What is a servo motor

Servo motor: It 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 is an electric-mechanical converter that can rotate continuously. As a servo motor for hydraulic valve controller, it is a micro motor with very low power. Permanent magnet DC servo motor and parallel excited DC servo motor are the most commonly used.

The function of servo motor: servo motor can control speed and position accuracy very accurately.

Classification of servo motors: DC servo motors and AC servo motors.

The output speed of the DC servo motor is proportional to the input voltage, and can realize forward and reverse speed control. It has the advantages of large starting torque, wide speed regulation range, good linearity of mechanical characteristics and adjustment characteristics, and convenient control, but the wear and sparks of the commutation brush will affect its service life. The brushless DC servo motor that has appeared in recent years avoids brush friction and commutation interference, so it has high sensitivity, small dead zone, low noise, long life, and little interference to surrounding electronic equipment.

The transfer function of the output speed/input voltage of a DC servo motor can be approximately regarded as a first-order delay link, and its electromechanical time constant is generally between about ten milliseconds and several tens of milliseconds. However, the time constant of some low-inertia DC servo motors (such as hollow cup rotor type, printed winding type, slotless type) is only a few milliseconds to twenty milliseconds.

The rated speed of low-power DC servo motors is above 3000r/min, or even greater than 10000r/min. Therefore, as a hydraulic valve controller, a high-speed reducer is required. However, DC torque servo motors (i.e. low-speed DC servo motors) can work at a low speed of several tens of rpm, or even under long-term stall conditions, so they can directly drive the controlled device without deceleration.

DC servo motors are divided into brushed and brushless motors.

Brushed motors have low cost, simple structure, large starting torque, wide speed range, easy control, and require maintenance, but maintenance is convenient (replacing carbon brushes), but they generate electromagnetic interference and have requirements for the environment. Therefore, they can be used in general industrial and civil occasions that are sensitive to cost.

Brushless motors are small in size, light in weight, high in output, fast in response, high in speed, small inertia, smooth in rotation, and stable in torque. The control is complex and easy to realize intelligentization. Its electronic commutation method is flexible, and it can be square wave commutation or sine wave commutation. The motor is maintenance-free, highly efficient, has a low operating temperature, low electromagnetic radiation, and a long life, and can be used in various environments.

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

Working Principle of AC 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 the functional difference between AC servo motors and brushless DC servo motors?

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.

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 volume and weight at the same power.