Servo motor selection comparison

A servo motor is an engine that controls the operation of mechanical components in a servo system. It is an auxiliary motor indirect speed change device.

The servo motor controls speed and position very accurately, and can convert voltage signals into torque and speed to drive the controlled object. The rotor speed of the servo motor is controlled by the input signal and can respond quickly. In the automatic control system, it is used as an actuator, and has the characteristics of small electromechanical time constant, high linearity, and starting voltage. It can convert the received electrical signal into angular displacement or angular velocity output on the motor shaft. It is divided into two categories: DC and AC servo motors. Its main characteristics are that there is no self-rotation when the signal voltage is zero, and the speed decreases uniformly with the increase of torque.

How it works

The servo system is an automatic control system that enables the output controlled quantities such as the position, orientation, and state of an object to follow any changes in the input target (or given value). The servo mainly relies on pulses for positioning. Basically, it can be understood that when the servo motor receives one pulse, it will rotate the angle corresponding to one 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 for each rotation angle, thus forming a response to 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 back. In this way, the rotation of the motor can be controlled very accurately, thereby achieving precise positioning, which can reach 0.001mm.

1. DC servo motors are divided into brushed DC servo motors and brushless DC servo motors.

**(1) Brushed motor** has low cost, simple structure, large starting torque, wide speed range, easy control, requires maintenance, but maintenance is inconvenient (replacing carbon brushes), generates electromagnetic interference, and has requirements for the environment. Therefore, it can be used in general industrial and civil occasions that are sensitive to cost.

(2) Brushless motors are small in size, light in weight, high in output, fast in response, high in speed, small inertia, smooth rotation, and stable torque. They are easy to control and easy to realize intelligentization. Their electronic commutation is flexible and can be square wave commutation or sine wave commutation. The motors are maintenance-free, highly efficient, have low operating temperatures, low electromagnetic radiation, and long life, and can be used in various environments.

2. AC servo motors are also brushless motors, which are divided into synchronous AC motors and asynchronous AC motors.

At present, synchronous motors are generally used in motion control. They have a wide power range and can achieve very high power. They have large inertia, low maximum rotation speed, and decrease rapidly as the power increases. Therefore, they are suitable for applications with low-speed and stable operation.

3. 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 the rotor. The accuracy of the servo motor is determined by the accuracy of the encoder (number of lines).

4. The functional difference between AC servo motor and brushless DC servo motor: AC servo is better because it is sine wave controlled and has small torque pulsation. DC servo is trapezoidal wave. But DC servo is simpler and cheaper.

Selection Comparison

AC servo motor

The rotor of AC servo motor is usually made into squirrel cage type, but in order to make the servo motor have a wider speed regulation range, linear mechanical characteristics, no "self-rotation" phenomenon and fast response performance, it should have the two characteristics of large rotor resistance and small moment of inertia compared with ordinary motors. There are two forms of rotor structures that are widely used at present: one is a squirrel cage rotor with high resistivity bars made of high resistivity conductive materials. In order to reduce the moment of inertia of the rotor, the rotor is made slender; the other is a hollow cup rotor made of aluminum alloy. The cup wall is very thin, only 0.2-0.3mm. In order to reduce the magnetic resistance of the magnetic circuit, a fixed inner stator should be placed in the hollow cup rotor. The hollow cup rotor has a small moment of inertia, quick response, and smooth operation, so it is widely used.

When there is no control voltage in the AC servo motor, there is only a pulsating magnetic field generated by the excitation winding in the stator, and the rotor is stationary. When there is a control voltage, a rotating magnetic field is generated in the stator, and the rotor rotates in the direction of the rotating magnetic field. Under the condition of constant load, the speed of the motor changes with the size of the control voltage. When the phase of the control voltage is opposite, the servo motor will reverse.

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 systems have become the main development direction of contemporary high-performance servo systems, making the original DC servos face the crisis of being eliminated. After the 1990s, the commercialized AC servo systems in various countries around the world use fully digitally controlled sinusoidal wave motor servo drives. 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 There are no brushes and commutators, so it works reliably and requires low maintenance.

⑵It is more convenient to dissipate heat from the stator winding.

⑶ Small inertia, easy to improve the speed of the system.

⑷ Suitable for high-speed and high-torque working conditions.

⑸ Smaller size and weight at the same power.