As the importance of control motors increases, their usage also increases year by year. A stepper motor is a type of control motor that can perform speed and positioning control without using a feedback loop; this is known as open-loop motor control.
Its applications are primarily focused on OA (Office Automation) and FA (Software Automation) machines, which have strong capabilities in handling office tasks, and are also widely used in medical devices, measuring instruments, automobiles, and game consoles. In terms of quantity, OA machine applications account for approximately 75% of the total number of stepper motors used.
I. Stepper Motor Principle
A stepper motor is a motor that uses the principle of electromagnetism to convert pulse signals into linear or angular displacement. With each electrical pulse, the motor rotates by an angle, causing the machine to move a small distance.
II. Classification
Motors can be classified in various ways. For example, when classified by voltage type, there are AC driven and DC driven motors; when classified by the relationship between rotational speed and power supply frequency, there are synchronous motors and asynchronous motors.
The following diagram shows the positional relationship of stepper motors in the small motor series.
As shown in the diagram above, stepper motors are DC-driven synchronous motors, but they cannot be directly driven by DC or AC power supplies; a driver is required. Therefore, stepper motors require a drive circuit to operate. This is similar to brushless DC motors, which also require a drive circuit to connect the motor stator to the DC power supply.
III. Stepper Motor Characteristics
The basic characteristics of a stepper motor include its static characteristics, continuous motion characteristics (dynamic characteristics), starting characteristics, and braking characteristics (transient characteristics). These will be described in detail below:
1. Static torque characteristics
When a stepper motor's coils are energized with direct current, the relationship between the electromagnetic torque of the loaded rotor (the restoring electromagnetic torque generated by balancing the load torque is called the static torque) and the rotor power angle is called the angle-static torque characteristic, which is the motor's static characteristic. See the diagram below:
Because the rotor is a permanent magnet, the resulting air gap magnetic flux density is sinusoidal, so theoretically the static torque curve is a sine wave. This angle-static torque characteristic is an important indicator of a stepper motor's ability to generate electromagnetic torque; the larger the maximum torque, the better, and the closer the torque waveform is to a sine wave, the better. In reality, there is cogging torque under the magnetic poles, which distorts the resultant torque. For example, the cogging torque of a two-phase motor is a fourth harmonic of the static torque angle period. Added to the sinusoidal static torque, the torque shown in the diagram above would be:
TL = TMsin[(θL/θM)π/2]
Where TL and TM represent the load torque and maximum static torque (or holding torque), respectively, and the corresponding power angles are θL and θM. The change in this displacement angle determines the positional accuracy of the stepper motor. According to the above formula:
θL=(2θM/π)arcsin(TL/TM)
The step angle θs of PM-type permanent magnet stepper motors and HB hybrid stepper motors were discussed in previous lessons, namely: θs = 180°/PNr. Converting the angle to mechanical angles (radians), it becomes the following formula:
θs=π/(2Nr)
In the above formula, Nr is the number of rotor teeth or pole pairs, so for a two-phase motor, θM = θs.
2. Dynamic torque characteristics
Dynamic torque characteristics include drive pulse frequency-torque characteristics and drive pulse frequency-inertia characteristics.
1) Pulse frequency-torque characteristics
The pulse frequency-torque characteristic is an important feature for selecting a stepper motor. As shown in the figure below, the vertical axis represents the dynamic torque, and the horizontal axis represents the response pulse frequency, which is expressed in pps, i.e., the number of pulses per second.
As shown in the figure, the dynamic torque generation of a stepper motor includes two torques: pull-out torque and pull-in torque. The former is called pull-out torque, and the latter is called starting torque. The pull-in torque ranges from zero to the maximum self-starting pulse frequency or maximum self-starting frequency range. The area enclosed by the pull-in curve is called the self-starting region. The motor starts and runs synchronously in both forward and reverse directions. The operating region is between the pull-in and pull-out regions. Within this region, the motor can run synchronously and continuously with a corresponding load. Load torques exceeding this range will prevent continuous operation, resulting in pull-out. Stepper motors use open-loop drive control, and there should be a margin between the load torque and the electromagnetic torque, which should be 50% to 80%.
2) Pulse frequency-inertia characteristics
When a stepper motor starts rapidly with an inertial load, it must have sufficient starting acceleration. Therefore, if the load inertia increases, the starting pulse frequency will decrease. For this reason, both factors must be considered when selecting a stepper motor.
The vertical axis of the graph below represents the maximum self-starting frequency, and the horizontal axis represents the load inertia. The curves show the relationship between the load inertia and the maximum self-starting pulse frequency. This example uses a PM-type claw-pole stepper motor (two-phase, step angle 7.5°). Under load PL, the relationship between the maximum self-starting pulse frequency PL and the load inertia Jc is as follows:
In the formula, JR represents the rotor inertia of the stepper motor, and Ps represents the maximum self-starting frequency under no-load conditions.
3. Transient torque characteristics
Due to the rotor inertia of the stepper motor, even if it runs one step under no load, it will generate an overshoot angle and oscillate back and forth between the overshoot angle and the undershoot angle. After the overshoot angle is reduced, it will come to rest at a predetermined angle. This is the transient response characteristic of the stepper motor.
The figure below illustrates the transient characteristics of a stepper motor. The vertical axis represents the rotor's movement angle, and the horizontal axis represents time. ΔT is the rise time, Δθ is the overshoot angle, and the time it takes for the rotor to move from free rest to the set position (typically reaching ±5% of the step angle error range) is called the settling time.
A shorter settling time results in better speed. To accelerate the mechanism's operation and shorten the settling time, the damping (braking) of the stepper motor becomes crucial. Methods to shorten the settling time include altering friction or changing the inertia drive, which will be discussed in detail later.
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