What is a Servo Motor and How Does it Work?

A servo motor is a kind of electric motor that is able to rotate or move with a definite location, speed, or torque according to an input signal from a controller.

Assun Motor has been manufacturing DC servo motors since 2003. They are the coreless and brushless type, meant for high-precision motion for industries like medical equipment, medical imaging,  telecommunications equipment, CNC machining, aerospace, mining, nanotechnology, etc. Do contact us using the button below to find out more.

Working Principles of a Servo Motor

The important parts of a servo motor are the DC motor, gear system, position sensor, and controller.

• The DC motor is connected with a gear mechanism which gives a feedback signal to a position sensor.
• The output of the motor is fed to the servo arm from the gearbox.
• The position of the potentiometer is changed with respect to the position of the motor.
• A change in resistance makes an equal change in voltage from the potentiometer. The pulse width modulated signal is fed through the control wire.
• Pulse width is transformed into an equal voltage that is compared with that of the signal from the potentiometer in an error amplifier. See “block diagram of a servo motor” below.
• The difference signal is amplified and provided to the DC motor. So, the signal applied to the DC servo motor is a damping wave.
• If the difference between the expected position of the pulse train and the present position is big, the motor moves fast. If the difference is less, the motor moves slowly.
• The required pulse train for regulating the servo motor can be generated by a timer IC such as 555 or a microcontroller to generate the required waveform.

Fig.1: Block diagram of a servo system

Servo Motor Control

The control of a servo motor is dependent on the type of motor, feedback method, and application requirements.

Analog Signal: Analog signals are usually applied to simple or low-cost servo system that does not require high precision. For example, a potentiometer can be used to produce an analog control signal for a servo motor.

Digital Signal: Digital control signals are discrete pulses or bits which are represented in a coded form. These are usually applied to complex or high-performance servo systems that require high accuracy, resolution, or communication. For example, a pulse-width modulation (PWM) signal can be applied to generate a digital control signal for a brushless DC servo motor.

Controller: The controller of a servo motor can be an external device or an integrated circuit. The controller receives the control signal from an external source and the feedback signal from the sensor and generates the appropriate control signal for driving the motor.

Fig. 2: Different parts of servo motor

The servo motor can be moved to an expected angular position by sending pulse width modulated (PWM) signals on the control wire. A pulse of width from 1 millisecond to 2 milliseconds in a repeated time frame is sent to the servo for around 50 times in a second. The width of the pulse determines the angular position.

For example, a pulse of 1 millisecond shifts the servo towards 0° and 2 milliseconds wide pulse would take it to 180°. A pulse of width 1.5 milliseconds will move the servo to 90°.

A train of such pulses (50 in one second) requires to be passed to the servo to get an angular position. Once the servo receives a pulse, it can attain the corresponding angular position for the next 20 milliseconds. So a pulse in every 20 millisecond time frame must be fed to the servo.

Fig. 3: Diagram showing angular rotation of a servo motor using PWM Wave.

Types of Servo Motors

● AC Servo Motor: AC Servo Motor is a kind of electric motor that is operated by alternating current (AC). It can be of two types such as synchronous and asynchronous.

● Synchronous AC Servo Motor: Synchronous AC Servo Motor is more effective, accurate, and responsive than asynchronous motor, but it has a more complicated controller and a position sensor.

● Asynchronous AC Servo Motor: Asynchronous AC Servo Motor is simpler, cheaper, and more rugged than synchronous motor, but it has lower efficiency, preciseness, and speed.

● DC Servo Motor: DC Servo Motor is operated by Direct Current (DC). DC servo motor can be of two types, such as brushed and brushless.

● Brushed DC Servo Motor: Brushed DC Servo Motor is simple, inexpensive, and easy to control, but it has lower efficiency, lifespan, and speed due to friction and wear of the brushes.

● Brushless DC Servo Motor: Brushless DC Servo Motor is more efficient, durable, and faster than brushed motor, but it needs a more sophisticated controller and a position sensor.

● Linear Servo Motor: Linear Servo Motor generates linear motion instead of rotary motion. Linear servo motor can be of two types, such as iron-core and ironless.

● Iron-Core Linear Servo Motor: Iron-core Linear Servo Motor has high force density, stiffness, and precision, but it also has high cogging force, weight, and heat generation.

● Ironless linear Servo Motor: Ironless linear Servo Motor has only magnets, but no iron cores.

Applications of Servo Motors

• Robotics: Servo motors are used to provide precise motion and force for robots.
• CNC Machinery: Servo motors allow CNC machines to perform accurate and complex machining activities, such as cutting, drilling, and engraving.
• Automated Manufacturing: Servo motors are used to control the movement and position of various components and devices in automated manufacturing systems, such as conveyors, feeders, loaders, etc.
• Medical equipment: Servo motors allow medical equipment to do accurate and safe operations and treatments.
• Electric mobility: Servo motors are widely used in electric mobility and electric vehicles.

Conclusion

Servo motors are reliable kinds of motors that are very useful for large-scale applications where precise control of position or speed is very important. They are used in various types of industries including robotics, automation, and manufacturing.

If you need help choosing a suitable DC servo motor for your project, contact us using the contact button below. Or you could check our range of servo DC motors.