A stepper motor is a special type of motor used for control. It rotates step by step at fixed angles (called "step angles"). Its characteristic is that there is no accumulated error (100% accuracy), so it is widely used in various open-loop control systems.
A stepper motor requires an electronic drive to operate; this drive is called a stepper motor driver. It converts the pulse signals from the control system into angular displacement of the stepper motor. In other words, for every pulse signal sent by the control system, the driver causes the stepper motor to rotate one angular increment. Therefore, the speed of the stepper motor is directly proportional to the frequency of the pulse signals.
Controlling the frequency of the stepper pulse signal allows for precise speed adjustment of the motor; controlling the number of stepper pulses allows for precise positioning of the motor. When a stepper motor is driven by a microstepping driver, its step angle is reduced. For example, when the driver operates in 10-microstep mode, its step angle is only one-tenth of the motor's inherent step angle. That is, when the driver operates in full-step mode without microstepping, the motor rotates 1.8° for each stepper pulse; while with a microstepping driver operating in 10-microstep mode, the motor only rotates 0.18°. This is the basic concept of microstepping. The microstepping function is entirely generated by the driver precisely controlling the motor's phase current and is independent of the motor itself.
The main advantages of microstepping drivers are: complete elimination of low-frequency oscillations in the motor. Low-frequency oscillations are an inherent characteristic of stepper motors (especially reactive motors), and microstepping is the only way to eliminate them. If your stepper motor sometimes needs to operate in the resonance zone (such as traversing arcs), choosing a microstepping driver is the only option. Increased output torque. Especially for three-phase reactive motors, the torque is increased by approximately 30-40% compared to non-microstepped motors. Improved motor resolution. The increased resolution is self-evident due to the reduced step angle and improved step uniformity.
The above explains the basic principles of stepper motors. Next, we will summarize our practical experience in designing and selecting stepper motor drivers:
Select holding torque.
Holding torque, also known as static torque, refers to the torque with which the stator locks the rotor in place when a stepper motor is energized but not rotating. Since the torque of a stepper motor at low speeds is close to its holding torque, and the torque of a stepper motor decreases rapidly with increasing speed, and the output power also changes with speed, holding torque is one of the most important parameters for measuring the load capacity of a stepper motor. For example, a stepper motor with a holding torque of 1 N·m can generally be understood as having a holding torque of 1 N·m.
Select number of phases
Two-phase stepper motors are low in cost, with a minimum step angle of 1.8 degrees. They exhibit greater vibration at low speeds and a rapid drop in torque at high speeds, making them suitable for high-speed applications where precision and stability requirements are not high. Three-phase stepper motors have a minimum step angle of 1.5 degrees, less vibration than two-phase stepper motors, better low-speed performance than two-phase stepper motors, and a maximum speed 30% to 50% higher than two-phase stepper motors. They are suitable for high-speed applications where precision and stability requirements are high. Five-phase stepper motors have an even smaller step angle and better low-speed performance than three-phase stepper motors, but are more expensive. They are suitable for medium-to-low speed applications where precision and stability requirements are high.
Select a stepper motor
The principle of selecting the motor first and then the driver should be followed. First, clarify the load characteristics, and then find the stepper motor that best matches the load characteristics by comparing the static torque and torque-frequency curves of different models. When high precision is required, a mechanical reduction gear should be used to ensure that the motor operates in a state of highest efficiency and lowest noise. Avoid operating the motor in the vibration zone; if necessary, this can be achieved by changing the voltage, current, or increasing damping. Regarding power supply voltage, it is recommended that 57 motors use DC 24V-36V, 86 motors use DC 46V, and 110 motors use a voltage higher than DC 80V. For loads with large rotational inertia, a motor with a larger frame size should be selected. For loads with large inertia and high operating speed, the motor should be gradually increased in frequency and speed to prevent motor step loss, reduce noise, and improve positioning accuracy when stopped. Since the torque of a stepper motor is generally below 40Nm, if it exceeds this torque range and the operating speed is greater than 1000RPM, a servo motor should be considered. Generally, AC servo motors can operate normally at 3000RPM, and DC servo motors can operate normally at 10000RPM.
Select drive and subdivision number
It's best to avoid full-step mode, as it results in significant vibration. Choose drivers with low current, high inductance, and low voltage. Use drivers with current greater than the operating current. Use microstepping drivers when low vibration or high precision is required. For high-torque motors, use high-voltage drivers to achieve good high-speed performance. In applications where the actual operating speed of the motor is usually high and precision and smoothness requirements are not high, it's unnecessary to choose a high microstepping driver to save costs. When the actual operating speed of the motor is usually very low, a larger microstepping should be selected to ensure smooth operation and reduce vibration and noise. In short, when selecting the microstepping, the actual operating speed of the motor, the load torque range, the reducer settings, precision requirements, vibration and noise requirements, etc., should be comprehensively considered.
