A servo motor is an engine that controls the operation of mechanical components in a servo system; it is a type of auxiliary motor with indirect speed change.
Servo motors can be either AC or DC. Initially, DC servo motors were preferred because only this type of motor could control large currents via sequential operation. As transistors became capable of controlling large currents and switching large currents at higher frequencies, AC servo motors became more commonly used. Early servos were designed specifically for servo amplifiers. Today, one type of motor is designed for applications where, in practice, servo amplifiers or variable frequency drives can be used. This means the motor can be used in a servo system in one application and in another using a variable frequency drive. Some companies also require any closed-loop system that does not utilize stepper motors for servo systems, so it is possible to simply connect an AC induction motor to a speed controller, which is called a servo motor.
Servo motors can control speed and position with extremely high accuracy. They convert voltage signals into torque and speed to drive the controlled object. The rotor speed of a servo motor is controlled by the input signal and can respond quickly. In automatic control systems, they are used as actuators and have characteristics such as a small electromechanical time constant and high linearity. They can convert received electrical signals into angular displacement or angular velocity output on the motor shaft. Servo motors are divided into two main categories: DC and AC servo motors. Their main characteristic is that they do not rotate when the signal voltage is zero, and their speed decreases uniformly as the torque increases.
Since the Indramat division of Rexroth GmbH of MANNESMANN in Germany officially launched the MAC permanent magnet AC servo motor and drive system at the Hannover Trade Fair in 1978, it marked the entry of this new generation of AC servo technology into the practical application stage. By the mid-to-late 1980s, various companies had complete product lines. The entire servo device market shifted to AC systems. Early analog systems had shortcomings in areas such as zero drift, interference immunity, reliability, accuracy, and flexibility, and could not fully meet the requirements of motion control. In recent years, with the application of microprocessors and new digital signal processors (DSPs), digital control systems have emerged, where the control part can be entirely controlled by software. These are respectively called DC servo systems and three-phase permanent magnet AC servo systems.
DC servo motor
1. Structure: Basically the same as a DC motor. It is made thinner and longer to reduce the moment of inertia.
2. Working principle: Same as DC motor.
3. Power supply method: separately excited. The excitation winding and armature are powered by two independent power sources:
U1 is the excitation voltage, and U2 is the armature voltage.
The mechanical characteristic formula for a DC servo motor is the same as that for a separately excited DC motor:
Based on mechanical properties, we can know that:
(1) When U1 (i.e. magnetic flux ￠) remains constant, under a certain load, U2↑, n↑.
(2) When U2=0, the motor stops immediately.
Reverse rotation: The polarity of the armature voltage changes, causing the motor to reverse.
AC servo motor stator
The stator of this motor is a laminated structure, with its two windings wound in space at 90-degree electrical angles. A winding voltage source, known as the main winding (also called the reference or fixed phase), is energized. The other winding, called the control belt winding (or control phase), is powered by a variable control voltage that is 90 degrees out of phase with the voltage across the main winding. This control voltage is provided by a servo amplifier.
AC servo motor rotor
The rotor is typically a squirrel-cage type with a small diameter and long length to maintain the lowest possible mechanical inertia. It has high resistance to achieve the most linear torque-speed characteristics possible. For very low-power applications, a drag cup rotor (Figure 2) is used to further reduce rotor inertia. This type of rotor is a special form of the squirrel-cage rotor where the conductors are drag cup-shaped and made of a non-magnetic conductive material (such as copper, aluminum, or alloys). The slotted rotor laminations are replaced by a set of fixed annular laminations, which provides a low magnetic reluctance path for the magnetic flux.
With long lifespan, light weight, high torque-to-weight ratio, high reliability, no radio noise, and simple drive circuitry, AC servo motors are widely used in instrument servo systems, computers, tracking and guidance systems, self-balancing recorders, remote positioning equipment, process controllers, robots, special-purpose machine tools, and many other applications requiring precise angular motion.
One of the best types of servo motors uses a digital proportional remote control system. These devices actually consist of three parts:
The system employs integrated circuits, a servo motor, a reduction gearbox, and a potentiometer mechanism. Figure 24 shows a block diagram of this system. The circuit's drive input is a pulse signal with a period of 15ms and a pulse width of 1-2ms. The width of the input pulse controls the position of the servo motor output. For example, a 1ms pulse width corresponds to the leftmost position; 1.5ms to the middle position; and 2ms to the rightmost position.
