What is the structure of a servo motor? The simplest servo control unit is a servo motor plus a servo controller. Today, we will analyze the servo motor and the servo controller.
The principle of motor operation
Right-hand screw law (Ampere's law) – Electric current generates magnetism
Ampere's law, also known as the right-hand screw law, is a rule that describes the relationship between the direction of a current and the direction of the magnetic field lines it generates. For a current-carrying straight conductor: grasp the conductor with your right hand, with your thumb pointing in the direction of the current; the direction your four fingers point indicates the direction of the magnetic field lines. For a current-carrying solenoid: grasp the solenoid with your right hand, bending your four fingers in the direction of the current; the end your thumb points to is the north pole (N) of the solenoid.
Fleming's Left-Hand Law - Magnetism
The rule for determining the direction of the force on a current-carrying conductor in an external magnetic field. Also known as the electric motor rule. With your left hand outstretched, thumb perpendicular to the other four fingers, palm facing the N pole, the four fingers indicate the direction of the current, and the thumb indicates the direction of the force on the current-carrying conductor in the external magnetic field.
DC servo motor structure
Servo Control Unit
The word SERVO originates from Latin and originally means "slave." It refers to the control of a mechanical system's position, torque, speed, or acceleration through closed-loop control. It is the execution unit in an automatic control system, which converts the electrical signals from the host controller into angular displacement or angular velocity outputs on the motor shaft.
1. Controller: The device that outputs action command signals.
2. Driver: A device that receives control commands and drives a motor.
3. Servo motor: A device that drives the controlled object and detects its status.
Types of servo motors
Servo motors can be broadly categorized into the following three types:
1. Synchronous type: Employs a permanent magnet synchronous motor. It generates electricity during power outages, making braking easy. However, due to limitations in manufacturing processes and materials, the motor's capacity is restricted. [Rotor: Permanent magnet; Stationary element: Coil]
2. Induction type: Induction motors are similar in construction to general-purpose motors, featuring a robust build and good torque performance at high speeds. However, they are more prone to overheating, and most capacity motors ( 7.5KW and above) are of this type. [Both the rotor and stationary are coils.]
3. DC type: DC servo motor. It suffers from dust generation due to carbon brush wear, making it unsuitable for cleanroom environments. Small capacity motors are preferred. [Rotor: coil; Stationary: permanent magnet; Commutator: magnetic brush]
SM synchronous servo motor
※Key Advantages: 1. Maintenance-free. 2. Excellent environmental resistance. 3. Excellent torque characteristics, constant torque. 4. Can generate brakes during power outages. 5. Small size and light weight. 6. High efficiency.
※Disadvantages: 1. The AMP is more complex in structure than the DC type. 2. The motor and AMP must be used in a 1:1 ratio. 3. The permanent magnet may be demagnetized.
IM inductive servo motor
※Key Advantages: 1. Easy maintenance. 2. Excellent environmental resistance. 3. Excellent torque characteristics at high speeds. 4. Can be manufactured in large capacity with high efficiency. 5. Robust construction.
※Disadvantages: 1. Small capacity models have poor efficiency. 2. AMP has a more complex structure than DC type. 3. Cannot dynamically brake during power outages. 4. Characteristics are affected by temperature changes. 5. AMP and motor must be used in a 1:1 ratio.
DC servo motor
※Key Advantages: 1. Simple servo driver construction. 2. Can generate electricity for braking during power outages. 3. Small size and low price. 4. High efficiency.
※Disadvantages: 1. The commutator requires regular maintenance. 2. Carbon brush wear produces carbon dust, making it unsuitable for applications requiring high cleanliness. 3. Due to issues with the commutator's carbon brushes, torque is poor at high speeds. 4. Permanent magnets may demagnetize.
Servo control principle
The most distinctive feature of a servo system is its control method based on feedback signals, which allows for the comparison of command values with target values, thereby significantly reducing errors.
What is a feedback signal? After issuing an instruction to the controlled object, it accurately tracks and ascertains the current value, and provides feedback on the deviation value of the control content at any time. After the target object reaches the destination, it provides feedback on the position value, and so on.
Control process: The position detection of the mechanical body is a closed system, called a fully closed loop. Conversely, the loop system for detecting the motor shaft end is called a semi-closed loop.
Internal structure of a servo driver
Rectifier section: The rectifier section converts AC power into DC power, and after capacitor filtering, it produces a stable and pulsation-free DC power supply.
Inverter: The SPWM signal from the control unit drives the IGBT to convert the DC power supply into an SPWM waveform to drive the servo motor.
Control Section: The servo unit adopts a fully digital structure, achieving software-based closed-loop control through high-performance hardware support. Currently, all servos utilize DSP (Digital Signal Processing) chips, capable of executing position, speed, torque, and current controller functions. It provides PWM control signals to the power drive unit and can receive and process position and current feedback, and has a communication interface.
Encoder: The servo motor is equipped with a high-performance angle measurement encoder, which can accurately measure the rotor position and the motor speed.
Inverters are increasingly employing new power electronic semiconductor devices. Currently, servo control systems are increasingly using high-frequency power semiconductor devices for their output, primarily high-power transistors (GTRs), power MOSFETs, and insulated-gate transistors (IGPTs). The application of these advanced devices significantly reduces power consumption in the servo unit's output circuit, improves system response speed, and reduces operating noise. Particularly noteworthy is the emergence of a new type of module in servo control systems that integrates control circuit functions with high-power electronic switching devices, called Intelligent Power Modules (IPMs). This device integrates input isolation, energy-saving braking, over-temperature, over-voltage, and over-current protection, as well as fault diagnosis, into a single, compact module. Its input logic levels are fully compatible with TTL signals and can directly interface with microprocessor outputs. Its application significantly simplifies servo unit design and enables the miniaturization and micro-miniaturization of servo systems.
