A servo motor is an engine that controls the operation of mechanical components in a servo system. The rotor speed of a servo motor is controlled by the input signal and can respond quickly. In automatic control systems, it serves as an actuator and possesses characteristics such as a small electromechanical time constant, high linearity, and low starting voltage. It can convert received electrical signals into angular displacement or angular velocity output on the motor shaft. Servo motors are broadly classified into two categories: DC and AC servo motors.
Servo motor working principle:
1. A servomechanism is an automatic control system that enables the output controlled variables, such as the position, orientation, and state of an object, to follow any changes in the input target (or given value).
Servo systems primarily rely on pulses for positioning. Essentially, a servo motor receives one pulse and rotates by the angle corresponding to that pulse, thus achieving displacement. Because the servo motor itself has the function of emitting pulses, it emits a corresponding number of pulses for each angle it rotates. This creates a feedback loop, or closed loop, between the pulses sent to and received by the servo motor. In this way, the system knows how many pulses were sent to and received by the servo motor, allowing for very precise control of the motor's rotation and achieving accurate positioning down to 0.001mm .
2. DC servo motors are divided into brushed motors and brushless motors.
Brushed motors are low-cost, simple in structure, have high starting torque, wide speed range, and are easy to control. However, they require maintenance, which is inconvenient (replacing carbon brushes), generates electromagnetic interference, and has environmental requirements. Therefore, they can be used in cost-sensitive general industrial and civilian applications.
Brushless motors are small in size, lightweight, powerful, fast-responding, high-speed, low-inertia, smooth-rotating, and stable in torque. While their control is complex, they are easily made intelligent. Their electronic commutation is flexible, allowing for either square wave or sine wave commutation. The motors are maintenance-free, highly efficient, operate at low temperatures, have minimal electromagnetic radiation, and a long lifespan, making them suitable for various environments.
The working principle of servo motors and their differences from stepper motors.
AC servo motors are also brushless motors, and they are divided into synchronous and asynchronous motors. Currently, synchronous motors are generally used in motion control because they have a wide power range and can achieve very high power. They have high inertia, low maximum rotational speed, and their speed decreases rapidly as power increases. Therefore, they are suitable for applications requiring low-speed, stable operation.
The rotor inside a servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver creates an electromagnetic field, causing the rotor to rotate under the influence of this magnetic field. Simultaneously, the motor's built-in encoder feeds back signals to the driver. The driver compares the feedback value with the target value and adjusts the rotor's rotation angle accordingly. The accuracy of a servo motor depends on the accuracy (line count) of the encoder.
The functional differences between AC servo motors and brushless DC servo motors: AC servo motors are generally better because they use sinusoidal wave control, resulting in less torque ripple. DC servo motors use trapezoidal wave control. However, DC servo motors are simpler and cheaper.
The main advantages are:
(1) It has no brushes or commutator, so it is reliable in operation and requires little maintenance.
(2) The stator winding is relatively easy to dissipate heat.
(3) Low inertia makes it easy to improve the speed of the system.
(4) Suitable for high-speed and high-torque working conditions.
(5) It has a smaller volume and weight for the same power.
The difference between servo motors and stepper motors
With the emergence of fully digital AC servo systems, AC servo motors are increasingly being used in digital control systems. To adapt to the development trend of digital control, most motion control systems use stepper motors or fully digital AC servo motors as actuators.
Stepper motors, as an open-loop control system, are fundamentally linked to modern digital control technology. They are widely used in current domestic digital control systems. With the emergence of fully digital AC servo systems, AC servo motors are also increasingly being applied in digital control systems. To adapt to the development trend of digital control, most motion control systems use stepper motors or fully digital AC servo motors as actuators. Although they are similar in control methods (pulse trains and direction signals), they differ significantly in performance and application scenarios. A comparison of their performance is presented below.
I. Different control precision
Two-phase hybrid stepper motors typically have step angles of 1.8 ° and 0.9 °, while five-phase hybrid stepper motors typically have step angles of 0.72 ° and 0.36 °. Some high-performance stepper motors can have even smaller step angles after microstepping. For example, the two-phase hybrid stepper motors manufactured by Sanyo can have their step angles set to 1.8 °, 0.9 °, 0.72 °, 0.36°, 0.18 °, 0.09 °, 0.072 ° , and 0.036 ° via DIP switches, making them compatible with both two-phase and five-phase hybrid stepper motors.
The control precision of the AC servo motor is ensured by the rotary encoder at the rear end of the motor shaft.
II. Different Low-Frequency Characteristics
Stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency is related to the load and driver performance, and is generally considered to be half of the motor's no-load starting frequency. This low-frequency vibration, determined by the working principle of stepper motors, is very detrimental to the normal operation of the machine. When stepper motors operate at low speeds, damping techniques should generally be used to overcome low-frequency vibration, such as adding a damper to the motor or using microstepping technology in the driver.
AC servo motors operate very smoothly, without vibration even at low speeds. AC servo systems feature resonance suppression to compensate for insufficient mechanical rigidity, and internal frequency analysis (FFT) capabilities to detect mechanical resonance points, facilitating system adjustments.
III. Different Moment-Frequency Characteristics
The output torque of a stepper motor decreases as the speed increases, and drops sharply at higher speeds. Therefore, its maximum operating speed is generally between 300 and 600 RPM. AC servo motors provide constant torque output, meaning they can output rated torque up to their rated speed (generally 2000 or 3000 RPM), and provide constant power output above the rated speed.
IV. Different Overload Capacities
Stepper motors generally lack overload capacity. AC servo motors, on the other hand, have strong overload capacity. Taking the Sanyo AC servo system as an example, it possesses both speed and torque overload capabilities. Its maximum torque is two to three times its rated torque, which can be used to overcome the inertial torque of inertial loads at startup. Because stepper motors lack this overload capacity, a motor with a larger torque is often selected to overcome this inertial torque during selection. However, the machine does not require such a large torque during normal operation, resulting in wasted torque.
V. Different operating performance
Stepper motors are controlled in an open-loop manner. Excessive starting frequency or load can easily lead to missed steps or stalling. Excessive stopping speed can cause overshoot. Therefore, to ensure control accuracy, the acceleration and deceleration issues must be properly addressed. AC servo drive systems, on the other hand, use closed-loop control. The driver can directly sample the feedback signal from the motor encoder, internally forming position and speed loops. Generally, the missed steps or overshoot issues of stepper motors are not present, resulting in more reliable control performance.
VI. Different speed response performance
Stepper motors require 200-400 milliseconds to accelerate from a standstill to their operating speed (typically several hundred revolutions per minute). AC servo systems offer better acceleration performance. For example, a Sanyo 400W AC servo motor can accelerate from a standstill to its rated speed of 3000 RPM in just a few milliseconds, making it suitable for control applications requiring rapid start and stop.
AC servo systems outperform stepper motors in many aspects, but stepper motors are still frequently used as actuators in less demanding applications. When choosing a servo motor, factors such as requirements and cost should be considered. This article should have given you a general understanding of servo motors. Understanding the working principles of these devices is crucial for maximizing their functionality and effectiveness.
