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Stepping motors are a type of control motors that move and rotate at a fixed angle like a clock by switching the phase through which current flows. Positioning of stepping motors can be achieved without a sensor. They are also known as a pulse motor, step motor, or stepper motor.
Stepping motors can be classified into 2-phase, 3-phase, and 5-phase motors based on structure.
The angle at which a stepping motor turns with one pulse, like a clock, is called the basic step angle. The finer it is, the movement is smoother and more accurate.
A driver is required to drive the stepping motor. One thing to keep in mind when designing a driver is that, in the case of a 2-phase stepping motor, the drive circuit of the driver differs depending on whether it is bipolar drive or unipolar drive. 3-phase and 5-phase stepping motors require a driver that can supply current in two directions.
Furthermore, a 2-phase and a 3-phase stepping motors have a simple winding structure, so even if the manufacture of the driver is different, the motor can be rotated with the same drive circuit. However, the 5-phase stepping motors have a complicated winding structure and there are a number of phase combinations and phase switching sequences of phases to rotate the motor. So, care must be taken with the driver to be combined.
First of all, pulse command signals are electric signals in which voltage ON and OFF (HI/LOW) repeats. One pulse is counted for each HIGH/LOW cycle.
Pulse signals are input to the driver from the host controller, and the rotation angle is controlled by these pulse signals.
Speed control of stepping motors
The rotational speed is controlled by the density of the number of pulses. When one pulse rotates the motor by one basic step angle, sending 10 pulses per second is a larger angle rotated per second than sending one pulse per second. So the higher pulse frequency, the faster the rotation speed.
Position control of stepping motor
Latest stepping drivers have a microstepping mode, which enables better positioning with step angles smaller than the full step angle of the motor by finely controlling the amount of current flowing to the phase, thus enabling finer positioning control.
Advantage 1: Easy to control
Easy operation by simply turning the driver's transistor on and off correctly. Easy to increase the rotation speed by simply speeding up the ON/OFF timing.
Advantage 2: Systems can be simplified
The motor can be easily controlled by the pulse input to the driver and its frequency, so a complicated controller is not required. In addition the most attractive feature of stepping motors is that the system can be simplified by being able to control the position and speed without a detector.
Advantage 3: Inexpensive systems can be constructed
The system can be built inexpensively as the driver is simple and does not require a detector and a complicated controller.
Advantage 4: Stable stopping is possible
A stepping motor is a motor that can be stopped by magnetic force. In other words, they generate a stopping force (holding torque), which enables them to stop stably.
Caution 1: Step-out
Stepping motors do not require sensors, but cannot check if they are moving as commanded or not (in the event of step-out). They are less reliable than servo motors with closed-loop control.
Caution 2: High heat generation
Stepping motors generate holding torque even during deceleration, generating heat.
Caution 3: Vibrations
Stepping motors, rotating at a fixed angle and moving one step at a time like a stairway, cannot move without causing vibration.
| Stepping motor | Servo motor | |||
| Control | Open-loop control | Closed-loop control | ||
| Torque | No sensor | Generates a constant torque in the direction that would slow down the motor. | Motor current sensor | Controls the motor current to generate a command torque in the direction that would accelerate the motor. |
| Rotational speed | No sensor | Rotates at a speed synchronized with the input pulse frequency. When not synchronized, it will step-out. | Motor speed sensor | Ensures that the motor rotates at the commanded speed by checking the motor speed. |
| Position (angle) | No sensor | Moves step by step with the input pulse. When the motor cannot keep up, it will step-out. | Motor position sensor | Ensures that the motor reaches the commanded target position by checking the motor position. |
| Characteristics | Less expensive and easy to use. High torque at low speeds | Expensive but has various functions. High torque at low to high speeds | ||
| Friction | Fast and accurate positioning even with high friction. | Precise positioning is possible, but it takes long with high friction. | ||
| Resonance and vibration | Speed ripple is high and resonance cannot be suppressed by the motor. | Speed ripple and resonance can be suppressed by the driver. | ||
| Inertia ratio | When the inertia ratio is high, the motor cannot be controlled. | The motor can be controlled even when the inertia ratio is high. | ||
| Advantages and applications |
The same position can be kept. | Stable operation with low vibration and noise. | ||
| Low-speed start/stop operation for short distances | High-speed/acceleration start/stop operation | |||
| Light processing operation | Precision and heavy processing operation | |||
Date of publication: March 15, 2021
