Analog, hybrid, digital. The analog and hybrid input parts are analog inputs, the difference is that the input of the hybrid servo system enters the analog regulator after the digital deflector. These three ways of servo system have position feedback and speed feedback.
The present servo drive technology is an important part of numerical control technology. In combination with the CNC device, the static and dynamic characteristics of the servo system directly affect the displacement speed, positioning accuracy and machining accuracy of the machine tool. Now, the DC servo system is replaced by the AC digital servo system; The position, speed and current loop of the servo motor are digitized; A new control theory is adopted to realize a high speed response system which is not affected by mechanical load changes.
Its main new developments in technology are:
a. Feedforward control technology. In the past servo system, the difference between the detector signal and the position instruction is multiplied by the position loop gain as the speed instruction. This control method always has a tracking lag error, which makes the machining accuracy deteriorate when machining corners and arcs. The so-called feedforward control is to add the speed command control mode on the original control system, so that the tracking lag error of the servo system is greatly reduced.
b. Nonlinear control technology of mechanical static friction. For some CNC machine tools with large static friction, the new digital servo system has the nonlinear control function to compensate the static friction of the machine tool drive system.
c. The position ring and speed ring (including the current ring) of the servo system are controlled by software, such as digital mediation and vector control. In order to adapt to different types of machine tools, different precision and different speeds, pre-adjust the acceleration and deceleration performance.
d. Adopt high resolution position detection device. For example, the high-resolution pulse encoder has a subdivision circuit composed of a microprocessor, which greatly improves the resolution, and the incremental position detection is more than 10000 p\/r (pulse number\/revolution); Absolute position detection is above 1000000 p\/r.
e. Compensation technology has been developed and applied. Modern CNC systems have the compensation function, can be a variety of compensation for the servo system, such as lead screw pitch error compensation, tooth side clearance compensation, axial motion error compensation, space error compensation and thermal deformation compensation.
In addition, servo motor and stepper motor are used in numerical control systems, here is to introduce the difference between the two:
Stepper motor is a discrete motion device, which is intrinsically related to modern digital control technology. At present, in the domestic digital control system, the stepper motor is widely used. With the emergence of full digital AC servo system, AC servo motor is more and more used in digital control system, especially in the motion control system most of the stepper motor or full digital AC servo motor as the execution motor. Although the two are similar in the control mode (pulse train and direction signal), there are big differences in the performance and application:
1. The control accuracy is different: the step Angle of the two-phase hybrid stepping motor is generally 3.6°, 1.8°, and the step Angle of the five-phase hybrid stepping motor is generally 0.72°, 0.36°. There are also some high-performance stepper motors with smaller step angles. For example, a stepper motor produced by Sitong Company for slow wire machine tools has a step Angle of 0.09°; The step Angle of the three-phase hybrid stepper motor produced by BERGER LAHR can be set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, 0.036° through the dip switch, which is compatible with the step Angle of the two-phase and five-phase hybrid stepper motor. The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft, taking the Panasonic full digital AC servo motor as an example, for the motor with a standard 2500 line encoder, due to the quadrupled frequency technology used inside the drive, its pulse equivalent is 360°\/10000=0.036°. For a motor with a 17-bit encoder, the driver receives 217=131072 pulse motor for one revolution, that is, its pulse equivalent is 360°\/131072=9.89 seconds. It is 1\/655 of the pulse equivalent of a stepper motor with a step Angle of 1.8°.
2. The low-frequency characteristics are different: the stepper motor is prone to low-frequency vibration at low speed, and the vibration frequency is related to the load and driver performance, and it is generally believed that the vibration frequency is half of the no-load take-off frequency of the motor. This low-frequency vibration phenomenon determined by the working principle of the stepper motor is very unfavorable to the normal operation of the machine. When the stepper motor works at a low speed, damping technology should generally be used to overcome low-frequency vibration, such as adding a damper to the motor, or using subdivision technology on the driver. The AC servo motor runs very smoothly and does not vibrate even at low speeds. The AC servo system has resonance suppression function, which can cover the lack of rigidity of the machine, and the frequency resolution function (FFT) in the system can detect the resonance point of the machine, which is easy to adjust the system.
3. The torque frequency characteristics are different: the output torque of the stepper motor decreases with the increase of the speed, and it will drop sharply at a higher speed, so its maximum working speed is generally 300 ~ 600RPM. Ac servo motor is a constant torque output, that is, within its rated speed (generally 2000RPM or 3000RPM), it can output rated torque, and above the rated speed is a constant power output.
4. The overload capacity is different: the stepper motor generally does not have the overload capacity, and the AC servo motor has a strong overload capacity. Take Panasonic AC servo system as an example, it has speed overload and torque overload capability. Its maximum torque is three times the rated torque and can be used to overcome the inertial torque of the inertial load at the moment of start. Because the stepper motor does not have this overload capacity, in order to overcome this inertia moment in the selection, it is often necessary to choose a motor with a larger torque, and the machine does not need so much torque during normal operation, and the phenomenon of torque waste appears.
5. The operation performance is different: the stepper motor is controlled by open loop control, the starting frequency is too high or the load is too large, it is easy to lose step or stop the phenomenon of overshoot when the speed is too high, so in order to ensure its control accuracy, it should deal with the problem of up and down speed. Ac servo drive system is closed-loop control, the driver can directly sample the feedback signal of the motor encoder, and the internal position ring and speed ring are formed, generally there will be no step loss or overkill phenomenon of the stepper motor, and the control performance is more reliable.
6. The speed response performance is different: it takes 200 to 400 milliseconds for the stepper motor to accelerate from rest to working speed (generally several hundred revolutions per minute). The acceleration performance of the AC servo system is better, taking the Panasonic MSMA 400W AC servo motor as an example, from the static acceleration to its rated speed of 3000RPM only a few milliseconds, can be used for control occasions requiring fast start and stop.
Ac servo system usually adopts three closed-loop structure forms:
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1. Speed closed-loop system: The system controls the speed and position by detecting the actual speed of the motor. The advantage of this system is that the speed response is fast and can be used for high-precision position control.
2. Position closed-loop system: The system controls the position and position error by detecting the actual position of the motor. The advantage of this system is that the position control precision is high and can be used for high-precision positioning.
3. Current closed-loop system: The system controls current and power by detecting the actual current of the motor. The advantage of this system is that it has a fast response speed and can be used for high-speed control.
The AC servo system has three closed-loop structures: position closed-loop, velocity closed-loop and current closed-loop. The position closed loop controls the output position of the motor by comparing the feedback position signal with the set position. The speed closed loop controls the output speed of the motor by comparing the feedback speed signal with the set speed. The current loop controls the output current of the motor by comparing the feedback current signal with the set current. These three closed-loop structures can improve the stability, accuracy and response speed of the system, and are suitable for different application scenarios.