In the actual production of the motor, due to the large number of simple program control processes controlled by switching quantities, and the actual production process and process are often changed, the traditional relay contact control system often cannot meet this requirement.

Therefore, there has been a control device that combines relay contact control and electronic technology, called sequence controller. It can change the control program according to the production needs, and is far simpler and cheaper than the electronic computer. It realizes the relay contact control through the insertion or programming of the combinational logic element. But its device is bulky and its function is also limited. With the development and application of large-scale integrated circuit and microprocessor technology.
The advantages of servo motors are simple structure, reliable operation and high efficiency; the disadvantages are large size and poor starting characteristics.
Due to the use of permanent magnet excitation, the excitation loss and related stray losses are eliminated, so the efficiency is high. And because there are no collector rings and brushes required by electromagnetic synchronous motors, its mechanical reliability is the same as that of induction (asynchronous) servo motors, but the power factor is much higher than that of asynchronous motors, so that the volume ratio of permanent magnet synchronous motors is higher. Asynchronous motors are smaller.