What is a servo system?
Servo system, also known as servo system, is the basic part of the automation system. It is an automatic control system with power amplification function composed of several components and components.
According to the physical properties of the components of the system, they are divided into electrical servo systems, electrical hydraulic servo systems, and electrical pneumatic servo systems.
The electric servo system is divided into DC servo system and AC servo system. In the 1970s, DC servo was widely used. DC motors had a wide speed range, convenient starting and stopping, large torque, low system energy consumption, and were widely used for control performance In relatively demanding servo systems.
What is excitation control?
DC servo systems are suitable for a wide range of power, including control objects from tens of watts to tens of kilowatts. Generally, from the perspective of improving system efficiency, DC servo systems are mostly used for control objects with a power of more than 100 watts. The output torque of a DC motor is related to the current applied to the armature and the magnetic flux generated by the exciting current. When the magnetic flux is fixed, the larger the armature current, the larger the motor torque. When the armature current is fixed, increasing the magnetic flux can increase the torque. Therefore, by changing the exciting current or the armature current, the torque of the DC motor can be controlled. When the armature current is controlled, it is called armature control. At this time, the control voltage is added to the armature. If the exciting current is controlled, the control voltage is added to the exciting winding, which is called exciting control.
In the armature control, the armature inductance is generally small, so the armature control can obtain good response characteristics. The disadvantage is that the load power must be provided by the control power of the armature, so a larger control power is required, which increases the complexity of the power amplification component. For example, for systems requiring larger control power, high-power amplifying components such as generator-motor units, motor amplifiers and thyristors must be used.
The excitation control requires a constant current power supply to the armature, so that the torque of the motor is controlled only by the excitation current. Constant current characteristics can be obtained by connecting a large resistance (10 times the armature resistance) in the armature circuit. For high-power control objects, the power consumption of the series resistor will become very large, which is very uneconomical. Therefore, the excitation control is limited to low-power applications. After the armature power source uses a constant current source, the slope of the mechanical characteristics is equal to zero, which causes the electromechanical time constant of the motor to increase, and the large amount of inductance in the field winding, which makes the dynamic characteristics of the field control poor and the response slow.