensures the reliability of the system operation.
4. Servo components
Servo assembly refers to an integrated servo mechanism consisting of servo motor, mechanical deceleration or coupling mechanism, servo controller and sensor. For example: the spindle drive module of CD-ROM, the joints of robots, and the electric power-assisted mechanism of automobiles. The basic requirements for the module are: small size, light weight (that is, high density), system integration and independence, interchangeability, reusability and high reproducibility. Servo components are our important research direction. Three-axis and four-axis components are distinctive, and these multi-axis servo controllers can usually be realized by an FPGA motion control IP core. In addition, electromagnetic compatibility, thermal analysis and design in servo components are also very important.
5, micro CNC machining center
CNC machining center is a great integration of servo control technology. The small CNC machining center system consists of five parts: lightweight machine tool body, high-density AC servo motor, high-precision encoder, servo unit and CNC system based on DSP+PC. Its remarkable feature is its lightweight appearance. Besides the automatic virtual tool library, it also has the functions and performance indexes of the conventional NC machining center. Compared with the conventional CNC machining center, multi-axis machining system has higher operating efficiency and lower use cost, and is competitive in many aspects such as operation convenience, product price and functional reorganization. It is a popular product that can meet the general needs of computer numerical control, automatic operation and high-precision machining, and is a powerful assistant for production, teaching, personal creation and labor. CNC machining center is a model of servo control technology application.
Small CNC machining center XYZ motion axis resolution is 0.00 1 (mm), repetition accuracy is 0.0 1 (mm), spindle speed (rate control) is 24000rpm, and the number of linkage axes is 4 or 5. It supports DNC, AUTO, EDIT and other working modes, G code and Mastercam, AutoCAD, Pro/E and other processing.