1 The basic concept of servo system
1.1 Nouns
The word "servo" is derived from the Greek word "slave." People want to be a "servo" as a docile tool that can be manipulated to comply with the requirements of control signals. Before the signal comes, the rotor is stationary; when the signal comes, the rotor turns immediately; when the signal disappears, the rotor can stop on its own. Because of its "servo" performance, hence the name - servo system.
1.2 Definition
Servo system - is an automatic control system that makes the position, orientation, status, etc. output of the object follow the arbitrary change of the input target value (or given value).
The main task of the servo is to amplify, transform and control the power according to the requirements of the control command, so that the force distance, speed, and position output of the drive device can be controlled flexibly and conveniently.
1.3 The composition of the servo system
The servo system shown in Figure 1 is a closed-loop automatic control system with feedback. It consists of a position detection part, an error amplification part, an execution part, and a controlled object.
1.4 Performance Requirements of Servo System
The servo system must have basic performances such as good controllability, high stability and strong speed response. Explain that good controllability means that the signal can be stopped immediately after it disappears. High stability means that the rotational speed decreases uniformly with the increase of torque; the rapid response should be fast, sensitive, and good in sound quality.
1.5 Kinds of Servo Systems
Usually classified according to the type of servo driver, there are three types of electrical, hydraulic or electrical-hydraulic.
If the servo system is divided according to the function, there are measuring servo and power servo systems; analog servo and power servo systems; position servo and acceleration servo systems.
The electrical servo system can be divided into two major categories of DC DC servo system and AC AC servo system according to electrical signals. AC AC servo system has two kinds of asynchronous motor servo system and synchronous motor servo system.
Only one of the electrical servo systems—an AC permanent magnet synchronous motor servo system—is discussed here.
2 AC permanent magnet synchronous motor servo system
The servo drive system can faithfully follow the control commands, such as CNC machine tools and industrial robots. The servo drive technology has an important influence on the performance of the product and even plays a key role. Therefore, it is necessary to further understand the position and role of the servo drive system.
2.1 AC Servo System
Electric servo technology is the most widely used, the main reason is that the control is convenient, flexible, easy to obtain drive energy, no pollution, maintenance is also relatively easy. Especially with the development of electronic technology and computer software technology, it provides a broad prospect for the development of electrical servo technology.
As early as the 1970s, small inertia servo DC motors had been put into practical use. By the end of the 1970s, the AC servo system began to develop and gradually became practical. AC servo motors were used more and more widely, and there was a tendency to replace the DC servo system to become the mainstream of electrical servo systems.
In the AC servo system, it can be divided into two kinds of synchronous and asynchronous AC servo systems.
AC servo system - → asynchronous type - - → two-phase asynchronous machine;
→Three-phase asynchronous machine (power-distance motor).
→ Synchronous → Magnetoresistive (switching);
→ Hysteresis (reaction type);
→ Permanent magnet type.
Permanent-magnet rotor synchronous servo motor due to continuous improvement of permanent magnet materials, the price continues to decline, the control is simpler than the asynchronous motor, easy to achieve high-performance reasons, so the permanent magnet synchronous motor AC servo system is more widely used.
At present, in the AC synchronous servo drive system, there are two major categories of AC permanent magnet synchronous servo motors that are commonly used. One type is called a brushless DC motor, which requires a square wave current to be directly into the stator winding. See the reference literature for details; if a class is called a three-phase permanent-magnet synchronous motor, it requires that the power input to the stator winding be a three-phase sinusoidal waveform. See the reference literature (2) for details. The former is abbreviated as BLDCM motor and the latter is abbreviated PMSM motor.
2.2 Take CNC machine tool as an example to see AC permanent magnet synchronous servo system
Figure 2 shows the composition of a servo control system for a CNC machine tool.
The system consists of computer numerical control (CNC), servo drive (SD), permanent magnet synchronous servo motor (SM) and position (speed) sensor (S).
The CNC is used to store parts program, perform various interpolation operations and software real-time control, and send out various control commands to the servo drive system of each coordinate axis.
After the SD and SM receive the CNC's control command, they quickly and smoothly adjust the movement speed and accurately perform the position control.
S stands for position and velocity sensor (or detector)
At present, the position and speed detectors commonly used in AC servo systems are optical and electromagnetic. Examples are optical encoders, magnetic encoders, rotary transformers (BR) and multi-turn absolute encoders. The latter two types can be used for a variety of detection functions. They can not only detect the position of the system and the rotor speed, but also detect the position of the system and the rotor speed, and can also detect the rotor pole position. It is sturdy and durable, not afraid of vibration, high temperature resistance, but there are complex signal processing circuit shortcomings.
Rotor magnetic pole position detection methods in brushless DC motors (BL, DCM) are generally done without contact, commonly used electromagnetic, photoelectric and indirect detection methods.
(1) Electromagnetic type: a. Differential transformer type; b. Proximity switch type;
(2) Magnetic type: Hall element integrated circuit and module fast;
(3) Optical: a. simple photoelectric (phototransistor);
b. Absolute photoelectric encoder disk;
c. Incremental optical encoder disk.
(4) Indirect: Indirect detection of the rotor pole position using the induced electromotive force (voltage) of the armature winding. It is used in applications where the accuracy is not high.
CNC machine tools are used for precision machining, so they have high requirements for the dynamic and static accuracy of the servo system, and have a wide range of speed and positioning accuracy. While the servo system structure of industrial robots is similar, but the opportunity to take the motor SM as an industrial robot The actuators for arms and waist and legs require a small size, light weight, and large torque. Because of the different movement posture of the industrial robot, the inertia and torque of the servo motor shaft will change greatly. Therefore, the adaptability has higher requirements.
2.3 AC Permanent Magnet Synchronous Motor AC Servo System
Figure 3 is an AC permanent magnet synchronous motor AC servo system. It is composed of permanent magnet synchronous servo motor SM1 speed and position sensor BR, PWM power inverter UI, and PI controller with speed control ASR and current controller ACR.
The working principle of the AC servo system is as follows: The speed command and the speed feedback signal are compared at the input of the ASR speed controller, and the ASR output signal is the current command signal multiplied by the multiplier to obtain the AC current command. After the AC current command value is compared with the current feedback signal, the difference is sent to the ACR current controller.
PWM or SPWM (Sinusoidal Pulse Width Modulation) waveform generation circuit and power inverter output three-phase variable-voltage alternating-current AC power to the stator windings of the permanent magnet synchronous motor SM, and maintain a good sinusoidal ac current in the input armature windings Sex. (The SM rotor is equipped with special shaped permanent magnets that generate a constant magnetic field, thereby increasing the SM's efficiency).
AC three-phase permanent magnet synchronous motor servo system example:
The brushless DC motor BLDCM can be used as an AC servo system, but it is not its only application and is mainly used as a large-scale equipment starting and speed regulating device. The same PM, SM can not only be used as an AC servo system, mainly for his control type permanent magnet synchronous motor variable frequency speed control system. However, three-phase SPWM, PM, and SM AC servo systems have superior low-speed servo performance, and are therefore widely used in CNC machines, industrial robots, and other high-performance servo systems.
Now take the AC servo system product of a company as an example.
Fig. 4 is a schematic diagram of an AC servo system controlled by SPWM three-phase PM and SM vectors.
In the picture, BR-resolver, FWC-weakening control link
ASR-Speed ​​Regulator, Π-Multiplier,
ACR-Current Regulators, Σ-Adders,
Resolvers are used in this system to detect pole position and rotor speed. The angular frequency requirement of the excitation signal of the resolver is much greater than that of the PM and SM motor rotors, and the frequency must be very stable. The phases of the sine and cosine excitation signals should be strictly orthogonal to improve the detection accuracy and reliability. Therefore, 3.795 is used in the system. MHz crystal oscillator.
The system also has a rotor pole position signal demodulation link and a rotor speed signal demodulation link.
The servo system controls the conversion of the DC measure q (torque current) d (demagnetization current) in the stationary coordinate system into the two-phase AC measure α and β in the rotating coordinate system, and then converts it to 3 through 2/3. The phase current works A, B, C, and the negative current of the reusing current constitute a current closed loop, so that the three-phase currents in the stator windings of the PM and SM motors are consistent with the commands.
3Siemens synchronous servo motor 1Fk6/1FT6 series
(Siemens SYNCHRONOUS SERVOMOTORS)
3.1 Siemens servo motor
Siemens servo motors are available in synchronous servo motors 1FK6/1FT6 and asynchronous servo motors IPA6/IPL6. Only the synchronous servo motor 1FK6/1FT6 series will be discussed here.
(1) The 1FK6 servo motor is a standard servo motor and can not be coupled with a permanent magnet synchronous motor. Natural cooling, protection class IP64. Power 0.5～5.2KW Torque 0.8～16.5NM
(2) 1FT6 servo motor is a permanent magnet synchronous motor. Protection class IP64
Natural cooling 0.5 ~ 15.5KW, 0.8 ~ 88NM.
Forced ventilation 6.9 to 34.6 kW, 17 to 160 NM.
Water cooling 11 ~ 27.6kW, 17 ~ 78NM.
For details, see Siemens product catalog data, DA65.3, 1998 (P.2/1~2/8)
3.2 1Fk6/1FT6 Synchronous Servo Motor Features:
1FK6/1FT6 is a three-phase rare earth permanent magnet synchronous motor.
(1) High static torque, large overload capacity, can be quickly accelerated.
Mmcx=1.6 to 3.0Me (rated torque).
(2) Excellent dynamic response quality.
Short rise time, no overshoot at expected position
(3) with very precise position resolution
In general, 1FK6 is used for low power range (≤5.2kW)
1FT6 for larger power range (≤ 27.6 kW)
3.3 Siemens permanent magnet synchronous servo motor power selection
Select motor power based on torque
Torque root mean square
Speed ​​average
In the formula: T-cycle, A-start, E-end, I-some instant,
Servo motor selection principle:
(1) All Mrms on the load curve must be below the motor torque limit curve S1;
(2) All nmean on the load curve must be less than the rated speed of the motor.
3.4 How does the inverter capacity match the synchronous motor?
(1) Multiple types of tuners can be selected
1Siemens MASTER DRIVES VC-IFX6
2Siemens MASTER DRIVES MC—IFK6/IFT6
3Siemens SIMO DRIVES-1FT6
(2) Selection principle of frequency converter capacity:
For synchronous servo motors 1FK6/1FT6.
Synchronous servo motor average current calculation formula:
Where: kTn—motor torque constant
Inverter capacity (rated current) selection formula: I0 ≤ Ivn
In the formula: I0—the current when the motor turns to stop,
Ivn—The rated current of the frequency converter.
In fact, the Siemens formula has a table providing what inverters the 1FK6/IET6 motor should match. No specific calculation is required.
See Siemens technical data DA65.11 1999 (p4/4→4/13) for details.
4 Comparison of three-phase sinusoidal permanent magnet synchronous servo systems (SPWM, PM, SM) and brushless DC motors (BLDCM)
Three-phase permanent magnet synchronous motor (PM, SM) is the key link in AC servo system. There are two main categories:
(1) Brushless DC motors (BLDCM), in which permanent magnetic rotors are used in place of the stator magnetic poles of brushed DC motors, and the armature of the original DC motor is changed into a stator. Brushed DC motors rely on mechanical commutators to convert DC currents into approximately trapezoidal AC currents to the armature windings, while BLDCMs directly input square wave currents (actually trapezoidal) to the stator. Turning the stator and rotor of a brushed DC motor upside down and using a permanent magnet rotor eliminates mechanical commutators and brushes, hence the name Brushless DC Motor.
See Figure 5 for a block diagram of the BLDCM system.
See Figure 6 for a block diagram of the AC servo motor control system.
The above two types of permanent magnet synchronous servo motors can also be distinguished by inducing electromotive force waveforms from the stator windings of the permanent magnet rotor magnetic field. Each phase of BLDCM stator induces a trapezoidal electromotive force. In order to generate a constant electromagnetic torque, the power inverter is required to input a three-phase symmetrical square wave current to the BLDCM stator, and each phase of the SPWM, PM, and SM stators is approximately a sine wave. Need to input three-phase symmetrical sine wave current to SPWM, PM, SM stator.
Figure 7 shows the comparative waveforms of PM, SM, and BLDCM motors.
In the figure, Bm—magnet flux density of permanent magnets,
Ea—Electromagnetic induction of each phase of the stator,
Ia - stator current per phase,
Pa, Pb, PC - stator per phase power,
P—total power.
In summary, the differences of the two types of permanent magnet AC synchronous servo motors are summed up as follows: Table 1
Comparing Fig. 5 and Fig. 6, the control principle is similar. The given command signal is added to the input of the AC servo system. The position feedback signal on the motor shaft is compared with the given position, and the movement of the servo is controlled according to the comparison result until the desired value is achieved. Location. The basic idea of ​​PM, SM and BLDCM servo systems is the same. However, the PM and SM servo systems require three-phase sinusoidal current input from the stator to achieve better smoothness and superior low-speed servo performance. Therefore, it is widely used in high performance servo drive systems such as CNC machine tools and industrial robots.
5 Conclusion (the development trend of AC servo system)
(1) Servo drive technology The DC servo system rapidly transforms into an AC servo system. It is foreseen that the AC servo system will completely replace the DC servo system in the near future.
(2) AC servo system develops in two directions:
One direction is a simple, low-cost AC servo system, such as simple CNC machine tools, office automation equipment, computer peripherals for home appliances, and industrial motion control with low performance requirements. It is a large-scale, non-negligible application area. It will rapidly develop and expand.
Another aspect is a higher-performance, all-digital intelligent software servo system to meet the needs of high-precision, numerically-controlled machine tools, robots and special processing equipment for fine feed. It will represent the development level and leading direction of the AC servo system, and will also become the mainstream of AC servo system development.