Servo mainly by pulse to locate, basically can be understood that the servo motor receives a pulse, it will rotate a pulse corresponding to the angle, in order to achieve displacement, because the servo motor itself has a pulse function, so each servo motor Rotating an angle will produce the corresponding number of pulses, thus forming a response with the pulse received by the servomotor, or by closing the loop so that the system knows how many pulses are sent to the servomotor and how many pulses are received at the same time In this way, it is possible to control the rotation of the motor very precisely for precise positioning, up to 0.001 mm. Stepper motors are discrete motion devices that are intrinsically linked to modern digital control technology. In the current domestic digital control system, the application of stepping motor is very wide. With the advent of all-digital AC servo systems, AC servo motors are also increasingly used in digital control systems. In order to adapt to the trend of digital control, most of the motion control system uses stepper motor or digital AC servo motor as the implementation of the motor. Although both are similar in control (burst and direction signals), there is a large difference in performance and application. Now use the performance of the two for a comparison. First, the control accuracy is different Two-phase hybrid stepping motor step angle is generally 3.6 °, 1.8 °, five-phase hybrid stepping motor step angle is generally 0.72 °, 0.36 °. There are also some high-performance stepper motors with smaller step angles. For example, a stepper motor manufactured by Stone Company for a slow-moving machine has a step angle of 0.09 °. The step angle of a three-phase hybrid stepping motor manufactured by BERGER LAHR can be obtained by DIP switches are set to 1.8 °, 0.9 °, 0.72 °, 0.36 °, 0.18 °, 0.09 °, 0.072 °, 0.036 °, compatible with the step angle of two-phase and five-phase hybrid stepping motors. The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the back of the motor shaft. Taking Panasonic digital AC servo motor as an example, for a motor with a standard 2500-line encoder, the pulse equivalent is 360 ° / 10000 = 0.036 ° due to the quadruple-frequency technique inside the driver. For a motor with a 17-bit encoder, the drive rotates one revolution for every 217 = 131 710 pulses received, that is, its pulse equivalent is 360 ° / 131072 = 9.89 seconds. 1/655 of the pulse equivalent of a stepper motor with a step angle of 1.8 °. Second, the low-frequency characteristics of asynchronous motor into the low-frequency vibration prone to low-speed phenomenon. Vibration frequency and load conditions and drive performance, the vibration frequency is generally considered to be half the motor no-load take-off frequency. This low-frequency vibration, which is determined by the working principle of the stepping motor, is very unfavorable for the normal operation of the machine. When the stepper motor work at low speed, the damping technology should generally be used to overcome the phenomenon of low frequency vibration, such as adding a damper on the motor, or the use of subdivision technology on the drive. The AC servo motor operates very smoothly and does not vibrate even at low speeds. AC servo system with resonance suppression function can cover the lack of mechanical rigidity, and the system has a frequency analysis function (FFT), can detect the mechanical resonance point, easy to system adjustment. Third, the characteristics of the moment frequency Not synchronous into the motor output torque decreases with increasing speed, and the sharp decline in the high speed, so the maximum working speed is generally 300 ~ 600RPM. AC servo motor is constant torque output, that is rated output speed (usually 2000RPM or 3000RPM), can output rated torque, above the rated speed for constant power output. Fourth, the overload capacity is not synchronous motor generally does not have overload. AC servo motor has a strong overload capacity. Take Matsushita AC servo system as an example, it has speed overload and torque overload capacity. Its maximum torque is three times the rated torque, which can be used to overcome the moment of inertia of the inertial load at start-up moment. Stepper motor because there is no such overload, in the selection in order to overcome this moment of inertia, often need to select a larger torque motor, and the machine does not require such a large torque during normal operation, it appeared torque Wasted phenomenon. Fifth, the performance of different stepping Motor control for the open-loop control, the starting frequency is too high or too large load is prone to lost or stalling phenomenon, excessive speed prone to overshoot phenomenon, so in order to ensure that its control Accuracy, should be handled well, slow down the problem. AC servo drive system is closed-loop control, the drive can directly sample the motor encoder feedback signal, the internal structure of the position loop and speed loop, generally do not appear stepping motor lost or overshoot phenomenon, the control performance is more reliable. Sixth, the speed response performance asynchronous motor into the work from the standstill speed (usually several hundred revolutions per minute) takes 200 ~ 400 milliseconds. AC servo system acceleration performance is better, to Panasonic MSMA 400W AC servo motor, for example, from standstill to its nominal speed of 3000RPM only a few milliseconds, can be used for quick start and stop control occasions. In summary, the AC servo system in many performance advantages over the stepper motor. However, some less demanding occasions are often used to do the implementation of the motor stepper motor. Therefore, the design of the control system should take into account control requirements, costs and other factors, the appropriate selection of motor control.
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