Application analysis of the hottest AC servo syste

Application analysis of AC servo system

1 Preface

at present, the AC permanent magnet servo drive system based on rare earth permanent magnets can provide the highest level of dynamic response and torque density. Therefore, the development trend of the drive system is to replace the traditional hydraulic, DC, stepping and AC variable-frequency drive with AC servo drive, so as to make the system performance reach a new level, including shorter cycle, higher productivity, better reliability and longer service life. In order to better improve the system performance with new motors, we must have a deep understanding of the characteristics of this technology. In fact, if we simply replace the traditional motor with new drive technology without new design, it will produce some unexpected problems, and sometimes even reduce the characteristics of the machine

2. Drive and mechanical connection selection

the success of all drive applications depends on the careful selection of all system parameters. Therefore, it is necessary to have a good understanding of the performance indicators of modern AC servo drive system (some performance parameters are very high, but it is difficult to fully understand them). In fact, the AC servo drive system does not simply depend on the motor, but a complete and complex control system, which leads to greater freedom in design, and more parameters need to be selected than the traditional drive device

from a conceptual point of view, a High-Performance AC permanent magnet servo motor drive system is similar to a standard magnetic induction motor, which adds a group of loudspeaker power amplifiers. Thus, the motor has a very short response time and limited inertia, so the motor can adapt to various control signals to the greatest extent. Like speakers, the final control quality depends more on the selected system parameters and driving conditions than on the motor itself

in the face of design selection, the system designer should not only consider various parameters of machinery, electronics and electricity, but also consider their mutual influence

in general, all systems require the following two basic choices:

mechanical aspects: the choice of mechanical linkage, the choice of transmission ratio, the choice of motion conversion mode, and the choice of coupling and clutch

Electronics: selection of feedback mode, sensor type and quantity, sensor installation mode, amplifier type, synchronization and control bus, etc

the following contents can help designers choose the functions of application features

3. Servo drive: operating rules, performance characteristics and limitations

all AC permanent magnet servo systems include: electric drive, servo motor and at least one feedback sensor. All these components operate in a closed-loop control system: the driver receives parameter information from the outside, then transmits the current to the motor, converts it into torque through the motor, and then drives the load. The load acts or accelerates according to its own characteristics. The sensor measures the position of the load, so that the driver compares the parameter information value with the actual position value, Then, by changing the motor current, the actual position value is consistent with the parameter information value

for example, if a constant speed is required, the driving device will continuously increase the motor current until the actual speed of the motor is consistent with the required speed. If the load suddenly increases, the speed will be reduced. The sensor can detect this speed change through the tensile test. The driver can meet the increase of load by increasing the motor torque and return to the set speed again. Through this example, we can draw the following conclusions:

speed accuracy is almost independent of load and motor, but only depends on the quality of sensor signal and the speed and control algorithm of driver

the time lag between load fluctuation and speed correction depends entirely on the speed, the resolution of the sensor and the parameter settings of the electric drive

modern AC permanent magnet servo drive system can respond to sensor signals in milliseconds or less time lag due to its very high closed-loop characteristics

however, at this point, the transmission time through the mechanical coupling usually becomes the main limitation of the dynamic response effect of the system

for example, suppose there is a system that uses a servo motor to drive a load with constant speed and large inertia through a synchronous toothed belt. The toothed belt is effective, fixed length and elastic. Imagine that in order to obtain the millisecond speed correction ability, we can draw the following conclusions:

1 Once the driver sends current to the motor, the motor must produce torque immediately

2. At first, the toothed belt will deform and stretch, and the load will not accelerate as fast as the motor

3. Thus, the motor reaches the set speed ahead of the load, and the sensor installed on the motor weakens the current, and then finally weakens the torque

4. With the continuous increase of the tension of the toothed belt, the speed of the motor slows down, forcing the driver to increase the current again, and a new cycle begins

in this example, the system is oscillating, the motor torque is pulsating, and the load speed is also pulsating. The result is noise, overheating and wear, none of which is due to the motor. However, shallow users will think that the motor is the source of noise. In fact, if the motor is replaced with an old-fashioned large base and large inertia motor, this problem may disappear, which gives people an illusion that the new drive system is not very effective

this simple understanding is wrong. In fact, analyzing the above example:

this instability is caused by the mismatch between the system reaction speed (high) and the mechanical transmission or reaction time (too long). That is, the motor reaction is faster than the time required for the system to adjust the new torque

the feasible solution is:

1 Or, reduce the reaction time of the mechanical system -- by enhancing the rigidity of the coupling and reducing the inertia of the system; Such as direct drive or replacing the toothed belt with a gearbox. Or, reduce the speed of the control system by giving up some control bandwidth; And this needs to be achieved with new technology

2. Of course, some qualities should be sacrificed, such as reducing the ability to respond quickly to sudden load fluctuations. In fact, the old driving devices are very slow. It uses the inertia of large motors to compensate for the lack of speed. On the other hand, because the inertia of AC servo motor is very small, a good control bandwidth is needed to ensure good rotation accuracy

all these can explain why the AC servo motor drive system has nothing to do with poor mechanical accuracy, such as reverse clearance, keyway and other factors. For this reason, the best motors are made into circular optical shafts without keyways and connected with tight fits with tapers. Its output shaft and flange need to be precision machined to eliminate flexible connectors. If flexible connector is necessary, it must have torsional rigidity, such as metal bellows type

conclusion: because the inertia and response time of the traditional drive system (permanent magnet DC motor, AC variable frequency motor) limit its use performance, the new high-level AC servo drive system with better application performance overcomes many mechanical limitations in traditional applications. Therefore, the design verification or system upgrading of mechanical systems today is more important than ever before

from the above simple example, we can also draw the following criteria:

speed accuracy only depends on the sensor, and has nothing to do with the motor

the following speed and the ability to compensate for sudden load fluctuations depend entirely on the rigidity and quality of mechanical connectors

in poor or modified application systems, the noise often heard is not from the motor or driver, but from the "original" mechanical connector. In fact, the noise is caused by the motor "catching" the correct torque. In this case, the motor may produce overheating independent of the load

in the same system, the old motor may work normally, because the inertia of the large base motor "masks" all its shortcomings

the analysis of the dynamic requirements of the application system is the basis for the selection of motors

to achieve this goal, this broad concept can be divided into two factors:

large signal bandwidth: This is the basis for generating sufficient torque and speed, which can force the load to reach the ideal running track in a very short time. It all depends on the motor, load torque and system inertia, and all parts of the system must be studied as infinite rigid parts

small signal bandwidth or control bandwidth, whose value is related to the reciprocal of stability time. Generally, it must be lower than any mechanical resonance frequency in the system, and its inverse value is the stability time of the control loop (such as the time required to reach the target position at the end of the motion command on the premise of meeting the required accuracy). Typically, it is impossible to make the setting time reach times of the lag time required for oscillation or resonance on all loads and connectors

for example, suppose there is a indexing shaft of a high-speed punch, and its rated speed rate is set at 10 times/second, that is, the workpiece position changes 10 times per second. If the resonance frequency of the whole connecting chain (shaft, reducer, transmission belt, ball screw, etc.) is 50Hz, the system stability time is about Ms, and only 40ms is left to move and punch. Due to the need for very high torque and acceleration performance, this application is almost impossible. However, if the rigidity of the transmission chain is enhanced (such as replacing the transmission belt with a filament rod), the resonance frequency of the transmission chain can be increased to 100Hz, the stability time can be reduced to MS, the movement time can be doubled, and the required torque can be halved, so there is no problem in application

4. Optimal drive design: transmission ratio, conversion mode, coupling

like all other motors, the size of AC servo motor is determined by the output torque rather than the output power. Therefore, in all applications, low motor speed will produce low rated power and relatively low efficiency. On the other hand, the AC servo motor has no minimum speed limit (its speed is only determined by the sensor used, for example, in some applications, its shaft speed is 1 revolution per year). Therefore, if someone proposes to use high-speed gears, it can only reduce the weight of the motor (such as electric traction) or improve efficiency. From the perspective of cost or dynamic performance, this scheme is not advocated. Wherever the motor directly acts on the load, the control bandwidth is maximized, because it has reached the maximized transmission rigidity. Therefore, these applications can provide the best position control and the following accuracy with the shortest stabilization time

before choosing the appropriate drive mode for a specific system, it is necessary to understand the available mechanical transmission modes. The most commonly used transmission modes are as follows:

conversion from rotation to rotation:

toothed belt

reducer with spiral wheel and parallel shaft

cycloid and epicycloid rotary reducer

harmonic drive

tangent lead screw reducer or Gleason gear

rotation to linear motion conversion:

toothed belt

gear rack

metal strip

ball screw

for any transmission system, the load parameters can be converted into the parameters of the motor shaft in the following way. If n= transmission ratio (ratio of motor to load speed, if from straight