I. What is the working principle of a stepper motor?
Most motors contain an iron core and winding coils. The windings have resistance, and when current flows through them, losses occur. The magnitude of these losses is proportional to the resistance and the square of the current; this is what we commonly call copper loss. If the current is not standard DC or a sine wave, harmonic losses will also occur. The iron core has hysteresis and eddy current effects, which also generate losses in an alternating magnetic field. The magnitude of these losses depends on the material, current, frequency, and voltage; this is called iron loss.
Both copper and iron losses manifest as heat, thus affecting motor efficiency. Stepper motors generally prioritize positioning accuracy and torque output, resulting in relatively low efficiency, high current consumption, and high harmonic content. The frequency of current alternation also varies with speed. Consequently, stepper motors commonly experience heat generation, and the situation is more severe than with general AC motors.
An electric motor converts electrical energy into mechanical energy, and a stepper motor is an open-loop control element that converts electrical pulse signals into angular or linear displacement. Under non-overload conditions, the motor's speed and stopping position depend only on the frequency and number of pulse signals, and are unaffected by load changes. That is, applying a pulse signal to the motor will cause it to rotate one step angle. This linear relationship, coupled with the fact that stepper motors only have periodic errors and no cumulative errors, makes controlling speed and position using stepper motors very simple.
II. Working Principle of Servo Motor
1. A servo system is an automatic control system that enables the output controlled variables, such as 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 corresponding angle, thus achieving displacement. Because the servo motor itself has the function of generating pulses, it generates a corresponding number of pulses for each angle of rotation. This creates a feedback loop, or closed loop, between the pulses received and the output. In this way, the system knows how many pulses were sent to the servo motor and how many were received, allowing for very precise control of the motor's rotation and achieving precise positioning down to 0.001mm. DC servo motors are divided into brushed 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 are suitable for 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.
2. AC servo motors are also brushless motors, and they are divided into synchronous and asynchronous motors. 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, a low maximum rotational speed, and their speed decreases rapidly as power increases. Therefore, they are suitable for low-speed, stable operation applications.
3. The rotor inside the servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver forms an electromagnetic field, and the rotor rotates under the influence of this magnetic field. At the same time, 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 the 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.
