Servo System (Servo System) is a servo system. It is an automatic control system that uses mechanical position or angle as the control object, such as CNC machine tools. The drive motor used in the servo system is required to have fast response speed, accurate positioning, and moment of inertia. (The servo motor used in the electromechanical system has a large moment of inertia, in order to be directly connected to mechanical parts such as a lead screw. The servo motor has a special purpose. The small inertia motor has a very high response speed, but this type of motor has a low overload capability and must be accelerated or decelerated when used in a feed servo system.
The moment of inertia reflects the acceleration characteristics of the system. When the servo motor is selected, the moment of inertia of the system cannot be greater than three times the moment of inertia of the motor. Larger characteristics, such a dedicated motor is called a servo motor. Of course, its basic working principle is no different from ordinary AC/DC motors. The dedicated drive unit of this type of motor is called a servo drive unit, sometimes referred to as a servo, and generally includes a current, speed and/or position closed loop.
A stepper motor is an actuator that converts electrical pulses into angular displacement. When the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (called the "step angle") in a set direction, and its rotation is run step by step at a fixed angle. The angular displacement can be controlled by controlling the number of pulses to achieve the purpose of accurate positioning. At the same time, the speed and acceleration of the motor rotation can be controlled by controlling the pulse frequency, thereby achieving the purpose of speed regulation. The stepping motor can be used as a special motor for control, and it is widely used in various open-loop control because it has no accumulation error (100% accuracy). More commonly used stepper motors include reactive stepper motors (VR), permanent magnet stepper motors (PM), hybrid stepper motors (HB), and single-phase stepper motors.
The rotor inside the servo motor is a permanent magnet. The U/V/W three-phase electric motor controlled by the driver forms an electromagnetic field. The rotor rotates under the action of the magnetic field. At the same time, the encoder feedback signal from the motor is supplied to the driver. The driver according to the feedback value and target. The values ​​are compared to adjust the angle at which the rotor rotates. The accuracy of the servo motor is determined by the accuracy (number of lines) of the encoder. Stepper motor and AC servo motor performance comparison Stepper motor is a discrete motion device, which has an essential relationship with modern digital control technology. In the current domestic digital control system, the application of stepper motors is very extensive. With the advent of all-digital AC servo systems, AC servo motors are increasingly being used in digital control systems. In order to adapt to the development trend of digital control, stepping motor or all-digital AC servo motor is mostly used as the execution motor in the motion control system. Although the two are similar in control mode (burst and direction signals), there are large differences in performance and application. Now compare the performance of the two.
The difference between stepper motor and servo motorFirst, the control precision is different, the two-phase hybrid stepping motor step angle is generally 3.6 & # 176;, 1.8 & # 176;, the five-phase hybrid stepping motor step angle is generally 0.72 & # 176;, 0.36 & # 176; There are also some high performance stepper motors with smaller step angles. For example, a stepping motor for a slow wire-cutting machine produced by Sitong has a step angle of 0.09° and a step of a three-phase hybrid stepping motor produced by BERGER LAHR. The angle can be 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 rear of the motor shaft. Take Panasonic's all-digital AC servo motor as an example. For a motor with a standard 2500-line encoder, the pulse equivalent is 360°/10000=0.036° because the driver uses quadruple frequency technology inside. For a motor with a 17-bit encoder, the drive receives one revolution per 217 = 131072 pulse motors, ie its pulse equivalent is 360 & #176; /131072 = 9.89 seconds. It is 1/655 of the pulse equivalent of the stepping motor with a step angle of 1.8°
Second, low-frequency characteristics Different stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency is related to the load condition and the performance of the driver. It is generally considered that the vibration frequency is half of the take-off frequency of the motor no-load. This low frequency vibration phenomenon, which is determined by the working principle of the stepper motor, is very detrimental to the normal operation of the machine. When the stepper motor works at low speed, damping technology should generally be used to overcome the low frequency vibration phenomenon, such as adding a damper to the motor or using subdivision technology on the drive. The AC servo motor runs very smoothly, and vibration does not occur even at low speeds. The AC servo system has a resonance suppression function, which can cover the rigidity of the machine, and has a frequency analysis function (FFT) inside the system, which can detect the resonance point of the machine and facilitate system adjustment.
Third, the moment frequency characteristics of the stepper motor output torque decreases with the increase of the speed, and will drop sharply at higher speeds, so its maximum working speed is generally 300 ~ 600RPM. The AC servo motor is a constant torque output, that is, it can output the rated torque within its rated speed (generally 2000RPM or 3000RPM), and it is a constant power output above the rated speed.
Fourth, the overload capacity of different stepper motors generally do not have overload capability. The AC servo motor has a strong overload capability. Take the 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 moment of inertia of the inertia load at the moment of starting. Because there is no such overload capability in the stepping motor, in order to overcome this moment of inertia during the selection, it is often necessary to select a motor with a large torque, and the machine does not need such a large torque during normal operation, and a torque appears. The phenomenon of waste.
V. Operational performance The control of the stepping motor is open-loop control. If the starting frequency is too high or the load is too large, it may be lost or blocked. When the speed is too high, the overshoot may occur, so to ensure its control. Accuracy should deal with the problem of rising and falling speed. The AC servo drive system is closed-loop control. The drive can directly sample the feedback signal of the motor encoder. The internal position loop and speed loop are formed. Generally, the stepless motor is lost or overshooted, and the control performance is more reliable.
Sixth, the speed response performance of the stepper motor from static acceleration to working speed (generally several hundred revolutions per minute) requires 200 to 400 milliseconds. The AC servo system has better acceleration performance. Taking the Panasonic MSMA 400W AC servo motor as an example, it takes only a few milliseconds to accelerate from standstill to its rated speed of 3000 RPM, which can be used for control applications requiring fast start and stop.
In summary, the AC servo system is superior to the stepper motor in many performance aspects. However, in some occasions where the requirements are not high, stepper motors are often used to perform the motor. Therefore, in the design process of the control system, it is necessary to comprehensively consider the control requirements, cost and other factors, and select the appropriate control motor.
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