The basic concept of servo is accurate, accurate and fast positioning. Frequency conversion is a necessary internal link of servo control. Frequency conversion also exists in servo driver (stepless speed regulation is needed). But servo controls both the current loop and the speed loop and the position loop, which is a big difference. In addition, the structure of the servo motor is different from that of the ordinary motor, which should meet the requirements of fast response and accurate positioning. Now on the market circulation of ac servo motor for permanent magnet synchronous ac servo, but this kind of motor restricted by technology, it's hard to do a lot of power, more than 10 KW synchronous servo and expensive, so in the case of field application allows much by ac asynchronous servo, then a lot of drive is high frequency converter, with encoder feedback closed loop control. The so-called servo is to meet the accurate, accurate, fast positioning, as long as the meet there is no servo frequency dispute.

一、Servo motor control principle and inverter control principle of the two have in common：

     Ac servo technology itself refers to and applies the technology of frequency conversion. Based on the servo control of DC motor, it is realized by imitating the control mode of DC motor through the PWM mode of frequency conversion. That is to say, ac servo motor must have this link of frequency conversion: Frequency is 50, 60 hz power frequency ac rectifier to dc first, and then through a control gate of the transistor (IGBT, IGCT, etc.) through the carrier frequency and PWM control inverter for adjustable frequency waveform is similar to the cosine of pulse power, because the frequency is adjustable, so the speed of ac motor is adjustable (n = 60 f/p, n speed, frequency, f p a logarithmic)   

二、Talk about inverter control principle：

    A simple frequency converter can only adjust the speed of ac motor, which can be open loop or closed loop depending on the control mode and frequency converter, which is the traditional V/F control mode. Now a lot of variable frequency has been through the establishment of mathematical model of ac motor stator magnetic field UVW3 phase into can control the motor speed and torque of the two current component, now most of the famous brand can control the torque converter is controlled by such way moment, UVW output of each phase with hall effect current detection device, after sampling feedback constitute a closed-loop feedback current loop PID regulation; ABB frequency conversion also proposes a direct torque control technology different from this one. Please refer to the relevant information for details. In this way, the speed of the motor can be controlled as well as the torque of the motor, and the control accuracy of the speed is better than that of v/ F control. The feedback of the encoder can also be added or not, and the control accuracy and response characteristics are much better when added.    

三、Talk about servo motor control principle：

    Drive: servo driver in the development of the premise of frequency conversion technology, inside the drive current loop, velocity loop and position loop (inverter without the ring) have been more accurate than general frequency conversion control technology and algorithm of computing, powerful in function than the traditional frequency many, the main point can be accurately position control. Through the upper controller sends a pulse sequence to control the speed and position (of course also some internal integration servo control unit directly or through the bus communication way to set the parameters such as position and speed) in the drive, drive internal algorithm and a faster and more accurate calculation and performance more superior electronics is superior in frequency converter.

    Aspects: motor servo motor material, structure and processing technology is far higher than the frequency inverter ac motor driven (general ac motor or constant torque and constant power and other kinds of variable frequency motor), that is to say, when the driver output current, voltage, the power of the frequency change quickly, the servo motor can vary according to the action of the power supply change response, response characteristic and the overload capacity is much higher than ac frequency converter drive motor, the motor of the serious differences and they have different performance. That is to say is not the frequency converter output so fast power signal, but the motor itself can not respond, so in the frequency conversion of the internal algorithm to protect the motor to do the corresponding overload setting. Of course, even if you do not set the output capacity of the inverter is still limited, some good performance of the inverter can directly drive the servo motor!!    

四、Talk about ac motor：

   Ac motors are generally divided into synchronous and asynchronous motors

    1、Ac synchronous motor: The rotor is composed of permanent magnet materials, so after rotation, the rotor also responds to frequency changes with the change of the stator rotating magnetic field. Moreover, the rotor speed is equal to the stator speed, so it is called 'synchronous'.。

    2、Ac induction motor: rotor is composed of induction coil and material. Cutting after rotation, the stator produce a rotating magnetic field, magnetic field induction coil of stator, rotor winding induced current, then the rotor magnetic field is induced, induced magnetic field to follow the change of the stator rotating magnetic field, but always less than the change of the stator, the rotor's magnetic field change once is no changing magnetic field cutting rotor induction coil, would be no induced current in the rotor winding, rotor magnetic field disappeared, rotor stall with the stator produce speed difference again regaining induced current... So a key parameter in an AC asynchronous motor is slip which is the ratio of the speed difference between the rotor and the stator。

    3、Corresponding to ac synchronous and asynchronous motor frequency converter, there are synchronous and asynchronous frequency converter, servo motor also has AC synchronous servo and AC asynchronous servo, of course, in the frequency converter, AC asynchronous frequency conversion is common, servo ac synchronous servo common。

五、Servo motor control principle and application in frequency converter control principle：

    Due to the performance and function differences between inverter and servo, the applications are not the same:    

    1、In the situation of speed control and torque control requirements are not very high in the general use of frequency converter, but also in the upper position of the feedback signal formed a closed loop with frequency conversion for position control, the accuracy and response are not high. Now some frequency conversion also accept the pulse train signal to control the speed, but it seems that can not directly control the position.

    2、Where strict position control requirements can only be used in servo, and the response speed of the servo is much greater than the frequency conversion, some requiring high precision and response speed servo control and can use the occasion of the movement of the frequency conversion control can almost all servo, the key is two things: one is the price of servo is far higher than the frequency, the second is the cause of the power: frequency conversion the biggest can do a few hundred KW, even higher, servo and dozens of KW.