The working principle of a DC servo motor is basically the same as that of a regular DC motor. It relies on the interaction between the armature airflow and the air gap magnetic flux to generate electromagnetic torque, causing the servo motor to rotate. Armature control is typically used, where the speed is changed by altering the voltage while maintaining a constant excitation voltage. The lower the voltage, the lower the speed; when the voltage is zero, rotation stops. Because the current is also zero when the voltage is zero, the motor does not generate electromagnetic torque and therefore does not exhibit self-rotation.
Working principle of DC servo motor
A DC servo motor is an assembly consisting of four main components: the DC motor itself, a position sensor, a gear assembly, and a control circuit. The desired speed of the DC motor depends on the applied voltage. To control the motor speed, a potentiometer generates a voltage that is applied to one of the inputs of an error amplifier.
In some circuits, a control panel is used to generate a DC reference voltage corresponding to the desired position or speed of the motor and applies it to a pulse with a voltage converter. The length of the pulse determines the voltage applied to the error amplifier as the desired voltage to produce the speed or position required by a digital control PLC or any other device.
Feedback sensors are typically potentiometers, which generate a voltage corresponding to the absolute angle of the motor shaft via a gear mechanism. The feedback voltage is applied to an input error comparator amplifier, which compares the voltage generated by the motor at its current position (feedback from the potentiometer) with the voltage generated at the desired position to reduce errors caused by positive or negative voltage. As long as an error exists, this error voltage increases with the output voltage applied to the armature. The comparator amplifier amplifies the error voltage and the corresponding armature power, filling the motor error to zero. If the error is negative, the armature voltage disappears, thus reversing the armature voltage and causing the armature to rotate in the opposite direction.
A DC servo motor has a direct current (DC) circuit with positive and negative terminals. The current flows in exactly the same direction between each of these terminals. The servo motor's moment of inertia should be small to ensure precision and accuracy. DC servo motors have a fast response, which can be achieved by maintaining a high torque-to-weight ratio. Furthermore, the speed characteristics of a DC servo motor should be linear.
Using a DC servo motor, current control is much simpler than using an AC servo motor because the only control requirement is the current armature amplitude. Motor speed is controlled by pulse-width modulation (PWM) with duty cycle control. Control flux is used to manage torque, thus achieving reliable consistency across each active cycle.
DC servo motors typically have greater inertia than squirrel-cage AC motors. This, along with increased brush friction resistance, is a major factor hindering their use in instrument servo systems. In small sizes, DC servo motors are primarily used in aircraft control systems, where weight and space constraints require the motor to deliver maximum power per unit volume. They are often used in intermittent operation or where exceptionally high starting torque is required. DC servo motors can also be used in electromechanical actuators, process controllers, programming devices, industrial automation robots, CNC machine tools, and many other similar applications.
This article provides a brief introduction to the working principle and characteristics of DC servo motors. A review of the full text reveals that a DC servo motor is an assembly composed of four main components: the DC motor itself, a position sensor, a gear assembly, and a control circuit. The required speed of the DC motor depends on the applied voltage. To control the motor speed, a potentiometer generates a voltage that is applied to one of the inputs of an error amplifier.
The advantages of DC servo motors include high control precision, fast response speed, stable torque output, and low noise. Furthermore, they can achieve closed-loop control by measuring the motor's position using an encoder. This closed-loop control method makes DC servo motors even more precise and better resistant to load disturbances and interference.
The disadvantages of DC servo motors are that they are more expensive than AC motors and require additional controllers and encoders to achieve closed-loop control. Furthermore, their lifespan is relatively short, requiring regular maintenance and parts replacement.
In summary, DC servo motors are high-precision, fast-response motors widely used in machining, automation control, and other fields. Their advantages include high control precision, fast response speed, stable torque output, and low noise.
