Working Principle of Servo Motor

The rotor inside the servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver forms an electromagnetic field. The rotor rotates under the action of this magnetic field. At the same time, the encoder of the motor feeds back the signal to the driver. The driver compares the feedback value with the target value and adjusts the angle of rotation of the rotor. The accuracy of the servo motor is determined by the accuracy of the encoder (number of lines).

What is a servo motor? How many types are there? What are its working characteristics?

A: Servo motors, also known as actuator motors, are used as actuators in automatic control systems to convert received electrical signals into angular displacement or angular velocity output on the motor shaft. They are divided into two categories: DC and AC servo motors. Their main features are that when the signal voltage is zero, there is no self-rotation, and the speed decreases uniformly as the torque increases.

What is the functional difference between AC servo motors and brushless DC servo motors?

Answer: AC servo is better because it uses a sine wave to control the ball screw and has a small torque pulsation. DC servo uses a trapezoidal wave. But DC servo is simpler and cheaper. Permanent magnet AC servo motor Since the 1980s, with the development of integrated circuits, power electronics technology and AC variable speed drive technology, permanent magnet AC servo drive technology has made outstanding progress. Famous electrical manufacturers in various countries have successively launched their own AC servo motors and servo drive series products and continuously improved and updated them. AC servo system has become the main development direction of contemporary high-performance servo system, making the original DC servo face the crisis of being eliminated. After the 1990s, the commercialized AC servo systems in various countries around the world use fully digitally controlled sine wave motor servo drive. The development of AC servo drive devices in the transmission field is changing with each passing day.

Compared with DC servo motors, permanent magnet AC servo motors have the following main advantages: ⑴ No brushes and commutators, so reliable operation, low maintenance and servicing requirements. ⑵ Stator winding heat dissipation is more convenient. ⑶ Small inertia, easy to improve the rapidity of the system. ⑷ Suitable for high-speed and high-torque working conditions. ⑸ Smaller volume and weight at the same power.

Servo and stepper motors

The servo mainly relies on pulses for positioning. Basically, it can be understood in this way: when the servo motor receives one pulse, it will rotate the angle corresponding to the pulse, thereby achieving displacement. Because the servo motor itself has the function of sending pulses, the servo motor will send a corresponding number of pulses every time it rotates one angle. This forms an echo with the pulses received by the servo motor, or a closed loop. In this way, the system will know how many pulses have been sent to the servo motor and how many pulses have been received. In this way, the rotation of the motor can be controlled very accurately, thereby achieving precise positioning, which can reach 0.001mm.

The stepper motor is a discrete motion device, which is essentially related to modern digital control technology. In the current domestic digital control system, the application of stepper motors is very extensive. With the emergence of fully digital AC servo systems, AC servo motors are also increasingly used in digital control systems. In order to adapt to the development trend of digital control, most motion control systems use stepper motors or fully digital AC servo motors as actuator motors. Although the two are similar in control mode (pulse train and direction signal), there are great differences in performance and application occasions. Now let's compare the performance of the two.

1. Different control accuracy

The step angle of a two-phase hybrid stepper motor is generally 3.6° and 1.8°, and the step angle of a five-phase hybrid stepper motor is generally 0.72° and 0.36°. There are also some high-performance stepper motors with smaller step angles. For example, a stepper motor for a slow wire-feeding machine tool produced by Sitong Company has a step angle of 0.09°; the step angle of a three-phase hybrid stepper motor produced by BERGER LAHR in Germany can be set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, 0.036° through a dial switch, which is compatible with the step angles of two-phase and five-phase hybrid stepper motors.

The control accuracy of AC servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft. Taking Panasonic's fully digital AC servo motor as an example, for a motor with a standard 2500-line encoder, due to the use of quadruple frequency technology inside the driver, its pulse equivalent is 360°/10000=0.036°. For a motor with a 17-bit encoder, the driver receives 217=131072 pulses for each motor rotation, 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. Different low-frequency characteristics

Stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency is related to the load conditions and driver performance. It is generally believed that the vibration frequency is half of the motor's no-load starting frequency. 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 a 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 driver.

The AC servo motor runs very smoothly and does not vibrate even at low speeds. The AC servo system has a resonance suppression function that can cover the lack of mechanical rigidity, and the system has a frequency resolution function (FFT) that can detect the resonance point of the machine to facilitate system adjustment.

3. Different torque-frequency characteristics

The output torque of a stepper motor decreases as the speed increases, and it drops sharply at higher speeds, so its maximum operating speed is generally between 300 and 600 RPM. AC servo motors have constant torque output, that is, they can output rated torque within their rated speed (generally 2000 RPM or 3000 RPM), and have constant power output above the rated speed.

4. Different overload capacity

Stepper motors generally do not have overload capacity. AC servo motors have strong overload capacity. Take Panasonic 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 inertia moment of the inertial load at the moment of startup. Because stepper motors do not have this overload capacity, in order to overcome this inertia moment, it is often necessary to select a motor with a larger torque when selecting a stepper motor. However, the machine does not need such a large torque during normal operation, which results in a torque waste phenomenon.

5. Different operating performance

The control of stepper motor is open-loop control. If the starting frequency is too high or the load is too large, it is easy to lose steps or stall. If the speed is too high when stopped, it is easy to overshoot. Therefore, in order to ensure its control accuracy, the speed increase and decrease problems should be handled well. The AC servo drive system is closed-loop control. The driver can directly sample the feedback signal of the motor encoder. The internal position loop and speed loop are formed. Generally, the stepper motor will not lose steps or overshoot, and the control performance is more reliable.

6. Different speed response performance

It takes 200 to 400 milliseconds for a stepper motor to accelerate from standstill to its operating speed (generally several hundred revolutions per minute). The acceleration performance of the AC servo system is better. For example, the Panasonic MSMA 400W AC servo motor accelerates from standstill to its rated speed of 3000RPM in just a few milliseconds, which can be used in control situations that require fast start and stop.

In summary, AC servo systems are superior to stepper motors in many aspects. However, stepper motors are often used as actuator motors in some occasions with low requirements. Therefore, in the design process of the control system, it is necessary to comprehensively consider control requirements, cost and other factors to select the appropriate control motor.