What is a servo motor?
Servo motors, also known as actuator motors, are actuators in automatic control systems. Their function is to convert received electrical signals into angular velocity or angular displacement outputs on the motor shaft.
Since their introduction, servo motors have proven their usefulness in numerous industries. For years, they have been involved in accomplishing large tasks. They may be small in size, yet they are incredibly powerful and energy-efficient. With these characteristics, servo motors are widely used in remote-controlled toy cars, airplanes, robots, and various industrial equipment. In recent years, servo motors have also been used in industrial applications, online manufacturing plants, pharmaceutical services, robotics, and the food service industry.
What are the different types of servo motors?
There are two types of servo motors: DC servo motors 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.
DC servo motors are ideal for small applications but cannot handle large current surges. AC servo motors, on the other hand, can handle much higher current surges and are widely used in industrial machinery. In terms of price, DC motors are cheaper than AC servo motors, so they are used more often. Furthermore, DC motors are specifically designed for continuous rotation, making them ideal for robot motion.
Working principle of servo motor
The working principle of a servo motor is relatively simple, yet it is highly efficient. The servo circuitry is built into the motor unit, which uses a flexible shaft, typically equipped with gears. Electrical signals control the motor and determine the amount of shaft movement. The internal setup of a servo motor is simple: a small DC motor, control circuitry, and a potentiometer. The DC motor is connected to a control wheel via gears; as the motor rotates, the resistance of the potentiometer changes, allowing the control circuitry to precisely adjust the motion and direction.
When the axis is in the correct (ideal) position, the motor stops receiving power. If the axis does not stop at the target position, the motor continues to run until it enters the correct direction. The target position is transmitted via a signal line using electrical pulses. Therefore, the motor speed is proportional to the actual and ideal position. As the motor approaches the desired position, it begins to rotate slowly, but when it reaches its maximum range, the speed is very fast. In other words, servo motors only need to complete the task as quickly as possible, making them highly efficient devices.
When there is no control voltage, an AC servo motor only has a pulsating magnetic field generated by the excitation winding in the air gap, and the rotor remains stationary due to the lack of starting torque. When a control voltage is applied and the control winding current and the excitation winding current are out of phase, a rotating magnetic field is generated in the air gap, producing electromagnetic torque and causing the rotor to rotate in the direction of the rotating magnetic field. However, servo motors are required not only to start under the influence of control voltage, but also to stop immediately after the voltage disappears. If a servo motor continues to rotate like a typical single-phase asynchronous motor after the control voltage disappears, a loss of control occurs; this phenomenon of self-rotation due to loss of control is called autorotation.
The basic working principle of a traditional DC servo motor is exactly the same as that of a regular DC motor. It relies on the interaction between the armature current and the air gap flux to generate electromagnetic torque, thus causing the servo motor to rotate. Armature control is typically used, meaning that the speed is adjusted by changing the armature voltage while keeping the excitation voltage constant. The lower the armature voltage, the lower the speed; when the armature voltage is zero, the motor stops. Since the armature current is also zero when the armature voltage is zero, the motor does not generate electromagnetic torque and therefore does not "rotate on its own."
Electric Motor Principle: A Diagram Explaining the Rotation Principle of an Electric Motor (Easy-to-Understand Version)
Differences between servo motors and stepper motors
1. Servo motors are high-speed and low-torque, while stepper motors are low-speed and high-torque.
Generally speaking, servo motors can easily reach rated speeds of several thousand RPM. For example, low-power servo motors, that is, those under 750W, are rated at 3000 RPM for Japanese models, while European servo motors can even reach 5000 or 6000 RPM. Stepper motors, due to their inherent characteristics, are generally only around 500 or 600 RPM. However, their torque is much greater than that of servo motors of the same specifications, so much so that a speed reducer can be omitted. In fact, stepper motors rarely use speed reducers; with speeds of only a few hundred RPM, adding a speed reducer would be practically useless.
2. The servo flange is a single piece, while the stepper flange is not.
Servo motor flanges are typically in multiples of 10, such as 40, 60, 80, 110, 180, etc., while stepper motor flanges are generally in multiples of 42, 57, 86, etc. You often hear "42 stepper" or "57 stepper" in engineering projects; these refer to the flange size. Stepper and servo motor flanges are not from the same series, which is a key factor in distinguishing between servo and stepper motors.
3. The servo's acceleration time is negligible, but the stepping time cannot be ignored.
The acceleration time of a servo motor is only a few milliseconds, which is negligible, while the acceleration time of a stepper motor is several hundred milliseconds. Don't underestimate this difference. Although we can't feel it intuitively, it has a crucial impact on the rhythm of mechanical equipment.
4. Servo motors have strong overload capacity, while stepper motors have almost no overload capacity.
Servo motors generally have overload capacity, such as 180% or 300%, while stepper motors have almost no overload capacity.
5. Servo motors are expensive, while stepper motors are cheaper.
Leaving aside that, if we only consider weight, stepper motors are about the same price as scrap metal, while servo motors are relatively expensive. Stepper motors are mainly used in applications with low speeds and infrequent starts and stops, especially in places with limited installation space; we can even think of stepper motors as cylinders. Servo motors, on the other hand, have a much wider range of applications than stepper motors, such as frequent starts and stops, and various high-speed devices.
