DC Servo Motor: Construction, Working & Its Applications

Servo motors are among the most efficient and precise motor machines. Its torque to inertia ratio is high. Hence, it can develop a significant twisting force in terms of its size and mass. This characteristic makes them suitable for use in areas that require quick changes in speed and reliable positioning. Servo motors can be supplied in various sizes and power capabilities by either AC or DC current. By using somewhat complex pulse width modulation (PWM) signals, their speed and force can be controlled. Further, the communication controllers and interfaces are often available on many servo motors, making them more versatile for integration with complex systems. These motors are of two types- AC and DC servo motors. You can read further information on DC motors in this blog by the Schneider Electric eShop team. 

What is a DC Servo Motor?

A DC servo motor is a specific type of servo motor that has the ability to take the direct current electrical supply and the outputs in terms of position, velocity or acceleration. These motors are used in applications where the starting and stopping of the motor are sensitive and swift; this includes numerically controlled machines and computers. 

DC Servo Motor Construction and Working

Construction 

• DC Motor: The core motor is a direct current motor that uses a field winding that is separately excited. According to the excitation mode, DC servo motors can be in two types, namely, the armature-controlled motor and the field-controlled motor. 
• Load: The load could be as simple as a fan or an industrial load coupled to the mechanical shaft of the motor. 
• Gearbox: Serves as a mechanical transducer to modify the motor’s output signal —acceleration, position, or velocity— to fit the task at hand. 
• Position Sensor: In general, it is a potentiometer that gives back information by generating a voltage corresponding to the angle of the motor shaft through a system of gears. 
• Comparator: Compares a reference signal from the signal obtained from the position sensor to generate an error signal that is given to an amplifier. 
• Amplifier: Increases the error signal coming out of the comparator section and gives it to the motor.
• Pulse Width Modulator (PWM): Compensates the feedback signal given to the motor to fine-tune the input towards it. 

Working

When there is an input signal, the shaft of the DC motor and gears turn. This change is captured by the position sensor, where the rotation of the gear causes it to change its resistance in order to generate a voltage change. This voltage, being its error signal, is supplied to the controller, which produces a PWM signal to control the motor's input. 

Transfer Function of DC Servo Motor

For a DC servo motor, the transfer function means the ratio of the Laplace transform of the output variable to the input variable. This function defines the manner in which the motor is able to transform electrical energy expressed at the armature terminals to mechanical energy in a controlled manner. More often, armature-controlled DC servo motors offer better performance and much faster response than field-controlled systems.

Specifications

• Shaft Speed: Stated in RPM (rotations per minute), it is the speed at which the shaft rotates.
• Terminal Voltage: The nominal voltage at which the motor is to be designed, manipulating the speed of the motor through the supply voltage fed into it. 
• Torque: Shaft torque created by the motor; it consists of start-up torque, which is the torque needed to initiate the spinning of the motor and the running torque, which is the torque capacity when the motor is operating continuously in a steady manner. 

AC Servo Motor vs DC Servo Motor

Power Source and Operation 

• AC Servo Motor: Runs on AC and is mostly used in the production of equipment that requires high voltage. 
• DC Servo Motor: Runs on direct current, and common applications are for low voltage equipment. 

Control Mechanism 

• AC Servo Motor: Has position feedback through the use of an encoder, which enables motor speed and position to be controlled. 
• DC Servo Motor: Consists of a potentiometer for feedback to control position and speed. 

Speed and Torque Characteristics 

• AC Servo Motor: It is able to give constant torque at variable speed and is therefore suitable for uses that need high torque in equal proportion with variable speed.
• DC Servo Motor: Offers good control in speed and has a high starting torque but has the drawback of causing a low torque at higher speeds. 

Read More -  Understanding Servo Motors: Working Principles & Applications

Conclusion

It can be concluded that the use of DC servo motors is seen in all the applications that demand accurate position, velocity or acceleration control. Thus, the construction of the motors, including such components as the motor, gearbox, position sensor, comparator, and amplifier, enables it to work with high accuracy and maximum speed. This blog must have helped you learn what a DC servo motor is. If you wish to buy these motors, shop online from Schneider Electric eShop today. 

Frequently Asked Questions

Q1. What are the characteristics of a DC servo motor?

Ans: DC servo motors are characterized by their precise control of angular position, velocity, and acceleration. They typically feature separate field windings and armatures, a feedback mechanism like a potentiometer or encoder, and a closed-loop control system. These motors deliver high torque at low speeds and exhibit quick response times.

Q2. What are the advantages of a DC servo motor?

Ans: DC servo motors offer high precision and responsiveness, making them ideal for applications requiring accurate positioning. They provide excellent torque performance at low speeds and can easily be controlled using simple electronic circuits. Additionally, their relatively simple construction allows for easier maintenance and lower operational costs.

Q3. What are the DC servo motor applications?

Ans: DC servo motors are widely used in applications such as robotics, CNC machinery, and automated manufacturing systems, where precise motion control is essential. They are also employed in aerospace, military equipment, and camera autofocus mechanisms, providing reliable performance in environments that demand high accuracy and quick response.