This section provides overview, applications, and principles of stepping motors. Also, please take a look at the list of 44 stepping motor manufacturers and their company rankings.
A stepping motor is a type of motor that's rotation angle can be controlled by pulse signals and can guarantee high positioning accuracy.
Also known as a pulse motor, the rotation angle is determined by the number of pulse signals, which are control signals, and the number of phases of the motor, while the rotation speed depends on the pulse frequency, which corresponds to the speed of the pulses. While relatively inexpensive and simple in motor configuration, it features high positioning accuracy and torque with open-loop control.
Stepping motors are used mainly in applications where positioning accuracy is required, as they are structurally suited for precise and reversible angle control. An example is a drive motor used to express two-dimensional movements of robot devices such as automatic transfer equipment.
By combining high-precision ball screws and stepping motors, the feed rate of a stage can be expressed with extremely high precision and repeatability. Also, for coating machines that spray a certain amount of paint depending on the valve opening, stepping motors can be used to precisely adjust the valve opening angle for more elaborate operations.
The inside of a stepping motor consists of a rotor connected to a shaft and multiple stators installed around the periphery of the rotor. The rotor section is further divided into two parts, each of which is magnetized so that its N-pole and S-pole are in opposite phases.
The stator is characterized by the presence of small teeth, the spacing between which is precisely controlled. 2-phase stepper motors, for example, are magnetized with the same polarity for the stator facing each other and in the opposite direction of the adjacent stator. Therefore, the stator attracts and repels the unevenness of the rotor, and the rotor is held in an energetically stable position relative to the stator's magnetization state.
Then, when current is applied to reverse the polarity of the stator, the rotor rotates by one stator. Repeated control of this process allows precise control of the angle of rotation according to the mechanical precision of the small teeth of the stator. 5-phase stepping motors control this sequentially in five steps, which allows finer angle control.
The output torque of a stepping motor varies depending on the rotation speed. Generally, the torque is high when the rotation speed is slow and low and when the rotation speed is high. When selecting a stepping motor, check the motor rotation speed-torque characteristics chart and select a motor so that the required torque at the operating rotation speed falls within the pull-out torque curve.
In particular, the pullout torque at high rotational speed is about 20% of the maximum excitation quiescent torque.
Torque characteristics also vary depending on the driver used, internal structure, and input voltage, even for motors of the same external dimensions, so motor selection should also take into consideration the manufacturer, driver combination, and input voltage.
A control device called a driver is required to operate a stepping motor. The driver controls the current voltage that flows to the stepping motor to control the rotation speed, amount of rotation, and other factors.
Drivers are available in constant-current and low-voltage drive systems, but the constant-current system is often used because of its superior torque characteristics at high speeds. Generally, a pulse train is input to the driver from a host control device as an indication value of rotation speed and amount and the motor accordingly.
Some drivers are equipped with a function called microstep. A stepping motor rotates with the same basic step angle as the minimum rotation angle, but a driver with the microstep function can adjust the current flowing to each coil to electrically subdivide the basic step angle and increase the resolution of rotation.
It also has the effect of reducing vibration and noise, overshoot at each step angle, and shock mitigation at startup and shutdown. The resolution of the microstep function can be selected by DIP switches, etc., depending on the intended use.
AC servo motors are often quoted in connection with stepping motors. However, they have major differences.
AC servo motors have a built-in encoder and feedback control, so the rotational torque is almost constant regardless of the rotation speed. Stepping motors, on the other hand, are not suitable for this application because their rotational torque decreases at high rotational speeds. Conversely, if low-speed rotation is the primary use, a stepping motor is suitable.
Stepping motors are mainly available in the market as open-loop control type, but there are also products that can be closed-controlled by attaching an encoder for improved efficiency. However, in such cases, it will be necessary to review the other advantages of these motors, which is their relatively small size, simple configuration, and low cost.
Applications that are more suitable for AC servo motors are those that require advanced rotation control using multiple motors. Since open-loop control cannot be expected to compensate for motor-to-motor motion by sensing, AC servo motors are better suited for this application than stepping motors, as is the case with high-speed rotary motion.
*Including some distributors, etc.
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