What Is Servo Motor and How It Works?

Have you ever thought about how robotic vehicles commonly used in a military operation or metal cutting and forming machine provide precise motion for milling, bending, lase for fabrication?

The answer can be found in this course.

Servo motor system application is mostly used in a closed loop system where precise position control is commonly found in industrial and commercial applications.

So what is a servo motor? And how it works?

Let us first determine what servo motor is and examine the unique features of types servo motor its applications.


Servo Motor Basics


Let us begin with servo motor basics. Servo motor is part of a closed-loop system. And it is comprised of several parts, namely, a control circuit, motor, shaft, amplifier and either an encoder or resolver.

A servo motor is a self-contained electrical device that rotates part of the machine with high efficiency and with great precision. The output shaft of this motor can be moved to a particular angle, position and velocity that regular motor does not have.

The servo motor utilizes regular motor and couples a sensor for position feedback. The controller is the most important part of the servo motor designed specifically for this purpose. The servo motor is a closed-loop mechanism that cooperates position feedback in order to control rotational or linear speed and position. The motor is controlled with an electrical signal, either analog or digital, which determines the amount of movement which represents the final command position for the shaft.

A type of encoder served as a sensor providing speed and position feedback. The circuit is built right inside the motor housing which usually feeding with a gear system.


Types of Servo Motors


Servo motor is classified into different types based on their application, such as AC servo motor and DC servo motor. There are three main considerations to evaluate servo motors.

First, based on their current type, AC or DC. And secondly, based on the type of commutation used, whether the motor uses brushes. And the third type of consideration is the motor rotating field, the rotor, whether the rotation is synchronous or asynchronous.

Let us discuss the first servo consideration. AC or DC consideration is the most basic classification of the motor based on the type of current it will use. Looking at the performance standpoint, the primary difference between AC and DC motors is the apparent ability to control speed. With the DC motor, the speed is directly proportional to the supplied voltage with the constant blow. And an AC motor, the speed is determined by the frequency of the applied voltage and the number of magnetic coils. Although AC and DC motors are used in servo systems, AC motor will withstand higher current and are more commonly used in servo applications such as robots in line manufacturing and other industrial applications where high repetition and high precision are required.

Brushed or brushless is the next step. A DC servo motor is commutated mechanically with brushes using a commutator or electrically without brushes. Brushed motors are generally less expensive and simpler to operate. While brushless designs are more reliable and have higher efficiency, and less noisy.

A commutator is a rotary electrical switch that periodically reverses the current direction between the rotor and Drive Circuit. A consists of a cylinder composed of multiple metal contact segments on the rotor. Two or more electrical contacts called the brushes made the soft conductive material such as carbon press against the commutator making the sliding contact with the segments of commutator as a rotates.

While the majority of the motor used in servo systems are AC brushless designs. Brushed permanent magnet DC motors are sometimes employed as servo motors for their simplicity and low cost.

The most common type of Brushed DC motor used in servo applications is the permanent magnet DC motor. Brushless DC motors replace the physical brushes and commutator with an electronic means of achieving commutation. Typically through use  Hall of Effect Sensors or encoder.

AC motor is generally brushless, although there are some designs such as the universal motor, which can run on either AC or DC power that does have brushes and mechanical commutators.

And the final classification for consideration is whether the servo motor application will use a synchronous or asynchronous rotating field. While DC motors are generally categorized as brushed or brushless. AC motors more often differentiated by the speed of rotating synchronous or asynchronous field. If we recall for the AC/DC consideration, that an AC motor, speed is determined by the frequency of supplied voltage and number of magnetic coils. The speed is referred to as the synchronous speed. Therefore the synchronous motor, the rotor rotates the same speed as the stator rotating magnetic field. However, in an asynchronous motor, normally referred to as an induction motor, the rotor rotates with the speed slower than the stator magnetic rotary field. However, the speed of an asynchronous motor can be very utilizing several control methods, such as changing the number of magnetic coils and AC frequency, just name a couple.


Working Principle


The working principle of a DC servo motor are the constructions of four components: a DC motor, a position sensing device, a gear assembly and a control circuit.

The desired speed of the DC motor is based on the voltage applied in order to control the motor speed, the potentiometer produces a voltage which supplies one of these inputs to air amplifier.

In some circuits, a control pulse used to produce DC reference voltage corresponding to the desired position, or speed of the motor, and it supplies to pull switch converter. The link of the pulse decides the voltage applied air amplifier as the desired voltage to produce the desired speed and position.

For digital control, a PLC or other motion controller is used for generating the pulses in terms of duty cycle to produce more accurate control. The feedback signal sensor is normally a potentiometer that produces a voltage corresponding to the absolute angle of the motor shaft through the gear mechanism. Then the feedback voltage value is applied to the input of the air comparator amplifier.

The amplifier compares the voltage generated from the current position of the motor, resulting from the potentiometer feedback and to the desired position of the motor producing in air, either a positive or a negative voltage. This air voltage is supplied to the armature of the motor. As the air increases, so does the output voltage applied to the motor armature. As long as air exists, the comparator amplifier amplifies the air voltage and correspondingly powers the armature. The motor rotates until the air becomes zero.  If the air is negative, the air armature voltage reverses and hence the armature rotates in the opposite direction.

Servo Motor Application

Servo motor applications are applied to many industrial and commercial systems and products such as robotics where servo motor is used in every joint of a robot to perform its precise angle of movement.

The camera autofocus uses a servo motor built into the camera that corrects the precise position of the lens that shot focus out images.

This the brief introduction of the servo motor.

And servo motor can be found on our window patching machines, like ZKT-750B, ZKT-700 high speed, ZKT-1080B, ZKT-700B, etc., to improve precision. And most of the servo motors we adopted are Japanese Yaskawa, for we always put the performance and reliability in the first place.

(Statements: This passage is basically summarized from Youtube Channel “REALPARS”. If this offends your rights, please contact us, and we will delete it.)

2 replies
  1. Dinesen says:

    Thank you so much for doing the job here, everyone will like your post.

    Best regards,
    Dinesen Cannon

  2. Henry Killingsworth says:

    It was really informative when you explained that servo motors are used to rotate a part of a machine. It seems like a servo motor would need to receive regular maintenance in order to keep it moving smoothly. You would want to prevent things like dust from accumulating because it can cause friction.


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